Measuring system and diffraction grating thereof

Abstract

The present disclosure provides a measurement system (shear interferometer) for determining an aberration map of a projection system (e.g., within a lithographic apparatus). The measurement system comprises: a first patterned device positionable in an object plane of the projection system; a second patterned device positionable in an image plane of the projection system; and a radiation detector arranged to receive radiation from the second patterned device. The second patterned device has a variable pitch such that an angular spacing between adjacent pairs of diffracted beams formed by the second patterned device is non-uniform. The pitch of the second patterned device may have an extremum at a first location on the second patterned device, and the pitch of the second patterned device may increase or decrease with a distance from the first location.

Description

Cross-references to related applications

[0001] This application claims priority to European Patent Application No. 23199378.3, filed on September 25, 2023, the entire contents of which are incorporated herein by reference. Technical Field

[0002] This invention relates to a measurement system for determining an aberration map of a projection system. In particular, the measurement system may be a phase-stepping measurement system. The projection system may be part of a photolithography apparatus. The invention also relates to a photolithography apparatus that includes such a measurement system for determining an aberration map of a projection system. Furthermore, the invention relates to a diffraction grating for such a measurement system to determine the aberration map of the projection system. Specifically, the invention relates to a two-dimensional diffraction grating for shearing a phase-stepping interferometry system, the diffraction grating being formed from an array of circular apertures or pinholes in a substrate. Background Technology

[0003] A photolithography apparatus is a machine configured to apply a desired pattern onto a substrate. For example, a photolithography apparatus can be used in the fabrication of integrated circuits (ICs). For example, a photolithography apparatus can project a pattern onto a layer of radiation-sensitive material (resist) provided on a substrate, such as a patterned device (e.g., a mask).

[0004] To project a pattern onto a substrate, a photolithography apparatus can use electromagnetic radiation. The wavelength of this radiation determines the minimum size of the feature that can be formed on the substrate. Photolithography apparatuses using extreme ultraviolet (EUV) radiation with wavelengths in the range of 4 nm to 20 nm (e.g., 6.7 nm or 13.5 nm) can be used to form features on a substrate that are smaller than those using radiation with a wavelength of, for example, 193 nm.

[0005] Radiation patterned by a patterning device is focused onto a substrate using a projection system. The projection system may introduce optical aberrations that cause the image formed on the substrate to deviate from the desired image (e.g., a diffraction-limited image of the patterning device).

[0006] It is desirable to provide methods and apparatus for accurately determining such aberrations caused by the projection system, so that these aberrations can be better controlled. Summary of the Invention

[0007] According to a first aspect of this disclosure, a measurement system for determining an aberration map of a projection system is provided, the measurement system comprising: a first patterning device capable of being positioned in an object plane of the projection system; a second patterning device capable of being positioned in an image plane of the projection system; a radiation detector arranged to receive radiation from the second patterning device; and a positioning device operable to move at least one of the first and second patterning devices in a shear direction; wherein the second patterning device has a variable pitch such that the angular spacing between adjacent diffraction beam pairs of the same order diffraction beams originating from the first patterning device formed by the second patterning device is [missing information]. The separation is non-uniform; and wherein, taking into account any reduction or magnification factor applied by the projection system, the pitch of the first patterning device and the second patterning device are matched such that the angular spacing between the zero-order diffraction beam and the first-order diffraction beam of the first patterning device is approximately matched with the angular spacing between the zero-order diffraction beam and the first-order diffraction beam of the second patterning device originating from the same-order diffraction beam of the first patterning device; and wherein, taking into account any reduction or magnification factor applied by the projection system, the variable pitch of the second patterning device is such that the angular spacing between other adjacent diffraction beam pairs of the second patterning device is not matched with the angular spacing between the zero-order diffraction beam and the first-order diffraction beam of the first patterning device.

[0008] As now discussed, the measurement system according to the first aspect of this disclosure is advantageous.

[0009] A measurement system can be used to measure aberrations caused by a projection system. Such a measurement system can be used within a photolithography apparatus and may include a shearing interferometer. The measurement system typically uses a mask-level patterning device (i.e., disposed in the object plane of the photolithography apparatus) and a wafer-level sensor (i.e., disposed in the image plane of the photolithography apparatus). The mask-level patterning device may, for example, include a diffraction grating. The wafer-level sensor includes a second patterning device (e.g., a two-dimensional diffraction grating) and a radiation detector (e.g., an array of sensing elements). The mask-level patterning device is irradiated with radiation to form a plurality of first diffraction beams, which are separated in a shearing direction. The projection system at least partially captures the plurality of first diffraction beams and images them onto the patterning device of the wafer-level sensor. The second patterning device receives the first diffraction beams from the projection system and forms a plurality of second diffraction beams from each of the first diffraction beams incident on the radiation detector.

[0010] In use, at least one of the mask-level patterning device and the wafer-level sensor device is scanned or stepped in the shear direction, such that the radiation intensity received by each part of the radiation detector varies with this movement in the shear direction, thereby forming an oscillating signal. The phase of the harmonics (e.g., the first harmonic) of the oscillating signal at multiple locations on the radiation detector (e.g., at each sensing element) is determined. Thus, a set of Zernike coefficients characterizing the aberration map of the projection system is determined.

[0011] In some known arrangements, taking into account any reduction factor applied by the projection system, the pitches of the first patterning device and the second patterning device are matched in the shear direction such that the pitch of the second patterned region in the shear direction is an integer multiple of the pitch of the first patterned region in the shear direction, or alternatively, the pitch of the first patterned region in the shear direction is an integer multiple of the pitch of the second patterned region in the shear direction.

[0012] With this known arrangement, the harmonics of the oscillating signal at the radiation detector typically have multiple distinct contributions, each with a different overlay with the detector (i.e., coinciding with a different set of sensing elements) and generally different intensities. In transmission deep ultraviolet (DUV) lithography apparatuses, a known arrangement uses a one-dimensional diffraction grating as a mask-level patterning device with a 50% duty cycle and a checkerboard grating as a wafer-level patterning device. With this arrangement, across most of the radiation detector area, the first harmonic of the oscillating (phase-stepped) signal at the radiation detector receives only two contributions of equal intensity. This greatly simplifies the reconstruction of the aberration map.

[0013] However, true checkerboard gratings require a transmission support layer, which is unsuitable for extreme ultraviolet (EUV) lithography systems because (a) EUV radiation is strongly absorbed by most materials, and (b) such a transmission support would be rapidly contaminated in the wafer fabrication environment of an EUV lithography system, rendering it nontransmissive to EUV. For these reasons, it is difficult to implement checkerboard grating arrangements for lithography systems using EUV radiation, and existing arrangements typically use wafer-level pinhole arrays instead of checkerboard gratings.

[0014] For wafer-level patterned devices, using pinhole array geometry generates unwanted interference beams that can cause first harmonics in the phase-stepped signal. Specifically, in addition to the two contributions from the checkerboard grating, there are numerous small but non-zero additional interference beams at the radiation detector. Furthermore, each of these contributions has a different overlap with the radiation detector, for example, with the circular portion of the radiation detector corresponding to the numerical aperture of the projection system.

[0015] The intensity of the additional interference beam contributing to the first harmonic of the oscillating (phase-stepped) signal is relatively small (compared to the intensity of the two contributions present when using a checkerboard). Therefore, first-type known aberration measurement systems for EUV radiation neglect these terms (i.e., assume they are zero) when reconstructing the wavefront to find a set of Zernike coefficients. This assumption affects the accuracy of wavefront measurements. Consequently, this negatively impacts the system imaging, overlay, and focusing performance of the lithography apparatus. Embodiments of this disclosure are designed to at least partially address the aforementioned problems of aberration measurement systems for EUV radiation.

[0016] Using the measurement system according to the first aspect, taking into account any reduction or magnification factors applied by the projection system, the pitches of the first and second patterned devices are matched such that the angular spacing between the zero-order and first-order diffraction beams of the first patterned device approximately matches the angular spacing between the zero-order and first-order diffraction beams of the second patterned device. Therefore, as with one-dimensional diffraction gratings using mask-level patterned devices with a 50% duty cycle and checkerboard gratings using wafer-level patterned devices, the same two contributions exist at the radiation detector to the first harmonic of the oscillating (phase-stepped) signal.

[0017] Furthermore, the second patterning device has a variable pitch, resulting in non-uniform angular spacing between adjacent diffraction beam pairs formed by the second patterning device. Moreover, considering any reduction or magnification factors applied by the projection system, the variation in the variable pitch of the second patterning device causes the angular spacing between other adjacent diffraction beam pairs of the second patterning device to mismatch with the angular spacing between the zero-order and first-order diffraction beams of the first patterning device. Therefore, advantageously, the additional contribution of the first harmonic to the oscillating (phase-stepped) signal at the radiation detector is eliminated or at least reduced. In particular, unwanted overlap between coherent diffraction beams (which could thus cause interference patterns at the radiation detector) is disturbed.

[0018] In use, the first patterning device can be configured to receive a radiation beam and form a plurality of first diffraction beams. In use, the projection system can be configured to project the first diffraction beams formed by the first patterning device onto a second patterning device. In use, the second patterning device can be configured to receive the first diffraction beams from the projection system and form a plurality of second diffraction beams from each first diffraction beam.

[0019] The pitch of the second patterning device can have an extreme value at a first position on the second patterning device, and the pitch of the second patterning device can be increased or decreased depending on the distance from the first position.

[0020] For example, the extreme value can be a minimum value, and the pitch of the second patterning device can increase with distance from the first position. Alternatively, the extreme value can be a maximum value, and the pitch of the second patterning device can decrease with distance from the first position.

[0021] The first position can, but is not necessarily, located at the center of the second patterning device.

[0022] The pitch of the second patterning device can vary linearly with the distance from the first position.

[0023] Alternatively, the pitch of the second patterning device can vary non-linearly with distance from the first position. Typically, the pitch of the second patterning device can be a monotonic function of the distance from the first position.

[0024] The pitch of the second patterning device can vary by at least 2%.

[0025] In some embodiments, the pitch of the second patterning device may vary by at least 3% on the second patterning device. In some embodiments, the pitch of the second patterning device may vary by 4% or more.

[0026] The second patterning device can be a two-dimensional diffraction grating.

[0027] The second patterning device can be a transmission grating. Alternatively, the second patterning device can be a pinhole grating.

[0028] The second patterned device may include a substrate having a two-dimensional array of circular through-holes.

[0029] The distance between the centers of adjacent through-holes can be referred to as the local pitch of the second patterned device.

[0030] The duty cycle of the second patterning device can also be variable. That is, for a pinhole grating, the ratio of the radius of the circular aperture to the distance between the centers of adjacent apertures can vary across the second patterning device. For example, multiple apertures can all be of fixed size, and the pitch (and duty cycle) can be varied by changing the distance between the centers of adjacent apertures across the entire second patterning device.

[0031] Typically, a wafer-level diffraction grating used as a measurement system for determining the aberration map of a projection system in a photolithography apparatus comprises multiple apertures distributed such that the distance between the centers of adjacent apertures is uniform across the diffraction grating. This is achieved by arranging the apertures such that the center of each circular through-aperture coincides with a position in a square position array. In contrast, a second patterned device according to the first aspect can be formed by arranging apertures such that the distance between the centers of adjacent apertures varies across the second patterned device.

[0032] The second patterning device may include at least 10 cells across that dimension or each dimension of the pattern of the second patterning device.

[0033] In some embodiments, the second patterning device may include about 3 to 100 unit cells across each dimension. For example, the second patterning device may include about 30 unit cells across each dimension.

[0034] The second patterning device and the radiation detector can together form a sensor device. The measurement system may further include: a controller configured to: control a positioning device to move at least one of the first patterning device and the sensor device in a shear direction, such that the radiation intensity received by each part of the radiation detector varies with the movement in the shear direction, thereby forming an oscillating signal; determine the phase of the harmonics of the oscillating signal at a plurality of locations on the radiation detector from the radiation detector; and determine a set of coefficients characterizing an aberration map of the projection system based on the phase of the harmonics of the oscillating signal at the plurality of locations on the radiation detector.

[0035] According to a second aspect of this disclosure, a photolithography apparatus is provided, the photolithography apparatus including a measurement system according to any one of the preceding claims.

[0036] According to a third aspect of this disclosure, a diffraction grating is provided for a phase-stepping measurement system for determining an aberration map of a projection system, wherein the pitch of the diffraction grating has an extremum at a first position on the diffraction grating, and wherein the pitch of the diffraction grating increases or decreases depending on the distance from the first position.

[0037] For example, the extremum can be a minimum value, and the pitch of the diffraction grating can be increased according to the distance from the first position. Alternatively, the extremum can be a maximum value, and the pitch of the diffraction grating can be decreased with the distance from the first position.

[0038] The first position can be, but is not necessarily, at the center of the diffraction grating.

[0039] Advantageously, using such a diffraction grating, the variation in pitch causes the angular spacing between adjacent diffraction beam pairs formed by the second patterning device to be non-uniform. Specifically, the variation in pitch causes the angular spacing between adjacent diffraction beam pairs formed by the second patterning device to increase or decrease for higher-order diffraction beams. This diffraction grating can be used as a second patterning device in a measurement system according to the first aspect of this disclosure.

[0040] In other words, the pitch of this diffraction grating can be matched with the pitch of the first patterned device (taking into account any reduction or magnification factors applied by the projection system), such that the angular interval between the zero-order and first-order diffraction beams of the first patterned device approximately matches the angular interval between the zero-order and first-order diffraction beams of the diffraction grating according to the third aspect. For example, this can be achieved by matching the extreme value of the diffraction grating pitch with the pitch of the first patterned device (taking into account any reduction or magnification factors applied by the projection system). That is, the extreme value of the diffraction grating pitch in the shear direction can be an integer multiple of the pitch of the first patterned region in that shear direction, or alternatively, the pitch of the first patterned region in the shear direction can be an integer multiple of the extreme value of the diffraction grating pitch in the shear direction.

[0041] Furthermore, variations in the pitch of the diffraction grating can cause the angular spacing between other adjacent diffraction beam pairs of the diffraction grating to mismatch with the angular spacing between the zero-order and first-order diffraction beams of the first patterning device (considering any reduction or magnification factors applied by the projection system).

[0042] In use, the diffraction grating can form the measurement system according to the first aspect of this disclosure. Therefore, for the reasons discussed above with reference to the measurement system according to the first aspect of this disclosure, the diffraction grating according to the third aspect of this disclosure is further advantageous.

[0043] The pitch of the diffraction grating can vary linearly with the distance from the first position.

[0044] Alternatively, the pitch of the diffraction grating device can vary non-linearly with the distance from the first position. Typically, the pitch of the diffraction grating can be a monotonic function of the distance from the first position.

[0045] The pitch of the diffraction grating can vary by at least 2% on the diffraction grating.

[0046] In some embodiments, the pitch of the diffraction grating may vary by at least 3%. In some embodiments, the pitch of the diffraction grating may vary by 4% or more.

[0047] A diffraction grating can be a two-dimensional diffraction grating.

[0048] A diffraction grating can be a transmission grating or a pinhole grating.

[0049] A diffraction grating may span that dimension of the diffraction grating pattern or include at least 10 unit cells per dimension.

[0050] In some embodiments, a diffraction grating may comprise approximately 3 to 100 unit cells across each dimension. For example, a diffraction grating may comprise approximately 30 unit cells across each dimension.

[0051] Diffraction gratings can be self-supporting.

[0052] Since the diffraction grating is self-supporting, no transmission support layer is required. This arrangement is particularly advantageous for use in phase-stepping measurement systems used to determine the aberration map of a projection system using EUV radiation, because using such a transmission support layer will significantly reduce the amount of EUV radiation transmitted through the two-dimensional diffraction grating.

[0053] A diffraction grating may include a substrate having a two-dimensional array of circular through-holes.

[0054] The distance between the centers of adjacent through-apertures can be referred to as the local pitch of a diffraction grating.

