Method and apparatus for measuring a topography of a surface of an object

Abstract

A method for determining at least one height of a surface of an object comprises projecting a patterned radiation beam onto an object so as to form an image of a pattern. The image of the pattern comprises first and second portions, each of the first and second portions comprising at least one line, wherein at least one line of the first portion is disposed at a non-zero angle to at least one line of the second portion. The method further comprises receiving a portion of the patterned radiation beam reflected from the substrate. The method further comprises determining at least one height from a combination of: (a) a part of the received portion of the patterned radiation beam reflected from the first portion of the pattern; and (b) a part of the received portion of the patterned radiation beam reflected from the second portion of the pattern.

Description

METHOD AND APPARATUS FOR MEASURING A TOPOGRAPHY OF A SURFACE OF AN OBJECTCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 24215498.7 which was filed on 26 November 2024 and which is incorporated herein in its entirety by reference.FIELD

[0002] The present invention relates to a method of measuring at least one height of a surface of an object. The present invention also relates a corresponding apparatus for measuring at least one height of a surface of an object. The present invention has particular application in the field of lithography. The apparatus may be referred to as a level sensor and may form part of a lithographic apparatus. The present invention also relates to a lithographic apparatus comprising the apparatus for measuring at least one height of a surface of an object. The present invention also relates to a new projection patterning device and / or splitting optics that may find use in the apparatus for measuring at least one height of a surface of an object.BACKGROUND

[0003] 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 use are 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 shape of the wafer may be determined using apparatus that may be referred to as a level sensor. This measurement of the shape of the wafer may be performed within the lithographic apparatus, forexample 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] It may be desirable to provide new methods and / or apparatus for determining a shape of a wafer that may at least partially address one or more problems associated with existing arrangements, whether identified herein or otherwise.SUMMARY

[0007] According to a first aspect of the present disclosure there is provided a method for determining at least one height of a surface of an object, the method comprising: projecting a patterned radiation beam onto an object so as to form an image of a pattern, the image of the pattern comprising a first portion and a second portion, each of the first and second portions comprising at least one line, wherein at least one line of the first portion is disposed at a non-zero angle to at least one line of the second portion; receiving a portion of the patterned radiation beam reflected from the substrate; and determining at least one height from a combination of: (a) a part of the received portion of the patterned radiation beam reflected from the first portion of the pattern; and (b) a part of the received portion of the patterned radiation beam reflected from the second portion of the pattern.

[0008] The method according to the first aspect is advantageous as it allows for a sensitivity of the determined height(s) to positioning errors of the image of the pattern in a first direction that is perpendicular to the plane of incidence of the patterned radiation beam at the object to be reduced, as now discussed.

[0009] The portion of the patterned radiation beam reflected from the substrate may form a second image of the pattern, for example on a detector. As the height of the object varies, the position of the second image of the pattern will also vary and, in turn, this may result in a change in an intensity distribution across the detector. In particular, as the height of the object varies, the position of the second image of the pattern will also vary in a direction that is parallel to the plane of incidence (which may be referred to as a second direction). However, the position of the second image of the pattern in the first direction (that is perpendicular to the plane of incidence) is independent of the height of the object. As the height of the object varies, the position of the second image of the pattern may vary relative to splitting optics arranged to split the reflected radiation into first and second portions. The splitting optics may comprise one or more prisms. For example, the splitting optics may comprise a prism for each line of the pattern.

[0010] In practice, such height measuring methods may be implemented by a level sensor, for example in a lithographic apparatus. Typically, such methods use a pattern that comprises one or more features, for example a plurality of parallel lines. As long as the images of the lines that are formed on the object have a component in the first direction (that is perpendicular to the plane of incidence), any change in a height of the object will result in a position of the lines shifting in adirection in the plane of incidence having a component perpendicular to their extent. One possible implementation would be to use patterns that comprise a plurality of parallel lines that, in a plane of the object, are all perpendicular to the plane of incidence (i.e. extending along the first direction). However, it is desirable for the lines to be oriented such that their images on the object are at an oblique angle to (i.e. neither parallel to nor perpendicular to) the plane of incidence. This may be, for example, so that the images of these features are not parallel to lithographic features that have been formed on the object (which may comprise a substrate or wafer).

[0011] For example, such patterns have previously comprised lines such that their images on the object are generally aligned with, but not exactly parallel to, the first direction that is perpendicular to the plane of incidence. For example, such features have previously been arranged such that their images on the object to extend in a direction inclined at an angle of the order of less than 10° to the first direction.

[0012] The linear range of such a level sensor is proportional to the pitch of the image of the pattern on the object in the second direction. In order to increase this linear range, this pitch may be increased. However, this can result in a relatively small number of (relatively thick) lines formed on the object, which increases the sensitivity of such a level sensor to variations in the reflectivity of the surface of the object and to variations in an intensity of the incident radiation. Therefore, it may be desirable for the images of the lines formed on the object to be more aligned with, but not exactly parallel to, the second direction. This allows for a larger number of thinner lines to be used. For example, the lines may be arranged to extend, when projected onto the plane of the object, in a direction which is closer to the second direction than to the first direction. However, the inventors of the present invention have realized that with such arrangements the height measurements become more sensitive to positioning errors of the image of the pattern in the first direction.

[0013] Since the image of the pattern comprises a first portion and a second portion wherein at least one line of the first portion is disposed at a non-zero angle to at least one line of the second portion, the at least one line of the first portion will be disposed at different angle to the first direction (that is perpendicular to the plane of incidence of the patterned radiation beam at the object) than the at least one line of the second portion. Therefore, by using a pattern comprising a first portion and a second portion (wherein at least one line of the first portion is disposed at a non-zero angle to at least one line of the second portion), height measurements based on the reflected portions from the first and second portions will have different sensitivities to positioning errors of the image of the pattern in the first direction. Therefore, advantageously, by determining a height of the substrate from a combination of the parts of the received portion of the patterned radiation beam reflected from the first and second portions of the image of the pattern, this sensitivity to positioning errors of the image of the pattern in the first direction can be eliminated or at least reduced. Advantageously, this may allow for the image of the pattern to comprise portions with lines that are closely aligned with the second direction, for example comprising lines that extend in a direction inclined at an angle of theorder of 10° or less to the second direction, which in turn increases the linear range of the method, without the disadvantage of an increase sensitivity to positioning errors of the image of the pattern in the first direction.

[0014] It will be appreciated that determining a height of the object comprises determining a height of the object relative to a reference height or position.

[0015] The part of the received portion of the patterned radiation beam reflected from the first portion of the image of the pattern and the part of the received portion of the patterned radiation beam reflected from the second portion of the image of the pattern may be combined in such a way so as to at least partially correct for a sensitivity of the determined height to positioning errors of the image of the pattern in a first direction that is perpendicular to the plane of incidence of the patterned radiation beam at the object.

[0016] It will be appreciated that such a combination of the part of the received portion of the patterned radiation beam reflected from the first portion of the image of the pattern and the part of the received portion of the patterned radiation beam reflected from the second portion of the image of the pattern may be achieved in a number of different ways, as now discussed.

[0017] It will be appreciated that at least partially correcting for the sensitivity of the height measurement to positioning errors of the image of the pattern the first direction may mean that the determined height is less sensitive to such positioning errors than a determination using reflected radiation from only one of the first and second portions of the image of the pattern would be.

[0018] In some embodiments, the part of the received portion of the patterned radiation beam reflected from the first portion of the image of the pattern and the part of the received portion of the patterned radiation beam reflected from the second portion of the image of the pattern may be combined in such a way so as to optimize or minimize a sensitivity of the determined height to positioning errors of the image of the pattern the first direction.

[0019] The part of the received portion of the patterned radiation beam reflected from the first portion of the image of the pattern and the part of the received portion of the patterned radiation beam reflected from the second portion of the image of the pattern may be combined such that a sensitivity of the determined height to positioning errors of the image of the pattern a first direction that is perpendicular to the plane of incidence of the patterned radiation beam at the object is less than a sensitivity to positioning errors of the image of the pattern in the first direction would be for a height determined using only one of: (a) the part of the received portion of the patterned radiation beam reflected from the first portion of the image of the pattern; or (b) the part of the received portion of the patterned radiation beam reflected from the second portion of the image of the pattern.

[0020] In some embodiments the sensitivity of the determined height to positioning errors of the first portion of the image of the pattern in the first direction may be generally equal and opposite to the sensitivity of the determined height to positioning errors of the second portion of the image of the pattern in the first direction. Such an arrangement may be achieved with the first portioncomprising a plurality of lines inclined at a first angle to the first direction and the second portion comprising a plurality of lines inclined at an equal but opposite angle to the first direction. With such embodiments, the contributions to the sensitivity of the determined height to positioning errors of the first and second portions of the image of the pattern in the first direction can at least partially cancel each other out, resulting in a measurement that is substantially independent of the position of the image of the pattern in the first direction.

[0021] The first portion of the image of the pattern may comprise at least one line disposed at a first angle to a first direction that is perpendicular to the plane of incidence of the patterned radiation beam at the object. The second portion of the image of the pattern may comprise at least one line disposed at a second angle to the first direction, the second angle being equal and opposite to the first angle.

[0022] Such embodiments may be used to implement the combination of: (a) a part of the received portion of the patterned radiation beam reflected from the first portion of the pattern; and (b) a part of the received portion of the patterned radiation beam reflected from the second portion of the pattern (to determine the at least one height) using either hardware or software, as discussed further below.

[0023] The first portion of the pattern may comprise at least one line that is perpendicular to a first direction that is perpendicular to the plane of incidence of the patterned radiation beam at the object.

[0024] That is, the first portion of the pattern comprises at least one line (for example a plurality of parallel lines) that is parallel to the second direction (i.e. lie in the plane of incidence). Advantageously, with such an arrangement the position of the first portion (in a direction perpendicular to the extent of the at least one line that is perpendicular to the first direction) is independent of the height of the object and is only dependent on a positioning error of the pattern in the first direction. Therefore, the first portion of the pattern can be used to make a direct measurement of a positioning error of the pattern in the first direction. In turn, this can be used to determine an error of a height determined using the second portion of the pattern (which may, for example, comprise lines that extend in a direction inclined at an angle of the order of less than 10° to the first direction). In this way, the first portion of the image of the pattern can act as a reference or calibration mark that can be used to correct for an error in the height determined using the second portion of the image of the pattern.

