Method and system for determining aberrations of a projection system - Patents.com

Abstract

Methods for determining one or more aberrations of a projection system (e.g., a projection system of a lithographic apparatus), and corresponding systems therefor, are disclosed. One method includes performing a phase-stepping or phase-scanning process using a first patterning device (at object level) that includes a specular diffraction grating. A calibration method is also disclosed for determining calibration data that characterizes differences between the aberrations of the projection system determined at object level using a diffuse grating and the aberrations of the projection system determined at object level using a specular grating.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to European Application No. 22199929.5, filed October 6, 2022, which is incorporated herein by reference in its entirety.

[0002] The present invention relates to a method for determining one or more aberrations of a projection system, which may be a projection system of a lithographic apparatus, and a corresponding system therefor. The present invention also relates to a calibration method for determining calibration data that characterizes the difference between the aberrations of the projection system determined at object level using a diffuse grating and the aberrations of the projection system determined at object level using a mirror grating. [Background technology]

[0003] A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus can, for example, project a pattern in a patterning device (such as a mask) onto a layer of radiation-sensitive material (resist) provided on the substrate.

[0004]

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

[0005]

[0005] The radiation patterned by the patterning device is focused onto the substrate using a projection system, which can 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 may be desirable to provide a method and apparatus for accurately determining such aberrations caused by a projection system, thereby allowing for better control of these aberrations. Summary of the Invention

[0007] According to a first aspect of the present disclosure, there is provided a method for determining one or more aberrations of a projection system, the method comprising performing a phase stepping or phase scanning process, illuminating a first patterning device with illumination radiation, the first patterning device including a specular diffraction grating arranged to form a plurality of first diffracted beams separated in a shear direction, and projecting, with a projection system, at least some of the plurality of first diffracted beams onto a sensor arrangement, wherein the sensor arrangement receives the first diffracted beams from the projection system and measures the first a second patterning device arranged to form a plurality of second diffracted beams from each of the diffracted beams, and a radiation detector arranged to receive at least a portion of the second diffracted beams; and moving at least one of the first and second patterning devices in a shear direction such that an intensity of radiation received by each portion of the radiation detector receiving the radiation varies as a function of movement in the shear direction, thereby forming an oscillation signal, the method also including determining one or more aberrations of the projection system from the oscillation signal determined by each portion of the radiation detector receiving the radiation.

[0008]

[0008] By specular diffraction grating, it will be understood that a reflective diffraction grating comprising a pattern of reflective portions and radiation-absorbing portions, the reflection from the reflective portions being primarily specular or normal. In other words, a specular diffraction grating is intended to mean a reflective diffraction grating that minimizes scattering of radiation incident thereon. It will be understood that this can be achieved by ensuring that the reflective portions of the specular diffraction grating have sufficient smoothness. In particular, any imperfections or surface roughness in the reflective portions of the specular diffraction grating may be small compared to the wavelength of the illumination radiation that the specular diffraction grating scatters during use.

[0009] It will be appreciated that this method is a form of shearing interferometry.

[0010] The method according to the first aspect may be suitable for use in a lithographic apparatus. The first patterning device may be arranged at reticle level (i.e. in an object plane of the lithographic apparatus) and the sensor apparatus may be arranged at wafer level (i.e. in an image plane of the lithographic apparatus). In particular, the method according to the first aspect may be suitable for use in an extreme ultraviolet (EUV) lithographic apparatus. That is, the illumination radiation may include EUV radiation.

[0011]

[0011] During a lithographic process, radiation is used to form a typically diffraction-limited image on a substrate (e.g. a resist-coated wafer, etc.) using a projection system. In practice, projection systems do not form perfect images and will be subject to some degree of aberrations. These aberrations describe distortions in the wavefront of light approaching a point in the image plane of the projection system from a spherical wavefront (which may generally depend on the position in the pupil plane and / or the angle at which the radiation approaches the image plane of the projection system). It is desirable to minimize aberrations and therefore it is desirable to have a measurement of the aberrations which can be used as part of a feedback loop to minimize aberrations as much as possible.

[0012]

[0012] In measuring the aberrations of the projection system of known EUV lithography apparatuses, the same (EUV) radiation used to expose the wafer during the lithography process is typically used for the aberration measurement. This is a clear advantage, since it is the aberrations caused by imaging with EUV radiation that are desirable to measure and minimize (because they affect the image formed on the wafer). However, such radiation is delivered to the reticle level of the lithography apparatus via an illumination system, which is arranged to optimize the angular distribution of EUV radiation at the reticle level for lithography imaging performance. In particular, while known EUV lithography apparatuses offer some control over the angular distribution of EUV radiation at the reticle level (sometimes referred to as the illumination mode), they do not provide uniform pupil-fill illumination (i.e., illumination of each field point at the reticle level with a uniform cone of radiation). The illumination modes achievable by the illumination system do not allow inspection of the entire pupil plane of the projection system. As a result, measurements of the aberrations of the projection system of known EUV lithography apparatus typically use a diffusely reflecting grating as the first patterning device (at reticle level), which is positioned to fill the entire pupil plane of the projection system so that the entire projection system is examined during the aberration measurement.

[0013]

[0013] However, the inventors of the present invention have recognised that such diffuse reflection gratings can introduce significant errors in the measurement of some aberrations (e.g. the amplitudes of some Zernike orders in the aberration map of the projection system). Typically, diffuse reflection gratings for use in aberration measurement systems are formed by creating a rough surface that is then imparted with a reflective grating pattern. For example, a uniform reflective layer may be applied to the rough surface, and a pattern of radiation-absorbing moieties is formed on the reflective layer. However, the inventors of the present invention have recognised that aberration measurements made with diffuse reflection gratings depend on the particular rough surface used to generate the diffusion.

[0014] However, contrary to the strong preconception in the art that diffusion should be used to fully test a projection system, the present inventors have surprisingly come to the realization that measurements using specular diffraction gratings can provide improved aberration measurements for at least some aberrations (i.e., Zernike orders). In particular, even when using illumination modes achievable by illumination systems of known lithography systems (e.g., those having illumination pupil fill of the order of 20%), the use of specular diffraction gratings can provide better aberration measurements for at least some aberrations (i.e., Zernike orders) than the use of known diffuse gratings.

[0015] Another advantage of using a specular diffraction grating at the reticle level for aberration measurements is that such a specular diffraction grating can be more conveniently provided on the reticle. While in principle, it is possible to form a diffusely reflecting grating on the reticle, in practice, such an arrangement is often impractical and is not currently standard practice (or even feasible). Therefore, it is currently standard practice to provide a diffusely reflecting grating elsewhere on the reticle stage, such as on a fiducial. However, if aberration measurements could be performed using a specular diffraction grating on the reticle, significant time savings could be achieved during aberration measurements. For example, currently, after exposing a target area of ​​a substrate to radiation patterned by the reticle, the reticle stage must be moved so that the fiducial (rather than the reticle) receives EUV radiation from the illumination system in order to perform the aberration measurements. Furthermore, after such aberration measurements, in order to expose a target area of ​​a substrate to radiation patterned by the reticle, the reticle stage must be moved so that the reticle (rather than the fiducial) receives EUV radiation from the illumination system. However, if the mirrored diffraction grating used for aberration measurement could be provided on the reticle, the reticle stage would not need to be moved as far between substrate exposure and aberration measurement, resulting in significant time savings. Furthermore, aberration measurements from the method according to the first aspect allow lower order Zernike coefficients (e.g., Z2, Z3, and Z4) to be used for reticle alignment. This may advantageously mean that it may not be necessary to use additional sensors for reticle alignment (which would also be provided elsewhere on the reticle stage).

[0016]

[0016] The above method may include performing a plurality of phase stepping processes or phase scanning processes, each of the plurality of phase stepping processes or phase scanning processes being performed using a different illumination mode so that different portions of the radiation detector receive radiation during different ones of the plurality of phase stepping processes or phase scanning processes, but otherwise being similar to other phase stepping processes or phase scanning processes, and the step of determining one or more aberrations of the projection system may include using vibration signals determined by each portion of the radiation detector receiving radiation from any one of the plurality of phase stepping processes or phase scanning processes.

[0017]

[0017] By performing multiple different phase stepping or phase scanning processes, each using a different illumination mode, each different phase stepping or phase scanning process can advantageously inspect a different portion of the pupil plane of the projection system. Note that aberrations or aberration maps are not determined for each individual phase stepping or phase scanning process and then combined. Rather, aberrations or aberration maps are determined using all of the multiple phase stepping or phase scanning processes. Such an embodiment advantageously allows for higher resolution measurement of the aberrations of the projection system.

[0018]

[0018] The above method may further include performing an additional phase stepping process or phase scanning process using a diffuse diffraction grating as the first patterning device but otherwise being similar to the above phase stepping process or phase scanning process or similar to each of the other phase stepping processes or phase scanning processes; determining one or more additional aberrations of the projection system from vibration signals determined by each portion of the radiation detector receiving the radiation during the additional phase stepping process or phase scanning process; and determining calibration data characterizing a difference between (a) the determined one or more aberrations of the projection system and (b) the determined one or more additional aberrations of the projection system.

[0019]

[0019] Such an arrangement may enable the generation of calibration data that can be used in subsequent measurements to correct for additional aberrations of the projection system that are generated using a diffuse diffraction grating as the first patterning device. This has the advantage that it allows for correcting and removing significant errors that may occur when measuring some aberrations (e.g. the amplitudes of some Zernike orders in the aberration map of the projection system) using such a diffuse diffraction grating as the first patterning device. This has the advantage that subsequent aberration measurements can be performed using a diffuse diffraction grating as the first patterning device, which may allow the subsequent aberration measurements to be performed faster, potentially increasing the throughput of the lithographic apparatus.

[0020]

[0020] The method may further include storing the determined calibration data in a memory.

[0021]

[0021] According to a second aspect of the present disclosure, there is provided a calibration method, the method comprising: performing a phase stepping process or a phase scanning process using an object-level specular diffraction grating to generate one or more vibration signals; determining one or more aberrations of the projection system from the one or more vibration signals; performing an additional phase stepping process or a phase scanning process using an object-level diffuse diffraction grating to generate one or more additional vibration signals; determining one or more additional aberrations of the projection system from the one or more additional vibration signals; and determining calibration data characterizing a difference between (a) the determined one or more aberrations of the projection system and (b) the determined one or more additional aberrations of the projection system.

[0022]

[0022] Such an arrangement may enable the generation of calibration data that can be used in subsequent measurements to correct for additional aberrations of the projection system that are generated using a diffuse diffraction grating as the first patterning device. This has the advantage that it allows for correcting and removing significant errors that may occur when measuring some aberrations (e.g. the amplitudes of some Zernike orders in the aberration map of the projection system) using such a diffuse diffraction grating as the first patterning device. This has the advantage that subsequent aberration measurements can be performed using a diffuse diffraction grating as the first patterning device, which may allow the subsequent aberration measurements to be faster, potentially increasing the throughput of the lithographic apparatus.

[0023]

[0023] The above method may further include performing a subsequent phase stepping process or phase scanning process which is performed using a diffuse diffraction grating as the first patterning device but is otherwise similar to the above phase stepping process or phase scanning process or similar to each of the other phase stepping processes or phase scanning processes, determining one or more raw subsequent aberrations of the projection system from vibration signals determined by each portion of the radiation detector receiving the radiation during the subsequent phase stepping process or phase scanning process, and determining one or more corrected subsequent aberrations from the one or more raw subsequent aberrations of the projection system and calibration data.

[0024]

[0024] The illumination radiation may include extreme ultraviolet radiation.

[0025] The first patterning device may be provided on the fiducial.

[0026] The first patterning device may be provided on a reticle.

[0027]

[0027] Determining either one or more aberrations of the projection system, one or more additional aberrations of the projection system, or one or more raw subsequent aberrations of the projection system from the vibration signals determined by the portions of the radiation detector that receive the radiation may comprise any known shearing interferometry technique, as needed or desired. For example, determining any of these aberrations may comprise any of the techniques disclosed in WO2019 / 149468, the contents of which are incorporated herein by reference in their entirety. For clarity, determining any of these aberrations may comprise any of the techniques disclosed as prior art in WO2019 / 149468 and / or any of the techniques taught in WO2019 / 149468.

[0028]

[0028] Either of the phase stepping or phase scanning processes may further include illuminating a first patterning device with illumination radiation, the first patterning device including a mirrored diffraction grating arranged to form a plurality of first diffracted beams separated in a second shear direction; projecting at least some of the plurality of first diffracted beams onto a sensor arrangement with a projection system; and moving at least one of the first and second patterning devices in the second shear direction such that the intensity of the radiation received by each portion of the radiation detector receiving the radiation varies as a function of movement in the second shear direction, thereby forming an oscillatory signal.

