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

Abstract

An apparatus for measuring a topography of a surface of an object comprises: an illumination system; an interference module; and a detection system. The illumination system is configured to emit primary radiation and project the primary radiation on the object so as to generate secondary radiation from a surface of the object via an even order non-linear optical process, the secondary radiation having a different wavelength to the primary radiation. The secondary radiation comprises a reflected portion and a transmitted portion. The interference module is operable to produce tertiary radiation having substantially the same wavelength as the secondary radiation, the tertiary radiation being incident on the object such that the tertiary radiation at least partially destructively interferes with the transmitted portion of the secondary radiation. The detection system is operable to receive the reflected portion of the secondary radiation and to determine a height of the substrate therefrom.

Description

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

[0001] The application claims priority of EP application 23208121.6 which was filed on 07 November, 2023 and EP application 24195897.4 which was filed on 22 August, 2024; and which are incorporated herein in their entirety by reference.FIELD

[0002] The present invention relates to an apparatus for measuring a topography of a surface of an object. The present invention also relates to a corresponding method for measuring a topography of a surface of an object. The present invention has particular application in the field of lithography. The object may be a substrate suitable for use within a lithographic apparatus. Such a substrate may comprise a silicon wafer coated with a photoresist. The apparatus may be referred to as a level sensor and may form part of a lithographic apparatus. The present invention also relates to an exposure apparatus (for example a lithographic apparatus) which comprises the apparatus for measuring a topography of a surface of a substrate. The present invention also relates to a metrology apparatus which comprises the apparatus for measuring a topography of a surface of a substrate. The present invention also relates to an exposure method (for example a lithographic exposure method) which uses the method or apparatus for measuring a topography of a surface of a substrate.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, maybe used to form smaller features on a substrate than a lithographic apparatus which uses, for example, radiation with a wavelength of 193 nm.

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

[0006] It may be desirable to provide new methods and / or apparatus for determining a topography 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 an apparatus for measuring a topography of a surface of an object, the apparatus comprising: a support for supporting an object such that it is positionable in a beam spot region; an illumination system configured to emit primary radiation and project the primary radiation on the beam spot region so as to generate secondary radiation from a surface of an object when disposed in the beam spot region via an even order nonlinear optical process, the secondary radiation having a different wavelength to the primary radiation and wherein the secondary radiation comprises a reflected portion that is reflected from the object and a transmitted portion that propagates into the object; an interference module operable to produce tertiary radiation having substantially the same wavelength as the secondary radiation and which is incident on the beam spot region such that the tertiary radiation at least partially destructively interferes with the transmitted portion of the secondary radiation; at least one detector; detection optics operable to receive the reflected portion of the secondary radiation reflected from the object and to direct it to the at least one detector; and a processor operable to determine a height of the object from reflected portion of the secondary radiation.

[0008] The apparatus may be referred to as a level sensor. The apparatus may form part of a lithographic apparatus. The support for supporting a substrate may comprise a substrate holder operable to secure the substrate. For example, the support may comprise a clamp for clamping the substrate to the support. The apparatus according to the first aspect is advantageous as it allows for a more accurate measurement of height of an object that comprises multiple layers relative to existing level sensors, as now discussed.

[0009] The object may be a substrate. The substrate may be a substrate within a lithographic apparatus. Such a substrate may comprise a silicon wafer coated with a photoresist. The lithographic apparatus may be used to expose the substrate to radiation that has been patterned by a reticle or mask. Before the substrate is exposed to the patterned radiation, a topology of the surface of the substrate maybe determined. For example a height map of the surface of the substrate may be determined, for example using a level sensor. Subsequently, the measured topography of a surface of a substrate can be used to control a height of the substrate while it is being exposed to the patterned radiation, for example to keep the substrate in a plane of best focus for the image of the patterning device.

[0010] One type of level sensor can be used to measure the height profile of the surface of the substrate by: projecting a patterned radiation beam onto a beam spot region; moving the substrate relative to the beam spot region; receiving a portion of the patterned radiation beam reflected from the object and determining the first measurement of the height therefrom. As the height of the substrate varies, the position of the pattern of the reflected radiation may vary, for example relative to splitting optics arranged to split the reflected radiation into first and second portions. When radiation is incident upon the surface of a silicon wafer, which typically comprises a multilayered stack, a portion of the radiation penetrates into the layers beneath the resist layer. This penetration causes multiple back reflections, resulting in an imbalance in the signal readout for such a level sensor. This effect, the difference between a measured height and a real height due to interference effects of underlying layers, may be referred to as “Apparent Surface Depression” (ASD) or “Height Process Dependency” (HPD).

[0011] The apparatus according to the first aspect is advantageous, as now discussed.

[0012] The inventors have realized that centro-symmetric media such as the top resist layer and the wafer as a whole do not generate radiation via even order (for example second order) nonlinear processes such as second harmonic generation (SHG). In contrast, the interface between an object when disposed in the beam spot region and a surrounding environment has a broken centro-symmetry and therefore satisfies the conditions for even order nonlinear processes such as SHG. Therefore, the reflected portion of the secondary radiation is a signal that originates from the surface of the object.

[0013] Furthermore, the tertiary radiation produced by the interference module at least partially destructively interferes with the transmitted portion of the secondary radiation. Advantageously, this suppresses back reflections of the transmitted portion and, in turn, reducing ASD or HPD errors.

[0014] The processor may be operable to implement any of the features of the method according to the fourth aspect of the present disclosure.

[0015] The processor may alternatively be referred to as a height determination unit or height determination device.

[0016] 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.

[0017] The at least one detector may form part of a detection system.

[0018] The illumination system may comprise a radiation source operable to produce primary radiation comprising a first wavelength.

[0019] With such an arrangement, the even order non-linear optical process may be second harmonic generation (SHG). In such embodiments, two photons of primary radiation (having a frequency of coi) may be combined to produce a single secondary photon having twice the frequency.

[0020] Additionally or alternatively, in other embodiments, the illumination system may comprise a second radiation source operable to produce primary radiation having a second wavelength. With such an arrangement, the even order non-linear optical process may be sum-frequency generation (SFG). In such embodiments, a photon of primary radiation having a first frequency (coi) may be combined with a photon of primary radiation having a second frequency (002) to produce a single secondary photon having a frequency of CO1+CO2. Note that SHG is a special case of SFG in which coi=co2.

[0021] The radiation source may be a pulsed radiation source and may be operable to produce pulsed primary radiation.

[0022] Advantageously, this may allow for high peak power to be achieved, which may increase the strength of non-linear processes, while maintaining a lower average power to reduce the risk of damage to the object.

[0023] The pulses may be short pulses, for example having a duration of the order of nanoseconds or less. Such short pulses are particularly well suited for generating non-linear optical processes.

[0024] In general, there is a correlation between the sensitivity of the at least one detector and the peak power of the primary radiation. It will be appreciated that the peak power of the primary radiation may be chosen so as to generate secondary radiation with a sufficiently high power to be measured by the at least one detector.

[0025] The even order non-linear optical process may be a second order non-linear optical process. Such processes may be referred to as three-wave mixing processes and / or %(2)processes.

[0026] The even order non-linear optical process may be second harmonic generation.

[0027] The interference module is operable to produce tertiary radiation having substantially the same wavelength as the secondary radiation. In some embodiments, the tertiary radiation may be generated from the primary radiation using a similar non-linear optical process to that used to generate the secondary radiation.

[0028] The interference module may comprise a non-linear crystal disposed in a path of the primary radiation.

[0029] For example, the interference module may comprise a relatively thin non-linear crystal. An example of such a non-linear crystal is a barium borate (BBO) crystal. As the primary radiation passes through the non-linear crystal, the tertiary radiation is generated. This may be via an even order nonlinear optical process such as, for example, second harmonic generation (SHG).

[0030] An optical path length between the non-linear crystal and the object may be adjustable.

[0031] It may be that an optical path length between the non-linear crystal and the object can be adjusted so that the tertiary radiation and the transmitted portion of the secondary radiation are substantially out of phase.

[0032] The primary and tertiary radiation may be substantially in phase at the non-linear crystal. However, given that the primary and tertiary radiation have different wavelengths, as they propagate away from the non-linear crystal the relative phase of the primary and tertiary radiation will oscillate asa function of optical path length. The primary and secondary radiation may be substantially in phase at the interface between the object when disposed in the beam spot region and the surrounding environment.

[0033] The optical path length between the non-linear crystal and the object may be adjusted so that the tertiary radiation and the transmitted portion of the secondary radiation are out of phase by 7t radians (180 degrees). For embodiments wherein the secondary and tertiary radiation are both formed from the primary radiation via SHG this may be achieved by selecting an optical path length between the non-linear crystal and the object of A / 2 (2n + 1), where A is the wavelength of the primary radiation and n is an integer.

[0034] It will be appreciated that as the height of the object varies, so too will the optical path length between the non-linear crystal and the object unless the non-linear crystal is also moved to compensate for such height variation.

[0035] In some embodiments, first a coarse measurement of the topology of the object (for example a wafer) may be made. This may then be used to control the height of the non-linear crystal during the main measurement process so as to keep the tertiary radiation out of phase with the transmitted portion of the secondary radiation.

[0036] The non-linear crystal may be rotatable about an axis.

[0037] Advantageously, this can allow for the amplitude of the tertiary radiation to be tuned so as to generally match that of the transmitted portion of the secondary radiation. As the non-linear crystal rotates about the axis (which may, for example, be normal to a surface of the crystal), an intensity or amplitude of the tertiary radiation may vary between zero and a maximum value.

[0038] A thickness of the non-linear crystal may be chosen so that a maximum intensity or amplitude of the tertiary radiation is of the order of twice an expected amplitude of the transmitted portion of the secondary radiation.

[0039] It may be that an orientation of the non-linear crystal can be adjusted so that an amplitude of the tertiary radiation is generally equal to an amplitude of the transmitted portion of the secondary radiation.