[0025] In such embodiments, a determined height from the first portion of the image of the pattern may be independent of a height of the surface of the object (and may only be dependent on the positioning errors of the first portion of the image of the pattern in the second direction). The determined height from the second portion of the image of the pattern may be dependent on both (a) a height of the surface of the object and (b) a positioning error of the image of the pattern in the second direction. With such embodiments, the height determined from the first portion of the image may beused to calibrate the height determined from the second portion of the image (i.e. to at least partially correct for a sensitivity to the positioning error of the image of the second portion of the pattern in the second direction).

[0026] Such embodiments may be used to implement the combination of: (a) a part of the received portion of the patterned radiation beam reflected from the first portion of the pattern; and (b) a part of the received portion of the patterned radiation beam reflected from the second portion of the pattern (to determine the at least one height) using software, as discussed further below.

[0027] The first and second portions may comprise at least one line disposed at an angle of less than 45° to the plane of incidence of the patterned radiation beam at the object.

[0028] Advantageously, with such arrangements a larger number of thinner lines can be used whilst still providing an arrangement with a significant linear range. In some embodiments, at least one of the first and second portions comprises at least one line disposed at an angle of less than 30° to the plane of incidence of the patterned radiation beam at the object. In some embodiments, at least one of the first and second portions comprises at least one line disposed at an angle of less than 20° to the plane of incidence of the patterned radiation beam at the object. In some embodiments, at least one of the first and second portions comprises at least one line disposed at an angle of less than 10° to the plane of incidence of the patterned radiation beam at the object.

[0029] It will be appreciated that the combination of: (a) a part of the received portion of the patterned radiation beam reflected from the first portion of the image of the pattern; and (b) a part of the received portion of the patterned radiation beam reflected from the second portion of the image of the pattern (to determine the at least one height) may be implemented using either hardware or software, as now discussed.

[0030] At least one of the first and second portions of the image of the pattern may have a pitch in a first direction that is perpendicular to the plane of incidence of the patterned radiation beam at the object that is less than a pitch in a second direction that is perpendicular to the first direction.

[0031] The first and second portions of the image of the pattern may each form part of a region of the object that is entirely illuminated by the patterned radiation beam.

[0032] That is, the first and second portions of the image of the pattern each form part of a region of same illumination spot on the object. With such embodiments, splitting optics defining a similar pattern to the pattern that is projected onto the object can be used to split the radiation into two parts: a first detection beam and a second detection beam. Note that the first portion of the image of the pattern will, in general, contribute to both the first detection beam and the second detection beam and, similarly, the second portion of the image of the pattern will, in general, contribute to both the first detection beam and the second detection beam. The first and second detection beams may be directed to separate detectors and the height may be determined as being proportional to a difference in the intensities determined by the two detectors.

[0033] The first portion and the second portion may be disposed in separate but adjacent regions of the image of the pattern.

[0034] A region of the image of the pattern containing the first portion may at least partially spatially overlap with a region of the image of the pattern containing the second portion. The line(s) of the first portion may be interposed with the line(s) of the second portion in a first direction that is perpendicular to the plane of incidence of the patterned radiation beam at the object.

[0035] Combining: (a) the part of the received portion of the patterned radiation beam reflected from the first portion of the image of the pattern; and (b) the part of the received portion of the patterned radiation beam reflected from the second portion of the image of the pattern may comprise: generating from the first and second portions a first detection beam and a second detection beam; determining an intensity of the first detection beam and the second detection beam; and determining the at least one height from a combination of the intensities of the first detection beam and the second detection beam.

[0036] In some embodiments, a plurality of separate regions of the object may be illuminated by the patterned radiation beam and the first and second portions of the image of the pattern may form different ones of said plurality of regions.

[0037] That is, the first and second portions of the image of the pattern form different illumination spots on the object.

[0038] With such embodiments, a splitting optics (for example a prism) may be provided for each of the plurality of separate regions (also referred to as illumination spots) defining a similar pattern to the pattern that is projected onto that illumination spot. Each such splitting optics can be used to split the radiation into two parts: a first detection beam and a second detection beam. Note that for such embodiments, the first portion of the image of the pattern will contribute to a first set of first detection beam and second detection beam whereas the second portion of the image of the pattern will contribute to a second set of first detection beam and second detection beam. The first and second detection beams from each set (one set per illumination spot) may be directed to separate detectors and a height may be determined from each one (for example being proportional to a difference in the intensities determined by the two detectors).

[0039] In turn, the at least one height may be determined from a combination of this plurality of determined heights (using a suitable algorithm).

[0040] Combining: (a) the part of the received portion of the patterned radiation beam reflected from the first portion of the image of the pattern; and (b) the part of the received portion of the patterned radiation beam reflected from the second portion of the image of the pattern may comprise: from each of a plurality of separate regions of the object that are illuminated by the patterned radiation beam: generating a first detection beam and a second detection beam; determining an intensity of the first detection beam and the second detection beam; and determining a height from a combination ofthe intensities of the first detection beam and the second detection beam; and determining the at least one height from a combination of these determined heights.

[0041] The method may further comprise moving the object in a second direction that is parallel to the plane of incidence of the patterned radiation beam at object. The second direction may be referred to as a scanning direction.

[0042] According to a second aspect of the present disclosure there is provided an apparatus for determining at least one height of a surface of an object, the apparatus comprising: a projection unit operable to form an image of a pattern on the object with a radiation beam, wherein the image of the pattern comprises a first portion and a second portion, each of the first and second portions comprising at least one line, wherein at least one line of the first portion is disposed at a non-zero angle to at least one line of the second portion; a detection unit operable to receive a portion of the radiation beam reflected from the object; and a processing unit operable to implement the method according to the first aspect of the present disclosure so as to determine a height of the surface of the object.

[0043] The apparatus according to the second aspect of the present disclosure may be referred to as a level sensor. The apparatus according to the second aspect of the present disclosure is arranged to implement the method according to the first aspect (i.e. to determining at least one height from a combination of: (a) a part of the received portion of the patterned radiation beam reflected from the first portion of the image of the pattern; and (b) a part of the received portion of the patterned radiation beam reflected from the second portion of the image of the pattern).

[0044] It will be appreciated that the object does not form part of the apparatus according to the second aspect of the present disclosure. The projection unit may be operable to form the image of the pattern with the radiation beam in a beam spot region in which an object is, in use, positionable. Further, the detection unit may be operable to receive a portion of the radiation beam reflected from an object when disposed in the beam spot region.

[0045] The projection unit may comprise a projection patterning device which defines the pattern.

[0046] The projection patterning device may be referred to as a projection grating. In general, the image of the pattern (that is formed on the object) may comprise a distorted version of the pattern defined by the projection grating. In general, the image of the pattern (that is formed on the object) may be dependent on (a) the pattern defined by the projection grating; (b) an angle of incidence of the radiation on the projection grating; and (c) an angle of incidence of the radiation on the object.

[0047] Note that in some embodiments, the projection patterning device may be illuminated with a radiation beam that is propagating in a direction aligned with a normal to the projection patterning device. Such an illumination scheme may be referred to as a normal illumination scheme. It will be appreciated that with such an illumination scheme the image of the pattern will be thepattern defined by the projection grating that has been elongated in a direction parallel to the plane of incidence by a factor of 1 / cos 0Swhere 0Sis the angle of incidence on the object.

[0048] In some alternative embodiments, the projection patterning device may be illuminated with a radiation beam that is propagating in a direction that is at a non-zero angle to the projection patterning device such that the pattern defined by the projection patterning device is the same as the image of the pattern formed on the object (with no scaling factor in the direction parallel to the plane of incidence). Such an illumination scheme may be referred to as a Scheimpflung illumination scheme.

[0049] In general, the projection patterning device may be illuminated with a radiation beam that is propagating in any direction.

[0050] The projection unit may further comprise first imaging optics arranged to form an image of the projection patterning device on the object.

[0051] The projection patterning device may define at least one continuous patterned region and the first and second portions of the image of the pattern may each form part of at least one such continuous patterned region.

[0052] The or each continuous patterned region may correspond to an illumination spot that is formed on the object.

[0053] The first portion and the second portion may be disposed in separate but adjacent regions of the at least one patterned region.

[0054] A region of the image of the pattern containing the first portion may at least partially spatially overlap with and a region of the image of the pattern containing the second portion.

[0055] Note that for such embodiments it may be that the individual lines of the first portion do not spatially overlap the individual lines of the second portion.

[0056] The line(s) of the first portion may be interposed with the line(s) of the second portion in a first direction that is perpendicular to the plane of incidence of the patterned radiation beam at the object.

[0057] That is, moving across the image of the pattern in the first direction (the x-direction), the image of the pattern may comprise a line of the first portion then a line of the second portion, then a line of the first portion and so on. Such embodiments may alternatively be described as the line(s) of the first portion being interleaved with the line(s) of the second portion in the first direction (the x- direction). Note that with such embodiments, the line(s) of the first portion may abut the line(s) of the second portion. Alternatively, a gap may be provided between the line(s) of the first portion and the line(s) of the second portion.

[0058] The projection patterning device may define a plurality of discrete patterned regions. The first portion of the image of the pattern may be defined by a first set of the plurality of patterned regions and the second portion of the image of the pattern may be defined by a second, different set of the plurality of patterned regions.

[0059] Each continuous patterned region may correspond to an illumination spot that is formed on the object. Each set of the plurality of patterned regions may comprise at least one of the plurality of patterned regions.

[0060] The first portion of the image of the pattern may comprise at least one line disposed at a first angle to a first direction that is perpendicular to the plane of incidence of the patterned radiation beam at the object and the second portion of the image of the pattern may comprise at least one line disposed at a second angle to the first direction, the second angle being equal and opposite to the first angle.

[0061] In some embodiments, the first portion of the image of the pattern comprises a plurality of parallel lines, each disposed at a first angle to the first direction and the second portion of the image of the pattern comprises a plurality of parallel lines disposed at a second angle to the first direction, the second angle being equal and opposite to the first angle.

[0062] The first portion of the image of the pattern may comprise at least one line that is perpendicular to a first direction that is perpendicular to the plane of incidence of the patterned radiation beam at the object.

[0063] In some embodiments, the first portion of the image of the pattern comprises a plurality of parallel lines that are perpendicular to the first direction.

