Method and apparatus for measuring one or more properties of a surface of an object

Abstract

A method of measuring properties of a surface of an object is disclosed. The method comprises making a plurality of sets of measurements of the object using a sensor (for example a level sensor), wherein a height of the object relative to the sensor is different for each of the plurality of sets measurements (for example a different height of the wafer stage may be used for each). For each of a plurality of positions on the surface of the object, a first quantity (indicative of a fine height measurement) and a second quantity (indicative of a coarse height measurement) are determined. For each of the plurality of positions on the surface of the object, the method further comprises: selecting one of the plurality of sets of measurements (based on the second quantity); and determining a weighting factor in dependence on the selected set of measurements for that position.

Description

2024P00445EP 1METHOD AND APPARATUS FOR MEASURING ONE OR MORE PROPERTIES OF A SURFACE OF AN OBJECTCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 25166738.2 which was filed on 27 March 2025 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to a method of measuring one or more properties of a surface of an object. The method may be used as a first measurement, pre-scan or calibration step for measuring aspects of a substrate within an exposure apparatus, for example a lithographic apparatus. In particular, the method may be used once when a first substrate of a particular lithographic process is loaded into such a lithographic apparatus. The method may have particular application for high topography lithographic processes such as 3D-NAND applications, and advanced packaging semiconductor processes. The present invention also relates to a method of determining a topography of a surface of an object. The present invention also relates to an exposure method which uses the method for of measuring one or more properties of a surface of an object and / or the method for measuring a topography of a surface of an object. The present invention also relates to a corresponding apparatus for measuring a topography of a surface of a substrate. The present invention has particular application in the field of lithography. The substrate may be a substrate within an exposure apparatus, for example a lithographic apparatus. The present invention also relates to an exposure apparatus comprising the apparatus for measuring a topography of a surface of a substrate.BACKGROUND

[0003] An exposure apparatus, for example a lithographic apparatus, is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may, for example, project a pattern (also often referred to as “design layout” or “design”) of a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g., a wafer).

[0004] As semiconductor manufacturing processes continue to advance, the dimensions of circuit elements have continually been reduced while the amount of functional elements, such as transistors, per device has been steadily increasing over decades, following a trend commonly referred to as ‘Moore’s law’. To keep up with Moore’s law the semiconductor industry is chasing technologies that enable to create increasingly smaller features. To project a pattern on a substrate a lithographic apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features which are patterned on the substrate. Typical wavelengths currently in use2024P00445EP 2are 365 nm (i-line), 248 nm, 193 nm and 13.5 nm. A lithographic apparatus, which uses extreme ultraviolet (EUV) radiation, having a wavelength within a range of 4 nm to 20 nm, for example 6.7 nm or 13.5 nm, may be used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

[0005] Before exposure of a wafer to patterned radiation in a lithographic apparatus, a topography of the wafer may be determined using apparatus that may be referred to as a level sensor. This measurement of the topography of the wafer may be performed within the lithographic apparatus, for example once the wafer has been clamped to a wafer stage. This information can be used during subsequent exposure of the wafer in order to keep the part of the wafer that is being exposed in a plane of best focus.

[0006] Traditionally, the substrate exposed in a lithographic apparatus comprises a silicon wafer coated in a photoresist. Additionally or alternatively, the substrate may comprise a metal, or an optically transparent material such as glass, quartz or a polymer. More recently, lithographic processes (for example advanced packaging semiconductor processes) have been proposed wherein the substrate has a composite structure. In particular, the substrate may comprise a base substrate to which a plurality of separate, previously formed sub-assemblies (for example silicon chips, optoelectronic devices etc.) have been applied. These sub -assemblies may be of the order of 10-100 pm high. A space between the sub-assemblies is filled with a filler material (for example an epoxy resin). Such a composite substrate may be loaded into a lithographic apparatus and various connection layers may be formed to form connections between groups of individual sub-assemblies. Such composite substrates may be considered to be an example of a high topography lithographic process. Another example of a high topography lithographic process is 3D-NAND manufacturing, in which memory cells may be stacked vertically (i.e. in a direction normal to a plane of the substrate. Determining a topography of such high topography lithographic processes (advanced packaging semiconductor processes and 3D-NAND substrates) presents new additional challenges.

[0007] It may be desirable to provide new methods and / or apparatus for determining a topography of an object (for example a substrate) that may at least partially address one or more problems associated with existing arrangements, whether identified herein or otherwise.SUMMARY

[0008] According to a first aspect of the present disclosure there is provided a method of measuring one or more properties of a surface of an object, the method comprising: (a) making a plurality of sets of measurements of the object using a sensor, wherein a height of the object relative to the sensor is different for each of the plurality of sets measurements, and wherein each set of measurements comprises, for each of a plurality of positions on the surface of the object, determining a first quantity and a second quantity by: projecting a radiation beam onto the position to form a periodic pattern thereon; receiving a portion of the radiation beam reflected from the position and2024P00445EP 3forming an image of the periodic pattern at a sensor region; determining the first quantity as being dependent on a phase of the pattern relative to the sensor region; and determining the second quantity as being proportional to an intensity of the radiation received at the sensor region; (b) for each of the plurality of positions on the surface of the object: selecting one of the plurality of sets of measurements in dependence on the second quantity; and determining a weighting factor for that position in dependence on the selected set of measurements for that position.

[0009] In some embodiments, determining the first quantity and the second quantity may comprise: splitting the received radiation into a first portion and a second portion such that the relative intensities of the first and second portions are dependent on a height of the position relative to the sensor; determining an intensity of each of the first and second portions; determining the first quantity as being proportional to a difference of the intensities of the first and second portions; and determining the second quantity as being proportional to a sum of the intensities of the first and second portions.

[0010] According to a second aspect of the present disclosure there is provided a method measuring one or more properties of a surface of an object, the method comprising: (a) making a plurality of sets of measurements of the object using a sensor, wherein a height of the object relative to the sensor is different for each of the plurality of sets measurements, and wherein each set of measurements comprises, for each of a plurality of positions on the surface of the object, determining a first quantity and a second quantity by: projecting a radiation beam onto the position to form a periodic pattern thereon; receiving a portion of the radiation beam reflected from the position and forming an image of the periodic pattern at a sensor region; splitting the received radiation into a first portion and a second portion such that the relative intensities of the first and second portions are dependent on a height of the position relative to the sensor; determining an intensity of each of the first and second portions; determining the first quantity as being proportional to a difference of the intensities of the first and second portions; and determining the second quantity as being proportional to a sum of the intensities of the first and second portions; (b) for each of the plurality of positions on the surface of the object: selecting one of the plurality of sets of measurements in dependence on the second quantity; and determining a weighting factor for that position in dependence on the selected set of measurements for that position.

[0011] The method according to the first and second aspects of the present disclosure may be used as a first measurement, pre-scan or calibration step for measuring aspects of a silicon wafer within a lithographic apparatus. In particular, the method according to the first and second aspects of the present disclosure may be used once when a first wafer of a particular lithographic process is loaded into such a lithographic apparatus. The method according to the first and second aspects of the present disclosure may have particular application for advanced packaging semiconductor processes.

[0012] The method according to the first and second aspects of the present disclosure is advantageous, as now discussed.2024P00445EP 4

[0013] In general, in a lithographic process a substrate may be exposed to radiation that has been patterned by a reticle or mask. Before the substrate is exposed to the patterned radiation, a topography of the surface of the substrate may be determined. For example a height map of the surface of the substrate may be determined, for example using a level sensor. Subsequently, the measured topography of the surface of the substrate can be used to control a height of the substrate while it is being exposed to the patterned radiation, for example to keep the substrate in a plane of best focus for the image of the patterning device. Typically, the substrate comprises a silicon wafer coated in a photoresist.

[0014] More recently, lithographic processes (for example advanced packaging semiconductor processes) have been proposed wherein the substrate has a composite structure. In particular, the substrate may comprise a base substrate to which a plurality of separate, previously formed subassemblies (for example silicon chips, opto-electronic devices etc.) have been applied. These subassemblies may be of the order of 10-100 pm high. A space between the sub-assemblies is filled with a filler material (for example an epoxy resin). Such a composite substrate may be loaded into a lithographic apparatus and various connection layers may be formed to form connections between groups of individual sub-assemblies.

[0015] The use of such composite substrates within lithographic apparatuses poses a number of additional challenges, as now discussed.

[0016] First, there will, in general, be a variety of different materials (the different subcomponents and the filler material) and each of these may have a different response when measured using known level sensors. For example, some level sensors may be subject to errors due to a fraction of the patterned radiation that is received being reflected from an interface at a non-zero depth below the surface of a substrate. Such errors can be much more easily corrected for or controlled if all parts of the substrate are subject to errors of the same magnitude. However, since a composite substrate comprises a variety of different materials, a magnitude of such errors can be significantly different for different parts of the substrate. In practice, with known systems, this may manifest itself as the height measurements for some portions of the composite substrate (for example the portions comprising filler material) being unacceptably inaccurate. Furthermore, if these measurements are subsequently used to control a position of the substrate during a subsequent exposure process in a known way, they can result in unacceptable lithographic imaging.

[0017] Second, such a composite substrate may be subject to earlier processing errors, which may mean that one or more sub-assemblies are missing from the composite substrate. If a height profile of such a composite substrate is determined (for example using a level sensor) then the determined height of the substrate in the proximity of the missing sub-assembly will typically be incorrect. Either: (a) the composite substrate may have a height that actually differs from the height the composite substrate would have in the presence of the sub-assembly by a significant amount (for example of the order of 10-100 pm); or (b) an erroneous height measurement will be obtained since2024P00445EP 5this region of the substrate may have been in-filled with filler material (resulting in a height measurement that is unacceptably inaccurate; as discussed above).

[0018] If the composite structure is controlled in dependence on a height profile (for example determined using a level sensor) in a conventional way then the presence of the erroneous height measurements (from regions of filler material or missing sub-assemblies) will result in sub-optimal imaging of other (more critical regions). This is because, with such a conventional control scheme, the lithographic apparatus will try to control the substrate in such a way that no portion of the substrate is out of focus by an unacceptable amount. For example, the height of a substrate may be controlled while it is being exposed to the patterned radiation in such a way that a first portion of the substrate (corresponding to a sub-assembly and which is within the field) is out of focus in one direction and a second portion of the substrate (corresponding to filler material or a missing subassembly and which is also within the field) is out of focus in another direction. As such, there is an imaging impact on the (critical) first portion due to the erroneous height measurements from the (non-critical) second portion. Furthermore, in addition to this imaging compromise of the critical portions of the substrate, the erroneous height measurements will result in additional movement of the substrate during exposure of the substrate (as the lithographic apparatus tries to keep the non-critical regions in acceptable focus). In turn, this may excite additional dynamic disturbances, which may lead to additional imaging errors in the critical regions of the substrate.

[0019] The methods according to the first and second aspects of the present disclosure are advantageous as they effectively combine the first and second quantities so as to determine a weighting factor for each of the plurality of positions on the surface of the object. The weighting factor for each position is dependent on the selected set of measurements for that position. As discussed further below, the selected set of measurements for each position (and therefore the weighting factor for that position) will, in general, be dependent on the optical properties of the material from which that position on the substrate is formed. Therefore, as discussed further below, the weighting factor for each position on the surface of the object may be indicative of the part of the composite substrate that that position corresponds to. Therefore, effectively, the methods according to the first and second aspects of the present disclosure provide robust methods of mapping out a surface of a substrate by determining which regions correspond to the same types of material.Advantageously, this information may be used (for all subsequent substrates for this lithographic process) to ensure that greater weight is given to height measurements in regions that correspond to sub-assemblies (which, in turn, can improve the overall imaging performance of the lithographic process). Furthermore, this information may also be used (for all subsequent substrates for this exposure process) to identify one or more regions of the substrate where height measurements are expected to be accurate, which may be used for an accurate coarse height measurement before a standard height map is generated.2024P00445EP 6

[0020] Particularly advantageously, the methods according to the first and second aspects of the present disclosure do not require any a priori knowledge of the layout of the wafer for the lithographic process. That is, a user is not required to input the details of the layout of the wafer (e.g. where the sub-assemblies are and where the filler material is). Rather, this information is generated automatically. This is advantageous as (a) it is more user-friendly and (b) it is not prone to errors that may be caused by user input of this information.

[0021] The sensor may be a level sensor.

[0022] The first quantity (which is proportional to a difference of the intensities of the first and second portions) may be referred to as a height measurement. For example, this may be a height measurement as determined by a level sensor.

[0023] The second quantity (which is proportional to a sum of the intensities of the first and second portions) may be referred to as an intensity measurement.

[0024] Determining the weighting factor for each position in dependence on the selected set of measurements for that position (the selected set, in turn, being generated with a different height of the object relative to the sensor) may be considered to be a process whereby a height map of the object is filtered into areas of varying degrees of interest (for example areas of interest and areas that are not of interest) for a subsequent exposure. Effectively, the determined weighting factor for each position on the surface of the object is dependent which set of measurements is selected for that position. This filtering of the substrate into areas of varying degrees of interest may be done in various different ways.

[0025] The one of the plurality of sets of measurements that is selected for each of the plurality of positions on the surface of the object may be the set of measurements for which the second quantity is largest.

[0026] That is, the second quantity (which is proportional to an intensity of the radiation received at the sensor region and / or proportional to a sum of the intensities of the first and second portions) may be generally maximized.

[0027] By selecting the set of measurements for which the second quantity is largest a shift of the image of the periodic pattern (for example a plurality of lines) relative to the splitting optics can be minimized.

[0028] In some embodiments, the plurality of sets of measurements of the object may be made at a plurality of equally-spaced heights of the object (relative to the sensor) with a height step or difference between adjacent heights. With such embodiments, if the height step is less than twice a pitch of the image of the periodic pattern (for example a plurality of lines) then by selecting the set of measurements for which the second quantity is largest a shift of the image of the lines relative to the splitting optics can be maintained so as to be less than one pitch.

[0029] Since the first quantity may be referred to as a height measurement and the second quantity may be referred to as an intensity measurement, selecting the set of measurements for which2024P00445EP 7the second quantity is largest may be considered to be verifying the height of each of the plurality of positions based on the corresponding intensity measurement for that position so as to provide a corrected height.

[0030] Determining a weighting factor for each of the plurality of positions may comprise: determining a nominal range of heights of the object relative to the sensor, the nominal range of heights containing the height of the object when one or more of the plurality of sets of measurements were made; and determining different weighting factors for: a first set of positions on the surface of the object, wherein the selected set of measurements for each of the first set of positions on the surface of the object were made when the height of the object fell within the determined range of heights; and a second set of positions on the surface of the object, wherein the selected set of measurements for each of the second set of positions on the surface of the object were made when the height of the object fell outside of the determined range of heights.

[0031] The nominal range of heights may correspond to the height of the object relative to the sensor for one or more of the plurality of sets of measurements. For example, the nominal range of heights may contain the height of the object that was used when only one of the plurality of sets of measurements were made.

[0032] The weighting factors for the first set of positions may all be the same (for example 1). Alternatively, the weighting factors for the first set of positions may be dependent on how close to the endpoints of the nominal range they are. The first set of positions may correspond to points that are expected to be of interest during a lithographic process.

[0033] The weighting factors for the second set of positions may all be the same (for example 0). Alternatively, the weighting factors for the second set of positions may be dependent on how far outside the nominal range of heights they are. The second set of positions may correspond to points that are not expected to be of interest during a lithographic process.

[0034] Determining the nominal range of heights of the object relative to the sensor may comprise selecting one of the plurality of sets of measurements and determining the nominal range of heights in dependence on the height of the object relative to the sensor when that set of measurements was made.

[0035] For example, the nominal range of heights of the object relative to the sensor may be ht+ R / 2, where R is the extent of the height range and where htis the height of object relative to the sensor when the selected set of measurements was made.

[0036] For example, as discussed further below, the plurality of sets of measurements of the object may be made at a plurality of equally-spaced heights of the object (relative to the sensor), with a height step Ah between adjacent heights. For such embodiments, the nominal range of heights of the object relative to the sensor may be h[ + Ah / 2, where h[ is the height of object relative to the sensor when the selected set of measurements was made.2024P00445EP 8

[0037] The one of the plurality of sets of measurements that is selected may be the one that was selected for the greatest number of the plurality of positions on the surface of the object.

[0038] In general, with a sufficiently dense and even sampling of positions across the surface of a composite substrate, one may expect more points to correspond to sub -assemblies than correspond to filler material.

[0039] Each of the positions of the first set of positions on the surface of the object may have the same, non-zero weighting factor.

[0040] Each of the positions of the second set of positions on the surface of the object may have a smaller weighting factor than each of the positions of the first set of positions on the surface of the object.

[0041] Each of the positions of the second set of positions on the surface of the object may have a weighting factor of zero.

[0042] The determined weighting factor for each position on the surface of the object may be dependent on an expected impact that that position will have on a lithographic process.

[0043] For example, in the case of a composite substrate, regions corresponding to subassemblies may have be assigned a first weighting factor (for example 1) and regions corresponding to filler material may be assigned a second weighting factor (for example 0). In a further example, in the case of a substrate comprising a high-topography pattern, for example 3D-NAND, regions to be exposed, which may correspond to cells to be patterned may be assigned a first weighting factor (for example 1) and regions not requiring exposure, which may correspond to peripheral circuits may be assigned a second weighting factor (for example 0).

[0044] The method may further comprise, for each of the plurality of positions on the surface of the object: determining a height for that position based on: (a) a value of the first quantity from the set of measurements that was selected for that position; and (b) the height of the object relative to the sensor when that set of measurements was made.

[0045] For example, for each position on the object, the height, which may be referred to as a correct height, hc, may be determined to be hc=hs+ , where hsis the height of the object relative to the sensor when the set of measurements that was selected for that position was made and Qi is the first quantity from the set of measurements that was selected for that position.

[0046] Making each of the plurality of sets of measurements of the object may comprise scanning one of the object and the sensor relative to the other such that the sensor can determine the first and second quantities for each of the plurality of positions on the surface of the object.

[0047] The plurality of sets of measurements of the object may cover a relative height range of 1 mm.2024P00445EP 9

[0048] The plurality of sets of measurements of the object may be made at a plurality of equally-spaced heights of the object, relative to the sensor, with a height step or difference between adjacent heights in the range 10 to 15 pm.

[0049] The plurality of sets of measurements of the object may be made at a plurality of equally-spaced heights of the object, relative to the sensor, wherein a height step or difference between adjacent heights is of the order of a linear range of the sensor.

[0050] The plurality of sets of measurements of the object may be made at a plurality of equally-spaced heights of the object, relative to the sensor, wherein a height step or difference between adjacent heights that is less than twice a pitch of the image of the periodic pattern in a plane of the sensor region.

[0051] If this condition is met (i.e. a height step Ah between adjacent heights that is less than twice a pitch p of the image of the periodic pattern in a plane of the sensor region), and the set of measurements that maximizes the second quantity (intensity) is selected, then (at least for regions of the substrate corresponding to sub-assemblies) it can be ensured that a shift of the image of the periodic pattern relative to splitting optics of the sensor can be less than one pitch, p.

[0052] The surface of the object may comprise at least one first region and at least one second region wherein the materials of the at least one first region have different optical properties to the materials of the at least one second region.

