WO2024256096A1

WO2024256096A1 - Aberration determination

Info

Publication number: WO2024256096A1
Application number: PCT/EP2024/063042
Authority: WO
Inventors: Dirk Jan Willem DIKKEN; Derick Yun Chek CHONG
Original assignee: ASML Netherlands BV
Current assignee: ASML Netherlands BV
Priority date: 2023-06-12
Filing date: 2024-05-13
Publication date: 2024-12-19
Anticipated expiration: 2025-12-12
Also published as: CN121263743A

Abstract

A method of determining aberration of a projection system of a lithographic apparatus, comprising: illuminating a mask grating located in an object plane of the projection system, forming an image of the mask grating at a sensor grating, using a detector to detect resulting interference patterns and output a measured intensity signal; using a wavefront fitting algorithm to fit to the measured intensity signal and obtain an estimate of aberration, the algorithm including an assumption that only [0,0] order diffraction beam convolutions contribute to the wavefront; determining a pupil of the illumination used, then: calculating 1st harmonic contributions of each diffraction order of the light diffracted by the mask grating, thereby providing intensity signals for those 1st harmonic contributions; obtaining a modelled intensity signal; obtaining a cleaned intensity signal; and obtaining an improved estimate of the aberration of the projection system.

Description

ABERRATION DETERMINATION

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority of EP application 23178756.5 which was filed on 12 June 2023 and which is incorporated herein in its entirety by reference.

FIELD

[0002] The present invention relates to a method of determining aberration of a projection system of a lithographic apparatus.

BACKGROUND

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

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

[0005] Radiation that has been patterned by the patterning device is focussed onto the substrate using a projection system. The projection system may introduce optical aberrations, which cause the image formed on the substrate to deviate from a desired image (for example a diffraction limited image of the patterning device).

[0006] It may be desirable to provide a method for accurately determining aberrations caused by a projection system.

SUMMARY

[0007] According to a first aspect of the present disclosure, there is provided a method of determining aberration of a projection system of a lithographic apparatus, the method comprising illuminating a mask grating located in an object plane of the projection system, using the projection system to form an image of the mask grating at a sensor grating, and using an imaging detector located below the sensor grating to detect resulting interference patterns and provide an output measured intensity signal; the method further comprising using a wavefront fitting algorithm to fit to the measured intensity signal and thereby obtain an estimate of aberration of the projection system, the wavefront fitting algorithm including an assumption that only [0,0] order diffraction beam convolutions contribute to the wavefront, determining a pupil of the illumination used to illuminate the mask grating, then a) using the estimated aberration of the projection system, diffraction amplitude and phase data from a model representing known three-dimensional structures of the mask grating and the sensor grating, and the illumination pupil, to calculate 1st harmonic contributions of each diffraction order of the light diffracted by the mask grating and projected by the projection system and convoluted with the diffraction orders of the sensor grating, except contributions of the [0,0] order diffraction beam convolutions, thereby providing intensity signals for those 1^st harmonic contributions, b) summing the intensity signals for the 1^st harmonic contributions to obtain a modelled intensity signal, c) subtracting the summed intensity signal from the measured intensity signal to obtain a cleaned intensity signal, and d) using the wavefront fitting algorithm to fit to the cleaned intensity signal and thereby obtain an improved estimate of the aberration of the projection system.

[0008] Advantageously, embodiments of the invention remove or reduce the contribution of the higher order diffraction beams to the first harmonic signal. This allows an improved estimated aberration of the projection system to be determined.

[0009] The steps a to d may be repeated using the improved estimate of the aberration of the projection system in order to generate a further improved estimate of the aberration of the projection system.

[0010] The steps a to d may be repeated multiple times until a fit provided by the wavefront fitting algorithm satisfies a predetermined metric.

[0011] The predetermined metric may be based on an amount of structure present in the cleaned intensity signal.

[0012] The steps a to d may be repeated multiple times until convergence of the estimate of the aberration of the projection system occurs.

[0013] The illumination pupil may be determined using a sensor located in the lithographic apparatus.

[0014] The illumination pupil may be estimated based upon an offset of the measured intensity signal.

[0015] The wavefront fitting algorithm may include a further assumption that only 0^th and +-l^st diffraction orders contribute to the wavefront.

[0016] According to a second aspect of the present disclosure, there is provided a lithographic apparatus comprising a projection system, a shearing interferometer comprising a mask grating a sensor grating and an imaging detector located below the sensor grating, and a processor configured to receive signals from the imaging detector, wherein the processor is configured to cause a radiation beam to illuminate the mask grating such that the projection system forms an image of the mask grating at the sensor grating, and interference patterns can be detected by the imaging detector to provide an output measured intensity signal; the processor being further configured to use a wavefront fitting algorithm to fit to the measured intensity signal and thereby obtain an estimate of aberration of the projection system, the wavefront fitting algorithm including an assumption that only [0,0] order diffraction beam convolutions contribute to the wavefront; determining a pupil of the illumination used to illuminate the mask grating, then; using the estimated aberration of the projection system, diffraction amplitude and phase data from a model representing known three-dimensional structures of the mask grating and the sensor grating, and the illumination pupil, to calculate 1st harmonic contributions of each diffraction order of the light diffracted by the mask grating and projected by the projection system and convoluted with the diffraction orders of the sensor grating, except contributions of the [0,0] order diffraction beam convolutions, thereby providing intensity signals for those 1^st harmonic contributions; summing the intensity signals for the 1^st harmonic contributions to obtain a modelled intensity signal; subtracting the summed intensity signal from the measured intensity signal to obtain a cleaned intensity signal; and using the wavefront fitting algorithm to fit to the cleaned intensity signal and thereby obtain an improved estimate of the aberration of the projection system.

[0017] Advantageously, embodiments of the invention remove or reduce the contribution of the higher order diffraction beams to the first harmonic signal. This allows an improved estimated aberration of the projection system to be determined.

[0018] The processor may be configured to repeat steps a to d using the improved estimate of the aberration of the projection system in order to generate a further improved estimate of the aberration of the projection system.

[0019] The processor may be configured to repeat steps a to d multiple times until a fit provided by the wavefront fitting algorithm satisfies a predetermined metric.