[0064] At least one of the first and second portions of the image of the pattern may comprise at least one line disposed at an angle of less than 45° to the plane of incidence of the patterned radiation beam at the object.

[0065] In some embodiments, at least one of the first and second portions of the image of the pattern comprises at least one line disposed at an angle of less than 30° to the plane of incidence of the patterned radiation beam at the object. In some embodiments, at least one of the first and second portions of the image of the pattern comprises at least one line disposed at an angle of less than 20° to the plane of incidence of the patterned radiation beam at the object. In some embodiments, at least one of the first and second portions of the image of the pattern comprises at least one line disposed at an angle of less than 10° to the plane of incidence of the patterned radiation beam at the object.

[0066] At least one of the first and second portions of the image of the pattern may have a pitch in a first direction that is perpendicular to the plane of incidence of the patterned radiation beam at the object that is less than a pitch in a second direction that is perpendicular to the first direction.

[0067] The detection unit may comprise splitting optics arranged to receive the beam of radiation reflected from a surface of the object and to split it into a plurality of detection beams. The splitting optics may comprise a plurality of splitting elements, each corresponding to a different one of a plurality of lines of the pattern.

[0068] The splitting optics may be referred to as a detection grating (or a plurality of detection gratings). Each splitting element may be operable to split the radiation received from a corresponding line of the pattern into a plurality of portions (e.g. two portions). Each splitting element may have asubstantially similar shape to a corresponding line of the pattern. In this way, the splitting optics may be said to be matched to a projection patterning device of the apparatus.

[0069] The detection unit may further comprise second imaging optics arranged to receive radiation reflected from the object and to form a second image of the pattern on the splitting optics.

[0070] In some embodiments, the splitting optics may be arranged to generate, from the portion of the radiation beam reflected from the object that originates from the first and second portions, a first detection beam and a second detection beam. The apparatus may further comprise: a first detector arranged to determine an intensity of the first detection beam; and a second detector arranged to determine an intensity of the second detection beam.

[0071] In some embodiments, the splitting optics may be arranged to generate, for the portion of the radiation beam reflected from the object that originates from each of a plurality of separate regions of the object that are illuminated by the patterned radiation beam: a first detection beam and a second detection beam. For each of the plurality of separate regions of the object that are illuminated by the patterned radiation beam, the apparatus may further comprise: a first detector arranged to determine an intensity of the first detection beam; and a second detector arranged to determine an intensity of the second detection beam.

[0072] The apparatus may further comprise a support for supporting a substrate.

[0073] The apparatus may further comprise a movement mechanism operable to cause relative movement of the support relative to the projection unit and / or the detection unit.

[0074] In particular, the movement mechanism operable to cause relative movement of the support relative to a beam spot region in a second direction that is parallel to the plane of incidence of the patterned radiation beam at a beam spot region of the apparatus. The second direction may be referred to as a scanning direction.

[0075] The apparatus may further comprise a radiation source operable to produce the radiation beam.

[0076] According to a third aspect of the present disclosure there is provided a lithographic apparatus comprising the apparatus according to the second aspect of the present disclosure.

[0077] According to a fourth aspect of the present disclosure there is provided a projection patterning device for use in the apparatus according to the second aspect of the present disclosure, the projection patterning device comprising: a first portion a second portion, each of the first and second portions comprising a plurality of lines, wherein the lines of the first portion are disposed at non-zero angle to the lines of the second portion and wherein a region containing the first portion and at least partially spatially overlaps with a region containing the second portion and wherein the plurality of lines of the first portion are interposed with the plurality of lines of the second portion in a first direction.

[0078] According to a fifth aspect of the present disclosure there is provided splitting optics for use in the apparatus according to the second aspect of the present disclosure, the splitting opticscomprising: a first portion a second portion, each of the first and second portions comprising a plurality of splitting elements, wherein the splitting elements of the first portion are disposed at a nonzero angle to the splitting elements of the second portion and wherein a region containing the first portion at least partially spatially overlaps with a region containing the second portion and wherein the splitting elements of the first portion are interposed with the splitting elements of the second portion in a first direction.

[0079] According to a sixth aspect of the present disclosure there is provided a kit of parts comprising: at least one projection patterning device according to the fourth aspect of the present disclosure; and at least one splitting optics according to the fifth aspect of the present disclosure.BRIEF DESCRIPTION OF THE DRAWINGS

[0080] 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;Figure 3 is a schematic view of a portion of a substrate, showing a beam spot region or measurement location; a first image of a pattern of a projection grating comprising two lines;Figure 4 is a schematic representation of a method of measuring at least one height of a surface of an object (for example a substrate) according to an embodiment of the present disclosure;Figure 5 is a first example of a pattern comprising a first portion and a second portion and which may be used in the method shown schematically in Figure 4;Figure 6 is a second example of a pattern comprising a first portion and a second portion and which may be used in the method shown schematically in Figure 4;Figure 7 is a third example of a pattern comprising a first portion and a second portion and which may be used in the method shown schematically in Figure 4;Figure 8 is a fourth example of a pattern comprising a first portion and a second portion and which may be used in the method shown schematically in Figure 4;Figure 9A shows an example time-dependent variation in the position of the image of a pattern in a first direction that is perpendicular to the plane of incidence of a patterned radiation beam at an object (i.e. the x-direction in Figure 2);Figure 9B shows a time dependent error in a height measurement (in the z-direction) of a substrate using a pattern of the form of a first one of the separate regions of the pattern shown in Figure 8;Figure 9C shows a time dependent error in a height measurement (in the z-direction) of a substrate using a pattern of the form of a second one of the separate regions of the pattern shown in Figure 8;Figure 9D shows a combined time dependent error in a height measurement (in the z- direction) of a substrate using a pattern of the form shown in Figure 6;Figure 9E shows a combined time dependent error in a height measurement (in the z- direction) of a substrate W using a pattern of the form shown in Figure 7;Figure 10 schematically shows a first embodiment of a plurality of sub-steps of one of the steps of the method shown schematically in Figure 4;Figure 11 schematically shows a second embodiment of a plurality of sub-steps of one of the steps of the method shown schematically in Figure 4;Figure 12 is a schematic representation of an apparatus for measuring at least one height of a surface of an object according to an embodiment of the present disclosure, which may form part of the lithographic apparatus shown in Figure 1 and which may implement the method shown in Figure 4;Figure 13 shows a first embodiment of splitting optics, which may form part of the apparatus shown in Figure 12, which is arranged to generate, from a portion of radiation beam reflected from an object that originates from first and second portions of a first detection beam and a second detection beam; andFigure 14 shows a second embodiment of splitting optics, which may form part of the apparatus shown in Figure 12, which is arranged to receive a plurality of radiation beams, each from a different one of a plurality of separate regions of an object that are illuminated by a patterned radiation beam and which is arranged to generate, for the portion of the radiation beam reflected from the object that originates from each of the separate illuminated regions: a first detection beam and a second detection beam.DETAILED DESCRIPTION

[0081] 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).

[0082] 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.

[0083] 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 firstpositioner 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.

[0084] 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 a desired spatial and angular intensity distribution in its cross section at a plane of the patterning device MA.

[0085] 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.

[0086] 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.

[0087] The lithographic apparatus LA may also be of a type having two or more substrate supports WT (also named “dual stage”). In such “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.

[0088] 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.

[0089] 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 be located 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.

[0090] 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.

[0091] 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 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.

[0092] 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 isimparted with a pattern by a projection grating PGR of the projection unit LSP. The projection grating PGR may alternatively be referred to as a projection patterning device PGR. The radiation source LSO may be, for example, a narrowband orbroadband 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.

[0093] 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 W having 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.

[0094] 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.

[0095] 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.

[0096] 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).

[0097] 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.

[0098] 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.