[0053] For example, the at least one first region may correspond to sub-assemblies of a composite substrate and may comprise first materials (e.g. silicon); and at least one second region may correspond to regions of filler material on such a composite substrate and may comprise a second material (e.g. glass, epoxy, etc.).

[0054] In general, some of the plurality of positions on the surface of the object will correspond to a first region and some of the plurality of positions on the surface of the object will correspond to a second region. With such an arrangement, one may expect that, in general, a first set of measurements may be selected for positions on the surface of the object that correspond to a first region (sub-assemblies) and that different sets of measurements may be selected for positions on the surface of the object that correspond to a second region (filler material).

[0055] The object may comprise a composite substrate comprising: comprise a base substrate; a plurality of sub-assemblies supported by the base substrate; and a filler material between adjacent sub-assemblies.

[0056] Lithographic processes such as, for example, advanced packaging semiconductor processes have been proposed wherein the substrate has such a composite structure. The plurality of sub-assemblies (for example silicon chips, opto-electronic devices etc.) are separate, previously formed sub-assemblies that have been applied to / mounted on the base substrate. These subassemblies may have a height of the order of 10-100 pm. A space between the sub-assemblies is filled with the filler material (for example an epoxy resin). Such a composite substrate may be loaded2024P00445EP 10into a lithographic apparatus and various connection layers may be formed to form connections between groups of individual sub-assemblies.

[0057] The selected set of measurements for each position (and therefore the weighting factor for that position) will, in general, be dependent on the optical properties of the material from which that position on the substrate is formed.

[0058] The weighting factor for each position on the surface of the object may be indicative of the part of the surface of the object that that position corresponds to.

[0059] The method may further comprise storing the determined weighting factors for each of the plurality of positions on the surface of the object in memory.

[0060] As explained above, effectively, the methods according to the first and second aspects of the present disclosure provide robust methods of mapping out a surface of a substrate by determining which regions correspond to the same types of material. Advantageously, this information can be stored in memory and may be used (for all subsequent substrates for this lithographic process). The information may be used (for all subsequent substrates for this lithographic process) to ensure that greater weight is given to height measurements in regions that correspond to sub-assemblies (which, in turn, can improve the overall imaging performance of the lithographic process). Furthermore, this information may also be used (for all subsequent substrates for this lithographic process) to identify one or more regions of the substrate where height measurements are expected to be accurate, which may be used for an accurate coarse height measurement before a standard height map is generated.

[0061] It will be appreciated that the full multi-level scan of the object that is performed by the methods according to the first and second aspects of the invention will take a significant amount of time. However, by doing this and determining the weighting factors once, and then storing these in memory, capture and level sensing (coarse and fine height measurements) of subsequent substrates for this lithographic process will be more robust and potentially faster. Therefore, overall the method may lead to an improvement in lithographic throughput.

[0062] The method may further comprise selecting one or more regions on the surface of the object suitable for making a subsequent course height measurement of another similar object, the one or more regions on the surface of the object being selected in dependence on the weighting factors for each of a plurality of positions on the surface of the object.

[0063] The one or more regions on the surface of the object may be selected based on the weighting factors of the plurality of positions on the surface of the object. For example, the one or more regions on the surface of the object may be selected as regions containing positions with nonzero weighting factors.

[0064] Such a coarse height measurement may be referred to as “capture”.

[0065] The method may further comprise storing the one or more selected regions on the surface of the object that are suitable for making a subsequent course height measurement of another similar object in memory.2024P00445EP 11

[0066] According to a third aspect of the present disclosure there is provided a method of determining a topography of a surface of an object, the method comprising: obtaining weighting factors for each of a plurality of positions on the surface of the object as determined using the method of the first and / or second aspect of the present disclosure; for at least some of the plurality of positions on the surface of the object, determining a height of the object relative to a sensor; and combining the obtained weighting factors and the determined heights so as to determine a height profile of the surface of the object.

[0067] The method according to the third aspect is advantageous because it uses the weighting factor for each of the plurality of positions on the surface of the object as determined using a method according to the first and / or second aspects of the present disclosure. By combining these weighting factors with height measurements of the object the determined height profile can, for example, give greater weight to height measurements in regions that correspond some portions of the object (for example portions corresponding to sub-assemblies, which, in turn, can improve the overall imaging performance of the lithographic process).

[0068] In some embodiments, a height of the object relative to a sensor may be determined for each of the plurality of positions on the surface of the object. It will be appreciated that some of the plurality of positions on the surface of the object may have a weighting factor of zero (and therefore such height measurements may be not used in a subsequent exposure process). However, in some embodiments, the plurality of height measurements (as determined, for example, using a level sensor) may be determined by scanning one of the object and the sensor relative to the other using a scanning route such that the sensor can measure a height for each of the plurality of positions on the surface of the object. Therefore, since the scanning route passes over each of the plurality of positions on the surface of the object, a height of the object relative to a sensor may be determined for each of the plurality of positions on the surface of the object (even if some of these positions has a weighting factor of zero).

[0069] Alternatively, in some embodiments, a height of the object relative to a sensor may only be determined for a sub-set of the plurality of positions on the surface of the object (for example those positions on the surface of the object with a non-zero weighting factor).

[0070] Obtaining the weighting factors for each of a plurality of positions on the surface of the object may comprise using the method of the first and / or second aspect of the present disclosure so as to determine the weighting factors for each of a plurality of positions on the surface of the object.

[0071] For at least a first object or wafer, the method may comprise first determining the weighting factors (using the method according to the first or second aspects) and then using this to generate the height profile (using the method according to the third aspect).

[0072] Additionally or alternatively, obtaining the weighting factors for each of a plurality of positions on the surface of the object may comprise retrieving the weighting factors from memory.2024P00445EP 12

[0073] Furthermore, as discussed above with reference to the first and second aspects of the present disclosure, the weighting factors may be used (for all subsequent substrates for this lithographic process) to identify one or more regions of the substrate where height measurements are expected to be accurate, which may be used for an accurate coarse height measurement before a height map or height profile is generated.

[0074] The method may further comprise making a course height measurement of at least one region on the surface of the object before the height is determined for at least some of the at least some of the plurality of positions on the surface of the object. The at least one region on the surface of the object may be determined in dependence on the weighting factors for each of a plurality of positions on the surface of the object.

[0075] The one or more regions on the surface of the object may be selected based on the weighting factors of the plurality of positions on the surface of the object. For example, the one or more regions on the surface of the object may be selected as regions containing positions with nonzero weighting factors.

[0076] Such a coarse height measurement may be referred to as “capture”.

[0077] The method may further comprising selecting at least one region on the surface of the object for such a course height measurement to be made. Alternatively, the method may comprise retrieving from memory details of at least one region on the surface of the object for such a course height measurement to be made.

[0078] Determining a height of the object relative to a sensor for the at least some of the plurality of positions on the surface of the object may comprise: projecting a radiation beam onto the position to form a periodic pattern thereon; receiving a portion of the radiation beam reflected from the position and forming an image of the periodic pattern at a sensor region; determining the height as being dependent on a phase of the pattern relative to the sensor region.

[0079] Determining a height of the object relative to a sensor for the at least some of the plurality of positions on the surface of the object may comprise: projecting a radiation beam onto the position to form a periodic pattern thereon receiving a portion of the radiation beam reflected from the position and forming an image of the periodic pattern at a sensor region; splitting the received radiation into a first portion and a second portion such that the relative intensities of the first and second portions are dependent on a height of the position relative to the sensor; determining an intensity of each of the first and second portions; and determining the height as being proportional to a difference of the intensities of the first and second portions.

[0080] In some embodiments, the height may be proportional to a ratio of the first quantity of the methods according to the first and / or second aspects of the present disclosure to the second quantity of the methods according to the first and / or second aspects of the present disclosure (which, in turn, may be proportional to an intensity of the radiation received at the sensor region). Advantageously, by determining the height of the object in this way using a ratio of the first quantity to the second2024P00445EP 13quantity, the determination of the height can be substantially independent of the intensity of the radiation beam. For example, the height may be determined as a differential measurement.

[0081] According to a fourth aspect of the present disclosure there is provided an exposure method comprising: measuring a topography of a surface of a substrate using the method of the third aspect of the present disclosure and / or the seventh aspect of the present disclosure; patterning a radiation beam using a patterning device; and projecting the patterned radiation onto the substrate so as to form an image of the patterning device on the substrate; wherein a position of the substrate while the patterned radiation is being projected onto the substrate is controlled in dependence on the measured topography of the surface of the substrate.

[0082] Advantageously, the measured topography of a surface of a substrate can be used to control a height of the substrate while it is being exposed to the patterned radiation, for example to keep the substrate in a plane of best focus for the image of the patterning device. It will be appreciated that the image of the patterning device formed on the substrate may be a diffraction limited image.

[0083] It will be appreciated that it is known to control the height of a substrate while it is being exposed to the patterned radiation in dependence on a height profile within an exposure apparatus, which may be a lithographic apparatus. However, there are particular advantages to using a height profile / measured topography of the surface of the substrate determined using the method according to the third aspect of the present disclosure, as now discussed.

[0084] Note that, in general, it is not possible to keep all portions of the substrate in perfect focus. Therefore, in practice, the height of a substrate while it is being exposed to patterned radiation will be controlled in such a way that no portion of the substrate is out of focus by an unacceptable amount. For example, the height of a substrate may be controlled while it is being exposed to the patterned radiation in such a way that a first portion of the substrate (that is within the field) is out of focus in one direction and a second portion of the substrate (that is also within the field) is out of focus in another direction. As such, there is an imaging impact on each of the first and second portions, however, the imaging impact for each of the first and second portions is acceptable.Traditionally, this is preferable to an alternative arrangement wherein the height of a substrate is controlled while it is being exposed to the patterned radiation in such a way that the first portion of the substrate is perfectly in focus and the second portion of the substrate is out of focus by a larger, unacceptable amount.

[0085] As discussed above, by using the weighting factors determined using a method according to the first and / or second aspect of the present disclosure, portions of the substrate that either: (a) comprise a material (for example a filler material) that has optical properties that differ significantly from the materials of the critical portions of the substrate for the exposure process (and may therefore yield an inaccurate height that differs from the correct height due to ASD or HPD errors); or (b) are missing a sub-assembly (and therefore may have a height that actually differs from the height the2024P00445EP 14composite substrate would have in the presence of the sub-assembly by a significant amount) can be either: (i) ignored (in the case of zero weighting factors); or (ii) given less significance as the substrate is being exposed subsequently. This is advantageous since it allows for these portions to be (at least partially) ignored, which, in turn, may allow other (more critical) portions of the substrate to be in better focus while being imaged / exposed (for example the height of a substrate can be controlled while it is being exposed to the patterned radiation in such a way that these more critical portions of the substrate are in better focus and the portion of the substrate with lower weighting factors can be out of focus by a larger amount). Furthermore, in addition to this imaging benefit, by placing less importance on such “less-critical” portions of the substrate fewer unwanted / less-critical moves of the substrate may be made during exposure of the substrate. In turn this reduces the risk of exciting much additional dynamic disturbances, which may lead to additional imaging errors.

[0086] The exposure may be a scanning exposure such that patterning a radiation beam using a patterning device comprises moving the patterning device through the radiation beam and projecting the patterned radiation onto the substrate so as to form an image of the patterning device on the substrate comprises moving the substrate such that the image of the patterning device is generally stationary relative to the substrate.

[0087] That is, in order to image the pattern onto a target region of the substrate, the patterning device is moved or scanned through an illumination region in a scanning direction. It will be appreciated that the substrate is also scanned relative to an illumination region in the plane of the substrate. The movement of the substrate is such than an aerial image of the patterning device is static relative to the substrate and it will be appreciated that a direction and / or speed of the substrate may, in general, differ from that of the patterning device (for example if the image is inverted and / or if a reduction faction is applied by the projection system).

[0088] According to a fifth aspect of the present disclosure there is provided an apparatus for measuring a topography of a surface of a substrate, the apparatus comprising: a support for supporting a substrate that is disposable in a beam spot region; projection optics operable to form a first image of a pattern on a substrate when disposed in the beam spot region with a radiation beam; detection optics operable to receive a portion of the radiation beam reflected from the substrate; and a processor operable to determine a height of the substrate from the radiation beam reflected from the substrate and further operable to implement the method of any one of the first, second, third, fourth, seventh and / or eighth aspects of the present disclosure.

[0089] The apparatus may be referred to as a level sensor. The apparatus may form part of a lithographic apparatus. The support for supporting a substrate may comprise a substrate holder operable to secure the substrate. For example, the support may comprise a clamp for clamping the substrate to the support.

[0090] The apparatus may further comprise a movement mechanism operable to cause relative movement of the support relative and the beam spot region.2024P00445EP 15

[0091] The movement mechanism may be operable to move the support relative to the beam spot region. Additionally or alternatively, the movement mechanism may be operable to move the beam spot region relative to the support. For example, movement of the beam spot region may be achieved by moving the projection optics. For such embodiments the movement mechanism may also be operable to move the detection optics.

[0092] The projection optics may comprise: a projection patterning device; and first imaging optics arranged to form an image of the projection patterning device on the beam spot region.

[0093] The projection patterning device may comprise a grating. The grating may comprise a plurality of lines. The lines may be of uniform thickness. The grating may have a 50% duty cycle.

[0094] The detection optics may be operable to receive a portion of the radiation beam reflected from the substrate and to split the reflected radiation into first and second portions such that a first portion of the radiation, which corresponds to a first portion of the first image, is spatially separate from a second portion of the radiation, which corresponds to a second portion of the first image. The apparatus may further comprise: a first detector arranged to determine an intensity of the first portion of the radiation; and a second detector arranged to determine an intensity of the second portion of the radiation; and the processor may be operable to determine a height profile of the substrate by combining the intensity of the first portion of the radiation and the intensity of the second portion of the radiation.

[0095] Advantageously, since the detection optics is operable to split the reflected radiation into first and second portions and the processor is operable to determine the height of the substrate by combining the intensities of the first and second portions, the determination of the height is substantially independent of the intensity of the radiation beam. For example, the height may be determined as a differential measurement.

[0096] The detection optics may comprise: splitting optics arranged to split the reflected radiation into first and second portions; and second imaging optics arranged to receive radiation reflected from an object supported by the support and to form a second image of the pattern on the splitting optics.

[0097] The first imaging optics may be generally equivalent to the second imaging optics.

[0098] The splitting optics, the beam spot region and the projection patterning device may all be in optically conjugate planes. It will be appreciated that two planes are optically conjugate if all of the radiation passing through each distinct point in the first plane is imaged onto a distinct point in the second plane.