[0040] The interference module may be configured such that the tertiary radiation is p-polarized and such that the tertiary radiation is incident on the object disposed in the beam spot region at an angle of incidence substantially equal to Brewster’ s angle.

[0041] Advantageously, this minimizes the amount of tertiary radiation that is reflected from the object and which could interfere destructively with the reflected portion of the secondary radiation.

[0042] The illumination system may be configured such that the primary radiation is s-polarized.

[0043] With such an arrangement, the secondary radiation will be p-polarized. Furthermore, for embodiments wherein the tertiary radiation is generated from the primary radiation using a non-linear optical process, the tertiary radiation will also be p-polarized.

[0044] The detection optics may comprise at least one component that is operable to distinguish the reflected portion of the secondary radiation from the primary radiation.

[0045] For example, such at least one component may distinguish the reflected portion of the secondary radiation from the primary radiation using wavelength. For example, the at least one component may comprise a filter arranged to generally block the wavelength of the primary radiation and to transmit the reflected portion of the secondary radiation. Additionally or alternatively, the at least one component may comprise any type of chromatic optics such as, for example, a diffraction grating of a prism that is configured to direct the primary radiation and the reflected portion of the secondary radiation along different optical paths.

[0046] Additionally or alternatively, such at least one component may distinguish the reflected portion of the secondary radiation from the primary radiation using polarization. For example, in some embodiments the primary radiation may be s-polarized and the reflected portion of the secondary radiation may be p-polarized.

[0047] The processor may be operable to employ any known method for determining the height of the substrate from reflected portion of the secondary radiation.

[0048] The illumination system may comprise projection optics operable to form a first image of a pattern on the beam spot region with the primary and tertiary radiation.

[0049] The projection optics may comprise: a projection patterning device; and first imaging optics arranged to form an image of the projection patterning device on the beam spot region using the primary and tertiary radiation.

[0050] The apparatus may further comprise a movement mechanism operable to cause relative movement of the support and the beam spot region.

[0051] This may allow the object or substrate to be stepped or scanned through the beam spot region.

[0052] The detection optics may be operable to receive the reflected portion of the secondary radiation and to split the reflected radiation into first and second portions. The at least one detector may comprise: a first detector arranged to determine an intensity of the first portion of the radiation; and a second detector arranged to determine an intensity of the second portion of the radiation. The processor may be operable to determine a height profile of the object from the intensity of the first portion of the radiation and the intensity of the second portion of the radiation.

[0053] 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 the relative values of the intensities of the first and second portions of the radiation. For example, as the height of the object varies, the position of the second image of the pattern may vary relative to the splitting optics that are arranged to split the reflected radiation into the first and second portions. By splitting the reflected radiation into first and second portions and determining the height of the substrate from the intensities of the first and second portions,the determination of the height can be substantially independent of the intensity of the radiation beam. For example, the height may be determined as a differential measurement.

[0054] The detection optics may comprise: splitting optics arranged to split the reflected portion of the secondary radiation into first and second portions; and second imaging optics arranged to receive the reflected portion of the secondary radiation and to form a second image of the pattern on the splitting optics.

[0055] The detection optics may comprise a filter configured to minimize transmission of scattered radiation. Advantageously, the suppression of scattered radiation further reduces measurement errors caused by interference effects due to HPD.

[0056] With such embodiments, the height may be determined as a differential measurement. With such embodiments, the detection system may be said to be a balance detection system.

[0057] According to a second aspect of the present disclosure there is provided an exposure apparatus comprising the apparatus of the first aspect of the present disclosure.

[0058] The exposure apparatus may comprise a lithographic apparatus.

[0059] According to a third aspect of the present disclosure there is provided a metrology apparatus comprising the apparatus of the first aspect of the present disclosure.

[0060] According to a fourth aspect of the present disclosure there is provided a method of measuring a topography of a surface of an object, the method comprising: projecting primary radiation onto an object disposed in a beam spot region so as to generate secondary radiation from a surface of the object via an even order non-linear optical process, wherein the secondary radiation has a different wavelength to the primary radiation and wherein the secondary radiation comprises a reflected portion that is reflected from the object and a transmitted portion that propagates into the object; producing tertiary radiation having substantially the same wavelength as the secondary radiation and projecting tertiary radiation onto the object disposed in the beam spot region such that the tertiary radiation at least partially destructively interferes with the transmitted portion of the secondary radiation; receiving the reflected portion of the secondary radiation; and determining a height of the object from the reflected portion of the secondary radiation.

[0061] The method according to the fourth aspect of the disclosure may be carried out using the apparatus according to the first aspect of the disclosure. The method according to the fourth aspect is advantageous as it allows for a more accurate measurement of height of an object that comprises multiple layers relative to existing level sensors, as now discussed.

[0062] As with the apparatus according to the first aspect, the method also exploits the fact that centro-symmetric media such as the top resist layer and the wafer as a whole do not generate radiation via even order (for example second order) nonlinear processes such as second harmonic generation (SHG). In contrast, the interface between an object when disposed in the beam spot region and a surrounding environment has a broken centro-symmetry and therefore satisfies the conditions for evenorder nonlinear processes such as SHG. Therefore, the reflected portion of the secondary radiation is a signal that originates from the surface of the object (rather from within the multilayer stack of the wafer).

[0063] Furthermore, the tertiary radiation produced at least partially destructively interferes with the transmitted portion of the secondary radiation. Advantageously, this suppresses back reflections of the transmitted portion reducing ASD or HPD errors and allowing for a more accurate measurement of the height.

[0064] The primary radiation may comprise a first wavelength.

[0065] For example, the primary radiation may comprise monochromatic radiation.

[0066] With such an arrangement, the even order non-linear optical process may be second harmonic generation (SHG). In such embodiments, two photons of primary radiation (having a frequency of coi) may be combined to produce a single secondary photon having twice the frequency.

[0067] Additionally or alternatively, in other embodiments, the primary radiation may also comprise a second wavelength. With such an arrangement, the even order non-linear optical process may be sum-frequency generation (SFG). In such embodiments, a photon of primary radiation having a first frequency (coi) may be combined with a photon of primary radiation having a second frequency (coz) to produce a single secondary photon having a frequency of coi+coz- Note that SHG is a special case of SFG in which coi=co2-

[0068] The primary radiation may comprise pulsed radiation.

[0069] Advantageously, this may allow for high peak power to be achieved, which may increase the strength of non-linear processes, while maintaining a lower average power to reduce the risk of damage to the object.

[0070] The pulses may be short pulses, for example having a duration of the order of nanoseconds or less. Such short pulses are particularly well suited for generating non-linear optical processes.

[0071] In general, there is a correlation between the sensitivity of the detector(s) used and the peak power of the primary radiation. It will be appreciated that the peak power of the primary radiation may be chosen so as to generate secondary radiation with a sufficiently high power to be measured by the detector(s).

[0072] The even order non-linear optical process may be a second order non-linear optical process. Such processes may be referred to as three-wave mixing processes and / or2)processes.

[0073] The even order non-linear optical process may be second harmonic generation.

[0074] In some embodiments, the tertiary radiation may be generated from the primary radiation using a similar non-linear optical process to that used to generate the secondary radiation.

[0075] Producing the tertiary radiation may comprise directing the primary radiation through a non-linear crystal.

[0076] For example, the crystal may comprise a relatively thin non-linear crystal. An example of such a non-linear crystal is a barium borate (BBO) crystal. As the primary radiation passes through thenon-linear crystal, the tertiary radiation is generated. This may be via an even order non-linear optical process such as, for example, second harmonic generation (SHG).

[0077] The method may further comprise a calibration process of matching the tertiary radiation with the transmitted portion of the secondary radiation such that they at least partially destructively interfere with each other.

[0078] It will be appreciated that the calibration process may be performed before all other steps of the method. The calibration process may be performed once per object (for example once per wafer).

[0079] The calibration process may comprise adjusting a relative phase of the tertiary radiation and the transmitted portion of the secondary radiation.

[0080] Adjusting the relative phase of the tertiary radiation and the transmitted portion of the secondary radiation may comprise adjusting an optical path length between a non-linear crystal used to generate the tertiary radiation and the object.

[0081] The calibration process may comprise adjusting an amplitude or intensity of the tertiary radiation so as to substantially match that of the transmitted portion of the secondary radiation.

[0082] Adjusting the amplitude of the tertiary radiation may comprise rotating a non-linear crystal used to generate the tertiary radiation about an axis.

[0083] The calibration process may comprise: projecting primary radiation onto an object disposed in a beam spot region so as to generate secondary radiation from a surface of the object via an even order non-linear optical process, wherein the secondary radiation has a different wavelength to the primary radiation and wherein the secondary radiation comprises a reflected portion that is reflected from the object and a transmitted portion that propagates into the object; producing tertiary radiation having substantially the same wavelength as the secondary radiation and projecting tertiary radiation onto the object disposed in the beam spot region such that the tertiary radiation at least partially destructively interferes with the transmitted portion of the secondary radiation; adjusting one or more parameters of the tertiary radiation and simultaneously monitoring an intensity of the reflected portion of the secondary radiation; and determining calibration values for the one or more parameters that minimize the intensity of the reflected portion of the secondary radiation.

[0084] When destructive interference between the tertiary radiation with the transmitted portion of the secondary radiation is maximized, the intensity of the reflected portion of the secondary radiation will be minimized. The one or more parameters may include a relative phase of the tertiary radiation and the transmitted portion of the secondary radiation and / or an amplitude or intensity of the tertiary radiation.

[0085] The method may further comprise a coarse measurement process comprising making a coarse measurement of the height of the object before measuring the height of the object.

[0086] It will be appreciated that the course measurement process may be performed before all other steps of the method. Alternatively, the course measurement process may be performed after the calibration process but prior to the other steps of the method.

[0087] The method may further comprise controlling a position of a non-linear crystal used to generate the tertiary radiation so as maintain an optical path length between the non-linear crystal and the object substantially constant during a measurement of a height profile of the object.