[0055] The duty cycle of a diffraction grating can also be variable. That is, for a pinhole grating, the ratio of the radius of the circular aperture to the distance between the centers of adjacent apertures can be varied on the second patterned device. For example, multiple apertures can all be of fixed size, and the pitch (and duty cycle) can be varied by changing the distance between the centers of adjacent apertures across the diffraction grating.

[0056] Typically, a wafer-level diffraction grating used as a measurement system for determining the aberration map of a projection system in a photolithography apparatus comprises multiple apertures distributed such that the distance between the centers of adjacent apertures is uniform across the diffraction grating. This is achieved by arranging the apertures such that the center of each circular through-aperture coincides with a position in a square position array. In contrast, a diffraction grating according to the third aspect can be formed by arranging the apertures such that the distance between the centers of adjacent apertures varies across the diffraction grating.

[0057] The substrate may include a radiation-absorbing layer, and the through-hole may extend through the radiation-absorbing layer.

[0058] The radiation-absorbing layer can be formed, for example, of aluminum nitride (AlN). Alternatively, the radiation-absorbing layer can be formed of a metal such as, for example, chromium (Cr), nickel (Ni), or cobalt (Co).

[0059] In some embodiments, the substrate may further include a support layer adjacent to the radiation-absorbing layer, and the through-hole may extend through both the support layer and the radiation-absorbing layer.

[0060] The support layer can be formed, for example, from SiN. The radiation-absorbing layer can be formed, for example, from a metal such as chromium (Cr), nickel (Ni), or cobalt (Co). Attached Figure Description

[0061] Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings, in which: - Figure 1 A lithography system, including a lithography apparatus and a radiation source, is described; - Figure 2 This is a schematic diagram of a measurement system according to an embodiment of the present invention; - Figure 3A and Figure 3B It can be formed Figure 2 A schematic diagram of a patterned device and sensor assembly, which is part of a measurement system; - Figure 4 This is a schematic diagram of a measurement system according to an embodiment of the present invention. The measurement system includes a first patterned region and a second patterned region. The first patterned region is arranged to receive radiation and form a plurality of first diffraction beams. - Figures 5A to 5C The following are respectively shown by Figure 4 The measurement system shown has a second set of different second diffraction beams formed by the second patterned region, which are generated by different first diffraction beams formed by the first patterned region. - Figure 6A The scattering efficiency of a one-dimensional diffraction grating with a 50% duty cycle is shown, and this one-dimensional diffraction grating can represent... Figure 4 The first patterned region of the measurement system shown; - Figure 6B The scattering efficiency of a two-dimensional diffraction grating in checkerboard form with a 50% duty cycle is shown, and this two-dimensional diffraction grating can represent... Figure 4 The second patterned region of the measurement system shown; - Figure 6C It shows when using Figure 6A The first patterned area shown and Figure 6B When the second patterned region is shown, Figure 4 The diagram shows the interferometric intensity of the measurement system. Each interferometric intensity represents a second interferometric beam that contributes to the first harmonic of the oscillating phase-stepped signal and has different overlap with the circle representing the numerical aperture of the projection system PS at the radiation detector. - Figure 7A , Figure 7B and Figure 7C It shows Figure 4 The numerical aperture portion of the projection system of the measurement system shown is composed of... Figure 4 The three different first diffraction beams shown are filled; - Figures 8A to 8C It shows Figure 4 The illustrated measurement system includes a portion of a radiation detector corresponding to the numerical aperture of the projection system, and this portion is filled with three second diffraction beams originating from... Figure 7BThe first diffraction beam represented; - Figures 9A to 9C It shows Figure 4 The portion of the radiation detector of the measurement system shown corresponds to the numerical aperture of the projection system of the measurement system and is filled with three second diffraction beams originating from... Figure 7A The first diffraction beam represented; - Figures 10A to 10C It shows Figure 4 The portion of the radiation detector of the measurement system shown corresponds to the numerical aperture of the projection system of the measurement system and is filled with three second diffraction beams originating from... Figure 7C The first diffraction beam represented; - Figure 11A It shows Figure 4 The portion shown is a part of the radiation detector of the measurement system, which corresponds to the numerical aperture of the projection system of the measurement system, and represents... Figure 8B and Figure 9A The overlap between the second diffraction beams shown and Figure 8A and Figure 10B The overlap between the second diffraction beams is shown; - Figure 11B It shows Figure 4 The portion shown is a part of the radiation detector of the measurement system, which corresponds to the numerical aperture of the projection system of the measurement system, and represents... Figure 8B and Figure 10C The overlap between the second diffraction beams shown and Figure 8C and Figure 9B The overlap between the first and second diffraction beams is shown. - Figure 12 The cell of a grating comprising a circular pinhole array and having a 50% duty cycle (by area) is shown; - Figure 13A The scattering efficiency of a one-dimensional diffraction grating with a 50% duty cycle is shown, and this one-dimensional diffraction grating can represent... Figure 4 The first patterned region of the measurement system shown; - Figure 13B It shows including Figure 12 The scattering efficiency of two-dimensional diffraction of the unit cell can be expressed as... Figure 4 The second patterned region of the measurement system shown; - Figure 13C It shows when using Figure 13A The first patterned area shown and Figure 13B When the second patterned region is shown, Figure 4The diagram shows the interferometric intensity of the measurement system. Each interferometric intensity represents a second interferometric beam that contributes to the first harmonic of the oscillating phase-stepped signal and has different overlap with the circle representing the numerical aperture of the projection system at the radiation detector. - Figure 14A It is composed of such Figure 12 The representation shown depicts 21 second diffraction beams generated by the second patterned region of the unit cell, which correspond to... Figure 13B The diffraction efficiency is contained within the white dashed line in the image. - Figure 14B This represents 16 interference beams, each consisting of... Figure 14A The interference generation of paired second diffraction beams is shown, where each interference beam corresponds to... Figure 13C The interference intensity contained within the white dashed line in the image; - Figure 15A This is a schematic diagram of a set of second diffraction beams generated by a second patterning device, which have a uniform pitch with each of the first diffraction beams (e.g., ...). Figure 4 (as shown) Figure 15A Equivalent to Figures 5A to 5C The combination; - Figure 15B This is a schematic diagram of a set of second diffraction beams generated by a second patterning device according to an embodiment of the present disclosure, the second diffraction beams having a non-uniform pitch (e.g., ...) with each of the first diffraction beams in the first diffraction beams. Figure 4 (as shown) - Figure 16A This is a schematic diagram of a first example diffraction grating with non-uniform pitch according to an embodiment of the present disclosure. The first example diffraction grating can be used as a second (wafer-level) patterning device for a measurement system for determining aberrations. The first diffraction grating is a two-dimensional pinhole array grating. - Figure 16B This is a schematic diagram of a second example diffraction grating with a non-uniform pitch. The second example diffraction grating is a linear (one-dimensional) transmission diffraction grating. - Figure 17 It shows Figure 16B The diffraction spectrum of a linear grating of the type shown, but with a uniform pitch, exhibits a change with diffraction order. The intensity of the changing diffraction pattern, where the diffraction order... Indicates the scattering angle ,in ,in For uniform pitch, and The wavelength is the radiation wavelength; the diffraction spectrum 90 shows three cases: (a) a grating consisting of a single unit cell; (b) a grating consisting of N=30 unit cells; and (c) a grating consisting of an infinite number of unit cells; - Figure 18A It shows Figure 16B The diffraction spectrum of a linear diffraction grating of the type shown, wherein the pitch of the grating increases from the center of the grating and has a 4% pitch variation across the grating; the diffraction spectrum shows the variation with diffraction order. The intensity of the changing diffraction pattern, where the diffraction order... Indicates the scattering angle ,in ,in This is the minimum pitch of the diffraction grating; the diffraction spectra are shown again in three cases 92: (a) a grating comprising a single unit cell; (b) a grating comprising N=30 unit cells; and (c) a grating comprising an infinite number of unit cells; and - Figure 18B It shows Figure 18A The magnified portion of the diffraction spectrum is shown. Detailed Implementation

[0062] Figure 1 A lithography system including a radiation source SO and a lithography apparatus LA is shown. The radiation source SO is configured to generate an EUV radiation beam B and provide the EUV radiation beam B to the lithography apparatus LA. The lithography apparatus LA includes an irradiation system IL, a support structure MT configured to support a patterned device MA (e.g., a mask), a projection system PS, and a substrate stage WT configured to support a substrate W.

[0063] The illumination system IL is configured to modulate the EUV radiation beam B before it is incident on the patterned device MA. Furthermore, the illumination system IL may include a faceted field mirror device 10 and a faceted pupil mirror device 11. Together, the faceted field mirror device 10 and the faceted pupil mirror device 11 provide the desired cross-sectional shape and intensity distribution for the EUV radiation beam B. In addition to the faceted field mirror device 10 and the faceted pupil mirror device 11, the illumination system IL may also include other mirrors or devices, or may include other mirrors or devices in place of the faceted field mirror device and the faceted pupil mirror device.

[0064] After such adjustment, the EUV radiation beam B interacts with the patterned device MA. This interaction generates a patterned EUV radiation beam B'. A projection system PS is configured to project the patterned EUV radiation beam B' onto the substrate W. For this purpose, the projection system PS may include multiple mirrors 13, 14 configured to project the patterned EUV radiation beam B' onto the substrate W held by the substrate stage WT. The projection system PS can apply a reduction factor to the patterned EUV radiation beam B', thereby forming an image with features smaller than the corresponding features on the patterned device MA. For example, a reduction factor of 4 or 8 can be applied. Although the projection system PS... Figure 1 The projection system PS is shown as having only two mirrors 13 and 14, but the projection system PS may include a different number of mirrors (e.g., six or eight mirrors).

[0065] The substrate W may include a previously formed pattern. In this case, the photolithography apparatus LA aligns the image formed by the patterned EUV radiation beam B' with the pattern previously formed on the substrate W.

[0066] A relative vacuum, i.e., a small amount of gas (e.g., hydrogen) at a pressure much lower than atmospheric pressure, can be provided in the radiation source SO, the irradiation system IL, and / or the projection system PS.

[0067] The radiation source SO can be a laser-generated plasma (LPP) source, a discharge-generated plasma (DPP) source, a free-electron laser (FEL) or any other radiation source capable of generating EUV radiation.

[0068] Generally, the projection system PS has a potentially non-uniform optical transfer function, which affects the pattern imaged on the substrate W. For unpolarized radiation, this effect can be well described by two scalar maps that describe the transmittance (apodization) and relative phase (aberration) of radiation leaving the projection system PS as functions of its position in the pupil plane. These scalar maps, which can be called transmittance maps and relative phase maps, can be represented as linear combinations of a complete set of basis functions. A particularly convenient set is the Zernike polynomials, which form a set of orthogonal polynomials defined on the unit circle. Determining each scalar map may involve determining the coefficients in this expansion. Since the Zernike polynomials are orthogonal on the unit circle, the Zernike coefficients can be obtained from the measurement scalar map by successively calculating the inner product of the measured scalar map with each Zernike polynomial and dividing that inner product by the square of the norm of that Zernike polynomial. In the following text, unless otherwise stated, any reference to Zernike coefficients will be understood to refer to the Zernike coefficients of the relative phase map (also referred to herein as the aberration map). It should be understood that other sets of basis functions may be used in alternative embodiments. For example, some embodiments may use Tatian-Zernike polynomials, such as for fuzzy aperture systems.

[0069] A wavefront aberration map represents the wavefront distortion of light from a point on the image plane of the projection system PS approaching from a spherical wavefront (this distortion is a function of the position in the pupil plane, or alternatively a function of the angle at which the radiation approaches the image plane of the projection system PS). As discussed, this wavefront aberration map... It can be represented as a linear combination of Zernike polynomials: (1) in and These are the coordinates in the pupil plane. It is the nth Zernike polynomial, and These are coefficients. It should be understood that in the following text, the Zernike polynomial and coefficients are labeled with an exponent, which is often referred to as the Noll index. Therefore, It is a Zernike polynomial with Noor exponent n, and These are coefficients with a Noel exponent of n. Then, the wavefront aberration map can be constructed using a set of coefficients in this expansion. These coefficients can be characterized as Zernike coefficients.

[0070] It should be understood that only a finite number of Zernike orders are considered. Different Zernike coefficients in the phase diagram can provide information about different forms of aberrations caused by the projection system PS. A Zernike coefficient with a Noll exponent of 1 can be called the first Zernike coefficient, a Zernike coefficient with a Noll exponent of 2 can be called the second Zernike coefficient, and so on.

[0071] The first Zernike coefficient relates to the mean value of the measured wavefront (which may be referred to as the piston term). The first Zernike coefficient may be unrelated to the performance of the projection system PS and therefore may not be able to be determined using the methods described herein. The second Zernike coefficient relates to the tilt of the measured wavefront in the x-direction. The tilt of the wavefront in the x-direction corresponds to its placement in the x-direction. The third Zernike coefficient relates to the tilt of the measured wavefront in the y-direction. The tilt of the wavefront in the y-direction corresponds to its placement in the y-direction. The fourth Zernike coefficient relates to the defocus of the measured wavefront. The fourth Zernike coefficient corresponds to its placement in the z-direction. Higher-order Zernike coefficients relate to other forms of aberrations caused by the projection system (e.g., astigmatism, coma, spherical aberration, and other effects).

[0072] Throughout this specification, the term "aberration" should be intended to include all forms of deviation from a perfect spherical wavefront. That is, the term "aberration" can relate to image placement (e.g., second, third, and fourth Zernike coefficients) and / or higher-order aberrations, such as aberrations related to Zernike coefficients with a Noll index of 5 or greater. Furthermore, any reference to aberration diagrams of the projection system can include all forms of deviation from a perfect spherical wavefront, including deviations due to image placement.

[0073] Transmission maps and relative phase maps depend on the field and the system. That is, in general, each projection system PS will have a different Zernike expansion for each field point (i.e., for each spatial location in its image plane).

[0074] As will be described in further detail below, the relative phase of the projection system PS in its pupil plane can be determined by projecting radiation from the object plane of the projection system PS (i.e., the plane of the patterned device MA) through the projection system PS and measuring the wavefront (i.e., the trajectory of points with the same phase) using a shearing interferometer. The shearing interferometer may include a diffraction grating, such as a two-dimensional diffraction grating, in the image plane of the projection system (i.e., the substrate stage WT), and a detector arranged to detect the interference pattern in a plane conjugate with the pupil plane of the projection system PS.

[0075] The projection system PS includes multiple optical elements (including mirrors 13 and 14). As already explained, although the projection system PS... Figure 1The image is shown with only two mirrors 13 and 14, but the projection system PS can include a different number of mirrors (e.g., six or eight mirrors). The lithography apparatus LA also includes an adjustment component PA for adjusting these optical elements to correct for aberrations (any type of phase change across the pupil plane throughout the field). To achieve this, the adjustment component PA can be operated to manipulate the optical elements within the projection system PS in one or more different ways. The projection system can have a coordinate system in which the optical axis of the projection system extends in the z-direction (understandably, the direction of this z-axis varies along the optical path through the projection system, e.g., at each mirror or optical element). The adjustment component PA can be used to perform any combination of the following: displacing one or more optical elements; tilting one or more optical elements; and / or deforming one or more optical elements. Displacement of the optical elements can be in any direction (x, y, z, or a combination thereof). Tilting of the optical elements is typically achieved by rotating about an axis in the x or y direction away from a plane perpendicular to the optical axis, but rotation about the z-axis can be used for non-rotationally symmetric optical elements. Deformation of optical elements can be performed, for example, by applying force to the sides of the optical element using an actuator and / or by heating a selected area of ​​the optical element using a heating element. Generally, the projection system PS cannot be adjusted to correct apodization (transmission variation across the pupil plane). When designing a mask MA for a lithography apparatus LA, the transmission map of the projection system PS can be used.