[0099] An image of the projection patterning device is formed on the splitting optics, the position of that image being indicative of a height of the object. In particular, a position of that image relative to the splitting optics is indicative of a height of the object. As explained above, the projection patterning device may comprise a grating comprising a plurality of lines. The splitting optics may comprise a plurality of prisms and the image of each line may be imaged onto one of the plurality of2024P00445EP 16generally triangular prisms such that a first portion of the line is incident in a first surface of the prism and a second portion of the line is incident in a second surface of the prism. The first portion of the line is directed to the first detector and the second portion of the line is directed to the second detector. As the line moves relative to the prism (as a result of a change in height of the object), the amount of radiation directed to each of the detectors changes.[000100] The apparatus may further comprise a radiation source operable to produce the radiation beam.[000101] According to a sixth aspect of the present disclosure there is provided an exposure apparatus comprising the apparatus according to the fifth aspect of the present disclosure.[000102] The exposure apparatus may further comprise: an illumination system operable to illuminate an illumination region; a support structure configured to support a patterning device such that the patterning device is positionable in the illumination region; a substrate table configured to support a substrate; and a projection system operable to form an image of a patterning device supported by the support structure on a substrate supported by the substrate table.[000103] According to a seventh aspect of the present disclosure there is provided a method of determining a topography of a surface of an object, the method comprising: determining a plurality of first regions on the surface of the object that correspond to a device or a chip; for each of a plurality of positions on the surface of the object determining a height of the surface; and determining a weighting factor for each of the first regions in dependence on the height(s) of the surface determined for any of the plurality of positions on the surface of the object that correspond to that first region; and combining the obtained weighting factors and the determined heights so as to determine a height profile of the surface of the object.[000104] The method according to the seventh aspect of the present disclosure is advantageous as it uses knowledge of the first regions where devices or chips are expected to, or intended to, be disposed.[000105] The first regions on the surface of the object may be referred to as critical regions. Other regions on the surface of the object may be referred to as non-critical regions.[000106] The method then involves determining a weighting factor for each of the first regions in dependence on the height(s) of the surface determined for positions on the surface that correspond to those first regions. For example, it may be expected that the height of all of the first regions should be the same nominal value. For such an embodiment, (a) a first region may be assigned a weight of 1 if an average of the heights determined for all of the plurality of position that correspond to that first region is close to the nominal value; and (b) a first region may be assigned a weight of 0 if an average of the heights determined for all of the plurality of position that correspond to that first region is not close to the nominal value. It will be appreciated that in other embodiments, it may be expected that the heights of a first set of the first regions should be a first nominal value and the heights of a second set of the first regions should be a second nominal value. For such embodiments, the weighting2024P00445EP 17factors for each of the first regions in the first set may be dependent on how close the average of the heights determined for all of the plurality of position that correspond to that first region is to the first nominal value and the weighting factors for each of the first regions in the second set may be dependent on how close the average of the heights determined for all of the plurality of position that correspond to that first region is to the second nominal value.[000107] Determining the plurality of first regions on the surface of the object that correspond to a device or a chip may comprise using the method of the first and / or second aspect of the present disclosure so as to determine weighting factors for each of a plurality of positions on the surface of the object and determining each region containing weighting factors above a threshold value to be a first region.[000108] As discussed above, positions on the surface of the object that have a determined weighting factor (as determined by the method according to the first and / or second aspects of the present disclosure) above a threshold value may correspond to critical regions of the object (where devices or chips are typically disposed). For example, positions on the surface of the object that have a determined weighting factor of 1 (as determined by the method according to the first and / or second aspects of the present disclosure) may correspond to critical regions of the object.[000109] In some embodiments of the method according to the seventh aspect of the present disclosure, determining the plurality of first regions on the surface of the object that correspond to a device or a chip may comprise retrieving the first regions so determined using the method of the first and / or second aspects of the present disclosure from memory.[000110] Determining the plurality of first regions on the surface of the object that correspond to a device or a chip may comprise retrieving the first regions from memory.[000111] Determining the plurality of first regions on the surface of the object that correspond to a device or a chip may comprise retrieving the first regions that are input by a user.[000112] Determining a height of the surface may comprise determining a first quantity and a second quantity by: projecting a radiation beam onto the position to form a periodic pattern thereon; receiving a portion of the radiation beam reflected from the position and forming an image of the periodic pattern at a sensor region; determining the first quantity as being dependent on a phase of the pattern relative to the sensor region; and determining the second quantity as being proportional to an intensity of the radiation received at the sensor region; and determining the height in dependence on the first and second quantities.[000113] Determining the first quantity and the second quantity may comprise: splitting the received radiation into a first portion and a second portion such that the relative intensities of the first and second portions are dependent on a height of the position relative to the sensor; determining an intensity of each of the first and second portions; determining the first quantity as being proportional to a difference of the intensities of the first and second portions; and determining the second quantity as being proportional to a sum of the intensities of the first and second portions.2024P00445EP 18[000114] Determining a weighting factor for each of the first regions may comprise: determining an average height for each of the first regions, the average height being an average of the height(s) of the surface determined for any of the plurality of positions on the surface of the object that correspond to that first region; determining a mean value of the average heights for the plurality of first regions; and determining the weighting factor of each of the first regions in dependence on a magnitude of a difference between the average height of that first region and the mean value.[000115] A weighting factor for each of the first regions for which the magnitude of the difference between the average height of that first region and the mean value is below a threshold value may be determined to be the same, non-zero weighting factor.[000116] For example, the non-zero weighting factor may be one.[000117] A weighting factor for each of the first regions for which the magnitude of the difference between the average height of that first region and the mean value is above a threshold value may be determined to be a weighting factor of zero.[000118] Determining a weighting factor for each of the first regions may further comprise: determining a spread of the average heights for the plurality of first regions; and wherein the threshold value is dependent on the determined spread. The spread of the average heights for the plurality of first regions may, for example, be represented by a variance or standard deviation of the average heights for the plurality of first regions.[000119] According to an eighth aspect of the present disclosure there is provided a method of determining one or more configurations of an apparatus for a coarse measurement of a height of an object, the method comprising: determining a plurality of first regions on the surface of the object that correspond to a device or a chip; and determining at least one configuration of the apparatus wherein a coarse measurement spot overlaps with one of the plurality of first regions.[000120] The coarse measurement may be referred to as a capture.[000121] At least one of the determined configurations of the apparatus may be such that a number of fine measurement spots that overlaps with one of the plurality of first regions is maximized.[000122] At least one of the determined configurations of the apparatus may be such that a number of fine measurement spots that overlaps with one the first region that overlaps with the coarse measurement spot is maximized.BRIEF DESCRIPTION OF THE DRAWINGS[000123] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:Figure 1 depicts a schematic overview of a lithographic apparatus;Figure 2 is a schematic illustration of a level or height sensor which may form part of the lithographic apparatus shown in Figure 1;2024P00445EP 19Figure 3 illustrates that, when radiation is incident upon the surface of a silicon wafer, which typically comprises a multilayered stack, a portion of the radiation penetrates into the layers beneath the resist layer, which causes multiple back reflections, resulting in an error in a measured height;Figure 4A is a schematic representation of a method of measuring one or more properties of a surface of an object according to an embodiment of the present disclosure;Figure 4B schematically illustrates a first step of the method shown in Figure 4A, which comprises making a plurality of sets of measurements of the object using a sensor;Figure 4C schematically illustrates that each of the plurality of sets of measurements shown in Figure 4B comprises making measurements at each of a plurality of positions on the surface of the object;Figure 4D schematically illustrates that the measurement made at each of the plurality of positions on the surface of the object comprises (a) a step of determining a first quantity and (b) a step 116 of determining a second quantity;Figure 5 A schematically illustrates that each of the plurality of sets of measurements shown in Figure 4B comprises a plurality of sub-steps;Figure 5B schematically illustrates that one of the steps shown in Figure 5A in turn comprises a plurality of sub-steps;Figure 6 is a schematic view of a portion of a substrate, showing a beam spot region or measurement location and a first image of a pattern of a projection grating comprising two lines;Figure 7 schematically shows single target region (which may, for example correspond to one of the target regains shown in Figure 1) of a composite substrate for an advanced packaging semiconductor process;Figure 8 schematically illustrates a third step of the method shown in Figure 4A; Figure 9A shows measurements of first and second quantities at each of three different heights of the object (relative to the sensor), i.e. corresponding to three sets of measurements of the object, for 18 distinct positions on the surface of the object; the three plots in the first (top) row of Figure 9A show the first quantityat a first height h±of the object (Q^1"1), a second height h2of the object (Qi2"1), and a third height h3of the object (Q^3"1) (each of the first, second and third heights being relative to the sensor) i.e. corresponding to three first sets of measurements of the object, each having 18 measurement points corresponding to 18 distinct positions on the surface of the object; the three plots in the second row of Figure 9A show the second quantity Q2at the first height h1of the object (Q^1"1)’ thesecond height h2of the object (Q^), and the third height h3of the object (Q23i e- corresponding to three second sets of measurements of the object, for the same 18 distinct positions on the surface of the object;2024P00445EP 20Figure 9B shows the three plots in the first row of Figure 9 A, i.e. the first quantity ft at the first, second and third heights hlth2, h3of the object respectively but with each plot only showing the positions on the surface of the object W for which the corresponding set of measurements was selected;Figure 9C shows two plots of correct heights for all of the 18 positions on the object determined from the measurements of the first and second quantities shown in Figure 9A, an example nominal range of heights Ahnomof the object relative to the sensor (that corresponds to and includes the nominal or correct coarse height h2) is also shown in Figure 9C;Figure 10 schematically shows the method shown in Figure 4A further comprising some additional, optional steps; the optional steps are shown in dotted lines;Figure 11 schematically shows a new method of determining a topography of a surface of an object according to an embodiment of the present disclosure; in general, optional steps of the method are shown in dotted lines;Figure 12 schematically shows that, optionally, a step, which forms part of the method shown in Figure 11, of determining a height of the object relative to a sensor for the at least some of the plurality of positions on the surface of the object comprising a plurality of steps;Figure 13 schematically shows that one of the steps shown in Figure 12 in turn may comprise a plurality of steps;Figure 14 schematically shows a new lithographic exposure method according to an embodiment of the present disclosure;Figure 15 schematically shows an apparatus for measuring a topography of a surface of a substrate according to an embodiment of the present disclosure;Figure 16 schematically shows a new method of determining a topography of a surface of an object (for example a resist-coated silicon wafer) according to an embodiment of the present disclosure;Figure 17A schematically shows a substrate comprising a plurality of target regions which the method shown in 16 may be used to determine a topography of a surface thereof;Figure 17B shows the substrate shown in Figure 17A but wherein one of the devices or chips 816 is missing from one of the target regions;Figure 17C shows: a histogram of the average heights of each of the first regions of the substrate shown in Figure 17B determined using the method shown in Figure 16; a curve, which may, for example, be a Gaussian or normal distribution and which may represent a best fit (for example a least squares fit) to the histogram data; and a mean value and a standard deviation of the average heights for the plurality of first regions;Figure 18 schematically shows various optional sub-steps which may form part of the method shown in Figure 16;2024P00445EP 21Figure 19 schematically shows a method of determining one or more configurations of an apparatus for a coarse measurement of a height of an object according to an embodiment of the present disclosure;Figure 20A schematically shows an arrangement of beam spots for a level sensor comprising a coarse measurement spot and a plurality of fine measurement spots;Figure 20B schematically shows a first configuration that may be determined using the method shown in Figure 19; andFigure 20C schematically shows a second configuration that may be determined using the method shown in Figure 19.DETAILED DESCRIPTION[000124] In the present document, the terms “radiation” and “beam” are used to encompass all types of electromagnetic radiation, including ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range of about 5-100 nm).[000125] The term “reticle”, “mask” or “patterning device” as employed in this text may be broadly interpreted as referring to a generic patterning device that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate. The term “light valve” can also be used in this context. Besides the classic mask (transmissive or reflective, binary, phase-shifting, hybrid, etc.), examples of other such patterning devices include a programmable mirror array and a programmable LCD array.[000126] Figure 1 schematically depicts a lithographic apparatus LA. The lithographic apparatus LA includes an illumination system (also referred to as illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation or EUV radiation), a mask support (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA in accordance with certain parameters, a substrate support (e.g., a wafer table) WT constructed to hold a substrate (e.g., a resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support in accordance with certain parameters, and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.[000127] In operation, the illumination system IL receives a radiation beam from a radiation source SO, e.g. via a beam delivery system BD. The illumination system IL may include various types of optical components, such as refractive, reflective, magnetic, electromagnetic, electrostatic, and / or other types of optical components, or any combination thereof, for directing, shaping, and / or controlling radiation. The illuminator IL may be used to condition the radiation beam B to have a2024P00445EP 22desired spatial and angular intensity distribution in its cross section at a plane of the patterning device MA.[000128] The term “projection system” PS used herein should be broadly interpreted as encompassing various types of projection system, including refractive, reflective, catadioptric, anamorphic, magnetic, electromagnetic and / or electrostatic optical systems, or any combination thereof, as appropriate for the exposure radiation being used, and / or for other factors such as the use of an immersion liquid or the use of a vacuum. Any use of the term “projection lens” herein may be considered as synonymous with the more general term “projection system” PS.[000129] The lithographic apparatus LA may be of a type wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the projection system PS and the substrate W - which is also referred to as immersion lithography. More information on immersion techniques is given in US6952253, which is incorporated herein by reference.[000130] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also named “dual stage”). In such a “multiple stage” machine, the substrate supports WT may be used in parallel, and / or steps in preparation of a subsequent exposure of the substrate W may be carried out on the substrate W located on one of the substrate support WT while another substrate W, on the other substrate support WT, is being used for exposing a pattern on the other substrate W.[000131] In addition to the substrate support WT, the lithographic apparatus LA may comprise a measurement stage. The measurement stage is arranged to hold a sensor and / or a cleaning device. The sensor may be arranged to measure a property of the projection system PS or a property of the radiation beam B. The measurement stage may hold multiple sensors. The cleaning device may be arranged to clean part of the lithographic apparatus, for example a part of the projection system PS or a part of a system that provides the immersion liquid. The measurement stage may move beneath the projection system PS when the substrate support WT is away from the projection system PS.[000132] In operation, the radiation beam B is incident on the patterning device, e.g. mask, MA which is held on the mask support MT, and is patterned by the pattern (design layout) present on patterning device MA. Having traversed the mask MA, the radiation beam B passes through the projection system PS, which focuses the beam onto a target portion C of the substrate W. With the aid of the second positioner PW and a position measurement system IF, the substrate support WT can be moved accurately, e.g., so as to position different target portions C in the path of the radiation beam B at a focused and aligned position. Similarly, the first positioner PM and possibly another position sensor (which is not explicitly depicted in Figure 1) may be used to accurately position the patterning device MA with respect to the path of the radiation beam B. Patterning device MA and substrate W may be aligned using mask alignment marks Ml, M2 and substrate alignment marks Pl, P2. Although the substrate alignment marks Pl, P2 as illustrated occupy dedicated target portions, they may be2024P00445EP 23located in spaces between target portions C. Substrate alignment marks Pl, P2 are known as scribelane alignment marks when these are located between the target portions C.[000133] To clarify the invention, a Cartesian coordinate system is used. The Cartesian coordinate system has three axes, i.e., an x-axis, a y-axis and a z-axis. Each of the three axes is orthogonal to the other two axes. A rotation around the x-axis is referred to as an Rx-rotation. A rotation around the y-axis is referred to as an Ry -rotation. A rotation around the z-axis is referred to as an Rz-rotation. The x-axis and the y-axis define a horizontal plane, whereas the z-axis is in a vertical direction. The Cartesian coordinate system is not limiting the invention and is used for clarification only. Instead, another coordinate system, such as a cylindrical coordinate system, may be used to clarify the invention. The orientation of the Cartesian coordinate system may be different, for example, such that the z-axis has a component along the horizontal plane.[000134] A topography measurement system, level sensor or height sensor, and which may be integrated in the lithographic apparatus, is arranged to measure a topography of a top surface of a substrate (or wafer). A map of the topography of the substrate, also referred to as a height map, may be generated from these measurements indicating a height of the substrate as a function of the position on the substrate. This height map may subsequently be used to correct the position of the substrate during transfer of the pattern on the substrate, in order to provide an aerial image of the patterning device in focus on the substrate. It will be understood that “height” in this context refers to a dimension broadly out of the plane to the substrate (also referred to as Z-axis). Typically, the level or height sensor performs measurements at a fixed location (relative to its own optical system) and a relative movement between the substrate and the optical system of the level or height sensor results in height measurements at locations across the substrate.[000135] An example of a level or height sensor LS as known in the art is schematically shown in Figure 2, which illustrates only the principles of operation. In this example, the level sensor LS comprises an optical system, which includes a projection unit LSP and a detection unit LSD. The projection unit LSP comprises a radiation source LSO providing a beam of radiation LSB which is imparted with a pattern by a projection grating PGR of the projection unit LSP. The projection grating PGR may alternatively be referred to as a patterning device PGR. The radiation source LSO may be, for example, a narrowband or broadband radiation source, such as a supercontinuum light source, polarized or non-polarized, pulsed or continuous, such as a polarized or non-polarized laser beam. The radiation source LSO may include a plurality of radiation sources having different colors, or wavelength ranges, such as a plurality of LEDs. The radiation source LSO of the level sensor LS is not restricted to visible radiation, but may additionally or alternatively encompass UV and / or IR radiation and any range of wavelengths suitable to reflect from a surface of a substrate.[000136] The projection grating PGR is a periodic grating comprising a periodic structure resulting in a beam of radiation BE1 having a periodically varying intensity. The beam of radiation BE1 with the periodically varying intensity is directed towards a measurement location MLO on a substrate W2024P00445EP 24having an angle of incidence ANG with respect to an axis perpendicular (Z-axis) to the incident substrate surface between 0 degrees and 90 degrees, typically between 70 degrees and 80 degrees. The measurement location MLO may alternatively be referred to as the beam spot region MLO. At the measurement location MLO, the patterned beam of radiation BE1 is reflected by the substrate W (indicated by arrows BE2) and directed towards the detection unit LSD.[000137] In order to determine the height level at the measurement location MLO, the level sensor further comprises a detection system comprising a detection grating DGR, a detector DET and a processing unit (not shown) for processing an output signal of the detector DET. The detection grating DGR may be identical to the projection grating PGR. The detector DET produces a detector output signal indicative of the light received, for example indicative of the intensity of the light received, such as may be output by a photodetector, or representative of a spatial distribution of the intensity received, such as may be output by a camera or sensor array. The detector DET may comprise any combination of one or more detector types.[000138] By means of triangulation techniques, the height level at the measurement location MLO can be determined. The detected height level is typically related to the signal strength as measured by the detector DET, the signal strength having a periodicity that depends, amongst others, on the design of the projection grating PGR and the (oblique) angle of incidence ANG.[000139] The projection unit LSP and / or the detection unit LSD may include further optical elements, such as lenses and / or mirrors, along the path of the patterned beam of radiation between the projection grating PGR and the detection grating DGR (not shown).[000140] In an embodiment, the detection grating DGR may be omitted, and the detector DET may be placed at the position where the detection grating DGR is located. Such a configuration provides a more direct detection of the image of the projection grating PGR.[000141] In order to cover the surface of the substrate W effectively, a level sensor LS may be configured to project an array of measurement beams BE1 onto the surface of the substrate W, thereby generating an array of measurement areas MLO or spots covering a larger measurement range.[000142] Various height sensors of a general type are disclosed for example in US7265364 and US7646471, both incorporated by reference. A height sensor using UV radiation instead of visible or infrared radiation is disclosed in US2010233600A1, incorporated by reference. In W02016102127A1, incorporated by reference, a compact height sensor is described which uses a multi-element detector to detect and recognize the position of a grating image, without needing a detection grating.[000143] In general, the detection unit LSD may be arranged such that the reflected radiation BE2 is split into first and second portions and the height of the substrate W is determined by combining the intensities of the first and second portions. For example, the height may be determined as a differential measurement. Advantageously, with such an arrangement, the determination of the height2024P00445EP 25of the substrate W can be substantially independent of the intensity of the radiation beam BE1. In practice, the splitting of the radiation into first and second portions may be achieved in a number of different ways.[000144] For example, in some known arrangements, a combination of a polarizer and a shear plate (for example in the form of a Wollaston prism) are used to form two laterally shifted images of the projection grating PGR (each having a different polarization state) on a detection grating DGR. An example of such an arrangement is shown schematically in Figure 5 of US2010233600A1. For example, the projection grating PGR may have a pitch P and a duty cycle of 50% such that the beam of radiation BE1 having a periodically varying intensity comprises a plurality of lines having a thickness of P / 2, adjacent lines being separated by P / 2. The polarizer and a shear plate are arranged to form two images of the projection grating PGR (each having a different polarization state) on the detection grating DGR, one image being laterally shifted relative to the other by P / 2. Downstream of the detection grating DGR the two separate polarization states are each directed to a different detector. The height of the substrate W is determined as being proportional to the difference in the intensities of the two separate polarization states.[000145] In some other known arrangements, rather than splitting the reflected radiation BE2 using two images of the projection grating PGR but having different polarization states, a single image of the projection grating PGR is formed on splitting optics that is arranged to split that single image into first and second portions. Examples of such arrangements are shown schematically in Figure 6 of US2010233600A1 and Figure 2 of W02016102127A1. For example, such arrangements generally comprise splitting optics that is arranged to split the reflected radiation into first and second portions. The splitting optics may be a ruled grating with a triangular grating profile which acts as a series of wedges or prisms to redirect the reflected radiation BE2 (according to Snell's law). Such splitting optics may be considered to comprise a plurality of prisms and the image of each line of the projection grating PGR may be imaged onto one of the plurality of generally triangular prisms such that a first portion of the line is incident in a first surface of the prism and a second portion of the line is incident in a second surface of the prism. The first portion of the line is directed to the first detector and the second portion of the line is directed to the second detector. As the line moves relative to the prism (as a result of a change in height of the substrate W), the amount of radiation directed to each of the detectors changes. Embodiments of the present disclosure have particular application for level sensors using splitting optics of this type.