[0099] 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.[000100] 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 height of the substrate W can be substantially independent of the intensity of the radiation beam BE 1. In practice, the splitting of the radiation into first and second portions may be achieved in a number of different ways.[000101] 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.[000102] 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 suchthat 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.[000103] Figure 3 is a schematic view of a portion of a substrate W, showing the 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 parts (the top half and bottom half respectively of each line Li, L2in Figure 3). The first image of the projection grating PGR may be considered to comprise a first part 10 (comprising the top parts of the two lines Li, L2) and a second part 20 (comprising the bottom parts of the two lines Li, L2).[000104] The first and second parts 10, 20 of the first image correspond to first and second parts of radiation reflected by the substrate W that are split, for example by splitting optics. That is, the first and second parts as indicated in Figure 3 may be considered to be a projection of the division effected by the splitting optics (for example by the detection grating DGR) back onto the substrate W (only indicated here to illustrate the advantages of the methods of embodiments of the present disclosure). In particular, the division of the first image into first and second parts 10, 20 that is shown in Figure 3 represents the situation when the height of the substrate W is zero (relative to a reference height) as the first and second parts 10, 20 are of substantially equal size.[000105] The portion of the patterned radiation beam BE2 reflected from the substrate W may form a second image of the pattern, for example on the detection unit LSD. As the height of the object W varies, the position of the second image of the pattern will also vary and, in turn, this may result in a change in an intensity distribution across the detector DET. In particular, as the height of the object W varies, the position of the second image of the pattern will also vary in a direction that is parallel to the plane of incidence (the y-z plane in Figures 2 and 3). However, the position of the second image of the pattern in a direction that is perpendicular to the plane of incidence (the x- direction in Figures 2 and 3) is independent of the height of the object W. As the height of the object W varies, the position of the second image of the pattern may vary relative to splitting optics (for example detection grating DGR) arranged to split the reflected radiation into first and second parts 10, 20. The splitting optics may comprise one or more prisms. For example, the splitting optics may comprise a prism for each line Li, L2of the pattern. Put differently, as the height of the object W varies the projection of the splitting optics onto the plane of the object W will change and so too will the relative sizes of the first and second parts 10, 20 of the first image.[000106] As indicated by arrow 30, during the height measurement the substrate W may be moved relative to the beam spot region (measurement location MLO) in a scanning direction (the y- direction in Figure 3). Note that although in this example the scan direction is shown as being in they -direction, in other embodiments the scan direction may be in another direction in the x-y plane. In particular, in general, the scan direction is not necessarily in the plane of incidence of the radiation beam BE1. Furthermore, although in this example the scan direction is shown as being linear, in other embodiments a non-linear scan trajectory may be used. The x and y directions shown in Figure 3 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 tend to be aligned with the x and / or y directions. Note that the lines Li, L2of the projection grating PGR are arranged at a non-zero angle to both the x and y directions in Figure 3. This is to minimize the effects of scattering of the incident radiation beam BE1 from features on the substrate (other than specular reflection) on the height measurement.[000107] Each of the two lines Li, L2has (in a plane of the object W) 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 for embodiments wherein the projection grating PGR is illuminated with the beam of radiation LSB at normal incidence the thickness t of each of the lines Li, L2in the first image (formed on the substrate W) may be larger than a thickness of each of the corresponding lines on the projection grating PGR (by a factor of l / cos(ANG)).[000108] In this example, a space between the two lines in the direction parallel to the plane of incidence (the y-direction in Figures 2 and 3) is also t such that the pitch p of the first image (in the y- direction, in the plane of the object W) is 2t.[000109] Some embodiments of the present disclosure relate to a method of measuring at least one height of a surface of an object (for example a substrate W), as now discussed with reference to Figure 4.[000110] Figure 4 is a schematic representation of a method 100 of determining at least one height of a surface of an object W.[000111] The method 100 comprises a step 110 of projecting a patterned radiation beam (for example beam of radiation BE1) onto an object W so as to form an image of a pattern. In particular, the image of the pattern comprises a first portion and a second portion, each of the first and second portions comprising at least one line, wherein at least one line of the first portion is disposed at a nonzero angle to at least one line of the second portion. This is in contrast to the example described above with reference to Figure 3, which comprised two parallel lines Li, L2.[000112] Some examples of patterns 200, 210, 220, 230 that comprise such a first portion 202, 212, 222, 232 and a second portion 204, 214, 224, 234 are shown in Figures 5 to 8. In Figures 5 to 8 the first portion 202, 212, 222, 232 and second portion 204, 214, 224, 234 are distinguished by different fill patterns. In each case, the first portion 202, 212, 222, 232 comprises a first set of generally parallel lines and the second portion 204, 214, 224, 234 comprises a second set of generally parallel lines. In each case, the first set of lines (of the first portion 202, 212, 222, 232) are disposed at a non-zero angle to the second set of lines (of the second portion 204, 214, 224, 234). In each case,the patterns 200, 210, 220, 230 are shown as they are imaged onto the plane of the substrate W and, as in Figure 3, the y-direction is in the plane of incidence whereas the x-direction is out of the plane of incidence.[000113] In some embodiments, a plurality of separate regions of the object W may be illuminated by the patterned radiation beam and the first and second portions of the image of the pattern form different ones of said plurality of regions. For example, the embodiment of the pattern 200 shown in Figure 5 comprises two separate regions 200a, 200b. Similarly, the embodiment of the pattern 230 shown in Figure 8 comprises four separate regions 230a, 230b, 230c, 230d. The separate regions of the object W that are illuminated may be referred to as separate illumination spots on the object W.[000114] The method 100 further comprises a step 120 of receiving a portion of the patterned radiation beam (for example reflected radiation BE2) reflected from the substrate W.[000115] The method 100 further comprises a step 130 of determining at least one height from a combination of: (a) a part of the received portion of the patterned radiation beam reflected from the first portion 202, 212, 222, 232 of the pattern 200, 210, 220, 230; and (b) a part of the received portion of the patterned radiation beam reflected from the second portion 204, 214, 224, 234 of the pattern 200, 210, 220, 230.[000116] The method 100 shown schematically in Figure 4 and described above is advantageous as it allows for a sensitivity of the determined height(s) to positioning errors of the image of the pattern in a first direction that is perpendicular to the plane of incidence of the patterned radiation beam at the object (for example the x-direction in Figures 2-3 and 5-8) to be reduced, as now discussed.[000117] The portion of the patterned radiation beam reflected from the substrate (for example reflected radiation BE2) may form a second image of the pattern, for example on a detection unit LSD. As the height of the object W varies, the position of the second image of the pattern will also vary and, in turn, this may result in a change in an intensity distribution across the detector DET. In particular, as the height of the object W varies, the position of the second image of the pattern will also vary in a direction that is parallel to the plane of incidence (i.e. parallel to the y-direction in Figures 2 and 3, which may be referred to as a second direction). However, the position of the second image of the pattern in the first direction (that is perpendicular to the plane of incidence, i.e. the x- direction in Figures 2 and 3) is independent of the height of the object W. As the height of the object W varies, the position of the second image of the pattern may vary relative to splitting optics (for example detection grating DGR) arranged to split the reflected radiation BE2 into first and second portions (cf. the first and second parts 10, 20 of the radiation discussed above with reference to Figure 3). The splitting optics may comprise one or more prisms. For example, the splitting optics may comprise a prism for each line of the pattern 200, 210, 220, 230.[000118] In practice, such height measuring methods may be implemented by a level sensor LS, for example in a lithographic apparatus LA. Typically, such methods use a pattern that comprises one or more features, for example a plurality of parallel lines (for example lines Li, L2shown in Figure 3). The inventors of the present invention have realized that, as long as the images of the lines that are formed on the object W have a component in the first direction (that is perpendicular to the plane of incidence, i.e. the x-direction in Figures 2 and 3), any change in a height of the object W will result in a position of the lines shifting in a direction in the plane of incidence having a component perpendicular to their extent. One possible implementation to allow for height measurements to be made from a translation of the pattern in a direction perpendicular to the extent of the lines would be to use patterns that comprise a plurality of parallel lines that, in a plane of the object W, are all perpendicular to the plane of incidence (i.e. extending along the first direction of x-direction). However, as discussed above, it is desirable for the lines to be oriented such that their images on the object W are at an oblique angle to (i.e. neither parallel to nor perpendicular to) the plane of incidence (the y-z plane). This may be, for example, so that the images of these features are not parallel to lithographic features that have been formed on the object W (which may comprise a substrate or wafer).[000119] For example, as shown in the example shown in Figure 3, such patterns have previously comprised lines Li, L2such that their images on the object W are generally aligned with, but not exactly parallel to, the first direction that is perpendicular to the plane of incidence (the x-direction). For example, such features have previously been arranged such that their images on the object W to extend in a direction inclined at an angle of the order of less than 10° to the first direction (the x- direction).[000120] The linear range of such a level sensor LS is proportional to the pitch p of the image of the pattern on the object W in the second direction (i.e. the y-direction, see Figure 3). In order to increase this linear range, this pitch p may be increased. However, this can result in a relatively small number of (relatively thick) lines formed on the object W, which increases the sensitivity of such a level sensor to variations in the reflectivity of the surface of the object W and to variations in an intensity of the incident radiation PGR. Therefore, it may be desirable for the images of the lines formed on the object to be more aligned with, but not exactly parallel to, the second direction (i.e. the y-direction). This allows for a larger number of thinner lines to be used. For example, the lines may be arranged to extend, when projected onto the plane of the object W, in a direction which is closer to the second direction (the y-direction) than to the first direction (the x-direction). However, the inventors of the present invention have realized that with such arrangements the height measurements become more sensitive to positioning errors of the image of the pattern in the first direction.[000121] Since the image of the pattern 200, 210, 220, 230 comprises a first portion 202, 212, 222, 232 and a second portion 204, 214, 224, 234 wherein at least one line of the first portion 202, 212, 222, 232 is disposed at a non-zero angle to at least one line of the second portion 204, 214, 224,234, the at least one line of the first portion 202, 212, 222, 232 will be disposed at a different angle to the first direction (that is perpendicular to the plane of incidence of the patterned radiation beam at the object, i.e. the x-direction) than the at least one line of the second portion 204, 214, 224, 234.Therefore, by using a pattern comprising a first portion 202, 212, 222, 232 and a second portion 204, 214, 224, 234 (wherein at least one line of the first portion 202, 212, 222, 232 is disposed at a nonzero angle to at least one line of the second portion 204, 214, 224, 234), height measurements based on the reflected portions from the first and second portions will have different sensitivities to positioning errors of the image of the pattern in the first direction (the x-direction). Therefore, advantageously, by determining a height of the substrate W from a combination of the parts of the received portion of the patterned radiation beam BE2 reflected from the first and second portions of the image of the pattern portion 200, 210, 220, 230, this sensitivity to positioning errors of the image of the pattern 200, 210, 220, 230 in the first direction can be eliminated or at least reduced.Advantageously, this may allow for the image of the pattern 200, 210, 220, 230 to comprise portions with lines that are closely aligned with the second direction (i.e. the y-direction), for example comprising lines that extend in a direction inclined at an angle of the order of 10° or less to the second direction, which in turn increases the linear range of the method 100, without the disadvantage of an increase in sensitivity to positioning errors of the image of the pattern in the first direction (i.e. the x- direction).[000122] It will be appreciated that determining a height of the object W may comprise determining a height of the object W relative to a reference height or position.[000123] In some embodiments of the new method 100, the part of the received portion of the patterned radiation beam reflected from the first portion 202, 212, 222, 232 of the image of the pattern 200, 210, 220, 230 and the part of the received portion of the patterned radiation beam reflected from the second portion 204, 214, 224, 234 of the image of the pattern 200, 210, 220, 230 may be combined in such a way so as to at least partially correct for a sensitivity of the determined height to positioning errors of the image of the pattern in a first direction that is perpendicular to the plane of incidence of the patterned radiation beam BE1 at the object W (i.e. the x-direction). This is now discussed with reference to Figures 9A to 9E.[000124] Figure 9A shows an example time-dependent variation 300 in the position of the image of the pattern 200, 210, 220, 230 in a first direction that is perpendicular to the plane of incidence of the patterned radiation beam BE1 at the object W (i.e. the x-direction). For this illustration, the variation 300 in the x-direction is a sinusoidal oscillation although it will be appreciated that, in general, the time-dependent variation 300 in the x-direction may comprise a plurality of components and the amplitudes of these may vary with time.