[0076] In some embodiments, the adjusting component PA is operable to move the support structure MT and / or the substrate stage WT. The adjusting component PA is operable to displace the support structure MT and / or the substrate stage WT (in any direction or combination thereof in the x, y, z directions) and / or tilt (by rotating about an axis in the x or y direction).

[0077] The projection system PS, which forms part of a lithography apparatus, can undergo periodic calibration processes. For example, when the lithography apparatus is manufactured in a factory, an initial calibration process can be performed to set up the optical elements (e.g., mirrors) that form the projection system PS. After the lithography apparatus is installed at the location where it will be used, the projection system PS can be calibrated again. Further calibration of the projection system PS can be performed at regular intervals. For example, under normal use, the projection system PS can be calibrated every few months (e.g., every three months).

[0078] Calibrling a projection system PS may involve passing radiation through the projection system PS and measuring the resulting projected radiation. The measurement of the projected radiation can be used to determine aberrations in the projected radiation caused by the projection system PS. Aberrations caused by the projection system PS can be determined using a measurement system. In response to the determined aberrations, the optical elements forming the projection system PS can be adjusted to correct the aberrations caused by the projection system PS.

[0079] Figure 2 This is a schematic diagram of a measurement system 12, which can be used to determine aberrations caused by a projection system PS. Measurement system 12 includes an illumination system IL, a measurement patterning device MA', a sensor device 21, and a controller CN. Measurement system 12 can be formed as part of a photolithography apparatus. For example, Figure 2 The irradiation system IL and projection system PS shown can be Figure 1 The photolithography apparatus shown includes an illumination system IL and a projection system PS. For ease of explanation, Figure 2 Additional components of the photolithography apparatus are not shown.

[0080] The measuring patterning device MA' is arranged to receive radiation from the irradiation system IL. The sensor device 21 is arranged to receive radiation from the projection system PS. During normal operation of the photolithography apparatus, Figure 2 The measurement patterning device MA' and sensor device 21 shown can be located in relation to... Figure 2 The different positions are shown. For example, during normal use of the photolithography apparatus, a patterning device MA configured to form a pattern to be transferred to a substrate W can be positioned to receive radiation from an illumination system IL, and the substrate W can be positioned to receive radiation from a projection system PS (e.g., as shown). Figure 1 (As shown). The measuring patterning device MA' and the sensor device 21 can be moved to their positions. Figure 2 The location shown is used to determine the aberrations caused by the projection system PS. The measuring patterning device MA' can be supported by a support structure MT, such as... Figure 1 The support structure is shown. The sensor device 21 can be constructed from a substrate stage (such as...). Figure 1 The substrate stage (WT) shown is used for support. Alternatively, the sensor device 21 may be supported by a measurement stage (not shown), which may be separate from the sensor stage (WT).

[0081] Figure 3A and Figure 3B The measurement patterning device MA' and sensor device 21 are shown in more detail below. Figure 2 , Figure 3A and Figure 3B Cartesian coordinates are used consistently in this system. Figure 3A This is a schematic diagram of measuring the patterned device MA' in the xy plane, and Figure 3B This is a schematic diagram of sensor device 21 in the xy plane.

[0082] The measuring patterned device MA' includes multiple patterned regions 15a to 15c. Figure 2 and Figure 3AIn the illustrated embodiment, the measuring patterning device MA' is a reflective patterning device MA'. Patterned regions 15a to 15c each include a reflective diffraction grating. Radiation incident on the patterned regions 15a to 15c of the measuring patterning device MA' is at least partially scattered by it and received by the projection system PS. In contrast, radiation incident on the remaining portion of the measuring patterning device MA' is not reflected or scattered towards the projection system PS (e.g., it may be absorbed by the measuring patterning device MA').

[0083] The illumination system IL uses radiation to illuminate the measurement patterning device MA'. Although Figure 2 Not shown, but the illumination system IL can receive radiation from the radiation source SO and modulate that radiation to illuminate the measurement patterning device MA'. For example, the illumination system IL can adjust the radiation to provide radiation with the desired spatial and angular distribution. Figure 2 In the illustrated embodiment, the illumination system IL is configured to form individual measurement beams 17a to 17c. Each measurement beam 17a to 17c illuminates a corresponding patterned region 15a to 15c of the measurement patterning device MA'.

[0084] To perform the determination of aberrations caused by the projection system PL, the mode of the illumination system IL can be changed so that the measurement patterned device MA' is illuminated with separate measurement beams 17a to 17c. For example, during normal operation of the lithography apparatus, the illumination system IL can be configured to illuminate the patterned device MA with a radiation slit. However, the mode of the illumination system IL can be changed such that the illumination system IL is configured to form separate measurement beams 17a to 17c in order to perform the determination of aberrations caused by the projection system PL. In some embodiments, different patterned regions 15a to 15c can be illuminated at different times. For example, a first subset of the patterned regions 15a to 15c can be illuminated at a first time to form a first subset of the measurement beams 17a to 17c, while a second subset of the patterned regions 15a to 15c can be illuminated at a second time to form a second subset of the measurement beams 17a to 17c.

[0085] In other embodiments, the mode of the illumination system IL can remain unchanged in order to perform the determination of aberrations caused by the projection system PS. For example, the illumination system IL can be configured to illuminate the measurement patterning device MA' with a radiation slit (e.g., the radiation slit substantially corresponds to the illumination area used during substrate exposure). The measurement patterning device MA' can then form separate measurement beams 17a to 17c, since only the patterned regions 15a to 15c reflect or scatter radiation toward the projection system PS.

[0086] In the figure, the Cartesian coordinate system is shown as being maintained by the projection system PS. However, in some embodiments, the characteristics of the projection system PS may lead to transformations of the coordinate system. For example, the projection system PS may form an image of the measuring patterned device MA', which is magnified, rotated, and / or mirrored relative to the measuring patterned device MA'. In some embodiments, the projection system PS may rotate the image of the measuring patterned device MA' about approximately 180° around the z-axis. In this embodiment, Figure 2 The relative positions of the first measurement beam 17a and the third measurement beam 17c shown can be interchanged. In other embodiments, the image can be mirrored around an axis located in the xy plane. For example, the image can be mirrored around the x-axis or around the y-axis.

[0087] In embodiments where the projection system PS rotates the image measured by the patterning device MA' and / or the image is mirrored by the projection system PS, it is assumed that the projection system transforms the coordinate system. That is, the coordinate system referred to herein is defined relative to the image projected by the projection system PS, and any rotation and / or mirroring of the image results in a corresponding rotation and / or mirroring of the coordinate system. For ease of illustration, the coordinate system is shown in the figures as being maintained by the projection system PS. However, in some embodiments, the coordinate system may be transformed by the projection system PS.

[0088] Patterned regions 15a to 15c modify measurement beams 17a to 17c. Specifically, patterned region 15a in 15c induces spatial modulation of measurement beams 17a to 17c and causes diffraction within measurement beams 17a to 17c. Figure 3B In the illustrated embodiment, patterned regions 15a to 15c each comprise two distinct portions. For example, the first patterned region 15a comprises a first portion 15a' and a second portion 15a'". The first portion 15a' includes a diffraction grating aligned parallel to the u-direction, and the second portion 15a' includes a grating aligned parallel to the v-direction. The u-direction and v-direction are as follows: Figure 3A As depicted. Both the u and v directions are aligned at approximately 45° relative to the x and y directions, and are perpendicular to each other. Figure 3A The second patterned region 15b and the third patterned region 15c shown are the same as the first patterned region 15a, and each includes a first portion and a second portion of the diffraction gratings that are perpendicularly aligned with each other.

[0089] The first and second portions of the patterned regions 15a to 15c can be illuminated by the measurement beams 17a to 17c at different times. For example, the first portion of each patterned region 15a to 15c can be illuminated by the measurement beams 17a to 17c at a first time. At a second time, the second portion of each patterned region 15a to 15c can be illuminated by the measurement beams 17a to 17c. As described above, in some embodiments, different patterned regions 15a to 15c can be illuminated at different times. For example, the first portion of a first subset of patterned regions 15a to 15c can be illuminated at a first time, and the second portion of a second subset of patterned regions 15a to 15c can be illuminated at a second time. The first subset and the second portion of the second subset of patterned regions can be illuminated at the same or different times. Generally, any schedule for illuminating different portions of the patterned regions 15a to 15c can be used.

[0090] The modified measurement beams 17a to 17c are received by the projection system PS. The projection system PS forms an image of patterned regions 15a to 15c on the sensor device 21. The sensor device 21 includes a plurality of diffraction gratings 19a to 19c and a radiation detector 23. The diffraction gratings 19a to 19c are arranged such that each diffraction grating 19a to 19c receives a corresponding modified measurement beam 17a to 17c output from the projection system PL. The modified measurement beams 17a to 17c incident on the diffraction gratings 19a to 19c are further modified by the diffraction gratings 19a to 19c. The modified measurement beams transmitted at the diffraction gratings 19a to 19c are incident on the radiation detector 23.

[0091] Radiation detector 23 is configured to detect the spatial intensity distribution of radiation incident on it. For example, radiation detector 23 may include an array of single detector elements or sensing elements. For example, radiation detector 23 may include an active pixel sensor, such as a CMOS (Complementary Metal-Oxide-Semiconductor) sensor array. Alternatively, radiation detector 23 may include a CCD (Charge-Coupled Device) sensor array. Diffraction gratings 19a to 19c and portions of radiation sensor 23 receiving modified measurement beams 17a to 17c form detector regions 25a to 25c. For example, the first diffraction grating 19a and a first portion of radiation sensor 23 receiving the first measurement beam 17a together form a first detector region 25a. Measurements can be performed on a given measurement beam 17a to 17c at the respective detector regions 25a to 25c (as shown). As described above, in some embodiments, the modified measurement beams 17a to 17c and the relative positioning of the coordinate system can be transformed by a projection system PS.

[0092] Modifications to the measurement beams 17a to 17c occurring in the patterned regions 15a to 15c of the detector regions 25a to 25c and the diffraction gratings 19a to 19c result in the formation of an interference pattern on the radiation detector 23. The interference pattern is related to the derivative of the phase of the measurement beam and depends on the aberrations caused by the projection system PS. Therefore, the interference pattern can be used to determine the aberrations caused by the projection system PS.

[0093] Typically, the diffraction gratings 19a to 19c of each detector region 25a to 25c include a two-dimensional transmission diffraction grating. Figure 3B In the illustrated embodiment, detector regions 25a to 25c each include diffraction gratings 19a to 19c arranged in a checkerboard pattern. As further described below, embodiments of this disclosure are particularly applicable to arrangements in which detector regions 25a to 25c each include two-dimensional transmission diffraction gratings 19a to 19c that are not arranged in a checkerboard pattern.

[0094] Irradiation of the first portion of the patterned regions 15a to 15c can provide information related to aberrations in the first direction, and illumination of the second portion of the patterned regions 15a to 15c can provide information related to distortions in the second direction.

[0095] In some embodiments, the measuring patterning device MA' and / or sensor device 21 are sequentially scanned and / or stepped in two vertical directions. For example, the measuring patterning device MA' and / or sensor device 21 may step relative to each other in the u and v directions. When the second portions 15a” to 15c” of the patterned regions 15a to 15c are illuminated, the measuring patterning device MA' and / or sensor device 21 may step in the u direction, while when the first portions 15a’ to 15c’ of the patterned regions 15a to 15c are illuminated, the measuring patterning device MA' and / or sensor device 21 may step in the v direction. That is, the measuring patterning device MA' and / or sensor device 21 may step in a direction perpendicular to the alignment of the illuminated diffraction grating.

[0096] The measuring patterning device MA' and / or sensor device 21 can be stepped at a distance corresponding to a fraction of the grating period of the diffraction grating. Measurements taken at different step positions can be analyzed to derive information about the wavefront in the step direction. For example, the phase of the first harmonic of the measured signal (which may be called the phase-stepped signal) can contain information about the derivative of the wavefront in the step direction. Therefore, stepping the measuring patterning device MA' and / or sensor device 21 in both the u and v directions (perpendicular to each other) allows information about the wavefront to be derived in both perpendicular directions (in particular, it provides information about the derivative of the wavefront in each of the two perpendicular directions), thereby allowing the reconstruction of the entire wavefront.

[0097] In addition to stepping the patterned device MA' and / or sensor device 21 in a direction perpendicular to the alignment of the irradiated diffraction grating (as described above), the patterned device MA' and / or sensor device 21 can also be scanned relative to each other. The scanning of the patterned device MA' and / or sensor device 21 can be performed in a direction parallel to the alignment of the irradiated diffraction grating. For example, the patterned device MA' and / or sensor device 21 can be scanned in the u direction while irradiating the first portion 15a' to 15c of the patterned regions 15a to 15c, and the patterned device MA' and / or sensor device 21 can be scanned in the v direction while irradiating the second portion 15a” to 15c” of the patterned regions 15a to 15c. Scanning the patterned device MA' and / or sensor device 21 in a direction parallel to the alignment of the irradiated diffraction grating allows for the averaging of measurements across the diffraction grating, thereby compensating for any variations in the diffraction grating in the scanning direction. The scanning of the measurement patterning device MA' and / or sensor device 21 can be performed at a time different from the stepping of the aforementioned measurement patterning device MA' and / or sensor device 21.

[0098] It should be understood that various different arrangements of patterned regions 15a to 15c and detector regions 25a to 25c can be used to determine aberrations caused by the projection system PS. Patterned regions 15a to 15c and / or detector regions 25a to 25c may include diffraction gratings. In some embodiments, patterned regions 15a to 15c and / or detector regions 25a to 25c may include components other than diffraction gratings. For example, in some embodiments, patterned regions 15a to 15c and / or detector regions may include a single slit or pinhole opening through which at least a portion of the measurement beam 17a to 17c can propagate. Typically, the patterned regions and / or detector regions may include any arrangement for modifying the measurement beam.

[0099] The controller CN receives measurements taken at the sensor device 21 and determines the aberrations caused by the projection system PS based on the measurements. The controller can be configured to control one or more components of the measurement system 12. For example, the controller CN can control a positioning device PW operable to move the sensor device 21 and / or the measuring patterning device MA' relative to each other. The controller can control adjustment components PA used to adjust components of the projection system PS. For example, the adjustment components PA can adjust the optics of the projection system PS to correct for aberrations caused by the projection system PS, and these aberrations are determined by the controller CN.

[0100] In some embodiments, the controller CN is operable to control the adjustment component PA for adjusting the support structure MT and / or the substrate stage WT. For example, the adjustment component PA can adjust the support structure MT and / or the substrate stage WT to correct aberrations caused by placement errors of the patterned device MA and / or the substrate W (and these aberrations are determined by the controller CN).

[0101] Determining aberrations (which may be caused by placement errors in the projection system PS, patterned device MA, or substrate W) may include fitting measurements taken by sensor device 21 to a Zernike polynomial to obtain Zernike coefficients. Different Zernike coefficients can provide information about different forms of aberrations caused by the projection system PS. Zernike coefficients can be determined independently at different locations in the x and / or y directions. For example, in Figure 2 , Figure 3A and Figure 3B In the illustrated embodiment, Zernike coefficients can be determined for each measurement bundle 17a to 17c.

[0102] In some embodiments, the measuring patterning device MA' may include more than three patterned regions, the sensor device 21 may include more than three detector regions, and more than three measurement bundles may be formed. This allows the Zernike coefficients to be determined at more locations. In some embodiments, the patterned regions and detector regions may be distributed at different locations in the x and y directions. This allows the Zernike coefficients to be determined at locations separate in both the x and y directions.

[0103] Although Figure 2 , Figure 3A and Figure 3B In the illustrated embodiment, the measuring patterning device MA' includes three patterned regions 15a to 15c, and the sensor device 21 includes three detector regions 25a to 25c. However, in other embodiments, the measuring patterning device MA' may include more or fewer than three patterned regions 15a to 15c, and / or the sensor device 21 may include more or fewer than three detector regions 25a to 25c.