[0088] It will be appreciated that as the height of the object varies, so too will the optical path length between the non-linear crystal and the object unless the non-linear crystal is also moved to compensate for such height variation.

[0089] The position of the non-linear crystal used to generate the tertiary radiation may be controlled in dependence on the coarse measurement of the height of the object.

[0090] That is, first a coarse measurement of the topology of the object (for example a wafer) is made. This is then used to control the height of the non-linear crystal during the main measurement process so as to keep the tertiary radiation out of phase with the transmitted portion of the secondary radiation.

[0091] The produced tertiary radiation may be p-polarized and the tertiary radiation may be incident on the object disposed in the beam spot region at an angle of incidence substantially equal to Brewster’s angle.

[0092] Advantageously, this minimizes the amount of tertiary radiation that is reflected from the object and which could interfere destructively with the reflected portion of the secondary radiation.

[0093] The primary radiation may be s-polarized.

[0094] With such an arrangement, the secondary radiation will be p-polarized. Furthermore, for embodiments wherein the tertiary radiation is generated from the primary radiation using a non-linear optical process, the tertiary radiation will also be p-polarized.

[0095] Receiving the reflected portion of the secondary radiation may comprise separating the reflected portion of the secondary radiation from the primary radiation.

[0096] For example, this may be achieved by exploiting the fact that the secondary radiation and the primary radiation have different wavelengths. For example, this may use at least one component comprising a filter arranged to generally block the wavelength of the primary radiation and to transmit the reflected portion of the secondary radiation. Additionally or alternatively, this may use at least one component comprising any type of chromatic optics such as, for example, a diffraction grating of a prism that is configured to direct the primary radiation and the reflected portion of the secondary radiation along different optical paths.

[0097] Additionally or alternatively, separation of the reflected portion of the secondary radiation from the primary radiation may be achieved by exploiting the fact that the secondary radiation and the primary radiation have different (and orthogonal) polarization states.

[0098] Projecting the primary radiation onto the object may comprise forming a first image of a pattern on the object disposed in the beam spot region with the primary radiation. Projecting the tertiary radiation onto the object may comprise forming a first image of a pattern on the object disposed in the beam spot region with the tertiary radiation.

[0099] The method may further comprise: splitting the received reflected portion of the secondary radiation into first and second portions; and determining an intensity of each of the first and second portions of the radiation; and the height of the object may be determined from the intensity of the first portion of the radiation and the intensity of the second portion of the radiation.

[0100] Splitting the received reflected portion of the secondary radiation into first and second portions may comprise forming a second image of the pattern on splitting optics with the reflected portion of the secondary radiation and using the splitting optics to direct radiation from first and second portions of the second image so as to be spatially separate.

[0101] The method may further comprise moving the object relative to the beam spot region.

[0102] Moving the object relative to the beam spot region may comprise scanning the object relative to the beam spot region. Such scanning may be at a constant speed or velocity or at a variable velocity. As used herein scanning of an object is intended to mean continuous movement of the object. Alternatively, moving the object relative to the beam spot region may comprise stepping the object relative to the beam spot region. As used herein stepping of an object is intended to mean movement of the object in a plurality of successive (temporally separated) steps.

[0103] The object may be supported by a support such as a wafer stage within a lithographic apparatus. Moving the object relative to the beam spot region may comprise moving said support.

[0104] According to a fifth aspect of the present disclosure there is provided an exposure method comprising: measuring a topography of a surface of a substrate using the method of the fourth aspect of the present disclosure; patterning an exposure radiation beam using a patterning device; and projecting the patterned exposure radiation onto the substrate so as to form an image of the patterning device on the substrate; wherein a position of the substrate while the patterned exposure radiation is being projected onto the substrate is controlled in dependence on the measured topography of the surface of the substrate.

[0105] The exposure method may comprise a lithographic exposure method.

[0106] The exposure may be a scanning exposure such that: patterning an exposure radiation beam using a patterning device comprises moving the patterning device through the exposure radiation beam; and projecting the patterned exposure radiation onto the substrate so as to form an image of the patterning device on the substrate comprises moving the substrate such that the image of the patterning device is generally stationary relative to the substrate.

[0107] It will be appreciated that since the patterning device is moving through the exposure radiation beam, the image of the patterning device (formed in a plane of the substrate) will also be moving. Therefore, the substrate is moved such that the image of the patterning device is generally stationary relative to the substrate. For example, in some embodiments the patterning device may be moved at a first speed in a scanning direction and the image of the patterning device may be an inverted image which is scaled by a reduction factor. For such embodiments, the substrate is moved in anopposite direction at a second speed, the second speed being the first speed divided by the reduction factor.BRIEF DESCRIPTION OF THE DRAWINGS

[0108] 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 illustration of a new type of apparatus for measuring a topography of a surface of an object, i.e., a new level sensor, according to an embodiment of the present disclosure;Figure 4 illustrates that, when radiation is incident upon the surface of a silicon wafer, which typically comprises a multilayered stack, a portion of the radiation penetrates into the layers beneath the resist layer, which causes multiple back reflections, resulting in an error in a measured height;Figure 5 is a schematic illustration of a specific embodiment of the new apparatus shown in Figure 3, having a specific illumination system and specific detection optics;Figure 6 is a schematic illustration of a new method of measuring a topography of a surface of an object according to an embodiment of the present disclosure, which may be carried out using the apparatus as shown in Figure 3 and / or Figure 5;Figure 7 is a schematic illustration of sub-steps which a calibration process that may form part of the method shown in Figure 6 may comprise; andFigure 8 is a schematic illustration of an exposure method according to an embodiment of the present disclosure that comprises measuring a topography of a surface of a substrate using the method shown in Figure 6.Figure 9 illustrates that, when radiation is incident upon the surface of a silicon wafer comprising a multilayered stack including a hard structured layer, a portion of the radiation penetrates into the layers beneath the resist layer, which causes multiple back reflections and additionally scattering from the hard structured layer, resulting in an error in a measured height.Figure 10 is a schematic illustration of an embodiment of an apparatus according to the disclosure.DETAILED DESCRIPTION

[0109] 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 186 nm) and EUV (extreme ultra-violet radiation, e.g. having a wavelength in the range of about 5-100 nm).

[0110] 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.

[0111] Figure 1 schematically depicts a lithographic apparatus LA. The lithographic apparatus LA includes an illumination system (also referred to as illuminator) IL configured to condition a radiation beam B (e.g., UV radiation, DUV radiation or EUV radiation), a mask support (e.g., a mask table) MT constructed to support a patterning device (e.g., a mask) MA and connected to a first positioner PM configured to accurately position the patterning device MA in accordance with certain parameters, a substrate support (e.g., a wafer table) WT constructed to hold a substrate (e.g., a resist coated wafer) W and connected to a second positioner PW configured to accurately position the substrate support in accordance with certain parameters, and a projection system (e.g., a refractive projection lens system) PS configured to project a pattern imparted to the radiation beam B by patterning device MA onto a target portion C (e.g., comprising one or more dies) of the substrate W.

[0112] 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.

[0113] 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.

[0114] 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.

[0115] 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 carriedout 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.

[0116] 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.

[0117] 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 scribe-lane alignment marks when these are located between the target portions C.

[0118] 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.

[0119] 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 infocus 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.

[0120] An example of a level or height sensor LS as known in the art is schematically shown in Figure 2, which illustrates only the principles of operation. In this example, the level sensor LS comprises an optical system, which includes a projection unit LSP and a detection unit LSD. The projection unit LSP comprises a radiation source LSO providing a beam of radiation LSB which is imparted with a pattern by a projection grating PGR of the projection unit LSP. The projection grating PGR may alternatively be referred to as a patterning device PGR. The radiation source LSO may be, for example, a narrowband or broadband radiation source, such as a supercontinuum light source, polarized or non-polarized, pulsed or continuous, such as a polarized or non-polarized laser beam. The radiation source LSO may include a plurality of radiation sources having different colors, or wavelength ranges, such as a plurality of LEDs. The radiation source LSO of the level sensor LS is not restricted to visible radiation, but may additionally or alternatively encompass UV and / or IR radiation and any range of wavelengths suitable to reflect from a surface of a substrate.

[0121] 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.

[0122] 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.

[0123] 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.

[0124] The proj ection 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).

[0125] 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.

[0126] 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.

[0127] 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.

[0128] 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 BEL In practice, the splitting of the radiation into first and second portions may be achieved in a number of different ways.

[0129] 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.

[0130] 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 generallycomprise splitting optics that is arranged to split the reflected radiation into first and second portions. The splitting optics may be a ruled grating with a triangular grating profile which acts as a series of wedges or prisms to redirect the reflected radiation BE2 (according to Snell’s law). Such splitting optics may be considered to comprise a plurality of prisms and the image of each line of the projection grating PGR may be imaged onto one of the plurality of generally triangular prisms such that a first portion of the line is incident in a first surface of the prism and a second portion of the line is incident in a second surface of the prism. The first portion of the line is directed to the first detector and the second portion of the line is directed to the second detector. As the line moves relative to the prism (as a result of a change in height of the substrate W), the amount of radiation directed to each of the detectors changes.

[0131] Some embodiments of the present disclosure relate to a new type of apparatus for measuring a topography of a surface of an object, i.e., a new level sensor. Such a new apparatus 100 is now described with reference to Figure 3.

[0132] The new apparatus 100 for measuring a topography of a surface of an object comprises: a support 110; an illumination system 120; an interference module 130; at least one detector 140; 150 detection optics; and a processor 160. The apparatus 100 may be referred to as a level sensor. The apparatus 100 may form part of a lithographic apparatus (for example of the type shown in Figure 1). The apparatus 100 is advantageous as it allows for a more accurate measurement of height of an object (such as a substrate W) that comprises one or more layers relative to existing level sensors, as discussed further below.

[0133] The support 110 is for supporting an object W such that it is positionable in a beam spot region 170. The support 110 may comprise an object holder operable to secure the object. For example, the support 110 may comprise a clamp for clamping the object to the support.