[0029] For example, the first patterning device may include a first portion positioned to shear the illumination radiation in a shear direction and a second portion positioned to shear the illumination radiation in a second shear direction. While a first portion of the first patterning device is illuminated, at least one of the first patterning device and the second patterning device is stepped in the shear direction. This generates first phase stepping data, which may be related to at least one order of a gradient of the aberration map in the shear direction. While a second portion of the first patterning device is illuminated, at least one of the first patterning device and the second patterning device is stepped in the second shear direction. This generates second phase stepping data, which may be related to at least one order of a gradient of the aberration map in the second shear direction. Combining the first and second phase stepping data determines one or more aberrations of the projection system. The first and second phase stepping data may be combined to determine an aberration (or relative phase) map of the projection system.

[0030]

[0030] Determining either one or more aberrations of the projection system, one or more additional aberrations of the projection system, or one or more raw subsequent aberrations of the projection system from the vibration signal determined by each portion of the radiation detector receiving the radiation may include equalizing the phase of the harmonics of the vibration signal received by each portion of the radiation detector receiving the radiation to the sum of at least one difference in the aberration map between a pair of positions in the pupil plane of the projection system.

[0031]

[0031] The harmonic of the vibration signal may be the first harmonic of the vibration signal.

[0032]

[0032] According to a third aspect of the present disclosure, there is provided a computer-readable medium carrying a computer program including computer-readable instructions configured to cause a computer to execute a method relating to the first aspect of the present disclosure or the second aspect of the present disclosure.

[0033]

[0033] According to a fourth aspect of the present disclosure, there is provided a computer apparatus comprising a memory storing processor-readable instructions and a processor arranged to read and execute the instructions stored in the memory, wherein the processor-readable instructions comprise instructions arranged to control a computer to perform a method according to the first aspect of the present disclosure or the second aspect of the present disclosure.

[0034] According to a fifth aspect of the present disclosure, there is provided a measurement system for determining one or more aberrations of a projection system, the measurement system comprising: a first patterning device including a specular diffraction grating; a sensor arrangement including a second patterning device and a radiation detector; a positioning apparatus configured to move at least one of the first patterning device and the sensor arrangement; and a controller, wherein the first patterning device and the sensor arrangement are positionable such that the projection system is capable of forming an image of the first patterning device on the second patterning device by illumination radiation, and wherein the radiation detector detects an image of the illumination radiation after it has passed through the second patterning device. The radiation detector is positionable so as to receive radiation, and the controller is configured to control the positioning device to move at least one of the first patterning device and the sensor device in a shear direction, thereby controlling the positioning device to move the first patterning device and the sensor device in a shear direction, so that the intensity of the radiation received by each portion of the radiation detector varies as a function of the movement in the shear direction to form a vibration signal; determine from the radiation detector the phases of harmonics of the vibration signal at multiple positions on the radiation detector; and determine at least one coefficient characterizing at least one aberration of the projection system from the phases of the harmonics of the vibration signal at multiple positions on the radiation detector.

[0035]

[0035] The measurement system according to the fifth aspect may be suitable for use in a lithographic apparatus. The first patterning device may be arranged at reticle level (i.e. in an object plane of the lithographic apparatus) and the sensor apparatus may be arranged at wafer level (i.e. in an image plane of the lithographic apparatus). In particular, the measurement system according to the fifth aspect may be suitable for use in an extreme ultraviolet (EUV) lithographic apparatus. That is, the illumination radiation may include EUV radiation.

[0036] It will be appreciated that the measurement system described above is in the form of a shearing interferometer.

[0037]

[0037] The controller may be configured to perform the method according to the first aspect of the present disclosure or the second aspect of the present disclosure.

[0038] The system may further include an illumination system operable to illuminate the first patterning device with illumination radiation.

[0039]

[0039] The illumination radiation may include extreme ultraviolet radiation.

[0040] The first patterning device may be provided on a fiducial.

[0041] The first patterning device may be provided on a reticle.

[0042] According to a sixth aspect of the present disclosure, there is provided a lithographic apparatus including a measurement system according to the fifth aspect of the present disclosure. [Brief explanation of the drawings]

[0043]

[0043] Some embodiments of the present invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which: [Figure 1] 1 depicts a lithography system comprising a lithographic apparatus and a radiation source. [Figure 2] 1 is a schematic diagram of a measurement system according to an embodiment of the present invention; [Figure 3A] 3 is a schematic diagram of a patterning device that may form part of the measurement system of FIG. 2; [Figure 3B] FIG. 3 is a schematic diagram of a sensor device that may form part of the measurement system of FIG. 2; [Figure 4] 1 is a schematic diagram of a measurement system according to an embodiment of the present invention, the measurement system including a first patterned area and a second patterned area, the first patterned area being arranged to receive radiation and form a plurality of first diffracted beams. [Figure 5A]5 shows different sets of second diffracted beams formed by the second patterned area of ​​the measurement system shown in FIG. 4, where the different sets of second diffracted beams are generated by different first diffracted beams formed by the first patterned area. [Figure 5B] 5 shows different sets of second diffracted beams formed by the second patterned area of ​​the measurement system shown in FIG. 4, where the different sets of second diffracted beams are generated by different first diffracted beams formed by the first patterned area. [Figure 5C] 5 shows different sets of second diffracted beams formed by the second patterned area of ​​the measurement system shown in FIG. 4, where the different sets of second diffracted beams are generated by different first diffracted beams formed by the first patterned area. [Figure 6A] 1 is a schematic diagram of a novel measurement system for determining one or more aberrations of a projection system (e.g., the projection system of the lithography system shown in FIG. 1 ), where, for ease of explanation, the measurement system is shown as a linear arrangement in which the optical axis is maintained in a single direction (it will be understood that in practice the first patterning device and components in the projection system are reflective and in practice the optical axis will change with each such reflection). [Figure 6B] 1 shows a typical measurement system for the aberrations of a projection system of a known EUV lithography apparatus that uses a diffusely reflecting grating as the first patterning device (at reticle level). [Figure 7] 6A is a schematic diagram of a novel method for determining one or more aberrations of a projection system (eg, the projection system of the lithography system shown in FIG. 1), which method can be implemented by the measurement system shown in FIG. 6A. [Figure 8]7 shows the results of a simulation performed to evaluate the performance of the method shown schematically in Fig. 7. The first plot shows the aberration footprint of the projection system input into the simulation for the first 25 Zernike orders, the second plot shows the reconstructed aberration footprint determined using the method of Fig. 7 (i.e., using a specular diffraction grating) for the first 25 Zernike orders at an illumination pupil fill of 20.7%, and the third plot shows the difference between the second and first plots. [Figure 9] 8 is a schematic diagram of a second novel method for determining one or more aberrations of a projection system, which method can be performed by the measurement system shown in FIG. 6A and is a variation of the method shown in FIG. 7. [Figure 10] FIG. 10 is another schematic diagram illustrating a second novel method for determining one or more aberrations of a projection system as shown in FIG. 9. [Figure 11] FIG. 6B is a schematic diagram of a third novel method for determining one or more aberrations of a projection system, which method can be performed by the measurement system shown in FIG. 6A and is a variation of the method shown in FIG. 7; this third method is a calibration method using shearing interferometry. [Figure 12] 12 is a schematic diagram of a fourth novel method for determining one or more aberrations of a projection system, which method can be performed by the measurement system shown in FIG. 6A and is a variation of the method shown in FIG. 11. DETAILED DESCRIPTION OF THE INVENTION

[0044] 1 shows a lithography system including a radiation source SO and a lithography apparatus LA. The radiation source SO is configured to generate a beam of EUV radiation B and to provide this beam of EUV radiation B to the lithography apparatus LA. The lithography apparatus LA comprises an illumination system IL, a support structure MT configured to support a patterning device MA (e.g. a mask), a projection system PS, and a substrate table WT configured to support a substrate W. In this specification, the patterning device MA may alternatively be referred to as a reticle. The support structure MT may alternatively be referred to as a reticle stage. In this specification, the substrate W may alternatively be referred to as a wafer. The substrate table WT may alternatively be referred to as a substrate stage or a wafer stage.

[0045] The illumination system IL is configured to condition the EUV radiation beam B before it is incident on the patterning device MA. Furthermore, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. Together, the facetted field mirror device 10 and the facetted pupil mirror device 11 impart a desired cross-sectional shape and a desired intensity distribution to the EUV radiation beam B. The illumination system IL may include other mirrors or devices in addition to or instead of the facetted field mirror device 10 and the facetted pupil mirror device 11.

[0046]

[0046] Having been so conditioned, the EUV radiation beam B interacts with the patterning device MA. This interaction results in a patterned EUV radiation beam B'. The projection system PS is configured to project this patterned EUV radiation beam B' onto the substrate W. To this end, the projection system PS may include a number of mirrors 13, 14 configured to project the patterned EUV radiation beam B' onto the substrate W held by a substrate table WT. The projection system PS may apply a demagnification factor to the patterned EUV radiation beam B', thereby forming an image having smaller features than corresponding features on the patterning device MA. For example, a demagnification factor of 4 or 8 may be applied. Although in Figure 1 the projection system PS is depicted as having only two mirrors 13, 14, the projection system PS may include a different number of mirrors (e.g. 6 or 8 mirrors).

[0047]

[0047] The substrate W may include a previously formed pattern, in which case the lithographic apparatus LA aligns the image formed by the patterned EUV radiation beam B' with the previously formed pattern on the substrate W.

[0048] A relative vacuum, ie a small amount of gas (eg hydrogen) at a pressure significantly below atmospheric pressure, may be provided within the source SO, illumination system IL and / or projection system PS.

[0049]

[0049] The source SO may be a laser-produced plasma (LPP) source, a discharge-produced plasma (DPP) source, a free-electron laser (FEL) or any other source capable of producing EUV radiation.

[0050] In general, the projection system PS has an optical transfer function, which may not be constant and may affect the pattern imaged onto the substrate W. For unpolarized radiation, such effects can be described quite well by two scalar maps, which represent the transmission (apodization) and relative phase (aberration) of radiation exiting the projection system PS as a function of position in its pupil plane. These scalar maps, sometimes called transmission and relative phase maps, may be expressed as linear combinations of a complete set of basis functions. A particularly convenient set are 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 such an expansion. Because the Zernike polynomials are orthogonal on the unit circle, the Zernike coefficients may be found from a measured scalar map by calculating the dot product of the measured scalar map with each Zernike polynomial in turn and dividing this by the square of the norm of that Zernike polynomial. Hereinafter, unless otherwise indicated, any reference to Zernike coefficients will be understood to mean the Zernike coefficients of the relative phase map (also referred to herein as the aberration map). It will be understood that in alternative embodiments, other sets of basis functions may be used. For example, in some embodiments, Tatian Zernike polynomials may be used, e.g., for an obstructed aperture system.

[0051]

[0051] A wavefront aberration map represents the wavefront distortion (as a function of position in the pupil plane, or as a function of the angle at which the radiation approaches the image plane of the projection system PS) of light approaching a point in the image plane of the projection system PS from a spherical wavefront. As mentioned above, this wavefront aberration map W(x,y) may be expressed as a linear combination of Zernike polynomials as follows:

number

[0052] It will be understood that only a finite number of Zernike orders are considered. The different Zernike coefficients of the phase map may provide information about different forms of aberration due to the projection system PS. The Zernike coefficient with a Noll index of 1 may be referred to as the first Zernike coefficient, the Zernike coefficient with a Noll index of 2 may be referred to as the second Zernike coefficient, and so on.

[0053] The first Zernike coefficient Z1 relates to the average value of the measured wavefront (which is sometimes called the piston). The first Zernike coefficient may be unrelated to the performance of the projection system PS and therefore may not be determined using the methods described herein. The second Zernike coefficient Z2 relates to the tilt of the measured wavefront in the x direction. The tilt of the wavefront in the x direction is equal to the disposition in the x direction. The third Zernike coefficient Z3 relates to the tilt of the measured wavefront in the y direction. The tilt of the wavefront in the y direction is equal to the disposition in the y direction. The fourth Zernike coefficient Z4 relates to the defocus of the measured wavefront. The fourth Zernike coefficient is equal to the disposition in the z direction. Higher order Zernike coefficients relate to other forms of aberration due to the projection system (e.g., astigmatism, coma, spherical aberration and other effects).

[0054] Throughout this description, the term "aberration" should be intended to include all forms of wavefront deviation from a perfect spherical wavefront. That is, the term "aberration" may relate to image geometry (e.g., second, third, and fourth Zernike coefficients) and / or higher order aberrations, such as those related to Zernike coefficients having Noll indices of 5 or greater. Furthermore, any reference to an aberration map of a projection system may include all forms of wavefront deviation from a perfect spherical wavefront, including those due to image geometry.