[0020] The predetermined metric may be based on an amount of structure present in the cleaned intensity signal.

[0021] The processor may be configured to repeat steps a to d multiple times until convergence of the estimate of the aberration of the projection system occurs.

[0022] The wavefront fitting algorithm may include a further assumption that only 0^th and +-l^st diffraction orders contribute to the wavefront.

[0023] Features of different aspects of the invention may be combined together.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings, in which:

Figure 1 depicts a lithographic system comprising a lithographic apparatus and a radiation source, including a processor configured to perform a method according to an embodiment of the invention;

Figure 2 is a schematic illustration of a measurement system used by an embodiment of the invention; Figure 3 schematically depicts diffraction orders that form part of a signal measured by the measurement system;

Figure 4 schematically depicts other diffraction orders that form part of a signal measured by the measurement system;

Figure 5 schematically depicts unwanted diffraction orders that form part of a signal measured by the measurement system;

Figure 6 is a flow chart which depicts an aberration measurement method according to an embodiment of the invention; and

Figure 7 is a flow chart which depicts an optional additional step of the aberration measurement method according to an embodiment of the invention.

DETAILED DESCRIPTION

[0025] Figure 1 shows a lithographic system 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 patterning device MA (e.g., a mask), a projection system PS and a substrate table WT configured to support a substrate W.

[0026] The illumination system IL is configured to condition the EUV radiation beam B before the EUV radiation beam B is incident upon the patterning device MA. Thereto, the illumination system IL may include a facetted field mirror device 10 and a facetted pupil mirror device 11. The faceted field mirror device 10 and faceted pupil mirror device 11 together provide the EUV radiation beam B with a desired cross-sectional shape and a desired intensity distribution. The illumination system IL may include other mirrors or devices in addition to, or instead of, the faceted field mirror device 10 and faceted pupil mirror device 11.

[0027] After being thus conditioned, the EUV radiation beam B interacts with the patterning 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 the substrate W. For that purpose, the projection system PS may comprise a plurality of mirrors 13, 14 which are configured to project the patterned EUV radiation beam B’ onto the substrate W held by the substrate table WT. The projection system PS may apply a reduction factor to the patterned EUV radiation beam B’ thus forming an image with features that are smaller than corresponding features on the patterning device MA. For example, a reduction factor of 4 or 8 may be applied. Although the projection system PS is illustrated as having only two mirrors 13, 14 in Figure 1, the projection system PS may include a different number of mirrors (e.g. six or eight mirrors). [0028] The substrate W may include previously formed patterns. Where this is the case, the lithographic apparatus LA aligns the image, formed by the patterned EUV radiation beam B’, with a pattern previously formed on the substrate W.

[0029] A relative vacuum, i.e. a small amount of gas (e.g. 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.

[0030] 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 that is capable of generating EUV radiation.

[0031] In general, the projection system PS has an optical transfer function which may be non- uniform, which can affect the pattern which is imaged on the substrate W. Such effects can be fairly well described by two scalar maps, which describe the transmission (apodization) and relative phase (aberration) of radiation exiting the projection system PS as a function of position in a pupil plane thereof. These scalar maps, which may be referred to as the transmission map and the relative phase map, may be expressed as a linear combination of a complete set of basis functions. A particularly convenient set is the Zernike polynomials, which form a set of orthogonal polynomials defined on a unit circle. A determination of each scalar map 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 a measured scalar map by calculating the inner product of the measured scalar map with each Zernike polynomial in turn and dividing this by the square of the norm of that Zernike polynomial. In the following, unless stated otherwise, any reference to Zernike coefficients will be understood to mean the Zernike coefficients of a relative phase map (also referred to herein as an aberration map). It will 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 obscured aperture systems.

[0032] The wavefront aberration map represents the distortions of the wavefront of light approaching a point in an image plane of the projection system PS from a spherical wavefront (as a function of position in the pupil plane or, alternatively, the angle at which radiation approaches the image plane of the projection system PS). As discussed, this wavefront aberration map W (x, y) may be expressed as a linear combination of Zernike polynomials:

where x and y are coordinates in the pupil plane, Z_n(x, y) is the nth Zernike polynomial and c_n is a coefficient. It will be appreciated that in the following, Zernike polynomials and coefficients are labelled with an index which is commonly referred to as a Noll index. Therefore, Z_n (x, y) is the Zernike polynomial having a Noll index of n and c_n is a coefficient having a Noll index of n. The wavefront aberration map may then be characterized by the set of coefficients c_n in such an expansion, which may be referred to as Zernike coefficients.

[0033] It will be appreciated that only a finite number of Zernike orders are taken into account. Different Zernike coefficients of the phase map may provide information about different forms of aberration which are caused by the projection system PS. The Zernike coefficient having a Noll index of 1 may be referred to as the first Zernike coefficient, the Zernike coefficient having a Noll index of 2 may be referred to as the second Zernike coefficient and so on.

[0034] The first Zernike coefficient relates to a mean value (which may be referred to as a piston) of a measured wavefront. The first Zernike coefficient may be irrelevant to the performance of the projection system PS and as such may not be determined using the methods described herein. The second Zernike coefficient relates to the tilt of a measured wavefront in the x-direction. The tilt of a wavefront in the x-direction is equivalent to a placement in the x-direction. The third Zernike coefficient relates to the tilt of a measured wavefront in the y-direction. The tilt of a wavefront in the y-direction is equivalent to a placement in the y-direction. The fourth Zernike coefficient relates to a defocus of a measured wavefront. The fourth Zernike coefficient is equivalent to a placement in the z- direction. Higher order Zernike coefficients relate to other forms of aberration which are caused by the projection system (e.g. astigmatism, coma, spherical aberrations and other effects).

[0035] Throughout this description the term “aberrations” should be intended to include all forms of deviation of a wavefront from a perfect spherical wavefront. That is, the term “aberrations” may relate to the placement of an image (e.g. the second, third and fourth Zernike coefficients) and/or to higher order aberrations such as those which relate to Zernike coefficients having a Noll index of 5 or more. 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 those due to image placement. [0036] 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 (i.e. for each spatial location in its image plane).

