Method and system for determining aberrations of projection system

Abstract

Methods and corresponding systems for determining one or more aberrations of a projection system, such as a projection system of a lithographic apparatus, are disclosed. A method includes performing a phase step or phase scan process using a first patterning device (at an object stage) that includes a specular diffraction grating. Also disclosed is a calibration method for determining calibration data characterizing any difference between an aberration of the projection system determined using the diffusion grating at the object level and an aberration of the projection system determined using the mirror grating at the object level.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to European application 22199929.5 filed on October 6, 2022, and the entire contents of this European application are incorporated herein by reference. Technical Field

[0003] The present invention relates to a method for determining one or more aberrations of a projection system and a corresponding system. The projection system may be a projection system of a lithographic apparatus. The present invention also relates to a calibration method for determining calibration data, the calibration data characterizing any difference between: an aberration of the projection system determined at object level using a diffuse grating, and an aberration of the projection system determined at object level using a mirror grating. Background Art

[0004] A lithographic apparatus is a machine configured to apply a desired pattern onto a substrate. A lithographic apparatus may, for example, be used in the manufacture of integrated circuits (ICs). For example, a lithographic apparatus may project a pattern at a patterning device (e.g., a mask) onto a layer of radiation-sensitive material (resist) disposed on a substrate.

[0005] To project a pattern on a substrate, a lithography apparatus may use electromagnetic radiation. The wavelength of this radiation determines the minimum size of features that can be formed on the substrate. A lithography apparatus that uses extreme ultraviolet (EUV) radiation (having a wavelength in the range of 4 nm to 20 nm, such as 6.7 nm or 13.5 nm) may be used to form smaller features on a substrate.

[0006] The radiation that has been patterned by the patterning device is focused onto a substrate using a projection system. The projection system may introduce optical aberrations that cause the image formed on the substrate to deviate from the desired image (eg, the diffraction limited image of the patterning device).

[0007] It may be desirable to provide methods and apparatus for accurately determining such aberrations caused by a projection system so that these aberrations can be better controlled. Summary of the invention

[0008] 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, the phase stepping or phase scanning process comprising: illuminating a first pattern forming device with illumination radiation, wherein the first pattern forming device comprises a mirror diffraction grating, the mirror diffraction grating being arranged to form a plurality of first diffraction beams, the first diffraction beams being separated in a shear direction; projecting at least a portion of the plurality of first diffraction beams onto a sensor device with the projection system, the sensor device comprising: a second pattern forming device, the second pattern forming device being arranged to receive the first diffraction beam from the projection system and to form a plurality of second diffraction beams from each of the first diffraction beams; and a radiation detector, the radiation detector being arranged to receive at least a portion of the second diffraction beam; and moving at least one of the first pattern forming device and the second pattern forming device in the shear direction such that the intensity of radiation received by each portion of the radiation detector receiving radiation varies in accordance with the movement in the shear direction to form an oscillating signal; and determining one or more aberrations of the projection system based on the oscillating signal determined by each portion of the radiation detector receiving radiation.

[0009] It will be appreciated that a specular diffraction grating is intended to mean a reflective diffraction grating comprising a pattern of reflective portions and radiation absorbing portions, wherein reflection from the reflective portions is predominantly specular or regular reflection. In other words, a specular diffraction grating is intended to mean a reflective diffraction grating that causes minimal diffusion of radiation incident thereon. It will be appreciated that the above may be achieved by ensuring that the reflective portion of the specular diffraction grating is sufficiently smooth. In particular, any defects or surface roughness of the reflective portion of the specular diffraction grating may be small compared to the wavelength of the illuminating radiation scattered by the specular diffraction grating in use.

[0010] It will be appreciated that the method described is a form of shearing interferometry.

[0011] The method according to the first aspect may be applicable within a lithographic apparatus. The first patterning device may be arranged at a reticle level (i.e. in an object plane of the lithographic apparatus), and the sensor device may be arranged at a wafer level (i.e. in an image plane of the lithographic apparatus). In particular, the method according to the first aspect may be applicable within an extreme ultraviolet (EUV) lithographic apparatus. That is, the illuminating radiation may comprise EUV radiation.

[0012] During the lithography process, a projection system is used to form a typically diffraction limited image on a substrate (such as, for example, a wafer coated with resist) using radiation. In practice, the projection system will not form a perfect image, but will be affected by a certain degree of aberrations. Such aberrations represent deformations of the wavefront of light approaching a point in the image plane of the projection system from a spherical wavefront (and may typically depend on the position in the pupil plane, or alternatively on 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 perform measurements of said aberrations, which may be used as part of a feedback loop to minimize aberrations as much as possible.

[0013] Aberration measurements of projection systems of known EUV lithography apparatuses are typically performed using the same (EUV) radiation as that used for exposure of the wafer during the lithography process. This is clearly advantageous because it is desirable to measure and minimize aberrations caused when imaging with EUV radiation (because these aberrations affect the image formed on the wafer). However, the radiation is provided to the reticle level of the lithography apparatus via an illumination system, which is arranged to optimize the angular distribution of the EUV radiation at the reticle level for lithography imaging performance. In particular, while known EUV lithography apparatuses allow the angular distribution of the EUV radiation at the reticle level (which may be referred to as the illumination mode) to be controlled to a certain extent, it does not allow uniform pupil filling illumination (i.e., illumination of each field point at the reticle level with a uniform radiation cone). The illumination mode that can be achieved by the illumination system will not detect the entire pupil plane of the projection system. Therefore, measurements of aberrations of projection systems of known EUV lithography apparatuses typically use a diffuse reflection grating as the first patterning device (at the reticle level). The diffuse reflection grating is arranged to fill the entire pupil plane of the projection system so that the entire projection system is probed during aberration measurements.

[0014] However, the inventors of the present invention have recognized that such diffuse reflection diffraction gratings can introduce significant errors in the measurement of some aberrations, such as the amplitudes of some Zernike orders in the aberration map of the projection system. Typically, diffuse reflection gratings for aberration measurement systems are formed by forming a rough surface to which a reflective grating pattern is applied. For example, a uniform reflective layer may be applied to the rough surface and a pattern of radiation absorbing portions may be formed above the reflective layer. However, the inventors of the present invention have recognized that aberration measurements performed using diffuse reflection diffraction gratings are dependent on the specific rough surface used to induce the diffusion.

[0015] However, while there is a strong bias in the art that the projection system should be fully probed using diffusion, the inventors of the present invention have come to the surprising realization that measurements made using a specular diffraction grating can provide improved aberration measurements, at least for some aberrations (i.e., Zernike orders). In particular, even when using illumination patterns that can be achieved by illumination systems of known lithography systems (e.g., with an illumination pupil filling of approximately 20%), the use of a specular diffraction grating can provide better aberration measurements, at least for some aberrations (i.e., Zernike orders), than using known diffuse gratings.

[0016] Another advantage of using a mirror diffraction grating for aberration measurement at the reticle level is that it may be easier to place such a mirror diffraction grating on the reticle. Although in principle a diffuse reflection grating can be formed on the reticle, in practice, such an arrangement will most likely be impractical and is not currently standard practice (and may not even be feasible). Therefore, currently, standard practice is to place the diffuse reflection grating elsewhere on the reticle stage, such as on a reference piece or the like. However, if aberration measurements can be performed using a mirror diffraction grating on the reticle, significant time can be saved in aberration measurements. For example, currently, after exposing a target area of ​​the substrate to radiation patterned by the reticle, in order to perform aberration measurements, the reticle stage should be moved so that the reference piece (rather than the reticle) receives EUV radiation from the illumination system. In addition, after such aberration measurements, in order to expose the target area of ​​the substrate to radiation patterned by the reticle, the reticle stage should be moved so that the reticle (rather than the reference piece) receives EUV radiation from the illumination system. However, if the mirrored diffraction grating used for aberration measurement can be provided on the reticle, a lot of time will be saved, as the reticle stage will not need to be moved that far between exposure of the substrate and aberration measurement. In addition, using aberration measurements from the method according to the first aspect, low-order Zernike coefficients (such as, for example, Z2, Z3 and Z4) can be used for reticle alignment. Advantageously, this can mean that an additional sensor (again provided elsewhere on the reticle stage) may no longer need to be used for reticle alignment.

[0017] The method may include: performing multiple phase stepping or phase scanning processes, using different illumination modes to perform each of the multiple phase stepping or phase scanning processes, so that different parts of the radiation detector receive radiation during different processes in the multiple phase stepping or phase scanning processes, but each of the multiple phase stepping or phase scanning processes is similar to other phase stepping or phase scanning processes in other aspects; and wherein the step of determining one or more aberrations of the projection system may include using the oscillation signal determined by each part of the radiation detector that receives radiation according to any one of the multiple phase stepping or phase scanning processes.

[0018] Advantageously, by performing a plurality of different phase stepping or phase scanning processes, each process using a different illumination mode, each different phase stepping or phase scanning process can detect a different portion of the projection system pupil plane. It should be noted that the aberration or aberration map is not determined for each individual phase stepping or phase scanning process and then combined. Instead, the aberration or aberration map is determined using all the multiple phase stepping or phase scanning processes. Advantageously, such an embodiment allows for a higher resolution measurement of the aberration of the projection system.

[0019] The method may also include: performing an additional phase stepping or phase scanning process, using a diffuse diffraction grating as the first pattern forming device to perform the additional phase stepping or phase scanning process, but otherwise similar to the or each other phase stepping or phase scanning process; determining one or more additional aberrations of the projection system based on the oscillation signal determined by each portion of the radiation received by the radiation detector during the additional phase stepping or phase scanning process; and determining calibration data characterizing any differences 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.

[0020] Such an arrangement may be utilized to generate calibration data which may be used in subsequent measurements to correct for additional aberrations of the projection system resulting from the use of a diffuse diffraction grating as the first patterning device. Advantageously, this may allow potentially significant errors in the measurement of 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 to be corrected out. This may allow subsequent aberration measurements to be made using a diffuse diffraction grating as the first patterning device, which advantageously may make subsequent aberration measurements faster, potentially increasing the throughput of the lithographic apparatus.

[0021] The method may further comprise storing the determined calibration data in a memory.

[0022] According to a second aspect of the present disclosure, a calibration method is provided, the method comprising: performing a phase stepping or phase scanning process using an object-level mirror diffraction grating to generate one or more oscillation signals; determining one or more aberrations of the projection system based on the one or more oscillation signals; performing an additional phase stepping or phase scanning process using an object-level diffuse diffraction grating to generate one or more additional oscillation signals; determining one or more additional aberrations of the projection system based on the one or more additional oscillation signals; and determining calibration data characterizing any differences 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.

[0023] Such an arrangement allows calibration data to be generated which can be used in subsequent measurements to correct for additional aberrations of the projection system resulting from the use of a diffuse diffraction grating as the first patterning device. Advantageously, this may allow potentially significant errors in the measurement of 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 to be corrected out. This may allow subsequent aberration measurements to be made using a diffuse diffraction grating as the first patterning device, which advantageously may make subsequent aberration measurements faster, potentially increasing the throughput of the lithographic apparatus.

[0024] The method may also include: performing a subsequent phase stepping or phase scanning process, using the diffuse diffraction grating as the first pattern forming device to perform the subsequent phase stepping or phase scanning process, but the subsequent phase stepping or phase scanning process is similar to the or each other phase stepping or phase scanning process in other aspects; determining one or more original subsequent aberrations of the projection system based on the oscillation signal determined by each portion of the radiation received by the radiation detector during the subsequent phase stepping or phase scanning process; and determining one or more corrected subsequent aberrations based on the one or more original subsequent aberrations of the projection system and the calibration data.

[0025] The illuminating radiation may include extreme ultraviolet radiation.

[0026] The first patterning device may be disposed on a reference member.

[0027] The first patterning device may be disposed on a reticle.

[0028] Determine one or more aberrations of the projection system, one or more additional aberrations of the projection system, or any of one or more original subsequent aberrations of the projection system according to an oscillating signal determined by each portion of the received radiation of the radiation detector, and any known shearing interferometry technique may be included as needed or required. For example, the determination of any of these aberrations may include any technique disclosed in WO2019 / 149468, the contents of which are hereby incorporated herein by reference in their entirety. For the avoidance of doubt, the determination of any phase difference in these aberrations may include any technique disclosed as prior art in WO2019 / 149468 and / or any technique taught by WO2019 / 149468.