[0055]

[0055] The transmission map and relative phase map are field and system dependent, i.e., in general, each projection system PS will have a different Zernike expansion at each field point (i.e. at each spatial location in its image plane).

[0056]

[0056] As will be explained in more detail below, the relative phase in the pupil plane of the projection system PS can be determined by projecting radiation from the object plane of the projection system PS (i.e. the plane of the patterning device MA) through the projection system PS and measuring the wavefront (i.e. the locus of points of identical phase) using a shearing interferometer. The shearing interferometer may comprise a diffraction grating, for example a two-dimensional grating, in the image plane of the projection system (i.e. the substrate table WT), and a detector positioned to detect the interference pattern in a plane conjugate to the pupil plane of the projection system PS.

[0057] The projection system PS comprises a number of optical elements (including mirrors 13, 14). As already explained, in FIG. 1 the projection system PS is shown as having only two mirrors 13, 14, but the projection system PS may include a different number of mirrors (e.g., six or eight mirrors). The lithographic apparatus LA further comprises adjustment means PA for adjusting these optical elements to correct for aberrations (any type of phase variation across the pupil plane across the field). To achieve this, the adjustment means PA may be operable to manipulate the optical elements in the projection system PS in one or more different ways. The projection system may have a coordinate system whose optical axis extends in the z-direction (it will be understood that the direction of this z-axis varies along the optical path through the projection system, for example for each mirror or optical element). The adjustment means PA may be operable to displace one or more optical elements, tilt one or more optical elements, and / or deform one or more optical elements in any combination. The displacement of the optical elements may be in any direction (x, y, z or a combination thereof). Tilting of an optical element is typically performed out of a plane perpendicular to the optical axis by rotating it about an axis in the x or y direction, although rotation about the z axis can be used for non-rotationally symmetric optical elements. Deformation of the optical element can be achieved, for example, by exerting a force on the side of the optical element using an actuator and / or by heating selected areas of the optical element using a heating element. In general, it may not be possible to adjust the projection system PS to correct for apodization (variation in transmission across the pupil plane). A transmission map of the projection system PS can be used when designing a mask MA for the lithographic apparatus LA.

[0058] In some embodiments, the adjustment means PA may be operable to move the support structure MT and / or the substrate table WT. The adjustment means PA may be operable to displace the support structure MT and / or the substrate table WT (in either or a combination of the x, y and z directions) and / or to tilt the support structure MT and / or the substrate table WT (by rotating about an axis in the x or y direction).

[0059] The projection system PS, which constitutes part of the lithographic apparatus, may be subjected to a calibration process periodically. For example, when the lithographic apparatus is manufactured in a factory, an initial calibration process may be carried out to set up the optical elements (e.g., mirrors) that constitute the projection system PS. After the lithographic apparatus has been installed at the location where it will be used, the projection system PS may be calibrated again. Further calibrations of the projection system PS may be carried out at regular intervals. For example, in normal use, the projection system PS may be calibrated every few months (e.g., every three months).

[0060] Calibrating the projection system PS may include passing radiation through the projection system PS and measuring the resulting projection radiation. The measurements of the projection radiation may be used to determine aberrations in the projection radiation caused by the projection system PS. The aberrations caused by the projection system PS may be determined using a measurement system. In response to the determined aberrations, optical elements comprising the projection system PS may be adjusted to correct for the aberrations caused by the projection system PS.

[0061]

[0061] Figure 2 is a schematic diagram of a measurement system 10 that can be used to determine aberrations due to a projection system PS. The measurement system 10 comprises an illumination system IL, a measurement patterning device MA', a sensor arrangement 21, and a controller CN. The measurement system 10 may form part of a lithographic apparatus. For example, the illumination system IL and the projection system PS shown in Figure 2 may be the illumination system IL and the projection system PS of the lithographic apparatus shown in Figure 1. For simplicity, additional components of the lithographic apparatus are not shown in Figure 2.

[0062] The measurement patterning device MA′ is positioned to receive radiation from the illumination system IL. The sensor apparatus 21 is positioned to receive radiation from the projection system PS. During normal use of the lithographic apparatus, the measurement patterning device MA′ and the sensor apparatus 21 shown in FIG. 2 may be in positions different from those shown in FIG. 2. For example, during normal use of the lithographic apparatus, a patterning device MA configured to form a pattern to be transferred onto a substrate W (e.g., as shown in FIG. 1) may be positioned to receive radiation from the illumination system IL, and the substrate W may be positioned to receive radiation from the projection system PS. The measurement patterning device MA′ and the sensor apparatus 21 may be moved to the positions shown in FIG. 2 to determine aberrations caused by the projection system PS. The measurement patterning device MA′ may be supported by a support structure MT, such as the support structure shown in FIG. 1. The sensor apparatus 21 may be supported by a substrate table, such as the substrate table WT shown in FIG. 1. Alternatively, the sensor device 21 may be supported by a measurement table (not shown), which may be a separate table from the sensor table WT.

[0063]

[0063] The measurement patterning device MA' and the sensor arrangement 21 are shown in more detail in Figures 3A and 3B. Cartesian coordinates are used throughout Figures 2, 3A and 3B. Figure 3A is a schematic illustration of the measurement patterning device MA' in the xy-plane, and Figure 3B is a schematic illustration of the sensor arrangement 21 in the xy-plane.

[0064] The measurement patterning device MA' includes a plurality of patterned regions 15a-15c. In the embodiment shown in Figures 2 and 3A, the measurement patterning device MA' is a reflective patterning device MA'. The patterned regions 15a-15c each include a reflective diffraction grating. Radiation incident on the patterned regions 15a-15c of the measurement patterning device MA' is at least partially scattered thereby and received by the projection system PS. In contrast, radiation incident on the remainder of the measurement patterning device MA' is not reflected or scattered towards the projection system PS (e.g., may be absorbed by the measurement patterning device MA').

[0065]

[0065] The illumination system IL illuminates the measurement patterning device MA' with radiation. Although not shown in Figure 2, the illumination system IL can receive radiation from a radiation source SO and condition the received radiation to illuminate the measurement patterning device MA'. For example, the illumination system IL can condition the received radiation to provide radiation having a desired spatial and angular distribution. In the embodiment shown in Figure 2, the illumination system IL is configured to form separate measurement beams 17a-17c. Each measurement beam 17a-17c illuminates a respective patterned area 15a-15c of the measurement patterning device MA'.

[0066] To determine the aberrations due to the projection system PL, the mode of the illumination system IL may be changed to illuminate the measurement patterning device MA' with separate measurement beams 17a-17c. For example, during normal operation of the lithographic apparatus, the illumination system IL may be configured to illuminate the patterning device MA with a slit of radiation. However, to determine the aberrations due to the projection system PL, the mode of the illumination system IL may be changed so that the illumination system IL forms separate measurement beams 17a-17c. In some embodiments, different patterned areas 15a-15c may be illuminated at different times. For example, a first subset of the patterned areas 15a-15c may be illuminated at a first time to form a first subset of measurement beams 17a-17c, and a second subset of the patterned areas 15a-15c may be illuminated at a second time to form a second subset of measurement beams 17a-17c.

[0067] In other embodiments, the mode of the illumination system IL may not be changed in order to determine the aberrations caused by the projection system PL. For example, the illumination system IL may be configured to illuminate the measurement patterning device MA' with a slit of radiation (e.g. substantially corresponding to the illumination area used during exposure of a substrate). Separate measurement beams 17a-17c may then be formed by the measurement patterning device MA', such that only the patterned regions 15a-15c reflect or scatter radiation towards the projection system PS.

[0068] In the figures, the Cartesian coordinate system is shown as being preserved throughout the projection system PS. However, in some embodiments, coordinate system transformations may occur due to properties of the projection system PS. For example, the projection system PS may form an image of the measurement patterning device MA′ that is magnified, rotated, and / or mirrored with respect to the measurement patterning device MA′. In some embodiments, the projection system PS may rotate the image of the measurement patterning device MA′ by approximately 180 degrees about the z-axis. In such an embodiment, the relative positions of the first measurement beam 17a and the third measurement beam 17c shown in FIG. 2 may be swapped. In other embodiments, the image may be mirrored about an axis that may lie in the x-y plane. For example, the image may be mirrored about the x-axis or the y-axis.

[0069] In embodiments in which the projection system PS rotates the image of the measurement patterning device MA′ and / or in which the image is reflected by the projection system PS, the projection system is considered to transform the coordinate system. That is, the coordinate systems referred to herein are defined relative to the image projected by the projection system PS, and any rotation and / or reflection of the image causes a corresponding rotation and / or reflection of the coordinate system. For ease of explanation, the drawings show the coordinate system as being preserved by the projection system PS. However, in some embodiments, the coordinate system may be transformed by the projection system PS.

[0070]

[0070] Patterned regions 15a-15c modify measurement beams 17a-17c. In particular, patterned regions 15a-15c cause spatial modulation of measurement beams 17a-17c, causing diffraction of measurement beams 17a-17c. In the embodiment shown in Figure 3B, patterned regions 15a-15c each include two separate portions. For example, first patterned region 15a includes first portion 15a' and second portion 15a''. First portion 15a' includes a diffraction grating aligned parallel to the u direction, and second portion 15a'' includes a diffraction grating aligned parallel to the v direction. The u and v directions are shown in Figure 3A. Both the u and v directions are aligned at approximately 45 degrees with respect to both the x and y directions and are aligned perpendicular to each other. The second patterned region 15b and the third patterned region 15c shown in FIG. 3A are identical to the first patterned region 15a, each including a first portion and a second portion, and their diffraction gratings are aligned perpendicular to each other.

[0071] First and second portions of patterned regions 15a-15c may be illuminated by measurement beams 17a-17c at different times. For example, a first portion of each of patterned regions 15a-15c may be illuminated by measurement beams 17a-17c at a first time point. At a second time point, a second portion of each of patterned regions 15a-15c may be illuminated by measurement beams 17a-17c. As mentioned above, in some embodiments, different patterned regions 15a-15c may be illuminated at different times. For example, a first portion of a first subset of patterned regions 15a-15c may be illuminated at a first time point, and a first portion of a second subset of patterned regions 15a-15c may be illuminated at a second time point. The second portions of the first and second subsets of patterned regions may be illuminated simultaneously or at different times. In general, any schedule for illuminating different portions of patterned regions 15a-15c may be used.

[0072] The modified measurement beams 17a-17c are received by the projection system PS. The projection system PS forms an image of the patterned areas 15a-15c on a sensor arrangement 21. The sensor arrangement 21 comprises a plurality of diffraction gratings 19a-19c and a radiation detector 23. The diffraction gratings 19a-19c are arranged such that each diffraction grating 19a-19c receives a respective modified measurement beam 17a-17c output from the projection system PL. The projection system PS is arranged to form an image of each of the patterned areas 15a-15c on one of the diffraction gratings 19a-19c of the sensor arrangement 21. The modified measurement beams 17a-17c incident on the diffraction gratings 19a-19c are further modified by the diffraction gratings 19a-19c. The modified measurement beams transmitted by the diffraction gratings 19a-19c are incident on the radiation detector 23.

[0073] The radiation detector 23 is configured to detect a spatial intensity profile of radiation incident on the radiation detector 23. The radiation detector 23 may, for example, comprise an array of individual detector elements or sensing elements. For example, the radiation detector 23 may comprise an active pixel sensor (such as a CMOS (Complementary Metal Oxide Semiconductor) sensor array). Alternatively, the radiation detector 23 may comprise a CCD (Charge Coupled Device) sensor array. The diffraction gratings 19a-19c and the portions of the radiation sensor 23 from which the modified measurement beams 17a-17c are received define detector areas 25a-25c. For example, the first diffraction grating 19a and the first portion of the radiation sensor 23 from which the first measurement beam 17a is received together define the first detector area 25a. Measurements of a given measurement beam 17a-17c may be made at respective detector areas 25a-25c (as shown). As mentioned above, in some embodiments, the relative positioning of the modified measurement beams 17a-17c and the coordinate system may be transformed by the projection system PS.

[0074]

[0074] As a result of the modifications of the measurement beams 17a-17c that occur at the diffraction gratings 19a-19c in the patterned regions 15a-15c and detector regions 25a-25c, an interference pattern is formed at the radiation detector 23. This 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.

[0075]

[0075] Typically, the diffraction gratings 19a-19c of each detector region 25a-25c include two-dimensional transmission diffraction gratings. In the embodiment shown in Figure 3B, each detector region 25a-25c includes a diffraction grating 19a-19c configured in a checkerboard shape. In alternative embodiments, each detector region 25a-25c may include a two-dimensional transmission diffraction grating 19a-19c that is not configured in a checkerboard shape, and the two-dimensional transmission diffraction gratings 19a-19c may instead be, for example, a pinhole array.