[0104] Now for reference Figure 4 This describes a method for determining aberrations caused by the projection system PS.

[0105] Typically, the measuring patterning device MA' includes at least one first patterned region 15a to 15c, and the sensor device 21 includes at least one second patterned region 19a to 19c.

[0106] Figure 4 This is a schematic diagram of measurement system 30, which can be used to determine aberrations caused by the projection system PS. Measurement system 30 can be connected to... Figure 2The measurement system 12 shown is the same; however, the measurement system 30 may have different numbers of first patterned regions (on the measuring patterning device MA') and second patterned regions (in the sensor device 21). Therefore, Figure 4 The measurement system 30 shown may include the above-mentioned Figure 2 Any characteristics of the measurement system 12 shown, which will not be further described below.

[0107] exist Figure 4 In this process, only a single first patterned region 31 is provided on the measuring patterning device MA', and a single second patterned region 32 is provided in the sensor device 21.

[0108] The measuring patterned device MA' is irradiated by radiation 33 from the irradiation system IL. For ease of understanding, Figure 4 Only a single line is shown (e.g., it could represent a single ray of the incident radiation beam, such as the main ray). However, it should be understood that radiation 33 will encompass the angular range incident on the first patterned region 31 of the measuring patterning device MA'. That is, each point on the first patterned region 31 of the measuring patterning device MA' can be illuminated by a light cone. Typically, each point is illuminated by substantially the same angular range, characterized by the radiation intensity in the pupil plane of the illumination system IL (not shown).

[0109] The first patterned region 31 is arranged to receive radiation 33 and form a plurality of first diffraction beams 34, 35, and 36. The central first diffraction beam 35 corresponds to the 0th order diffraction beam of the first patterned region 31, and the other two first diffraction beams 34 and 36 correspond to the ±1st order diffraction beams of the first patterned region 31. It should be understood that there are usually more and higher order diffraction beams as well. Similarly, for ease of understanding, Figure 4 Only three first diffraction beams, 34, 35, and 36, are shown in the image.

[0110] It should also be understood that, since the incident radiation 33 includes a radiation cone that converges at a point on the first patterned region 31, each of the first diffraction beams 34, 35, 36 also includes a radiation cone that diverges from that point on the first patterned region 31.

[0111] To generate the first diffraction beams 34, 35, and 36, the first patterned region 31 can be in the form of a diffraction grating. For example, the first patterned region 31 can typically be... Figure 3A The patterned region 15a is shown in the form of the patterned region shown. In particular, at least a portion of the first patterned region 31 may be... Figure 3A The first portion 15a' of the patterned region 15a shown is in the form of a diffraction grating aligned parallel to the u direction (note, Figure 4(Shown in the zv plane). Therefore, the first diffraction beams 34 to 36 are separated in the shear direction (i.e., the v direction).

[0112] As now described, the first diffracted beams 34 to 36 are at least partially captured by the projection system PS. How much of the first diffracted beams 34 to 36 are captured by the projection system PS will depend on: the pupil filling of the incident radiation 33 from the illumination system IL; the angular spacing of the first diffracted beams 34 to 36 (which in turn depends on the pitch of the first patterned region 31 and the wavelength of the radiation 33); and the numerical aperture of the projection system PS.

[0113] The measurement system 30 can be arranged such that the first diffraction beam 35 corresponding to the 0th order diffraction beam substantially fills the numerical aperture of the projection system PS, which can be represented by a circular region of the pupil plane 37 of the projection system PS, and the first diffraction beams 34, 36 corresponding to the ±1st order diffraction beams significantly overlap with the first diffraction beam 35 corresponding to the 0th order diffraction beam. With this arrangement, substantially all of the first diffraction beams in the first diffraction beam 35 corresponding to the 0th order diffraction beam and most of the first diffraction beams in the first diffraction beams 34, 36 corresponding to the ±1st order diffraction beams are captured by the projection system PS and projected onto the sensor device 21. (Furthermore, with this arrangement, a large number of diffraction beams generated by the first patterned region 31 are at least partially projected onto the sensor device 21).

[0114] As discussed now, the role of the first patterned region 31 is to introduce spatial coherence.

[0115] Typically, two radiation rays 33 from the irradiation system IL, incident at different incident angles on the same point of the measuring patterning device MA', are incoherent. By receiving the radiation 33 and forming multiple first diffraction beams 34, 35, 36, the first patterned region 31 can be considered to form multiple replica beams of the incident radiation cone 33 (these replica beams typically have different phases and intensities). Within any of these replica beams or the first diffraction beams 34, 35, 36, two radiation rays originating from the same point on the measuring patterning device MA' but at different scattering angles are incoherent (due to the characteristics of the irradiation system IL). However, for a given radiation ray within any of the first diffraction beams 34, 35, 36, there exists a corresponding radiation ray spatially coherent with that given ray in each of the other first diffraction beams 34, 35, 36. For example, the principal ray of each of the first diffraction beams 34, 35, 36 (which corresponds to the principal ray of the incident radiation 33) is coherent and, if combined, may interfere at the amplitude level.

[0116] The measurement system 30 uses this coherence to determine the aberration map of the projection system PS.

[0117] The projection system PS projects a portion of the first diffraction beams 34, 35, and 36 (captured by the numerical aperture of the projection system) onto the sensor device 21.

[0118] exist Figure 4 In this embodiment, sensor device 21 includes a single second patterned region 32. As further described below (see reference...) Figures 5A to 5C The second patterned region 32 is arranged to receive the first diffraction beams 34 to 36 from the projection system PS, and to form a plurality of second diffraction beams from each of the first diffraction beams. To achieve this, the second patterned region 32 includes a two-dimensional transmission diffraction grating. Figure 4 In this diagram, all radiation transmitted through the second patterned region 32 is represented by a single arrow 38. This radiation 38 is received by the detector region 39 of the radiation detector 23 and used to determine the aberration map.

[0119] Each of the first diffraction beams 34 to 36 incident on the patterned region 32 will diffract to form a plurality of second diffraction beams. Since the second patterned region 32 comprises a two-dimensional diffraction grating, a two-dimensional array of secondary diffraction beams is generated from each incident first diffraction beam (the principal rays of these secondary diffraction beams are separated in both the shear direction (v direction) and the direction perpendicular to it (u direction). In the following text, the n-th order diffraction order in the shear direction (v direction) and the m-th order diffraction order in the non-shear direction (u direction) will be referred to as the (n, m)-th order diffraction order of the second patterned region 32. In the following text, when the order of the second diffraction beam in the non-shear direction (u direction) is not important, the (n, m)-th order diffraction order of the second patterned region 32 can be simply referred to as the n-th order second diffraction beam.

[0120] Figures 5A to 5C A set of second diffraction beams generated by each of the first diffraction beams 34 to 36 is shown. Figure 5A A set of second diffraction beams 35a to 35e generated by a first diffraction beam 35 is shown, the first diffraction beam 35 corresponding to the 0th order diffraction beam of the first patterned region 31. Figure 5B A set of second diffraction beams 36a to 36e generated by a first diffraction beam 36 is shown, the first diffraction beam 36 corresponding to the -1st order diffraction beam of the first patterned region 31. Figure 5C A set of second diffraction beams 34a to 34e generated by a first diffraction beam 34 is shown, the first diffraction beam 34 corresponding to the +1 order diffraction beam of the first patterned region 31.

[0121] exist Figure 5A In this context, the second diffraction beam 35a corresponds to the 0th order diffraction beam (of the second patterned region 32 in the shear direction), while the second diffraction beams 35b and 35c correspond to ±1st order diffraction beams, and the second diffraction beams 35d and 35e correspond to ±2nd order diffraction beams. It should be understood that... Figures 5A to 5C Shown in the vz plane, the second diffraction beam shown can, for example, correspond to the 0th order diffraction beam of the second patterned region 32 in the non-shear direction (i.e., the u direction). It should also be understood that multiple replica beams will exist among these second diffraction beams, representing entry or exit. Figures 5A to 5C The higher-order diffraction beams in the non-shearing direction of the page.

[0122] exist Figure 5B In the second diffraction beam 36a, the second diffraction beam corresponds to the 0th order diffraction beam (of the second patterned region 32 in the shear direction), while the second diffraction beams 36b and 36c correspond to the ±1st order diffraction beams and the second diffraction beams 36d and 36e correspond to the ±2nd order diffraction beams.

[0123] exist Figure 5C In the second diffraction beam 34a, the second diffraction beam corresponds to the 0th order diffraction beam (of the second patterned region 32 in the shear direction), while the second diffraction beams 34b and 34c correspond to the ±1st order diffraction beams and the second diffraction beams 34d and 34e correspond to the ±2nd order diffraction beams.

[0124] from Figures 5A to 5C As can be seen, several second diffraction beams in the second diffraction beam overlap each other in space. For example, the second diffraction beam 35b, which corresponds to the -1st order diffraction beam of the 0th order diffraction beam 35 originating from the first patterned region 31 and corresponds to the second patterned region 32, overlaps with the 0th order diffraction beam 36a, which corresponds to the -1st order diffraction beam 36 originating from the first patterned region 31 and corresponds to the second patterned region 32. Figure 4 and Figures 5A to 5C All lines can be considered to represent a single radiation ray originating from a single input ray 33 from the illumination system IL. Therefore, as described above, these lines represent spatially coherent rays that, if spatially overlapping at the radiation detector 23, will produce an interference pattern. Furthermore, interference occurs between rays passing through different portions of the pupil plane 37 of the projection system PS (separated in the shear direction). Therefore, the interference of radiation originating from a single input ray 33 depends on the phase difference between the two different portions of the pupil plane.

[0125] In known arrangements, this spatial overlap and spatial coherence of the second diffraction beams at the radiation detector 23 is achieved by matching the first patterned region 31 and the second patterned region 32, such that the angular spacing (in the shear direction) between different second diffraction beams originating from a given first diffraction beam is the same as the angular spacing (in the shear direction) between different first diffraction beams when they converge on the second patterned region 32. Typically, this spatial overlap and spatial coherence of the second diffraction beams at the radiation detector 23 is achieved by matching the pitch of the first patterned region 31 and the second patterned region 32 in the shear direction. It should be understood that this matching of the pitch of the first patterned region 31 and the second patterned region 32 in the shear direction takes into account any reduction factor imposed by the projection system PS. As used herein, the pitch of a two-dimensional diffraction grating in a particular direction is defined as follows.

[0126] It should be understood that a one-dimensional diffraction grating comprises a series of lines formed by repeating patterns of reflectivity or transmittance in directions perpendicular to these lines. In the direction perpendicular to the lines, the smallest non-repeating segment forming the repeating pattern is called a unit cell, and the length of this unit cell is called the pitch of the one-dimensional diffraction grating. Typically, such a one-dimensional diffraction grating will have a diffraction pattern such that the incident radiation beam will be diffracted, thereby forming a one-dimensional array of angularly spaced (but possibly spatially overlapping) diffracted beams. The first patterned region 31 forms a one-dimensional array of angularly spaced first diffracted beams 34 to 36 offset in the shear direction (angularly spaced).

[0127] It should be understood that a two-dimensional diffraction grating comprises a two-dimensional repeating pattern of reflectivity or transmittance. The smallest non-repeating segment forming this repeating pattern can be called a unit cell. The unit cell can be square, and the fundamental pitch of such a two-dimensional diffraction grating can be defined as the length of the square unit cell. Typically, such a two-dimensional diffraction grating will have a diffraction pattern such that the incident radiation beam will be diffracted, thereby forming a two-dimensional array of diffracted beams that are angularly spaced (but may overlap spatially). The axis of this two-dimensional (square) array of diffracted beams is parallel to the sides of the unit cell. The angular spacing between adjacent diffracted beams in these two directions can be given by the ratio of the wavelength of radiation to the pitch of the grating. Therefore, the smaller the pitch, the larger the angular spacing between adjacent diffracted beams.

[0128] In some embodiments, the axis of the unit cell of the two-dimensional second patterned region 32 may be arranged at a non-zero angle with the shear direction and non-shear direction defined by the first patterned region 31. For example, the axis of the unit cell of the two-dimensional second patterned region 32 may be arranged at 45° with the shear direction and non-shear direction defined by the first patterned region 31. As previously described, by ensuring that the angular spacing (in the shear direction) between different second diffraction beams originating from a given first diffraction beam is the same as the angular spacing (in the shear direction) between different first diffraction beams when they converge on the second patterned region 32, spatial overlap and spatial coherence of the second diffraction beams at the radiation detector 23 are achieved, thereby allowing wavefront measurement. For the arrangement where the axis of the unit cell of the two-dimensional second patterned region 32 is arranged at a non-zero angle (e.g., 45°) with the shear direction and non-shear direction, the following definition of pseudo-cell and pseudo-pitch may be useful. A pseudo-cell is defined as the smallest non-repeating square forming the repeating pattern of the diffraction grating, and the pseudo-cell is oriented such that its sides are parallel to the shear direction and non-shear direction (as defined by the first patterned region 31). The pseudo-pitch can be defined as the length of the square pseudo-cell. This can be referred to as the pitch of the two-dimensional diffraction grating in the shear direction. It is precisely this pseudo-pitch that should match (an integer multiple or a fraction) the pitch of the first patterned region 31.

[0129] The diffraction pattern of a diffraction grating can be considered as a two-dimensional array of pseudo-diffraction beams spaced angularly (but possibly overlapping spatially), the axes of which are parallel to the sides of the pseudo-cell. Since this square is not a cell (a cell is defined as the smallest square of any orientation that forms the repeating pattern of the diffraction grating), the pseudo-pitch will be larger than the pitch (or fundamental pitch). Therefore, in the diffraction pattern (in the direction parallel to the sides of the pseudo-cell), the pitch between adjacent pseudo-diffraction beams will be smaller than the pitch between adjacent diffraction beams in the diffraction pattern (in the direction parallel to the sides of the cell). This can be understood as follows: some pseudo-diffraction beams correspond to diffraction beams in the diffraction pattern, and others are non-physical and do not represent diffraction beams generated by the diffraction grating (and are only produced by using pseudo-cells larger than the real cell).

[0130] Taking into account any reduction (or magnification) factor applied by the projection system PS, the pitch of the second patterned region 32 in the shear direction should be an integer multiple of the pitch of the first patterned region 31 in the shear direction, or the pitch of the first patterned region 31 in the shear direction should be an integer multiple of the pitch of the second patterned region 32 in the shear direction. Figures 5A to 5C In the example shown, the pitch of the first patterned region 31 and the second patterned region 32 is substantially equal in the shear direction (considering any scaling factor).

[0131] As from Figures 5A to 5CAs can be seen, each point on detector region 39 of radiation detector 23 typically receives several coherently summed contributions. For example, a point on detector region 39 receiving a second diffraction beam 35b corresponding to a -1st order diffraction beam originating from the 0th order diffraction beam 35 of the first patterned region 31 in the second patterned region 32 overlaps with: (a) a second diffraction beam 36a corresponding to a 0th order diffraction beam originating from the -1st order diffraction beam 36 of the first patterned region 31 in the second patterned region 32; and (b) a second diffraction beam 34d corresponding to a -2nd order diffraction beam originating from the +1st order diffraction beam 34 of the first patterned region 31 in the second patterned region 32. It should be understood that when considering the higher-order diffraction beams of the first patterned region 31, more beams will be coherently summed at each point on detector region 39 to determine the intensity of radiation measured by that portion of detector region 39 (e.g., a corresponding pixel in a two-dimensional array of sensing elements).

[0132] Typically, multiple different second diffraction beams contribute to the radiation received by each portion of detector region 39. The intensity of the radiation from this coherent sum is given by the following equation: (2) Here, DC is a constant term (which corresponds to the incoherent sum of different diffraction beams), and this sum is the sum of all different pairs of second diffraction beams. It is the interference intensity of the second diffraction beam pair, and It is the phase difference between the third diffraction beam pairs.