[000125] Figure 9B shows a time dependent error 310 in a height measurement (in the z- direction) of a substrate W using a pattern of the form of a first one of the separate regions 230a of the pattern 230 shown in Figure 8. Similarly, Figure 9C shows a time dependent error 320 in a heightmeasurement (in the z-direction) of a substrate W using a pattern of the form of a second one of the separate regions 230b of the pattern 230 shown in Figure 8. Note that the errors 310, 320 have a similar time dependence to the time-dependent variation 300 in the x-direction of the image, however, since the lines of the first region 230a of the pattern 230 are inclined in an opposite direction to the x- direction to the lines of the second region 230b of the pattern 230, the two errors 310, 320 are out of phase with each other.[000126] Figure 9D shows a combined time dependent error 330 in a height measurement (in the z-direction) of a substrate W using a pattern 210 of the form shown in Figure 6. Figure 9E shows a combined time dependent error 340 in a height measurement (in the z-direction) of a substrate W using a pattern 220 of the form shown in Figure 7. Note that the errors 310, 320, 330, 340 are all shown with axes having the same scale. It can be seen from Figures 9D and 9E that using a pattern 210, 220 that comprises two sets of lines, one set of lines disposed at a first angle to the x-direction and a second set of lines disposed at an equal but opposite angle to the x-direction, the error contributions from the two sets of lines partially mutually cancel each other out. As a result, the errors 330, 340 from such patterns 210, 220 are significantly reduced relative to the errors 310, 320 that result from using a pattern 230a, 230b having a single set of parallel lines.[000127] It will be appreciated that such a combination of the part of the received portion of the patterned radiation beam reflected from the first portion 202, 212, 222, 232 of the image of the pattern and the part of the received portion of the patterned radiation beam reflected from the second portion 204, 214, 224, 234 of the image of the pattern may be achieved in a number of different ways, as now discussed.[000128] It will be appreciated that at least partially correcting for the sensitivity of the height measurement to positioning errors of the image of the pattern the first direction (the x-direction) may mean that the determined height is less sensitive to such positioning errors than a determination using reflected radiation from only one of the first and second portions of the image of the pattern would be. [000129] In some embodiments, the part of the received portion of the patterned radiation beam reflected from the first portion 202, 212, 222, 232 of the image of the pattern and the part of the received portion of the patterned radiation beam reflected from the second portion 204, 214, 224, 234 of the image of the pattern may be combined in such a way so as to optimize or minimize a sensitivity of the determined height to positioning errors of the image of the pattern in the first direction (i.e. the x-direction).[000130] In some embodiments, the part of the received portion of the patterned radiation beam reflected from the first portion 202, 212, 222, 232 of the image of the pattern and the part of the received portion of the patterned radiation beam reflected from the second portion 204, 214, 224, 234 of the image of the pattern may be combined such that a sensitivity of the determined height to positioning errors of the image of the pattern a first direction that is perpendicular to the plane of incidence of the patterned radiation beam BE1 at the object W (e.g. the x-direction) is less than asensitivity to positioning errors of the image of the pattern in the first direction would be for a height determined using only one of: (a) the part of the received portion of the patterned radiation beam reflected from the first portion 202, 212, 222, 232 of the image of the pattern; or (b) the part of the received portion of the patterned radiation beam reflected from the second portion 204, 214, 224, 234 of the image of the pattern.[000131] As explained above (with reference to Figures 9A to 9E), in some embodiments the sensitivity of the determined height to positioning errors of the first portion 202, 212, 222, 232 of the image of the pattern in the first direction (e.g. the x-direction) may be generally equal and opposite to the sensitivity of the determined height to positioning errors of the second portion 204, 214, 224, 234 of the image of the pattern in the first direction (e.g. the x-direction). Such an arrangement may be achieved with the first portion 202, 212, 222, 232 comprising a plurality of lines inclined at a first angle to the first direction (e.g. the x-direction) and the second portion 204, 214, 224, 234 comprising a plurality of lines inclined at an equal but opposite angle to the first direction (e.g. the x-direction). With such embodiments, the contributions to the sensitivity of the determined height to positioning errors of the first and second portions of the image of the pattern in the first direction can at least partially cancel each other out, resulting in a measurement that is substantially independent of the position of the image of the pattern in the first direction (e.g. the x-direction).[000132] Therefore, in some embodiments of the method 100 (see Figures 6 to 8), the first portion 212, 222, 232 of the image of the pattern may comprise at least one line disposed at a first angle to a first direction that is perpendicular to the plane of incidence of the patterned radiation beam BE at the object W (e.g. the x-direction) and the second portion 214, 224, 234 of the image of the pattern comprises at least one line disposed at a second angle to the first direction, the second angle being equal and opposite to the first angle.[000133] Such embodiments may be used to implement the combination of: (a) a part of the received portion of the patterned radiation beam reflected from the first portion 212, 222, 232 of the pattern; and (b) a part of the received portion of the patterned radiation beam reflected from the second portion 214, 224, 234 of the pattern (to determine the at least one height) using either hardware or software, as discussed further below.[000134] In some embodiments of the method 100 (see Figure 5), the first portion 202 of the pattern 200 comprises at least one line that is perpendicular to the x-direction (that is perpendicular to the plane of incidence) of the patterned radiation beam at the object W.[000135] That is, the first portion 202 of the pattern comprises at least one line (for example a plurality of parallel lines) that is parallel to the y -direction (i.e. are parallel to the plane of incidence). Advantageously, with such an arrangement the position of the second image of the first portion is independent of the height of the object W and is only dependent on a positioning error of the pattern in the first direction (the x-direction). Therefore, the first portion 202 of the pattern 200 can be used to make a direct measurement of a positioning error of the pattern 200 in the first direction (the x-direction). In turn, this can be used to determine an error of a height determined using the second portion 204 of the pattern 200 (which may, for example, comprise lines that extend in a direction inclined at an angle of the order of less than 10° to the first direction). In this way, the first portion 202 of the image of the pattern 200 can act as a reference or calibration mark that can be used to correct for an error in the height determined using the second portion 204 of the image of the pattern 200.[000136] In such embodiments, a determined height from the first portion 202 may be independent of a height of the surface of the object W (and may only be dependent on the positioning errors of the pattern 200 in the x-direction). The determined height from the second portion of the image of the pattern may be dependent on both (a) a height of the surface of the object W and (b) a positioning error of the image of the pattern 200 in the x-direction. With such embodiments, the height determined from the first portion 202 of the image may be used to calibrate the height determined from the second portion 204 of the image (i.e. to at least partially correct for a sensitivity to the positioning error of the image of the second portion 204 of the pattern in the x-direction).[000137] Such embodiments may be used to implement the combination of: (a) a part of the received portion of the patterned radiation beam reflected from the first portion 202 of the pattern 200; and (b) a part of the received portion of the patterned radiation beam reflected from the second portion 204 of the pattern 200 (to determine the at least one height) using software.[000138] In some embodiments of the method, at least one of the first and second portions comprises at least one line disposed at an angle of less than 45° to the plane of incidence of the patterned radiation beam at the object W (i.e. an angle of less than 45° to the y -direction).[000139] Advantageously, with such arrangements a larger number of thinner lines can be used whilst still providing an arrangement with a significant linear range. In some embodiments, at least one of the first and second portions comprises at least one line disposed at an angle of less than 30° to the plane of incidence of the patterned radiation beam at the object. In some embodiments, at least one of the first and second portions comprises at least one line disposed at an angle of less than 20° to the plane of incidence of the patterned radiation beam at the object. In some embodiments, at least one of the first and second portions comprises at least one line disposed at an angle of less than 10° to the plane of incidence of the patterned radiation beam at the object.[000140] It will be appreciated that the combination of: (a) a part of the received portion of the patterned radiation beam reflected from the first portion of the image of the pattern; and (b) a part of the received portion of the patterned radiation beam reflected from the second portion of the image of the pattern (to determine the at least one height) may be implemented using either hardware or software, as now discussed.[000141] In some embodiments of the method 100, at least one of the first and second portions of the image of the pattern has a pitch in a first direction that is perpendicular to the plane of incidence of the patterned radiation beam at the object is less than a pitch in a second direction that isperpendicular to the first direction. That is, the pitch in the x-direction may be less than the pitch in the y -direction.[000142] In some embodiments of the method 100 (see Figures 6 and 7), the first portion 212, 222 and the second portion 214, 224 of the image of the pattern 210, 220 may each form part of a region of the object W that is entirely illuminated by the patterned radiation beam. That is, the first and second portions of the image of the pattern each form part of a region of the same illumination spot on the object W. With such embodiments, splitting optics defining a similar pattern to the pattern that is projected onto the object W can be used to split the radiation into two parts: a first detection beam and a second detection beam. Note that the first portion 212, 222 of the image of the pattern will, in general, contribute to both the first detection beam and the second detection beam and, similarly, the second portion 214, 224 of the image of the pattern will, in general, contribute to both the first detection beam and the second detection beam. The first and second detection beams may be directed to separate detectors and the height may be determined as being proportional to a difference in the intensities determined by the two detectors. Therefore, with such embodiments the combination of the radiation from the two portions of the pattern may be considered to be implemented using hardware.[000143] In some embodiments of the method 100 (see Figure 6), the first portion 212 and the second portion 214 are disposed in separate but adjacent regions of the image of the pattern 210. [000144] In some embodiments of the method 100 (see Figure 7), a region of the image of the pattern 220 containing the first portion 222 may at least partially spatially overlap with a region of the image of the pattern 220 containing the second portion 224. Note that for such embodiments it may be that the individual lines of the first portion 222 do not spatially overlap the individual lines of the second portion 224. For example, the line(s) of the first portion 222 may be interposed with the line(s) of the second portion 224 in the first direction (that is perpendicular to the plane of incidence of the patterned radiation beam at the object, i.e. the x-direction). That is, moving across the image of the pattern 220 in the first direction (the x-direction), the image of the pattern 200 may comprise a line of the first portion 222 then a line of the second portion 224, then a line of the first portion 222 and so on. Such embodiments may alternatively be described as the line(s) of the first portion 222 being interleaved with the line(s) of the second portion 224 in the first direction (the x-direction). Note that with such embodiments, the line(s) of the first portion 222 may abut the line(s) of the second portion 224. Alternatively, a gap may be provided between the line(s) of the first portion 222 and the line(s) of the second portion 224.[000145] As shown in Figure 7, a region of the image of the pattern 220 containing the first portion 222 (comprising three lines in this example) partially spatially overlaps with a region of the image of the pattern 220 containing the second portion 224 (comprising three lines in this example). In this example, an extent, in the x-direction, of the region of the image of the pattern 220 containing the first portion 222 partially overlaps an extent, in the x-direction, of the region of the image of thepattern 220 containing the second portion 224. In this example, an extent, in the y-direction, of the region of the image of the pattern 220 containing the first portion 222 entirely overlaps with an extent, in the y-direction, of the region of the image of the pattern 220 containing the second portion 224. Note that, in this embodiment, the individual lines of the first portion 222 do not spatially overlap the individual lines of the second portion 224. For the embodiment shown in Figure 7, the line(s) of the first portion 222 may be considered to be interposed with the line(s) of the second portion 224 in the first direction (the x-direction).[000146] In some embodiments of the method 100, the step 130 of combining: (a) the part of the received portion of the patterned radiation beam reflected from the first portion of the image of the pattern; and (b) the part of the received portion of the patterned radiation beam reflected from the second portion of the image of the pattern may comprise a plurality of steps, as shown schematically in Figure 10. In particular, this combination step 130 may comprise: a step 140 of generating from the radiation received from the first and second portions a first detection beam and a second detection beam. The combination step 130 may further comprise: a step 142 of determining an intensity of the first detection beam and the second detection beam. The combination step 130 may further comprise: a step 144 of determining the at least one height from a combination of the intensities of the first detection beam and the second detection beam. The embodiment of the combination step 130 shown in Figure 10 may be considered to be a hardware implementation.[000147] In some embodiments of the method 100 (see Figures 5 and 8) a plurality of separate regions of the object W may be illuminated by the patterned radiation beam and the first portion 202, 232 and the second portion 204, 234 of the image of the pattern 200, 230 may form different ones of said plurality of regions. That is, the first and second portions of the image of the pattern form different illumination spots on the object W.