[000146] One type of level sensor (see, for example, Figure 2 as described above) can be used to measure the height profile of the surface of a substrate W by: projecting a patterned radiation beam BE1 onto a beam spot region MLO; moving the substrate W relative to the beam spot region MLO; receiving a portion of the patterned radiation beam BE2 reflected from the object W and determining the first measurement of the height therefrom. As the height of the substrate W varies, the position of the pattern of the reflected radiation may vary, for example relative to splitting optics arranged to split2024P00445EP 26the reflected radiation into first and second portions. As shown in Figure 3, when radiation is incident upon the surface of a silicon wafer W, which typically comprises a multilayered stack, a portion of the radiation BE1 penetrates into the layers beneath the resist layer. This penetration causes multiple back reflections, resulting in an imbalance in the signal readout for such a level sensor. As a result, the measured height, hm, differs from the actual height, , of the wafer W by an amount A. This amount, A, which is the difference between a measured height and a real height due to interference effects of underlying layers, may be referred to as “Apparent Surface Depression” (ASD) error or “Height Process Dependency” (HPD) error.[000147] Note that, in some embodiments, a level sensor may be provided which does not comprise gratings. For such embodiments, the level sensor may project light onto the substrate W without patterning it.[000148] Some embodiments of the present disclosure relate to a method of measuring one or more properties of a surface of an object. An embodiment of a method 100 of measuring one or more properties of a surface of an object is shown schematically in Figure 4A and is now discussed with reference to Figures 4A to 10.[000149] As shown schematically in Figures 4A and 4B, the method 100 comprises a step 110 of making a plurality of sets 110a-l lOn of measurements of the object using a sensor. For example, the object may be a substrate W within a lithographic apparatus LA and the sensor may comprise a level sensor LS.[000150] A height of the object relative to the sensor is different for each of the plurality of sets 110a-l lOn of measurements. As discussed below, the individual measurements will ultimately be used to determine a topography of the object and, in general, a height of (a surface of) the object relative to the sensor will vary (it is this variation that the measurements are ultimately used to determine). As used here, as will be appreciated by the skilled person, a height of the object relative to the sensor being different for each of the plurality of sets 110a- 11 On of measurements is intended to mean that an average height or global position of the object relative to the sensor is different for each of the plurality of sets 110a-l lOn of measurements. For example, the object may be a substrate W within a lithographic apparatus LA and a relative height or position of a wafer stage WT may be different for each of the plurality of sets 110a-l lOn of measurements.[000151] As shown schematically in Figures 4C and 4D, each set 110a-l lOn of measurements comprises making measurements 112a- 112m at each of a plurality of positions on the surface of the object. In particular, this comprises a step 114 of determining a first quantity and a step 116 of determining a second quantity.[000152] Each measurement 112i of the first and second quantities (steps 114, 116) may be made using an existing type of level sensor (for example a level sensor LS of the type shown in Figure 2), as now discussed with reference to Figures 5A, 5B and 6.2024P00445EP 27[000153] As shown schematically in Figure 5, each measurement 112i of the first and second quantities (steps 114, 116) may comprise the following steps. In particular, each measurement 112i of the first and second quantities (steps 114, 116) may comprise a step 210 of projecting a radiation beam onto the position (on the object) to form a periodic pattern thereon. Each measurement 112i of the first and second quantities (steps 114, 116) may further comprise a step 220 of receiving a portion of the radiation beam reflected from the position and forming an image of the periodic pattern at a sensor region. Each measurement 112i of the first and second quantities (steps 114, 116) may further comprise a step 230 of determining the first quantity and the second quantity (from the image of the periodic pattern formed at the sensor region).[000154] Optionally, at step 230, the first quantity may be determined as being dependent on a phase of the pattern relative to the sensor region. Optionally, at step 230, the second quantity may be determined as being proportional to an intensity of the radiation received at the sensor region.[000155] Additionally or alternatively, optionally, and as shown schematically in Figure 5B, the step 230 of determining the first quantity and the second quantity may comprise the following steps. In particular, the step 230 of determining the first quantity and the second quantity may comprise a step 232 of splitting the received radiation into a first portion and a second portion such that the relative intensities of the first and second portions are dependent on a height of the position relative to the sensor. The step 230 of determining the first quantity and the second quantity may further comprise a step 234 of determining an intensity of each of the first and second portions. The step 230 of determining the first quantity and the second quantity may further comprise a step 236 of determining the first quantity as being proportional to a difference of the intensities of the first and second portions. The step 230 of determining the first quantity and the second quantity may further comprise a step 238 of determining the second quantity as being proportional to a sum of the intensities of the first and second portions.[000156] These steps may be carried out by a level sensor LS of the type shown in Figure 2, as now discussed with reference to Figure 6. Figure 6 is a schematic view of a portion of a substrate W, showing a beam spot region or measurement location MLO. A first image of the pattern of the projection grating PGR comprising two lines Li, L2is also shown. In addition, each of the two lines comprises two portions (the top half and bottom half respectively of each line Li, L2in Figure 6). The first image of the projection grating PGR may be considered to comprise a first portion Pi (comprising the top portions of the two lines Li, L2) and a second portion P2(comprising the bottom portions of the two lines Li, L2).[000157] The first and second portions Pi, P2of the first image correspond to first and second portions of radiation reflected by the substrate W that are split, for example by splitting optics (see, for example, the splitting optics 648 shown in Figure 15, as discussed below). That is, the first and second portions Pi, P2as indicated in Figure 6 may be considered to be a projection of the division effected by the splitting optics back onto the substrate W (only indicated here to illustrate the methods2024P00445EP 28of embodiments of the present disclosure). The intensities of the reflected radiation from the first and second portions Pi, P2 of the first image, which may be referred to as the first and second intensities , / 2, may be used to determine the first and second quantities. For example, the first quantity, Q±, may be determined (at step 236) as being proportional to a difference of the intensities of the first and second portions, i.e. Q1oc I2- Z1, and the second quantity, Q2, may be determined (at step 238) as being proportional to a sum of the intensities of the first and second portions, i.e. Q2oc Z2+ .[000158] Note that the division of the first image into first and second portions Pi, P2 is dependent on the relative phase of the pattern that is formed on the sensor region relative to, for example, splitting optics disposed in the sensor region. Therefore, for such embodiments, the first quantity, Q±, is effectively determined as being dependent on a phase of the pattern (that is formed on the sensor region) relative to the sensor region and the second quantity, Q2, is effectively determined as being proportional to a total intensity of the radiation received at the sensor region. Note that the division of the first image into first and second portions Pi, P2 that is shown in Figure 6 represents the situation when the height of the substrate W is zero (relative to a reference height) as the first and second portions Pi, P2 are of substantially equal size.[000159] As indicated by arrow A, during the method 100 (for each measurement 112i of the first and second quantities at one of the plurality of positions on the surface of the object) the substrate W may be moved relative to the beam spot region (measurement location MLO) in a scanning direction (the y-direction in Figure 6). The x and y directions shown in Figure 6 represent the sides of the target portions C (e.g., comprising one or more dies) of the substrate W (see Figure 1) and, in general, features formed on the substrate W tend to be aligned with the x and / or y directions. Note that the lines Li, L2 of the projection grating PGR are arranged at a non-zero angle to both the x and y directions in Figure 6. This is to minimize the effects of scattering of the incident radiation beam BE1 from features on the substrate W (other than specular reflection) on the height measurement.[000160] Each of the two lines Li, L2has a thickness, t, in the direction in which the substrate W is moved (i.e. the y-direction) relative to the beam spot region MLO. Note that the thickness t of each of the lines Li, L2in the first image (formed on the substrate W) will in general be larger than a thickness of each of the corresponding lines on the projection grating PGR (by a factor of l / cos(ANG)). A space between the two lines in the y-direction is also t such that the pitch p of the first image (in the y-direction) is 2t.[000161] Note that only 2 lines Li, L2 have been illustrated in Figure 6 for simplicity. However, in practice the pattern may comprise any number of such lines and the splitting optics may comprise a corresponding number of splitting members (for example prisms).[000162] Note that, as explained above, in some alternative embodiments, a level sensor may be provided which does not comprise gratings. For such embodiments, the level sensor may project light onto the substrate W without patterning it. Such an embodiment may be considered to project a single “line” onto the substrate W.2024P00445EP 29[000163] The method 100 further comprises (see Figure 4A) a step 120 of, for each of the plurality of positions on the surface of the object, selecting one of the plurality of sets 110a- 11 On of measurements in dependence on the second quantity (as determined at step(s) 116).[000164] The method 100 further comprises (see Figure 4A) a step 130 of, for each of the plurality of positions on the surface of the object, determining a weighting factor for that position in dependence on the selected set of measurements for that position (as determined at step 120).[000165] The new method 100 shown in Figure 4A may be used as a first measurement, pre-scan or calibration step for measuring aspects of a silicon wafer within a lithographic apparatus LA. In particular, the method 100 according shown in Figure 4A may be used once when a first wafer of a particular lithographic process is loaded into such a lithographic apparatus LA. The method shown in Figure 4A may have particular application for high topography lithographic processes such as, for example, advanced packaging semiconductor processes and 3D-NAND lithographic process.[000166] The method 100 shown in Figure 4A is advantageous, as now discussed.[000167] In general, in a lithographic process a substrate W may be exposed to radiation that has been patterned by a reticle or mask MA. Before the substrate W is exposed to the patterned radiation, a topography of the surface of the substrate W may be determined. For example a height map of the surface of the substrate may be determined, for example using a level sensor LS. Subsequently, the measured topography of the surface of the substrate can be used to control a height of the substrate W while it is being exposed to the patterned radiation, for example to keep the substrate W in a plane of best focus for the image of the patterning device. Typically, the substrate W comprises a silicon wafer coated in a photoresist.[000168] As described above with reference to Figures 2 and 6, one known type of level sensor measures a phase of an image of a periodic pattern reflected from the substrate W (for example a plurality of lines) relative to a periodic sensing structure (for example splitting optics comprising a plurality of splitting elements). The image and the sensing structure have the same pitch in the plane of incidence. Typically, the height h measured by such a level sensor is determined as:where a is a gain. Such a level sensor can measure the height of a substrate (for example relative to a reference height) with very high accuracy; this may be referred to as a fine height measurement. If there is a relative movement between the image and the sensing structure by an integer multiple of the pitch of these periodic structures then the same relative phase will be measured (i.e. the output of such a level sensor is periodic). For example, if the image and the sensing structure illustrated in Figure 6 were to be shifted by the pitch, p, of the pattern then only one of the lines will be received by the splitting optics and used in the height determination. This will result in a decrease in each of the first and second2024P00445EP 30intensities , 12, but will yield the same height measurement when equation (1) is used. Therefore, such a level sensor is provided with some way of determining an overlap between the image and the sensing structure (i.e. which period of the periodic output is being measured). Preferably, a shift of the image of the lines relative to the splitting optics is maintained so as to be less than one pitch (so the overlap is maximized). One way to ensure this is to make a separate course measurement of height that may be known as “capture”. For example an image of a single line may be formed on the substrate W and a reflected portion of this may be collected and a second image of the line is formed. A relative position of the second image of the line and the sensor may be determined. For example, the second image may be projected into a prism and split into two parts, the intensity of each of which is measured. The height of the substrate W (relative to the level sensor) may be varied (for example by moving a wafer stage WT) until the two parts have equal intensity (or a difference in their intensities is negligible). This may be referred to as a course measurement. Once this condition has been met, the shift of the image of the lines relative to the splitting optics will be less than one pitch.[000169] Such a capture measurement may be made at least once after a substrate W has been loaded into the lithographic apparatus LA. Alternatively, a plurality of such capture measurements may be made for each substrate. In some arrangements, capture measurements may be made before, or even simultaneously to, each height measurement (or set of height measurements) made using the level sensor.[000170] More recently, lithographic processes (for example advanced packaging semiconductor processes) have been proposed wherein the substrate has a composite structure. In particular, the substrate may comprise a base substrate to which a plurality of separate, previously formed subassemblies (for example silicon chips, opto-electronic devices etc.) have been applied. These subassemblies may be of the order of 10-100 pm high. A space between the sub-assemblies is filled with a filler material (for example an epoxy resin). Such a composite substrate may be loaded into a lithographic apparatus and various connection layers may be formed to form connections between groups of individual sub-assemblies. Advanced packaging semiconductor processes may be considered to be a high topography lithographic processes. Another example of such a high topography process is a 3D-NAND lithographic process.[000171] A single target region 300 (which may, for example correspond to one of the target regains C shown in Figure 1) of a composite substrate for an advanced packaging semiconductor process is shown schematically in Figure 7. The target region 300 comprises a plurality of separate, previously formed sub-assemblies 302, 304, 306, 308, 310, 312, 314 (for example silicon chips, optoelectronic devices etc.) have been applied. A space between the sub-assemblies is filled with a filler material 316 (for example an epoxy resin).[000172] As discussed above, the height measurement made by some known level sensors may be an average height of the radiation reflected from the substrate W. In one ideal scenario, all of the radiation is reflected by an upper surface of the substrate W, in which case the measured height2024P00445EP 31corresponds to the height of this surface. As shown in Figure 3, in a more realistic scenario, the substrate W may comprise a multilayered stack and a portion of the radiation penetrates into the layers beneath the resist layer causing multiple back reflections. As a result, the measured height, hm, differs from the actual height, ha, of the surface of the substrate (this may be referred to as “Apparent Surface Depression” (ASD) error or “Height Process Dependency” (HPD) error). Due to the variation in the optical properties of the different materials used, a height of a new composite substrate measured using a typical level sensor LS will typically result in large height variations between the different materials used and, in addition, there may be significant variation in ASD or HPD errors for the different materials. For example, in general ASD or HPD errors for height measurements made on regions 302, 304, 306, 308, 310, 312, 314 comprising sub-assemblies may be significantly different to such errors for height measurements made on regions 316 comprising filler material.[000173] The use of such composite substrates within lithographic apparatuses poses a number of additional challenges, as now discussed.[000174] First, there will, in general, be a variety of different materials (the different subcomponents and the filler material) and each of these may have a different response when measured using known level sensors. For example, some level sensors may be subject to errors due to a fraction of the patterned radiation that is received being reflected from an interface at a non-zero depth below the surface of a substrate. Such errors can be much more easily corrected for or controlled if all parts of the substrate W are subject to errors of the same magnitude. However, since a composite substrate comprises a variety of different materials, a magnitude of such errors can be significantly different for different parts of the substrate. In practice, with known systems, this may manifest itself as the height measurements for some portions of the composite substrate (for example the portions 316 comprising filler material) being unacceptably inaccurate. Furthermore, if these measurements are subsequently used to control a position of the substrate W during a subsequent exposure process in a known way, they can result in unacceptable lithographic imaging.[000175] Second, such a composite substrate may be subject to earlier processing errors, which may mean that one or more sub-assemblies are missing from the composite substrate. If a height profile of such a composite substrate is determined (for example using a level sensor LS) then the determined height of the substrate W in the proximity of the missing sub-assembly will typically be incorrect. Either: (a) the composite substrate may have a height that actually differs from the height the composite substrate would have in the presence of the sub-assembly by a significant amount (for example of the order of 10-100 pm); or (b) an erroneous height measurement will be obtained since this region of the substrate may have been in-filled with filler material (resulting in a height measurement that is unacceptably inaccurate; as discussed above).[000176] If the composite structure is controlled in dependence of a height profile (for example determined using a level sensor LS) in a conventional way then the presence of the erroneous height measurements (from regions of filler material or missing sub-assemblies) will result in sub-optimal2024P00445EP 32imaging of other (more critical regions). This is because, with such a conventional control scheme, the lithographic apparatus W will try to control the substrate in such a way that no portion of the substrate is out of focus by an unacceptable amount. For example, the height of a substrate may be controlled while it is being exposed to the patterned radiation in such a way that a first portion of the substrate 302, 304, 306, 308, 310, 312, 314 (corresponding to a sub-assembly and which is within the field) is out of focus in one direction and a second portion 316 of the substrate (corresponding to filler material or a missing sub-assembly and which is also within the field) is out of focus in another direction. As such, there is an imaging impact on the (critical) first portion 302, 304, 306, 308, 310, 312, 314 due to the erroneous height measurements from the (non-critical) second portion 316. Furthermore, in addition to this imaging compromise of the critical portions 302, 304, 306, 308, 310, 312, 314 of the substrate W, the erroneous height measurements will result in additional movement of the substrate W during exposure of the substrate (as the lithographic apparatus tries to keep the non-critical regions 316 in acceptable focus). In turn, this may excite additional dynamic disturbances, which may lead to additional imaging errors in the critical regions 302, 304, 306, 308, 310, 312, 314 of the substrate W. Another method, which gives less weight (for example no weight) to regions of a substrate that are missing a sub-assembly (and therefore may have a height that actually differs from the height the composite substrate would have in the presence of the sub-assembly by a significant amount) is discussed further below with reference to Figures 16 to 18.[000177] Note that similar challenges are posed for level sensing of other high topography lithographic processes such as a 3D-NAND lithographic process.[000178] Note that although herein the method 100 shown in Figure 4A and discussed above is particularly advantageous for high topography lithographic processes (such as advanced packaging semiconductor processes and 3D-NAND lithographic processes) it also offers advantages for other processes. For example, the method 100 shown in Figure 4A and discussed above is also advantageous for a silicon or transparent substrate comprising one or more patterned layers that have already been formed thereon. Note that for use cases with transparent substrates, previous production steps (layers) on such transparent substrates can cause local (level sensing) measurement artefacts, for example due to local deposition of materials. In particular, for the reasons discussed above, method 100 shown in Figure 4 A and discussed above is particularly advantageous may offer a particular advantage for processes wherein the substrate comprises a variety of different materials which may have a different response when measured using known level sensors.[000179] The method 100 shown in Figure 4A and discussed above is advantageous as it effectively combines the first and second quantities ft, Q2so as to determine a weighting factor for each of the plurality of positions on the surface of the object. The weighting factor for each position is dependent on the selected set of measurements for that position. As discussed further below, the selected set of measurements for each position (and therefore the weighting factor for that position) will, in general, be dependent on the optical properties of the material from which that position on the2024P00445EP 33substrate is formed. Therefore, as discussed further below, the weighting factor for each position on the surface of the object may be indicative of the part 302, 304, 306, 308, 310, 312, 314, 316 of the composite substrate that that position corresponds to. Therefore, effectively, the method 100 shown in Figure 4A provides a robust method of mapping out a surface of a substrate by determining which regions correspond to the same types of material. Advantageously, this information may be used (for all subsequent substrates for this lithographic process) to ensure that greater weight is given to height measurements in regions 302, 304, 306, 308, 310, 312, 314 that correspond to sub-assemblies (which, in turn, can improve the overall imaging performance of the lithographic process). Furthermore, this information may also be used (for all subsequent substrates for this lithographic process) to identify one or more regions 302, 304, 306, 308, 310, 312, 314 of the substrate where height measurements are expected to be accurate, which may be used for an accurate coarse height measurement before a standard height map is generated.[000180] Particularly advantageously, the method 100 shown in Figure 4A does not require any a priori knowledge of the layout of the wafer for the lithographic process. That is, a user is not required to input the details of the layout of the wafer (e.g. where the sub-assemblies are and where the filler material is). Rather, this information is generated automatically. This is advantageous as (a) it is more user-friendly and (b) it is not prone to errors that may be caused by user input of this information.[000181] The first quantity Q1(which is proportional to a difference of the intensities of the first and second portions) may be referred to as a height measurement. For example, this may be a height measurement as determined by a level sensor (for example as determined using equation (1)).[000182] The second quantity Q2(which is proportional to a sum of the intensities of the first and second portions) may be referred to as an intensity measurement.[000183] The weighting factor for each position on the object is determined (at step 130) in dependence on the selected set of measurements for that position (as determined at step 120), the selected set, in turn, being generated with a different height of the object relative to the sensor. This determination of a weighting factor for each position on the object may be considered to be a process whereby a height map of the object is filtered into areas of varying degrees of interest (for example areas 302, 304, 306, 308, 310, 312, 314 of interest and areas 316 that are not of interest) for a subsequent exposure. Effectively, the determined weighting factor for each position on the surface of the object is dependent which set of measurements is selected for that position. This filtering of the substrate into areas of varying degrees of interest may be done in various different ways, as now discussed.[000184] In some embodiments of the method 100 shown in Figure 4A, the one of the plurality of sets of measurements that is selected for each of the plurality of positions on the surface of the object W may be the set of measurements for which the second quantity Q2is largest.2024P00445EP 34[000185] That is, the second quantity Q2(which is proportional to an intensity of the radiation received at the sensor region and / or proportional to a sum of the intensities of the first and second portions , / 2) may be generally maximized.[000186] By selecting the set of measurements for which the second quantity Q2is largest a shift of the image of the periodic pattern (for example a plurality of lines Li, L2; see Figure 6) relative to the splitting optics can be minimized.[000187] In some embodiments, the plurality of sets of measurements 110a-l lOn of the object W may be made at a plurality of equally-spaced heights of the object (relative to the sensor) with a height step or difference between adjacent heights. With such embodiments, if the height step is less than twice a pitch p of the image of the periodic pattern (for example a plurality of lines Li, L2; see Figure) then by selecting the set of measurements for which the second quantity Q2is largest a shift of the image of the lines relative to the splitting optics can be maintained so as to be less than one pitch.[000188] Since the first quantity Q1may be referred to as a height measurement and the second quantity Q2may be referred to as an intensity measurement, selecting the set of measurements for which the second quantity Q2is largest may be considered to be verifying the height of each of the plurality of positions based on the corresponding intensity measurement for that position so as to provide a corrected height.[000189] In some embodiments of the method 100 shown in Figure 4 A, the step 130 of determining a weighting factor for each of the plurality of positions may comprise the following steps, as now discussed with reference to Figures 8, 9 A and 9B.[000190] Step 130 of the method 100 shown in Figure 4A may comprise a step 240 of determining a nominal range of heights of the object relative to the sensor, the nominal range of heights containing the height of the object when one or more of the plurality of sets 110a- 11 On of measurements were made. In an example implementation, application of the nominal range of heights may comprise a filtering step.[000191] Step 130 of the method 100 shown in Figure 4A may further comprise a step 242 of determining different weighting factors for: a first set of positions on the surface of the object W, wherein the selected set of measurements for each of the first set of positions on the surface of the object W were made when the height of the object fell within the determined range of heights (determined at step 240). In an example implementation, determining the weighting factors for the first set of positions may comprise all or part of a masking step. In an example implementation, the weighting factors may indicate that the first set of positions on the surface of the object is “valid” or “true”, for example the weighting factors may be equal to 1.