[0133] The phase difference between a pair of second diffracted beams depends on two contributions: (a) the first contribution relates to different portions of the pupil plane 37 of the projection system PS from which they originate; and (b) the second contribution relates to the position within the unit cell of each of the first patterned region 31 and the second patterned region 32 from which they originate.

[0134] The first of these contributions can be understood as arising from the fact that different coherent radiation beams have passed through different parts of the projection system PS, and are therefore related to the aberrations that need to be determined (in fact, they are related to the difference between two points separated in the shear direction in the aberration map).

[0135] The second contribution can be understood as arising from the fact that the relative phase of the multiple radiation rays produced by a single ray incident on the diffraction grating will depend on which part of the grating's unit cell the ray is incident on. Therefore, this does not contain information related to aberrations. As described above, in some embodiments, the measuring patterning device MA' and / or sensor device 21 are sequentially scanned and / or stepped in the shear direction. This causes a change in the phase difference between all pairs of interfering radiation beams received by the radiation detector 23. Typically, the phase difference between the second diffraction beam pairs changes when the amount by which the measuring patterning device MA' and / or sensor device 21 is sequentially stepped in the shear direction is equal to a fraction of the pitch (in the shear direction) of the first patterned region 31 and the second patterned region 32. If the amount by which the measuring patterning device MA' and / or sensor device 21 is stepped in the shear direction is equal to an integer multiple of the pitch (in the shear direction) of the first patterned region 31 and the second patterned region 32, the phase difference between the second diffraction beam pairs remains constant. Therefore, when the patterned device MA' and / or sensor device 21 are sequentially scanned and / or stepped in the shear direction, the intensity received by each part of the radiation detector 23 will oscillate. The first harmonic of this oscillating signal measured by the radiation detector 23 (which may be referred to as the phase-stepped signal) depends on the contribution of the adjacent first diffraction beams 34 to 36 (i.e., the first diffraction beams with a phase difference of ±1) to equation (1). Due to this phase-stepping technique, the contributions generated by the first diffraction beams with different phase differences will contribute to the higher harmonics of the signal determined by the radiation detector 23.

[0136] For example, in the three overlapping second diffraction beams (35b, 36a, and 34d) mentioned above, only two of the three possible pairs of these diffraction beams contribute to the first harmonic of the phase-stepped signal: (a) second diffraction beams 35b and 36a (originating from the 0th-order diffraction beam 35 and the -1st-order diffraction beam 36 of the first patterned region 31, respectively); and (b) second diffraction beams 35a and 34d (originating from the 0th-order diffraction beam 35 and the +1st-order diffraction beam 34 of the first patterned region 31, respectively).

[0137] Each pair of second diffraction beams will produce an interference term of the form shown in equation (2), which contributes to the first harmonic of the phase-stepped signal, i.e., an interference term of the form shown below: (3) in It is the magnitude of the interference term. It is the pitch (in the shear direction) of the first patterned region 31 and the second patterned region 32. The relative positions of the first patterned region 31 and the second patterned region 32 in the shearing direction are parameterized, and It is the difference between the aberration values ​​at two locations in the pupil plane of the projection system PS, corresponding to the origins of the two second diffraction beams. The amplitude of the interference term. The frequency of the first harmonic of the phase-stepped signal is proportional to the product of the recombination scattering efficiency of the two second diffracted beams, as discussed further below. The frequency of the first harmonic of the phase-stepped signal is determined by the pitch of the first patterned region 31 and the second patterned region 32 in the shear direction. The reciprocal of is given. The phase of the phase-stepped signal is given by . (The difference between the values ​​of the aberration maps at two locations in the pupil plane of the projection system PS, which correspond to the locations of the origins of the two second diffraction beams) is given.

[0138] As discussed now, the interference intensity of a pair of second diffraction beams It is proportional to the product of the recombination scattering efficiency of the two second diffraction beams.

[0139] Typically, the scattering efficiency of the diffracted beams generated by a diffraction grating will depend on the geometry of the grating. These diffraction efficiencies can be normalized to the efficiency of the 0th-order diffracted beam, which describes the relative intensity of the diffracted beams. As used herein, the recombination scattering efficiency of the second diffracted beam is given by the product of the scattering efficiency of the first diffracted beam from which it originates and the scattering efficiency of the diffraction order of its corresponding second patterned region 32.

[0140] exist Figures 3A to 5C In the above description of the illustrated embodiments, in Figure 3A When the first portion 15a' of the patterned region 15a shown is illuminated, the shear direction corresponds to the v direction, and the non-shear direction corresponds to the u direction. It should be understood that when... Figure 3A When the second portion 15a” of the patterned region 15a shown is irradiated, the shearing direction corresponds to the u direction, and the non-shearing direction corresponds to the v direction. Although in these above embodiments, both the u direction and the v direction (which define the two shearing directions) are aligned at approximately 45° relative to the x and y directions of the lithography apparatus LA, it is understood that in alternative embodiments, these two shearing directions can be arranged at any angle relative to the x and y directions of the lithography apparatus LA (which can correspond to the non-scanning and scanning directions of the lithography apparatus LA). Generally, the two shearing directions will be perpendicular to each other. Hereinafter, the two shearing directions will be referred to as the x direction and the y direction. However, it should be understood that these shearing directions can be arranged at any angle relative to the x and y directions of the lithography apparatus LA.

[0141] Figure 6A The scattering efficiency of the first patterned region 31 is shown. The first patterned region 31 is Figure 3AThe first portion 15a' of the patterned region 15a shown has a 50% duty cycle. The horizontal axis represents the diffraction order in the shear direction. Figure 6A The diffraction efficiencies shown are normalized to the efficiency of the 0th-order diffraction beam, making the efficiency of the 0th-order diffraction beam 100%. Using this geometry (50% duty cycle), the efficiency of even-order diffraction beams (excluding the 0th-order diffraction beam) is zero. The efficiency of the ±1st-order diffraction beam is 63.7%.

[0142] Figure 6B The scattering efficiency of the second patterned region 32 is shown, the second patterned region 32 having Figure 3B The diffraction grating 19a shown is in the form of a checkerboard pattern with a 50% duty cycle. The horizontal axis represents the diffraction order in the shear direction. The vertical axis represents the diffraction order in the non-shear direction. Figure 6B The diffraction efficiency shown is normalized to the efficiency of the (0, 0) order diffraction beam, making the efficiency of the (0, 0) order diffraction beam 100%.

[0143] As mentioned above, the first harmonic of the oscillating phase-stepped signal depends only on the contribution of the first diffraction beam, with a phase difference of ±1, to equation (1). For example, from... Figure 6A As can be seen, with a 50% duty cycle grating on the patterned device MA', only two pairs of first diffraction beams differing in order by ±1 are either the 0th-order beam or either the +1st-order or -1st-order beam. Furthermore, for this geometry of the first patterned region 31, the scattering efficiency is symmetrical, such that the efficiency of the ±1st-order diffraction beams is the same (63.7%). Therefore, the interference intensity of all second diffraction beam pairs contributing to the first harmonic of the oscillating phase-stepped signal can be determined as follows. . Figure 6B The second copy of the scattering efficiency map of the second patterned region 32 shown is weighted by the scattering efficiency of the ±1st order diffracted beams of the first patterned region 31, and then compared with... Figure 6B The scattering efficiency map of the second patterned region 32 is overlaid, but offset in the shear direction by one pair of diffraction order intervals (of the first patterned region 31). Here, the pitch of the first patterned region 31 and the second patterned region 32 in the shear direction is equal (considering any reduction factor imposed by the projection system PS), and therefore, in this example, the second copy of the scattering efficiency map of the second patterned region 32 is offset in the shear direction by one diffraction order of the second patterned region 31. The product of the scattering efficiencies of the two overlaid scattering efficiency maps is then determined. Figure 6C The interference intensity of all second-order diffraction beam pairs contributing to the first harmonic of the oscillating phase-stepped signal is shown. picture.

[0144] Notice, Figure 6CEach interference intensity shown This actually represents two different pairs of second diffraction beams. For example, Figure 6C The left-hand pixel shown represents both: (a) the interference between the second diffraction beams 35a and 34b, and (b) the interference between the second diffraction beams 35b and 36a. Similarly, Figure 6C The right-hand pixel shown represents both of the following: (a) interference between second diffraction beams 35a and 36c, and (b) interference between second diffraction beams 35c and 34a. Typically, each pixel in this diagram represents two pairs of second diffraction beams: (a) a first pair of second diffraction beams, comprising one second diffraction beam originating from a first diffraction beam 35 corresponding to the 0th order diffraction order of the first patterning device 31 and another second diffraction beam originating from a first diffraction beam 34 corresponding to the +1st order diffraction order of the first patterned region 31; and (b) a second pair of second diffraction beams, comprising one second diffraction beam originating from a first diffraction beam 35 corresponding to the 0th order diffraction order of the first patterning device 31 and another second diffraction beam originating from a first diffraction beam 36 corresponding to the -1st order diffraction order of the first patterned region 31.

[0145] generally, Figure 6C Each interference intensity shown represents two distinct pairs of second diffraction beams: (a) a pair of second diffraction beams comprising an n-order second diffraction beam generated by the first diffraction beam 35 (which corresponds to the 0-order diffraction beam of the first patterned region 31); and (b) another pair of second diffraction beams comprising an (n+1)-order second diffraction beam generated by the first diffraction beam 35. Therefore, each interference intensity can be characterized by the two diffraction orders ((n, m) order and (n+1, m) order) of the contributing first diffraction beam 35, and can be expressed as... In the following text, when it is clear that m=0 or the value of m is unimportant, the interference intensity can be expressed as: .

[0146] although Figure 6C The interference intensity shown Each interference intensity (or ) represents two different pairs of second diffraction beams, but Figure 6C The interference intensity shown Each interference intensity in the diagram represents a second diffracted beam that contributes to the first harmonic of the oscillating phase-stepped signal, and the second diffracted beam has different overlaps at the radiation detector 23 with the circle representing the numerical aperture of the projection system PS, as now described.

[0147] Figure 7A , Figure 7B and Figure 7CA portion of the pupil plane 37 of the projection system PS is shown, corresponding to the numerical aperture of the projection system PS filled by the first diffraction beams 34, 35, and 36. Figure 7A , 7B In each of the figures in 7C, the numerical aperture of the projection system PS is represented by circle 40, and the portion of the pupil plane 37 of the projection system PS filled by the first diffraction beams 34, 35, and 36 is respectively represented by... Figure 7A , Figure 7B and Figure 7C The shaded area of ​​circle 40 is shown in the diagram. (As shown from...) Figure 7B As can be seen from the example shown, the central first diffraction beam 35, corresponding to the 0th order diffraction beam, essentially fills the numerical aperture of the projection system PS. (As can be seen from...) Figure 7A and Figure 7C As can be seen, each of the two first diffraction beams 34, 36 corresponding to the ±1st order diffraction beams of the first patterned region 31 is shifted so that they only partially fill the numerical aperture. It should be understood that this shift of the first order diffraction beams 34, 36 relative to the numerical aperture is actually very small and is exaggerated here for ease of understanding.

[0148] Figures 8A to 10C The diagram shows a portion of the radiation detector 23 filled with various second diffraction beams. Figures 8A to 10C In each of them, the numerical aperture of the projection system PS is represented by a circle 40, and the portion of the circle 40 filled by the second diffraction beam is shown through a shaded area. Figures 8A to 8C The portion of circle 40 is shown to be filled with diffraction beams 35b, 35a, and 35c of order (-1, 0), (0, 0), and (1, 0), which originate from the first diffraction beam 35 corresponding to the 0th order diffraction beam of the first patterned region 31. Figures 9A to 9C The portion of circle 40 is shown to be filled with diffraction beams 34b, 34a, and 34c of orders (-1, 0), (0, 0), and (1, 0), which originate from the first diffraction beam 34 corresponding to the first diffraction beam of the first patterned region 31. Figures 10A to 10C The portion of circle 40 is shown to be filled with (-1, 0), (0, 0) and (1, 0) order diffraction beams 36b, 36a, 36c, which originate from the first diffraction beam 36 corresponding to the -1 order diffraction beam of the first patterned region 31.

[0149] from Figure 8B , Figure 9A , Figure 8A and Figure 10B As can be seen, the area of ​​the radiation detector is Figure 11ARegion 41, as shown, receives contributions from both: (a) interference between the second diffraction beams 35a and 34b; and (b) interference between the second diffraction beams 35b and 36a. Similarly, from Figure 8B , Figure 10C , Figure 8C and Figure 9B As can be seen, the area of ​​the radiation detector is Figure 11B Region 42 shown receives contributions from both: (a) interference between the second diffraction beams 35a and 36c; and (b) interference between the second diffraction beams 35c and 34a.

[0150] generally, Figure 6C The interference intensity shown Each interference intensity can be considered to represent a radiation beam formed by multiple interfering second interference beams, each such radiation beam being formed by multiple interfering second interference beams propagating in different directions, such that the overlap of each such radiation beam at the radiation detector 23 with the circle representing the numerical aperture of the projection system PS is different.

[0151] Typically, the second diffracted beam can be considered to form multiple radiation beams, each of which is formed by a set of interfering second diffracted beams. Each such radiation beam may be referred to herein as an interference beam. Each such interference beam formed by the multiple interfering second interference beams can be considered to propagate in different directions, such that the overlap of each interference beam at the radiation detector 23 with the circle representing the numerical aperture of the projection system PS is different. Although they can be considered to propagate in different directions and have different overlaps with the circle representing the numerical aperture of the projection system PS, there is significant overlap between the different interference beams at the radiation detector 23. Figure 6C The interference intensity shown Each interference intensity can be considered to represent a different interference bundle (formed by a second interference bundle of multiple interferences).

[0152] As mentioned above, Figure 6C The interference intensity shown Each interference intensity (or The symbols () represent two distinct pairs of second diffraction beams. However, for a given position on the radiation detector, both pairs of contributing second diffraction beams comprise two interference rays originating from the same two points in the pupil plane 37 of the projection system PS. Specifically, for the position (x, y) on the radiation detector (these coordinates correspond to the coordinates of the pupil plane 37 of the projection system PS and the x-direction corresponding to the shear direction), there exists a contribution with interference intensity. The two pairs of interfering second diffraction beams each include a ray originating from the second diffraction beam at position (x-ns, y-ms) in the pupil plane 37 and a ray originating from the second diffraction beam at position (x-(n+1)s, y-ms) in the pupil plane 37, where s is the shear distance. The shear distance s corresponds to the distance between two coherent rays of adjacent first diffraction beams 34 to 36 in the pupil plane 37. Therefore, both pairs of contributing second diffraction beams produce an interference term of the form of expression (3), where It is the difference between the aberration values ​​at these two locations in the pupil plane 37.

[0153] from Figure 6C As can be seen, for the second patterned region 32 in the form of a checkerboard with a 50% duty cycle, only two sets of second diffraction beams contribute to the first harmonic of the phase-stepped signal, and the interference intensity of the two ( , Both were 25.8%. This is due to the geometry of the chessboard, such as... Figure 6A As can be seen, this results in a diffraction efficiency diagram where, except for the (-1, 0), (0, 0), and (1, 0) order diffraction beams that shift in the shear direction, the diffraction efficiency of every other diffraction beam is 0%. That is, except for the (0, 0) diffraction order, the grating efficiencies of the even-numbered (n, m) diffraction orders (n±m) are all zero. Since these grating efficiencies are zero, except for the interference intensity... and Furthermore, all interference intensities contributing to the first harmonic of the phase-stepped signal are zero.

[0154] The portion of the radiation detector 23 corresponding to the numerical aperture of the projection system PS (by...) Figures 7A to 11B The circle 40 in the diagram can be considered to comprise three segments, each distinguished by an interference beam that contributes to the first harmonic of the oscillating phase-stepped signal in these segments. These three segments can be considered as: the first crescent-shaped segment, possessing only interference intensity... The interference beam contributes to it; the second crescent-shaped portion only has interference intensity. The interference beam contributes to it; and the central part where the two interference beams work together. The first crescent segment is... Figure 11B The white portion shown; the second crescent segment is Figure 11A The white portion shown; and the central portion is the overlap between the two regions 41 and 42. The central portion, which is the overlap between the two regions 41 and 42, can be referred to as the three-beam region of the radiation detector 23.