[0037] As will be described in further detail below, the relative phase of the projection system PS in its pupil plane may be determined by projecting radiation from an object plane of the projection system PS (i.e. the plane of the patterning device MA), through the projection system PS and using a shearing interferometer 16 to measure a wavefront (i.e. a locus of points with the same phase). The shearing interferometer 16 may be provided on a substrate table WT of the lithographic apparatus LA, and thus may be in an image plane of the projection system PS. The shearing interferometer 16 may comprise a diffraction grating, for example a two dimensional diffraction grating, and a detector arranged to detect an interference pattern in a plane that is conjugate to a pupil plane of the projection system PS (see Figure 2). The shearing interferometer 16 may be referred to as a sensor. [0038] The projection system PS comprises a plurality of optical elements (including mirrors 13, 14). As already explained, although the projection system PS is illustrated as having only two mirrors 13, 14 in Figure 1, the projection system PS may include a different number of mirrors (e.g. six or eight mirrors). The lithographic apparatus LA further comprises adjusting system PA for adjusting these optical elements so as to correct for aberrations (any type of phase variation across the pupil plane throughout the field). To achieve this, the adjusting system PA may be operable to manipulate optical elements within the projection system PS in one or more different ways. The projection system may have a co-ordinate system wherein its optical axis extends in the z direction (it will be appreciated that the direction of this z axis changes along the optical path through the projection system, for example at each mirror or optical element). The adjusting system PA may be operable to do any combination of the following: displace one or more optical elements; tilt one or more optical elements; and/or deform one or more optical elements. Displacement of optical elements may be in any direction (x, y, z or a combination thereof). Tilting of optical elements is typically out of a plane perpendicular to the optical axis, by rotating about axes in the x or y directions although a rotation about the z axis may be used for non-rotationally symmetric optical elements. Deformation of an optical element may be performed for example by using actuators to exert force on sides of the optical element and/or by using heating elements to heat selected regions of the optical element. In general, it may not be possible to adjust the projection system PS to correct for apodizations (transmission variation across the pupil plane). The transmission map of a projection system PS may be used when designing masks MAs for the lithographic apparatus LA.

[0039] In some embodiments, the adjusting system PA may be operable to move the support structure MT and/or the substrate table WT. The adjusting system PA may be operable to displace (in any of the x, y, z directions or a combination thereof) and/or tilt (by rotating about axes in the x or y directions) the support structure MT and/or the substrate table WT.

[0040] The adjusting system PA may receive input from a processor 18. The processor 18 may receive a signal from the shearing interferometer 16. The processor 18 may use that signal to determine a wavefront aberration map of the projection system PS. The processor 18 may use that signal to determine adjustments to be made by the adjusting system PA to the projection system PS.

[0041] The projection system PS may periodically undergo a calibration process. For example, when a lithographic apparatus is manufactured in a factory the optical elements (e.g. mirrors) which form the projection system PS may be set up by performing an initial calibration process. After installation of a lithographic apparatus at a site at which the lithographic apparatus is to be used, the projection system PS may once again be calibrated. Further calibrations of the projection system PS may be performed at regular intervals. For example, under normal use the projections system PS may be calibrated every few months (e.g. every three months).

[0042] Figure 2 is a schematic illustration of a measurement system 30 which may be used to determine a wavefront aberration map of the projection system PS. The measurement system 30 comprises an illumination system IL, a mask grating 31, a sensor 16 and a processor 18. The measurement system 30 may form part of a lithographic apparatus. For example, the illumination system IL and the projection system PS which are shown in Figure 2 may be the illumination system IL and projection system PS of the lithographic apparatus which is shown in Figure 1. For ease of illustration additional components of a lithographic apparatus are not shown in Figure 2.

[0043] The mask grating 31 is arranged to receive radiation from the illumination system IL. The sensor 16 is arranged to receive radiation from the projection system PS. During normal use of a lithographic apparatus, the mask grating 31 and the sensor 16 which are shown in Figure 2 may be located in positions that are different to the positions in which they are shown in Figure 2. For example, during normal use of a lithographic apparatus a patterning device MA which is configured to form a pattern to be transferred to a substrate W may be positioned to receive radiation from the illumination system IL and a substrate W may be positioned to receive radiation from the projection system PS (as is shown, for example, in Figure 1). The mask grating 31 and the sensor 16 may be moved into the positions in which they are shown in Figure 2 in order to determine aberrations which are caused by the projection system PS. The mask grating 31 may be supported by a support structure MT, such as the support structure which is shown in Figure 1. The sensor 16 may be supported by a substrate table, such as the substrate table WT which is shown in Figure 1. Alternatively the sensor 16 may be supported by a measurement table (not shown) which may be separate to the substrate table WT.

[0044] The mask grating 31 comprises a reflective mask grating 31. The reflective mask grating 31 may be referred to as a mask grating 31. The mask grating 31 may comprise one or more additional diffraction gratings, including for example an orthogonal diffraction grating. These are omitted for ease of illustration. Radiation which is incident on the reflective mask grating 31 is diffracted and is received by the projection system PS. The mask grating 31 is an example of a mask grating located in an object plane of the projection system PS.

[0045] The illumination system IL illuminates the mask grating 31 with radiation. Whilst not shown in Figure 2, the illumination system IL may receive radiation from a radiation source SO and condition the radiation so as to illuminate the mask grating 31. For example, the illumination system IL may condition the radiation so as to provide radiation having a desired angular distribution. An illumination pupil provided by the illumination system IL may be measured using a detector (not depicted). An illumination pupil provided by the illumination system IL may be estimated, as described further below.

[0046] In the Figures the Cartesian co-ordinate system is shown as being conserved through the projection system PS. However, in some embodiments the properties of the projection system PS may lead to a transformation of the co-ordinate system. For example, the projection system PS may form an image of the mask grating 31 which is magnified, rotated and/or mirrored relative to the mask grating 31. [0047] The mask grating 31 is illuminated by a measurement beam 33 of EUV radiation beam, and diffracts the measurement beam. The diffracted beams 34-36 are received by the projection system PS. The projection system PS forms an image of the mask grating 31 on the sensor 16. The sensor 16 comprises a transmissive diffraction grating 32 (which may be referred to as a sensor grating 32) and a radiation detector 23. The sensor grating 32 is arranged such that the image of the mask grating 31 is formed on the sensor grating 32. The sensor grating 32 is transmissive, and thus diffracts the incident radiation onto the radiation detector 23.