[000192] Step 130 of the method 100 shown in Figure 4A may further comprise a step 244 of determining different weighting factors for a second set of positions on the surface of the object W, wherein the selected set of measurements for each of the second set of positions on the surface of the2024P00445EP 35object were made when the height of the object fell outside of the determined range of heights (determined at step 240). In an example implementation, determining the weighting factors for the second set of positions on the surface of the object W may comprise all or part of a masking step. In an example implementation, the weighting factors may indicate that the second set of positions on the surface of the object is “invalid” or “false”, for example the weighting factors may be less than 1.[000193] The nominal range of heights (determined at step 240) may correspond to the height of the object relative to the sensor for one or more of the plurality of sets of measurements 110a, 1 lOn. For example, the nominal range of heights may contain the height of the object that was used when only one HOi of the plurality of sets 110a- 11 On of measurements were made.[000194] The weighting factors for the first set of positions may all be the same (for example 1). Alternatively, the weighting factors for the first set of positions may be dependent on how close to the endpoints of the nominal range they are. The first set of positions may correspond to points that are expected to be of interest during a lithographic process (for example, they may fall within regions 302, 304, 306, 308, 310, 312, 314 corresponding to sub-assemblies as discussed above).[000195] The weighting factors for the second set of positions may all be the same (for example 0). Alternatively, the weighting factors for the second set of positions may be dependent on how far outside the nominal range of heights they are. The second set of positions may correspond to points that are not expected to be of interest during a lithographic process (for example, they may fall within regions 316 of filler material as discussed above).[000196] Determining the nominal range of heights of the object relative to the sensor (at step 240) may comprise selecting one 1 lOi of the plurality of sets of measurements 110a- 11 On and determining the nominal range of heights in dependence on the height of the object relative to the sensor when that set 1 lOi of measurements was made.[000197] For example, the nominal range of heights of the object relative to the sensor may be ht+ R / 2, where R is the extent of the height range and where htis the height of object relative to the sensor when the selected set of measurements was made.[000198] For example, as discussed further below, the plurality of sets of measurements 110a-l lOn of the object may be made at a plurality of equally-spaced heights of the object (relative to the sensor), with a height step Ah between adjacent heights. For such embodiments, the nominal range of heights of the object relative to the sensor may be ht+ Ah / 2, where htis the height of object relative to the sensor when the selected set of measurements was made.[000199] In some embodiments, the one 1 lOi of the plurality of sets of measurements that is selected is the one that was selected for the greatest number of the plurality of positions on the surface of the object.[000200] In general, with a sufficiently dense and even sampling of positions across the surface of a composite substrate W, one may expect more points to correspond to sub-assemblies (i.e. within regions 302, 304, 306, 308, 310, 312, 314) than correspond to filler material (i.e. within region 316).2024P00445EP 36[000201] As example of how step 130 and / or steps 240, 242, 244 may be carried out in practice is now discussed with reference to Figures 9A and 9B.[000202] Figure 9A shows measurements of the first and second quantities at each of three different heights of the object (relative to the sensor), i.e. corresponding to three sets of measurements 110a- 110c of the object, for 15 distinct positions on the surface of the object W. The three plots in the first (top) row of Figure 9A show the first quantityat a first height h1of the object (Q^1"1), a second height h2of the object (Qi2"1), and a third height h3of the object (Q^3"1) (each of the first, second and third heights being relative to the sensor) i.e. corresponding to three first sets of measurements 110a of the object, each having 18 measurement points corresponding to 18 distinct positions on the surface of the object W. The three plots in the second row of Figure 9A show the second quantity Q2at the first height h1of the object (Q^)’ the second height h2of the object (Q^), and the third height h3of the object (Q3), i.e. corresponding to three second sets of measurements 110b of the object, for the same 18 distinct positions on the surface of the object W. The second height h2of the object is intermediate the first height h^ of the object and the third height h3of the object such that h1> h2> h3. The three of sets of measurements 110a- 110c of the object may be made at a three of equally-spaced heights of the object (relative to the sensor), with a height step Ah between adjacent heights, such that h2= h3+ Ah and h2= h1- Ah.[000203] From the three plots of the second quantity Q^\at the three heightsh2, h3, it can be seen that for three of the positions on the object, the second quantity is largest (maximum) in the first set of measurements 110a, thereby indicating that these three positions have been measured in the correct period and the corresponding height h1is correct for these three positions(accordingly this set 110a of measurements may be selected for these three positions on the object at step 120).Similarly, it can be seen that for eleven of the positions on the object, the second quantity is largest in the second set of measurements 110b, thereby indicating that these eleven positions have been measured in the correct period and the corresponding height h2is correct for these eleven positions (accordingly, this set 110b of measurements may be selected for these eleven positions on the object at step 120). Similarly, it can be seen that for four positions on the object, the second quantity is largest in the third set of measurements 110c, thereby indicating that these four positions have been measured in the correct period and the corresponding height h3is correct for these four positions (and accordingly this set 110b of measurements may be selected for these four positions on the object at step 120).[000204] Since the second set of measurements 110b of the object was selected for the greatest number of the plurality of positions on the surface of the object, second height h2of the object (relative to the sensor), the nominal range of heights of the object relative to the sensor (determined at2024P00445EP 37step 240) may be a range of heights that encompasses the second height h2of the object (relative to the sensor).[000205] For example, determining the nominal range of heights of the object relative to the sensor (at step 240) may comprise selecting the second set 110b of measurements and determining the nominal range of heights in dependence on the height h2of the object relative to the sensor when that set 110b of measurements was made.[000206] In some cases the three plots of the first quantityat the three heights h±, h2, h3, may be substantially the same. This may be the case if, for example, the height step Ah between adjacent heights is an integer multiple of the pitch p of the pattern used by the level sensor LS. As stated above, the first quantity Q1may be referred to as a height measurement. However, note that the first quantityis determined with the object at the first height h1and is therefore effectively a height relative to the first height h±. Similarly, the first quantityis determined with the object at the second height h2and is therefore effectively a height relative to the second height h2. Similarly, the first quantityis determined with the object at the third height h3and is therefore effectively a height relative to the third height h3.[000207] A corrected height profile may be determined for all of the plurality of positions on the object (at which the plurality of sets of measurements 110a-l lOn are made), as now discussed with reference to Figures 9B and 9C.[000208] A nominal or correct coarse height may be selected, corresponding to the height of the object relative to the sensor for the set of measurements that was selected (at step 120) for the largest number of positions on the object. For example, in the simple example shown in Figure 9A, as discussed above, the second set 110b of measurements was selected for more positions on the object than the first and third sets 110a, 110c of measurements were. Therefore, the second height h2may be selected as the nominal or correct coarse height.[000209] A correct height may be determined for each position on the object as follows. The correct height is selected based on: (a) the value of first quantityQ[2\ from the set of measurements that was selected for that position; and (b) the height h±, h2, h3of the object relative to the sensor when that set of measurements was made. For example, for each position on the object, the correct height, hc, may be determined to be:hc= hs+ (2)where hsis the height of the object relative to the sensor when the set of measurements that was selected for that position was made andis the first quantity from the set of measurements that was selected for that position.2024P00445EP 38[000210] For example, in the simple example shown in Figure 9 A, for the three of the positions on the object wherein the second quantity is largest in the first set of measurements at a first height h1110a (and therefore this set 110a of measurements is selected for these three positions on the object at step 120), the correct height is h1+for the eleven positions on the object wherein the second quantity is largest in the second set of measurements at a second height h2110b (and therefore this set 110b of measurements is selected for these eleven positions on the object at step 120) the correct height is h2+; and for the four positions on the object wherein the second quantity is largest in the third set of measurements at a third height h3110c (and therefore this set 110c of measurements is selected for these four positions on the object at step 120) the correct height is h3+.[000211] Note that all of these correct heights can be expressed relative to the nominal or correct coarse height h2as follows. For the three of the positions on the object wherein the second quantity is largest in the first set of measurements 110a, the correct height is h2+ Ah + for the eleven positions on the object wherein the second quantity is largest in the second set of measurements 110b the correct height is h2+ Qj ; and for the four positions on the object wherein the second quantity is largest in the third set of measurements 110c the correct height is h3- Ah +.[000212] Figure 9B shows the three plots in the first row of Figure 9A, i.e. the first quantity Q1at the first, second and third heights h1(h2, h3of the object respectively but with each plot only showing the positions on the surface of the object W for which the corresponding set of measurements was selected (at step 120). Figure 9C shows two plots of the correct heights hcfor all of the 18 positions on the object determined from the measurements of the first and second quantities shown in Figure 9 A. The second plot of Figure 9C shows these correct heights hcfor all of the 18 positions on the object together with an example nominal range of heights Ahnomof the object relative to the sensor (that corresponds to and includes the nominal or correct coarse height h2).[000213] In the above example, the nominal range of heights of the object relative to the sensor (determined at step 240) may be a range of heights containing the height h2of the object when the second set 110b of measurements were made. This nominal range of heights Ahnomof the object relative to the sensor (determined at step 240) is also shown in Figure 9C.[000214] In this example, all of the eleven positions on the object wherein the second quantity is largest in the second set of measurements 110b fall within the nominal range of heights Ahnomof the object relative to the sensor and were made when the height of the object was h2, which falls within the determined range of heights Ahnom. Therefore, all of the eleven positions on the object wherein the second quantity is largest in the second set of measurements 110b may be considered to be a first set of positions on the surface of the object W. Weighting factors are determined for this first set of positions at step 242.2024P00445EP 39[000215] In this example, the three positions on the object wherein the second quantity is largest in the first set of measurements 110a and the four positions on the object wherein the second quantity is largest in the third set of measurements 110c fall outside of the nominal range of heights Ahnomof the object relative to the sensor and were made when the height of the object was either h^ or h3, which both fall outside of the determined range of heights Ahnom. Therefore, all four of these positions on the object may be considered to be a second set of positions on the surface of the object W.Weighting factors are determined for this second set of positions at step 244.[000216] In some embodiments of the method 100 shown in Figure 4 A, each of the positions of the first set of positions on the surface of the object W may have the same, non-zero weighting factor.[000217] In some embodiments of the method 100 shown in Figure 4A, each of the positions of the second set of positions on the surface of the object W may have a smaller weighting factor than each of the positions of the first set of positions on the surface of the object W.[000218] In some embodiments, each of the positions of the second set of positions on the surface of the object W may have a weighting factor of zero.[000219] In some embodiments of the method 100 shown in Figure 4A, the determined weighting factor for each position on the surface of the object may be dependent on an expected impact that that position will have on a lithographic process. For example, in the case of a composite substrate, regions 302, 304, 306, 308, 310, 312, 314 corresponding to sub-assemblies may have be assigned a first weighting factor (for example 1) and regions 316 corresponding to filler material may be assigned a second weighting factor (for example 0). In a further example, in the case of a substrate comprising a high-topography pattern, for example, 3D-NAND, regions to be exposed, which may correspond to cells to be patterned may be assigned a first weighting factor (for example 1) and regions not requiring exposure, which may correspond to peripheral circuits may be assigned a second weighting factor (for example 0).[000220] In some embodiments of the method 100 shown in Figure 4A, making each of the plurality of sets 110a-l lOn of measurements of the object W may comprises scanning one of the object W and the sensor relative to the other such that the sensor can determine the first and second quantities ft, Q2for each of the plurality of positions on the surface of the object W.[000221] In some embodiments of the method 100 shown in Figure 4A, the plurality of sets 110a-1 lOn of measurements of the object W may cover a relative height range of 1 mm. Note any relative height range allowed by the wafer stage may be used.[000222] In some embodiments of the method 100 shown in Figure 4A, the plurality of sets 110a-1 lOn of measurements of the object W may be made at a plurality of equally-spaced heights of the object W, relative to the sensor, with a height step Ah or difference between adjacent heights in the range 10 to 15 pm.[000223] In some embodiments of the method 100 shown in Figure 4A, the plurality of sets 110a-1 lOn of measurements of the object W may be made at a plurality of equally-spaced heights of the2024P00445EP 40object W, relative to the sensor, and a height step Ah or difference between adjacent heights may be of the order of a linear range of the sensor.[000224] In some embodiments of the method 100 shown in Figure 4A, the plurality of sets 110a-1 lOn of measurements of the object W may be made at a plurality of equally-spaced heights of the object W, relative to the sensor, and a height step Ah or difference between adjacent heights may be less than twice a pitch p of the image of the periodic pattern in a plane of the sensor region. If this condition is met (i.e. a height step Ah between adjacent heights that is less than twice a pitch p of the image of the periodic pattern in a plane of the sensor region), and the set 1 lOi of measurements that maximizes the second quantity Q2(intensity) is selected, then (at least for regions 302, 304, 306, 308, 310, 312, 314 of the substrate corresponding to sub-assemblies) it can be ensured that a shift of the image of the periodic pattern relative to splitting optics of the sensor can be less than one pitch, p.[000225] In some embodiments of the method 100 shown in Figure 4A, the surface of the object W may comprise at least one first region 302, 304, 306, 308, 310, 312, 314 and at least one second region 316 wherein the materials of the at least one first region 302, 304, 306, 308, 310, 312, 314 have different optical properties to the materials of the at least one second region 316. For example, the at least one first region 302, 304, 306, 308, 310, 312, 314 may correspond to sub-assemblies of a composite substrate and may comprise first materials (e.g. silicon); and at least one second region 316 may correspond to regions of filler material on such a composite substrate and may comprise a second material (e.g. glass, epoxy, etc.).[000226] In general, some of the plurality of positions on the surface of the object W (at which the measurements 112a- 112m are made for each set 110a-l lOn of measurements) will correspond to a first region 302, 304, 306, 308, 310, 312, 314 and some of the plurality of positions on the surface of the object W will correspond to a second region 316. With such an arrangement, one may expect that, in general, a first set of measurements 1 lOi may be selected for positions on the surface of the object W that correspond to a first region 302, 304, 306, 308, 310, 312, 314 (sub-assemblies) and that different sets of measurements 1 lOj, 110k may be selected for positions on the surface of the object W that correspond to a second region 316 (filler material).[000227] In some embodiments of the method 100 shown in Figure 4A, the object W may comprises a composite substrate comprising: comprise a base substrate; a plurality of sub-assemblies supported by the base substrate; and a filler material between adjacent sub-assemblies. Lithographic processes with higher structures than traditional lithographic apparatuses have been proposed; these are referred to herein as high topography lithographic processes. For example, advanced packaging semiconductor processes have been proposed wherein the substrate has such a composite structure (as discussed above with reference to Figure 7). The plurality of sub-assemblies (for example silicon chips, opto-electronic devices etc.) are separate, previously formed sub-assemblies that have been applied to / mounted on the base substrate. These sub-assemblies may have a height of the order of 10-100 pm. A space between the sub-assemblies is filled with the filler material (for example an epoxy2024P00445EP 41resin). Such a composite substrate may be loaded into a lithographic apparatus LA and various connection layers may be formed to form connections between groups of individual sub -assemblies. Another example of a high topography lithographic process is 3D-NAND manufacturing, in which memory cells may be stacked vertically (i.e. in a direction normal to a plane of the substrate.[000228] The selected set of measurements for each position (and therefore the weighting factor for that position) will, in general, be dependent on the optical properties of the material from which that position on the substrate W is formed.[000229] In some embodiments of the method 100 shown in Figure 4A, the weighting factor for each position on the surface of the object W (as determined at step 130) may be is indicative of the part of the surface of the object W that that position corresponds to.[000230] The method 100 shown in Figure 4A, may further comprise one or more additional, optional steps, as now described with reference to Figure 10. In Figure 10, the optional steps are shown in dotted lines.[000231] Optionally, in some embodiments of the method 100 shown in Figure 4A, the method 100 may further comprise a step 140 storing the determined weighting factors (determined at step 130) for each of the plurality of positions on the surface of the object in memory.[000232] As explained above, effectively, the method 100 shown in Figure 4A provides a robust method of mapping out a surface of a substrate W by determining which regions correspond to the same types of material. Advantageously, this information can be stored in memory and may be used (for all subsequent substrates for this lithographic process). The information may be used (for all subsequent substrates for this lithographic process) to ensure that greater weight is given to height measurements in regions 302, 304, 304, 306, 308, 310, 312, 314 that correspond to sub-assemblies (which, in turn, can improve the overall imaging performance of the lithographic process).Furthermore, this information may also be used (for all subsequent substrates for this lithographic process) to identify one or more regions of the substrate where height measurements are expected to be accurate, which may be used for an accurate coarse height measurement before a standard height map is generated.[000233] It will be appreciated that the full multi-level scan of the object W that may be performed by the method 100 shown in Figure 4 A will take a significant amount of time. However, by doing this and determining the weighting factors once, and then storing these in memory (at step 140), capture and level sensing (coarse and fine height measurements) of subsequent substrates for this lithographic process will be more robust and potentially faster. Therefore, overall the method 100 shown in Figure 4A may lead to an improvement in lithographic throughput.[000234] Optionally, in some embodiments of the method 100 shown in Figure 4A, the method 100 may further comprise a step 150 of selecting one or more regions on the surface of the object W that are suitable for making a subsequent course height measurement of another similar object W. These2024P00445EP 42one or more regions on the surface of the object W may be selected in dependence on the weighting factors for each of a plurality of positions on the surface of the object W (as determined at step 130).[000235] The one or more regions on the surface of the object W (that are suitable for making a subsequent course height measurement of another similar object W) may be selected based on the weighting factors of the plurality of positions on the surface of the object W (as determined at step 130). For example, the one or more regions on the surface of the object W (that are suitable for making a subsequent course height measurement of another similar object W) may be selected as regions containing positions with non-zero weighting factors.[000236] Such a coarse height measurement may be referred to as “capture”.[000237] Optionally, in some embodiments of the method 100 shown in Figure 4A, the method 100 may further comprise a step 160 of storing the one or more selected regions on the surface of the object that are suitable for making a subsequent course height measurement of another similar object (as determined at step 150) in memory.[000238] Some embodiments of the present disclosure relate to new methods of determining a topography of a surface of an object W. An example of such a new method 400 of determining a topography of a surface of an object W is now described with reference to Figure 11. In general, optional steps of the method 400 shown in Figure 11 are shown in dotted lines.[000239] The method 400 comprises a step 410 of obtaining weighting factors for each of a plurality of positions on the surface of the object W as determined using the method 100 shown in Figure 4A (or Figure 10). The method 400 further comprises a step 420 of, for at least some of the plurality of positions on the surface of the object W, determining a height of the object W relative to a sensor. The method 400 comprises a step 430 of combining the obtained weighting factors and the determined heights so as to determine a height profile of the surface of the object W.[000240] The method 400 shown in Figure 11 is advantageous because it uses the weighting factor for each of the plurality of positions on the surface of the object as determined using the method 100 shown in Figure 4A (or Figure 10). By combining these weighting factors with height measurements of the object W (for example determined using a known level sensor LS) the determined height profile can, for example, give greater weight to height measurements in regions that correspond some portions of the object (for example portions 302, 304, 306, 308, 310, 312, 314 corresponding to subassemblies, which, in turn, can improve the overall imaging performance of the lithographic process).[000241] In some embodiments, a height of the object W relative to a sensor may be determined for each of the plurality of positions on the surface of the object. It will be appreciated that some of the plurality of positions on the surface of the object W may have a weighting factor of zero (and therefore such height measurements may be not used in a subsequent exposure process). However, in some embodiments, the plurality of height measurements (as determined, for example, using a level sensor) may be determined by scanning one of the object W and the sensor relative to the other using a scanning route such that the sensor can measure a height for each of the plurality of positions on the2024P00445EP 43surface of the object W. Therefore, since the scanning route passes over each of the plurality of positions on the surface of the object W, a height of the object W relative to a sensor may be determined for each of the plurality of positions on the surface of the object (even if some of these positions has a weighting factor of zero).[000242] Alternatively, in some embodiments, a height of the object W relative to a sensor may only be determined for a sub-set of the plurality of positions on the surface of the object W (for example those positions on the surface of the object with a non-zero weighting factor).[000243] The step 410 of obtaining the weighting factors for each of a plurality of positions on the surface of the object W may comprise a step 412 of using the method 100 shown in Figure 4A (or Figure 10) so as to determine the weighting factors for each of a plurality of positions on the surface of the object W. For at least a first object W or wafer, the method 400 may comprise first determining the weighting factors (using the method 100 shown in Figures 4A and / or 10) and then using this to generate the height profile (at step 430 of the method 400 shown in Figure 11).[000244] Additionally or alternatively, the step 410 of obtaining the weighting factors for each of a plurality of positions on the surface of the object W may comprise a step 414 of retrieving the weighting factors from memory. For example, the weighting factors for each of a plurality of positions on the surface of the object W may have previously been stored in memory (for example at step 140, as discussed above, with reference to Figure 10).[000245] As discussed above, the weighting factors generated by the method 100 shown in Figure 4A and / or Figure 10 may be used (for all subsequent substrates for this lithographic process) to identify one or more regions of the substrate W where height measurements are expected to be accurate, which may be used for an accurate coarse height measurement before a height map or height profile is generated.[000246] Optionally, in some embodiments, the method 400 shown in Figure 11 may further comprise a step 418 of making a course height measurement of at least one region on the surface of the object W before the height is determined for at least some of the at least some of the plurality of positions on the surface of the object W (at step 420). The at least one region on the surface of the object W (at which the course height measurement is made at step 418) may be determined in dependence on the weighting factors for each of a plurality of positions on the surface of the object (as obtained at step 410).[000247] The one or more regions on the surface of the object W (at which the course height measurement is made at step 418) may be selected based on the weighting factors of the plurality of positions on the surface of the object W. For example, the one or more regions on the surface of the object may be selected as regions containing positions with non-zero weighting factors.[000248] Such a coarse height measurement may be referred to as “capture”.[000249] The method 400 shown in may further comprise a step 416 of selecting at least one region on the surface of the object W for such a course height measurement to be made (at step 418).2024P00445EP 44Alternatively, the method 400 may comprise a step 417 of retrieving from memory details of at least one region on the surface of the object W for such a course height measurement to be made. For example, the at least one region on the surface of the object W for such a course height measurement to be made may have previously been determined (for example at step 150, as discussed above, with reference to Figure 10) and stored in memory (for example at step 160, as discussed above, with reference to Figure 10).[000250] As shown in Figure 12, optionally, the step 420 of determining a height of the object W relative to a sensor for the at least some of the plurality of positions on the surface of the object W may comprise the following steps. In particular, step 420 may comprise: a step 422 of projecting a radiation beam onto the position to form a periodic pattern thereon; a step 424 of receiving a portion of the radiation beam reflected from the position and forming an image of the periodic pattern at a sensor region; and a step 426 of determining the height (from the image of the periodic pattern formed at the sensor region).