[0134] The object may be a substrate W. The substrate W may be a substrate suitable for use within a lithographic apparatus. Such a substrate may comprise a silicon wafer coated with a photoresist. The lithographic apparatus may be suitable to expose the substrate W to exposure radiation that has been patterned by a reticle or mask MA. Before the substrate W is exposed to the patterned radiation, a topology of the surface of the substrate W may be determined. For example a height map of the surface of the substrate W may be determined, for example using the new apparatus 100. Subsequently, the measured topography of a surface of a substrate W can be used to control a height of the substrate W while it is being exposed to the patterned exposure radiation, for example to keep the substrate W in a plane of best focus for the image of the patterning device MA.

[0135] The illumination system 120 is configured to emit primary radiation 182 and project the primary radiation on the beam spot region 170 so as to generate secondary radiation from a surface of an object W when disposed in the beam spot region 170 via an even order non-linear optical process. The secondary radiation has a different wavelength to the primary radiation 182. The secondary radiation comprises a reflected portion 184a that is reflected from the object W and a transmitted portion 184b that propagates into the object W.

[0136] The interference module 130 is operable to produce tertiary radiation 186 having substantially the same wavelength as the secondary radiation 184a, 184b and which is incident on the beam spot region 170 such that the tertiary radiation 186 at least partially destructively interferes with the transmitted portion of the secondary radiation 184b.

[0137] The at least one detector 140 may form part of a detection system.

[0138] The detection optics 150 is operable to receive the reflected portion 184a of the secondary radiation reflected from the object W and to direct it to the at least one detector 140.

[0139] The processor 160 is operable to determine a height of the substrate from reflected portion of the secondary radiation 184a. It will be appreciated that determining a height of the object W comprises determining a height of the object W relative to a reference height or position.

[0140] The processor may 160 be operable to implement any of the features of the methods shown in Figures 6, 7 and / or 8 and described below. The processor 160 may alternatively be referred to as a height determination unit or height determination device.

[0141] One type of level sensor LS (see, for example, Figure 2 as described above) can be used to measure the height profile of the surface of a substrate W by: projecting a patterned radiation beam BE1 onto a beam spot region MLO; moving the substrate W relative to the beam spot region MLO; receiving a portion of the patterned radiation beam BE2 reflected from the object W and determining the first measurement of the height therefrom. As the height of the substrate W varies, the position of the pattern of the reflected radiation may vary, for example relative to splitting optics arranged to split the reflected radiation into first and second portions. As shown in Figure 4, when radiation is incident upon the surface of a silicon wafer W, which typically comprises a multilayered stack, a portion of the radiation BE1 penetrates into the layers beneath the resist layer. This penetration causes multiple back reflections, resulting in an imbalance in the signal readout for such a level sensor LS. As a result, the measured height, hm, differs from the actual height, ha, of the wafer W by an amount A. This amount, A, which is the difference between a measured height and a real height due to interference effects of underlying layers, may be referred to as “Apparent Surface Depression” (ASD) error or “Height Process Dependency” (HPD) error.

[0142] The new apparatus 100 is advantageous as it is less sensitive to such errors in the height measurement, as now discussed.

[0143] The inventors have realized that centro-symmetric media such as the top resist layer and the wafer as a whole do not generate radiation via even order (for example second order) nonlinear processes such as second harmonic generation (SHG). In contrast, the interface between an object W when disposed in the beam spot region 170 and a surrounding environment has a broken centrosymmetry and therefore satisfies the conditions for even order nonlinear processes such as SHG. Therefore, the reflected portion of the secondary radiation 184a is a signal that originates from the surface of the object W.

[0144] Furthermore, the tertiary radiation 186 produced by the interference module 130 at least partially destructively interferes with the transmitted portion of the secondary radiation 184b. Advantageously, this suppresses back reflections of the transmitted portion 184b, reducing ASD or HPD errors.

[0145] The illumination system 120 may comprise a radiation source 122 operable to produce primary radiation 182 comprising a first wavelength. With such an arrangement, the even order nonlinear optical process may be second harmonic generation (SHG). In such embodiments, two photons of primary radiation 182 (having a frequency of coi) may be combined to produce a single secondary photon having twice the frequency.

[0146] Additionally or alternatively, in other embodiments, the illumination system 120 may comprise a second radiation source (not shown) operable to produce primary radiation having a second wavelength. With such an arrangement, the even order non-linear optical process may be sum-frequency generation (SFG). In such embodiments, a photon of primary radiation 182 having a first frequency (coi) may be combined with a photon of primary radiation 182 having a second frequency (coz) to produce a single secondary photon having a frequency of 0) 1+0)2. Note that SHG is a special case of SFG in which C01=C02-

[0147] The radiation source 122 may be a pulsed radiation source and is operable to produce pulsed primary radiation 182. Advantageously, this may allow for high peak power to be achieved, which may increase the strength of non-linear processes, while maintaining a lower average power to reduce the risk of damage to the object W. The pulses may be short pulses, for example having a duration of the order of nanoseconds or less. Such short pulses are particularly well suited for generating non-linear optical processes.

[0148] In general, there is a correlation between the sensitivity of the at least one detector 140 and the peak power of the primary radiation 182. It will be appreciated that the peak power of the primary radiation 182 may be chosen so as to generate secondary radiation 184a with a sufficiently high power to be measured by the at least one detector 140.

[0149] In some embodiments, a frequency of the primary radiation 182 may be selected so as to match a molecular vibrational oscillation mode of a material from which a top layer of the object W is formed (for example a resist). Advantageously, this can result in a significant enhancement of the secondary radiation 184a, 184b. Typically, resist layers consist of either polymers or metallo-organic materials and therefore are typically rich in free C-H groups. Therefore, the frequency of the primary radiation 182 may be selected so as to coincide with a resonant excitation of the C-H stretch vibration mode at around 3 pm. Advantageously, the resulting secondary radiation 184a, 184b is significantly enhanced by such a selection compared to off-resonant excitation.

[0150] In some embodiments, the even order non-linear optical process that generates the secondary radiation 184a, 184b may be second order non-linear optical process. Such processes may be referred to as three-wave mixing processes and / or %(2)processes.

[0151] In some embodiments, the even order non-linear optical process that generates the secondary radiation 184a, 184b may be second harmonic generation.

[0152] The interference module is operable to produce tertiary radiation having substantially the same wavelength as the secondary radiation. In some embodiments, the tertiary radiation may be generated from the primary radiation using a similar non-linear optical process to that used to generate the secondary radiation.

[0153] The interference module 130 may comprise a non-linear crystal disposed in a path of the primary radiation 182. For example, the interference module 130 may comprise a relatively thin nonlinear crystal. An example of such a non-linear crystal is a barium borate (BBO) crystal. As the primary radiation 182 passes through the non-linear crystal, the tertiary radiation 186 is generated. This may be via an even order non-linear optical process such as, for example, second harmonic generation (SHG).

[0154] In some embodiments, an optical path length between the interference module 130 (for example a non-linear crystal) and the object W is adjustable. This is indicated schematically by arrow 132 in Figure 3.

[0155] In particular, the optical path length between the non-linear crystal 130 and the object W can be adjusted so that the tertiary radiation 186 and the transmitted portion of the secondary radiation 184b are substantially out of phase.

[0156] The primary 182 and tertiary radiation 186 may be substantially in phase at the interference module 130 (for example a non-linear crystal). However, given that the primary radiation 182 and tertiary radiation 186 have different wavelengths, as they propagate away from the non-linear crystal 130 the relative phase of the primary radiation 182 and tertiary radiation 186 will oscillate as a function of optical path length. The primary radiation 182 and secondary radiation 184a, 184b may be substantially in phase at the interface between the object W when disposed in the beam spot region 170 and the surrounding environment.

[0157] In some embodiments, the optical path length between the non-linear crystal 130 and the object W may be adjusted so that the tertiary radiation 186 and the transmitted portion of the secondary radiation are out of phase by 7t radians (180 degrees). For embodiments wherein the secondary radiation 184a, 184b and tertiary radiation 186 are both formed from the primary radiation 182 via SHG this may be achieved by selecting an optical path length between the non-linear crystal 130 and the object W of A / 2(2n + 1), where A is the wavelength of the primary radiation 182 and n is an integer.

[0158] It will be appreciated that as the height of the object W varies, so too will the optical path length between the non-linear crystal 130 and the object W unless the non-linear crystal is also moved to compensate for such height variation.

[0159] In some embodiments, first a coarse measurement of the topology of the object W (for example a wafer) may be made. This may then be used to control the height of the non-linear crystal 130 during the main measurement process so as to keep the tertiary radiation 186 out of phase with the transmitted portion of the secondary radiation 184b.

[0160] In some embodiments, the interference module 130 (for example a non-linear crystal) may be rotatable about an axis. This is indicated schematically by arrow 134 in Figure 3. The axis may, for example, be normal to a surface of the non-linear crystal. The axis may, for example, be generally parallel to a propagation direction of the primary radiation 182.

[0161] Advantageously, this can allow for the amplitude of the tertiary radiation 186 to be tuned so as to generally match that of the transmitted portion of the secondary radiation 184b. As the nonlinear crystal 130 rotates about the axis (which may, for example, be normal to a surface of the crystal), an intensity or amplitude of the tertiary radiation 186 may vary between zero and a maximum value.

[0162] A thickness of the non-linear crystal 130 may be chosen so that a maximum intensity or amplitude of the tertiary radiation 186 is of the order of twice an expected amplitude of the transmitted portion of the secondary radiation 184b.

[0163] In some embodiments, an orientation of the non-linear crystal 130 may be adjusted so that an amplitude of the tertiary radiation 186 is generally equal to an amplitude of the transmitted portion of the secondary radiation 184b.

[0164] The interference module 130 may be configured such that the tertiary radiation 186 is p- polarized and such that the tertiary radiation 186 is incident on the object W disposed in the beam spot region 170 at an angle of incidence substantially equal to Brewster’s angle. Advantageously, this minimizes the amount of tertiary radiation 186 that is reflected from the object W and which could interfere destructively with the reflected portion of the secondary radiation 184b.