[0048] The radiation detector 23 is configured to detect the spatial intensity profile of radiation which is incident on the radiation detector 23. The radiation detector 23 may, for example, comprise an array of individual detector elements or sensing elements. For example, the radiation detector 23 may comprise an active pixel sensor such as, for example, a CMOS (complementary metal-oxide- semiconductor) sensor array. Alternatively, the radiation detector 23 may comprise a CCD (charge- coupled device) sensor array.

[0049] The diffraction of the measurement beam 33 which occurs at the mask grating 31 and the diffraction which occurs at the sensor grating 32 results in interference patterns being formed on the radiation detector 23. The interference patterns are related to the derivative of the phase of the diffracted beams 34-36 and depend on aberrations caused by the projection system PS. The interference patterns may therefore be used to determine aberrations which are caused by the projection system PS (i.e. to determine a wavefront aberration map of the projection system).

[0050] In some embodiments, the mask grating 31 and/or the sensor 16 is sequentially scanned and/or stepped. The mask grating 31 and/or the sensor 16 may be stepped by distances which correspond with a fraction of the grating period of the diffraction gratings. Measurements which are made at different stepping positions may be analysed in order to derive information about a wavefront in the stepping direction. For example, the phase of the first harmonic of the measured signal (which may be referred to as a phase stepping signal) may contain information about the derivative of a wavefront in the stepping direction. Stepping may be performed in two directions orthogonal to the z- direction.

[0051] The processor 18 receives measurements made at the sensor 16 and determines, from the measurements, aberrations which are caused by the projection system PS. The processor 18 may be configured to control one or more components of the measurement system 30. For example, the processor 18 may control a positioning apparatus which is operable to move the sensor 16 and/or the mask grating 31 relative to each other. The processor may control an adjusting system PA for adjusting components of the projection system PS. For example, the adjusting system PA may adjust optical elements of the projection system PS so as to correct for aberrations which are caused by the projection system PS and which are determined by the processor 18.

[0052] In some embodiments, the processor 18 may be operable to control the adjusting system PA for adjusting the support structure MT and/or the substrate table WT. For example, the adjusting system PA may adjust support structure MT and/or substrate table WT so as to correct for aberrations which are caused by placement errors of patterning device MA and/or substrate W (and which are determined by the processor 18).

[0053] 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 the measurements which are made by the sensor 16 to Zernike polynomials in order to obtain Zernike coefficients. This may be referred to as wavefront fitting. Different Zernike coefficients may provide information about different forms of aberration which are caused by the projection system PS. Zernike coefficients may be determined independently at different positions in the x and/or the y-directions.

[0054] The mask grating 31 is irradiated with radiation 33 from the illumination system IL. For ease of understanding only a single line (which may, for example, represent a single ray, for example the chief ray, of an incident radiation beam) is shown in Figure 2. However, it will be appreciated that the radiation 33 will comprise a range of angles incident on the mask grating 31 of the mask grating 31. That is, each point on the mask grating 31 of the mask grating 31 may be illuminated by a cone of light. In general, each point is illuminated by substantially the same range of angles, this being characterized by the intensity of radiation in a pupil plane of the illumination system IL (not shown).

[0055] The mask grating 31 is arranged to receive the radiation 33 and to form a plurality of first diffraction beams 34, 35, 36. A central first diffraction beam 35 corresponds to a 0^th order diffraction beam of the mask grating 31 and the other two first diffraction beams 34, 36 correspond to the ±l^st order diffraction beams of the mask grating 31. Higher order diffraction beams will also be present, as discussed further below. Again for ease of understanding, only three first diffraction beams 34, 35, 36 are shown in Figure 4.

[0056] It will also be appreciated that, as the incoming radiation 33 comprises a cone of radiation converging on a point on the mask grating 31, each of the first diffraction beams 34, 35, 36 also comprises a cone of radiation diverging from that point on the mask grating 31.

[0057] To achieve the generation of the first diffraction beams 34, 35, 36, the mask grating 31 may be of the form of a diffraction grating, e.g. a diffraction grating which extends in the x-direction. As a result, the first diffraction beams 34-36 are separated in a shearing direction, which is the x-direction.

[0058] The first diffraction beams 34-36 are at least partially captured by the projection system PS, as now described. How much of the first diffraction beams 34-36 is captured by the projection system PS will be dependent on: the pupil fill of the incident radiation 33 from the illumination system IL; the angular separation of the first diffraction beams 34-36 (which in turn is dependent on the pitch of the mask grating 31 and the wavelength of the radiation 33); and the numerical aperture of the projection system PS.

[0059] The measurement system 30 may be arranged such that first diffraction beam 35 that corresponds to the 0^th order diffraction beam substantially fills the numerical aperture of the projection system PS, which may be represented by a circular region of a pupil plane 37 of the projection system PS, and the first diffraction beams 34, 36 that correspond to the ±l^st order diffraction beams overlap significantly with the first diffraction beam 35 that corresponds to the 0^th order diffraction beam. With such an arrangement, substantially all of the first diffraction beam 35 that corresponds to the 0^th order diffraction beam and most of the first diffraction beams 34, 36 that correspond to the ±l^st order diffraction beams is captured by the projection system PS and projected onto the sensor 16. Furthermore, with such an arrangement a large number of diffraction beams generated by the mask grating 31 are at least partially projected onto the sensor 16.

[0060] The role of the mask grating 31 is to introduce spatial coherence, as now discussed.