[0076]

[0076] Illuminating a first portion of the patterned regions 15a-15c can provide information related to the gradient in a first direction of the aberration map of the projection system PS, and illuminating a second portion of the patterned regions 15a-15c can provide information related to the gradient in a second direction of the aberration map of the projection system PS.

[0077] In some embodiments, the measurement patterning device MA' and / or sensor apparatus 21 are scanned and / or stepped sequentially in two perpendicular directions. For example, the measurement patterning device MA' and / or sensor apparatus 21 may be stepped in the u direction and the v direction relative to each other. While the second portions 15a''-15c'' of the patterned regions 15a-15c are illuminated, the measurement patterning device MA' and / or sensor apparatus 21 may be stepped in the u direction, and while the first portions 15a'-15c' of the patterned regions 15a-15c are illuminated, the measurement patterning device MA' and / or sensor apparatus 21 may be stepped in the v direction. That is, the measurement patterning device MA' and / or sensor apparatus 21 may be stepped in a direction perpendicular to the arrangement of the diffraction gratings being illuminated.

[0078]

[0078] The measurement patterning device MA' and / or the sensor arrangement 21 may be stepped by a distance corresponding to a fraction of the grating period of the diffraction grating. Measurements made at different stepping positions may be analyzed to derive information about the wavefront in the stepping direction. For example, the phase of the first harmonic of the measurement signal (which may also be called a phase stepping signal) may contain information about the derivative of the wavefront in the stepping direction (i.e., the gradient of the wavefront map in the stepping direction). Thus, by stepping the measurement patterning device MA' and / or the sensor arrangement 21 in both the (mutually perpendicular) u and v directions, information about the wavefront can be derived in two perpendicular directions (in particular, information about the derivative of the wavefront in each of the two perpendicular directions is provided), which allows the entire wavefront to be reconstructed.

[0079] In addition to stepping the measurement patterning device MA′ and / or sensor apparatus 21 in a direction perpendicular to the array of the illuminated diffraction grating (as described above), the measurement patterning device MA′ and / or sensor apparatus 21 may be scanned relative to one another. The scanning of the measurement patterning device MA′ and / or sensor apparatus 21 may be in a direction parallel to the lines of the illuminated diffraction grating. For example, the measurement patterning device MA′ and / or sensor apparatus 21 may be scanned in the u direction while a first portion 15a′-15c′ of the patterned areas 15a-15c is illuminated, and the measurement patterning device MA′ and / or sensor apparatus 21 may be scanned in the v direction while a second portion 15a″-15c″ of the patterned areas 15a-15c is illuminated. By scanning the measurement patterning device MA′ and / or sensor apparatus 21 in a direction parallel to the array of the illuminated diffraction grating, measurements can be averaged across the entire diffraction grating, thereby accounting for any variations in the diffraction grating in the scan direction. The scanning of the measurement patterning device MA' and / or the sensor arrangement 21 may occur at a different time than the stepping of the measurement patterning device MA' and / or the sensor arrangement 21 described above.

[0080] It will be appreciated that a variety of different arrangements of patterned regions 15a-15c and detector regions 25a-25c can be used to determine aberrations due to projection system PS. Patterned regions 15a-15c and / or detector regions 25a-25c may include diffraction gratings. In some embodiments, patterned regions 15a-15c and / or detector regions 25a-25c may include components other than diffraction gratings. For example, in some embodiments, patterned regions 15a-15c and / or detector regions may include a single slit or pinhole aperture through which at least a portion of measurement beam 17a-17c can propagate. In general, patterned regions and / or detector regions may include any arrangement that operates to modify the measurement beam.

[0081] [000081] The controller CN receives measurements made by the sensor arrangement 21 and determines from the measurements the aberrations due to the projection system PS. The controller may be configured to control one or more components of the measurement system 10. For example, the controller CN may control a positioning device PW operable to move the sensor arrangement 21 and / or the measurement patterning device MA' relative to each other. The controller may control adjusting means PA for adjusting components of the projection system PS. For example, the adjusting means PA may adjust optical elements of the projection system PS to correct the aberrations due to the projection system PS and determined by the controller CN.

[0082] In some embodiments, the controller CN may be operable to control the adjusting means PA for adjusting the support structure MT and / or the substrate table WT. For example, the adjusting means PA may adjust the support structure MT and / or the substrate table WT to correct aberrations due to positioning errors of the patterning device MA and / or the substrate W (wherein the aberrations are determined by the controller CN).

[0083] Determining the aberrations (which may be due to the projection system PS or to positioning errors of the patterning device MA or the substrate W) may involve fitting the measurements made by the sensor arrangement 21 to Zernike polynomials to obtain Zernike coefficients. Different Zernike coefficients may provide information about different forms of aberrations due to the projection system PS. The Zernike coefficients may be determined separately at different positions in the x and / or y directions. For example, in the embodiment shown in Figures 2, 3A and 3B, a Zernike coefficient may be determined for each measurement beam 17a-17c.

[0084] In some embodiments, the measurement patterning device MA' may include more than three patterned areas, the sensor apparatus 21 may include more than three detector areas, and more than three measurement beams may be formed. This may allow the Zernike coefficients to be determined at more positions. In some embodiments, the patterned areas and detector areas may be distributed at various positions in both the x and y directions. This may allow the Zernike coefficients to be determined at positions separated in both the x and y directions.

[0085]

[0085] In the embodiment shown in Figures 2, 3A and 3B, the measurement patterning device MA' includes three patterned areas 15a-15c and the sensor apparatus 21 includes three detector areas 25a-25c, but in other embodiments, the measurement patterning device MA' may include more or less than three patterned areas 15a-15c and / or the sensor apparatus 21 may include more or less than three detector areas 25a-25c.

[0086]

[0086] Any method known in the art may be used to determine the aberrations due to the projection system PS. Examples of such methods are disclosed in WO2019 / 149468, both of which are hereby incorporated by reference in their entirety. A method for determining the aberrations due to the projection system PS will now be briefly described with reference to Figure 4.

[0087]

[0087] In general, the measurement patterning device MA' comprises at least one first patterned area 15a-15c and the sensor arrangement 21 comprises at least one second patterned area 19a-19c.

[0088]

[0088] Figure 4 is a schematic diagram of a measurement system 30 that can be used to determine aberrations due to the projection system PS. The measurement system 30 may be the same as the measurement system 10 shown in Figure 2, but may have a different number of first patterned areas (on the measurement patterning device MA') and a different number of second patterned areas (in the sensor arrangement 21). The measurement system 30 shown in Figure 4 may therefore include any of the features of the measurement system 10 shown in Figure 2 above, which will not be described further below.

[0089] In FIG. 4, only a single first patterned area 31 is provided on the measurement patterning device MA′, and a single second patterned area 32 is provided on the sensor arrangement 21.

[0090]

[0090] The measurement patterning device MA' is illuminated with radiation 33 from the illumination system IL. For ease of understanding, only a single line is shown in Figure 4 (such a line may, for example, represent a single ray (e.g., chief ray) of an incident radiation beam). It will be understood, however, that the radiation 33 includes a range of angles that are incident on the first patterned area 31 of the measurement patterning device MA'. That is, each point on the first patterned area 31 of the measurement patterning device MA' may be illuminated by a cone of light (although, as will be explained further below, such a cone is generally not completely filled with light). Generally, each point is illuminated over substantially the same angular range, which is characterised by the intensity of the radiation in a pupil plane (not shown) of the illumination system IL.

[0091] The first patterned area 31 is arranged to receive radiation 33 and form a plurality of first diffracted beams 34, 35, 36. The central first diffracted beam 35 corresponds to the zeroth diffracted order beam of the first patterned area 31, and the other two first diffracted beams 34, 36 correspond to the ±1st diffracted order beams of the first patterned area 31. It will be understood that in general, there will also be more, higher order diffracted beams. Again, for ease of understanding, only three first diffracted beams 34, 35, 36 are shown in Figure 4.

[0092]

[0092] It will also be understood that when the incident radiation 33 comprises a (partially filled) cone of radiation converging at a point on the first patterned area 31, each of the first diffracted beams 34, 35, 36 also comprises a (partially filled) cone of radiation emanating from that point on the first patterned area 31.

[0093] To achieve the generation of the first diffracted beams 34, 35, and 36, the first patterned region 31 may take the form of a diffraction grating. For example, the first patterned region 31 may take the overall form of the patterned region 15a shown in FIG. 3A. In particular, at least a portion of the first patterned region 31 may take the form of the first portion 15a' of the patterned region 15a shown in FIG. 3A, i.e., a diffraction grating aligned parallel to the u direction (note that FIG. 4 is shown in the z-v plane). Thus, the first diffracted beams 34-36 are separated in the shear direction, which is the v direction.

[0094] The first diffracted beams 34-36 are at least partially captured by the projection system PS, as will be explained below. How much of the first diffracted beams 34-36 is captured by the projection system PS will depend on the pupil fill of the incident radiation 33 from the illumination system IL, the angular separation of the first diffracted beams 34-36 (which in turn depends on the pitch of the first patterned area 31 and the wavelength of the radiation 33), and the numerical aperture of the projection system PS.

[0095] Typically, substantially all of the first diffracted beam 35, corresponding to the zeroth diffracted beam order, and most of the first diffracted beams 34, 36, corresponding to the ±1st diffracted beam orders, are captured by the projection system PS and projected onto the sensor arrangement 21. (Furthermore, such an arrangement ensures that multiple diffracted beams generated by the first patterned area 31 are at least partially projected onto the sensor arrangement 21.)

[0096]

[0096] As will be discussed below, the role of the first patterned region 31 is to introduce spatial coherence.

[0097] In general, if two rays of radiation 33 from the illumination system IL are incident on the same point on the measurement patterning device MA′ at different angles of incidence, these two rays are not coherent. The first patterned area 31 can be thought of as receiving radiation 33 and forming multiple first diffracted beams 34, 35, 36, thereby forming multiple copies of the incident radiation cone 33 (the copies generally have different phases and intensities). These copies, or two radiation rays in any one of the first diffracted beams 34, 35, 36 that arise from the same point on the measurement patterning device MA′ at different scattering angles, are not coherent (due to the properties of the illumination system IL). However, for a given radiation ray in any one of the first diffracted beams 34, 35, 36, there exists a corresponding radiation ray in each of the other first diffracted beams 34, 35, 36 that is spatially coherent with the given ray. For example, the chief rays of each of the first diffracted beams 34, 35, 36 (corresponding to the chief ray of the incident radiation 33) are coherent and may interfere at the amplitude level when combined.

[0098] This coherence is utilized by the measurement system 30 to determine the aberration map of the projection system PS.

[0099] The projection system PS projects parts of the first diffracted beams 34, 35, 36 (captured by the numerical aperture of the projection system) onto the sensor arrangement 21.

[0100] [000100] In Figure 4, the sensor arrangement 21 comprises a single second patterned area 32. As will be explained further below (with reference to Figures 5A-5C), the second patterned area 32 is arranged to receive these first diffracted beams 34-36 from the projection system PS and to form a plurality of second diffracted beams from each of the first diffracted beams. To achieve this, the second patterned area 32 comprises a two-dimensional transmission grating. In Figure 4, all radiation transmitted by the second patterned area 32 is represented as a single arrow 38. This radiation 38 is received by a detector area 39 of the radiation detector 23 and used to determine the aberration map.

[0101] [000101] Each of the first diffracted beams 34-36 incident on the patterned region 32 is diffracted to form a plurality of second diffracted beams. Because the second patterned region 32 includes a two-dimensional diffraction grating, a two-dimensional array of second diffracted beams is generated from each incident first diffracted beam (the chief rays of these second diffracted beams are separated in both the shearing direction (v direction) and the direction perpendicular thereto (u direction)). Hereinafter, a diffraction order that is nth in the shearing direction (v direction) and mth in the non-shearing direction (u direction) will be referred to as the (n,m)th diffraction order of the second patterned region 32. Hereinafter, if the order of the second diffracted beam in the non-shearing direction (u direction) is not important, the (n,m)th diffraction order of the second patterned region 32 may also be simply referred to as the nth second diffracted beam.

[0102] 5A-5C show a set of second diffracted beams generated by each of the first diffracted beams 34-36. FIG. 5A shows a set of second diffracted beams 35a-35e generated by the first diffracted beam 35, which corresponds to the 0th order diffracted beam of the first patterned region 31. FIG. 5B shows a set of second diffracted beams 36a-36e generated by the first diffracted beam 36, which corresponds to the −1st order diffracted beam of the first patterned region 31. FIG. 5C shows a set of second diffracted beams 34a-34e generated by the first diffracted beam 34, which corresponds to the +1st order diffracted beam of the first patterned region 31.