[0029] Any phase stepping or phase scanning process may also include: illuminating the first pattern forming device with illumination radiation, wherein the first pattern forming device includes a mirror diffraction grating, and the mirror diffraction grating is arranged to form a plurality of first diffraction beams, which are separated in a second shear direction; projecting at least a portion of the plurality of first diffraction beams onto the sensor device using the projection system; and moving at least one of the first pattern forming device and the second pattern forming device in the second shear direction so that the radiation intensity received by each portion of the received radiation of the radiation detector varies according to the movement in the second shear direction to form an oscillating signal.

[0030] For example, the first patterning device may include: a first portion, the first portion being arranged to shear the illumination radiation in a shearing direction; and a second portion, the second portion being arranged to shear the illumination radiation in a second shearing direction. During irradiation of the first portion of the first patterning device, at least one of the first patterning device and the second patterning device is stepped in the shearing direction. This generates first phase stepping data, which may be correlated to a gradient of an aberration map in the shearing direction at least in order. During irradiation of the second portion of the first patterning device, at least one of the first patterning device and the second patterning device is stepped in the second shearing direction. This generates second phase stepping data, which may be correlated to a gradient of an aberration map in the second shearing direction at least in one order. The first phase stepping data and the second phase stepping data are combined to determine one or more aberrations of the projection system. The first phase stepping data and the second phase stepping data may be combined to determine an aberration (or relative phase) map of the projection system.

[0031] Determining any of the one or more aberrations of the projection system, the one or more additional aberrations of the projection system, or the one or more original subsequent aberrations of the projection system based on the oscillating signal determined by each portion of the received radiation of the radiation detector may include: making the phase of the harmonics of the oscillating signal received by each portion of the received radiation of the radiation detector equal to the sum of at least one aberration mapping difference between a pair of positions in a pupil plane of the projection system.

[0032] The harmonic of the oscillation signal may be a first harmonic of the oscillation signal.

[0033] According to a third aspect of the present disclosure, a computer-readable medium carrying a computer program is provided, wherein the computer program includes computer-readable instructions configured to cause a computer to implement the method according to the first aspect of the present disclosure or the second aspect of the present disclosure.

[0034] According to a fourth aspect of the present disclosure, a computer device is provided, 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 include instructions arranged to control the computer to implement the method according to the first aspect of the present disclosure or the second aspect of the present disclosure.

[0035] 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 pattern forming device, wherein the first pattern forming device comprises a mirror diffraction grating; a sensor device, the sensor device comprising a second pattern forming device and a radiation detector, the first pattern forming device and the sensor device being positionable so that the projection system can form an image of the first pattern forming device on the second pattern forming device using illumination radiation, and so that the radiation detector is arranged to receive the illumination radiation after the illumination radiation has been transmitted through the second pattern forming device; a positioning device, the positioning device being configured to move at least one of the first pattern forming device and the sensor device; and a controller, the controller being configured to: control the positioning device to move at least one of the first pattern forming device and the sensor device in a shear direction so that the intensity of radiation received by each portion of the radiation detector varies depending on the movement in the shear direction to form an oscillating signal; determine from the radiation detector the phase of a harmonic of the oscillating signal at a plurality of positions on the radiation detector; and determine at least one coefficient characterizing at least one aberration of the projection system based on the phase of the harmonic of the oscillating signal at the plurality of positions on the radiation detector.

[0036] The measurement system according to the fifth aspect may be adapted for use within a lithographic apparatus. The first patterning device may be arranged at a reticle level (i.e. in an object plane of the lithographic apparatus), and the sensor device may be arranged at a wafer level (i.e. in an image plane of the lithographic apparatus). In particular, the measurement system according to the fifth aspect may be adapted for use within an extreme ultraviolet (EUV) lithographic apparatus. That is, the illuminating radiation may comprise EUV radiation.

[0037] It will be appreciated that the measurement system is in the form of a shearing interferometer.

[0038] The controller may be configured to implement the method according to the first aspect of the present disclosure or the second aspect of the present disclosure.

[0039] The system may further comprise an illumination system operable to illuminate the first patterning device with illumination radiation.

[0040] The illuminating radiation may include extreme ultraviolet radiation.

[0041] The first patterning device may be disposed on a reference member.

[0042] The first patterning device may be disposed on a reticle.

[0043] According to a sixth aspect of the present disclosure, there is provided a lithographic apparatus, the lithographic apparatus comprising the measurement system according to the fifth aspect of the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

[0045] Figure 1 Describing a lithographic system including a lithographic apparatus and a radiation source;

[0046] Figure 2 is a schematic illustration of a measurement system according to an embodiment of the present invention;

[0047] Figure 3A and Figure 3B It can be formed Figure 2 Schematic illustration of a patterning device and a sensor device as part of a measurement system;

[0048] Figure 4 is a schematic illustration of a measurement system according to an embodiment of the present invention, the measurement system comprising 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;

[0049] FIG. 5A to FIG. 5C Each is shown by Figure 4 Different groups of second diffraction beams formed by the second patterned area of ​​the measurement system shown in FIG. 5 , the groups of second diffraction beams having been generated by different first diffraction beams formed by the first patterned area;

[0050] Fig. 6A is used to determine the projection system (e.g. Figure 1 a schematic illustration of a novel measurement system for one or more aberrations of the lithography system (shown in Figure 1 ), wherein, for ease of illustration only, the measurement system is illustrated as a linear arrangement with the optical axis maintained in a single direction (it being understood that in practice the first patterning device and components within the projection system are reflective and that in practice the optical axis changes at each such reflection);

[0051] Figure 6B A typical measurement system for aberrations of a projection system of a known EUV lithography apparatus using a diffuse reflection grating as first patterning device (at reticle level) is shown;

[0052] Figure 7 is to determine the projection system (e.g. Figure 1 A schematic diagram of a novel method for detecting one or more aberrations of the projection system of the lithography system shown in FIG. 1 , and the method can be performed by Fig. 6A The measurement system shown in FIG.

[0053] Figure 8 Shown for evaluation Figure 7 The performance of the method schematically shown in FIG. 1 is the result of a simulation that has been performed; the first graph shows the aberration footprint or aberration imprint of the projection system input to the simulation for the first 25 Zernike orders; the second graph shows the aberration footprint or aberration imprint of the projection system input to the simulation for the first 25 Zernike orders; Figure 7 a reconstructed aberration footprint or aberration footprint determined using the method of FIG. 1 (i.e., using a mirror diffraction grating) using an illumination pupil filling of 20.7%; and a third graph showing the difference between the second graph and the first graph;

[0054] Fig. 9 is a schematic diagram of a second novel method for determining one or more aberrations of a projection system, which method can be obtained by Fig. 6A The measurement system shown in the figure is implemented and is Figure 7 A variation of the method shown in;

[0055] Fig.10 Yes Fig. 9 Another schematic diagram of a second novel method for determining one or more aberrations of a projection system shown in;

[0056] Fig.11 is a schematic diagram of a third novel method for determining one or more aberrations of a projection system, which method can be obtained by Fig. 6A The measurement system shown in FIG. Figure 7 A third method is a calibration method using shearing interferometry, a variation of the method shown in

[0057] Fig.12 is a schematic diagram of a fourth new method for determining one or more aberrations of a projection, and the fourth new method can be obtained by Fig. 6A The measurement system shown in FIG. 1 is implemented, and the fourth new method is Fig.11 A variation of the method shown in . DETAILED DESCRIPTION

[0058] Figure 1 A lithographic system is shown comprising a radiation source SO and a lithographic apparatus LA. The radiation source SO is configured to generate an EUV radiation beam B and to supply the EUV radiation beam B to the lithographic apparatus LA. The lithographic apparatus LA comprises an illumination system IL, a support structure MT configured to support a pattern forming device MA (e.g., a mask), a projection system PS, and a substrate table WT configured to support a substrate W. The pattern forming device MA is alternatively referred to herein as a reticle. The support structure MT is alternatively referred to as a reticle stage. The substrate W is alternatively referred to herein as a wafer. The substrate table WT is alternatively referred to as a substrate stage or a wafer stage.

[0059] 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 comprise a faceted field mirror arrangement 10 and a faceted pupil mirror arrangement 11. The faceted field mirror arrangement 10 and the faceted pupil mirror arrangement 11 together provide an EUV radiation beam B having a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may comprise other mirrors or arrangements in addition to or instead of the faceted field mirror arrangement 10 and the faceted pupil mirror arrangement 11.

[0060] After being so conditioned, the EUV radiation beam B interacts with the pattern forming device MA. As a result of this interaction, a patterned EUV radiation beam B' is generated. The projection system PS is configured to project the patterned EUV radiation beam B' onto a substrate W. For this purpose, the projection system PS may include a plurality of mirrors 13, 14, which are configured to project the patterned EUV radiation beam B' onto a substrate W, which is held by a substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B', thereby forming an image having features that are smaller than corresponding features on the pattern forming device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is Figure 11. Although illustrated in FIG. 1 as having only two mirrors 13, 14, the projection system PS may comprise a different number of mirrors (eg six or eight mirrors).

[0061] The substrate W may include a pre-formed pattern. Where this is the case, the lithographic apparatus LA aligns the image formed by the patterned EUV radiation beam B′ with the pattern previously formed on the substrate W.

[0062] A relative vacuum, ie a small amount of gas (eg hydrogen) at a pressure well below atmospheric pressure, may be provided in the radiation source SO, in the illumination system IL and / or in the projection system PS.

[0063] The radiation source SO may be a laser produced plasma (LPP) source, a discharge produced plasma (DPP) source, a free electron laser (FEL) or any other radiation source capable of generating EUV radiation.

[0064] Typically, the projection system PS has a possibly non-uniform optical transfer function, which may affect the pattern imaged onto the substrate W. For unpolarized radiation, such effects can be fairly well described by two scalar mappings that describe the transmission (apodization) and relative phase (aberration) of the radiation as a function of the position in the pupil plane of the radiation exiting the projection system PS. These scalar mappings, which may be referred to as transmittance mappings and relative phase mappings, can be expressed as linear combinations of a complete set of basis functions. A particularly suitable set is the Zernike polynomials, which form a set of orthogonal polynomials defined on the unit circle. The determination of each scalar mapping may involve determining the coefficients in such an expansion. Since the Zernike polynomials are orthogonal on the unit circle, the Zernike coefficients may be obtained from the measured scalar mappings by successively calculating the inner product of the measured scalar mapping with each Zernike polynomial and dividing such inner product by the square of the norm of the Zernike polynomial. In the following, unless otherwise stated, any reference to Zernike coefficients should be understood to mean Zernike coefficients of a relative phase map (also referred to herein as an aberration map). It should be appreciated that in alternative embodiments, other sets of basis functions may be used. For example, some embodiments may use Tatian Zernike polynomials, for example for a shielded aperture system.

[0065] The wavefront aberration map represents the deformation of the wavefront of light approaching a point in the image plane of the projection system PS from a spherical wavefront (as a function of position in the pupil plane, or alternatively, as a function of the angle at which the radiation approaches the image plane of the projection system PS). As discussed, such a wavefront aberration map W(x,y) can be expressed as a linear combination of Zernike polynomials:

[0066]

[0067] where x and y are coordinates in the pupil plane, z n (x,y) is the nth Zernike polynomial, and Z n It should be understood that in the following, Zernike polynomials and coefficients are labeled with indices commonly known as Nohl indices. Thus, z n (x, y) is a Zernike polynomial with Nohl index n, and Z n is the coefficient with Knoll index n. Then, the wavefront aberration map can be expanded into the coefficient group Z in the following formula: n To characterize, the coefficients can be called Zernike coefficients.

[0068] It will be appreciated that only a limited number of Zernike orders are considered. Different Zernike coefficients of the phase map may provide information about different forms of aberrations caused by the projection system PS. Zernike coefficients having a Noelle index of 1 may be referred to as first Zernike coefficients, Zernike coefficients having a Noelle index of 2 may be referred to as second Zernike coefficients, etc.

[0069] The first Zernike coefficient Z1 is related to the average value of the measured wavefront (which may be referred to as a translation term). The first Zernike coefficient may not be related to the performance of the projection system PS, and thus the method described herein may not be used to determine the first Zernike coefficient. The second Zernike coefficient Z2 is related to the tilt of the measured wavefront in the x-direction. The tilt of the wavefront in the x-direction is equivalent to placement in the x-direction. The third Zernike coefficient Z3 is related to the tilt of the measured wavefront in the y-direction. The tilt of the wavefront in the y-direction is equivalent to placement in the y-direction. The fourth Zernike coefficient Z4 is related to the defocus of the measured wavefront. The fourth Zernike coefficient is equivalent to placement in the z-direction. Higher-order Zernike coefficients are related to other forms of aberrations caused by the projection system, such as astigmatism, coma, spherical aberration, and other effects.