[0061] In general, two rays of radiation 33 from the illumination system IL that are incident on the same point of the mask grating 31 at different angles of incidence are not coherent. By receiving the radiation 33 and forming a plurality of first diffraction beams 34, 35, 36, the mask grating 31 may be considered to form a plurality of copies of the incident radiation cone 33 (the copies having, in general different phases and intensities). Within any one of these copies, or first diffraction beams 34, 35, 36, two rays of radiation which originate from the same point on the mask grating 31 but at different scattering angles, are not coherent (due to the properties of the illumination system IL). However, for a given ray of radiation within any one of the first diffraction beams 34, 35, 36 there is a corresponding ray of radiation in each of the other first diffraction beams 34, 35, 36 that is spatially coherent with that given ray. For example, the chief rays of each of the first diffraction beams 34, 35, 36 (which correspond to the chief ray of the incident radiation 33) are coherent and could, if combined, interfere at the amplitude level.

[0062] This coherence is exploited by the measurement system 30 to determine a wavefront aberration map of the projection system PS.

[0063] In Figure 2, the sensor 16 comprises the single sensor grating 32. In Figure 2, all radiation that is transmitted by the sensor grating 32 is represented as a single arrow 38. This radiation 38 is received by the radiation detector 23 and is used to determine the aberration map.

[0064] Each of the first diffraction beams 34-36 that is incident on the sensor grating 32 will diffract to from a plurality of second diffraction beams.

[0065] Figures 3 and 4 show a set of second diffraction beams produced by the first diffraction beams 34-36.

[0066] Figure 3 shows a set of second diffraction beams 40 produced by the first diffraction beams 34-36. The central diffraction beam 41 generated by the sensor grating 32 is in fact a pair of diffraction beams which will interfere with each other, thereby providing phase information regarding the projection system PS. The pair of diffraction beams are:

• 0^th order diffraction of the 0^th order diffraction beam 35 generated by the mask grating, and

• +l^st order diffraction of the -1^st order diffraction beam 36 generated by the mask grating.

This is schematically indicated in Figure 3 by using solid lines for diffraction orders that contribute to the pair of central diffraction beams 41. This combination of diffraction beams may be referred to as a 0 order diffraction beam convolution.). The term 0 order diffraction beam convolution may be interpreted as referring to a sum of the diffraction orders generated by the mask grating 31 and the diffraction orders generated by the sensor grating 32, when the outcome of the sum is zero (as depicted in Figure 3). Other diffraction beams which do not form part of the central diffraction beam 41 are depicted by dashed lines.

[0067] In Figure 4, the central diffraction beam pair 41 is generated using different diffraction beams:

• -1^st order diffraction of the -1st order diffraction beam 34 generated by the mask grating.

This is schematically indicated in Figure 4 by using solid lines for diffraction orders that contribute to the central diffraction beams 41. This combination of diffraction beams may also be referred to as a 0 order diffraction beam convolution (again the sum of the mask grating and sensor grating diffraction orders is zero). Other diffraction beams are depicted by dashed lines.

[0068] Figures 3 and 4 depict diffraction in the x-direction. Corresponding diffraction in the y- direction is also used, for example using mask and sensor gratings which extend in the y-direction. When considering diffraction in the x and y directions together, the 0 order diffraction beam convolution may be expressed in the form [x,y] and may be referred to as a [0,0] order diffraction beam convolution. [0069] The diffraction beam pairs 41 of Figures 3 and 4 are first harmonics, as explained further above. The first harmonics are discriminated from other harmonics using phase stepping, as explained further below.

[0070] All of the lines in Figures 2-4 may be considered to represent a single ray of radiation that originates from a single input ray 33 from the illumination system IL. Therefore, as explained above, these lines represent spatially coherent rays that, if spatially overlapping at radiation detector 23 will produce an interference pattern. Furthermore, the interference is between rays which have passed though different parts of the pupil plane 37 of the projection system PS (which are separated in the shearing direction). Therefore, the interference of radiation that originates from a single input ray 33 is dependent on phase differences between two different parts of the pupil plane.

[0071] This spatial overlapping and spatial coherence of the second diffraction beams 41 at radiation detector 23 is achieved by matching the mask and sensor gratings 31, 32 such that the angular separation (in the shearing direction) between different second diffraction beams that originate from a given first diffraction beam is the same as the angular separation (in the shearing direction) between different first diffraction beams as they converge on the sensor grating 32. This spatial overlapping and spatial coherence of the second diffraction beams at radiation detector 23 is achieved by matching the pitches of the mask grating 31 and the sensor grating 32 in the shearing direction. It will be appreciated that this matching of the pitches of the mask and sensor gratings 31, 32 in the shearing direction takes into account any reduction factor applied by the projection system PS. [0072] In addition to the diffraction beams 41 depicted in Figures 3 and 4, higher order diffraction beams of the mask grating 31 are also incident upon the detector 23. Thus, there will be more beams that should be summed coherently at each point on the detector 23 in order to determine the intensity of radiation as measured by that part of the detector (for example a corresponding pixel in a two dimensional array of sensing elements).

[0073] In general, a plurality of different second diffraction beams contributes to the radiation received by each part of the detector. The intensity of radiation from such a coherent sum is given by:

I = DC + Spairs { t } Yi cos(Ac|>i), (2) where DC is a constant term (which is equivalent to the incoherent sum of the different diffraction beams), the sum is over all pairs of different second diffraction beams, y_t is an interference strength for that pair of second diffraction beams and Ac[>i is a phase difference between that pair of second diffraction beams. In this context, second diffraction beams means diffraction beams generated by the sensor grating 32.

[0074] The phase difference A between a pair of second diffraction beams is dependent on two contributions: (a) a first contribution relates to the different part of the pupil plane 37 of the projection system PS from which they originate; and (b) a second contribution relates to the position within unit cells of each of the mask and sensor gratings 31, 32 from which they originate.

[0075] The first of these contributions can be understood to arise from the fact that the different coherent radiation beams have passed through different parts of the projection system PS and are therefore related to the aberrations that it is desired to determine (in fact they are related to a difference between two points in the aberration map that are separated in the shearing direction).