[0103] 5A, second diffracted beam 35a corresponds to the zeroth diffracted order beam (in the shearing direction of second patterned region 32), while second diffracted beams 35b, 35c correspond to the ±1st diffracted order beams, and second diffracted beams 35d, 35e correspond to the ±2nd diffracted order beams. It will be understood that Figures 5A-5C are shown in the vz-plane, and the illustrated second diffracted beams may correspond, for example, to the zeroth diffracted order beam of second patterned region 32 in the non-shearing direction (i.e., u-direction). It will also be understood that there will be multiple copies of these second diffracted beams into and out of the page of Figures 5A-5C, representing higher diffracted order beams in the non-shearing direction.

[0104] [000104] In Figure 5B, the second diffracted beam 36a corresponds to the zeroth diffracted order beam (in the shearing direction of the second patterned region 32), while the second diffracted beams 36b, 36c correspond to the ±1st diffracted order beams, and the second diffracted beams 36d, 36e correspond to the ±2nd diffracted order beams.

[0105] [000105] In Figure 5C, the second diffracted beam 34a corresponds to the zeroth diffracted order beam (in the shearing direction of the second patterned region 32), while the second diffracted beams 34b, 34c correspond to the ±1st diffracted order beams, and the second diffracted beams 34d, 34e correspond to the ±2nd diffracted order beams.

[0106] 5A-5C show that some of the second diffracted beams spatially overlap one another. For example, second diffracted beam 35b, which arises from zeroth order diffracted beam 35 of first patterned area 31 and corresponds to the −1st order diffracted beam of second patterned area 32, overlaps with second diffracted beam 36a, which arises from −1st order diffracted beam 36 of first patterned area 31 and corresponds to the zeroth order diffracted beam of second patterned area 32. All the lines in FIGS. 4 and 5A-5C can be considered to represent a single radiation beam arising from a single input beam 33 from illumination system IL. Thus, as mentioned above, these lines represent spatially coherent rays that, when spatially overlapping at radiation detector 23, produce an interference pattern. Furthermore, this interference is between rays (separated in the shear direction) that have passed through different parts of pupil plane 37 of projection system PS. Therefore, the interference of radiation resulting from a single input beam 33 depends on the phase difference between two different parts of the pupil plane.

[0107] [000107] This spatial overlap and spatial coherence of the second diffracted beams at the radiation detector 23 is achieved by aligning the first and second patterned areas 31, 32 such that the angular separation (in the shear direction) between the different second diffracted beams arising from a given first diffracted beam is the same as the angular separation (in the shear direction) between the different first diffracted beams when focused onto the second patterned area 32. This spatial overlap and spatial coherence of the second diffracted beams at the radiation detector 23 is achieved by matching the pitch in the shear direction of the first and second patterned areas 31, 32. It will be appreciated that such matching of the pitch in the shear direction of the first and second patterned areas 31, 32 takes into account the demagnification factor applied by the projection system PS. As used herein, the pitch in a particular direction of a two-dimensional diffraction grating is defined as follows:

[0108] [000108] It will be understood that a one-dimensional diffraction grating comprises a series of lines formed in a repeating pattern (of reflectance or transmittance) in a direction perpendicular to the lines. The smallest non-repeating section forming the repeating pattern in a direction perpendicular to the lines is called a unit cell, and the length of this unit cell is called the pitch of the one-dimensional diffraction grating. In general, such a one-dimensional diffraction grating has a diffraction pattern such that an incident radiation beam is diffracted to form a one-dimensional array of angularly spaced (but potentially spatially overlapping) diffracted beams. The first patterned region 31 forms such a one-dimensional array of angularly spaced first diffracted beams 34-36, which are offset (angularly spaced) in the shear direction.

[0109] [000109] It will be understood that a two-dimensional diffraction grating comprises a two-dimensional repeating pattern of reflectance or transmittance. The smallest non-repeating section forming this repeating pattern can be called a unit cell. The unit cell can be rectangular, and the fundamental pitch of such a two-dimensional diffraction grating can be defined as the length of a rectangular unit cell. In general, such a two-dimensional diffraction grating has a diffraction pattern such that an incident radiation beam is diffracted to form a two-dimensional array of angularly spaced (but potentially spatially overlapping) diffracted beams. The axes of this two-dimensional (rectangular) array of diffracted beams are parallel to the sides of the unit cell. The angular separation between adjacent diffracted beams in these two directions can be given by the ratio of the wavelength of the radiation to the pitch of the grating. Thus, the smaller the pitch, the greater the angular separation between adjacent diffracted beams.

[0110] [000110] In some embodiments, the axes of the unit cells of the two-dimensional second patterned region 32 may be oriented at a non-zero angle relative to the shearing and non-shearing directions defined by the first patterned region 31. For example, the axes of the unit cells of the two-dimensional second patterned region 32 may be oriented at 45 degrees relative to the shearing and non-shearing directions defined by the first patterned region 31. As mentioned above, spatial overlap and spatial coherence of the second diffracted beams at the radiation detector 23, which enables wavefront measurements, is achieved by ensuring that the angular separation (in the shearing direction) between different second diffracted beams arising from a given first diffracted beam is the same as the angular separation (in the shearing direction) between the different first diffracted beams when focused onto the second patterned region 32. In arrangements where the axes of the unit cells of the two-dimensional second patterned region 32 are oriented at a non-zero angle (e.g., 45 degrees) to the shear and non-shear directions, it may be useful to define the pseudo-unit cell and pseudo-pitch as follows: The pseudo-unit cell is defined as the smallest non-repeating rectangle that forms the repeating pattern of the diffraction grating, oriented with its sides parallel to the shear and non-shear directions (defined by the first patterned region 31). The pseudo-pitch may be defined as the length of a rectangular pseudo-unit cell, which may be referred to as the shear direction pitch of the two-dimensional diffraction grating. This pseudo-pitch should be matched to (an integer multiple or a fraction of) the pitch of the first patterned region 31.

[0111] [000111] The diffraction pattern of a diffraction grating can be thought of as forming a two-dimensional array of angularly spaced (but potentially spatially overlapping) pseudo-diffracted beams, with the axes of this two-dimensional (rectangular) array of pseudo-diffracted beams parallel to the sides of the pseudo-unit cell. Because this square is not a unit cell (defined as the smallest square, in any orientation, that constitutes the repeating pattern of the diffraction grating), the pseudo-pitch is larger than the pitch (or fundamental pitch). Thus, there is less separation between adjacent pseudo-diffracted beams of the diffraction pattern (in a direction parallel to the sides of the pseudo-unit cell) than there is between adjacent diffracted beams of the diffraction pattern (in a direction parallel to the sides of the unit cell). This can be understood as follows: some of the pseudo-diffracted beams correspond to diffracted beams of the diffraction pattern, while other pseudo-diffracted beams are non-material and do not represent diffracted beams generated by the diffraction grating (and arise only from the use of a pseudo-unit cell that is larger than a true unit cell).

[0112] [000112] Taking into account any de-magnification (or magnification) factor applied by the projection system PS, the shear direction pitch of the second patterned region 32 must be an integer multiple of the shear direction pitch of the first patterned region 31, or vice versa. In the example shown in Figures 5A-5C, the shear direction pitch of the first and second patterned regions 31, 32 are substantially equal (taking into account any de-magnification factor).

[0113] 5A-5C, each point on the detector area 39 of the radiation detector 23 will generally receive several coherently added contributions. For example, a point on the detector area 39 receiving a second diffracted beam 35b arising from the zeroth order diffracted beam 35 of the first patterned area 31 and corresponding to the −1st order diffracted beam of the second patterned area 32 will overlap with both (a) a second diffracted beam 36a arising from the −1st order diffracted beam 36 of the first patterned area 31 and corresponding to the zeroth order diffracted beam of the second patterned area 32, and (b) a second diffracted beam 34d arising from the +1st order diffracted beam 34 of the first patterned area 31 and corresponding to the −2nd order diffracted beam of the second patterned area 32. It will be appreciated that when taking into account the higher order diffracted beams of the first patterned region 31, many more beams must be coherently summed at that portion of the detector region 39 to determine the intensity of radiation measured at each point on the detector region 39 (e.g., a corresponding pixel in a two-dimensional array of sensing elements).

[0114] [000114] In general, there are multiple different second diffracted beams that contribute to the radiation received by each part of the detector area 39. The intensity of the radiation from such a coherent sum is given by:

number

[0115] [000115] The phase difference ΔΦ between a pair of second diffracted beams idepends on two contributions: (a) a first contribution related to the different parts of the pupil plane 37 of the projection system PS from which the beams originate, and (b) a second contribution related to the positions within the unit cell of each of the first and second patterned regions 31, 32 from which the beams originate.

[0116] [000116] The first of these contributions can be understood to arise from the fact that different coherent radiation beams are passing through different parts of the projection system PS and are therefore the beams that are relevant to the aberrations that we wish to determine (indeed, they are relevant to the difference between two points in the aberration map separated in the shear direction).

[0117] [000117] The second of these contributions can be understood to arise from the fact that the relative phases of multiple radiation beams resulting from a single beam incident on a diffraction grating depend on which part of the unit cell of the grating the beam is incident on. It therefore does not contain information about aberrations. As mentioned above, in some embodiments the measurement patterning device MA' and / or sensor arrangement 21 are sequentially scanned and / or stepped in the shear direction. This will change the phase difference between every pair of interfering radiation beams received by the radiation detector 23. Since the measurement patterning device MA' and / or sensor arrangement 21 are sequentially stepped in the shear direction by an amount equivalent to a fraction of the pitch (in the shear direction) of the first and second patterned areas 31, 32, in general the phase difference between every pair of second diffracted beams will change. When the measurement patterning device MA' and / or sensor arrangement 21 are stepped in the shear direction by an amount equal to an integer multiple of the pitch (in the shear direction) of the first and second patterned areas 31, 32, the phase difference between the pair of second diffracted beams remains the same. Therefore, as the measurement patterning device MA' and / or sensor arrangement 21 are sequentially scanned and / or stepped in the shear direction, the intensity received by each portion of the radiation detector 23 will oscillate. The first harmonic of this oscillating signal (which may also be referred to as a phase-stepped signal) measured by the radiation detector 23 depends on the contributions to equation (1) arising from adjacent first diffracted beams 34-36, i.e., first diffracted beams differing in order by ±1. Due to this phase-stepping technique, the contributions arising from the different amounts of the differing orders will contribute to higher harmonics of the signal determined by the radiation detector 23.

[0118] [000118] For example, of the three overlapping second diffracted beams (35b, 36a, and 34d) mentioned above, only two of the three possible pairs of these diffracted beams will contribute to the first harmonic of the phase stepping signal: (a) second diffracted beams 35b and 36a (arising from the 0th and −1st diffracted order beams 35 and 36, respectively, of the first patterned region 31), and (b) second diffracted beams 35a and 34d (arising from the 0th and +1st diffracted order beams 35 and 34, respectively, of the first patterned region 31).

[0119] [000119] Each pair of second diffracted beams will result in an interference term of the form shown in equation (2), i.e., an interference term of the form:

number

[0120] 3A-5C, when a first portion 15a' of patterned area 15a shown in FIG. 3A is illuminated, the shearing direction corresponds to the v direction, and the non-shearing direction corresponds to the u direction. It will be understood that when a second portion 15a'' of patterned area 15a shown in FIG. 3A is illuminated, the shearing direction corresponds to the u direction, and the non-shearing direction corresponds to the v direction. In these above-described embodiments, the u and v directions (which define the two shearing directions) are both aligned at approximately 45 degrees relative to both the x and y directions of lithographic apparatus LA, but it will be understood that in alternative embodiments, the two shearing directions may be aligned at any angle relative to the x and y directions of lithographic apparatus LA (which may correspond to the non-scanning and scanning directions of lithographic apparatus LA). In general, the two shearing directions will be perpendicular to each other.

[0121] [000121] The wavefront reconstruction can be performed as described in WO2019 / 149468, or using more conventional techniques, as follows: A first phase-stepping process may be performed using a first shearing direction (v direction, using a first portion 15a' of patterned region 15a shown in FIG. 3A), and the phase of the first harmonic of the oscillating phase-stepping signal is related to the ΔW term in equation (3). For this first phase-stepping process, for each pixel or each sensing element of detector region 39 receiving radiation 38, the ΔW term in equation (3) is related to the slope of the wavefront in the first shearing direction (v direction). A second phase-stepping process may be performed using a second shearing direction (u direction, using a second portion 15a'' of patterned region 15a shown in FIG. 3A), and the phase of the first harmonic of the oscillating phase-stepping signal is related to the ΔW term in equation (3). For this second phase-stepping process, for each pixel or sensing element of the detector area 39 receiving radiation 38, the ΔW term in equation (3) relates to the gradient of the wavefront in the second shear direction (u direction). Thus, for each pixel or sensing element of the detector area 39 receiving radiation 38, two phases (of the harmonics of the phase-stepping signal) are determined. By combining these phases determined for all pixels of the detector area 39, a wavefront (relative phase) map of the projection system PS is determined.