[0070] Throughout this specification, the term "aberration" shall be intended to include all forms of deviation of a wavefront from a perfect spherical wavefront. That is, the term "aberration" may relate to the placement of an image (e.g., second, third, and fourth Zernike coefficients) and / or to higher-order aberrations, such as those associated with Zernike coefficients having a Noelle index of 5 or greater. Furthermore, any reference to an aberration map for a projection system may include all forms of deviation of a wavefront from a perfect spherical wavefront, including deviations due to image placement.

[0071] The transmission map and the relative phase map are field and system dependent. That is, in general, each projection system PS will have a different Zernike expansion for each field point (ie for each spatial location in its image plane).

[0072] As will be described in further detail below, the relative phase of the projection system PS in its pupil plane can be determined by projecting radiation from an object plane of the projection system PS (i.e. the plane of the patterning device MA), via the projection system PS and using a shearing interferometer to measure the wavefront (i.e. the locus of points having the same phase). The shearing interferometer may comprise a diffraction grating, e.g. a two-dimensional diffraction grating, located in the image plane of the projection system (i.e. the substrate table WT), and a detector arranged to detect the interference pattern in a plane conjugate to the pupil plane of the projection system PS.

[0073] The projection system PS comprises a plurality of optical elements (including the mirrors 13, 14). As already explained, although the projection system PS is Figure 1 13, 14, but the projection system PS may include a different number of mirrors (e.g., six or eight mirrors). The lithographic apparatus LA also includes an adjustment device PA for adjusting these optical elements so as to correct aberrations (any type of phase change across the pupil plane throughout the field). To achieve the above, the adjustment device PA may be operable to manipulate the optical elements within the projection system PS in one or more different ways. The projection system may have a coordinate system in which its optical axis extends in the z-direction (it should be understood that the direction of such a z-axis changes, for example, at each mirror or optical element along the optical path through the projection system). The adjustment device PA is operable to perform any combination of the following operations: displacing one or more optical elements; tilting one or more optical elements; and / or deforming one or more optical elements. The displacement of the optical element may be performed in any direction (x, y, z, or a combination thereof). The tilting of the optical element is typically performed by rotating around an axis in the x or y direction out of a plane perpendicular to the optical axis, but rotation around the z-axis may be used for non-rotationally symmetric aspherical optical elements. Deformation of the optical element may be performed, for example, by using actuators to exert forces on the sides of the optical element and / or by using heating elements to heat selected areas of the optical element. Typically, it is not possible to adjust the projection system PS to correct for apodization (variations in transmission across the pupil plane). A transmission map of the projection system PS may be used when designing a mask MA for the lithographic apparatus LA.

[0074] In some embodiments, the adjustment apparatus PA may be operable to move the support structure MT and / or the substrate table WT. The adjustment apparatus PA may be operable to displace (in any of the x, y, z directions or a combination thereof) and / or tilt (by rotating about an axis in the x or y direction) the support structure MT and / or the substrate table WT.

[0075] The projection system PS forming part of the lithographic apparatus may periodically undergo a calibration process. For example, when the lithographic apparatus is manufactured in a factory, the optical elements (e.g., mirrors) forming the projection system PS may be set up by performing an initial calibration process. After installation of the lithographic apparatus at the site where the lithographic apparatus is to be used, the projection system PS may be calibrated again. Further calibration of the projection system PS may be performed at regular time intervals. For example, under normal use, the projection system PS may be calibrated every few months (e.g., every three months).

[0076] Calibrating the projection system PS may include passing radiation through the projection system PS and measuring the resulting projected radiation. The measurement of the projected radiation may be used to determine aberrations of the projected radiation caused by the projection system PS. A measurement system may be used to determine the aberrations caused by the projection system PS. In response to the determined aberrations, optical elements forming the projection system PS may be adjusted to correct for the aberrations caused by the projection system PS.

[0077] Figure 2 is a schematic illustration of a measurement system 10 that may be used to determine aberrations caused by a projection system PS. The measurement system 10 comprises an illumination system IL, a measurement pattern forming device MA', a sensor device 21 and a controller CN. The measurement system 10 may form part of a lithographic apparatus. For example, Figure 2 The illumination system IL and projection system PS shown in FIG. Figure 1 For ease of illustration, Figure 2 Further components of the lithographic apparatus are not shown.

[0078] The measurement patterning device MA' is arranged to receive radiation from the illumination system IL. The sensor device 21 is arranged to receive radiation from the projection system PS. During normal use of the lithographic apparatus, Figure 2 The measurement pattern forming device MA' and the sensor device 21 shown in FIG. 2 can be located in a manner similar to that of the sensor device 21. Figure 2 For example, during normal use of the lithographic apparatus, a patterning device MA configured to form a pattern to be transferred to a substrate W 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 (e.g., as shown in FIG. Figure 1 The measurement pattern forming device MA' and the sensor device 21 can be moved to where they are Figure 2 The measurement pattern forming device MA' may be formed by a support structure MT (such as Figure 1The sensor device 21 may be supported by a substrate table (such as a support structure shown in FIG. Figure 1 Alternatively, the sensor device 21 may be supported by a measurement table (not shown) which may be detachable from the sensor table WT.

[0079] The measurement pattern forming device MA' and the sensor device 21 are Figure 3A and Figure 3B is shown in more detail in . Figure 2 , Figure 3A and Figure 3B Cartesian coordinates are used consistently throughout. 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 device 21 in the xy plane.

[0080] The measurement pattern forming device MA' comprises a plurality of patterned areas 15a to 15c. Figure 2 and Figure 3A In the embodiment shown in , the measurement patterning device MA' is a reflective patterning device MA'. The patterned areas 15a to 15c each comprise a reflective diffraction grating. Radiation incident on the patterned areas 15a to 15c of the measurement patterning device MA' is thereby at least partially scattered and received by the projection system PS. In contrast, radiation incident on the remaining part of the measurement patterning device MA' is not reflected or scattered towards the projection system PS (e.g. it may be absorbed by the measurement patterning device MA').

[0081] The illumination system IL illuminates the measurement patterning device MA' with radiation. Figure 2 Although not shown, the illumination system IL may receive radiation from the radiation source SO and condition the radiation so as to illuminate the measurement patterning device MA'. For example, the illumination system IL may condition the radiation so as to provide radiation having a desired spatial and angular distribution. Figure 2 In the embodiment shown in FIG. 1 , the illumination system IL is configured to form separate measurement beams 17a to 17c. Each measurement beam 17a to 17c illuminates a respective patterned area 15a to 15c of the measurement patterning device MA'.

[0082] In order to perform a determination of aberrations caused by the projection system PL, the mode of the illumination system IL may be changed so as to illuminate the measurement patterning device MA' with separate measurement beams 17a to 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 narrow strip of radiation. However, the mode of the illumination system IL may be changed so that the illumination system IL is configured to form separate measurement beams 17a to 17c in order to perform a determination of aberrations caused by the projection system PL. In some embodiments, different patterned areas 15a to 15c may be illuminated at different times. For example, a first subset of the patterned areas 15a to 15c may be illuminated at a first time so as to form measurement beams 17a to 17c of the first subset, and a second subset of patterned areas 15a to 15c may be illuminated at a second time so as to form measurement beams 17a to 17c of the second subset.

[0083] In other embodiments, the mode of the illumination system IL may not be changed in order to perform the determination of 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 narrow strip of radiation, e.g. corresponding approximately to the illumination area used during exposure of the substrate. Separate measurement beams 17a to 17c may then be formed by the measurement patterning device MA', since only the patterned areas 15a to 15c reflect or scatter radiation towards the projection system PS.

[0084] In these figures, the Cartesian coordinate system is shown as being conserved through the projection system PS. However, in some embodiments, the properties of the projection system PS may result in a transformation of the coordinate system. For example, the projection system PS may form an image of the measurement patterning device MA' that is magnified, rotated and / or mirrored relative 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° about the z-axis. In such an embodiment, Figure 2 The relative positions of the first measurement beam 17a and the third measurement beam 17c shown in FIG. 1 may be exchanged. In other embodiments, the image may be mirrored around an axis that may be in the xy plane. For example, the image may be mirrored around the x-axis or around the y-axis.

[0085] In embodiments where the projection system PS causes the image of the measurement pattern forming device MA' to be rotated and / or the image to be mirrored by the projection system PS, the projection system is considered to transform the coordinate system. That is, the coordinate system referred to herein is defined relative to the image projected by the projection system PS, and any rotation and / or mirroring of the image will cause a corresponding rotation and / or mirroring of the coordinate system. For ease of illustration, the coordinate system is shown in the figure as being conserved by the projection system PS. However, in some embodiments, the coordinate system may be transformed by the projection system PS.

[0086] The patterned areas 15a to 15c modify the measuring beams 17a to 17c. Specifically, the patterned areas 15a to 15c cause spatial modulation of the measuring beams 17a to 17c and cause diffraction in the measuring beams 17a to 17c. Figure 3B In the embodiment shown in FIG. 1 , each of the patterned regions 15a to 15c includes two different portions. For example, the first patterned region 15a includes a first portion 15a′ and a second portion 15a″. The first portion 15a′ includes a diffraction grating aligned parallel to the u direction, and the second portion 15a″ includes a diffraction grating aligned parallel to the v direction. The u and v directions are aligned parallel to the v direction. Figure 3A Both the u-direction and the v-direction are aligned at approximately 45° relative to both the x-direction and the y-direction, and are aligned perpendicularly to each other. Figure 3A The second patterned region 15b and the third patterned region 15c shown in FIG. 1 are identical to the first patterned region 15a and each include a first portion and a second portion whose diffraction gratings are vertically aligned with each other.

[0087] The first and second portions of the patterned areas 15a to 15c may be irradiated with the measurement beams 17a to 17c at different times. For example, the first portion of each of the patterned areas 15a to 15c may be irradiated by the measurement beams 17a to 17c at a first time. At a second time, the second portion of each of the patterned areas 15a to 15c may be irradiated by the measurement beams 17a to 17c. As mentioned above, in some embodiments, different patterned areas 15a to 15c may be irradiated at different times. For example, the first portion of the first subset of the patterned areas 15a to 15c may be irradiated at a first time, and the first portion of the second subset of the patterned areas 15a to 15c may be irradiated at a second time. The second portion of the first subset of the patterned areas and the second portion of the second subset of the patterned areas may be irradiated at the same time or at different times. In general, any schedule for irradiating different portions of the patterned areas 15a to 15c may be used.

[0088] The modified measurement beams 17a to 17c are received by the projection system PS. The projection system PS forms an image of the patterned areas 15a to 15c on the sensor device 21. The sensor device 21 includes a plurality of diffraction gratings 19a to 19c and a radiation detector 23. The diffraction gratings 19a to 19c are arranged so that each diffraction grating 19a to 19c receives a corresponding modified measurement beam 17a to 17c output from the projection system PL. The projection system PS is arranged to form an image of each of the patterned areas 15a to 15c on a corresponding one of the diffraction gratings 19a to 19c of the sensor device 21. The modified measurement beams 17a to 17c incident on the diffraction gratings 19a to 19c are further modified by the diffraction gratings 19a to 19c. The modified measurement beams transmitted at the diffraction gratings 19a to 19c are incident on the radiation detector 23.

[0089] The radiation detector 23 is configured to detect a spatial intensity distribution of radiation incident on the radiation detector 23. The radiation detector 23 may, for example, include an array of individual detector elements or sensing elements. For example, the radiation detector 23 may include an active pixel sensor, such as, for example, a CMOS (complementary metal oxide semiconductor) sensor array. Alternatively, the radiation detector 23 may include a CCD (charge coupled device) sensor array. The diffraction gratings 19a to 19c and the portion of the radiation sensor 23 receiving the modified measurement beams 17a to 17c form detector regions 25a to 25c. For example, the first diffraction grating 19a and the first portion of the radiation sensor 23 receiving the first measurement beam 17a together form a first detector region 25a. The measurement of a given measurement beam 17a to 17c may be performed at the corresponding detector region 25a to 25c (as illustrated). As described above, in some embodiments, the relative positioning of the modified measurement beams 17a to 17c and the coordinate system may be transformed by the projection system PS.

[0090] The modification of the measurement beam 17a to 17c occurring at the patterned regions 15a to 15c and the diffraction gratings 19a to 19c of the detector regions 25a to 25c results in an interference pattern being formed on the radiation detector 23. The interference pattern is related to the derivative of the phase of the measurement beam and depends on the aberrations caused by the projection system PS. Therefore, the interference pattern can be used to determine the aberrations caused by the projection system PS.