[000251] For example, at step 426, the height may be determined (from the image of the periodic pattern formed at the sensor region) as being dependent on a phase of the pattern relative to the sensor region.[000252] Additionally or alternatively, as shown in Figure 13, at step 426, determining the height of the object W relative to a sensor for the at least some of the plurality of positions on the surface of the object may comprise the following optional steps. In particular, step 426 may comprise a step 440 of splitting the received radiation into a first portion P1and a second portion P2such that the relative intensities 4, 4 of the first and second portions are dependent on a height of the position relative to the sensor. Step 426 may further comprise a step 442 of determining an intensity 4, 4 of each of the first and second portions. Step 426 may further comprise a step 444 of determining the height as being proportional to a difference of the intensities 4, 4 °f the first and second portions Px, P2. In some embodiments, the height may be determined as a differential measurement (according to equation (1)). Advantageously, by determining the height of the object in this way, the determination of the height can be substantially independent of the intensity of the radiation beam.[000253] Some embodiments of the present disclosure relate to a new lithographic exposure method. An example of such a new lithographic exposure method 500 is shown schematically in Figure 14 and is now described.[000254] The new lithographic exposure method 500 comprises a step 400 of measuring a topography of a surface of a substrate W using the method 400 shown in Figure 11.[000255] The new lithographic exposure method 500 further comprises a step 510 of patterning a radiation beam B using a patterning device MA. This may be achieved, for example using a radiation beam B and a patterning device MA generally of the form shown in Figure 1 and as described above.[000256] The new lithographic exposure method 500 further comprises a step 520 of projecting the patterned radiation onto the substrate W so as to form an image of the patterning device on the2024P00445EP 45substrate W. Again, this may be achieved, for example, projection system PS of the form shown in Figure 1 and as described above.[000257] The new lithographic exposure method 500 is carried out such that while the patterned radiation is being projected onto the substrate W (at step 520), a position of the substrate W is controlled in dependence on the measured topography of the surface of the substrate (as determined at step 400).[000258] Advantageously, the measured topography of a surface of a substrate W (as measured using the method 400 shown in Figure 11 and as described above) can be used to control a height of the substrate W while it is being exposed to the patterned radiation, for example to keep the substrate W in a plane of best focus for the image of the patterning device MA. It will be appreciated that the image of the patterning device MA formed on the substrate W may be a diffraction limited image.[000259] It will be appreciated that it is known to control the height of a substrate W while it is being exposed to the patterned radiation in dependence on a height profile within lithographic apparatus LA. However, there are particular advantages to using a height profile / measured topography of the surface of the substrate determined using the method 400 shown in Figure 11 and as described above, as now discussed.[000260] Note that, in general, it is not possible to keep all portions of the substrate W in perfect focus. Therefore, in practice, the height of a substrate W while it is being exposed to patterned radiation will be controlled in such a way that no portion of the substrate W is out of focus by an unacceptable amount. For example, the height of a substrate W may be controlled while it is being exposed to the patterned radiation in such a way that a first portion of the substrate W (that is within the field) is out of focus in one direction and a second portion of the substrate W (that is also within the field) is out of focus in another direction. As such, there is an imaging impact on each of the first and second portions, however, the imaging impact for each of the first and second portions is acceptable. Traditionally, this is preferable to an alternative arrangement wherein the height of a substrate W is controlled while it is being exposed to the patterned radiation in such a way that the first portion of the substrate W is perfectly in focus and the second portion of the substrate W is out of focus by a larger, unacceptable amount.[000261] As discussed above, by using the weighting factors determined using a method 100 of the type shown in Figures 4A and / or 10, portions of the substrate W that either: (a) comprise a material (for example a filler material) that has optical properties that differ significantly from the materials of the critical portions 302, 304, 306, 308, 310, 312, 314 of the substrate W for the lithographic process (and may therefore yield an inaccurate height that differs from the correct height due to ASD or HPD errors); or (b) are missing a sub-assembly (and therefore may have a height that actually differs from the height the composite substrate would have in the presence of the sub-assembly by a significant amount) can be either: (i) ignored (in the case of zero weighting factors); or (ii) given less significance as the substrate W is being exposed subsequently. This is advantageous since it allows for these2024P00445EP 46portions 316 to be (at least partially) ignored, which, in turn, may allow other (more critical) portions 302, 304, 306, 308, 310, 312, 314 of the substrate W to be in better focus while being imaged / exposed (for example the height of a substrate W can be controlled while it is being exposed to the patterned radiation in such a way that these more critical portions 302, 304, 306, 308, 310, 312, 314 of the substrate W are in better focus and the portion of the substrate W with lower weighting factors can be out of focus by a larger amount). Furthermore, in addition to this imaging benefit, by placing less importance on such “less-critical” portions 316 of the substrate W fewer unwanted / less-critical moves of the substrate W may be made during exposure of the substrate. In turn this reduces the risk of exciting much additional dynamic disturbances, which may lead to additional imaging errors.Another method, which gives less weight (for example no weight) to regions of a substrate that are missing a sub-assembly (and therefore may have a height that actually differs from the height the composite substrate would have in the presence of the sub-assembly by a significant amount) is discussed further below with reference to Figures 16 to 18.[000262] In some embodiments of the exposure method 500 shown in Figure 14, the lithographic exposure may be a scanning exposure such that patterning a radiation beam B using a patterning device MA (at step 510) comprises moving the patterning device MA through the radiation beam B and projecting the patterned radiation onto the substrate W so as to form an image of the patterning device MA on the substrate W (at step 520) comprises moving the substrate W such that the image of the patterning device MA is generally stationary relative to the substrate W.[000263] That is, in order to image the pattern onto a target region C of the substrate W (see Figure 1), the patterning device MA may be moved or scanned through an illumination region in a scanning direction. It will be appreciated that the substrate W is also scanned relative to an illumination region in the plane of the substrate W. The movement of the substrate W may be such than an aerial image of the patterning device MA is static relative to the substrate W and it will be appreciated that a direction and / or speed of the substrate W may, in general, differ from that of the patterning device MA (for example if the image is inverted and / or if a reduction faction is applied by the projection system PS).[000264] Some embodiments of the present disclosure relate to an apparatus for measuring a topography of a surface of a substrate W. An example embodiment of such an apparatus 600 for measuring a topography of a surface of a substrate W is shown schematically in Figure 15.[000265] The apparatus 600 comprises: a support 610; projection optics 620; detection optics 640; and a processor 670. The support 610 is suitable for supporting a substrate W that is disposable in a beam spot region 680. The projection optics 620 is operable to form a first image of a pattern on a substrate W when disposed in the beam spot region 680 with a radiation beam 622. The detection optics 640 is operable to receive a portion 642 of the radiation beam reflected from the substrate W. The processor 670 is operable to determine a height of the substrate W from the radiation beam 642 reflected from the substrate W and further operable to implement the method 100 described above2024P00445EP 47with reference to Figures 4A to 10 and / or the method 400 described above with reference to Figures 11 to 13.[000266] The apparatus 600 may be referred to as a level sensor. The apparatus may have, or be operable to implement, any feature of level sensors described above (in particular with reference to Figures 3 and 6. The apparatus 600 may form part of a lithographic apparatus LA. The support 610 for supporting a substrate W may comprise a substrate holder operable to secure the substrate W. For example, the support 610 may comprise a clamp for clamping the substrate W to the support 610.[000267] Optionally, the apparatus 600 may further comprise a movement mechanism 630 operable to cause relative movement of the support 610 relative and the beam spot region 680. This movement is indicated rather schematically by arrow 632. It will be appreciated that the movement may be in any direction and the movement mechanism 630 may be operable to cause relative movement of the support 610 relative and the beam spot region 680 in at least three independent directions.[000268] The movement mechanism 630 may be operable to move the support 610 relative to the beam spot region 680. Additionally or alternatively, the movement mechanism 630 may be operable to move the beam spot region 680 relative to the support 610. For example, movement of the beam spot region 680 may be achieved by moving the projection optics 620. For such embodiments the movement mechanism 630 may also be operable to move the detection optics 640.[000269] In some embodiments, the projection optics 620 may comprise: a projection patterning device 624; and first imaging optics 626 arranged to form an image of the projection patterning device 624 on the beam spot region 680. The projection patterning device 624 may comprise a grating. The grating may comprise a plurality of lines. The lines may be of uniform thickness. The grating may have a 50% duty cycle.[000270] In some embodiments, the detection optics 640 may be operable to receive a portion 642 of the radiation beam reflected from the substrate W and to split the reflected radiation into first and second portions 644, 646 such that a first portion 644 of the radiation, which corresponds to a first portion of the first image, is spatially separate from a second portion 646 of the radiation, which corresponds to a second portion of the first image.[000271] The apparatus 600 may further comprise: a first detector 650 arranged to determine an intensity of the first portion of the radiation 644; and a second detector 660 arranged to determine an intensity of the second portion of the radiation 646. The controller 670 may be arranged to receive a first signal Si indicative of the first intensity from the first detector 650 and a second signal S2 indicative of the second intensity from the second detector 660. The processor 670 may be operable to determine a height profile of the substrate W by combining the intensity of the first portion of the radiation 644 and the intensity of the second portion of the radiation 646.[000272] Advantageously, since the detection optics 640 is operable to split the reflected radiation into first and second portions 644, 646 and the processor 670 is operable to determine the height of the substrate W by combining the intensities of the first and second portions 644, 646, the2024P00445EP 48determination of the height W is substantially independent of the intensity of the radiation beam 622. For example, the height may be determined as a differential measurement (for example according to equation (1)).[000273] In some embodiments of the apparatus 600, the detection optics 640 may comprise: splitting optics 648 arranged to split the reflected radiation 642 into first and second portions 644, 646; and second imaging optics 649 arranged to receive radiation 642 reflected from an object W supported by the support 610 and to form a second image of the pattern on the splitting optics 648.[000274] The first imaging optics 626 may be generally equivalent to the second imaging optics 649.[000275] The splitting optics 648, the beam spot region 680 and the projection patterning device 624 may all be in optically conjugate planes. It will be appreciated that two planes are optically conjugate if all of the radiation passing through each distinct point in the first plane is imaged onto a distinct point in the second plane.[000276] An image of the projection patterning device 624 is formed on the splitting optics 648, the position of that image being indicative of a height of the object W. In particular, a position of that image relative to the splitting optics 648 is indicative of a height of the object W. As explained above, the projection patterning device 624 may comprise a grating comprising a plurality of lines. The splitting optics 648 may comprise a plurality of prisms and the image of each line may be imaged onto one of the plurality of generally triangular prisms such that a first portion of the line is incident in a first surface of the prism and a second portion of the line is incident in a second surface of the prism. The first portion of the line is directed to the first detector 650 and the second portion of the line is directed to the second detector 660. As the line moves relative to the prism (as a result of a change in height of the object), the amount of radiation directed to each of the detectors 650, 660 changes.[000277] In some embodiments, the apparatus 600 may further comprise a radiation source 690 operable to produce the radiation beam 622.[000278] Some embodiments of the present disclosure relate to a lithographic apparatus comprising the apparatus 600 as shown in Figure 15 and as described above.[000279] The lithographic apparatus may be generally of the form of, and / or comprise any combination of features of, the lithographic apparatus LA shown schematically in Figure 15. In particular, the lithographic apparatus LA may comprise: an illumination system IL operable to illuminate an illumination region; a support structure MT configured to support a patterning device MA such that the patterning device MA is positionable in the illumination region; a substrate table WT configured to support a substrate W; and a projection system PS operable to form an image of a patterning device MA supported by the support structure MT on a substrate W supported by the substrate table WT.2024P00445EP 49[000280] Some embodiments of the present disclosure relate to a new method of determining a topography of a surface of an object (for example a resist-coated silicon wafer), as now discussed with reference to Figures 16 to 18.[000281] Figure 16 schematically shows a new method 700 of determining a topography of a surface of an object (for example a resist-coated silicon wafer).[000282] The method 700 comprises a step 710 of determining a plurality of first regions on the surface of the object that correspond to a device or a chip. The first regions on the surface of the object may be referred to as critical regions. For example, the first regions may be regions of the substrate where devices or chips are expected to, or intended to, be disposed. Other regions on the surface of the object may be referred to as non-critical regions.[000283] In some embodiments of the method 700 shown in Figure 16, the object may comprise a composite substrate, for example of the type used in advanced packaging semiconductor processes. For example, the substrate may comprise a base substrate to which a plurality of separate, previously formed sub-assemblies (for example silicon chips, opto-electronic devices etc.) have been applied. These sub-assemblies may be of the order of 10-100 pm high. A space between the sub-assemblies may be filled with a filler material (for example an epoxy resin). Such a composite substrate may be loaded into a lithographic apparatus and various connection layers may be formed to form connections between groups of individual sub-assemblies. Such composite substrates may be considered to be an example of a high topography lithographic process. Another example of a high topography lithographic process is 3D -N AND manufacturing, in which memory cells may be stacked vertically (i.e. in a direction normal to a plane of the substrate. Determining a topography of such high topography lithographic processes (advanced packaging semiconductor processes and 3D-NAND substrates) presents new additional challenges.[000284] By way of example, Figure 17A schematically shows a substrate 800 comprising a plurality of target regions 810. In this simplified example eleven generally rectangular target regions 810 are shown in the (circular) surface of the substrate 800, however, it will be appreciated that in practice there may be more (or fewer) target regions on the substrate. Each target region comprises three chips or devices 812, 814, 816 surrounded by a filler material 818. Note that these are only shown for one of the eleven target regions 810 in Figure 17A to aid the clarity of the figure. For the example embodiment of a substrate 800 shown in Figure 17 A, at step 710 of the method 700 shown in Figure 16 each chip or device 812, 814, 816 of each target region 810 may be determined to be a first region. That is, 33 first regions may be determined at step 710.[000285] The method 700 further comprises a step 720 of, for each of a plurality of positions on the surface of the object, determining a height of the surface.[000286] The method 700 further comprises a step 730 of determining a weighting factor for each of the first regions in dependence on the height(s) of the surface determined for any of the plurality of positions on the surface of the object that correspond to that first region. As discussed further below,2024P00445EP 50this weighting factor may be indicative of whether or not a device or chip 812, 814, 816 is present in that first region.[000287] The method 700 further comprises a step 740 of combining the obtained weighting factors and the determined heights so as to determine a height profile of the surface of the object. As discussed further below, weighting factors may be used to ensure that first regions where no device or chip 812, 814, 816 is present do not contribute to the height profile of the surface of the object, or contribute to a lesser extent than first regions where a device or chip 812, 814, 816 is present.[000288] The method 700 shown in Figure 16 is advantageous as it uses knowledge of the first regions where devices or chips are expected to, or intended to, be disposed.[000289] The method then involves determining a weighting factor for each of the first regions in dependence on the height(s) of the surface determined for positions on the surface that correspond to those first regions. For example, it may be expected that the height of all of the first regions should be the same nominal value. For such an embodiment, (a) a first region may be assigned a weight of 1 if an average of the heights determined for all of the plurality of position that correspond to that first region is close to the nominal value; and (b) a first region may be assigned a weight of 0 if an average of the heights determined for all of the plurality of position that correspond to that first region is not close to the nominal value. It will be appreciated that in other embodiments, it may be expected that the heights of a first set of the first regions should be a first nominal value and the heights of a second set of the first regions should be a second nominal value (for example each of the different devices or chips 812, 814, 816 in each target region 810 may have a different nominal height value). For such embodiments, the weighting factors for each of the first regions in the first set may be dependent on how close the average of the heights determined for all of the plurality of position that correspond to that first region is to the first nominal value and the weighting factors for each of the first regions in the second set may be dependent on how close the average of the heights determined for all of the plurality of position that correspond to that first region is to the second nominal value.[000290] In some embodiments, determining (at step 710) the plurality of first regions on the surface of the object that correspond to a device or a chip may comprise using the method 100 of measuring one or more properties of a surface of an object is shown schematically in Figure 4A and as discussed above with reference to Figures 4A to 10. In particular, the method 100 shown in Figure 4A may be used so as to determine weighting factors for each of a plurality of positions on the surface of the object (as obtained at step 410). Each region containing weighting factors above a threshold value may be determined (at step 710) to be a first region.[000291] As discussed above, positions on the surface of the object that have a determined weighting factor (as determined by the method 100 shown in Figure 4A) above a threshold value may correspond to critical regions of the object (where devices or chips 812, 814, 816 are typically disposed). For example, positions on the surface of the object that have a determined weighting factor2024P00445EP 51of 1 (as determined by the method 100 shown in Figure 4A) may correspond to critical regions of the object.[000292] In some embodiments of the method 700, determining (at step 710) the plurality of first regions on the surface of the object that correspond to a device or a chip may comprise retrieving the first regions so determined using the method 100 shown in Figure 4 A from memory.[000293] In some embodiments of the method 700, determining (at step 710) the plurality of first regions on the surface of the object that correspond to a device or a chip may comprise retrieving the first regions from memory.[000294] In some embodiments of the method 700, determining (at step 710) the plurality of first regions on the surface of the object that correspond to a device or a chip may comprise retrieving the first regions that are input by a user. That is, a user with a priori knowledge of the layout of the substrate may input the positions of the first regions (which may, for example, be the expected positions of chips or devices 812, 814, 816).[000295] In some embodiments of the method 700, determining (at step 720) a height of the surface may be achieved using any height-determining methods and / or apparatus discussed above. For example, in some embodiments of the method, determining (at step 720) a height of the surface may use apparatus generally of the form of the apparatus LS, 600 shown in Figures 2 and 15. Additionally or alternatively, determining a(at step 720) a height of the surface may comprise (as shown schematically in Figure 4C), a step 114 of determining a first quantity and / or a step 116 of determining a second quantity. Such a measurement 112i of the first and second quantities (steps 114, 116) may be made using a level sensor (for example a level sensors LS, 600 of the type shown in Figures 2 and 15), as discussed above with reference to Figures 5A, 5B and 6.[000296] In some embodiments of the method 700, determining (at step 720) a height of the surface may comprise determining a first quantity and a second quantity by: projecting a radiation beam 622 onto the position 680 to form a periodic pattern thereon; receiving a portion of the radiation beam 642 reflected from the position 680 and forming an image of the periodic pattern at a sensor region; determining the first quantity as being dependent on a phase of the pattern relative to the sensor region; and determining the second quantity as being proportional to an intensity of the radiation received at the sensor region. The height may be determined in dependence on the first and second quantities (as discussed above).[000297] For such embodiments, determining the first quantity and the second quantity may comprise: splitting the received radiation 642 into a first portion 644 and a second portion 646 such that the relative intensities of the first and second portions 644, 646 are dependent on a height of the position (on the object) relative to the sensor; determining an intensity of each of the first and second portions 644, 646 (for example using two sensors 650, 660); determining the first quantity as being proportional to a difference of the intensities of the first and second portions 644, 646; and2024P00445EP 52determining the second quantity as being proportional to a sum of the intensities of the first and second portions 644, 646.[000298] As shown schematically in Figure 18, in some embodiments of the method 700 shown in Figure 16, determining (at step 730) a weighting factor for each of the first regions may comprise various optional sub-steps. In particular, determining (at step 730) a weighting factor for each of the first regions may comprise: a step 732 of determining an average height for each of the first regions, the average height being an average of the height(s) of the surface determined for any of the plurality of positions on the surface of the object that correspond to that first region. Determining (at step 730) a weighting factor for each of the first regions may further comprise: a step 734 of determining a mean value of the average heights for the plurality of first regions. Determining (at step 730) a weighting factor for each of the first regions may further comprise: a step 736 of determining the weighting factor of each of the first regions in dependence on a magnitude of a difference between the average height of that first region and the mean value.[000299] In some embodiments of the method 700 shown in Figure 16, determining a weighting factor for each of the first regions (at step 730) may further comprise: a step 738 of determining a spread of the average heights for the plurality of first regions. The spread of the average heights for the plurality of first regions may, for example, be represented by a variance or standard deviation of the average heights for the plurality of first regions.[000300] To illustrate the sub-steps 732, 734, 736, 738 shown in Figure 18, consider the substrate shown in Figure 17A but wherein one of the devices or chips 816 is missing. Such an arrangement is shown schematically in Figure 17B, wherein one of the devices or chips 816 is missing from one of the target regions 810a. At step 720, a height is measured at a plurality of different positions on the substrate 800. At step 732, an average height may be determined for each of the (33) first regions, the average height being an average of the height(s) of the surface determined for any of the plurality of positions on the surface of the object that correspond to that first region. Figure 17C shows a histogram 850 of all of these average heights. Also shown in Figure 17C is a curve 852, which may, for example, be a Gaussian or normal distribution and which may represent a best fit (for example a least squares fit) to the histogram 850 data. Also indicated are a mean value 854 and a standard deviation 856 of the average heights for the plurality of first regions.[000301] Note that the average heights of most of the first regions are well described by the curve 852. However, there is one first region whose average height, which is represented by data point 858, and which corresponds to the first region in the target regions 810a shown in Figure 17B which is missing one of the devices or chips 816. Some embodiments of the method 700 shown in Figure 16 determine the first region corresponding to this data point 858 should be given a smaller weight (for example zero weight) since this average height is too far from the mean value 854. Other data points with average heights that are closer to the mean value 854 may be assigned a larger weight (for example a weight of 1).2024P00445EP 53[000302] In some embodiments of the method 700 shown in Figure 16, a weighting factor (determined at step 830) for each of the first regions for which the magnitude of the difference between the average height of that first region and the mean value 854 is below a threshold value may be determined to be the same, non-zero weighting factor. For example, the non-zero weighting factor may be one.[000303] In some embodiments of the method 700 shown in Figure 16, a weighting factor for each of the first regions for which the magnitude of the difference between the average height of that first region and the mean value 854 is above a threshold value may be determined to be a weighting factor of zero.[000304] In some embodiments, the threshold value that is used to determine if the average heights of the first regions are closer to or far from the mean value 854 may be dependent on the determined spread 856. For example, in some embodiments a first region may be assigned a weight of 0 if its average height is more than 3CT, where a is the standard deviation of the curve 852.[000305] Some embodiments of the present disclosure relate to a new method of determining one or more configurations of an apparatus for a coarse measurement of a height of an object, as now discussed with reference to Figures 19 to 20C.[000306] Figure 19 schematically shows a method 900 of determining one or more configurations of an apparatus for a coarse measurement of a height of an object. The method 900 comprises: a step 910 of determining a plurality of first regions on the surface of the object that correspond to a device or a chip. The method 900 further comprises: a step 920 of determining at least one configuration of the apparatus wherein a coarse measurement spot overlaps with one of the plurality of first regions. The coarse measurement may be referred to as a capture.