[0165] The illumination system 120 may be configured such that the primary radiation 182 is s- polarized. With such an arrangement, the secondary radiation 184a, 184b will be p-polarized. Furthermore, for embodiments wherein the tertiary radiation 186 is generated from the primary 182 radiation using a non-linear optical process, the tertiary radiation 186 will also be p-polarized.

[0166] The detection optics 150 may comprise at least one component 152 that is operable to distinguish the reflected portion of the secondary radiation 184a from the primary radiation 182.

[0167] For example, such at least one component 152 may distinguish the reflected portion of the secondary radiation 184a from the primary radiation 182 using wavelength as a discriminator (for example using optical filters). For example, the at least one component 152 may comprise a filter arranged to generally block the wavelength of the primary radiation 182 and to transmit the reflected portion of the secondary radiation 184a. Additionally or alternatively, the at least one component 152 may comprise any type of chromatic optics such as, for example, a diffraction grating of a prism that is configured to direct the primary radiation 182 and the reflected portion of the secondary radiation 184a along different optical paths.

[0168] Additionally or alternatively, such at least one component 152 may distinguish the reflected portion of the secondary radiation 184a from the primary radiation 182 using polarization as a discriminator. For example, in some embodiments the primary radiation 182 may be s-polarized and the reflected portion of the secondary radiation 184a may be p-polarized.

[0169] In some embodiments, the apparatus 100 may further comprise a movement mechanism 190 operable to cause relative movement of the support 110 and the beam spot region 170. In particular, the movement mechanism 190 may be operable to cause relative movement of the support 110 and the beam spot region 170 in the plane of the substrate W. This movement is indicated schematically by arrow 192. This may allow the object or substrate to be stepped or scanned through the beam spot region. Although represented as a single arrow 192 in the cross section of Figure 3, in general, the movement mechanism 190 may be operable to cause relative movement of the support 110 and the beam spot region 170 in two independent directions (which may be referred to as the x and y directions) in the plane of the substrate W. Additionally or alternatively, the movement mechanism 190 may be operable to cause relative movement of the support 110 and the beam spot region 170 in a direction generally perpendicular to the plane of the substrate W (which may be referred to as the z-direction). This movement is indicated schematically by arrow 194. This may, for example, allow for an optical path length between the interference module 130 (for example a non-linear crystal) and the object W to be adjustable (for example so as to ensure that the tertiary radiation 186 and the transmitted portion of the secondary radiation 184b are substantially out of phase).

[0170] The processor 160 may be operable to employ any known method for determining the height (topology value) of the substrate W from reflected portion of the secondary radiation 184a.

[0171] The processor 160 may be further operable to control any one of the following: a position of the support 110 relative to the beam spot region 170 (via the movement mechanism 190); a position of the interference module 130 relative to the object W (i.e., the movement indicated schematically by arrow 132); and / or an orientation of the interference module 130 relative to the object W (i.e., the movement indicated schematically by arrow 134). Alternatively, one or more additional controllers may be provided to control these variables.

[0172] The new type of apparatus 100 for measuring a topography of a surface of an object W may use the methods for determining the height of the substrate W from reflected portion of the secondary radiation 184a from any known type of height sensor. As explained above, 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.

[0173] A specific embodiment of the new apparatus 100 shown in Figure 3, having a specific illumination system 120 and specific detection optics 150 is now described with reference to Figure 5.

[0174] In the embodiment shown in Figure 5, the illumination system 120 comprises projection optics 124 operable to form a first image of a pattern on the beam spot region 170 with the primary radiation 182 and the tertiary radiation 186.

[0175] The projection optics 124 may comprise: a projection patterning device 126; and first imaging optics 128 arranged to form an image of the projection patterning device 126 on the beam spot region 170 using the primary and tertiary radiation.

[0176] In the embodiment shown in Figure 5, the detection optics 150 is operable to receive the reflected portion of the secondary radiation 184a and to split the reflected radiation into first and second portions 188a, 188b.

[0177] The detection optics 150 comprises: splitting optics 154 arranged to split the reflected portion of the secondary radiation 184a into first and second portions 188a, 188b; and second imaging optics 156 arranged to receive the reflected portion of the secondary radiation 184a and to form a second image of the pattern on the splitting optics 154. With such embodiments, the height may be determined as a differential measurement. With such embodiments, the detection system may be said to be a balanced detection system.

[0178] The at least one detector 140 comprises: a first detector 142 arranged to determine an intensity of the first portion of the radiation 188a; and a second detector 144 arranged to determine an intensity of the second portion of the radiation 188b. The first detector 142 may be operable to send a first signal si that is indicative of the intensity of the first portion of the radiation 188a to the processor 160. The second detector 144 may be operable to send a second signal S2 that is indicative of the intensity of the second portion of the radiation 188b to the processor 160.

[0179] In practice, the splitting of the reflected portion of the secondary radiation 184a into first and second portions 188a, 188b may be achieved in a number of different ways, as now discussed.

[0180] 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 patterning device 126 (each having a different polarization state) on a detection grating (for example, the splitting optics 154). An example of such an arrangement is shown schematically in Figure 5 of US2010233600A1. For example, the projection patterning device 126 may have a pitch P and a duty cycle of 50% such that the beams of radiation 182, 184a, 184b, 186 have a periodically varying intensity comprising a plurality of lines having a thickness of P / 2, adjacent lines being separated by P / 2. The polarizer and a shear plate may be arranged to form two images of the projection patterning device 126 using the reflected portion of the secondary radiation 184a (each having a different polarization state) on the splitting optics 154 (a grating), one image being laterally shifted relative to the other by P / 2. Downstream of the splitting optics 154 the two separate polarization states 188a, 188b are each directed to a different detector 142, 144. The height of the substrate W is determined as being proportional to the difference in the intensities of the two separate polarization states 188a, 188b.

[0181] In some other known arrangements, rather than splitting the reflected portion of the secondary radiation 184a using two images of the projection patterning device 126 but having different polarization states, a single image of the projection patterning device 126 is formed (using the reflected portion of the secondary radiation 184a) on splitting optics 154 that is arranged to split that single imageinto first and second portions 188a, 188b. 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 154 that is arranged to split the reflected portion of the secondary radiation 184a into first and second portions 188a, 188b. The splitting optics 154 may be a ruled grating with a triangular grating profile which acts as a series of wedges or prisms to redirect the reflected portion of the secondary radiation 184a (according to Snell’s law). Such splitting optics 154 may be considered to comprise a plurality of prisms and the image of each line of the projection patterning device 126 may be imaged onto one of the plurality of generally triangular prisms such that a first portion of the line is incident in a first surface of the prism and a second portion of the line is incident in a second surface of the prism. The first portion of the line is directed to the first detector 142 and the second portion of the line is directed to the second detector 144. 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 142, 144 changes.

[0182] The processor 160 is operable to determine a height of the object W from the intensity of the first portion of the radiation 188a and the intensity of the second portion of the radiation 188b.

[0183] As the height of the object W varies, while moving the spot over the object surface, the position of the second image of the pattern will also vary. In turn, this may result in a change in the relative values of the intensities of the first and second portions of the radiation 188a, 188b. For example, as the height of the object varies, the position of the second image of the pattern may vary relative to the splitting optics 154 that are arranged to split the reflected radiation into the first and second portions 188a, 188b. By splitting the reflected radiation 184a into first and second portions 188a, 188b and determining the height of the substrate W from the intensities of the first and second portions 188a, 188b, the determination of the height can be substantially independent of the intensity of the radiation beam 184a. For example, the height may be determined as a differential measurement.

[0184] Some embodiments of the present disclosure relate to an exposure apparatus comprising the apparatus 100 as shown in Figure 3 and / or Figure 5 and as described above.

[0185] The exposure apparatus may comprise a lithographic apparatus of the type shown in Figure 1. The lithographic apparatus FA may further comprise: an illumination system IL operable to illuminate an illumination region; a support structure MT configured to support a patterning device MA such that the patterning device MA is positionable in the illumination region; a substrate table WT configured to support a substrate W; and a projection system PS operable to form an image of a patterning device MA supported by the support structure MT on a substrate W supported by the substrate table WT.

[0186] Some embodiments of the present disclosure relate to a metrology apparatus comprising the apparatus 100 as shown in Figure 3 and / or Figure 5 and as described above.

[0187] Some embodiments of the present disclosure relate to new methods of measuring a topography of a surface of an object. Such a new method 200 is now described with reference to Figure6. The new method may be carried out using the apparatus 100 as shown in Figure 3 and / or Figure 5 and as described above.

[0188] The method 200 comprises a step 210 of projecting primary radiation 182 onto an object W disposed in a beam spot region 170 so as to generate secondary radiation 184a, 184b from a surface of the object W via an even order non-linear optical process. The secondary radiation 184a, 184b has a different wavelength to the primary radiation 182. The secondary radiation 184a, 184b comprises a reflected portion 184a that is reflected from the object W and a transmitted portion 184b that propagates into the object W.

[0189] The method 200 further comprises a step 220 of producing tertiary radiation 186 having substantially the same wavelength as the secondary radiation 184a, 184b and projecting the tertiary radiation 186 onto the object W disposed in the beam spot region 170 such that the tertiary radiation 186 at least partially destructively interferes with the transmitted portion of the secondary radiation 184b.

[0190] The method 200 comprises: a step 230 of receiving the reflected portion of the secondary radiation 184a; and a step 240 of determining a height of the object W from the reflected portion of the secondary radiation 184a.

[0191] The method 200 shown in Figure 6 may be carried out using the apparatus 100 shown in Figure 3. The new method 200 shown in Figure 6 is advantageous as it allows for a more accurate measurement of height of an object W that comprises multiple layers relative to existing level sensors, as now discussed.