[0091] Typically, the diffraction grating 19a to 19c of each of the detector zones 25a to 25c comprises a two-dimensional transmission type diffraction grating. Figure 3BIn the embodiment shown in , the detector zones 25a to 25c each include a diffraction grating 19a to 19c configured in the form of a chessboard. In an alternative embodiment, the detector zones 25a to 25c each may include a two-dimensional transmission type diffraction grating 19a to 19c that is not configured in the form of a chessboard, but may instead be, for example, a pinhole array.

[0092] Illuminating a first portion of the patterned areas 15a to 15c may provide information related to the gradient of the aberration map of the projection system PS in a first direction, and irradiating a second portion of the patterned areas 15a to 15c may provide information related to the gradient of the aberration map of the projection system PS in a second direction.

[0093] In some embodiments, the measurement patterning device MA' and / or the sensor device 21 are continuously scanned and / or stepped in two vertical directions. For example, the measurement patterning device MA' and / or the sensor device 21 may be stepped relative to each other in the u-direction and the v-direction. The measurement patterning device MA' and / or the sensor device 21 may be stepped in the u-direction when irradiating the second part 15a'' to 15c'' of the patterned areas 15a to 15c; and the measurement patterning device MA' and / or the sensor device 21 may be stepped in the v-direction when irradiating the first part 15a'-15c' of the patterned areas 15a to 15c. That is, the measurement patterning device MA' and / or the sensor device 21 may be stepped in a direction perpendicular to the alignment of the diffraction grating being illuminated.

[0094] The measurement pattern forming device MA' and / or the sensor device 21 can be stepped by a distance corresponding to a fraction of the grating period of the diffraction grating. The measurements made at different stepping positions can be analyzed in order to derive information about the wavefront in the stepping direction. For example, the phase of the first harmonic of the measurement signal (which can be called a phase-stepped signal) can contain information about the derivative of the wavefront in the stepping direction (i.e. the gradient of the wavefront map in the stepping direction). Stepping the measurement pattern forming device MA' and / or the sensor device 21 in both the u-direction and the v-direction (which are perpendicular to each other) thus allows to derive information about the wavefront in two perpendicular directions (specifically, it provides information about the derivative of the wavefront in each of the two perpendicular directions), thereby allowing to reconstruct the complete wavefront.

[0095] In addition to stepping the measurement patterning device MA' and / or the sensor device 21 in a direction perpendicular to the alignment of the diffraction grating being illuminated (as described above), the measurement patterning device MA' and / or the sensor device 21 may also be scanned relative to each other. The scanning of the measurement patterning device MA' and / or the sensor device 21 may be performed in a direction parallel to the lines of the diffraction grating being illuminated. For example, the measurement patterning device MA' and / or the sensor device 21 may be scanned in the u-direction when illuminating a first portion 15a' to 15c' of the patterned areas 15a to 15c, and may be scanned in the v-direction when illuminating a second portion 15a'' to 15c'' of the patterned areas 15a to 15c. Scanning the measurement patterning device MA' and / or the sensor device 21 in a direction parallel to the alignment of the diffraction grating being illuminated allows the measurements to be averaged across the diffraction grating, thereby taking into account any variations of the diffraction grating in the scanning direction. The scanning of the measurement patterning device MA' and / or the sensor device 21 may be performed at a different time than the stepping of the measurement patterning device MA' and / or the sensor device 21 described above.

[0096] It will be appreciated that various arrangements of the patterned areas 15a to 15c and the detector areas 25a to 25c may be used in order to determine aberrations caused by the projection system PS. The patterned areas 15a to 15c and / or the detector areas 25a to 25c may include a diffraction grating. In some embodiments, the patterned areas 15a to 15c and / or the detector areas 25a to 25c may include components other than a diffraction grating. For example, in some embodiments, the patterned areas 15a to 15c and / or the detector areas may include a single slit or pinhole opening through which at least a portion of the measurement beam 17a to 17c may propagate. In general, the patterned areas and / or the detector areas may include any arrangement for modifying the measurement beam.

[0097] The controller CN receives measurements made at the sensor device 21 and determines from the measurements aberrations caused by 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 device 21 and / or the measurement patterning device MA' relative to each other. The controller may control an adjustment device PA for adjusting components of the projection system PS. For example, the adjustment device PA may adjust optical elements of the projection system PS in order to correct aberrations caused by the projection system PS and determined by the controller CN.

[0098] In some embodiments, the controller CN may be operable to control an adjustment device PA for adjusting the support structure MT and / or the substrate table WT. For example, the adjustment device PA may adjust the support structure MT and / or the substrate table WT so as to correct aberrations caused by placement errors of the patterning device MA and / or the substrate W (and determined by the controller CN).

[0099] Determining aberrations (which may be caused by the projection system PS or by placement errors of the patterning device MA or the substrate W) may comprise fitting measurements made by the sensor apparatus 21 to Zernike polynomials to obtain Zernike coefficients. Different Zernike coefficients may provide information about different forms of aberrations caused by the projection system PS. The Zernike coefficients may be determined independently at different positions in the x and / or y direction. For example, at Figure 2 , Figure 3A and Figure 3B In the embodiment shown in , a Zernike coefficient can be determined for each measurement beam 17 a to 17 c.

[0100] In some embodiments, the measurement patterning device MA' may include more than three patterned areas, the sensor device 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 locations. In some embodiments, the patterned areas and the detector areas may be distributed at different locations in both the x-direction and the y-direction. This may allow the Zernike coefficients to be determined at separated locations in both the x-direction and the y-direction.

[0101] At the same time, Figure 2 , Figure 3A and Figure 3BIn the embodiment shown in , the measurement pattern forming device MA' includes three patterned areas 15a to 15c and the sensor device 21 includes three detector areas 25a to 25c. In other embodiments, the measurement pattern forming device MA' may include more or less than three patterned areas 15a to 15c and / or the sensor device 21 may include more or less than three detector areas 25a to 25c.

[0102] Any method known in the art may be used to determine the aberrations caused by the projection system PS. Examples of such methods are disclosed in WO2019 / 149468 and WO2019 / 149468, both of which are hereby incorporated herein by reference in their entirety. Figure 4 A method for determining the aberrations caused by the projection system PS is briefly described.

[0103] Typically, the measurement patterning device MA' comprises at least one first patterned area 15a to 15c and the sensor device 21 comprises at least one second patterned area 19a to 19c.

[0104] Figure 4 is a schematic illustration of a measurement system 30 that can be used to determine aberrations caused by the projection system PS. The measurement system 30 can be used with Figure 2 The measurement system 10 shown in FIG. 1 is identical to the measurement system 10 shown in FIG. 1 , however, it may have a different number of first patterned areas (on the measurement patterning device MA′) and second patterned areas (in the sensor device 21 ). Figure 4 The measurement system 30 shown in FIG. 3 may include the Figure 2 Any features of the measurement system 10 shown in FIG. 1 are not included, and those features will not be described further below.

[0105] exist Figure 4 In FIG. 2 , only a single first patterned area 31 is provided on the measurement patterning device MA′ and a single second patterned area 32 is provided in the sensor device 21 .

[0106] The measurement patterning device MA' is illuminated with radiation 33 from the illumination system IL. For ease of understanding, Figure 4Only a single line is shown in FIG. 1 (which may represent, for example, a single ray of an incident radiation beam, such as a principal ray). However, it will be appreciated that the radiation 33 will comprise a range of angles 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 (the cone, as explained further below, is typically not completely filled with light). Typically, each point is illuminated by substantially the same range of angles, which is characteristic of the radiation intensity in a pupil plane of the illumination system IL (not shown).

[0107] The first patterned area 31 is arranged to receive the radiation 33 and form a plurality of first diffraction beams 34, 35, 36. The central first diffraction beam 35 corresponds to the 0th order diffraction beam of the first patterned area 31, and the other two first diffraction beams 34, 36 correspond to the ±1st order diffraction beams of the first patterned area 31. It should be understood that there will generally be more, higher order diffraction beams. Again for ease of understanding, in Figure 4 Only the three first diffracted beams 34 , 35 , 36 are shown.

[0108] It will also be appreciated that, since the incident radiation 33 comprises a (partially filled) cone of radiation converging on 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 diverging from said point on the first patterned area 31 .

[0109] In order to achieve the generation of the first diffraction beams 34, 35, 36, the first patterned area 31 may be in the form of a diffraction grating. For example, the first patterned area 31 may generally have Figure 3A Specifically, at least a portion of the first patterned area 31 may have Figure 3A The first portion 15a' of the patterned region 15a is shown in the form of a diffraction grating aligned parallel to the u direction (note that Figure 4 . Shown in the zv plane). Therefore, the first diffracted beams 34 to 36 are separated in the shear direction, which is the v direction.

[0110] The first diffracted beams 34 to 36 are at least partially captured by the projection system PS as presently described. The number of first diffracted beams 34 to 36 captured by the projection system PS will depend on: the pupil filling of the incident radiation 33 from the illumination system IL; the angular spacing of the first diffracted beams 34 to 36 (which in turn depends on the pitch of the first patterned areas 31 and the wavelength of the radiation 33); and the numerical aperture of the projection system PS.

[0111] Typically, substantially all first diffraction beams 35 corresponding to the 0th order diffraction beam and most of the first diffraction beams 34, 36 corresponding to the ±1st order diffraction beams are captured by the projection system PS and projected onto the sensor device 21. (In addition, in the case of such an arrangement, a large number of diffraction beams generated by the first patterned area 31 are at least partially projected onto the sensor device 21).

[0112] The role of the first patterned area 31 is to introduce spatial coherence, as currently discussed.

[0113] Typically, two rays of radiation 33 from the illumination system IL that are incident on the same point of the measurement patterning device MA' at different angles of incidence are incoherent. By receiving the radiation 33 and forming a plurality of first diffracted beams 34, 35, 36, the first patterned area 31 can be considered to form a plurality of copies of the incident radiation cone 33 (the copies typically having different phases and intensities). Within any of these copies or first diffracted beams 34, 35, 36, two rays of radiation originating from the same point on the measurement patterning device MA' but at different scattering angles are incoherent (due to the nature of the illumination system IL). However, for a given radiation ray within any of the first diffracted beams 34, 35, 36, there is 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 ray of each of the first diffracted beams 34, 35, 36 (which corresponds to the chief ray of the incident radiation 33) is coherent and, when combined, interferes at the amplitude level.

[0114] This coherence is exploited by a measurement system 30 to determine an aberration map of the projection system PS.

[0115] The projection system PS projects parts of the first diffracted beam 34 , 35 , 36 , which are captured by the numerical aperture of the projection system, onto the sensor device 21 .

[0116] exist Figure 4 In the embodiment, the sensor device 21 comprises a single second patterned area 32. As further described below (refer to FIG. 5A to FIG. 5C ), the second patterned area 32 is arranged to receive these first diffraction beams 34 to 36 from the projection system PS and form a plurality of second diffraction beams from each of the first diffraction beams. To achieve this, the second patterned area 32 comprises a two-dimensional transmission type diffraction grating. Figure 4 In FIG. 3 , 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 is used to determine an aberration map.

[0117] Each of the first diffraction beams 34 to 36 incident on the second patterned area 32 will be diffracted into a plurality of second diffraction beams. Since the second patterned area 32 includes a two-dimensional diffraction grating, a two-dimensional array of secondary diffraction beams is generated from each incident first diffraction beam (the main rays of these secondary diffraction beams are separated in both the shear direction (v direction) and the direction perpendicular thereto (u direction)). Hereinafter, the diffraction order of nth order in the shear direction (v direction) and mth order in the non-shear direction (u direction) will be referred to as the (n, m) diffraction order of the second patterned area 32. Hereinafter, when it is not important what order the second diffraction beam is in the non-shear direction (u direction), the (n, m)th diffraction order of the second patterned area 32 may be simply referred to as the nth order second diffraction beam.

[0118] FIG. 5A to FIG. 5C A set of second diffracted beams generated by each of the first diffracted beams 34 to 36 is shown. Figure 5A A set of second diffraction beams 35 a to 35 e generated by the first diffraction beam 35 corresponding to the 0th order diffraction beam of the first patterned area 31 is shown. Figure 5B A set of second diffraction beams 36 a to 36 e generated by the first diffraction beam 36 corresponding to the −1 order diffraction beam of the first patterned area 31 is shown. Figure 5C A set of second diffraction beams 34 a to 34 e generated by the first diffraction beam 34 corresponding to the +1 order diffraction beam of the first patterned area 31 is shown.