[0155] for Figure 11A and Figure 11BThe overlap between the two regions 41 and 42 shown (for small shear angles, this overlap will form most of the circle 40) will cause the first harmonic of the oscillating phase-stepped signal to be proportional to the sum of the two cosines (see equation (2) and expression (3)): (4) The first cosine is the difference in the aberration map between the first two points in the pupil plane, and the second cosine is the difference in the aberration map between the second two points in the pupil plane (the phase step term is omitted here for clarity). Specifically, for a given position (x, y) on the radiation detector (x refers to the shear direction), the first two points include the corresponding points in the pupil plane (x, y) (as in equation (4)). (represented by) and another point offset by a shear distance (xs,y) in the first direction along the shear direction (as in equation (4) (as indicated). Similarly, the latter two points include the corresponding points in the pupil plane (x, y) (as in equation (4)). (represented by) and another point offset by a shear distance (x+s, y) in the second direction along the shear direction (as in equation (4)). (As indicated).

[0156] Existing wavefront reconstruction techniques utilize the fact that the two interference intensities in equation (4) are equal, allowing the sum of the two cosines to be rewritten using trigonometric identities as the cosine of half the difference between the aberration maps of two locations separated by twice the shear distance in the shear direction (i.e., This is then multiplied by a factor (approximately 1 for small shear distances). Therefore, this known technique involves determining a set of Zernike coefficients by making the phase of the first harmonic of the phase-stepped signal (within the overlap between the two regions 41, 42) equal to half the difference in aberration maps between positions in the pupil plane separated by twice the shear distance in the shear direction. Alternatively, in the first and second crescent segments, the phase of the first harmonic of the phase-stepped signal can be equal to the difference in aberration maps between positions in the pupil plane separated by the shear distance in the shear direction. However, as mentioned above, since... Figure 11A and Figure 11BThe overlap between the two regions 41 and 42 shown forms most of the circle 40 with a small shear angle, so some existing reconstruction algorithms only use the overlap between the two regions 41 and 42 (without using the first and second crescent segments of the radiation detector 23). Recall that the aberration map depends on the Zernike coefficients (see equation (1)). This is done for multiple locations on the radiation sensor (e.g., at multiple pixels in an array or at a single sensing element), first for the first shear direction and then for the second orthogonal direction. These constraints for the two shear orthogonal directions are solved simultaneously to obtain the set of Zernike coefficients.

[0157] As described above, the combination of a first patterned region 31 comprising a linear grating and a second patterned region 32 comprising a two-dimensional checkerboard is advantageous (because only two interference beams contribute to the first harmonic of the phase-stepped signal). Due to the geometry of the checkerboard, checkerboard gratings typically include a light-transmitting carrier or support layer. However, EUV radiation is strongly absorbed by most materials, and therefore there are currently no good light-transmitting materials for EUV radiation. Furthermore, such a light-transmitting carrier is disadvantageous in the wafer fabrication environment of an EUV lithography system because it is rapidly contaminated in such an environment. This will render the light-transmitting carrier nontransparent to EUV. This contamination problem can only be addressed through periodic cleaning operations, which affects system availability and, consequently, the throughput of the lithography system. For the reasons described above, it is difficult to implement a checkerboard grating arrangement for lithography systems using EUV radiation.

[0158] Therefore, existing EUV radiation aberration measurement systems use a geometry employing a circular pinhole array as a second patterning device 32. Figure 12 The cell 50 of this grating is shown, having a duty cycle of 50% (by area). Cell 50 includes a circular aperture 51 disposed in an EUV absorption film 52. The circular aperture 51 is a through-hole, representing a void in the EUV absorption film 52 through which EUV radiation is transmitted. To achieve a 50% (by area) duty cycle, the radius of aperture 51 is... Pitch with grating The ratio (i.e., the distance between the centers of adjacent apertures) is given by the following formula: (5) It is approximately 0.4. However, the geometry of this pinhole array (such as...) Figure 12 (As shown) This will produce unwanted interference beams, which will cause the first harmonic of the phase-stepped signal, as now referenced. Figures 13A to 13B The subject of discussion.

[0159] Figure 13A The scattering efficiency of the first patterned region 31 is shown. Figure 3AThe first portion 15a' of the patterned region 15a shown has a 50% duty cycle (with...). Figure 6A (The geometry shown is the same). Similarly, the diffraction efficiency is normalized to the efficiency of the 0th order diffraction beam, making the efficiency of the 0th order diffraction beam 100%. Figure 13B It shows having Figure 12 The scattering efficiency of the second patterned region 32 in the form of a pinhole array of the unit cell 50 shown. Figure 13B The diffraction efficiency shown is normalized to the efficiency of the (0, 0) order diffraction beam, making the efficiency of the (0, 0) order diffraction beam 100%.

[0160] Figure 13C The interference intensity of the interferometer beam that contributes to the first harmonic of the oscillating phase-stepped signal. Figure (This is based on) Figure 13A and Figure 13B Constructed based on scattering efficiency, and according to Figure 6A and Figure 6B scattering efficiency construction Figure 6C (Similar to the method).

[0161] from Figure 13C It can be seen from this that for those with Figure 12 The second patterned region 32 of the unit cell 50 shown, in addition to the two main interference beams (interference intensity) , Besides the 25.2% (which is 25.2%), there are also many with small but non-zero interference strengths. Additional interference beams. Furthermore, as now referenced... Figure 14A and Figure 14B The interference beams discussed have a different overlap at the radiation detector 23 than the circular portion of the radiation detector 23, which corresponds to the numerical aperture of the projection system PS.

[0162] Figure 14A It is composed of such Figure 12 The diagram shows a representation of 21 diffraction beams generated by the second patterned region 32 of the unit cell 50. In particular, these can be considered as second diffraction beams generated by this second patterned region 32 and originating from the 0th-order first diffraction beam 35. Figure 14A The diffraction efficiencies of these second diffraction beams are shown, and these diffraction efficiencies correspond to... Figure 13B The diffraction efficiency is contained within the dashed line. In fact, Figure 13B The dashed lines in the diagram correspond to 25 second diffraction beams; however, the diffraction efficiency of four of these second diffraction beams is negligible, and therefore they are not included. Figure 14A In addition, in Figure 14A In the diagram, the dashed circular line represents the region on the radiation detector 23 corresponding to the numerical aperture of the projection system PS.

[0163] Figure 14B This represents 16 interference beams, each consisting of... Figure 14A The pair of second diffraction beams shown are generated by interference with the second diffraction beams generated by the second patterned region 32 and originating from the ±1st order first diffraction beams 34, 36. Figure 14B The interference intensities of these interference beams are shown, and these interference intensities correspond to... Figure 13C The interference intensity contained within the dashed line. Similarly, Figure 13C The dashed lines in the diagram actually correspond to 20 different second diffraction beams; however, the interference intensity of four of these 20 beams is negligible, and therefore they are not included. Figure 14B In addition, in Figure 14B In the diagram, the dashed circular line represents the region on the radiation detector 23 corresponding to the numerical aperture of the projection system PS.

[0164] from Figure 13C and Figure 14B It can be seen from this that for those with Figure 12 The second patterned region 32 of the unit cell 50 shown, in addition to the two main interference beams (interference intensity) , Besides the 25.2% (which is 25.2%), there are many others with small but non-zero interference intensities. The additional interference beams. Due to the different interference intensities of these additional interference beams, for the region where multiple interference beams overlap in the radiation detector 23, the first harmonic of the oscillating phase-stepped signal will be proportional to the weighted sum of multiple cosines (see equation (4)), with different weights. Therefore, they cannot be easily combined using trigonometric identities. However, due to the interference intensities of the additional interference beams... (with interference intensity) , Compared to the smaller values, some known existing EUV radiation aberration measurement systems ignore these terms (i.e., assume they are zero) when reconstructing the wavefront to obtain the Zernike coefficient set.

[0165] This assumption affects the accuracy of wavefront measurements. Consequently, this negatively impacts system imaging, overlay, and focusing performance. Embodiments of the present invention are designed to at least partially address the aforementioned problems of aberration measurement systems for EUV radiation.

[0166] Embodiments of the present invention relate to solutions for problems arising from additional interference beams (besides having interference intensity) , A new solution to the problem caused by the presence of (beyond the two main interference beams). In particular, some embodiments of the invention relate to new and alternative two-dimensional diffraction gratings used as the second patterned region 32. Some embodiments of this disclosure relate to a new measurement system for determining the aberration map of the projection system PS, which is typically... Figure 2 The measurement system 12 shown or Figure 4 The measurement system 30 is shown and described above.

[0167] That is, the new measurement system includes: first patterning devices 15a, 15b, 15c, 31; second patterning devices 19a, 19b, 19c, 32; a radiation detector 23; and a positioning device. The first patterning devices 15a, 15b, 15c, 31 are capable of being positioned in the object plane of the projection system PS. The second patterning devices 19a, 19b, 19c, 32 are capable of being positioned in the image plane of the projection system PS. The radiation detector 23 is arranged to receive radiation 17a, 17b, 17c, 38 from the second patterning devices 19a, 19b, 19c, 32. The positioning device is operable to move at least one of the first and second patterning devices 15a, 15b, 15c, 31, 19a, 19b, 19c, 32 in the shear direction.

[0168] In contrast to the above arrangement, the second patterning devices 19a, 19b, 19c, 32 of the new measurement system have a variable pitch. Specifically, the second patterning devices 19a, 19b, 19c, 32 cause the angular spacing between adjacent diffraction beam pairs formed by the second patterning devices to be non-uniform. Reference will be made below. Figure 15A and Figure 15B This will be explained further.

[0169] Taking into account any reduction or magnification factors applied by the projection system PS, the pitches of the first patterning device and the second patterning devices 15a, 15b, 15c, 31, 19a, 19b, 19c, 32 are matched such that the angular spacing between the zero-order diffraction beam 35 and the first-order diffraction beam 34, 36 of the first patterning devices 15a, 15b, 15c, and 31 is approximately matched with the angular spacing between the zero-order diffraction beams 34a, 35a, 36a and the first-order diffraction beams 34b, 34c, 35b, 35c, 36b, 36c of the same-order diffraction beams 34, 35, 36 of the second patterning devices 19a, 19b, 19c, 32 originating from the first patterning devices 15a, 15b, 15c, 31. In other words, the angular spacing between the zero-order diffraction beam 35 and the first-order diffraction beams 34 and 36 of the first patterned devices 15a, 15b, 15c, 31 approximately matches the following values: (a) the angular spacing between the zero-order diffraction beam 35a and the first-order diffraction beams 35b and 35c of the second patterned devices 19a, 19b, 19c, 32 originating from the zero-order diffraction beam 35 of the first patterned devices 15a, 15b, 15c, 31; (b) the angular spacing between the zero-order diffraction beam 35a and the first-order diffraction beams 35b and 35c of the second patterned devices 19a, 19b, 19c, 32; (c) The angular spacing between the zero-order diffraction beam 34a and the first-order diffraction beams 34b and 34c of the +1st-order diffraction beam 34 originating from the first patterning devices 15a, 15b, 15c and 31; and the angular spacing between the zero-order diffraction beam 36a and the first-order diffraction beams 36b and 36c of the -1st-order diffraction beam 36 originating from the first patterning devices 15a, 15b, 15c and 31.

[0170] The following will refer to Figure 18A and Figure 18B Further discussion is given on how the pitches of the first patterning device and the second patterning devices 15a, 15b, 15c, 31, 19a, 19b, 19c, and 32 are matched.

[0171] Furthermore, considering any reduction or magnification factors imposed by the projection system PS, the variable pitch of the second patterning devices 19a, 19b, 19c, 32 causes the angular spacing between the other pairs of adjacent diffraction beams of the second patterning devices 19a, 19b, 19c, and 32 to mismatch with the angular spacing between the zero-order diffraction beam 35 and the first-order diffraction beams 34, 36 of the first patterning devices 15a, 15b, 15c, 31. Another consequence of the variable pitch of the second patterning devices 19a, 19b, 19c, 32 is a reduction in the intensity of the higher-order diffraction beams, as will be referred to below. Figure 18A and Figure 18B Further discussion is needed.

[0172] Figure 15AThis is a schematic diagram of a set of second diffraction beams generated by the second patterning device 32, which has a uniform pitch with each of the first diffraction beams 34 to 36 (e.g., ...). Figure 4 (As shown and as described above). Figure 15A Equivalent to the above Figures 5A to 5C The combination of .

[0173] Figure 15A It is shown that a set of second diffraction beams 35a to 35e are generated by the first diffraction beam 35, the first diffraction beam 35 corresponding to the 0th order diffraction beam of the first patterned region 31 (see...). Figure 5A A set of second diffraction beams 36a to 36e generated by the first diffraction beam 36, the first diffraction beam 36 corresponding to the -1st order diffraction beam of the first patterned region 31 (see...). Figure 5B ); and a set of second diffraction beams 34a to 34e generated by the first diffraction beam 34, the first diffraction beam 34 corresponding to the +1st order diffraction beam of the first patterned region 31 (see Figure 5C ). Figure 15A The spatial distribution (e.g., principal rays) of the second diffraction beams 34a to 34e, 35a to 35e, and 36a to 36e is shown below. Second diffraction beams from the same diffraction beam of the first patterned region 31 are shown as straight lines, separated from each other in the shear direction. Second diffraction beams from different diffraction beams of the first patterned region 31 are offset in a direction perpendicular to the shear direction, such that they are shown as different lines. It should be understood that this offset is merely for ease of understanding; otherwise, several second diffraction beams in the second diffraction beam would overlap each other. In practice, all these second diffraction beams 34a to 34e, 35a to 35e, and 36a to 36e should be in the same position in the non-scanning direction.

[0174] because Figure 15A Corresponding to the arrangement of the second patterning device 32 with a uniform pitch, the angular spacing between adjacent diffraction beam pairs formed by the second patterning device 32 is uniform. Therefore, in the spatial distribution of the second diffraction beams 34a to 34e, 35a to 35e, and 36a to 36e, there is a uniform distance 60 between adjacent diffraction beam pairs.

[0175] Figure 15B This is a schematic diagram of a set of second diffraction beams generated by the second patterning device 32, and the second patterning device is associated with each of the first diffraction beams 34 to 36 (e.g., ...). Figure 4 As shown and as described above, it has a non-uniform pitch. Because... Figure 15BCorresponding to the arrangement of the second patterning device 32 with a non-uniform pitch, the angular spacing between adjacent diffraction beam pairs originating from the same first diffraction beam 34 to 36 formed by the second patterning device 32 is non-uniform. Therefore, in the spatial distribution of the second diffraction beams 34a to 34e, 35a to 35e, and 36a to 36e, the distance between adjacent diffraction beam pairs is non-uniform.

[0176] A distance 62 exists between the second diffraction beam 35a, corresponding to the 0th order diffraction beam 35, and the second diffraction beams 35b and 35c, corresponding to the ±1st order diffraction beams 30, originating from the 0th order diffraction beam 30. Similarly, a distance 64 exists between the second diffraction beams 35b and 35c, corresponding to the ±1st order diffraction beams 35, and the second diffraction beams 35d and 35e, corresponding to the ±2nd order diffraction beams 35, originating from the 0th order diffraction beam 35. Similarly, a distance 66 exists between the second diffraction beams 35d and 35e, corresponding to the ±2nd order diffraction beams 35, originating from the 0th order diffraction beam 35, and the second diffraction beams 35d and 35e, corresponding to the ±3rd order diffraction beams 35, originating from the 0th order diffraction beam 35.

[0177] Because the pitch of the second patterning device 32 is non-uniform, the distances 62, 64, and 66 are different. In this embodiment, the spatial spacing 62, 64, and 66 between adjacent diffraction orders increases with the increase of the diffraction order.