[000148] With such embodiments, a splitting optics (for example a prism) may be provided for each of the plurality of separate regions (also referred to as illumination spots) defining a similar pattern to the pattern that is projected onto that illumination spot. Each such splitting optics can be used to split the radiation into two parts: a first detection beam and a second detection beam. Note that for such embodiments, the first portion 202, 232 of the image of the pattern will contribute to a first set of first detection beam and second detection beam whereas the second portion 204, 234 of the image of the pattern will contribute to a second set of first detection beam and second detection beam. The first and second detection beams from each set (one set per illumination spot) may be directed to separate detectors and a height may be determined from each one (for example being proportional to a difference in the intensities determined by the two detectors).[000149] In turn, the at least one height may be determined from a combination of this plurality of determined heights (using a suitable algorithm).[000150] In some embodiments of the method 100, the step 130 of combining: (a) the part of the received portion of the patterned radiation beam reflected from the first portion of the image of thepattern; and (b) the part of the received portion of the patterned radiation beam reflected from the second portion of the image of the pattern may comprise a plurality of steps, as shown schematically in Figure 11. In particular, this combination step 130 may comprise: a step 150 of determining a height from the radiation received from each of the plurality of separate regions or illumination spots on the object W. The combination step 130 may further comprise a step 152 of determining the at least one height from a combination of these determined heights (as determined at step 150). The embodiment of the combination step 130 shown in Figure 11 may be considered to be a software implementation.[000151] Note that the determining a height from the radiation received from each of the plurality of separate regions or illumination spots on the object W (at step 150) may comprise, for each of the separate regions or illumination spots on the object W, the following steps 154, 156, 158. A first step 154 comprises generating a first detection beam and a second detection beam; a second step 156 comprises determining an intensity of the first detection beam and the second detection beam; and a third step 158 comprises determining a height from a combination of the intensities of the first detection beam and the second detection beam.[000152] In some embodiments, the method 100 may further comprising moving the object W in a second direction that is parallel to the plane of incidence of the patterned radiation beam at object W (i.e. the y-direction). The second direction may be referred to as a scanning direction.[000153] Some embodiments of the present disclosure relate to a new apparatus for determining at least one height of a surface of an object W. An example of such a new apparatus 400 is shown schematically in Figure 12. Optional features of the apparatus 400 are generally shown in dotted lines in Figure 12[000154] The apparatus 400 comprises: a projection unit 410; a detection unit 420; and a processing unit 430 operable to determine a height of the surface of the object.[000155] The projection unit 410 is operable to form an image of a pattern on an object W with a radiation beam 412. The image of the pattern 200, 210, 220, 230 comprises a first portion 202, 212, 222, 232 and a second portion 204, 214, 224, 234. Each of the first and second portions comprises at least one line. At least one line of the first portion 202, 212, 222, 232 is disposed at a non-zero angle to at least one line of the second portion 204, 214, 224, 234.[000156] The detection unit 420 is operable to receive a portion 422 of the radiation beam reflected from the object W.[000157] The processing unit 430 is operable to implement the method 100 shown in Figure 4 and as described above so as to determine a height of the surface of the object W.[000158] The apparatus 400 may be referred to as a level sensor. The apparatus 400 is arranged to implement the method 100 shown in Figure 4 (i.e. to determining at least one height from a combination of: (a) a part of the received portion of the patterned radiation beam reflected from the first portion 202, 212, 222, 232 of the image of the pattern; and (b) a part of the received portion ofthe patterned radiation beam reflected from the second portion 204, 214, 224, 234 of the image of the pattern).[000159] It will be appreciated that the object W does not form part of the apparatus 400. The projection unit 410 may be operable to form the image of the pattern with the radiation beam 412 in a beam spot region 440 in which an object W is, in use, positionable. Further, the detection unit 420 may be operable to receive a portion 422 of the radiation beam reflected from an object W when disposed in the beam spot region 440.[000160] In some embodiments, the projection unit 410 may comprise a projection patterning device 414 which defines the pattern. The projection patterning device 414 may be referred to as a projection grating. In general, the image of the pattern (that is formed on the object W) may comprise a distorted version of the pattern defined by the projection grating 414. In general, the image of the pattern (that is formed on the object W) may be dependent on (a) the pattern defined by the projection grating 414; (b) an angle of incidence of the radiation on the projection grating; and (c) an angle of incidence of the radiation 412 on the object W.[000161] Note that in some embodiments, the projection patterning device 414 may be illuminated with a radiation beam 472 that is propagating in a direction aligned with a normal to the projection patterning device 414. Such an illumination scheme may be referred to as a normal illumination scheme. It will be appreciated that with such an illumination scheme the image of the pattern will be the pattern defined by the projection grating 414 that has been elongated in a direction parallel to the plane of incidence by a factor of 1 / cos 0Swhere 0Sis the angle of incidence on the object W.[000162] In some alternative embodiments, the projection patterning device 414 may be illuminated with a radiation beam 472 that is propagating in a direction that is at a non-zero angle to the projection patterning device 414 such that the pattern defined by the projection patterning device 414 is the same as the image of the pattern formed on the object W (with no scaling factor in the direction parallel to the plane of incidence). Such an illumination scheme may be referred to as a Scheimpflung illumination scheme.[000163] In general, the projection patterning device 414 may be illuminated with a radiation beam that is propagating in any direction.[000164] In some embodiments, the projection unit 410 may further comprise first imaging optics 416 arranged to form an image of the projection patterning device 414 on the object W.[000165] In some embodiments, the projection patterning device 414 may define at least one continuous patterned region and the first and second portions of the image of the pattern may each form part of at least one such continuous patterned region. The or each continuous patterned region may correspond to an illumination spot that is formed on the object W.[000166] In some embodiments (see Figure 6), the first portion 212 and the second portion 214 may be disposed in separate but adjacent regions of the at least one patterned region.[000167] In some embodiments (see Figure 7), a region of the image of the pattern 220 containing the first portion 222 may at least partially spatially overlap with a region of the image of the pattern 220 containing the second portion 224. For example, the line(s) of the first portion 222 may be interposed with the line(s) of the second portion 224 in the first direction (the x-direction). [000168] In some embodiments (see Figures 5 and 8), the projection patterning device 414 may define a plurality of discrete patterned region and the first portion 202, 232 of the image of the pattern may be defined by a first set of the plurality of patterned regions and the second portion 204, 234 of the image of the pattern may be defined by a second, different set of the plurality of patterned regions. Each continuous patterned region may correspond to an illumination spot that is formed on the object W. Each set of the plurality of patterned regions may comprise at least one of the plurality of patterned regions.[000169] In some embodiments (see Figures 6 to 8), the first portion 212, 222, 232 of the image of the pattern 200 comprises at least one line disposed at a first angle to a first direction that is perpendicular to the plane of incidence of the patterned radiation beam 412 at the object W and wherein the second portion 214, 224, 234 of the image of the pattern comprises at least one line disposed at a second angle to the first direction, the second angle being equal and opposite to the first angle. In some embodiments, the first portion 212, 222, 232 of the image of the pattern comprises a plurality of parallel lines, each disposed at a first angle to the first direction and the second portion 214, 224, 234 of the image of the pattern comprises a plurality of parallel lines disposed at a second angle to the first direction, the second angle being equal and opposite to the first angle.[000170] In some embodiments (see Figure 5), the first portion 202 of the image of the pattern comprises at least one line that is perpendicular to a first direction that is perpendicular to the plane of incidence of the patterned radiation beam 412 at the object W (i.e. parallel to the plane of incidence). In some embodiments, the first portion 202 of the image of the pattern comprises a plurality of parallel lines that are parallel to the plane of incidence of the patterned radiation 412.[000171] In some embodiments, at least one of the first and second portions of the image of the pattern may comprise at least one line disposed at an angle of less than 45° to the plane of incidence of the patterned radiation beam 412 at the object W. In some embodiments, at least one of the first and second portions of the image of the pattern comprises at least one line disposed at an angle of less than 30° to the plane of incidence of the patterned radiation beam 412 at the object W. In some embodiments, at least one of the first and second portions of the image of the pattern comprises at least one line disposed at an angle of less than 20° to the plane of incidence of the patterned radiation beam 412 at the object W. In some embodiments, at least one of the first and second portions of the image of the pattern comprises at least one line disposed at an angle of less than 10° to the plane of incidence of the patterned radiation beam 412 at the object W.[000172] In some embodiments, at least one of the first and second portions of the image of the pattern has a pitch in a first direction that is perpendicular to the plane of incidence of the patternedradiation beam 412 at the object W that is less than a pitch in a second direction that is perpendicular to the first direction. That is, at least one of the first and second portions of the image of the pattern has a pitch in the x-direction that is less than a pitch in the y-direction.[000173] In some embodiments, the detection unit 420 may comprise splitting optics 424 arranged to receive the beam of radiation 422 reflected from a surface of the object W and to split it into a plurality of detection beams. The splitting optics 424 may comprise a plurality of splitting elements, each corresponding to a different one of a plurality of lines of the pattern.[000174] The splitting optics 424 may be referred to as a detection grating (or a plurality of detection gratings). Each splitting element may be operable to split the radiation received from a corresponding line of the pattern into a plurality of portions (e.g. two portions). Each splitting element may have a substantially similar shape to a corresponding line of the pattern. In this way, the splitting optics 424 may be said to be matched to a projection patterning device 414 of the apparatus 400.[000175] In some embodiments, the detection unit 420 may further comprise second imaging optics 426 arranged to receive radiation 422 reflected from the object W and to form a second image of the pattern on the splitting optics 424.[000176] In some embodiments, the apparatus 400 may further comprise a support 450 for supporting a substrate W.[000177] In some embodiments, the apparatus 400 may further comprise a movement mechanism 460 operable to cause relative movement of the support relative 450 to the projection unit 410 and / or the detection unit 420. In particular, the movement mechanism 460 may be operable to cause relative movement of the support 450 relative to a beam spot region 440 in a direction that is parallel to the plane of incidence of the patterned radiation beam 412 at a beam spot region 440 of the apparatus 400 (i.e. the y-direction). This direction may be referred to as a scanning direction.[000178] In some embodiments, the apparatus 400 may further comprise a radiation source 470 operable to produce a radiation beam 472 (that may be patterned by the projection patterning device 414).[000179] As shown in Figure 13, in some embodiments the splitting optics 424 may be arranged to generate, from the portion 422 of the radiation beam reflected from the object W that originates from the first and second portions of the pattern 210, 220 a first detection beam 480 and a second detection beam 482. For such embodiments, the apparatus 400 may further comprise: a first detector 484 arranged to determine an intensity of the first detection beam 480; and a second detector 486 arranged to determine an intensity of the second detection beam 482. For such embodiments, the first detector 484 may be operable to send a first signal Si (which may be indicative of the intensity of the first detection beam 480) to the processing unit 430. Similarly, for such embodiments, the second detector 486 may be operable to send a second signal S2 (which may be indicative of the intensity of the second detection beam 482) to the processing unit 430.[000180] As shown in Figure 14, in some embodiments the splitting optics 424 receive a plurality of radiation beams 422a-422n, each from a different one of a plurality of separate regions of the object that are illuminated by the patterned radiation beam 412. The splitting optics 424 may be arranged to generate, for the portion of the radiation beam 422a-422n reflected from the object that originates from each of the separate illuminated regions: a first detection beam 480a-480n and a second detection beam 482a-482n. Such embodiments of the apparatus 400 may further comprise, for each of the separate illuminated regions, a first detector 484a, 484n and a second detector 486a, 486n. The first detector 484a, 484n is arranged to determine an intensity of the first detection beam 480a, 480n; and the second detector 486a-486n is arranged to determine an intensity of the second detection beam 482a-482n. For such embodiments, each first detector 484a-484n may be operable to send a first signal sai, sni(which may be indicative of the intensity of the first detection beam 480a-480n) to the processing unit 430. Similarly, for such embodiments, the second detector 486a-486n may be operable to send a second signal sa2, s„2 (which may be indicative of the intensity of the second detection beam 482a-482n) to the processing unit 430.[000181] Some embodiments of the present disclosure relate to a lithographic apparatus comprising the apparatus 400 described above with reference to Figures 12 to 14.[000182] Some embodiments of the present disclosure relate to a new projection patterning device for use in a level sensor. In particular, the new projection patterning device may be for use in the method 100 described above with reference to Figure 4 and / or an apparatus 400 described above with reference to Figures 12 to 14.