[0192] As with the apparatus 100 shown in Figure 3, the method 200 shown in Figure 6 also exploits the fact that centro-symmetric media such as the top resist layer and the wafer W as a whole do not generate radiation via even order (for example second order) nonlinear processes such as second harmonic generation (SHG). In contrast, the interface between an object W when disposed in the beam spot region 170 and a surrounding environment has a broken centro-symmetry and therefore satisfies the conditions for even order nonlinear processes such as SHG. Therefore, the reflected portion of the secondary radiation 184a is a signal that originates from the surface of the object W (rather from within the multilayer stack of the wafer).

[0193] Furthermore, the tertiary radiation 186 produced at step 220 (for example produced by an interference module) at least partially destructively interferes with the transmitted portion of the secondary radiation 184b. Advantageously, this suppresses back reflections of the transmitted portion of the secondary radiation 184b, reducing ASD or HPD errors and allowing for a more accurate measurement of the height.

[0194] The primary radiation 182 produced at step 210 may comprise a first wavelength. For example, the primary radiation 182 may comprise monochromatic radiation. With such an arrangement, the even order non-linear optical process may be second harmonic generation (SHG). In suchembodiments, two photons of primary radiation 182 (having a frequency of coi) may be combined to produce a single secondary photon having twice the frequency.

[0195] Additionally or alternatively, in other embodiments, the primary radiation 182 may also comprise a second wavelength. With such an arrangement, the even order non-linear optical process may be sum-frequency generation (SFG). In such embodiments, a photon of primary radiation 182 having a first frequency (co i) may be combined with a photon of primary radiation 182 having a second frequency (CO2) to produce a single secondary photon having a frequency of coi+coz- Note that SHG is a special case of SFG in which coi=co2-

[0196] The primary radiation 182 produced at step 210 may comprise pulsed radiation. Advantageously, this may allow for high peak power to be achieved, which may increase the strength of non-linear processes, while maintaining a lower average power to reduce the risk of damage to the object W. The pulses may be short pulses, for example having a duration of the order of nanoseconds or less. Such short pulses are particularly well suited for generating non-linear optical processes.

[0197] In general, there is a correlation between the sensitivity of the detector(s) used and the peak power of the primary radiation 182. It will be appreciated that the peak power of the primary radiation 182 produced at step 210 may be chosen so as to generate secondary radiation 184a with a sufficiently high power to be measured by the detector(s).

[0198] The even order non-linear optical process that generates the secondary radiation 184a, 184b at step 210 may be a second order non-linear optical process. Such processes may be referred to as three-wave mixing processes and / or2)processes.

[0199] The even order non-linear optical process that generates the secondary radiation 184a, 184b at step 210 may be second harmonic generation.

[0200] In some embodiments, the tertiary radiation 186 generated at step 220 may be generated from the primary radiation 182 using a similar non-linear optical process to that used to generate the secondary radiation 184a, 184b.

[0201] The step 220 of producing the tertiary radiation 186 may comprise directing the primary radiation 182 through a non-linear crystal 130. For example, the crystal 130 may comprise a relatively thin non-linear crystal. An example of such a non-linear crystal is a barium borate (BBO) crystal. As the primary radiation 182 passes through the non-linear crystal 130, the tertiary radiation 186 may be generated. This may be via an even order non-linear optical process such as, for example, second harmonic generation (SHG).

[0202] In some embodiments, the method 200 may further comprise a calibration process 300 of matching the tertiary radiation 186 with the transmitted portion of the secondary radiation 184b such that they at least partially destructively interfere with each other. It will be appreciated that the calibration process 300 may be performed before all other steps of the method. The calibration process 300 may be performed once per object (for example once per wafer).

[0203] The calibration process 300 may comprise a step 310 of adjusting a relative phase of the tertiary radiation 186 and the transmitted portion of the secondary radiation 184b. In some embodiments, the step 310 of adjusting the relative phase of the tertiary radiation 186 and the transmitted portion of the secondary radiation 184b may comprise adjusting an optical path length between a non-linear crystal 130 used to generate the tertiary radiation 186 and the object W.

[0204] The calibration process 300 may comprise a step 320 of adjusting an amplitude or intensity of the tertiary radiation 186 so as to substantially match that of the transmitted portion of the secondary radiation 184b. In some embodiments, the step 320 of adjusting the amplitude of the tertiary radiation 186 may comprises rotating a non-linear crystal 130 used to generate the tertiary radiation 186 about an axis.

[0205] As shown in Figure 7, in some embodiments the calibration process 300 may comprise the following steps. Note that the many of the following steps used as part of the calibration process 300 are steps that are later used as part of the method 200 (except rather than determining a height at the end, as is done in the method 200, the calibration process 300 comprises determining calibration values).

[0206] The calibration process 300 may comprise a step 330 of projecting primary radiation 182 onto an object W disposed in a beam spot region 170 so as to generate secondary radiation 184a, 184b from a surface of the object W via an even order non-linear optical process. The secondary radiation 184a, 184b has a different wavelength to the primary radiation 182. The secondary radiation comprises a reflected portion 184a that is reflected from the object W and a transmitted portion 184b that propagates into the object W.

[0207] The calibration process 300 may further comprise a step 340 of producing tertiary radiation 186 having substantially the same wavelength as the secondary radiation 184a, 184b and projecting the tertiary radiation 186 onto the object W disposed in the beam spot region W such that the tertiary radiation 186 at least partially destructively interferes with the transmitted portion of the secondary radiation 184b.

[0208] The calibration process 300 may comprise a step 350 of adjusting one or more parameters of the tertiary radiation 186 and simultaneously monitoring an intensity of the reflected portion of the secondary radiation 184a.

[0209] The calibration process 300 may comprise a step 360 of determining calibration values for the one or more parameters that minimize the intensity of the reflected portion of the secondary radiation 184a.

[0210] When destructive interference between the tertiary radiation 186 with the transmitted portion of the secondary radiation 184b is maximized, the intensity of the reflected portion of the secondary radiation 184a will be minimized. The one or more parameters may include a relative phase of the tertiary radiation relative 186 and the transmitted portion of the secondary radiation 184b and / or an amplitude or intensity of the tertiary radiation 186. Therefore, the step 350 of adjusting one or more parameters of the tertiary radiation 186 and simultaneously monitoring an intensity of the reflectedportion of the secondary radiation 184a may comprise: step 310 of adjusting a relative phase of the tertiary radiation 186 and the transmitted portion of the secondary radiation 184b (as discussed above); and / or the step 320 of adjusting an amplitude or intensity of the tertiary radiation 186 (as discussed above).

[0211] Referring back to Figure 6, in some embodiments, the method 200 may further comprise a coarse measurement process 400 comprising making a coarse measurement of the height of the object W before measuring the height of the object. The coarse measurement process 400 may be referred to as a coarse pre-measurement process 400. It will be appreciated that the course measurement process 400 may be performed before all other steps of the method 200. Alternatively, the course measurement process 400 may be performed after the calibration process 300 but prior to the other steps of the method 200.

[0212] The method 200 may further comprise controlling a position of a non-linear crystal 130 used to generate the tertiary radiation 186 so as maintain an optical path length between the non-linear crystal 130 and the object W substantially constant during a measurement of a height profile of the object W. It will be appreciated that as the height of the object W varies, so too will the optical path length between the non-linear crystal 130 and the object W unless the non-linear crystal 130 is also moved to compensate for such height variation.

[0213] In particular, the position of the non-linear crystal 130 used to generate the tertiary radiation 186 may be controlled in dependence on the coarse measurement of the height of the object. That is, first a coarse measurement of the topology of the object W (for example a wafer) is made (at step 400). This is then used to control the height of the non-linear crystal 130 during the main measurement process (comprising steps 210, 220, 230, 240) so as to keep the tertiary radiation 186 out of phase with the transmitted portion of the secondary radiation 184b.

[0214] In some embodiments, the tertiary radiation 186 (produced at steps 220 and 340) is p- polarized and the tertiary radiation 186 is incident on the object W disposed in the beam spot region 170 at an angle of incidence substantially equal to Brewster’s angle. Advantageously, this minimizes the amount of tertiary radiation 186 that is reflected from the object W and which could interfere destructively with the reflected portion of the secondary radiation 184a.

[0215] In some embodiments, the primary radiation 182 (produced at steps 210 and 330) is s- polarized. With such an arrangement, the secondary radiation 184a, 184b will be p-polarized. Furthermore, for embodiments wherein the tertiary radiation 186 is generated from the primary radiation 182 using a non-linear optical process, the tertiary radiation 186 will also be p-polarized.

[0216] In some embodiments, receiving the reflected portion of the secondary radiation 184a (at step 230 and 350) may comprise separating the reflected portion of the secondary radiation 184a from the primary radiation 182.

[0217] For example, this may be achieved by exploiting the fact that the secondary radiation 184a, 184b and the primary radiation 182 have different wavelengths. For example, this may use at least onecomponent 152 comprising a filter arranged to generally block the wavelength of the primary radiation 182 and to transmit the reflected portion of the secondary radiation 184a. Additionally or alternatively, this may use at least one component 152 comprising any type of chromatic optics such as, for example, a diffraction grating of a prism that is configured to direct the primary radiation 182 and the reflected portion of the secondary radiation 184a along different optical paths.

[0218] Additionally or alternatively, separation of the reflected portion of the secondary radiation 184a from the primary radiation 182 may be achieved by exploiting the fact that the secondary radiation 184a, 184b and the primary radiation 182 have different (and orthogonal) polarization states.

[0219] In some embodiments of the method 200, projecting the primary radiation 182 onto the object W (at steps 210 and / or 330) may comprise forming a first image of a pattern on the object W disposed in the beam spot region 170 with the primary radiation 182. Similarly, in some embodiments of the method 200, projecting the tertiary radiation 186 onto the object W (at steps 220 and / or 340) may comprise forming a first image of a pattern on the object W disposed in the beam spot region 170 with the tertiary radiation 186.

[0220] In some embodiments, the method 200 may further comprise: splitting the received reflected portion of the secondary radiation 184a into first and second portions 188a, 188b; and determining an intensity of each of the first and second portions of the radiation 188a, 188b. For example, the steps 230 and 350 may comprise these sub-steps.