[000307] Figure 20 A schematically shows an arrangement 950 of beam spots for a level sensor. The arrangement 950 of beam spots may represent an arrangement of beam spots that are projected onto a substrate by an apparatus (level sensor) LS, 600 of the type shown in Figures 2 or 15. The arrangement 950 of beam spots comprises a coarse measurement spot 952 and a plurality of fine measurement spots 954. During a measurement of a substrate relative movement of the projection optics of the apparatus and the substrate causes the arrangement 950 of beam spots to move over a surface of the substrate. This relative movement is typically, though not necessarily, achieved by moving the substrate in a scanning direction (the y -direction in Figure 20 A). This movement may be such that the coarse measurement spot 952 moves over a portion of the substrate and, subsequently, the fine measurement spots 954 move over the same portion of the substrate.[000308] In order to measure a height of an object with some apparatus (level sensors) first a coarse measurement may be made and then, subsequently a fine measurement may be made. A single coarse measurement may be made for the arrangement 950 of beam spots shown in Figure 20 A.Subsequently, a plurality of fine measurements may be made, each fine measurement being made using a different one of the fine measurement spots 954.2024P00445EP 54[000309] The coarse (capture) measurement is made using the coarse measurement spot 952 and one or more of the fine measurement spots 954. In general, the more fine measurement spots 954 that are used for the coarse measurement, the more accurate and robust the coarse (capture) measurement will be. In general, the level sensor may have the functionality to individually select or deselect the fine measurement spots 954 for the coarse (capture) measurement.[000310] In some embodiments of the method 900 shown in Figure 19, the object may comprise a composite substrate, for example of the type used in advanced packaging semiconductor processes. For example, the substrate may comprise a base substrate to which a plurality of separate, previously formed sub-assemblies (for example silicon chips, opto-electronic devices etc.) have been applied. These sub-assemblies may be of the order of 10-100 pm high. A space between the sub-assemblies may be filled with a filler material (for example an epoxy resin). Such a composite substrate may be loaded into a lithographic apparatus and various connection layers may be formed to form connections between groups of individual sub-assemblies. Such composite substrates may be considered to be an example of a high topography lithographic process. Another example of a high topography lithographic process is 3D -N AND manufacturing, in which memory cells may be stacked vertically (i.e. in a direction normal to a plane of the substrate. Determining a topography of such high topography lithographic processes (advanced packaging semiconductor processes and 3D-NAND substrates) presents new additional challenges.[000311] As will be appreciated by the skilled person, for such embodiments it may be advantageous to ensure that, during a coarse height measurement (capture) the arrangement 950 of beam spots and the object are such that the coarse measurement spot 952 overlaps with a region that corresponds to a chip or device (rather than filler material). The method 900 shown in Figure 19 is advantageous as the method 900 ensures this via step 920 by determining at least one configuration of the apparatus wherein a coarse measurement spot 952 overlaps with one of the plurality of first regions. Advantageously, this can improve an accuracy of the capture or coarse height measurement.[000312] For a good coarse height measurement (capture) when processing a composite substrate of the type discussed above (for example as used in advanced packaging semiconductor processes and 3D-NAND substrates), it may be desirable for the coarse measurement spot 952 and at least one fine measurement spot 954 to overlap with one of the plurality of first regions (i.e. to overlap with a chiplet location). Furthermore, it may be desirable to deselect any fine measurement spots 954 that do not overlap with one of the plurality of first regions (i.e. do not overlap with a chiplet location) such that they are not used for the coarse height measurement (capture).[000313] By way of example, Figures 20B and 20C each schematically shows a configuration 960, 960’ showing a target region 970 of a substrate and a portion of the arrangement 950 of beam spots shown in Figure 20 A. In this simplified example the target region 970 comprises: six chips or devices 972, 974, 976, 978, 980, 982; and two regions of dummy silicon 984, 986, all surrounded by a filler material 988. For the example embodiment of arrangements 960, 960’ shown in Figures 20B and2024P00445EP 5520C, at step 910 of the method 900 shown in Figure 19 each chip or device 972, 974, 976, 978, 980, 982 of each target region 970 may be determined to be a first region at step 910. In both of the arrangements 960, 960’ shown in Figures 20B and 20C the coarse measurement spot 952 overlaps with one of the plurality of first regions 982.[000314] It will be appreciated from Figure 19 that an extent of the single coarse measurement spot 952 in a direction (the x-direction) that is perpendicular to the scanning direction is significantly less than an extent of the plurality of fine measurement spots 954 in this direction (the x-direction).[000315] In some embodiments of the method 900 shown in Figure 19 at least one of the determined configurations of the apparatus may be such that a number of fine measurement spots 954 that overlaps with one of the plurality of first regions 972, 974, 976, 978, 980, 982 is maximized; such an arrangement 960 is shown in Figure 20B. In the arrangement 960 shown in Figure 20B, the target region 970 of the substrate and the arrangement 950 of beam spots are such that a first set of fine measurement spots 954a each overlap with one of the plurality of first regions 972, 974, 976, 978, 980, 982 whereas a second set of fine measurement spots 954b do not (fully) overlap with one of the plurality of first regions 972, 974, 976, 978, 980, 982. Furthermore, in the arrangement 960 shown in Figure 20B, the target region 970 of the substrate and the arrangement 950 of beam spots are such that a number of the first set of fine measurement spots 954a is maximized (and a number of the second set of fine measurement spots 954b is minimized). In particular, in the arrangement 960 shown in Figure 20B, target region 970 of the substrate and the arrangement 950 of beam spots are such that: the coarse measurement spot 952 overlaps with one of the first regions 982; three fine measurement spots 954a each overlap with another one of the plurality of first regions 972; three fine measurement spots 954a each overlap with another one of the plurality of first regions 974; and three fine measurement spots 954a each overlap with another one of the plurality of first regions 976.[000316] It will be appreciated that the first set of fine measurement spots 954a (which each overlap with one of the plurality of first regions 972, 974, 976, 978, 980, 982) may be selected for use during a coarse height measurement (capture) whereas the second set of fine measurement spots 954b may be deselected such that they do not contribute to the coarse height measurement (capture). It will be appreciated an arrangement 960 of the type shown in Figure 20B may be used for embodiments for which it is the heights of different first regions (i.e. different chips) are expected to be approximately the same.[000317] In some embodiments of the method 900 shown in Figure 19 at least one of the determined configurations of the apparatus may be such that a number of fine measurement spots 954 that overlaps with the first region 982 that overlaps with the coarse measurement spot 952 is maximized; such an arrangement 960’ is shown in Figure 20C. In the arrangement 960’ shown in Figure 20C, the target region 970 of the substrate and the arrangement 950 of beam spots are such that a first set of fine measurement spots 954c each overlap with the first region 982 that overlaps with the coarse measurement spot 952 whereas a second set of fine measurement spots 954d do not overlap2024P00445EP 56with said first region 982. Furthermore, in the arrangement 960’ shown in Figure 20C, the target region 970 of the substrate and the arrangement 950 of beam spots are such that a number of the first set of fine measurement spots 954c is maximized (and a number of the second set of fine measurement spots 954d is minimized). In particular, in the arrangement 960’ shown in Figure 20C, target region 970 of the substrate and the arrangement 950 of beam spots are such that: the coarse measurement spot 952 and three fine measurement spots 954c each overlap with one of the first regions 982.[000318] It will be appreciated that the first set of fine measurement spots 954c (which each overlap with the first region 982 that overlaps with the coarse measurement spot 952) may be selected for use during a coarse height measurement (capture) whereas the second set of fine measurement spots 954d may be deselected such that they do not contribute to the coarse height measurement (capture).[000319] Although specific reference may be made in this text to the use of a lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-crystal displays (LCDs), thin-film magnetic heads, etc.[000320] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non-vacuum) conditions.[000321] Although specific reference may have been made above to the use of embodiments of the invention in the context of optical lithography, it will be appreciated that the invention, where the context allows, is not limited to optical lithography and may be used in other applications, for example imprint lithography.[000322] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that2024P00445EP 57such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.[000323] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below. Other aspects of the invention are set-out as in the following numbered clauses.1. A method of measuring one or more properties of a surface of an object, the method comprising:(a) making a plurality of sets of measurements of the object using a sensor, wherein a height of the object relative to the sensor is different for each of the plurality of sets measurements, and wherein each set of measurements comprises, for each of a plurality of positions on the surface of the object, determining a first quantity and a second quantity by:projecting a radiation beam onto the position to form a periodic pattern thereon;receiving a portion of the radiation beam reflected from the position and forming an image of the periodic pattern at a sensor region;determining the first quantity as being dependent on a phase of the pattern relative to the sensor region; anddetermining the second quantity as being proportional to an intensity of the radiation received at the sensor region;(b) for each of the plurality of positions on the surface of the object:selecting one of the plurality of sets of measurements in dependence on the second quantity; and determining a weighting factor for that position in dependence on the selected set of measurements for that position.2. The method of clause 1 wherein determining the first quantity and the second quantity comprises:splitting the received radiation into a first portion and a second portion such that the relative intensities of the first and second portions are dependent on a height of the position relative to the sensor; determining an intensity of each of the first and second portions;determining the first quantity as being proportional to a difference of the intensities of the first and second portions; anddetermining the second quantity as being proportional to a sum of the intensities of the first and second portions.3. A method measuring one or more properties of a surface of an object, the method comprising:2024P00445EP 58(a) making a plurality of sets of measurements of the object using a sensor, wherein a height of the object relative to the sensor is different for each of the plurality of sets measurements, and wherein each set of measurements comprises, for each of a plurality of positions on the surface of the object, determining a first quantity and a second quantity by:projecting a radiation beam onto the position to form a periodic pattern thereon; receiving a portion of the radiation beam reflected from the position and forming an image of the periodic pattern at a sensor region;splitting the received radiation into a first portion and a second portion such that the relative intensities of the first and second portions are dependent on a height of the position relative to the sensor; determining an intensity of each of the first and second portions;determining the first quantity as being proportional to a difference of the intensities of the first and second portions; anddetermining the second quantity as being proportional to a sum of the intensities of the first and second portions;(b) for each of the plurality of positions on the surface of the object:selecting one of the plurality of sets of measurements in dependence on the second quantity; and determining a weighting factor for that position in dependence on the selected set of measurements for that position.4. The method of any preceding clause wherein the one of the plurality of sets of measurements that is selected for each of the plurality of positions on the surface of the object is the set of measurements for which the second quantity is largest.5. The method of any preceding clause wherein determining a weighting factor for each of the plurality of positions comprises:determining a nominal range of heights of the object relative to the sensor, the nominal range of heights containing the height of the object when one or more of the plurality of sets of measurements were made; anddetermining different weighting factors for:a first set of positions on the surface of the object, wherein the selected set of measurements for each of the first set of positions on the surface of the object were made when the height of the object fell within the determined range of heights; anda second set of positions on the surface of the object, wherein the selected set of measurements for each of the second set of positions on the surface of the object were made when the height of the object fell outside of the determined range of heights.6. The method of clause 5 wherein determining the nominal range of heights of the object relative to the sensor comprises selecting one of the plurality of sets of measurements and determining the nominal range of heights in dependence on the height of the object relative to the sensor when that set of measurements was made.2024P00445EP 597. The method of clause 6 wherein the one of the plurality of sets of measurements that is selected is the one that was selected for the greatest number of the plurality of positions on the surface of the object.8. The method of any one of clauses 5 to 7 wherein each of the positions of the first set of positions on the surface of the object has the same, non-zero weighting factor.9. The method of any one of clauses 5 to 8 wherein each of the positions of the second set of positions on the surface of the object has a smaller weighting factor than each of the positions of the first set of positions on the surface of the object.10. The method of clause 9 wherein each of the positions of the second set of positions on the surface of the object has a weighting factor of zero.11. The method of any preceding clause wherein the determined weighting factor for each position on the surface of the object is dependent on an expected impact that that position will have on a lithographic process.12. The method of any preceding clause further comprising, for each of the plurality of positions on the surface of the object: determining a height for that position based on: (a) a value of the first quantity from the set of measurements that was selected for that position; and (b) the height of the object relative to the sensor when that set of measurements was made.13. The method of any preceding clause wherein making each of the plurality of sets of measurements of the object comprises scanning one of the object and the sensor relative to the other such that the sensor can determine the first and second quantities for each of the plurality of positions on the surface of the object.14. The method of any preceding clause wherein the plurality of sets of measurements of the object cover a relative height range of 1 mm.15. The method of any preceding clause wherein the plurality of sets of measurements of the object are made at a plurality of equally -spaced heights of the object, relative to the sensor, with a height step or difference between adjacent heights in the range 10 to 15 pm.16. The method of any preceding clause wherein the plurality of sets of measurements of the object are made at a plurality of equally-spaced heights of the object, relative to the sensor, wherein a height step or difference between adjacent heights is of the order of a linear range of the sensor. 17. The method of any preceding clause wherein the plurality of sets of measurements of the object are made at a plurality of equally-spaced heights of the object, relative to the sensor, wherein a height step or difference between adjacent heights that is less than twice a pitch of the image of the periodic pattern in a plane of the sensor region.18. The method of any preceding clause wherein the surface of the object comprises at least one first region and at least one second region and wherein the materials of the at least one first region have different optical properties to the materials of the at least one second region.2024P00445EP 6019. The method of any preceding clause wherein the object comprises a composite substrate comprising: comprise a base substrate; a plurality of sub-assemblies supported by the base substrate; and a filler material between adjacent sub-assemblies.20. The method of any preceding clause wherein the weighting factor for each position on the surface of the object is indicative of the part of the surface of the object that that position corresponds to.21. The method of any preceding clause further comprising storing the determined weighting factors for each of the plurality of positions on the surface of the object in memory.22. The method of any preceding clause further comprising selecting one or more regions on the surface of the object suitable for making a subsequent course height measurement of another similar object, the one or more regions on the surface of the object being selected in dependence on the weighting factors for each of a plurality of positions on the surface of the object.23. The method of any preceding clause further comprising storing the one or more selected regions on the surface of the object that are suitable for making a subsequent course height measurement of another similar object in memory.24. A method of determining a topography of a surface of an object, the method comprising: obtaining weighting factors for each of a plurality of positions on the surface of the object as determined using the method of any preceding clause;for at least some of the plurality of positions on the surface of the object, determining a height of the object relative to a sensor; andcombining the obtained weighting factors and the determined heights so as to determine a height profile of the surface of the object.25. The method of clause 24 wherein obtaining the weighting factors for each of a plurality of positions on the surface of the object comprises using the method of any one of clauses 1 to 23 so as to determine the weighting factors for each of a plurality of positions on the surface of the object.26. The method of clause 24 or clause 25 wherein obtaining the weighting factors for each of a plurality of positions on the surface of the object comprises retrieving the weighting factors from memory.27. The method of any one of clauses 24 to 26 further comprising making a course height measurement of at least one region on the surface of the object before the height is determined for at least some of the at least some of the plurality of positions on the surface of the object, wherein the at least one region on the surface of the object is determined in dependence on the weighting factors for each of a plurality of positions on the surface of the object.28. The method of any one of clauses 24 to 27 wherein determining a height of the object relative to a sensor for the at least some of the plurality of positions on the surface of the object comprises:projecting a radiation beam onto the position to form a periodic pattern thereon;2024P00445EP 61receiving a portion of the radiation beam reflected from the position and forming an image of the periodic pattern at a sensor region; anddetermining the height as being dependent on a phase of the pattern relative to the sensor region. 29. The method of any one of clauses 24 to 28 wherein determining a height of the object relative to a sensor for the at least some of the plurality of positions on the surface of the object comprises:projecting a radiation beam onto the position to form a periodic pattern thereon receiving a portion of the radiation beam reflected from the position and forming an image of the periodic pattern at a sensor region;splitting the received radiation into a first portion and a second portion such that the relative intensities of the first and second portions are dependent on a height of the position relative to the sensor; determining an intensity of each of the first and second portions; anddetermining the height as being proportional to a difference of the intensities of the first and second portions.30. An exposure method comprising:measuring a topography of a surface of a substrate using the method of any one of clauses 24 to 29 and / or the method of any one of clauses 39 to 49;patterning a radiation beam using a patterning device; andprojecting the patterned radiation onto the substrate so as to form an image of the patterning device on the substrate;wherein a position of the substrate while the patterned radiation is being projected onto the substrate is controlled in dependence on the measured topography of the surface of the substrate. 31. The exposure method of clause 30, wherein the exposure is a scanning exposure such that patterning a radiation beam using a patterning device comprises moving the patterning device through the radiation beam and projecting the patterned radiation onto the substrate so as to form an image of the patterning device on the substrate comprises moving the substrate such that the image of the patterning device is generally stationary relative to the substrate.32. An apparatus for measuring a topography of a surface of a substrate, the apparatus comprising:a support for supporting a substrate that is disposable in a beam spot region; projection optics operable to form a first image of a pattern on a substrate when disposed in the beam spot region with a radiation beam;detection optics operable to receive a portion of the radiation beam reflected from the substrate; anda processor operable to determine a height of the substrate from the radiation beam reflected from the substrate and further operable to implement the method of any preceding clause and / or the method of any one of clauses 39 to 52.2024P00445EP 6233. The apparatus of clause 32 further comprising a movement mechanism operable to cause relative movement of the support relative and the beam spot region.34. The apparatus of clause 32 or clause 33 wherein the projection optics comprises:a projection patterning device; andfirst imaging optics arranged to form an image of the projection patterning device on the beam spot region.35. The apparatus of any one of clauses 32 to 34 wherein the detection optics is operable to receive a portion of the radiation beam reflected from the substrate and to split the reflected radiation into first and second portions such that a first portion of the radiation, which corresponds to a first portion of the first image, is spatially separate from a second portion of the radiation, which corresponds to a second portion of the first image, and wherein the apparatus further comprises:a first detector arranged to determine an intensity of the first portion of the radiation; and a second detector arranged to determine an intensity of the second portion of the radiation; andwherein the processor is operable to determine a height profile of the substrate by combining the intensity of the first portion of the radiation and the intensity of the second portion of the radiation.36. The apparatus of any one of clauses 32 to 35 wherein the detection optics comprises: splitting optics arranged to split the reflected radiation into first and second portions; andsecond imaging optics arranged to receive radiation reflected from an object supported by the support and to form a second image of the pattern on the splitting optics.37. The apparatus of any one of clauses 32 to 36 further comprising a radiation source operable to produce the radiation beam.38. An exposure apparatus comprising the apparatus of any one of clauses 32 to 37.39. A method of determining a topography of a surface of an object, the method comprising:determining a plurality of first regions on the surface of the object that correspond to a device or a chip;for each of a plurality of positions on the surface of the object determining a height of the surface; anddetermining a weighting factor for each of the first regions in dependence on the height(s) of the surface determined for any of the plurality of positions on the surface of the object that correspond to that first region; andcombining the obtained weighting factors and the determined heights so as to determine a height profile of the surface of the object.40. The method of clause 39 wherein determining the plurality of first regions on the surface of the object that correspond to a device or a chip comprises using the method of any one of clauses 12024P00445EP 63to 23 so as to determine weighting factors for each of a plurality of positions on the surface of the object and determining each region containing weighting factors above a threshold value to be a first region.41. The method of clause 40 wherein determining the plurality of first regions on the surface of the object that correspond to a device or a chip comprises retrieving the first regions so determined using the method of any one of clauses 1 to 23 from memory.42. The method of any one of clauses 39 to 41 wherein determining the plurality of first regions on the surface of the object that correspond to a device or a chip comprises retrieving the first regions from memory.43. The method of any one of clauses 39 to 41 wherein determining the plurality of first regions on the surface of the object that correspond to a device or a chip comprises retrieving the first regions that are input by a user.44. The method of any one of clauses 39 to 43 wherein determining a height of the surface comprises determining a first quantity and a second quantity by:projecting a radiation beam onto the position to form a periodic pattern thereon;receiving a portion of the radiation beam reflected from the position and forming an image of the periodic pattern at a sensor region;determining the first quantity as being dependent on a phase of the pattern relative to the sensor region; anddetermining the second quantity as being proportional to an intensity of the radiation received at the sensor region; anddetermining the height in dependence on the first and second quantities.45. The method of clause 44 wherein determining the first quantity and the second quantity comprises:splitting the received radiation into a first portion and a second portion such that the relative intensities of the first and second portions are dependent on a height of the position relative to the sensor; determining an intensity of each of the first and second portions;determining the first quantity as being proportional to a difference of the intensities of the first and second portions; anddetermining the second quantity as being proportional to a sum of the intensities of the first and second portions.46. The method of any one of clauses 39 to 45 wherein determining a weighting factor for each of the first regions comprises:determining an average height for each of the first regions, the average height being an average of the height(s) of the surface determined for any of the plurality of positions on the surface of the object that correspond to that first region;determining a mean value of the average heights for the plurality of first regions; and2024P00445EP 64determining the weighting factor of each of the first regions in dependence on a magnitude of a difference between the average height of that first region and the mean value.47. The method of clause 46 wherein a weighting factor for each of the first regions for which the magnitude of the difference between the average height of that first region and the mean value is below a threshold value is determined to be the same, non-zero weighting factor.48. The method of clause 46 and / or clause 47 wherein a weighting factor for each of the first regions for which the magnitude of the difference between the average height of that first region and the mean value is above a threshold value is determined to be a weighting factor of zero.49. The method of any one of clauses 46 to 48 wherein determining a weighting factor for each of the first regions further comprises:determining a spread of the average heights for the plurality of first regions; andwherein the threshold value is dependent on the determined spread.50. A method of determining one or more configurations of an apparatus for a coarse measurement of a height of an object, the method comprising:determining a plurality of first regions on the surface of the object that correspond to a device or a chip; anddetermining at least one configuration of the apparatus wherein a coarse measurement spot overlaps with one of the plurality of first regions.51. The method of clause 50 wherein at least one of the determined configurations of the apparatus is such that a number of fine measurement spots that overlaps with one of the plurality of first regions is maximized.52. The method of clause 50 or clause 5 Iwherein at least one of the determined configurations of the apparatus is such that a number of fine measurement spots that overlaps with one the first region that overlaps with the coarse measurement spot is maximized.