[0119] exist Figure 5A , the second diffraction beam 35a corresponds to the 0th order diffraction beam (of the second patterned area 32 and in the shear direction), while the second diffraction beams 35b, 35c correspond to ±1st order diffraction beams and the second diffraction beams 35d, 35e correspond to ±2nd order diffraction beams. It will be appreciated that FIG. 5A to FIG. 5C The second diffraction beams shown are shown in the vz plane, and may correspond, for example, to the 0th order diffraction beams of the second patterned area 32 in the non-shear direction (i.e., the u direction). It will be further appreciated that there will be multiple copies of these second diffraction beams, representing the entry or exit in the non-shear direction. FIG. 5A to FIG. 5C Higher order diffraction beams of the page.

[0120] exist Figure 5B , the second diffraction beam 36a corresponds to the 0th order diffraction beam (of the second patterned area 32 and in the shear direction), while the second diffraction beams 36b, 36c correspond to ±1st order diffraction beams and the second diffraction beams 36d, 36e correspond to ±2nd order diffraction beams.

[0121] exist Figure 5C, the second diffraction beam 34a corresponds to the 0th order diffraction beam (of the second patterned area 32 and in the shear direction), while the second diffraction beams 34b, 34c correspond to ±1st order diffraction beams and the second diffraction beams 34d, 34e correspond to ±2nd order diffraction beams.

[0122] from FIG. 5A to FIG. 5C It can be seen that several second diffraction beams overlap each other in space. For example, the second diffraction beam 35b of the 0th order diffraction beam 35 originating from the first patterned area 31 and corresponding to the -1st order diffraction beam of the second patterned area 32 overlaps with the second diffraction beam 36a of the -1st order diffraction beam 36 originating from the first patterned area 31 and corresponding to the 0th order diffraction beam of the second patterned area 32. Figure 4 and FIG. 5A to FIG. 5C All of the lines in can be considered to represent individual radiation rays originating from a single input ray 33 from the illumination system IL. Thus, as explained above, these lines represent spatially coherent rays that would produce an interference pattern if spatially superimposed at the radiation detector 23. Furthermore, interference is between rays that have passed through different portions of the pupil plane 37 of the projection system PS (which are separated in the shear direction). Thus, interference of radiation originating from a single input ray 33 depends on the phase difference between two different portions of the pupil plane.

[0123] This spatial superposition and spatial coherence of the second diffraction beam at the radiation detector 23 is achieved by matching the first patterned area 31 and the second patterned area 32 so that the angular spacing (in the shear direction) between different second diffraction beams originating from a given first diffraction beam is the same as the angular spacing (in the shear direction) between the different first diffraction beams when they converge on the second patterned area 32. This spatial superposition and spatial coherence of the second diffraction beam at the radiation detector 23 is achieved by matching the pitch of the first patterned area 31 and the second patterned area 32 in the shear direction. It should be understood that this matching of the pitch of the first patterned area 31 and the second patterned area 32 in the shear direction takes into account any reduction factor applied by the projection system PS. As used herein, the pitch of a two-dimensional diffraction grating in a particular direction is defined as follows.

[0124] It will be appreciated that a one-dimensional diffraction grating comprises a series of lines formed by a repeating pattern (a repeating pattern having a reflectivity or transmittance) in a direction perpendicular to the lines. The smallest non-repeating segment used to form the repeating pattern in a direction perpendicular to the lines is referred to as a unit cell, and the length of such a unit cell is referred to as the pitch of the one-dimensional diffraction grating. Typically, such a one-dimensional diffraction grating will have a diffraction pattern such that an incident radiation beam will be diffracted so as to form a one-dimensional array of angularly spaced (but possibly spatially superimposed) diffraction beams. The first patterned region 31 forms such a one-dimensional array of angularly spaced first diffraction beams 34 to 36 that are offset (angularly spaced) in the shear direction.

[0125] It will be appreciated that a two-dimensional diffraction grating comprises a two-dimensional repeating pattern having a reflectivity or a transmittance. The smallest non-repeating segment used to form such a repeating pattern may be referred to as a unit cell. The unit cell may be a square, and the basic pitch of such a two-dimensional diffraction grating may be defined as the length of a square unit cell. Typically, such a two-dimensional diffraction grating will have a diffraction pattern such that an incident radiation beam will be diffracted to form a two-dimensional array of diffraction beams that are angularly spaced apart (but may be spatially superimposed). The axes of such a two-dimensional (square) array of diffraction beams are parallel to the sides of the unit cell. The angular spacing between adjacent diffraction beams in these two directions may be given by the ratio of the wavelength of the radiation to the pitch of the grating. Therefore, the smaller the pitch, the greater the angular spacing between adjacent diffraction beams.

[0126] In some embodiments, the axis of the unit cell of the two-dimensional second patterned area 32 may be arranged at a non-zero angle to the shear direction and the non-shear direction as defined by the first patterned area 31. For example, the axis of the unit cell of the two-dimensional second patterned area 32 may be arranged at 45° to the shear direction and the non-shear direction as defined by the first patterned area 31. As previously explained, the spatial superposition and spatial coherence of the second diffracted beams at the radiation detector 23 allowing measurement of the wavefront is achieved by ensuring that the angular spacing (in the shear direction) between different second diffracted beams originating from a given first diffracted beam is the same as the angular spacing (in the shear direction) between the different first diffracted beams when they converge on the second patterned area 32. For arrangements in which the axis of the unit cell of the two-dimensional second patterned area 32 is arranged at a non-zero angle (e.g. 45°) 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 square that forms a repeating pattern of the diffraction grating, which is oriented so that its sides are parallel to the shear and non-shear directions (as defined by the first patterned area 31). The pseudo pitch can be defined as the length of the square pseudo unit cell. This can be referred to as the pitch of the two-dimensional diffraction grating in the shear direction. This pseudo pitch should match (an integer multiple or fraction of) the pitch of the first patterned area 31.

[0127] The diffraction pattern of a diffraction grating can be considered to form a two-dimensional array of pseudo-diffraction beams that are angularly spaced (but possibly spatially superimposed), with the axes of this two-dimensional (square) array of pseudo-diffraction beams being parallel to the sides of the pseudo-unit cells. Since such a square is not a unit cell (defined with respect to the smallest square of any orientation used to form the repeating pattern of the diffraction grating), the pseudo-pitch will be larger than the pitch (or basic pitch). Therefore, the spacing between adjacent pseudo-diffraction beams in the diffraction pattern (in a direction parallel to the sides of the pseudo-unit cell) will be smaller than the spacing between adjacent diffraction beams in the diffraction pattern (in a direction parallel to the sides of the unit cell). This situation can be understood as follows. Some of the pseudo-diffraction beams correspond to diffraction beams in the diffraction pattern, and other pseudo-diffraction beams are non-physical and do not represent diffraction beams produced by the diffraction grating (and only appear due to the use of pseudo-unit cells that are larger than true unit cells).

[0128] Taking into account any reduction (or magnification) factor applied by the projection system PS shown, the pitch of the second patterned area 32 in the shear direction should be an integer multiple of the pitch of the first patterned area 31 in the shear direction, or the pitch of the first patterned area 31 in the shear direction should be an integer multiple of the pitch of the second patterned area 32 in the shear direction. FIG. 5A to FIG. 5CIn the example shown in , the pitches of the first patterned areas 31 and the second patterned areas 32 in the shear direction are substantially equal (taking into account any reduction factors).

[0129] As from FIG. 5A to FIG. 5C As can be seen, each point on the detector area 39 of the radiation detector 23 will typically receive several contributions that are coherently summed. For example, a point on the detector area 39 that receives a second diffraction beam 35b of the 0th order diffraction beam 35 originating from the first patterned area 31, corresponding to the -1st order diffraction beam of the second patterned area 32, is superimposed with: (a) a second diffraction beam 36a of the -1st order diffraction beam 36 originating from the first patterned area 31, corresponding to the 0th order diffraction beam of the second patterned area 32; and (b) a second diffraction beam 34d of the +1st order diffraction beam 34 originating from the first patterned area 31, corresponding to the -2nd order diffraction beam of the second patterned area 32. It should be appreciated that when considering higher order diffraction beams of the first patterned area 31, there will be more beams that should be coherently summed at each point on the detector area 39 in order to determine the radiation intensity as measured by the portion of the detector area 39 (e.g., the corresponding pixel in the two-dimensional array of sensing elements).

[0130] Typically, a plurality of different second diffracted beams contribute to the radiation received by each portion of the detector region 39. The intensity of the radiation resulting from such a coherent sum is given by:

[0131]

[0132] where DC is a constant term (which is equivalent to the incoherent sum of the different diffracted beams) involving all different pairs of second diffracted beams, γ i is the interference intensity of the second diffracted beam and Δϕ i is the phase difference between the pair of second diffracted beams.

[0133] The phase difference Δϕ between a pair of second diffracted beams i Relies on two contribution values: (a) a first contribution value is related to different parts of the pupil plane 37 of the projection system PS from which the pair of second diffraction beams originate; and (b) a second contribution value is related to the position within the unit cell of each of the first patterned area 31 and the second patterned area 32 from which the pair of second diffraction beams originate.

[0134] The first of these contributions can be understood as arising from the fact that different coherent radiation beams have passed through different parts of the projection system PS and are therefore associated with the aberration that one wishes to determine (in practice, it is associated with the difference between two points in the aberration map separated in the shear direction).

[0135] The second of these contributions can be understood as arising from the fact that the relative phases of multiple radiation rays caused by a single ray incident on the diffraction grating will depend on which part of the unit cell of the grating the ray is incident on. Therefore, this does not contain information related to the aberration. As explained above, in some embodiments, the measurement pattern forming device MA' and / or the sensor device 21 are continuously scanned and / or stepped in the shear direction. This causes the phase difference between all pairs of interfering radiation beams received by the radiation detector 23 to change. When the measurement pattern forming device MA' and / or the sensor device 21 are continuously stepped in the shear direction by an amount equivalent to a fraction of the pitch (in the shear direction) of the first patterned area 31 and the second patterned area 32, typically, the phase differences between the second diffraction beam pairs will all change. If the measurement pattern forming device MA' and / or the sensor device 21 are continuously stepped in the shear direction by an amount equivalent to an integer multiple of the pitch (in the shear direction) of the first patterned area 31 and the second patterned area 32, the phase difference between the second diffraction beam pairs will remain unchanged. Therefore, when the measurement pattern forming device MA' and / or the sensor device 21 are continuously scanned and / or stepped in the shear direction, the intensity received by each part of the radiation detector 23 will oscillate. The first harmonic of this oscillating signal as measured by the radiation detector 23 (which can be called a phase stepped signal) depends on the contribution to equation (1) generated by adjacent first diffraction beams 34 to 36, that is, first diffraction beams with an order difference of ±1. Due to such a phase stepping technique, the contributions generated by first diffraction beams with different order differences will contribute to the higher-order harmonics of the signal determined by the radiation detector 23.

[0136] For example, of the three superimposed second diffraction beams (35b, 36a, and 34d) discussed above, only two of the three possible pairs of these diffraction beams will contribute to the first harmonic of the phase stepping signal: (a) second diffraction beams 35b and 36a (which originate from the 0th-order diffraction beam 35 and the -1st-order diffraction beam 36 of the first patterned area 31, respectively); and (b) second diffraction beams 35a and 34d (which originate from the 0th-order diffraction beam 35 and the +1st-order diffraction beam 34 of the first patterned area 31, respectively).

[0137] Each pair of second diffracted beams will produce an interference term in the form shown in equation (2) that contributes to the first harmonic of the phase-stepped signal, i.e., an interference term of the form:

[0138]

[0139] Wherein γ is the amplitude of the interference term, p is the pitch (in the shear direction) of the first patterned area 31 and the second patterned area 32, v parameterizes the relative position of the first patterned area 31 and the second patterned area 32 in the shear direction and ΔW is the difference between the values ​​of the aberration map at two positions in the pupil plane of the projection system PS, the two positions corresponding to the positions from which the two second diffraction beams originate. The amplitude γ of the interference term is proportional to the product of the composite scattering efficiency of the two second diffraction beams, as discussed, for example, in WO2019 / 149468. The frequency of the first harmonic of the phase stepping signal is given by the inverse of the pitch p of the first patterned area 31 and the second patterned area 32 in the shear direction. The phase of the phase stepping signal is given by ΔW (the difference between the values ​​of the aberration map at two positions in the pupil plane of the projection system PS, the two positions corresponding to the positions from which the two second diffraction beams originate).

[0140] In the right FIG. 3A to FIG. 5C In the above description of the embodiment shown in Figure 3A The first portion 15a' of the patterned area 15a shown in FIG. 1 is illuminated, the shear direction corresponds to the v direction and the non-shear direction corresponds to the u direction. It should be understood that when Figure 3A When the second portion 15a'' of the patterned area 15a shown in Fig. 1 is illuminated, the shear direction corresponds to the u direction and the non-shear direction corresponds to the v direction. Although in these above embodiments, both the u direction and the v direction (which define the two shear directions) are aligned at approximately 45° relative to both the x direction and the y direction of the lithographic apparatus LA, it will be appreciated that in alternative embodiments, the two shear directions may be arranged at any angle to the x direction and the y direction of the lithographic apparatus LA (which may correspond to the non-scanning direction and the scanning direction of the lithographic apparatus LA). Typically, the two shear directions will be perpendicular to each other.