[0076] The second of these contributions can be understood to arise from the fact that the relative phases of multiple rays of radiation that arise from a single ray incident on a diffraction grating will depend on which part of the unit cell of that grating the ray was incident. This therefore does not contain information relating to the aberrations. As explained above, in some embodiments, the mask grating 31 and/or the sensor 16 are sequentially scanned and/or stepped in the shearing direction. This causes the phase differences between all of pairs of interfering radiation beams received by the radiation detector 23 to change. As the mask grating 31 and/or the sensor 16 are sequentially stepped in the shearing direction by an amount that is equivalent to a fraction of the pitches (in the shearing direction) of the mask and sensor gratings 31, 32, in general, the phase differences between pairs of second diffraction beams will all change. If the mask grating 31 and/or the sensor 16 are stepped in the shearing direction by an amount that is equivalent to an integer multiple of the pitches (in the shearing direction) of the mask and sensor gratings 31, 32 the phase differences between pairs of second diffraction beams will remain the same. Therefore, as the mask grating 31 and/or the sensor 16 are sequentially scanned and/or stepped in the shearing direction, the intensity received by each part of the radiation detector 23 will oscillate. The first harmonic of this oscillating signal (which may be referred to as a phase-stepping signal), as measured by the radiation detector 23, is dependent on the contributions to equation (1) from mask grating diffraction orders that differ by one diffraction order. In other words, the shearing angle of two interfering mask grating diffraction orders determines the modulation frequency. For example, 0^th and 2^nd mask grating diffraction orders will provide a second harmonic modulation frequency (there is a difference of 2 orders between them). 4^th and 5^th mask grating diffraction orders will provide a first harmonic modulation frequency (there is a difference of 1 order between them). Phase stepping provides discrimination of the first harmonic signal from higher harmonic signals.

[0077] The wavefront fitting model assumes that only two pairs of diffraction beams will contribute to the first harmonic of the phase stepping signal, as explained further below in connection with Figures 3 and 4.

[0078] Each pair of diffraction beams will result in an interference term of the form shown in equation (2), which contributes to the first harmonic of the phase stepping signal, i.e. an interference term of the form:

2TT (3) y cos( - v + AW)

P where y is an amplitude of the interference term, p is the pitch of the mask and sensor gratings 31, 32 (in the shearing direction), v parameterizes the relative positions of the mask and sensor gratings 31, 32 in the shearing direction and AW is a difference between values of the aberration map for two diffraction beams (which may be referred to as k- vectors). The amplitude y of the interference term is proportional to the product of the compound scattering efficiencies of the two diffraction beams, as discussed further below. The frequency of the first harmonic of the phase stepping signal is given by the inverse of the pitch p of the mask and sensor gratings 31 , 32 in the shearing direction. The phase of the phase stepping signal is given by AW (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 diffraction beams originate).

[0079] As noted above, phase-stepping is used to separate the first harmonic from other harmonics such that only the first harmonic of the signal output from the detector 23 is used. The first harmonic consists of diffraction beams that differ in order by ±1. The primary contributors to the first harmonic signal are 0^th, +1 and -1 order diffraction beams 34-36 generated by the mask grating 31, and some of the 0^th, +1 and -1 order diffraction beams generated by the sensor grating 32 as explained above in connection with Figures 3 and 4. A conventional aberration determining method may assume that these are the only diffraction beams which contribute to the first harmonic signal, and that only [0,0] order diffraction beam convolutions need to be modelled. Specifically, a conventional aberration determining method may assume that only convolutions of the diffraction beams which include a 0^th diffraction order component from the mask grating 31 or a 0^th diffraction order component from the sensor grating 32, and which provide [0,0] order diffraction beam convolutions, contribute to the first harmonic signal. However, in practice other order diffraction beams contribute to the first harmonic signal. An example of this is schematically depicted in Figure 5. In Figure 5, the -2^nd and -3^rd order diffraction beams 37, 38 generated by the mask grating 31 are depicted. At the sensor grating 32, +l^st order diffraction of the -2^nd order diffraction beam generates a diffraction beam 42. In addition, at the sensor grating 32, +2^nd order diffraction of the -3^rd order diffraction beam generates a diffraction beam 42. This diffraction beam pair will provide a first harmonic signal, and thus will contribute to the sensed first harmonic signal. Other diffraction orders will combine to provide a first harmonic signal. These diffraction orders reduce the accuracy with which the aberration is determined by a conventional method.

[0080] Embodiments of the invention remove or reduce the contribution of the higher order diffraction beams to the first harmonic signal. This allows an improved estimated aberration of the projection system to be determined.

[0081] In a conventional lithographic apparatus with transmissive optics, an assumption that only [0,0] order diffraction beam convolutions of the 0^th order and ±l^st order diffraction beams contribute to the first harmonic signal may be sufficient to obtain an accurate estimate of the aberration of the projection system. However, in an EUV lithographic apparatus, significant contributions to the 1^st order harmonic signal may arise from higher order diffraction beams. This may be for example due to a three- dimensional structure of the grating 31 provided on the mask grating 31, and a three-dimensional structure of the diffraction grating 32 provided on the sensor 16. Removing or reducing the effect of these unwanted contributions may be particularly desirable if projection of a pattern by the projection system PS with high accuracy is required, e.g. as may be the case for an EUV lithographic apparatus. Embodiments of the invention may also apply to DUV lithographic apparatus.

[0082] Figure 6 is a flow chart which sets out a method according to an embodiment of the invention.

[0083] In a first step the EUV radiation beam B, 33 is used to illuminate the mask grating 31. The radiation beam B may for example have a desired illumination mode (e.g. conventional, annular, dipole, quadrupole, etc.). The radiation beam B is diffracted by the mask grating 31. The mask grating diffraction provides 0^th order, +l^st and -1^st order diffraction as depicted in Figures 2-4. In addition however, the mask grating diffraction provides higher order diffraction as depicted in Figure 5. The sensor 16, including the sensor grating 32, is used to measure the wavefront of the radiation as transmitted by the projection system PS. Phase-stepping is used to discriminate the first harmonic signal from other harmonics, as explained further above. A measured signal is output from the shearing interferometer 23 to the processor 18.

[0084] In a second step, a wavefront is fitted by the processor to the first harmonic signal. The wavefront fit is applied using an assumption that only combinations of a 0^th order diffraction beam and a + 1^st or - 1^st order diffraction beams contribute to the measured signal (as explained above in connection with Figures 3 and 4). In other words, a design matrix is constructed which describes the intensity signal of the sheared Zernike polynomials. The design matrix is used to fit the measured sheared wavefront. In the construction of the design matrix it is assumed that only the Oth and +- 1 st diffraction orders contribute to the sheared intensity signal. In addition, it is assumed that only [0,0] diffraction order convolutions contribute to the first harmonic. The wavefront fit provides an estimated aberration of the projection system PS.