[000183] The new projection patterning device may be of any suitable shape to form an image on a substrate W of the form of any of the patterns 200, 210, 220, 230 discussed above. As discussed above, such a projection patterning device may itself define a distorted version of the patterns 200, 210, 220, 230 discussed above if, for example, it is illuminated at normal incidence.[000184] In general, the new projection patterning device may comprise a first portion a second portion, each of the first and second portions comprising a plurality of lines. The lines of the first portion may be disposed at non-zero angle to the lines of the second portion. In one embodiment, a region containing the first portion may at least partially spatially overlap a region containing the second portion. For example, the plurality of lines of the first portion may be being interposed with the plurality of lines of the second portion in the first direction (the x-direction).[000185] Some embodiments of the present disclosure relate to a new splitting optics for use in a level sensor. In particular, the new splitting optics may be for use in the method 100 described above with reference to Figure 4 and / or an apparatus 400 described above with reference to Figures 12 to 14. [000186] In general the splitting optics comprises: a first portion a second portion, each of the first and second portions comprising a plurality of splitting elements. The splitting elements of the first portion may be disposed at a non-zero angle to the splitting elements of the second portion. A region containing the first portion may at least partially spatially overlap with a region containing thesecond portion. The splitting elements of the first portion may be interposed with the splitting elements of the second portion in a first direction.[000187] Some embodiments of the present disclosure relate to a kit of parts for use in a level sensor. In particular, the new kit of parts may be for use in the method 100 described above with reference to Figure 4 and / or an apparatus 400 described above with reference to Figures 12 to 14. [000188] 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.[000189] 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.[000190] 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.[000191] 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 that such 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.[000192] 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 thatmodifications 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 in the following numbered clauses:1. A method for determining at least one height of a surface of an object, the method comprising: projecting a patterned radiation beam onto an object so as to form an image of a pattern, the image of the pattern comprising a first portion and a second portion, each of the first and second portions comprising at least one line, wherein at least one line of the first portion is disposed at a non-zero angle to at least one line of the second portion; receiving a portion of the patterned radiation beam reflected from the substrate; and determining at least one height from a combination of: (a) a part of the received portion of the patterned radiation beam reflected from the first portion of the pattern; and (b) a part of the received portion of the patterned radiation beam reflected from the second portion of the pattern.2. The method of clause 1 wherein the part of the received portion of the patterned radiation beam reflected from the first portion of the image of the pattern and the part of the received portion of the patterned radiation beam reflected from the second portion of the image of the pattern are combined in such a way so as to at least partially correct for a sensitivity of the determined height to positioning errors of the image of the pattern in a first direction that is perpendicular to the plane of incidence of the patterned radiation beam at the object.3. The method of clause 1 or clause 2 wherein the part of the received portion of the patterned radiation beam reflected from the first portion of the image of the pattern and the part of the received portion of the patterned radiation beam reflected from the second portion of the image of the pattern are combined such that a sensitivity of the determined height to positioning errors of the image of the pattern a first direction that is perpendicular to the plane of incidence of the patterned radiation beam at the object is less than a sensitivity to positioning errors of the image of the pattern in the first direction would be for a height determined using only one of: (a) the part of the received portion of the patterned radiation beam reflected from the first portion of the image of the pattern; or (b) the part of the received portion of the patterned radiation beam reflected from the second portion of the image of the pattern.4. The method of any preceding clause wherein the first portion of the image of the pattern comprises at least one line disposed at a first angle to a first direction that is perpendicular to the plane of incidence of the patterned radiation beam at the object and wherein the second portion of the image of the pattern comprises at least one line disposed at a second angle to the first direction, the second angle being equal and opposite to the first angle.5. The method of any preceding clause wherein the first portion of the pattern comprises at least one line that is perpendicular to a first direction that is perpendicular to the plane of incidence of the patterned radiation beam at the object.6. The method of any preceding clause wherein at least one of the first and second portions comprises at least one line disposed at an angle of less than 45° to the plane of incidence of the patterned radiation beam at the object.7. The method of any preceding clause wherein at least one of the first and second portions of the image of the pattern has a pitch in a first direction that is perpendicular to the plane of incidence of the patterned radiation beam at the object that is less than a pitch in a second direction that is perpendicular to the first direction.8. The method of any preceding clause wherein the first and second portions of the image of the pattern each form part of a region of the object that is entirely illuminated by the patterned radiation beam.9. The method of clause 8 wherein the first portion and the second portion are disposed in separate but adjacent regions of the image of the pattern.10. The method of clause 8 wherein a region of the image of the pattern containing the first portion at least partially spatially overlaps with a region of the image of the pattern containing the second portion and wherein the line(s) of the first portion are interposed with the line(s) of the second portion in a first direction that is perpendicular to the plane of incidence of the patterned radiation beam at the object.11. The method of any preceding clause wherein combining: (a) the part of the received portion of the patterned radiation beam reflected from the first portion of the image of the pattern; and (b) the part of the received portion of the patterned radiation beam reflected from the second portion of the image of the pattern comprises: generating from the first and second portions a first detection beam and a second detection beam; determining an intensity of the first detection beam and the second detection beam; and determining the at least one height from a combination of the intensities of the first detection beam and the second detection beam.12. The method of any preceding clause wherein a plurality of separate regions of the object are illuminated by the patterned radiation beam and wherein the first and second portions of the image of the pattern form different ones of said plurality of regions.13. The method of any preceding clause wherein combining: (a) the part of the received portion of the patterned radiation beam reflected from the first portion of the image of the pattern; and (b) the part of the received portion of the patterned radiation beam reflected from the second portion of the image of the pattern comprises: from each of a plurality of separate regions of the object that are illuminated by the patterned radiation beam: generating a first detection beam and a second detection beam; determining an intensity of the first detection beam and the second detection beam; and determining a height from a combination of the intensities of the first detection beam and the second detection beam; and determining the at least one height from a combination of these determined heights.14. The method of any preceding clause further comprising moving the object in a second direction that is parallel to the plane of incidence of the patterned radiation beam at object.15. An apparatus for determining at least one height of a surface of an object, the apparatus comprising: a projection unit operable to form an image of a pattern on the object with a radiation beam, wherein the image of the pattern comprises a first portion and a second portion, each of the first and second portions comprising at least one line, wherein at least one line of the first portion is disposed at a non-zero angle to at least one line of the second portion; a detection unit operable to receive a portion of the radiation beam reflected from the object; and a processing unit operable to implement the method of any one of clauses 1 to 14 so as to determine a height of the surface of the object.16. The apparatus of clause 15 wherein the projection unit comprises a projection patterning device which defines the pattern.17. The apparatus of clause 16 wherein the projection unit further comprises first imaging optics arranged to form an image of the projection patterning device on the object.18. The apparatus of clause 15 or clause 16 wherein the projection patterning device defines at least one continuous patterned region and wherein the first and second portions of the image of the pattern each form part of at least one such continuous patterned region.19. The apparatus of clause 18 wherein the first portion and the second portion are disposed in separate but adjacent regions of the at least one patterned region.20. The apparatus of clause 18 wherein a region of the image of the pattern containing the first portion at least partially spatially overlaps with and a region of the image of the pattern containing the second portion.21. The apparatus of clause 18 or clause 19 wherein with the line(s) of the first portion are interposed with the line(s) of the second portion in a first direction that is perpendicular to the plane of incidence of the patterned radiation beam at the object.22. The apparatus of clause 15 or clause 16 wherein the projection patterning device defines a plurality of discrete patterned regions and wherein the first portion of the image of the pattern is defined by a first set of the plurality of patterned regions and wherein the second portion of the image of the pattern is defined by a second, different set of the plurality of patterned regions.23. The apparatus of any one of clauses 15 to 22 wherein the first portion of the image of the pattern comprises at least one line disposed at a first angle to a first direction that is perpendicular to the plane of incidence of the patterned radiation beam at the object and wherein the second portion of the image of the pattern comprises at least one line disposed at a second angle to the first direction, the second angle being equal and opposite to the first angle.24. The apparatus of any one of clauses 15 to 23 wherein the first portion of the image of the pattern comprises at least one line that is perpendicular to a first direction that is perpendicular to the plane of incidence of the patterned radiation beam at the object.25. The apparatus of any one of clauses 15 to 24 wherein at least one of the first and second portions of the image of the pattern comprises at least one line disposed at an angle of less than 45° to the plane of incidence of the patterned radiation beam at the object.26. The apparatus of any one of clauses 15 to 25 wherein at least one of the first and second portions of the image of the pattern has a pitch in a first direction that is perpendicular to the plane of incidence of the patterned radiation beam at the object that is less than a pitch in a second direction that is perpendicular to the first direction.27. The apparatus of any one of clauses 15 to 26 wherein the detection unit comprises splitting optics arranged to receive the beam of radiation reflected from a surface of the object and to split it into a plurality of detection beams, wherein the splitting optics comprises a plurality of splitting elements, each corresponding to a different one of a plurality of lines of the pattern.28. The apparatus of clause 27 wherein the detection unit further comprises second imaging optics arranged to receive radiation reflected from the object and to form a second image of the pattern on the splitting optics.29. The apparatus of clause 27 or clause 28 wherein the splitting optics is arranged to generate, from the portion of the radiation beam reflected from the object that originates from the first and second portions, a first detection beam and a second detection beam and wherein the apparatus further comprises: a first detector arranged to determine an intensity of the first detection beam; and a second detector arranged to determine an intensity of the second detection beam.30. The apparatus of clause 27 or clause 28 wherein the splitting optics is arranged to generate, for the portion of the radiation beam reflected from the object that originates from each of a plurality of separate regions of the object that are illuminated by the patterned radiation beam: a first detection beam and a second detection beam; and wherein, for each of the plurality of separate regions of the object that are illuminated by the patterned radiation beam, the apparatus further comprises: a first detector arranged to determine an intensity of the first detection beam; and a second detector arranged to determine an intensity of the second detection beam.31. The apparatus of any one of clauses 15 to 30 further comprising a support for supporting a substrate.32. The apparatus of clause 31 further comprising a movement mechanism operable to cause relative movement of the support relative to the projection unit and / or the detection unit.33. The apparatus of any one of clauses 15 to 32 further comprising a radiation source operable to produce the radiation beam.34. A lithographic apparatus comprising the apparatus of any one of clauses 15 to 33.35. A projection patterning device for use in the apparatus of any one of clauses 15 to 33, the projection patterning device comprising: a first portion a second portion, each of the first and second portions comprising a plurality of lines, wherein the lines of the first portion are disposed at non-zero angle to the lines of the second portion and wherein a region containing the first portion and at least partially spatially overlaps with a region containing the second portion and wherein the plurality of lines of the first portion are interposed with the plurality of lines of the second portion in a first direction.36. Splitting optics for use in the apparatus of any one of clauses 15 to 33, the splitting optics comprising: a first portion a second portion, each of the first and second portions comprising a plurality of splitting elements, wherein the splitting elements of the first portion are disposed at a non-zero angle to the splitting elements of the second portion and wherein a region containing the first portion at least partially spatially overlaps with a region containing the second portion and wherein the splitting elements of the first portion are interposed with the splitting elements of the second portion in a first direction.37. A kit of parts comprising: at least one projection patterning device according to clause 35; and at least one splitting optics according to clause 36.