[0141] As explained in WO2019 / 149468, or using more conventional techniques, the wavefront can be reconstructed as follows. The first shear direction (v direction, using Figure 3A The first phase stepping process is performed by oscillating the first portion 15a' of the patterned area 15a shown in FIG. 1 , and the phase of the first harmonic of the oscillating phase stepping signal is related to the term ΔW in equation (3). For the first phase stepping process, for each pixel or sensing element of the detector area 39 that receives the radiation 38, the term ΔW in equation (3) is related to the gradient of the wavefront in the first shearing direction (v direction). The second shearing direction (u direction, using Figure 3AA second phase stepping process is performed for a second portion 15a'' of the patterned area 15a shown in Fig. 1 and the phase of the first harmonic of the oscillating phase stepping signal is related to the term Δ in equation (3). For the second phase stepping process, for each pixel or sensing element of the detector zone 39 receiving radiation 38, the term ΔW in equation (3) is related to the gradient of the wavefront in the second shearing direction (u direction). Therefore, two phases (of the harmonics of the phase stepping signal) are determined for each pixel or sensing element of the detector zone 39 receiving radiation 38. These determined phases for all pixels of the detector zone 39 are combined to determine the wavefront (relative phase) map of the projection system PS.

[0142] Some embodiments of the present disclosure relate to a method of determining one or more aberrations of a projection system PS that uses a mirror diffraction grating located at a reticle level or in an object plane of the projection system PS. Some other embodiments of the present disclosure relate to a corresponding measurement system for determining one or more aberrations of a projection system PS that uses a mirror diffraction grating located at a reticle level or in an object plane of the projection system PS. The measurement system may generally be the above Figure 2 and Figure 4 The measurement system shown in FIG. 1 and the mirror diffraction grating at the reticle level or in the object plane of the projection system PS can be used as Figures 2 to 3A Any one of the plurality of patterned regions 15a to 15c shown and described above and / or Figure 4 The first patterned region 31 is shown in FIG. 1 and described above.

[0143] It will be appreciated that a specular diffraction grating is intended to mean a reflective diffraction grating comprising a pattern of reflective portions and radiation absorbing portions, wherein reflection from the reflective portion is predominantly specular or regular reflection. In other words, a specular diffraction grating is intended to mean a reflective diffraction grating that causes minimal diffusion of radiation incident thereon. It will be appreciated that the above may be achieved by ensuring that the reflective portion of the specular diffraction grating is sufficiently smooth. In particular, any defects or surface roughness of the reflective portion of the specular diffraction grating may be small compared to the wavelength of the illuminating radiation scattered by the specular diffraction grating in use.

[0144] Fig. 6A1 is a schematic illustration 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 apparatus 120; a positioning apparatus 130; and a controller 140. For ease of illustration only, the measurement system 100 is illustrated as a linear arrangement, wherein the optical axis remains 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 in practice the optical axis changes at each such reflection.

[0145] The positioning device 130 is configured to move at least one of the first patterning device 110 and the sensor device 120. To achieve the above, the positioning device 130 may be operable to send control signals 132, 134 to one or both of the first patterning device 110 and the sensor device 120.

[0146] The first patterning device 110 may be positioned in the object plane of the projection system PS. During aberration measurement, 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 device 130). The first patterning device 110 comprises a mirror diffraction grating.

[0147] The sensor device 120 comprises: a second patterning device 122 and a radiation detector 124. The sensor device 120 is arranged at the wafer level. The sensor device 120 can be positioned so that the second patterning device 122 can be arranged in the image plane of the projection system PS. The first patterning device 110 and the sensor device 120 can be positioned (for example using the positioning device 130) so that the projection system PS can form an image of the first patterning device 110 on the second patterning device 122 using the illumination radiation 150, and so that the radiation detector 124 is arranged to receive the illumination radiation 150 after it has passed through the second patterning device 122.

[0148] The controller 140 is configured to control the positioning device 130 so as to move at least one of the first pattern forming device 110 and the sensor device 120. Specifically, the controller 140 may be configured to control the positioning device 130 during a phase stepping or phase scanning process so as to move at least one of the first pattern forming device 110 and the sensor device 120 in a shear direction, such that the radiation intensity received by each portion (e.g., each individual sensing element or pixel) of the radiation detector 124 varies according to the movement in the shear direction so as to form an oscillating signal.

[0149] The controller 140 is further configured to determine, from the radiation detector 124, phases of harmonics of the oscillating signal at a plurality of locations on the radiation detector 124 (e.g., each individual sensing element or pixel). The controller 140 is further configured to determine, based on the phases of the harmonics of the oscillating signal at a plurality of locations on the radiation detector 124, at least one coefficient characterizing at least one aberration of the projection system PS.

[0150] The measurement system 100 is suitable for use in an extreme ultraviolet (EUV) lithography apparatus. That is, the illuminating radiation 150 may comprise EUV radiation. It will be appreciated that the measurement system 100 is in the form of a shearing interferometer.

[0151] The measurement system 100 is suitable for Figure 1 within a lithographic apparatus LA of the type shown in, and is generally equivalent to Figure 2 and Figure 4 The measurement system shown in and described above can be combined with the measurement system described above. Figures 2 to 5B Any of the features described (eg, the controller 140 may be configured to implement any of a number of methods described below with reference to Figures 7 to 12 The first pattern forming device 110 is substantially equivalent to Figures 2 to 3A One of the plurality of patterned regions 15a to 15c shown in and described above and / or equivalent to Figure 4 The sensor device 120 is generally equivalent to the first patterned area 31 shown in FIG. Figures 2 to 3A One of the detector zones 25a to 25c shown in and described above and / or Figures 4 to 5B Detector region 39 shown in and described above.

[0152] 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 Figure 1Any of the illumination systems IL and / or radiation sources SO shown in .

[0153] In some embodiments, the first patterning device 110 may be disposed on a reference. In other embodiments, the first patterning device 110 may be disposed on a reticle.

[0154] Reference now Figures 7 to 12 Various methods are described that the controller 140 is configured to implement.

[0155] Reference now Figure 7 A new method 200 of determining one or more aberrations of a projection system PS is described. Figure 7 The method 200 shown in FIG. 2 may be performed by Fig. 6A The measurement system 100 shown in FIG. 2 is implemented in the form of shearing interferometry.

[0156] The method 200 comprises a step 210 of performing a phase stepping or phase scanning process, and a step 220 of determining one or more aberrations of the projection system PS (based on signals determined during the phase stepping or phase scanning process).

[0157] The phase stepping or scanning process comprises a step 212 of illuminating the first patterning device 110 with an illumination radiation 150. The first patterning device 110 shown comprises a specular diffraction grating arranged to form a plurality of first diffracted beams from the illumination radiation 150, the first diffracted beams being separated in a shear direction.

[0158] The phase stepping or phase scanning process further comprises a step 214 of projecting at least a portion of the plurality of first diffraction beams onto the sensor device 120 using the projection system PS. The sensor device 120 comprises: a second pattern forming device 122; and a radiation detector 124. The second pattern forming device 122 is arranged to receive the first diffraction beams from the projection system PS and form a plurality of second diffraction beams from each of the first diffraction beams. The radiation detector 124 is arranged to receive at least a portion of the second diffraction beams.

[0159] The phase stepping or phase scanning process also includes step 216, which moves at least one of the first pattern forming device 110 and the second pattern forming device 122 in the shear direction so that the radiation intensity received by each part of the radiation detector 124 that receives radiation varies according to the movement in the shear direction to form an oscillating signal.

[0160] At step 220, one or more aberrations of the projection system PS are determined from the oscillating signals determined by each portion of the radiation detector 124 that receives radiation.

[0161] The method 200 is applicable in a lithographic apparatus LA. The first patterning device 110 may be arranged at a reticle level (i.e., in an object plane of the lithographic apparatus LA), and the sensor device 120 may be arranged at a wafer level (e.g., the second patterning device 122 may be arranged in an image plane of the lithographic apparatus LA). In particular, the method 200 is applicable in an extreme ultraviolet (EUV) lithographic apparatus and the illumination radiation 150 may include EUV radiation.

[0162] During a lithography process, a projection system PS is used to form a typically diffraction limited image on a substrate W (such as, for example, a wafer coated with resist) using radiation B. In practice, the projection system PS will not be able to form a perfect image, but will be affected by a certain degree of aberrations. Such aberrations represent deformations of the wavefront of light approaching a point in the image plane of the projection system PS from a spherical wavefront (and may typically depend on the position in the pupil plane, or alternatively on the angle at which radiation approaches the image plane of the projection system PS). It is desirable to minimize aberrations, and therefore it is desirable to perform a measurement of the aberrations, which can be used as part of a feedback loop to minimize aberrations as much as possible.

[0163] Aberration measurements of the projection system of a known EUV lithography apparatus typically use for aberration measurements the same (EUV) radiation B as is used for wafer exposure during the lithography process. This is obviously advantageous, since it is desirable to measure and minimize aberrations caused when imaging with EUV radiation B (since these aberrations affect the image formed on the wafer). However, the radiation B is provided to the reticle level of the lithography apparatus via an illumination system IL, which is arranged to optimize the angular distribution of the EUV radiation at the reticle level for lithography imaging performance. In particular, although known EUV lithography apparatus allow the angular distribution of the EUV radiation at the reticle level (which may be referred to as the illumination mode) to be controlled to a certain extent (e.g. using an illumination system IL), the illumination system IL may be used to optimize the angular distribution of the EUV radiation at the reticle level for lithography imaging performance. Figure 1 10 and pupil facet mirror 11 shown in FIG. 1 ), but it does not allow for uniform pupil filling illumination (i.e. illuminating each field point at the reticle level with a uniform cone of radiation). The illumination pattern achievable by the illumination system IL will not probe the entire pupil plane of the projection system PS.

[0164] Therefore, measurements of aberrations of projection systems of known EUV lithography apparatuses typically use a diffuse reflection grating as the first patterning device (at the reticle level). Figure 6BA typical measurement system 100' for the aberrations of a projection system of a known EUV lithography apparatus is schematically shown in Figure 1, which uses a diffuse reflection grating 110' as a first patterning device (at the reticle level). Again, for ease of illustration only, the measurement system 100' is illustrated as a linear arrangement, wherein the optical axis remains in a single direction. The diffuse reflection grating 110' is arranged to fill the entire pupil plane of the projection system PS, so that the entire projection system PS is probed during the aberration measurement. This is in particular the case of a conventional EUV lithography apparatus, which uses a diffuse reflection grating 110' as a first patterning device (at the reticle level). Furthermore, for ease of illustration only, the measurement system 100' is illustrated as a linear arrangement, wherein the optical axis remains in a single direction. The diffuse reflection grating 110' is arranged to fill the entire pupil plane of the projection system PS, so that the entire projection system PS is probed during the aberration measurement. This is in particular the case of a conventional EUV lithography apparatus, which uses a diffuse reflection grating 110' as a first patterning device (at the reticle level). Figure 6B , where radiation propagates from a diffuse reflection grating 110' to the projection system PS 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.

[0165] However, the inventors of the present invention have recognized that such a diffuse reflection diffraction grating 110' can introduce significant errors in the measurement of some aberrations, such as the amplitudes of some Zernike orders in the aberration map of the projection system PS. Typically, the diffuse reflection grating 110' used in the aberration measurement system 100' is formed by forming a rough surface to which a reflective grating pattern is applied or coated. For example, a uniform reflective layer can be applied or coated to the rough surface, and a pattern of radiation absorbing portions is formed above the reflective layer. However, the inventors of the present invention have recognized that aberration measurements performed using the diffuse reflection diffraction grating 110' are dependent on the specific rough surface used to cause diffusion.

[0166] However, while there is a strong bias in the art that the projection system PS should be fully probed using diffusion, the inventors of the present invention have come to the surprising realization that measurements using a mirrored diffraction grating 110 can provide improved aberration measurements, at least for certain aberrations (i.e., Zernike orders). In particular, the use of a mirrored diffraction grating 110 can provide better aberration measurements, at least for certain aberrations (i.e., Zernike orders), than using a known diffuse grating 110′, even when using illumination modes that can be achieved by an illumination system IL of a known lithography system LA.