[0122] [000122] Some embodiments of the present disclosure relate to methods for determining one or more aberrations of a projection system PS using a reticle-level mirrored grating or a mirrored grating in the object plane of the projection system PS. Some other embodiments of the present disclosure relate to corresponding measurement systems for determining one or more aberrations of a projection system PS using a reticle-level mirrored grating or a mirrored grating in the object plane of the projection system PS. The measurement system may be generally the measurement system shown in Figures 2 and 4 above, where the reticle-level mirrored grating or a mirrored grating in the object plane of the projection system PS may be used as any of the plurality of patterned areas 15a-15c as described above and shown in Figures 2-3A and / or as the first patterned area 31 as described above and shown in Figure 4.

[0123] [000123] It will be understood that a specular diffraction grating is intended to mean a reflective diffraction grating that includes a pattern of reflective portions and radiation-absorbing portions, and that the reflection from the reflective portions is primarily specular or normal. In other words, a specular diffraction grating is intended to mean a reflective diffraction grating that minimizes scattering of radiation incident on it. It will be understood that this can be achieved by ensuring that the reflective portions of the specular diffraction grating have sufficient smoothness. In particular, any imperfections or surface roughness in the reflective portions of the specular diffraction grating may be small compared to the wavelength of the illumination radiation that the specular diffraction grating scatters during use.

[0124] 6A is a schematic diagram of a novel measurement system 100 for determining one or more aberrations of a projection system PS. The measurement system 100 comprises a first patterning device 110, a sensor arrangement 120, a positioning arrangement 130, and a controller 140. For ease of explanation, the measurement system 100 is shown in a linear arrangement in which the optical axis is maintained in a single direction. It will be appreciated that in practice the first patterning device 110 and components within the projection system PS are reflective and that in practice the optical axis will change with each such reflection.

[0125] [000125] The positioning arrangement 130 is configured to move at least one of the first patterning device 110 and the sensor arrangement 120. To achieve this, the positioning arrangement 130 may be operable to send control signals 132, 134 to one or both of the first patterning device 110 and the sensor arrangement 120.

[0126] [000126] The first patterning device 110 can be positioned in the object plane of the projection system PS. During the measurement of the aberrations, a phase stepping or phase scanning process may be performed, during which the first patterning device is positioned in the object plane of the projection system PS (e.g., using the positioning apparatus 130). The first patterning device 110 comprises a specular diffraction grating.

[0127] [000127] The sensor arrangement 120 comprises a second patterning device 122 and a radiation detector 124. The sensor arrangement 120 is provided at wafer level. The sensor arrangement 120 is positionable such that the second patterning device 122 can be located in an image plane of the projection system PS. The first patterning device 110 and the sensor arrangement 120 are positionable (e.g. using the positioning arrangement 130) such that the projection system PS can form an image of the first patterning device 110 on the second patterning device 122 using illumination radiation 150, and such that the radiation detector 124 is positioned to receive the illumination radiation 150 after it has passed through the second patterning device 122.

[0128] The controller 140 is configured to control the positioning arrangement 130 to move at least one of the first patterning device 110 and the sensor arrangement 120. In particular, the controller 140 may be configured to control the positioning arrangement 130 to move at least one of the first patterning device 110 and the sensor arrangement 120 in a shear direction during a phase stepping or phase scanning process, such that the intensity of radiation received by portions (e.g. individual sensing elements or pixels) of the radiation detector 124 varies as a function of the movement in the shear direction, thereby forming an oscillation signal.

[0129] [000129] The controller 140 is further configured to determine, from the radiation detector 124, phases of the harmonics of the vibration signal at a plurality of locations (e.g. individual sensing elements or pixels) on the radiation detector 124. The controller 140 is further configured to determine at least one coefficient characterizing at least one aberration of the projection system PS from the phases of the harmonics of the vibration signal at the plurality of locations on the radiation detector 124.

[0130] [000130] The measurement system 100 is suitable for use in an extreme ultraviolet (EUV) lithography apparatus, i.e. the illumination radiation 150 may include EUV radiation. It will be appreciated that the measurement system 100 is in the form of a shearing interferometer.

[0131] [000131] Measurement system 100 is suitable for use in a lithographic apparatus LA of the type shown in Figure 1, and generally corresponds to the measurement system shown in Figures 2 and 4 and described above, and may incorporate any of the features described above in relation to Figures 2-5B (as well as any methods that controller 140 may be configured to perform, which are described below with reference to Figures 7-12). First patterning device 110 generally corresponds to one of the plurality of patterned areas 15a-15c and shown in Figures 2-3A and described above, and / or to first patterned area 31 and shown in Figure 4 and described above. Sensor arrangement 120 generally corresponds to one of detector areas 25a-25c and shown in Figures 2-3A and described above, and / or to detector area 39 and shown in Figures 4-5B and described above.

[0132] [000132] In some embodiments, the measurement system 100 may further include an illumination system (not shown) operable to illuminate the first patterning device 110 with illumination radiation 150. Such an illumination system may include any of the illumination system IL and / or radiation source SO shown in Figure 1 .

[0133] [000133] In some embodiments, the first patterning device 110 may be provided on a fiducial. In other embodiments, the first patterning device 110 may be provided on a reticle.

[0134] [000134] Various methods that the controller 140 may be configured to perform will now be described with reference to Figures 7 to 12.

[0135] [000135] A novel method 200 for determining one or more aberrations of a projection system PS will now be described with reference to Figure 7. The method 200 shown in Figure 7 can be implemented by the measurement system 100 shown in Figure 6A. The method 200 is a form of shearing interferometry.

[0136] [000136] The method 200 includes step 210 of performing a phase stepping or phase scanning process, and step 220 of determining one or more aberrations of the projection system PS (from signals determined during the phase stepping or phase scanning process).

[0137] [000137] The phase stepping or phase scanning process includes illuminating 212 a first patterning device 110 with illumination radiation 150. The first patterning device 110 includes a specular diffraction grating arranged to form a plurality of first diffracted beams separated in a shear direction from the illumination radiation 150.

[0138] [000138] The phase stepping or phase scanning process further comprises the step 214 of projecting, with the projection system PS, at least some of the plurality of first diffracted beams onto the sensor arrangement 120. The sensor arrangement 120 comprises a second patterning device 122 and a radiation detector 124. The second patterning device 122 is positioned to receive the first diffracted beams from the projection system PS and to form a plurality of second diffracted beams from each of the first diffracted beams. The radiation detector 124 is positioned to receive at least some of the second diffracted beams.

[0139] [000139] The phase stepping or phase scanning process further comprises a step 216 of moving at least one of the first and second patterning devices 110, 122 in a shear direction such that the intensity of the radiation received by each portion of the radiation detector 124 that receives the radiation varies as a function of movement in the shear direction, thereby forming an oscillating signal.

[0140] [000140] In step 220, one or more aberrations of the projection system PS are determined from vibration signals determined by the portions of the radiation detector 124 that receive the radiation.

[0141] [000141] The method 200 is suitable for use in a lithographic apparatus LA. The first patterning device 110 may be located at reticle level (i.e., in an object plane of the lithographic apparatus LA) and the sensor apparatus 120 may be located at wafer level (e.g., the second patterning device 122 may be located in an image plane of the lithographic apparatus LA). In particular, the method 200 is suitable for use in an extreme ultraviolet (EUV) lithographic apparatus, and the illumination radiation 150 may comprise EUV radiation.

[0142] [000142] During a lithographic process, radiation B is used to form a typically diffraction-limited image on a substrate W (e.g. a resist-coated wafer or the like) using a projection system PS. In practice, the projection system PS will not form a perfect image and will be subject to some degree of aberrations. Such aberrations describe distortions in the wavefront of light approaching a spherical wavefront to a point in the image plane of the projection system PS (which may generally depend on the position in the pupil plane and / or the angle at which the radiation approaches the image plane of the projection system PS). It is desirable to minimize aberrations and therefore it is desirable to have a measurement of the aberrations which can be used as part of a feedback loop to minimize aberrations as much as possible.

[0143] [000143] When measuring aberrations in the projection system of known EUV lithography apparatuses, the same (EUV) radiation B that is used to expose the wafer during the lithography process is typically used for the aberration measurement. This is a clear advantage, since it is the aberrations induced when imaging with EUV radiation B that are desirable to measure and minimize (because they affect the image formed on the wafer). However, radiation B is delivered to the reticle level of the lithography apparatus via an illumination system IL, which is arranged to optimize the angular distribution of EUV radiation at reticle level for lithographic imaging performance. In particular, in known EUV lithography apparatuses, although the angular distribution of EUV radiation at reticle level (sometimes referred to as the illumination mode) can be controlled to some extent (e.g., using the field facet mirrors 10 and pupil facet mirrors 11 shown in Figure 1), it is not possible to achieve uniform pupil-fill illumination (i.e., to illuminate each field point at reticle level with a uniform cone of radiation). The illumination modes achievable by the illumination system IL do not result in examining the entire pupil plane of the projection system PS.

[0144] [000144] As a result, measuring the aberrations of the projection system of known EUV lithography apparatuses typically uses a diffusely reflective grating as the first patterning device (at reticle level). Figure 6B shows a schematic representation of a typical measurement system 100' for the aberrations of the projection system of a known EUV lithography apparatus, using a diffusely reflective grating 110' as the first patterning device (at reticle level). Again, for ease of explanation, the measurement system 100' is shown in a linear arrangement in which the optical axis is maintained in a single direction. The diffusely reflective grating 110' is positioned to fill the entire pupil plane of the projection system PS, so that the entire projection system PS is examined during the aberration measurement. This is shown in Figure 6B, where the radiation propagating from the diffusely reflective grating 110' into the projection system PS is represented by a solid cone. This is in contrast to a specular diffraction grating 110, which scatters radiation into discrete portions of the pupil plane of the projection system PS.

[0145] [000145] However, the present inventors have recognized that such a diffuse reflecting grating 110' can introduce significant errors in the measurement of some aberrations (e.g., the amplitudes of some Zernike orders in the aberration map of the projection system PS). Typically, a diffuse reflecting grating 110' for an aberration measurement system 100' is formed by creating a rough surface that imparts a reflective grating pattern to the rough surface. For example, a uniform reflective layer may be applied to the rough surface, and a pattern of radiation-absorbing portions is formed on the reflective layer. However, the present inventors have recognized that aberration measurements made with a diffuse reflecting grating 110' depend on the particular rough surface used to generate the diffusion.

[0146] [000146] However, contrary to the strong preconception that exists in the art that diffusion should be used to fully inspect a projection system PS, the present inventors have surprisingly come to the realization that measurements using a specular diffraction grating 110 can provide improved aberration measurements for at least some aberrations (i.e. Zernike orders). In particular, even when using illumination modes that are realizable by the illumination system IL of known lithography systems LA, the use of a specular diffraction grating 110 can provide better aberration measurements for at least some aberrations (i.e. Zernike orders) than the use of known diffuse gratings 110'.

[0147] [000147] Another advantage of using a specular diffraction grating 110 at the reticle level for aberration measurements is that such a specular diffraction grating 110 can be more conveniently provided on the reticle MA. While in principle it is possible to form a diffusely reflective grating on the reticle MA, in practice, such an arrangement is often impractical and is not currently standard practice (or even feasible). As a result, it is now standard practice to provide a diffusely reflective grating elsewhere on the reticle stage MT, such as on a fiducial. However, being able to perform aberration measurements using a specular diffraction grating 110 on the reticle MA could significantly reduce the time required for aberration measurements. For example, currently, after exposing a target area of ​​the substrate W to radiation patterned by the reticle MA, in order to perform aberration measurements, the reticle stage MT must be moved so that the fiducial (rather than the reticle MA) receives EUV radiation B from the illumination system IL. Furthermore, after such aberration measurements, in order to expose the target area of ​​the substrate W with the radiation patterned by the reticle MA, the reticle stage MT must be moved so that the reticle MA (rather than the fiducial) receives the EUV radiation B from the illumination system IL. However, if the mirrored diffraction grating 110 used for the aberration measurements could be provided on the reticle MA, the reticle stage MT would not need to be moved as far between exposing the substrate W and measuring the aberrations, thereby saving significant time. Furthermore, the aberration measurements from the method 200 shown in FIG. 7 allow lower-order Zernike coefficients (e.g., Z2, Z3, Z4, etc.) to be used for aligning the reticle MA. This may advantageously mean that it may not be necessary to use additional sensors (which would also be provided elsewhere on the reticle stage MT) for aligning the reticle MA.