[0085] In a third step, a model is used to calculate the first harmonic signal for each convolution order except the [0,0] order diffraction beam convolutions using the estimated aberration of the projection system PS. In other words, the model calculates the first harmonic signal that would be seen by the sensor 16 if only higher convolution orders contributed to that signal. An example of a higher convolution order that will contribute to the first harmonic signal is depicted in Figure 5 (as described further above).

[0086] There are three inputs to the model. The first input is the estimated aberration of the projection system PS, as mentioned above.

[0087] The second input is a model of diffraction caused by the 3 -dimensional structure of the mask grating 31 and the 3-dimensional structure of the sensor grating 32. The structure of these diffraction gratings 31, 32, specifically the width and height of lines of the gratings and the refractive indices of the materials used to form the gratings are known. The structure of the gratings 31, 32 does not change over time, and thus the model of the diffraction provided by those gratings may be determined for example at an earlier time. In general the model of diffraction caused by the 3- dimensional structures may be generated at any time, and may then be stored and used by the method when it is needed. The model may be referred to as mask grating 3D diffraction pupils and sensor grating 3D diffraction pupils.

[0088] The third input to the model is the illumination pupil. The illumination pupil may be measured, for example using a sensor in the lithographic apparatus. For example, a pinhole mask and a sensor in the substrate table may be used. Illumination pupil measurements are conventional and so are not described further here. . Alternatively, the illumination pupil may be estimated based on a DC map of the measured Zernikes. Estimation of the illumination pupil may be less accurate than measuring the pupil.

[0089] The model uses the three above mentioned inputs to calculate the first harmonic intensity signal for each diffraction beam convolution order except the [0,0] order diffraction beam convolutions. The model is an analytical model that calculates the intensity signal of each convolution order. The intensity signal of each convolution order is calculated separately:

1. The illumination pupil is multiplied with the mask grating 3D diffraction pupils

2. The data of step 1 is multiplied with the estimated aberration of the projection system (and optionally a polarization pupil) POB Jones pupil 3. Electric fields of respective convolution orders are calculated by performing a convolution of diffraction order pupils generated using steps 1 and 2 with the sensor grating 3D diffraction pupils

4. The Intensity signal of each convolution order is calculated by multiplying the data of step 3 with its complex conjugate

[0090] In a fourth step the outputs from the model for each convolution order are summed together to generate a modelled first harmonic intensity signal for all convolutions excluding [0,0] diffraction order convolutions. This may be referred to as the modelled intensity signal.

[0091] In a fifth step the modelled intensity signal is subtracted from the measured intensity signal. The result of the subtraction is a cleaned intensity signal in which an unwanted contribution to the measured intensity signal from higher diffraction order convolutions is removed. The cleaned intensity signal thus includes a reduced contribution from unwanted higher diffraction order convolutions. Some contribution from unwanted higher diffraction order convolutions may remain in the cleaned intensity signal. The cleaned intensity signal may also be referred to as a cleaned intensity modulation signal.

[0092] In a sixth step, a wavefront is fitted to the cleaned intensity signal using only Oth and +- 1 st diffraction orders and only [0,0] order diffraction beam convolutions. Fitting the wavefront provides an improved estimate of the aberration of the projection system PS. The improved estimate of the aberration of the projection system may be used to adjust the projection system using the adjusting system PA (see Figure 1).

[0093] The method may further comprise using a fit error metric to determine the accuracy with which wavefront was fitted to the cleaned intensity signal. If the metric indicates that the fit was sufficiently accurate, then the processing may be stopped and the projection system adjusted accordingly. If the fit was not sufficiently accurate, then a further iteration of the method may be performed. For clarity of representation the further iteration is schematically depicted in Figure 7.

[0094] An example of the fit error metric is determining how homogenous the cleaned intensity signal is. This may be referred to as quantifying the modulation of the cleaned intensity modulation signal. In the case that only the Oth and +- I st diffraction orders and only [0,0] order diffraction beam convolutions contribute to the cleaned intensity signal, the cleaned intensity modulation signal should not have any structure and should be constant over the measured pupil. The more structure is present in the cleaned intensity modulation signal the further you are from reducing the contributions of the higher convolution orders. The fit error metric may quantify the amplitude of modulations in the cleaned intensity modulation signal and compare this with a threshold.

[0095] In an alternative approach, convergence of the reconstructed wavefront results may be used to determine when to stop processing. For example, a difference between two consecutive iterations of the wavefront construction shall not be larger than a threshold value, e.g. 5 pm.

[0096] The further iteration closely corresponds with the steps described above and thus is not described in detail here. In the further iteration, the initial wavefront fit is to the cleaned intensity signal rather than to the measured intensity signal. The resulting estimated aberration is then used by the model to obtain a new modelled intensity signal. This modelled intensity signal is subtracted from the cleaned intensity signal to obtain a 2^nd iteration cleaned intensity signal. A wavefront is then fitted to this 2^nd iteration cleaned intensity signal.

[0097] The fit error metric may again be applied. If the fit is not sufficiently accurate then one or more further iterations (not depicted may be performed. Iterations may be performed until the fit error metric satisfies the accuracy requirement (e.g. the fit error metric is below a threshold).

[0098] The intensity signal may be referred to as an intensity map.

[0099] In described embodiments of the invention, the model used for the second step uses only [0,0] order diffraction beam convolutions in combination with only 0^th and +-l^st diffraction orders to characterise the aberration of the projection system. In other embodiments, the model used for the second step may use [0,0] order diffraction beam convolutions, in combination with 0^th and +-l^st and additional diffraction orders, to characterise the aberration of the projection system. However, the effect of including the additional diffraction orders for the [0,0] order diffraction beam convolutions may be marginal (i.e. may be minor or negligible).

[0100] Although specific reference may be made in this text to the use of lithographic apparatus in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may have other applications. Possible other applications include the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquidcrystal displays (LCDs), thin-film magnetic heads, etc.