[0167] Another advantage of using a mirror diffraction grating 110 for aberration measurement at the reticle level is that it may be easier to arrange such a mirror diffraction grating 110 on the reticle MA. Although in principle a diffuse reflection grating could be formed on the reticle MA, in practice such an arrangement would most likely be impractical and is not currently standard practice (and may not even be feasible). Therefore, currently, standard practice is to arrange the diffuse reflection grating elsewhere on the reticle stage MT, such as on a reference piece or the like. However, if aberration measurements can be performed using a mirror diffraction grating 110 on the reticle MA, significant time savings can be made in 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 should be moved so that the reference piece (rather than the reticle MA) receives EUV radiation B from the illumination system IL. Furthermore, after such aberration measurement, in order to expose a target area of ​​substrate W to radiation patterned by mask MA, reticle table MT should be moved so that mask MA (and not the reference) receives EUV radiation B from illumination system IL. However, if mirror diffraction grating 110 used for aberration measurement could be provided on reticle MA, considerable time would be saved since reticle table MT would not need to be moved as far between exposure of substrate W and aberration measurement. Figure 7 With the aberration measurement of method 200 shown in FIG. 2 , low-order Zernike coefficients (such as, for example, Z2, Z3, and Z4) can be used for reticle MA alignment. Advantageously, this can mean that an additional sensor (again provided elsewhere on reticle stage MT) may no longer need to be used for reticle MA alignment.

[0168] Figure 8 Shown as an evaluation Figure 7 250 shows the aberration footprint or aberration imprint of the projection system PS that was input into the simulation for the first 25 Zernike orders. The second graph 260 shows the aberration footprint or aberration imprint of the projection system PS that was input into the simulation for the first 25 Zernike orders. Figure 7 20 (i.e., using a mirror diffraction grating 110). The simulation used an illumination pattern in which 20.7% of the illumination pupil plane was filled with the illumination radiation 150 that illuminated the first patterning device 110 during step 212. Although only 20.7% of the illumination pupil plane was filled with illumination radiation 150, the illumination radiation 150 was distributed approximately uniformly over the illumination pupil plane. A third graph 270 shows the difference between the second graph 260 and the first graph 250. As demonstrated by the small values ​​of the third graph 270, it has been found that, although only approximately 20% of the illumination pupil plane was illuminated, using Figure 7The method 200 (ie, using the mirrored diffraction grating 110) has reconstructed the injected Zernike quite accurately.

[0169] Current reference Fig. 9 and Fig.10 A second new method 300 for determining one or more aberrations of a projection system PS is described. Fig. 9 The method 300 shown in FIG. Fig. 6A The measurement system 100 shown in FIG. Fig. 9 The method 300 shown in Figure 7 A variation of method 200 shown in and described above. Fig. 9 The method 300 shown in is a form of shearing interferometry.

[0170] The second method 300 comprises: performing a plurality of phase stepping or phase scanning processes 330A, 330B, 330C; and determining 320 one or more aberrations of the projection system PS therefrom.

[0171] like Fig.10 As illustrated in FIG. 1 , 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. In other respects, 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 can generally be Figure 7 2 and described above. Specifically, each of the multiple phase stepping or phase scanning processes 310A, 310B, 310C may include: a step 212 of irradiating the first patterning device 110 with the irradiation radiation 150; a step 214 of projecting at least a portion of the multiple first diffraction beams onto the sensor device 120 using the projection system PS; and a step 216 of moving at least one of the first patterning device 110 and the second patterning device 122 in a shear direction so that the radiation intensity received by each portion of the radiation detector 124 receiving the radiation varies according to the movement in the shear direction to form an oscillating signal.

[0172] The step 320 of determining one or more aberrations of the projection system PS comprises using an oscillating signal determined by each portion of the radiation detector that receives radiation according to any one of a plurality of phase stepping or phase scanning processes 310A, 310B, 310C.

[0173] Advantageously, by performing a plurality of different phase stepping or phase scanning processes 310A, 310B, 310C, each process using a different illumination mode 330A, 330B, 330C, each different phase stepping or phase scanning process 310A, 310B, 310C can detect a different portion of the pupil plane of the projection system PS. It should be noted that the aberration or aberration map is not determined for each individual phase stepping or phase scanning process 310A, 310B, 310C and then combined. Instead, the aberration or aberration map is determined using all the plurality of phase stepping or phase scanning processes 310A, 310B, 310C. Advantageously, such an embodiment allows for a higher resolution measurement of the aberrations of the projection system PS.

[0174] Current reference Fig.11 A third new method 400 for determining one or more aberrations of a projection system PS is described. Fig.11 The method 400 shown in FIG. 4 may be performed by Fig. 6A The measurement system 100 shown in FIG. Fig.11 The method 400 shown is Figure 7 A variation of method 200 is shown in and described above. Fig.11 The method 400 shown in FIG. 4 is a calibration method using shearing interferometry.

[0175] The calibration method 400 comprises a step 410 of determining one or more aberrations of the projection system PS using the object-level mirror diffraction grating 110 as part of a phase stepping or phase scanning process. Figure 7 The method 200 shown in or Fig. 9 and Fig.10 This step 410 is performed using the method 300 shown in FIG.

[0176] The calibration method 400 further includes: using ( Figure 6B The calibration method 400 further comprises a step 420 of determining one or more additional aberrations of the projection system PS based on the one or more additional oscillation signals.

[0177] The calibration method 400 also includes a step 440 of determining calibration data characterizing any differences between: (a) the determined one or more aberrations of the projection system PS (from step 410); and (b) the determined one or more additional aberrations of the projection system PS (from step 430).

[0178] Such an arrangement allows calibration data to be generated, which can be used in subsequent measurements to correct for additional aberrations of the projection system PS resulting from the use of the diffuse diffraction grating 110' as a first patterning device (at the object plane level). Advantageously, this may allow potentially significant errors in the measurement of 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 a first patterning device to be corrected away. This may allow subsequent aberration measurements to be made using the diffuse diffraction grating 110' as a first patterning device, which may advantageously make subsequent aberration measurements faster, potentially increasing the throughput of the lithographic apparatus LA.

[0179] Fig.11 The method 400 shown in may also include storing the determined calibration data in a memory.

[0180] Current reference Fig.12 A fourth new method 500 for determining one or more aberrations of a projection system PS is described. Fig.12 The method 500 shown in FIG. 5 may be performed by Fig. 6A The measurement system 100 shown in FIG. Fig.12 The method 500 shown in Fig.11 A variation of method 400 is shown in and described above. Fig.12 The method 500 shown in FIG. 5 is a calibration method using shearing interferometry.

[0181] Fig.12 The method 500 shown in FIG. 5 includes Fig.11 All steps 410, 420, 430, 440 of the method shown in .

[0182] Fig.12 The method 500 shown in FIG. 5 further includes a step 510 of performing a subsequent phase stepping or phase scanning process. The subsequent phase stepping or phase scanning process is performed using the diffuse diffraction grating 110' as the first patterning device, but is otherwise similar to the or each other phase stepping or phase scanning process (e.g., those performed at steps 410 and 420).

[0183] Fig.12 The method 500 shown in also includes a step 520 of determining one or more raw subsequent aberrations of the projection system PS from an oscillating signal determined by the radiation detector 122 for each portion of radiation received during a subsequent phase stepping or phase scanning process (step 510).

[0184] Fig.12The method 500 shown in also comprises a step 530 of determining one or more corrected subsequent aberrations of the projection system PS from the one or more original subsequent aberrations (determined at step 520) and the calibration data (determined at step 440).

[0185] It should be understood that any of the phase stepping or phase scanning processes described in any of the above methods 200, 300, 400, 500 (i.e., in any of steps 210, 210A to 210C, 410, 420 or 510) may also include a second phase stepping or phase scanning process, but which uses a second shear direction. Such a second phase stepping or phase scanning process may include: (a) illuminating an object-level pattern forming device with illumination radiation, wherein the first pattern forming device includes a diffraction grating arranged to form a plurality of first diffraction beams, and the first diffraction beams are separated in a second shear direction; (b) using a projection system PS, projecting at least a portion of the plurality of first diffraction beams onto the sensor device 120; and moving at least one of the first pattern forming device and the second pattern forming device in the second shear direction so that the radiation intensity received by each portion of the received radiation of the radiation detector varies according to the movement of the second shear direction to form an oscillating signal.

[0186] For example, the first patterning device may include: a first portion, the first portion being arranged to shear the illumination radiation in a shearing direction; and a second portion, the second portion being arranged to shear the illumination radiation in a second shearing direction. During irradiation of the first portion of the first patterning device, at least one of the first patterning device and the second patterning device is stepped in the shearing direction. This generates first phase stepping data, which may be related to a gradient of an aberration map in the shearing direction at least to one order. During irradiation of the second portion of the first patterning device, at least one of the first patterning device and the second patterning device is stepped in the second shearing direction. This generates second phase stepping data, which may be related to a gradient of an aberration map in the second shearing direction at least to one order. The first phase stepping data and the second phase stepping data are combined to determine one or more aberrations of the projection system. The first phase stepping data and the second phase stepping data may be combined to determine an aberration (or relative phase) map of the projection system.

[0187] Determining any of 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 original subsequent aberrations of the projection system (step 520) based on the oscillation signal determined by each portion of the received radiation of the radiation detector 122 may include any known shearing interferometry technique as needed or required. For example, the determination of any of these aberrations may include the above in combination with Figures 2 to 5C Additionally or alternatively, determination of any of these aberrations may include any of the techniques disclosed in WO2019 / 149468, the contents of which are hereby incorporated herein by reference in their entirety. For the avoidance of doubt, determination of any of these aberrations may include any of the techniques disclosed as prior art in WO2019 / 149468 and / or any of the techniques taught by WO2019 / 149468.

[0188] Determining any of 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 original subsequent aberrations of the projection system (step 520) from the oscillating signal determined by each portion of received radiation of the radiation detector 122 may include causing the phase of a harmonic of the oscillating signal received by each portion of received radiation of the radiation detector 122 to be equal 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 oscillating signal may be a first harmonic of the oscillating signal.

[0189] 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 implement Figures 7 to 12 One or more of the methods 200, 300, 400, 500 shown in and described above.

[0190] Some embodiments of the present disclosure relate to a computer device 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 include instructions arranged to control the computer to implement Figures 7 to 12 Instructions of one or more of methods 200, 300, 400, 500 shown in and described above.

[0191] In some embodiments, the diffraction grating discussed above may be self-supporting. In use, the diffraction grating may be provided with a support for the absorbent layer. The support may only contact a peripheral portion of the absorbent layer. That is, the support may be in the form of a frame and the support is not adjacent to a central portion of the absorbent layer. With such an arrangement, in the central portion of the absorbent layer, the absorbent layer may be considered to be self-supporting. To achieve this, the absorbent layer may be tensioned on the support (e.g., so that it remains substantially flat).

[0192] Advantageously, for embodiments in which the central portion of the absorber layer is self-supporting, the grating does not require, for example, a transmissive support layer. Such an arrangement is particularly beneficial for use in a phase stepping measurement system for determining an aberration map of a projection system using EUV radiation, since the use of such a transmissive support layer will significantly reduce the amount of EUV radiation transmitted by the diffraction grating.

[0193] In some embodiments, in the diffraction grating discussed above, the substrate (in which the holes are formed) includes a radiation absorbing layer. For example, the radiation absorbing layer may be formed of a metal such as chromium (Cr), nickel (Ni), cobalt (Co), or aluminum (Al).

[0194] In some embodiments, the absorber layer may include a ceramic. The ceramic may include a metal or metalloid component having a relatively high extinction coefficient for EUV radiation, and a non-metallic component. Both components may have a refractive index for EUV that is relatively close to 1. The ceramic may include aluminum nitride (AlN). In some embodiments, the absorber layer may include aluminum nitride (AlN).

[0195] In some embodiments, in the diffraction grating discussed above, the substrate (in which the holes are formed) further comprises a support layer. The through-holes may extend through both the support layer and the radiation absorbing layer. The support layer may be formed, for example, of SiN.

[0196] Although the above described embodiments use a first harmonic of the phase stepped signal, it will be appreciated that in alternative embodiments, higher order harmonics of the phase stepped signal may instead be used.

[0197] Although the above-described embodiments use a first patterned region 31 comprising a one-dimensional diffraction grating 31 with a 50% duty cycle, it should be appreciated that in alternative embodiments, other first patterned regions 31 may use different geometries. For example, in some embodiments, the first patterned region 31 may include a two-dimensional checkerboard diffraction grating with a 50% duty cycle.

[0198] Although specific reference may be made herein to the use of lithographic apparatus in IC manufacturing, it should be understood that the lithographic apparatus described herein may have other applications. Other possible applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memory, flat panel displays, liquid crystal displays (LCDs), thin film magnetic heads, etc.