[0178] A distance of 62 exists between the second diffraction beam 34a, corresponding to the 0th order diffraction beam originating from the +1st order diffraction beam 34, and the second diffraction beams 34b and 34c, corresponding to the ±1st order diffraction beams originating from the +1st order diffraction beam 33. Similarly, a distance of 64 exists between the second diffraction beams 34b and 34c, corresponding to the ±1st order diffraction beams originating from the +1st order diffraction beam 34, and the second diffraction beams 34d and 34e, corresponding to the ±2nd order diffraction beams originating from the +1st order diffraction beam 34. Similarly, a distance of 66 exists between the second diffraction beams 34d and 34e, corresponding to the ±2nd order diffraction beams originating from the +1st order diffraction beam 34, and the second diffraction beams 34d and 34e, corresponding to the ±3rd order diffraction beams originating from the +1st order diffraction beam 33.

[0179] A distance 62 exists between the second diffraction beam 36a, corresponding to the 0th order diffraction beam originating from the -1st order diffraction beam 36, and the second diffraction beams 36b and 36c, corresponding to the ±1st order diffraction beams originating from the -1st order diffraction beam 36. Similarly, a distance 64 exists between the second diffraction beams 36b and 36c, corresponding to the ±1st order diffraction beams originating from the -1st order diffraction beam 36, and the second diffraction beams 36d and 36e, corresponding to the ±2nd order diffraction beams originating from the -1st order diffraction beam 36. Similarly, a distance 66 exists between the second diffraction beams 36d and 36e, corresponding to the ±2nd order diffraction beams originating from the -1st order diffraction beam 36, and the second diffraction beams 36d and 36e, corresponding to the ±3rd order diffraction beams originating from the -1st order diffraction beam 36.

[0180] from Figure 15A and Figure 15B As can be seen, due to the uneven pitch of the second patterning device 32, therefore... Figure 15B In this process, the second diffraction beams no longer overlap each other in space.

[0181] from Figure 15B It can be seen that even though the second patterning device 32 has a non-uniform pitch, the second diffraction beam pairs (second diffraction beams 35a and 34b and second diffraction beams 35b and 36a) that contribute to the first interference beam still overlap in space. Figure 15B It can also be seen that even though the second patterning device 32 has a non-uniform pitch, the pairs of second diffraction beams that contribute to the second interference beams (second diffraction beams 35a and 36c and second diffraction beams 35c and 34a) still overlap spatially. However, the spatial overlap between pairs of second diffraction beams corresponding to other (unwanted) interference beams is at least partially disrupted by the non-uniform pitch.

[0182] As discussed now, the new measurement system of the second patterned devices 19a, 19b, 19c, 32 with variable pitch is advantageous.

[0183] The new measurement system allows for the use of pinhole array geometries for wafer-level patterned devices 19a, 19b, 19c, 32 without generating unwanted interference beams that contribute to the first harmonic of the phase-stepped signal. Specifically, when using pinhole arrays with uniform pitch, the large number of additional interference beams with small but non-zero intensities that were present are no longer present, or at least have significantly reduced intensities. This is due to both: (a) less spatial overlap between the second diffraction beams that contribute to the unwanted interference beams; and (b) a reduction in the intensity of the second diffraction beams that contribute to the unwanted interference beams.

[0184] Taking into account any reduction or magnification factors imposed by the projection system PS, using the new measurement system, the pitches of the first patterning device 31 and the second patterning device 32 are matched such that the angular spacing between the zero-order diffraction beam 35 and the first-order diffraction beams 34, 36 of the first patterning device 31 is approximately matched with the angular spacing between the zero-order diffraction beams 34a, 35a, 36a and the first-order diffraction beams 34b, 34c, 35b, 35c, 36b, 36c of the second patterning device, which originate from the same order diffraction beam of the first patterning device 31. Therefore, as with the use of a one-dimensional diffraction grating of a mask-level patterning device with a 50% duty cycle and a checkerboard grating of a wafer-level patterning device, the same two contributions exist at the radiation detector to the first harmonic of the oscillating (phase-stepped) signal.

[0185] Furthermore, the second patterning device 32 has a variable pitch, resulting in non-uniform angular spacing between adjacent diffraction beam pairs formed by the second device 31. Moreover, considering any reduction or magnification factors applied by the projection system PS, the variable pitch of the second patterning device 32 causes the angular spacing between other diffraction beam pairs of adjacent diffraction beam pairs originating from a given diffraction order of the first patterning device 31 to mismatch with the angular spacing between the zero-order diffraction beam 35 and the first-order diffraction beams 34, 36 of the first patterning device 31. Therefore, advantageously, the additional contribution of the first harmonic to the oscillating (phase-stepped) signal at the radiation detector 23 is eliminated or at least reduced. In particular, unwanted overlap between diffraction beams with coherent relationships (thus potentially leading to interference patterns at the radiation detector) is disrupted.

[0186] As referenced above Figure 4 As explained, in use, the first patterning device 31 can be configured to receive the radiation beam 33 and form a plurality of first diffraction beams 34, 35, and 36. In use, the projection system PS can be configured to project the first diffraction beams 34, 35, and 36 formed by the first patterning device 31 onto the second patterning device 32. In use, the second patterning device 32 can be configured to receive the first diffraction beams 34, 35, and 36 from the projection system PS and form a plurality of second diffraction beams from each of the first diffraction beams 34, 35, and 36.

[0187] Typically, the contributions from different diffraction orders of the first diffraction beams 34, 35, and 36 will result in different phase contributions from the projection system PS. Therefore, each pixel of the radiation detector 23 does not receive a clean phase signal from a single path through the projection optics PS, but rather a combination of paths around the 0th order first diffraction beam 35. This can be termed phase ambiguity.

[0188] Due to the variable-pitch second patterning device 32, the accuracy of wavefront fitting in measurement systems 12, 30 is expected to be significantly improved because the number of convolution orders or interference beams contributing to the first harmonic of the phase-stepped signal is significantly reduced. This will reduce phase ambiguity at the sensor level (i.e., at the radiation detector 23) and will ensure better matching with currently implemented wavefront fitting algorithms. This, in turn, will reduce Zernike gain and / or crosstalk errors, and also reduce bias errors in such measurement systems 12, 30. Pure Zernike wavefront aberration maps (e.g., The input is fed into the reconstruction algorithm, and the algorithm is evaluated on how well it reconstructs the polynomials into a linear combination of Zernike polynomials. Ideally, the reconstruction algorithm should output a set of Zernike coefficients. This makes for , And for , Any changes starting from 1 All of these can be referred to as gain error. Any change starting from 0. (in Both can be referred to as crosstalk error.

[0189] Furthermore, a higher diffraction order also indicates a decrease in diffraction intensity (see, for example, below). Figure 17 , Figure 18A and Figure 18B This is very useful because it will also limit the contribution of unwanted convolution orders or interference beams to the first harmonic of the phase-stepped signal.

[0190] The measured intensity signal at radiation detector 23 is the convolution of the grating functions of the first patterning devices 15a, 15b, 15c, 31 (mask-level) and the second patterning devices 19a, 19b, 19c, 32 (wafer-level). For example, if both the mask-level and wafer-level gratings are binary lines, the phase step signal is typically pyramidal. Similarly, a mask-level binary grating and a wafer-level checkerboard grating will produce a more cosine-shaped phase step signal. In both examples, using constant-pitch mask-level and wafer-level gratings, the measured phase step signal repeats periodically with the same amplitude. However, for wafer-level variable-pitch gratings (i.e., the second patterning devices 19a, 19b, 19c, 32 with variable pitch), the phase step signal does not repeat periodically. Furthermore, as the local pitches of the mask-level and wafer-level gratings become less matched, the amplitude of the phase step signal will attenuate. In short, for wafer-level periodic gratings, since the amplitude of the periodic phase step signal is the same for each period, the measurement method cannot distinguish alignments larger than the grating pitch. In contrast, in the case of wafer-level variable pitch gratings (i.e., second patterning devices 19a, 19b, 19c, 32 with variable pitch), the amplitude of the phase step signal will have a bell-shaped envelope. Based on the maximum value of this bell-shaped envelope, the optimal alignment position of the mask-level markings (i.e., the first patterning devices 15a, 15b, 15c, 31) can be determined even when the misalignment is greater than the pitch of the mask-level markings. Therefore, this type of grating / design concept (second patterning devices 19a, 19b, 19c, 32 with variable pitch) is expected to resolve undesirable periodic jumps that sometimes occur in such measurement systems 12, 30 (e.g., due to relative misalignment between the mask-level and wafer-level gratings).

[0191] The pitch of the second patterning device 32 may have an extreme value at a first position on the second patterning device 32, and the pitch of the second patterning device 32 may increase or decrease with the distance from the first position.

[0192] For example, the extreme value can be a minimum value, and the pitch of the second patterning device 32 can increase with the distance from the first position. Alternatively, the extreme value can be a maximum value, and the pitch of the second patterning device 32 can decrease with the distance from the first position.

[0193] The first position can, but is not necessarily, located at the center of the second patterning device.

[0194] The pitch of the second patterning device 32 can vary linearly with distance from the first position. Alternatively, the pitch of the second patterning device 32 can vary non-linearly with distance from the first position. Typically, the pitch of the second patterning device can be a monotonic function of distance from the first position.

[0195] The pitch of the second patterning device 32 may vary by at least 2%. In some embodiments, the pitch of the second patterning device 32 may vary by about 4% or more.

[0196] The second patterning device 32 can be a two-dimensional diffraction grating. The second patterning device 32 can be a transmission grating. The second patterning device 32 can be a pinhole grating.

[0197] The second patterning device 32 may include a substrate having a two-dimensional array of circular through-holes.

[0198] The distance between the centers of adjacent through-holes can be referred to as the local pitch of the second patterned device 32.

[0199] The duty cycle of the second patterning device 32 can also be variable. That is, for a pinhole grating, the ratio of the radius of the circular aperture to the distance between the centers of adjacent apertures can be varied on the second patterning device 32. For example, multiple apertures can all be of fixed size, and the pitch (and duty cycle) can be varied by changing the distance between the centers of adjacent apertures across the second patterning device.

[0200] Typically, a diffraction grating 32, used as a wafer-level grating for measurement systems 12, 30, for determining the aberration map of the projection system PS of a photolithography apparatus LA, comprises a plurality of apertures distributed such that the distance between the centers of adjacent apertures is uniform across the diffraction grating. This is achieved by arranging the apertures such that the center of each circular through-aperture coincides with a position in a square position array. In contrast, a second patterned device 32 according to an embodiment of the present disclosure can be formed by arranging apertures such that the distance between the centers of adjacent apertures varies across the second patterned device 32.

[0201] The second patterning device 32 may include at least 10 cells in each dimension of the pattern across the second patterning device.

[0202] The second patterned device 32 may include approximately 3 to 100 unit cells across each dimension. For example, the second patterned device 32 may include approximately 30 unit cells across each dimension.

[0203] Some aspects of this disclosure relate to lithography apparatuses including the aforementioned new measurement system.

[0204] Some aspects of this disclosure relate to a diffraction grating for use in a phase-stepping measurement system 12, 30 for determining an aberration map of a projection system PS.

[0205] Figure 16A This is a schematic diagram of a first example diffraction grating 70 with a non-uniform pitch, which can be used as a second patterning device 32. The diffraction grating 70 is a two-dimensional diffraction grating 70. The diffraction grating 70 is a transmission grating. The diffraction grating 70 is a pinhole grating.

[0206] The first example diffraction grating 70 includes a substrate with a two-dimensional array of circular through-apertures 72. The distance between the centers of adjacent through-apertures 72 can be referred to as the local pitch of the diffraction grating.

[0207] Multiple apertures 72 are of fixed size, and the pitch variation is achieved by changing the distance between the centers of adjacent apertures across the diffraction grating 70. Therefore, in this example, the duty cycle of the diffraction grating 70 is also variable. That is, the ratio of the radius of the circular aperture 72 to the distance between the centers of adjacent apertures 72 varies across the diffraction grating 70.

[0208] Typically, the diffraction gratings used as measurement systems 12 and 30 for determining the aberration map of the projection system PS of the photolithography apparatus LA include multiple apertures distributed such that the distance between the centers of adjacent apertures is uniform across the diffraction grating. This is achieved by arranging the apertures such that the center of each circular through-aperture in the circular through-apertures coincides with a position in a square position array. In contrast, Figure 16A The diffraction grating 70 shown can be formed by arranging apertures 72 such that the distance between the centers of adjacent apertures 72 varies across the diffraction grating 70.

[0209] exist Figure 16A In the example shown, the pitch of the diffraction grating 70 has a minimum at a first position on the diffraction grating 70, which in this example is the center of the diffraction grating. The pitch of the diffraction grating 70 increases in all directions from the first position with distance from the first position (the center of the grating 70), such as from... Figure 16AThe local pitches p1, p2, and p3 marked in the middle are seen (p1) <p2<p3)。

[0210] In an alternative embodiment, a maximum pitch may exist at the first position, and the pitch of the diffraction grating may decrease with distance from the first position. In other embodiments, the first position need not be at the center of the diffraction grating.

[0211] Advantageously, by using this diffraction grating 70, the variation in pitch results in non-uniform angular spacing between adjacent diffraction beam pairs formed by the second patterning device 32. In particular, the variation in pitch increases the angular spacing between adjacent diffraction beam pairs formed by the second patterning device 32 for higher-order diffraction beams. This diffraction grating can be used as the second patterning device 32 in the aforementioned new measurement systems 12, 30.

[0212] The pitches p1, p2, and p3 of the diffraction grating 70 can vary linearly with distance from a first position (the center of the diffraction grating 70). Alternatively, the pitches p1, p2, and p3 of the diffraction grating device 70 can vary non-linearly with distance from the first position (e.g., as a parabolic function). Typically, the pitches of the diffraction grating 70 can be a monotonic function of the distance from the first position.

[0213] The pitches p1, p2, and p3 of the diffraction grating 70 can vary by at least 2%. In some embodiments, the pitches p1, p2, and p3 of the diffraction grating 70 can vary by at least 3%. In some embodiments, the pitches p1, p2, and p3 of the diffraction grating 70 can vary by 4% or more.

[0214] Figure 16B This is a schematic diagram of a second example diffraction grating 80 with a non-uniform pitch. The diffraction grating 80 is a linear (one-dimensional) diffraction grating 80. The diffraction grating 80 is a transmission grating.

[0215] and Figure 16A The first example grating 70 shown is the same. Figure 16B The second example grating 80 shown has pitches p1, p2, p3 and a duty cycle that both vary across the grating 80, with the pitch increasing from the center of the grating 80. For simplicity, the diffraction pattern of this one-dimensional diffraction grating 80 is discussed to provide a qualitative teaching on how this variable pitch alters the diffraction pattern of the grating.

[0216] Figure 17 It shows Figure 16B The linear grating of the type shown exhibits a diffraction spectrum of 90°, but with a uniform pitch. The diffraction spectrum 90 shows the variation with diffraction order. The intensity of the changing diffraction pattern. Here, the diffraction order... express The scattering angle, where ,in It is a uniform pitch, and It is the wavelength of the radiation. Diffraction spectra 90 show three cases: (a) a grating consisting of a single unit cell; (b) a grating consisting of N=30 unit cells; and (c) a grating consisting of an infinite number of unit cells; the diffraction spectra of N=30 and N=∞ are almost indistinguishable.

[0217] Figure 18A It shows Figure 16B The diffraction spectrum 92 of a linear diffraction grating 80 of the type shown, wherein the pitch of the grating increases from the center of the grating 80 and has a pitch variation of 4% across the grating 80, is illustrated. The diffraction spectrum 92 shows the intensity of the diffraction pattern as a function of diffraction order. Here, the diffraction order express The scattering angle, where ,in This is the minimum pitch of the diffraction grating 80. The diffraction spectra 92 are shown again for three cases: (a) a grating comprising a single unit cell; (b) a grating comprising N=30 unit cells; and (c) a grating comprising an infinite number of unit cells. Figure 18B It shows Figure 18A The magnified portion of the diffraction spectrum 92 shown.