[0101] Although specific reference may be made in this text to embodiments of the invention in the context of a lithographic apparatus, embodiments of the invention may be used in other apparatus. Embodiments of the invention may form part of a mask inspection apparatus, a metrology apparatus, or any apparatus that measures or processes an object such as a wafer (or other substrate) or mask (or other patterning device). These apparatus may be generally referred to as lithographic tools. Such a lithographic tool may use vacuum conditions or ambient (non- vacuum) conditions.

[0102] Where the context allows, embodiments of the invention may be implemented in hardware, firmware, software, or any combination thereof. Embodiments of the invention may also be implemented as instructions stored on a machine -readable medium, which may be read and executed by one or more processors. A machine-readable medium may include any mechanism for storing or transmitting information in a form readable by a machine (e.g., a computing device). For example, a machine -readable medium may include read only memory (ROM); random access memory (RAM); magnetic storage media; optical storage media; flash memory devices; electrical, optical, acoustical or other forms of propagated signals (e.g. carrier waves, infrared signals, digital signals, etc.), and others. Further, firmware, software, routines, instructions may be described herein as performing certain actions. However, it should be appreciated that such descriptions are merely for convenience and that such actions in fact result from computing devices, processors, controllers, or other devices executing the firmware, software, routines, instructions, etc. and in doing that may cause actuators or other devices to interact with the physical world.

[0103] While specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The descriptions above are intended to be illustrative, not limiting. Thus it will be apparent to one skilled in the art that modifications may be made to the invention as described without departing from the scope of the claims set out below.

Claims

1. A method of determining aberration of a projection system of a lithographic apparatus, the method comprising: illuminating a mask grating located in an object plane of the projection system, using the projection system to form an image of the mask grating at a sensor grating, and using an imaging detector located below the sensor grating to detect resulting interference patterns and provide an output measured intensity signal; the method further comprising: using a wavefront fitting algorithm to fit to the measured intensity signal and thereby obtain an estimate of aberration of the projection system, the wavefront fitting algorithm including an assumption that only [0,0] order diffraction beam convolutions contribute to the wavefront; determining a pupil of the illumination used to illuminate the mask grating, then; a. using the estimated aberration of the projection system, diffraction amplitude and phase data from a model representing known three-dimensional structures of the mask grating and the sensor grating, and the illumination pupil, to calculate 1st harmonic contributions of each diffraction order of the light diffracted by the mask grating and projected by the projection system and convoluted with the diffraction orders of the sensor grating, except contributions of the [0,0] order diffraction beam convolutions, thereby providing intensity signals for those 1^st harmonic contributions; b. summing the intensity signals for the 1^st harmonic contributions to obtain a modelled intensity signal; c. subtracting the summed intensity signal from the measured intensity signal to obtain a cleaned intensity signal; and d. using the wavefront fitting algorithm to fit to the cleaned intensity signal and thereby obtain an improved estimate of the aberration of the projection system.

2. The method of claim 1 , wherein steps a to d are repeated using the improved estimate of the aberration of the projection system in order to generate a further improved estimate of the aberration of the projection system.

3. The method of claim 2, wherein steps a to d are repeated multiple times until a fit provided by the wavefront fitting algorithm satisfies a predetermined metric.

4. The method of claim 2, wherein the predetermined metric is based on quantifying a modulation of the cleaned intensity signal.

5. The method of claim 2, wherein steps a to d are repeated multiple times until convergence of the estimate of the aberration of the projection system occurs.

6. The method of any preceding claim, wherein the illumination pupil is determined using a sensor located in the lithographic apparatus.

7. The method of any of claims 1 to 5, wherein the illumination pupil is estimated based upon an offset of the measured intensity signal.

8. The method of any preceding claim, wherein the wavefront fitting algorithm includes a further assumption that only 0^th and +-l^st diffraction orders contribute to the wavefront.

9. A lithographic apparatus comprising a projection system, a shearing interferometer comprising a mask grating a sensor grating and an imaging detector located below the sensor grating, and a processor configured to receive signals from the imaging detector, wherein the processor is configured to: cause a radiation beam to illuminate the mask grating such that the projection system forms an image of the mask grating at the sensor grating, and interference patterns can be detected by the imaging detector to provide an output measured intensity signal; the processor being further configured to: use a wavefront fitting algorithm to fit to the measured intensity signal and thereby obtain an estimate of aberration of the projection system, the wavefront fitting algorithm including an assumption that only [0,0] order diffraction beam convolutions contribute to the wavefront; determining a pupil of the illumination used to illuminate the mask grating, then; a. using the estimated aberration of the projection system, diffraction amplitude and phase data from a model representing known three-dimensional structures of the mask grating and the sensor grating, and the illumination pupil, to calculate 1st harmonic contributions of each diffraction order of the light diffracted by the mask grating and projected by the projection system and convoluted with the diffraction orders of the sensor grating, except contributions of the [0,0] order diffraction beam convolutions, thereby providing intensity signals for those 1^st harmonic contributions; b. summing the intensity signals for the 1^st harmonic contributions to obtain a modelled intensity signal; c. subtracting the summed intensity signal from the measured intensity signal to obtain a cleaned intensity signal; and d. using the wavefront fitting algorithm to fit to the cleaned intensity signal and thereby obtain an improved estimate of the aberration of the projection system.

10. The lithographic apparatus of claim 9, wherein the processor is configured to repeat steps a to d using the improved estimate of the aberration of the projection system in order to generate a further improved estimate of the aberration of the projection system.

11. The lithographic apparatus of claim 10, wherein the processor is configured to repeat steps a to d multiple times until a fit provided by the wavefront fitting algorithm satisfies a predetermined metric.

12. The lithographic apparatus of claim 11, wherein the predetermined metric is based on based on quantifying a modulation of the cleaned intensity signal.

13. The lithographic apparatus of claim 10, wherein the processor is configured to repeat steps a to d multiple times until convergence of the estimate of the aberration of the projection system occurs.

14. The lithographic apparatus of any of claims 9 to 13, wherein the wavefront fitting algorithm includes a further assumption that only 0^th and +-l^st diffraction orders contribute to the wavefront.