[0148] [000148] FIG. 8 shows the results of a simulation performed to evaluate the performance of the method 200 shown in FIG. 7. A first plot 250 shows the aberration footprint of the projection system PS input into the simulation for the first 25 Zernike orders. A second plot 260 shows the reconstructed aberration footprint determined using the method 200 of FIG. 7 (i.e., using the specular diffraction grating 110) for the first 25 Zernike orders. The simulation used an illumination mode in which 20.7% of the illumination pupil plane was filled by the illumination radiation 150 used to illuminate the first patterning device 110 in step 212. Even though only 20.7% of the illumination pupil plane is filled by the illumination radiation 150, the illumination radiation 150 is distributed approximately uniformly across the illumination pupil plane. A third plot 270 shows the difference between the second plot 260 and the first plot 250. As can be seen from the small values ​​in the third plot 270, even though only about 20% of the illumination pupil plane is illuminated, the input Zernike is reconstructed very accurately using the method 200 of FIG. 7 (i.e., using the mirrored diffraction grating 110).

[0149] [000149] A second novel method 300 for determining one or more aberrations of a projection system PS will now be described with reference to Figures 9 and 10. The method 300 shown in Figure 9 may be implemented by the measurement system 100 shown in Figure 6A. The method 300 shown in Figure 9 is a variation of the above-described method 200 shown in Figure 7. The method 300 shown in Figure 9 is a form of shearing interferometry.

[0150] [000150] The second method 300 includes a step 320 of performing a plurality of phase stepping or phase scanning processes 330A, 330B, 330C and determining therefrom one or more aberrations of the projection system PS.

[0151] 10 , each of the multiple phase stepping or phase scanning processes 310A, 310B, 310C is performed using a different illumination mode 330A, 330B, 330C. As such, different portions of the radiation detector 124 receive radiation during different ones of the multiple phase stepping or phase scanning processes 310A, 310B, 310C. Otherwise, each of the multiple phase stepping or phase scanning processes 310A, 310B, 310C is similar to the other phase stepping or phase scanning processes. Each of the multiple phase stepping or phase scanning processes 310A, 310B, 310C may generally be in the form of the phase stepping or phase scanning process 210 shown in FIG. 7 and described above. In particular, each of the multiple phase stepping or phase scanning processes 310A, 310B, 310C may include a step 212 of illuminating the first patterning device 110 with illumination radiation 150, a step 214 of projecting at least some of the multiple first diffracted beams onto the sensor arrangement 120 with the projection system PS, and a step 216 of moving at least one of the first and second patterning devices 110, 122 in a shear direction such that the intensity of the radiation received by each portion of the radiation detector 124 that receives the radiation changes as a function of the movement in the shear direction, thereby forming an oscillation signal.

[0152] [000152] The step 320 of determining one or more aberrations of the projection system PS comprises using oscillation signals determined by portions of the radiation detector that receive radiation from any one of a plurality of phase stepping or phase scanning processes 310A, 310B, 310C.

[0153] [000153] By performing multiple different phase stepping or phase scanning processes 310A, 310B, 310C using different illumination modes 330A, 330B, 330C, each different phase stepping or phase scanning process 310A, 310B, 310C can advantageously examine a different portion of the pupil plane of the projection system PS. Note that aberrations or aberration maps are not determined for each individual phase stepping or phase scanning process 310A, 310B, 310C and then combined. Rather, the multiple phase stepping or phase scanning processes 310A, 310B, 310C are all used to determine the aberrations or aberration map. Such an embodiment advantageously allows for higher resolution measurements of the aberrations of the projection system PS.

[0154] [000154] A third novel method 400 for determining one or more aberrations of a projection system PS will now be described with reference to Figure 11. The method 400 shown in Figure 11 can be implemented by the measurement system 100 shown in Figure 6A. The method 400 shown in Figure 11 is a variation of the above-described method 200 shown in Figure 7. The method 400 shown in Figure 11 is a calibration method using shearing interferometry.

[0155] [000155] The calibration method 400 includes determining 410, as part of a phase stepping or phase scanning process, one or more aberrations of the projection system PS using the object-level mirror grating 110. This step 410 may be performed using either the method 200 shown in Figure 7 or the method 300 shown in Figures 9 and 10.

[0156] [000156] The calibration method 400 also includes a step 420 of performing an additional phase stepping or phase scanning process using an object-level diffuse grating 110' (of the type described above and shown in Figure 6B) to generate one or more additional vibration signals. The calibration method 400 also includes a step 430 of determining one or more additional aberrations of the projection system PS from the one or more additional vibration signals.

[0157] [000157] Calibration method 400 also includes step 440 of determining calibration data that characterizes a difference between (a) the one or more aberrations of the projection system PS determined (from step 410) and (b) one or more additional aberrations of the projection system determined (from step 430).

[0158] [000158] Such an arrangement allows the generation of calibration data that can be used in subsequent measurements to correct for additional aberrations of the projection system PS that are generated using the diffuse diffraction grating 110' as the first patterning device (at object plane level). This has the advantage that it allows correcting for and removing significant errors that may occur when measuring some aberrations (e.g., the amplitudes of some Zernike orders in the aberration map of the projection system) using such a diffuse diffraction grating 110' as the first patterning device. This has the advantage that subsequent aberration measurements can be performed using the diffuse diffraction grating 110' as the first patterning device, which may allow the subsequent aberration measurements to be faster and potentially increase the throughput of the lithographic apparatus LA.

[0159] [000159] The method 400 shown in Figure 11 may further include storing the determined calibration data in a memory.

[0160] [000160] A fourth novel method 500 for determining one or more aberrations of a projection system PS will now be described with reference to Figure 12. The method 500 shown in Figure 12 may be implemented by the measurement system 100 shown in Figure 6A. The method 500 shown in Figure 12 is a variation of the above-described method 400 shown in Figure 11. The method 500 shown in Figure 12 is a calibration method using shearing interferometry.

[0161] [000161] The method 500 shown in FIG. 12 includes all of steps 410, 420, 430, 440 of the method shown in FIG.

[0162] 12 further includes a step 510 of performing a subsequent phase stepping or phase scanning process using the diffuse diffraction grating 110′ as the first patterning device, but otherwise similar to the phase stepping or phase scanning process described above or each of the other phase stepping or phase scanning processes (e.g., those performed in steps 410 and 420).

[0163] [000163] The method 500 shown in Figure 12 also includes a step 520 of determining one or more subsequent raw aberrations of the projection system PS from the vibration signals determined by the portions of the radiation detector 122 that receive radiation during the subsequent phase stepping or phase scanning process (step 510).

[0164] [000164] The method 500 shown in Figure 12 also includes a step 530 of determining one or more corrected subsequent aberrations from the one or more raw subsequent aberrations of the projection system PS (determined in step 520) and the calibration data (determined in step 440).

[0165] [000165] It will be appreciated that any of the phase stepping or phase scanning processes described in any of the above methods 200, 300, 400, 500 (i.e., the processes described in any of steps 210, 210A-210C, 410, 420, or 510) may further include a second phase stepping or phase scanning process (but using a second shearing direction). Such a second phase stepping or phase scanning process may include: (a) illuminating an object-level patterning device with illumination radiation, the first patterning device including a diffraction grating arranged to form a plurality of first diffracted beams separated in a second shear direction; and (b) projecting at least some of the plurality of first diffracted beams onto the sensor arrangement 120 with the projection system PS; and moving at least one of the first and second patterning devices in the second shear direction such that the intensity of the radiation received by each portion of the radiation detector receiving the radiation varies as a function of movement in the second shear direction, thereby forming an oscillation signal.

[0166] For example, the first patterning device may include a first portion positioned to shear the illumination radiation in a shear direction and a second portion positioned to shear the illumination radiation in a second shear direction. While a first portion of the first patterning device is illuminated, at least one of the first patterning device and the second patterning device is stepped in the shear direction. This generates first phase stepping data, which may be related to at least one order of a gradient of the aberration map in the shear direction. While a second portion of the first patterning device is illuminated, at least one of the first patterning device and the second patterning device is stepped in the second shear direction. This generates second phase stepping data, which may be related to at least one order of a gradient of the aberration map in the second shear direction. Combining the first and second phase stepping data determines one or more aberrations of the projection system. The first and second phase stepping data may be combined to determine an aberration (or relative phase) map of the projection system.

[0167] [000167] Determining one or more aberrations of the projection system PS (steps 220, 230), one or more additional aberrations of the projection system (step 430), or one or more raw subsequent aberrations of the projection system (step 520) from the vibration signals determined by the portions of the radiation detector 122 that receive the radiation may include any known shearing interferometry technique, as needed or desired. For example, determining any of these aberrations may include any of the techniques disclosed above in relation to Figures 2-5C. Additionally or alternatively, determining any of these aberrations may include any of the techniques disclosed in WO2019 / 149468, the contents of which are incorporated herein by reference in their entirety. For clarity, determining any of these aberrations may include any of the techniques disclosed as prior art in WO2019 / 149468 and / or any of the techniques taught therein.

[0168] [000168] Determining either one or more aberrations of the projection system PS (steps 220, 230), one or more additional aberrations of the projection system (step 430), or one or more raw subsequent aberrations of the projection system (step 520) from the vibration signal determined by the portions of the radiation detector 122 that receive the radiation may comprise equalizing the phase of a harmonic of the vibration signal received by the portions of the radiation detector 122 that receive the radiation to the sum of at least one difference in the aberration map between a pair of positions in a pupil plane of the projection system PS. The harmonic of the vibration signal may be a first harmonic of the vibration signal.

[0169] [000169] Some embodiments of the present disclosure relate to a computer-readable medium carrying a computer program including computer-readable instructions configured to cause a computer to perform one or more of the above-described methods 200, 300, 400, 500 shown in Figures 7-12.

[0170] [000170] Some embodiments of the present disclosure relate to a computing device including a memory storing processor-readable instructions and a processor arranged to read and execute the instructions stored in the memory, where the processor-readable instructions include instructions arranged to control a computer to perform one or more of the above-described methods 200, 300, 400, 500 shown in Figures 7-12.

[0171] [000171] In some embodiments, the diffraction gratings described above may be freestanding. In use, the diffraction grating may be provided with a support for the absorbing layer. The support may contact only a peripheral portion of the absorbing layer. That is, the support may be in the form of a frame that is not adjacent to the central portion of the absorbing layer. With such an arrangement, the absorbing layer can be considered freestanding at its central portion. To achieve this, the absorbing layer may be tensioned on the support (e.g., to maintain it in a substantially planar state).

[0172] [000172] In embodiments where the central portion of the absorbing layer is freestanding, the grating has the advantage that it does not require, for example, a transmissive support layer. Such an arrangement is particularly beneficial for use in phase-stepping measurement systems for determining aberration maps of projection systems that use EUV radiation, since the use of such a transmissive support layer would significantly reduce the amount of EUV radiation transmitted by the grating.

[0173] [000173] In some embodiments, the diffraction gratings described above include a radiation-absorbing layer in the substrate (in which the apertures are formed). The radiation-absorbing layer may be formed of a metal such as, for example, chromium (Cr), nickel (Ni), cobalt (Co), or aluminum (Al).

[0174] [000174] In some embodiments, the absorbing layer may include a ceramic. The ceramic may include a metal or semi-metal component having a relatively high extinction coefficient for EUV radiation and a non-metal component. Both components may have a refractive index relatively close to 1 for EUV. The ceramic may include aluminum nitride (AlN). In some embodiments, the absorbing layer may include aluminum nitride (AlN).

[0175] [000175] In some embodiments, in the diffraction gratings described above, the substrate (in which the apertures are formed) further includes a support layer. The through apertures may extend through both the support layer and the radiation-absorbing layer. The support layer may be formed of, for example, SiN.

[0176] [000176] In the above embodiment, the first harmonic of the phase stepping signal is used, however, in alternative embodiments, it will be appreciated that higher harmonics of the phase stepping signal may be used instead.

[0177] [000177] While the above embodiments use a first patterned region 31 comprising a one-dimensional diffraction grating 31 with a 50% duty cycle, it will be appreciated that in alternative embodiments, the first patterned region 31 may use a different shape. For example, in some embodiments, the first patterned region 31 may comprise a two-dimensional checkerboard diffraction grating with a 50% duty cycle.