[0218] The following general characteristics of diffraction spectrum 92 can be obtained from Figure 18A and Figure 18B This is identified in the process. First, for N=∞, a variable-pitch linear grating 80 generates diffraction spectra 92, where higher diffraction orders have increased angular spacing (increased angular spacing is equivalent to decreased pitch). Recall that for a fixed-pitch diffraction grating, the larger the pitch, the smaller the angular spacing between adjacent diffraction beam pairs. This can be identified in... Figure 18B The most easily observed line shifts, representing the 3rd and 5th order diffraction orders with N=∞, are above the grating. The expected value is the uniform pitch.

[0219] Second, for a diffraction grating 80 comprising a finite number of unit cells (e.g., see the case of N=30), the amplitude of higher diffraction orders is significantly reduced in intensity compared to lower diffraction orders. For example, the first-order peak is reduced by about 50%, while the third-order and fifth-order peaks are reduced by about 20% and about 5%, respectively.

[0220] Third, the position of the peak of the diffraction beam depends not only on the pitch and its variation, but also on the number of unit cells used for the grating 80. It is important to note that this contrasts with the diffraction spectrum 90 of a uniformly pitched grating, where the position of the peak of the diffraction beam does not depend on the number of unit cells (although the width of the peak is). This is significant because it relates to the angular spacing between the zero-order diffraction beams 34a, 35a, 36a and the first-order diffraction beams 34b, 34c, 35b, 35c, 36b, 36c of the same-order diffraction beams 34, 35, 36 originating from the first patterned device 31. Therefore, the number of unit cells also affects how the pitch of the first patterned device 31 and the second patterned device 32 is matched (in order to ensure that the angular spacing between the zero-order diffraction beam 35 and the first-order diffraction beams 34, 36 of the first patterned device 31 is approximately matched with the angular spacing between the zero-order diffraction beams 34a, 35a, 36a and the first-order diffraction beams 34b, 34c, 35b, 35c, 36b, 36c of the second patterned device 32, which are of the same order as the diffraction beams 34, 35, 36 of the first patterned device 31).

[0221] For example, for infinite N, the minimum pitch The pitch should match that of the mask-level grating 31. However, this is not the case when N=30. The angular spacing between the zero-order diffraction beams 34a, 35a, 36a and the first-order diffraction beams 34b, 34c, 35b, 35c, 36b, 36c of the same-order diffraction beams 34, 35, 36 originating from the first patterning device 31, and the angular spacing between these diffraction order peaks, can be defined as the angular spacing between these diffraction order peaks. According to this definition, as can be seen from Figure 19B, the angular spacing between the zero-order diffraction beams 34a, 35a, 36a and the first-order diffraction beams 34b, 34c, 35b, 35c, 36b, 36c of the same-order diffraction beams 34, 35, 36 originating from the first patterning device 31, and the angular spacing between these beams ... The expected angular spacing of the uniform pitch grating. Therefore, for this case with a finite number of unit cells, the pitch of the first patterned device 31 should be greater than the minimum pitch. The value matches. Given the diffraction spectrum of the variable pitch grating 32, it is possible to determine exactly how much larger it is.

[0222] For example, such matching can be determined by: (a) determining the diffraction spectrum of the variable-pitch second patterned device 32; (b) determining the angular positions of the peaks of the zero-order diffraction beams 34a, 35a, 36a and the first-order diffraction beams 34b, 34c, 35b, 35c, 36b, 36c of the second patterned device 32 (e.g., by fitting curves to the peaks, such as least-squares fitting); (c) determining the angular spacing between the zero-order diffraction beams 34a, 35a, 36a and the first-order diffraction beams 34b, 34c, 35b, 35c, 36b, 36c of the second patterned device 32 derived from the same-order diffraction beams 34, 35, 36 of the first patterned device 31; and (d) determining the effective pitch of the second patterned device 32 as a pitch that will achieve the same angular spacing for adjacent diffraction orders of a uniform-pitch diffraction grating.

[0223] Figure 16A , Figure 16B The diffraction gratings 70 and 80 shown can be self-supporting. Since the diffraction gratings 70 and 80 are self-supporting, they do not require, for example, a transmission support layer. This arrangement is particularly advantageous for use in phase-stepping measurement systems 12 and 30, which are used to determine the aberration map of a projection system PS using EUV radiation, because using such a transmission support layer will significantly reduce the amount of EUV radiation transmitted through the two-dimensional diffraction grating.

[0224] In some embodiments, the diffraction gratings 70 and 80 described above may be self-supporting. In use, the diffraction gratings 70 and 80 may be provided with supports for the absorption layer. The supports may only contact the peripheral portion of the absorption layer. That is, the supports may be in the form of a frame and not adjacent to the central portion of the absorption layer. With this arrangement, the absorption layer can be considered self-supporting in the central portion of the absorption layer. To achieve this, the absorption layer may be tensioned on the supports (e.g., kept generally planar).

[0225] Advantageously, in embodiments where the central portion of the absorption layer is self-supporting, the grating does not require, for example, a transmission support layer. This arrangement is particularly advantageous for use in phase-stepping measurement systems used to determine aberration maps of projection systems using EUV radiation, because using such a transmission support layer would significantly reduce the amount of EUV radiation transmitted through the diffraction grating.

[0226] In some embodiments, in the diffraction grating described above, the substrate (in which an aperture is formed) includes a radiation-absorbing layer. The radiation-absorbing layer may be formed, for example, of a metal such as chromium (Cr), nickel (Ni), cobalt (Co), or aluminum (Al).

[0227] In some embodiments, the absorbing layer may comprise a ceramic. The ceramic may comprise a metallic or metalloid component and a nonmetallic component having a relatively high extinction coefficient for EUV radiation. Both components may have a refractive index relatively close to 1 for EUV. The ceramic may comprise aluminum nitride (AlN). In some embodiments, the absorbing layer may comprise aluminum nitride (AlN).

[0228] In some embodiments, in the diffraction grating described above, the substrate (in which the aperture is formed) may further include a support layer. The through-hole aperture may extend through the support layer and the radiation-absorbing layer. The support layer may be formed of SiN, for example.

[0229] Although the above embodiments use the first harmonic of the phase-stepped signal, it should be understood that, in alternative embodiments, higher harmonics of the phase-stepped signal may be used instead.

[0230] Although the above embodiments use a first patterned region 31 including a one-dimensional diffraction grating 31 with a 50% duty cycle, it should be understood that in alternative embodiments, other first patterned regions 31 may use different geometries. For example, in some embodiments, the first patterned region 31 may include a two-dimensional checkerboard diffraction grating with a 50% duty cycle.

[0231] While this article can specifically mention the use of photolithography equipment in IC manufacturing, it should be understood that the photolithography equipment described herein can have other applications. Other possible applications include the fabrication of integrated optical systems, the guiding and detection modes of magnetic domain memory, flat panel displays, liquid crystal displays (LCDs), thin-film magnetic heads, etc.

[0232] Although embodiments of the invention may be specifically referred to herein in the context of a lithography apparatus, these embodiments can also be used in other apparatuses. Embodiments of the invention can form part of a mask inspection apparatus, a metrology apparatus, or any apparatus for measuring or processing objects such as wafers (or other substrates) or masks (or other patterning devices). These apparatuses are generally referred to as lithography tools. Such lithography tools can use vacuum conditions or ambient (non-vacuum) conditions.

[0233] Where the context permits, embodiments of the present invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the present invention may also be implemented as instructions stored on a machine-readable medium that can be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a machine-readable (e.g., computing device) form. For example, a machine-readable medium may include read-only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustic, or other forms of propagation signals (e.g., carrier waves, infrared signals, digital signals, etc.). Furthermore, firmware, software, routines, and instructions may be described herein as performing certain actions. However, it should be understood that such descriptions are merely for convenience, and such actions are actually caused by a computing device, processor, controller, or other device implementing the firmware, software, routines, instructions, etc., and doing so enables the actuator or other device to interact with the physical world.

[0234] This invention can also be described by the following terms: 1. A measurement system for determining an aberration map of a projection system, the measurement system comprising: The first patterning device is capable of being positioned in the object plane of the projection system; The second patterning device can be positioned in the image plane of the projection system; A radiation detector, the radiation detector being arranged to receive radiation from the second patterning device; and The positioning device is operable to move at least one of the first patterning device and the second patterning device in the shearing direction; The second patterning device has a variable pitch, such that the angular spacing between adjacent diffraction beam pairs originating from the first patterning device and formed by the second patterning device is non-uniform; and Taking into account any reduction or magnification factors applied by the projection system, the pitches of the first patterning device and the second patterning device are matched such that the angular spacing between the zero-order and first-order diffraction beams in the first patterning device approximately matches the angular pitch between the zero-order and first-order diffraction beams of the second patterning device originating from the same-order diffraction beam of the first patterning device; and Taking into account any reduction or magnification factors applied by the projection system, the variable pitch of the second patterning device causes the angular spacing between other adjacent diffraction beam pairs of the second patterning device to be mismatched with the angular spacing between the zero-order diffraction beam and the first-order diffraction beam of the first patterning device. 2. The measurement system according to Clause 1, wherein the pitch of the second patterning device has an extreme value at a first position on the second patterning device, and wherein the pitch of the second patterning device increases or decreases with distance from the first position. 3. The measurement system according to Clause 2, wherein the pitch of the second patterning device varies linearly with distance from the first position. 4. The measurement system according to any one of the preceding clauses, wherein the pitch of the second patterning device varies by at least 2% on the second patterning device. 5. The measurement system according to any one of the preceding clauses, wherein the second patterning device is a two-dimensional diffraction grating. 6. The measurement system according to any one of the preceding clauses, wherein the second patterned device comprises a substrate having a two-dimensional array of circular through-holes. 7. The measurement system according to any one of the preceding clauses, wherein the second patterning device comprises at least 10 unit cells across a dimension of the pattern of the second patterning device or each dimension thereof. 8. The measurement system according to any one of the preceding clauses, wherein the second patterning device and the radiation detector together form a sensor device, the measurement system further comprising: The controller is configured to: The positioning device is controlled to move at least one of the first patterning device and the sensor device in the shear direction, such that the intensity of radiation received by each part of the radiation detector varies with the movement in the shear direction, thereby forming an oscillating signal; The phase of the harmonics of the oscillating signal at multiple locations on the radiation detector is determined from the radiation detector; and A set of coefficients characterizing the aberration map of the projection system is determined based on the phase of the harmonics of the oscillation signal at multiple locations on the radiation detector. 9. A photolithography apparatus comprising a measurement system according to any one of the preceding clauses. 10. A diffraction grating for a phase-stepping measurement system, the diffraction grating being used to determine an aberration map of a projection system, wherein the pitch of the diffraction grating has an extremum at a first position on the diffraction grating, and wherein the pitch of the diffraction grating increases or decreases with distance from the first position. 11. The diffraction grating according to Clause 10, wherein the pitch of the diffraction grating varies linearly with distance from the first position. 12. A diffraction grating according to Clause 10 or Clause 11, wherein the pitch of the diffraction grating varies by at least 2% on the diffraction grating. 13. A diffraction grating according to any one of clauses 10 to 12, wherein the diffraction grating is a two-dimensional diffraction grating. 14. A diffraction grating according to any one of clauses 10 to 13, wherein the diffraction grating comprises at least 10 unit cells across a dimension of the pattern of the diffraction grating or in each dimension. 15. A diffraction grating according to any one of clauses 10 to 14, wherein the diffraction grating is self-supporting. 16. The diffraction grating according to any one of Clauses 10 to 15, wherein the diffraction grating comprises a substrate having a two-dimensional array of circular through-apertures. 17. The diffraction grating according to Clause 16, wherein the substrate comprises: Radiation absorption layer And the through-hole extends through the radiation-absorbing layer. 18. The diffraction grating according to Clause 17, wherein the substrate further comprises: The support layer is adjacent to the radiation-absorbing layer. Furthermore, the through-hole extends through the support layer and the radiation-absorbing layer.

[0235] While specific embodiments of the invention have been described above, it should be understood that the invention can be practiced in ways other than those described. The foregoing description is intended to be illustrative and not restrictive. Therefore, those skilled in the art will understand that modifications can be made to the described invention without departing from the scope of the appended claims.

Claims

1. A measurement system for determining an aberration map of a projection system, the measurement system comprising: The first patterning device is capable of being positioned in the object plane of the projection system; The second patterning device can be positioned in the image plane of the projection system; A radiation detector is arranged to receive radiation from the second patterning device; as well as The positioning device is operable to move at least one of the first patterning device and the second patterning device in the shearing direction; The second patterning device has a variable pitch, such that the angular spacing between adjacent diffraction beam pairs originating from the first patterning device and formed by the second patterning device is non-uniform; and Taking into account any reduction or magnification factors applied by the projection system, the pitches of the first patterning device and the second patterning device are matched such that the angular spacing between the zero-order and first-order diffraction beams in the first patterning device approximately matches the angular pitch between the zero-order and first-order diffraction beams of the second patterning device originating from the same-order diffraction beam of the first patterning device; and Taking into account any reduction or magnification factors applied by the projection system, the variable pitch of the second patterning device causes the angular spacing between adjacent diffraction beam pairs of the other pairs of the second patterning device to be mismatched with the angular spacing between the zero-order diffraction beam and the first-order diffraction beam of the first patterning device.

2. The measurement system of claim 1, wherein the pitch of the second patterning device has an extreme value at a first position on the second patterning device, and wherein the pitch of the second patterning device increases or decreases with distance from the first position.

3. The measurement system of claim 2, wherein the pitch of the second patterning device varies linearly with the distance from the first position.

4. The measurement system according to any one of the preceding claims, wherein the pitch of the second patterning device varies by at least 2% on the second patterning device.

5. The measurement system according to any one of the preceding claims, wherein the second patterning device is a two-dimensional diffraction grating.

6. The measurement system according to any one of the preceding claims, wherein the second patterned device comprises a substrate having a two-dimensional array of circular through-holes.

7. The measurement system according to any one of the preceding claims, wherein the second patterning device comprises at least 10 unit cells across a dimension of the pattern of the second patterning device or each dimension thereof.

8. The measurement system according to any one of the preceding claims, wherein the second patterning device and the radiation detector together form a sensor device, the measurement system further comprising: The controller is configured as follows: The positioning device is controlled to move at least one of the first patterning device and the sensor device in the shear direction, such that the intensity of radiation received by each part of the radiation detector varies with the movement in the shear direction, thereby forming an oscillating signal; The phase of the harmonics of the oscillating signal at multiple locations on the radiation detector is determined from the radiation detector; as well as A set of coefficients characterizing the aberration map of the projection system is determined based on the phase of the harmonics of the oscillating signal at multiple locations on the radiation detector.

9. A photolithography apparatus comprising a measurement system according to any one of the preceding claims.

10. A diffraction grating for a phase-stepping measurement system, the diffraction grating being used to determine an aberration map of a projection system, wherein the pitch of the diffraction grating has an extreme value at a first position on the diffraction grating, and wherein the pitch of the diffraction grating increases or decreases with distance from the first position.

11. The diffraction grating of claim 10, wherein the pitch of the diffraction grating varies linearly with distance from the first position.

12. The diffraction grating according to claim 10 or claim 11, wherein the pitch of the diffraction grating varies by at least 2% on the diffraction grating.

13. The diffraction grating according to any one of claims 10 to 12, wherein the diffraction grating is a two-dimensional diffraction grating.

14. The diffraction grating according to any one of claims 10 to 13, wherein the diffraction grating is self-supporting.

15. The diffraction grating according to any one of claims 10 to 14, wherein the diffraction grating comprises a substrate having a two-dimensional array of circular through-apertures.

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