Claims

CLAIMS1. A method for determining at least one height of a surface of an object, the method comprising: projecting a patterned radiation beam onto an object so as to form an image of a pattern, the image of the pattern comprising a first portion and a second portion, each of the first and second portions comprising at least one line, wherein at least one line of the first portion is disposed at a non-zero angle to at least one line of the second portion; receiving a portion of the patterned radiation beam reflected from the substrate; and determining at least one height from a combination of: (a) a part of the received portion of the patterned radiation beam reflected from the first portion of the pattern; and (b) a part of the received portion of the patterned radiation beam reflected from the second portion of the pattern.

2. The method of claim 1 wherein the part of the received portion of the patterned radiation beam reflected from the first portion of the image of the pattern and the part of the received portion of the patterned radiation beam reflected from the second portion of the image of the pattern are combined in such a way so as to at least partially correct for a sensitivity of the determined height to positioning errors of the image of the pattern in a first direction that is perpendicular to the plane of incidence of the patterned radiation beam at the object.

3. The method of claim 1 or claim 2 wherein the part of the received portion of the patterned radiation beam reflected from the first portion of the image of the pattern and the part of the received portion of the patterned radiation beam reflected from the second portion of the image of the pattern are combined such that a sensitivity of the determined height to positioning errors of the image of the pattern a first direction that is perpendicular to the plane of incidence of the patterned radiation beam at the object is less than a sensitivity to positioning errors of the image of the pattern in the first direction would be for a height determined using only one of: (a) the part of the received portion of the patterned radiation beam reflected from the first portion of the image of the pattern; or (b) the part of the received portion of the patterned radiation beam reflected from the second portion of the image of the pattern.

4. The method of any preceding claim wherein the first portion of the image of the pattern comprises at least one line disposed at a first angle to a first direction that is perpendicular to the plane of incidence of the patterned radiation beam at the object and wherein the second portion of the image of the pattern comprises at least one line disposed at a second angle to the first direction, the second angle being equal and opposite to the first angle.

5. The method of any preceding claim wherein the first and second portions of the image of the pattern each form part of a region of the object that is entirely illuminated by the patterned radiation beam.

6. The method of claim 5 wherein the first portion and the second portion are disposed in separate but adjacent regions of the image of the pattern, or wherein a region of the image of the pattern containing the first portion at least partially spatially overlaps with a region of the image of the pattern containing the second portion and wherein the line(s) of the first portion are interposed with the line(s) of the second portion in a first direction that is perpendicular to the plane of incidence of the patterned radiation beam at the object.

7. The method of any preceding claim wherein combining: (a) the part of the received portion of the patterned radiation beam reflected from the first portion of the image of the pattern; and (b) the part of the received portion of the patterned radiation beam reflected from the second portion of the image of the pattern comprises: generating from the first and second portions a first detection beam and a second detection beam; determining an intensity of the first detection beam and the second detection beam; and determining the at least one height from a combination of the intensities of the first detection beam and the second detection beam.

8. The method of any preceding claim wherein combining: (a) the part of the received portion of the patterned radiation beam reflected from the first portion of the image of the pattern; and (b) the part of the received portion of the patterned radiation beam reflected from the second portion of the image of the pattern comprises: from each of a plurality of separate regions of the object that are illuminated by the patterned radiation beam: generating a first detection beam and a second detection beam; determining an intensity of the first detection beam and the second detection beam; and determining a height from a combination of the intensities of the first detection beam and the second detection beam; and determining the at least one height from a combination of these determined heights.

9. An apparatus for determining at least one height of a surface of an object, the apparatus comprising: a projection unit operable to form an image of a pattern on the object with a radiation beam, wherein the image of the pattern comprises a first portion and a second portion, each of the first andsecond portions comprising at least one line, wherein at least one line of the first portion is disposed at a non-zero angle to at least one line of the second portion; a detection unit operable to receive a portion of the radiation beam reflected from the object; and a processing unit operable to implement the method of any one of claims 1 to 8 so as to determine a height of the surface of the object.

10. The apparatus of claim 9 wherein the projection unit comprises; a projection patterning device which defines the pattern, and first imaging optics arranged to form an image of the projection patterning device on the object, wherein the projection patterning device defines at least one continuous patterned region and wherein the first and second portions of the image of the pattern each form part of at least one such continuous patterned region.

11. The apparatus of claim 10 wherein the first portion and the second portion are disposed in separate but adjacent regions of the at least one patterned region, or wherein a region of the image of the pattern containing the first portion at least partially spatially overlaps with and a region of the image of the pattern containing the second portion.

12. The apparatus of claim 10 or claim 11 wherein with the line(s) of the first portion are interposed with the line(s) of the second portion in a first direction that is perpendicular to the plane of incidence of the patterned radiation beam at the object.

13. The apparatus of any one of claims 9 to 12 wherein the first portion of the image of the pattern comprises at least one line disposed at a first angle to a first direction that is perpendicular to the plane of incidence of the patterned radiation beam at the object and wherein the second portion of the image of the pattern comprises at least one line disposed at a second angle to the first direction, the second angle being equal and opposite to the first angle.

14. The apparatus of any one of claims 9 to 12 wherein the detection unit comprises; splitting optics arranged to receive the beam of radiation reflected from a surface of the object and to split it into a plurality of detection beams, wherein the splitting optics comprises a plurality of splitting elements, each corresponding to a different one of a plurality of lines of the pattern, and second imaging optics arranged to receive radiation reflected from the object and to form a second image of the pattern on the splitting optics, wherein the splitting optics is arranged to generate, from the portion of the radiation beam reflected from the object that originates from the first and second portions, a first detection beam and a second detection beam and wherein the apparatus further comprises:a first detector arranged to determine an intensity of the first detection beam; and a second detector arranged to determine an intensity of the second detection beam.

15. A lithographic apparatus comprising the apparatus of any one of claims 9 to 14.

Images