Claims

1. 2024P00445EP 65CLAIMS1. A method of measuring one or more properties of a surface of an object, the method comprising:(a) making a plurality of sets of measurements of the object using a sensor, wherein a height of the object relative to the sensor is different for each of the plurality of sets measurements, and wherein each set of measurements comprises, for each of a plurality of positions on the surface of the object, determining a first quantity and a second quantity by:projecting a radiation beam onto the position to form a periodic pattern thereon; receiving a portion of the radiation beam reflected from the position and forming an image of the periodic pattern at a sensor region;determining the first quantity as being dependent on a phase of the pattern relative to the sensor region; anddetermining the second quantity as being proportional to an intensity of the radiation received at the sensor region;(b) for each of the plurality of positions on the surface of the object:selecting one of the plurality of sets of measurements in dependence on the second quantity; anddetermining a weighting factor for that position in dependence on the selected set of measurements for that position.

2. The method of claim 1 wherein determining the first quantity and the second quantity comprises:splitting the received radiation into a first portion and a second portion such that the relative intensities of the first and second portions are dependent on a height of the position relative to the sensor;determining an intensity of each of the first and second portions;determining the first quantity as being proportional to a difference of the intensities of the first and second portions; anddetermining the second quantity as being proportional to a sum of the intensities of the first and second portions.

3. A method measuring one or more properties of a surface of an object, the method comprising:(a) making a plurality of sets of measurements of the object using a sensor, wherein a height of the object relative to the sensor is different for each of the plurality of sets measurements, and wherein each set of measurements comprises, for each of a plurality of positions on the surface of the object, determining a first quantity and a second quantity by:projecting a radiation beam onto the position to form a periodic pattern thereon; receiving a portion of the radiation beam reflected from the position and forming an image of the periodic pattern at a sensor region;2024P00445EP 66splitting the received radiation into a first portion and a second portion such that the relative intensities of the first and second portions are dependent on a height of the position relative to the sensor;determining an intensity of each of the first and second portions;determining the first quantity as being proportional to a difference of the intensities of the first and second portions; anddetermining the second quantity as being proportional to a sum of the intensities of the first and second portions;(b) for each of the plurality of positions on the surface of the object:selecting one of the plurality of sets of measurements in dependence on the second quantity; anddetermining a weighting factor for that position in dependence on the selected set of measurements for that position.

4. The method of any preceding claim wherein the one of the plurality of sets of measurements that is selected for each of the plurality of positions on the surface of the object is the set of measurements for which the second quantity is largest.

5. The method of any preceding claim wherein determining a weighting factor for each of the plurality of positions comprises:determining a nominal range of heights of the object relative to the sensor, the nominal range of heights containing the height of the object when one or more of the plurality of sets of measurements were made; anddetermining different weighting factors for:a first set of positions on the surface of the object, wherein the selected set of measurements for each of the first set of positions on the surface of the object were made when the height of the object fell within the determined range of heights; anda second set of positions on the surface of the object, wherein the selected set of measurements for each of the second set of positions on the surface of the object were made when the height of the object fell outside of the determined range of heights.

6. The method of claim 5 wherein determining the nominal range of heights of the object relative to the sensor comprises selecting one of the plurality of sets of measurements and determining the nominal range of heights in dependence on the height of the object relative to the sensor when that set of measurements was made.2024P00445EP 677. The method of any preceding claim further comprising, for each of the plurality of positions on the surface of the object: determining a height for that position based on: (a) a value of the first quantity from the set of measurements that was selected for that position; and (b) the height of the object relative to the sensor when that set of measurements was made.

8. A method of determining a topography of a surface of an object, the method comprising: obtaining weighting factors for each of a plurality of positions on the surface of the object as determined using the method of any preceding claim;for at least some of the plurality of positions on the surface of the object, determining a height of the object relative to a sensor; andcombining the obtained weighting factors and the determined heights so as to determine a height profile of the surface of the object,wherein obtaining the weighting factors for each of a plurality of positions on the surface of the object comprises using the method of any one of claims 1 to 7 so as to determine the weighting factors for each of a plurality of positions on the surface of the object.

9. The method of claim 8 further comprising making a course height measurement of at least one region on the surface of the object before the height is determined for at least some of the at least some of the plurality of positions on the surface of the object, wherein the at least one region on the surface of the object is determined in dependence on the weighting factors for each of a plurality of positions on the surface of the object.

10. An apparatus for measuring a topography of a surface of a substrate, the apparatus comprising:a support for supporting a substrate that is disposable in a beam spot region;projection optics operable to form a first image of a pattern on a substrate when disposed in the beam spot region with a radiation beam;detection optics operable to receive a portion of the radiation beam reflected from the substrate; anda processor operable to determine a height of the substrate from the radiation beam reflected from the substrate and further operable to implement the method of any preceding claim and / or the method of claim 11.

11. A method of determining a topography of a surface of an object, the method comprising: determining a plurality of first regions on the surface of the object that correspond to a device or a chip;for each of a plurality of positions on the surface of the object determining a height of the surface; and2024P00445EP 68determining a weighting factor for each of the first regions in dependence on the height(s) of the surface determined for any of the plurality of positions on the surface of the object that correspond to that first region; andcombining the obtained weighting factors and the determined heights so as to determine a height profile of the surface of the object,wherein determining the plurality of first regions on the surface of the object that correspond to a device or a chip comprises using the method of any one of claims 1 to 7 so as to determine weighting factors for each of a plurality of positions on the surface of the object and determining each region containing weighting factors above a threshold value to be a first region.

12. The method of claim 11 wherein determining a height of the surface comprises determining a first quantity and a second quantity by:projecting a radiation beam onto the position to form a periodic pattern thereon; receiving a portion of the radiation beam reflected from the position and forming an image of the periodic pattern at a sensor region;determining the first quantity as being dependent on a phase of the pattern relative to the sensor region; anddetermining the second quantity as being proportional to an intensity of the radiation received at the sensor region; anddetermining the height in dependence on the first and second quantities.

13. The method of claim 12 wherein determining the first quantity and the second quantity comprises:splitting the received radiation into a first portion and a second portion such that the relative intensities of the first and second portions are dependent on a height of the position relative to the sensor;determining an intensity of each of the first and second portions;determining the first quantity as being proportional to a difference of the intensities of the first and second portions; anddetermining the second quantity as being proportional to a sum of the intensities of the first and second portions.

14. The method of any one of claims 12 to 13 wherein determining a weighting factor for each of the first regions comprises:determining an average height for each of the first regions, the average height being an average of the height(s) of the surface determined for any of the plurality of positions on the surface of the object that correspond to that first region;determining a mean value of the average heights for the plurality of first regions; and2024P00445EP 69determining the weighting factor of each of the first regions in dependence on a magnitude of a difference between the average height of that first region and the mean value.

15. A method of determining one or more configurations of an apparatus for a coarse measurement of a height of an object, the method comprising:determining a plurality of first regions on the surface of the object that correspond to a device or a chip; anddetermining at least one configuration of the apparatus wherein a coarse measurement spot overlaps with one of the plurality of first regions,wherein at least one of the determined configurations of the apparatus is such that at least one of:a number of fine measurement spots that overlaps with one of the plurality of first regions is maximized, anda number of fine measurement spots that overlaps with one the first region that overlaps with the coarse measurement spot is maximized.

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