[0221] In some embodiments of the method 200 the height of the object W may be determined (at step 240) from the intensity of the first portion of the radiation 188a and the intensity of the second portion of the radiation 188b.

[0222] In some embodiments of the method 200, splitting the received reflected portion of the secondary radiation 184a into first and second portions 188a, 188b may comprise forming a second image of the pattern on splitting optics 154 with the reflected portion of the secondary radiation and using the splitting optics to direct radiation from first and second portions of the second image so as to be spatially separate.

[0223] In some embodiments, the method 200 may further comprise moving the object W relative to the beam spot region 170. Moving the object W relative to the beam spot region 170 may comprise scanning the object W relative to the beam spot region 170. Such scanning may be at a constant speed or velocity or at a variable velocity. As used herein scanning of an object W is intended to mean continuous movement of the object W. Alternatively, moving the object W relative to the beam spot region 170 may comprise stepping the object W relative to the beam spot region 170. As used herein stepping of an object W is intended to mean movement of the object in a plurality of successive (temporally separated) steps.

[0224] The object W may be supported by a support 110 such as a wafer stage within a lithographic apparatus. Moving the object relative to the beam spot region 170 may comprise moving said support 110.

[0225] In some embodiments, the method 200 may comprise stepping (or scanning) the object W relative to the beam spot region 170 and making a plurality of height measurements, one at each of a plurality of different positions on the object W. In this way, a height map of the object may be determined.

[0226] For example, after the step 240 of determining a height of the object W from the reflected portion of the secondary radiation 184a, the method 200 may comprise a step 250 of moving the object W and the apparatus 100 relative to each other so as to position a different part of that object in the beam spot region 170. After the step 250 of moving the object W an apparatus relative to each other, steps 210, 220, 230 and 240 may be repeated.

[0227] It will be appreciated that, for such embodiments the calibration process 300 and / or the coarse measurement process 400 may also be performed for the same plurality of positions on the object W and the individual calibration values and / or coarse height measurements may be used during determination of the corresponding height measurement.

[0228] Some embodiments of the present disclosure relate to an exposure method comprising: measuring a topography of a surface of a substrate using the method 200 shown in Figure 6. An example of such an exposure method 500 is shown in Figure 8.

[0229] The exposure method 500 comprises a step 510 of patterning an exposure radiation beam B using a patterning device MA.

[0230] The exposure method 500 further comprises a step 520 of projecting the patterned exposure radiation B onto the substrate W so as to form an image of the patterning device MA on the substrate W. While the patterned exposure radiation B is being projected onto the substrate W, a position of the substrate W is controlled in dependence on a topography of the surface of the substrate W as measured using the method 200 shown in Figure 6.

[0231] The exposure method 500 may comprise a lithographic exposure method.

[0232] In some embodiments, the exposure is a scanning exposure such that: the step 510 of patterning the exposure radiation beam B using a patterning device MA comprises moving the patterning device MA through the exposure radiation beam B; and the step 520 of projecting the patterned exposure radiation B onto the substrate W so as to form an image of the patterning device MA on the substrate W comprises moving the substrate W such that the image of the patterning device MA is generally stationary relative to the substrate W.

[0233] It will be appreciated that since the patterning device MA is moving through the exposure radiation beam B, the image of the patterning device MA (formed in a plane of the substrate W, for example at wafer-level) will also be moving. Therefore, the substrate W is moved such that the image of the patterning device MA is generally stationary relative to the substrate W. For example, in some embodiments the patterning device MA may be moved at a first speed in a scanning direction and the image of the patterning device MA may be an inverted image which is scaled by a reduction factor. Forsuch embodiments, the substrate W is moved in an opposite direction at a second speed, the second speed being the first speed divided by the reduction factor.

[0234] Measurement errors caused by interference effects due to height process dependency (HPD), as described above and illustrated in Figure 4, may be further enhanced as a result of scattering of light by hard and / or metallic layers. This is shown in Figure 9, where a hard or metallic structure S is present in a multilayered stack on a silicon wafer W. As in Figure 4, a portion of the radiation BE1 penetrates into layers beneath the resist layer, causing multiple back reflections (BE2). In addition to these back-reflections, light incident on structure S is scattered. Structures which cause scattering typically have a larger refractive index than surrounding layers. Scattered rays are shown by the dashed lines in Figure 9. These scattered rays deviate from the main reflection direction. This distorts the beam profile of the reflected radiation. In the case that a differential detection scheme such as that shown in Figure 5 is used for recording the reflected light, the scattered light may reach either of the detectors and result in measurement errors.

[0235] A nonlinear interaction of the light reflected from the wafer surface with an interference module may be utilized further to reduce errors from light scattering. Figure 10 illustrates an embodiment of this principle, showing part of an apparatus 100 for measuring a topography of a surface of an object. The object may be a substrate W, which is supported on a support 110 as disclosed in earlier embodiments. The illumination system 120 is configured to emit at least one radiation beam, which may for example be primary radiation beam 182. The radiation is incident on the surface to be measured, namely a beam spot region 170 on the surface of an object W, for example a substrate including multiple patterned and processed layers of a stack. The primary radiation beam penetrates through the stack and reflects from the different interfaces therein. When incident on a hard or metallic structure S, with a relatively large refractive index, the primary radiation beam is both reflected and scattered in the direction of detection optics 150 and one or more detectors 140.

[0236] The power of the reflected light with no scattering may be given as Po, while the intensity of the light that is both reflected and scattered from a structure S issuch thatIt is typical that the power of scattered light is much lower than the power of the central part of the reflected beam,As such, when subjected to a nonlinear process, the power of the scattered light is further reduced. This is due to the fact that the power of the produced light through a nonlinear process scales with the power of the incoming beams to the power of the nonlinearity order of that process i.e., PnoniinearKP , where Pois the power of the fundamental light before interacting with the nonlinear medium. For instance, the power of the second harmonic of light is proportional to the square of the power of the fundamental beam PSHcc Po2.A nonlinear medium may therefore filter the reflected beam of intensity components arising from scattered light. In the embodiment of Figure 10, detection optics comprises filter 151. Filter 151 may comprise the same material as interference module 130, for example a crystal with high second order nonlinear coefficient for generating the second harmonic of a signal, such as barium borate (BBO).

[0237] The aforementioned features to reduce detection of and subsequent measurement errors caused by scattered light may be taken alone or in combination with any of the previously described embodiments. Advantageously, the accuracy of height measurements is improved.

[0238] 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, liquidcrystal displays (LCDs), thin-film magnetic heads, etc.

[0239] 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.

[0240] 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.