[0199] Although specific reference may be made herein to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or a mask (or other pattern forming device). These apparatuses may generally be referred to as lithographic tools. Such lithographic tools may use vacuum conditions or ambient (non-vacuum) conditions.

[0200] Where context permits, embodiments of the present invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the present invention may also be implemented as instructions stored on a machine-readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine-readable medium may include a read-only memory (ROM); a random access memory (RAM); a magnetic storage medium; an optical storage medium; a flash memory device; an electrical, optical, acoustic, or other form of propagation signal (e.g., a carrier wave, an infrared signal, a digital signal, etc.), and the like. In addition, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be understood that such descriptions are for convenience only, and such actions are actually generated by a computing device, a processor, a controller, or other device that executes firmware, software, routines, instructions, etc., and doing so may cause an actuator or other device to interact with the physical world.

[0201] Although specific embodiments of the present invention have been described above, it will be appreciated that the present invention may be practiced in ways other than those described. The foregoing description is intended to be illustrative, not restrictive. Therefore, it will be apparent to those skilled in the art that the present invention may be modified as described without departing from the scope of the claims set forth below.

[0202] aspect

[0203] 1. A method for determining one or more aberrations of a projection system, the method comprising:

[0204] Performing a phase stepping or phase scanning process, the phase stepping or phase scanning process comprising:

[0205] illuminating a first patterning device with an illumination radiation, wherein the first patterning device comprises a specular diffraction grating arranged to form a plurality of first diffracted beams, the first diffracted beams being separated in a shear direction;

[0206] Projecting at least a portion of the plurality of first diffraction beams onto a sensor device using the projection system, the sensor device comprising:

[0207] a second patterning device arranged 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

[0208] a radiation detector arranged to receive at least a portion of the second diffracted beam; and

[0209] moving at least one of the first patterning device and the second patterning device in the shear direction such that the intensity of radiation received by each portion of the radiation detector receiving radiation varies in dependence on the movement in the shear direction to form an oscillating signal; and

[0210] One or more aberrations of the projection system are determined from an oscillating signal determined by each portion of the radiation detector that receives radiation.

[0211] 2. The method according to aspect 1, wherein the method comprises:

[0212] 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 ones of the plurality of phase stepping or phase scanning processes, but each of the plurality of phase stepping or phase scanning processes being similar in other respects to the other phase stepping or phase scanning processes; and

[0213] Wherein the step of determining one or more aberrations of the projection system comprises using the oscillating signal determined by each portion of the radiation detector receiving radiation according to any of the plurality of phase stepping or phase scanning processes.

[0214] 3. The method according to aspect 1 or aspect 2, further comprising:

[0215] performing an additional phase stepping or phase scanning process, the additional phase stepping or phase scanning process being performed using a diffuse diffraction grating as the first patterning device but being similar in other respects to the or each other phase stepping or phase scanning process;

[0216] determining one or more additional aberrations of the projection system from the oscillating signal determined by each portion of the radiation detector receiving radiation during the additional phase stepping or phase scanning process; and

[0217] Calibration data is determined that characterizes any differences 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.

[0218] 4. The method according to clause 3, further comprising storing the determined calibration data in a memory.

[0219] 5. A calibration method, the method comprising:

[0220] performing a phase stepping or phase scanning process using the object-level mirror diffraction grating to generate one or more oscillating signals;

[0221] determining one or more aberrations of the projection system based on the one or more oscillating signals;

[0222] performing an additional phase stepping or phase scanning process using the object-level diffuse diffraction grating to generate one or more additional oscillation signals;

[0223] determining one or more additional aberrations of the projection system based on the one or more additional oscillating signals; and

[0224] Calibration data is determined that characterizes any differences 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.

[0225] 6. The method according to any one of aspects 3 to 5, further comprising:

[0226] performing a subsequent phase stepping or phase scanning process, the subsequent phase stepping or phase scanning process being performed using the diffuse diffraction grating as the first patterning device but being similar in other respects to the or each other phase stepping or phase scanning process;

[0227] determining one or more raw subsequent aberrations of the projection system from the oscillating signal determined by each portion of the radiation detector receiving radiation during the subsequent phase stepping or phase scanning process; and

[0228] One or more corrected subsequent aberrations are determined based on the one or more raw subsequent aberrations of the projection system and the calibration data.

[0229] 7. The method of any preceding aspect, wherein the illuminating radiation comprises extreme ultraviolet radiation.

[0230] 8. The method according to any preceding clause, wherein the first patterning device is disposed on a reference.

[0231] 9. The method according to any preceding clause, wherein the first patterning device is disposed on a reticle.

[0232] 10. The method according to any preceding aspect, wherein any phase stepping or phase scanning process further comprises:

[0233] illuminating the first patterning device with an illumination radiation, wherein the first patterning device comprises a specular diffraction grating arranged to form a plurality of first diffracted beams, the first diffracted beams being separated in a second shear direction;

[0234] projecting at least a portion of the plurality of first diffracted beams onto the sensor device using the projection system; and

[0235] At least one of the first patterning device and the second patterning device is moved in the second shear direction so that the intensity of radiation received by each portion of the radiation detector receiving radiation varies according to the movement in the second shear direction to form an oscillating signal.

[0236] 11. The method of any preceding aspect, wherein any of the determining of the one or more aberrations of the projection system, the determining of the one or more additional aberrations of the projection system, or the determining of the one or more original subsequent aberrations of the projection system based on the oscillation signal determined by each portion of the radiation detector receiving radiation comprises:

[0237] The phase of the harmonics of the oscillating signal received by each portion of the radiation detector receiving radiation is made equal to the sum of at least one aberration map difference between a pair of locations in a pupil plane of the projection system.

[0238] 12. A computer-readable medium carrying a computer program comprising computer-readable instructions configured to cause a computer to implement the method according to any one of aspects 1 to 11.

[0239] 13. A computer device comprising:

[0240] a memory storing processor-readable instructions, and

[0241] A processor arranged to read and execute instructions stored in the memory, wherein the processor-readable instructions include instructions arranged to control the computer to implement the method according to any one of aspects 1 to 11.

[0242] 14. A measurement system for determining one or more aberrations of a projection system, the measurement system comprising:

[0243] a first patterning device, wherein the first patterning device comprises a specular diffraction grating;

[0244] a sensor device comprising a second patterning device and a radiation detector, the first patterning device and the sensor device being positionable such that the projection system is capable of forming an image of the first patterning device on the second patterning device using illumination radiation, and such that the radiation detector is arranged to receive the illumination radiation after the illumination radiation has passed through the second patterning device;

[0245] a positioning device configured to move at least one of the first patterning device and the sensor device; and

[0246] A controller, the controller being configured to:

[0247] controlling the positioning device to move at least one of the first patterning device and the sensor device in a shear direction such that the intensity of radiation received by each portion of the radiation detector varies in dependence on the movement in the shear direction to form an oscillating signal;

[0248] determining from the radiation detector the phases of harmonics of the oscillating signal at a plurality of locations on the radiation detector; and

[0249] At least one coefficient characterizing at least one aberration of the projection system is determined from the phases of harmonics of the oscillating signal at the plurality of locations on the radiation detector.

[0250] 15. The system according to aspect 14, wherein the controller is configured to implement the method according to any one of aspects 1 to 11.

[0251] 16. The system of clause 14 or clause 15, further comprising an illumination system operable to illuminate the first patterning device with illumination radiation.

[0252] 17. The system of aspect 16, wherein the illuminating radiation comprises extreme ultraviolet radiation.

[0253] 18. The system of any one of aspects 14 to 17, wherein the first patterning device is disposed on a reference.

[0254] 19. The system of any one of clauses 14 to 18, wherein the first patterning device is disposed on a reticle.

[0255] 20. A lithographic apparatus comprising a measurement system according to any of clauses 14 to 19.

Claims

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 an illumination radiation, wherein the first patterning device comprises a specular diffraction grating arranged to form a plurality of first diffracted beams, the first diffracted beams being separated in a shear direction; Projecting at least a portion of the plurality of first diffraction beams onto a sensor device using the projection system, the sensor device comprising: a second patterning device arranged 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 arranged to receive at least a portion of the second diffracted beam; and moving at least one of the first patterning device and the second patterning device in the shear direction such that the intensity of radiation received by each portion of the radiation detector receiving radiation varies in dependence on the movement in the shear direction to form an oscillating signal; and One or more aberrations of the projection system are determined from an oscillating signal determined by each portion of the radiation detector that receives radiation.

2. The method according to claim 1, wherein: The method comprises: 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 ones of the plurality of phase stepping or phase scanning processes, but each of the plurality of phase stepping or phase scanning processes being similar in other respects to the other phase stepping or phase scanning processes; and Wherein the step of determining one or more aberrations of the projection system comprises using the oscillating signal determined by each portion of the radiation detector receiving radiation according to any of the plurality of phase stepping or phase scanning processes.

3. The method according to claim 1 or claim 2, further comprising: performing an additional phase stepping or phase scanning process, the additional phase stepping or phase scanning process being performed using a diffuse diffraction grating as the first patterning device but being similar in other respects to the or each other phase stepping or phase scanning process; determining one or more additional aberrations of the projection system from the oscillating signal determined by each portion of the radiation detector receiving radiation during the additional phase stepping or phase scanning process; as well as Calibration data is determined that characterizes any differences 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. A calibration method, the method comprising: performing a phase stepping or phase scanning process using the object-level mirror diffraction grating to generate one or more oscillating signals; determining one or more aberrations of the projection system based on the one or more oscillating signals; performing an additional phase stepping or phase scanning process using the object-level diffuse diffraction grating to generate one or more additional oscillation signals; determining one or more additional aberrations of the projection system based on the one or more additional oscillating signals; as well as Calibration data is determined that characterizes any differences 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.

5. The method according to any one of claims 3 to 4, further comprising: performing a subsequent phase stepping or phase scanning process, the subsequent phase stepping or phase scanning process being performed using the diffuse diffraction grating as the first patterning device but being similar in other respects to the or each other phase stepping or phase scanning process; determining one or more raw subsequent aberrations of the projection system from the oscillating signal determined by each portion of the radiation detector receiving radiation during the subsequent phase stepping or phase scanning process; and One or more corrected subsequent aberrations are determined based on the one or more raw subsequent aberrations of the projection system and the calibration data.

6. A method according to any preceding claim, wherein: The illuminating radiation includes extreme ultraviolet radiation.

7. A method according to any preceding claim, wherein: The first patterning device is disposed on a reference and / or on a reticle.

8. A method according to any preceding claim, wherein: Any phase stepping or phase sweeping process also includes: illuminating the first patterning device with an illumination radiation, wherein the first patterning device comprises a specular diffraction grating arranged to form a plurality of first diffracted beams, the first diffracted beams being separated in a second shear direction; projecting at least a portion of the plurality of first diffracted beams onto the sensor device using the projection system; and At least one of the first patterning device and the second patterning device is moved in the second shear direction so that the intensity of radiation received by each portion of the radiation detector receiving radiation varies according to the movement in the second shear direction to form an oscillating signal.

9. A method according to any preceding claim, wherein: Determining any of the one or more aberrations of the projection system, the one or more additional aberrations of the projection system, or the one or more original subsequent aberrations of the projection system based on the oscillating signal determined by each portion of the radiation detector receiving radiation comprises: The phase of the harmonics of the oscillating signal received by each portion of the radiation detector receiving radiation is made equal to the sum of at least one aberration map difference between a pair of locations in a pupil plane of the projection system.

10. A measurement system for determining one or more aberrations of a projection system, the measurement system comprising: a first patterning device, wherein the first patterning device comprises a specular diffraction grating; a sensor device comprising a second patterning device and a radiation detector, the first patterning device and the sensor device being positionable such that the projection system is capable of forming an image of the first patterning device on the second patterning device using illumination radiation, and such that the radiation detector is arranged to receive the illumination radiation after the illumination radiation 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; as well as A controller, the controller being configured to: controlling the positioning device to move at least one of the first patterning device and the sensor device in a shear direction such that the intensity of radiation received by each portion of the radiation detector varies in dependence on the movement in the shear direction to form an oscillating signal; determining, from the radiation detector, phases of harmonics of the oscillating signal at a plurality of locations on the radiation detector; as well as At least one coefficient characterizing at least one aberration of the projection system is determined from the phases of harmonics of the oscillating signal at the plurality of locations on the radiation detector.

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

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 according to claim 12, wherein: The illuminating radiation includes extreme ultraviolet radiation.

14. The system according to any one of claims 12 to 13, wherein: The first patterning device is disposed on a reference 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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