[0178] [000178] Although specific reference is made in this specification to the use of lithographic apparatus in IC manufacture, it should be understood that the lithographic apparatus described herein may have other applications, including the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0179] [000179] Although specific reference is made herein to embodiments of the invention in the context of lithography apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes objects such as wafers (or other substrates) or masks (or other patterning devices). These apparatus are sometimes referred to generically as lithography tools. Such lithography tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0180] Depending on the context, 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 may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include read-only memory (ROM), random-access memory (RAM), magnetic storage media, optical storage media, flash memory devices, or electrical, optical, acoustic, or other forms of propagated signals (e.g., carrier waves, infrared signals, digital signals, etc.). Also, firmware, software, routines, or instructions may be described herein as performing certain actions. However, it should be understood that such description is merely for convenience and that such actions are in fact due to computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc., and that in doing so, actuators or other devices may interact with the physical world.

[0181] [000181] While specific embodiments of the present invention have been described above, it will be understood that the invention may be practiced otherwise than as described. The foregoing description is intended to be illustrative and not limiting. Accordingly, it will be apparent to those skilled in the art that modifications may be made to the invention as described without departing from the scope of the appended claims.

[0182] [000182] Clause 1. A method for determining one or more aberrations of a projection system, the method comprising: performing a phase stepping or phase scanning process, the phase stepping or phase scanning process comprising: illuminating a first patterning device with illumination radiation, the first patterning device including a specular diffraction grating arranged to form a plurality of first diffracted beams separated in a shear direction; projecting, with a projection system, at least some of the plurality of first diffracted beams onto a sensor device, the sensor device comprising: a second patterning device positioned to receive the first diffracted beams from the projection system and to form a plurality of second diffracted beams from each of the first diffracted beams; and a radiation detector positioned to receive at least a portion of the second diffracted beam; and performing a phase stepping or phase scanning process, which includes moving at least one of the first and second patterning devices in a shear direction such that the intensity of radiation received by each portion of the radiation detector receiving the radiation varies as a function of the movement in the shear direction, thereby forming an oscillating signal; determining one or more aberrations of the projection system from vibration signals determined by portions of the radiation detector receiving the radiation; and A method comprising: 2. performing a plurality of phase stepping or phase scanning processes, each of the plurality of phase stepping or phase scanning processes being performed using a different illumination mode such that different portions of the radiation detector receive radiation during different of the plurality of phase stepping or phase scanning processes, but otherwise similar to other phase stepping or phase scanning processes; The method of claim 1, wherein the step of determining one or more aberrations of the projection system includes using vibration signals determined by each portion of the radiation detector receiving radiation from any one of a plurality of phase stepping processes or phase scanning processes. 3. performing an additional phase stepping or phase scanning process, the additional phase stepping or phase scanning process being performed using a diffusing diffraction grating as the first patterning device, but otherwise being similar to the phase stepping or phase scanning process described above, or each of the other phase stepping or phase scanning processes; determining one or more further aberrations of the projection system from oscillation signals determined by portions of the radiation detector receiving radiation during the further phase stepping or phase scanning process; 3. The method of claim 1 or claim 2, further comprising: determining calibration data characterizing a difference between (a) the determined one or more aberrations of the projection system; and (b) the determined one or more additional aberrations of the projection system. 4. The method of clause 3, further comprising storing the determined calibration data in a memory. 5. A calibration method comprising: performing a phase stepping or phase scanning process using an object-level mirror grating to generate one or more vibration signals; determining one or more aberrations of the projection system from the one or more vibration signals; and performing an additional phase stepping or phase scanning process using an object-level diffuse grating to generate one or more additional vibration signals; determining one or more additional aberrations of the projection system from the one or more additional vibration signals; and (a) determining calibration data that characterizes a difference between the determined one or more aberrations of the projection system and (b) the determined one or more additional aberrations of the projection system; A method comprising: 6. performing a subsequent phase stepping or phase scanning process, where the subsequent phase stepping or phase scanning process is performed using a diffusing diffraction grating as the first patterning device, but is otherwise similar to the phase stepping or phase scanning process described above, or each of the other phase stepping or phase scanning processes; determining one or more subsequent raw aberrations of the projection system from oscillation signals determined by portions of the radiation detector that receive radiation during the subsequent phase stepping or phase scanning process; 6. The method of any one of clauses 3 to 5, further comprising determining one or more corrected subsequent aberrations from the one or more raw subsequent aberrations of the projection system and from the calibration data. 7. The method of any preceding clause, wherein the illumination radiation comprises extreme ultraviolet radiation. 8. The method of any preceding clause, wherein the first patterning device is provided on a fiducial. 9. The method of any preceding clause, wherein the first patterning device is provided on a reticle. 10. Any phase stepping or phase scanning process illuminating a first patterning device with illumination radiation, the first patterning device including a specular diffraction grating arranged to form a plurality of first diffracted beams separated in a second shearing direction; projecting at least some of the plurality of first diffracted beams onto a sensor device with a projection system; The method of any preceding clause, further comprising moving at least one of the first and second patterning devices in a second shear direction such that the intensity of the radiation received by each portion of the radiation detector receiving the radiation varies as a function of movement in the second shear direction, thereby forming an oscillatory signal. 11. Determining either one or more aberrations of the projection system, one or more additional aberrations of the projection system, or one or more raw subsequent aberrations of the projection system from vibration signals determined by portions of the radiation detector that receive radiation; 10. A method according to any preceding clause, comprising equalising the phase of harmonics of the oscillation signals received by each portion of the radiation detector receiving the radiation to the sum of at least one difference in an aberration map between a pair of positions in a pupil plane of the projection system. 12. A computer readable medium carrying a computer program comprising computer readable instructions configured to cause a computer to carry out the method of any one of clauses 1 to 11. 13. A memory storing processor-readable instructions; a processor configured to read and execute instructions stored in the memory, A computer apparatus, wherein the processor-readable instructions include instructions arranged to control a computer to perform a method according to any one of clauses 1 to 11. 14. A measurement system for determining one or more aberrations of a projection system, comprising: a first patterning device including a specular diffraction grating; a sensor arrangement including a second patterning device and a radiation detector, wherein the first patterning device and the sensor arrangement are positionable such that the projection system can form an image of the first patterning device on the second patterning device with illumination radiation, and the radiation detector is positioned to receive the illumination radiation after it has passed through the second patterning device; a positioning device configured to move at least one of the first patterning device and the sensor device; a controller; The above controller is controlling the positioning arrangement to move at least one of the first patterning device and the sensor arrangement in a shear direction, whereby the intensity of radiation received by portions of the radiation detector varies as a function of the movement in the shear direction to form an oscillation signal; determining, from the radiation detector, phases of harmonics of the vibration signal at a plurality of locations on the radiation detector; and determining at least one coefficient characterizing at least one aberration of the projection system from the phases of the harmonics of the vibration signal at the plurality of positions on the radiation detector. 15. The system of clause 14, wherein the controller is configured to perform the method of any one of clauses 1 to 11. 16. The system of clause 14 or clause 15, further comprising an illumination system operable to illuminate the first patterning device with illumination radiation. 17. The system of clause 16, wherein the illumination radiation comprises extreme ultraviolet radiation. 18. The system of any one of clauses 14-17, wherein the first patterning device is disposed on a fiducial. 19. The system of any one of clauses 14 to 18, wherein the first patterning device is provided on a reticle. 20. A lithographic apparatus comprising a measurement system according to any one of clauses 14 to 19.

Claims

1. 1. A method for determining one or more aberrations of a projection system, comprising the steps of: performing a phase stepping or phase scanning process, the phase stepping or phase scanning process comprising: illuminating a first patterning device with illumination radiation, the first patterning device including a specular diffraction grating arranged to form a plurality of first diffracted beams separated in a shear direction; projecting at least some of the plurality of first diffracted beams onto a sensor device with the projection system, the sensor device comprising: a second patterning device positioned to receive the first diffracted beams from the projection system and to form a plurality of second diffracted beams from each of the first diffracted beams; and a radiation detector positioned to receive at least a portion of the second diffracted beam; and moving at least one of the first and second patterning devices in the shear direction such that an intensity of radiation received by each portion of the radiation detector receiving radiation varies as a function of the movement in the shear direction, thereby forming an oscillatory signal; determining one or more aberrations of the projection system from the vibration signals determined by the portions of the radiation detector receiving radiation; A method comprising:

2. The method includes performing a plurality of phase stepping or phase scanning processes; each of the plurality of phase stepping or phase scanning processes is similar to the other of the plurality of phase stepping or phase scanning processes, but is performed using a different illumination mode such that different portions of the radiation detector receive radiation during different ones of the plurality of phase stepping or phase scanning processes; 2. The method of claim 1, wherein determining one or more aberrations of the projection system comprises using the oscillation signals determined by portions of the radiation detector receiving radiation from any one of the plurality of phase stepping or phase scanning processes.

3. performing an additional phase stepping or phase scanning process using a diffusing grating as the first patterning device but otherwise being similar to the phase stepping or phase scanning process or each other phase stepping or phase scanning process; determining one or more additional aberrations of the projection system from the oscillation signals determined by portions of the radiation detector receiving radiation during the additional phase stepping or phase scanning process; (a) determining calibration data characterizing a difference between the determined one or more aberrations of the projection system and (b) the determined one or more additional aberrations of the projection system; 3. The method of claim 1 or claim 2, further comprising:

4. 1. A calibration method comprising: performing a phase stepping or phase scanning process using an object-level mirror grating to generate one or more vibration signals; determining one or more aberrations of the projection system from the one or more vibration signals; performing an additional phase stepping or phase scanning process using an object-level diffusive grating to generate one or more additional vibration signals; determining one or more additional aberrations of the projection system from the one or more additional vibration signals; and (a) determining calibration data characterizing a difference between the determined one or more aberrations of the projection system and (b) the determined one or more additional aberrations of the projection system; A method comprising:

5. performing a subsequent phase stepping or phase scanning process using the diffusing diffraction grating as the first patterning device but otherwise similar to the phase stepping or phase scanning process or each other phase stepping or phase scanning process; determining one or more subsequent raw aberrations of the projection system from the oscillation signals determined by portions of the radiation detector receiving radiation during the subsequent phase stepping or phase scanning process; determining one or more corrected subsequent aberrations from the one or more raw subsequent aberrations of the projection system and the calibration data; The method of any one of claims 3 to 4, further comprising:

6. The method of any preceding claim, wherein the illumination radiation comprises extreme ultraviolet radiation.

7. The method according to any one of claims 1 to 6, wherein the first patterning device is provided on a fiducial and / or on a reticle.

8. Any phase stepping or phase scanning process illuminating the first patterning device with illumination radiation, the first patterning device including a specular diffraction grating positioned to form a plurality of first diffracted beams separated in a second shear direction; projecting at least some of the plurality of first diffracted beams onto the sensor device with the projection system; moving at least one of the first and second patterning devices in the second shear direction such that an intensity of radiation received by each portion of the radiation detector receiving radiation varies as a function of the movement in the second shear direction, thereby forming an oscillatory signal; The method of any one of claims 1 to 7, further comprising:

9. Determining either the one or more aberrations of the projection system, the one or more additional aberrations of the projection system, or the one or more raw subsequent aberrations of the projection system from the vibration signals determined by the portions of the radiation detector receiving radiation comprises:

9. A method according to any preceding claim, comprising equalising the phase of harmonics of the oscillation signal received by each portion of the radiation detector receiving radiation to the sum of at least one difference in an aberration map between a pair of positions in a pupil plane of the projection system.

10. 1. A measurement system for determining one or more aberrations of a projection system, comprising: a first patterning device including a specular diffraction grating; a sensor apparatus comprising a second patterning device and a radiation detector, wherein the first patterning device and the sensor apparatus are positionable such that the projection system can form an image of the first patterning device on the second patterning device with illumination radiation, and the radiation detector is positioned to receive the illumination radiation after it has passed through the second patterning device; a positioning device configured to move at least one of the first patterning device and the sensor device; a controller; The controller controlling the positioning arrangement to move at least one of the first patterning device and the sensor arrangement in a shear direction, whereby an intensity of radiation received by portions of the radiation detector varies as a function of the movement in the shear direction to form an oscillation signal; determining, from the radiation detector, phases of harmonics of the vibration signal at a plurality of locations on the radiation detector; determining at least one coefficient characterizing at least one aberration of the projection system from the phases of harmonics of the vibration signal at the plurality of locations on the radiation detector; and a measurement system configured to:

11. The system of claim 10, wherein the controller is configured to perform the method of any one of claims 1 to 9.

12. 12. The system of claim 10 or claim 11, further comprising an illumination system operable to illuminate the first patterning device with illumination radiation.

13. The system of claim 12 , wherein the illumination radiation comprises extreme ultraviolet radiation.

14. The system of any one of claims 12 to 13, wherein the first patterning device is provided on a fiducial and / or on a reticle.

15. A lithographic apparatus comprising a measurement system according to any one of claims 10 to 14.

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