[0241] Aspects of the invention are set out in the clauses below.1. An apparatus for measuring a topography of a surface of an object, the apparatus comprising: a support for supporting an object such that it is positionable in a beam spot region; an illumination system configured to emit primary radiation and project the primary radiation on the beam spot region so as to generate secondary radiation at a surface of an object when disposed in the beam spot region via an even order non-linear optical process, the secondary radiation having a different wavelength to the primary radiation and wherein the secondary radiation comprises a reflected portion that is reflected from the object and a transmitted portion that propagates into the object; an Interference module operable to produce tertiary radiation having substantially the same wavelength as the secondary radiation and which is incident on the beam spot region such that the tertiary radiation at least partially destructively interferes with the transmitted portion of the secondary radiation; at least one detector; detection optics operable to receive the reflected portion of the secondary radiation reflected from the object and to direct it to the at least one detector; and a processor operable to determine a height of the object from reflected portion of the secondary radiation.2. The apparatus of clause 1 wherein the illumination system comprises a radiation source operable to produce primary radiation comprising a first wavelength.3. The apparatus of clause 2 wherein the radiation source is a pulsed radiation source and is operable to produce pulsed primary radiation.4. The apparatus of any preceding clause wherein the even order non-linear optical process is a second order non-linear optical process.5. The apparatus of any preceding clause wherein the even order non-linear optical process is second harmonic generation.6. The apparatus of any preceding clause wherein the interference module comprises a non-linear crystal disposed in a path of the primary radiation.7. The apparatus of clause 6 wherein an optical path length between the non-linear crystal and the object is adjustable.8. The apparatus of clause 6 or clause 7 wherein an optical path length between the non-linear crystal and the object can be adjusted so that the tertiary radiation and the transmitted portion of the secondary radiation are substantially out of phase.9. The apparatus of any one of clauses 6 to 8 wherein the non-linear crystal is rotatable about an axis.10. The apparatus of clause 9 wherein an orientation of the non-linear crystal can be adjusted so that an amplitude of the tertiary radiation is generally equal to an amplitude of the transmitted portion of the secondary radiation.11. The apparatus of any preceding clause wherein the interference module is configured such that the tertiary radiation is p-polarized and such that the tertiary radiation is incident on the object disposed in the beam spot region at an angle of incidence substantially equal to Brewster’s angle.12. The apparatus of any preceding clause wherein the illumination system is configured such that the primary radiation is s-polarized.13. The apparatus of any preceding clause wherein the detection optics comprises at least one component that is operable to distinguish the reflected portion of the secondary radiation from the primary radiation.14. The apparatus of any preceding clause wherein the illumination system comprises projection optics operable to form a first image of a pattern on the beam spot region with the primary and tertiary radiation.15. The apparatus of clause 14 wherein the projection optics comprises: a projection patterning device; and first imaging optics arranged to form an image of the projection patterning device on the beam spot region using the primary and tertiary radiation.16. The apparatus of any preceding clause further comprising a movement mechanism operable to cause relative movement of the support and the beam spot region.17. The apparatus of any preceding clause wherein the detection optics is operable to receive the reflected portion of the secondary radiation and to split the reflected radiation into first and second portions, and wherein the at least one detector comprises: a first detector arranged to determine an intensity of the first portion of the radiation; and a second detector arranged to determine an intensity of the second portion of the radiation; andwherein the processor is operable to determine a height profile of the object from the intensity of the first portion of the radiation and the intensity of the second portion of the radiation.18. The apparatus of any preceding clause when dependent either directly or indirectly on clause 14 wherein the detection optics comprises: splitting optics arranged to split the reflected portion of the secondary radiation into first and second portions; and second imaging optics arranged to receive the reflected portion of the secondary radiation and to form a second image of the pattern on the splitting optics.19. The apparatus of any preceding clause, wherein the detection optics comprises a filter configured to minimize transmission of scattered radiation.20. The apparatus of clause 19, wherein the filter comprises a non-linear crystal.21. The apparatus of clause 19 or 20, wherein the filter is configured to effect a frequency shift of radiation incident thereon.22. An exposure apparatus comprising the apparatus of any preceding clause.23. A metrology apparatus comprising the apparatus of any one of clauses 1 to 21.24. A method of measuring a topography of a surface of an object, the method comprising: projecting primary radiation onto an object disposed in a beam spot region so as to generate secondary radiation from a surface of the object via an even order non-linear optical process, wherein the secondary radiation has a different wavelength to the primary radiation and wherein the secondary radiation comprises a reflected portion that is reflected from the object and a transmitted portion that propagates into the object; producing tertiary radiation having substantially the same wavelength as the secondary radiation and projecting tertiary radiation onto the object disposed in the beam spot region such that the tertiary radiation at least partially destructively interferes with the transmitted portion of the secondary radiation; receiving the reflected portion of the secondary radiation; and determining a height of the object from the reflected portion of the secondary radiation.25. The method of clause 24 wherein the primary radiation comprises a first wavelength.26. The method of clause 24 or clause 25 wherein the primary radiation comprises pulsed radiation.27. The method of any one of clauses 24 to 26 wherein the even order non-linear optical process is a second order non-linear optical process.28. The method of any one of clauses 24 to 27 wherein the even order non-linear optical process is second harmonic generation.29. The method of any one of clauses 24 to 28 wherein producing the tertiary radiation comprises directing the primary radiation through a non-linear crystal.30. The method of any one of clauses 24 to 29 further comprising a calibration process of matching the tertiary radiation with the transmitted portion of the secondary radiation such that they at least partially destructively interfere with each other.31. The method of clause 30 wherein the calibration process comprises adjusting a relative phase of the tertiary radiation and the transmitted portion of the secondary radiation.32. The method of clause 31 wherein adjusting the relative phase of the tertiary radiation and the transmitted portion of the secondary radiation comprises adjusting an optical path length between a nonlinear crystal used to generate the tertiary radiation and the object.33. The method of any one of clauses 30 to 32 wherein the calibration process comprises adjusting an amplitude or intensity of the tertiary radiation so as to substantially match that of the transmitted portion of the secondary radiation.34. The method of clause 33 wherein adjusting the amplitude of the tertiary radiation comprises rotating a non-linear crystal used to generate the tertiary radiation about an axis.35. The method of any one of clauses 30 to 34 wherein the calibration process comprises: projecting primary radiation onto an object disposed in a beam spot region so as to generate secondary radiation from a surface of the object via an even order non-linear optical process, wherein the secondary radiation has a different wavelength to the primary radiation and wherein the secondary radiation comprises a reflected portion that is reflected from the object and a transmitted portion that propagates into the object; producing tertiary radiation having substantially the same wavelength as the secondary radiation and projecting tertiary radiation onto the object disposed in the beam spot region such that the tertiary radiation at least partially destructively interferes with the transmitted portion of the secondary radiation; adjusting one or more parameters of the tertiary radiation and simultaneously monitoring an intensity of the reflected portion of the secondary radiation; and determining calibration values for the one or more parameters that minimize the intensity of the reflected portion of the secondary radiation.36. The method of any one of clauses 24 to 35 further comprising a coarse measurement process comprising making a coarse measurement of the height of the object before measuring the height of the object.37. The method of any one of clauses 24 to 36 further comprising controlling a position of a non-linear crystal used to generate the tertiary radiation so as maintain an optical path length between the nonlinear crystal and the object substantially constant during a measurement of a height profile of the object.38. The method of clause 37 when dependent on clause 36 wherein the position of the non-linear crystal used to generate the tertiary radiation is controlled in dependence on the coarse measurement of the height of the object.39. The method of any one of clauses 24 to 38 wherein the produced tertiary radiation is p-polarized and wherein the tertiary radiation is incident on the object disposed in the beam spot region at an angle of incidence substantially equal to Brewster’ s angle.40. The method of any one of clauses 24 to 39 wherein the primary radiation is s-polarized.41. The method of any one or clauses 24 to 40 wherein receiving the reflected portion of the secondary radiation comprises separating the reflected portion of the secondary radiation from the primary radiation.42. The method of any one of clauses 24 to 41 wherein projecting the primary radiation onto the object comprises forming a first image of a pattern on the object disposed in the beam spot region with the primary radiation; and projecting the tertiary radiation onto the object comprises forming a first image of a pattern on the object disposed in the beam spot region with the tertiary radiation.43. The method of any one of clauses 24 to 42 further comprising: splitting the received reflected portion of the secondary radiation into first and second portions; and determining an intensity of each of the first and second portions of the radiation; and wherein the height of the object is determined from the intensity of the first portion of the radiation and the intensity of the second portion of the radiation.44. The method of any one of clauses 24 to 43 when dependent either directly or indirectly on clause 39 wherein splitting the received reflected portion of the secondary radiation into first and second portions comprises forming a second image of the pattern on splitting optics with the reflected portion of the secondary radiation and using the splitting optics to direct radiation from first and second portions of the second image so as to be spatially separate.45. The method of any one of clauses 24 to 44 further comprising moving the object relative to the beam spot region.46. An exposure method comprising: measuring a topography of a surface of a substrate using the method of any one of clauses 24 to 45; patterning an exposure radiation beam using a patterning device; and projecting the patterned exposure radiation onto the substrate so as to form an image of the patterning device on the substrate; wherein a position of the substrate while the patterned exposure radiation is being projected onto the substrate is controlled in dependence on the measured topography of the surface of the substrate.47. The exposure method of clause 46, wherein the exposure is a scanning exposure such that patterning an exposure radiation beam using a patterning device comprises moving the patterning device through the exposure radiation beam and projecting the patterned exposure radiation onto the substrate so as to form an image of the patterning device on the substrate comprises moving the substrate such that the image of the patterning device is generally stationary relative to the substrate.

[0242] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

CLAIMS1. An apparatus for measuring a topography of a surface of an object, the apparatus comprising: a support for supporting an object such that it is positionable in a beam spot region; an illumination system configured to emit primary radiation and project the primary radiation on the beam spot region so as to generate secondary radiation at a surface of an object when disposed in the beam spot region via an even order non-linear optical process, the secondary radiation having a different wavelength to the primary radiation and wherein the secondary radiation comprises a reflected portion that is reflected from the object and a transmitted portion that propagates into the object; an interference module operable to produce tertiary radiation having substantially the same wavelength as the secondary radiation and which is incident on the beam spot region such that the tertiary radiation at least partially destructively interferes with the transmitted portion of the secondary radiation; at least one detector; detection optics operable to receive the reflected portion of the secondary radiation reflected from the object and to direct it to the at least one detector; and a processor operable to determine a height of the object from reflected portion of the secondary radiation.

2. The apparatus of claim 1 wherein the illumination system comprises a radiation source operable to produce primary radiation comprising a first wavelength.

3. The apparatus of claim 2 wherein the radiation source is a pulsed radiation source and is operable to produce pulsed primary radiation.

4. The apparatus of any preceding claim wherein the even order non-linear optical process is a second order non-linear optical process.

5. The apparatus of any preceding claim wherein the even order non-linear optical process is second harmonic generation.

6. The apparatus of any preceding claim wherein the interference module comprises a non-linear crystal disposed in a path of the primary radiation.

7. The apparatus of any preceding claim wherein the illumination system comprises projection optics operable to form a first image of a pattern on the beam spot region with the primary and tertiary radiation.

8. The apparatus of claim 7 wherein the projection optics comprises: a projection patterning device; and first imaging optics arranged to form an image of the projection patterning device on the beam spot region using the primary and tertiary radiation.

9. The apparatus of any preceding claim wherein the detection optics is operable to receive the reflected portion of the secondary radiation and to split the reflected radiation into first and second portions, and wherein the at least one detector comprises: a first detector arranged to determine an intensity of the first portion of the radiation; and a second detector arranged to determine an intensity of the second portion of the radiation; and wherein the processor is operable to determine a height profile of the object from the intensity of the first portion of the radiation and the intensity of the second portion of the radiation.

10. The apparatus of any preceding claim wherein the detection optics comprises: splitting optics arranged to split the reflected portion of the secondary radiation into first and second portions; and second imaging optics arranged to receive the reflected portion of the secondary radiation and to form a second image of the pattern on the splitting optics.

11. The apparatus of any preceding claim wherein the detection optics comprises a filter configured to minimize transmission of scattered radiation.

12. A method of measuring a topography of a surface of an object, the method comprising: projecting primary radiation onto an object disposed in a beam spot region so as to generate secondary radiation from a surface of the object via an even order non-linear optical process, wherein the secondary radiation has a different wavelength to the primary radiation and wherein the secondary radiation comprises a reflected portion that is reflected from the object and a transmitted portion that propagates into the object; producing tertiary radiation having substantially the same wavelength as the secondary radiation and projecting tertiary radiation onto the object disposed in the beam spot region such that the tertiary radiation at least partially destructively interferes with the transmitted portion of the secondary radiation; receiving the reflected portion of the secondary radiation; and determining a height of the object from the reflected portion of the secondary radiation.

13. The method of claim 12 wherein the primary radiation comprises a first wavelength.

14. The method of claim 12 or 13 wherein the even order non-linear optical process is a second order non-linear optical process.

15. The method of any one of claims 12 to 14 further comprising a calibration process of matching the tertiary radiation with the transmitted portion of the secondary radiation such that they at least partially destructively interfere with each other.

Images