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`10/13
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`7/13
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`

`2020P0021 LEP
`
`43
`
`ABSTRACT
`
`A measurement system,
`
`the measurement system comprising: a sensor apparatus; an illumination
`
`system arranged to illuminate the sensor apparatus with radiation, the sensor apparatus comprising a
`
`patterned region arranged to receive a radiation beam and to form a plurality of diffraction beams, the
`
`diffraction beams being separated in a shearing direction; the sensor apparatus comprising a radiation
`
`detector; wherein the patterned region is arranged such that at least some of the diffraction beams form
`
`interference patterns on the radiation detector; wherein the sensor apparatus comprises a plurality of
`
`patterned regions, and wherein pitches of the patterned regions are different in adjacent patterned
`
`regions.
`
`(Fig. 12]
`
`wi
`
`10
`
`

`

`2020P0021 LEP
`
`42
`
`wi
`
`10
`
`21.
`
`The method of any of claims 17 to 20, further comprising moving the at least one of the
`
`patterning device and the sensor apparatus in the shearing direction in phase steps in a range of 4-9 to
`
`form the plurality of oscillating signals.
`
`22,
`
`A computer readable medium carrying a computer program comprising computer readable
`
`instructions configured to cause a computer to carry out a method according to any one ofclaims 16
`to 21.
`
`23.
`
`A computer apparatus comprising:
`
`a memory storing processor readable instructions, and
`
`a processor arranged to read and execute instructions siored in said memory, wherein said
`
`processor readable instructions comprise instructions arranged to control the computer to carry out the
`
`method according to any one of claims 16 to 21.
`
`

`

`2020P0021 LEP
`
`41
`
`the patterned region comprising a second patterned region arrangedto receivethe first diffraction
`
`beams from the projection system and to form a plurality of second diffraction beams from each of the
`
`first diffraction beams; and
`
`wi
`
`10
`
`a radiation detector arranged to receive at least a portion of the second diffraction beams,
`
`wherein the first and second patterned regions in the set are matched by matching the pitches
`
`of the first and second patterned regions in the shearing direction such that at least some of the second
`
`diffraction beams formed from at least one ofthe first diffraction beamsare spatially coherent with a
`seconddiffraction beam formed fromat least one otherfirst diffraction beam to forminterference
`
`patterns on the radiation detector;
`
`wherein the patterning device comprises a plurality of first patterned regions and the sensor
`
`apparatus comprises a plurality of second patterned regions such that there is a plurality of sets, each
`
`sel comprising one ofthe plurality of first patterned regions and oneofthe plurality of second
`
`patterned regions, and
`
`wherein the pitches of the first patterned regions are different in adjacent sets and/or the
`
`pitches of the second patterned regions are different in adjacentsets.
`
`18.
`
`The method of claim 17, further comprising moving at least one of the patterning device and
`
`the sensor apparatus in the shearing direction such that an intensity of radiation received byeach part
`
`of the radiation detector varies as a function of the movement in the shearing direction so as to form a
`
`plurality of oscillating signals corresponding to the different pitches of the first patterned regions in
`
`adjacent sects and/or the different pitches of the second patterned regions in adjacentsets;
`
`determining fromthe radiation detector phases of harmonics of the oscillating signals at a
`
`plurality of positions on the radiation detector; and
`
`determining a set of coefficients that characterize an aberration map ofthe projection system
`
`fromthe phase of the harmonics of the oscillating signals at the plurality of positions on the radiation
`detector.
`
`19.
`
`The methodof cither of claims 17 or 18, further comprising determining the sct of
`
`coefficients that characterize the aberration map of the projection system by equating the phases of the
`
`30
`
`harmonics ofthe oscillating signals to a difference in the aberration map between positions in the
`
`pupil plane that are separated in the shearing direction by twice a shearing distance which corresponds
`
`to the distance in the pupil plane between two adjacentfirst diffraction beams and solving to find the
`set of coefficients.
`
`20.
`
`The method of any of claims 17 to 19, further comprising determining the set of coefficients
`
`that characterize the aberration map of the projection system by simultaneously solving constraints for
`
`the shearing direction and for a second, orthogonaldirection.
`
`

`

`2020P0021 LEP
`
`40
`
`determinea set of coefficients that characterize an aberration map of the projection
`
`system from the phase of the harmonics ofthe oscillating signals at the plurality of positions on the
`radiation detector.
`
`wi
`
`10
`
`30
`
`12.
`
`The measurement system of claim 11, wherein the set of coefficients that characterize the
`
`aberration map of the projection system are determined by equating the phases of the harmonics of the
`
`oscillating signals to a difference in the aberration map between positions in the pupil plane that are
`
`separated in the shearing direction by twice a shearing distance which correspondsto the distance in
`
`the pupil plane between two adjacentfirst diffraction beams and solving to find the set of coefficients.
`
`13.
`
`The measurement system of claim 12, wherein the set of coefficients that characterize the
`
`aberration map of the projection sysiem are determined by simultaneously solving constraints for the
`
`shearing direction and for a second, orthogonal direction.
`
`14.
`
`The measurement system of any of claims 9-13, wherein the plurality offirst patterned
`
`regionsandthe plurality of second patterned regions are gratings.
`
`15.
`
`A lithographic apparatus comprising the measurement system of anyone of claims 1 to 14.
`
`16.
`
`A method for measurement, the method comprising:
`
`illuminating a sensor apparatus with radiation, wherein the sensor apparatus comprises a
`
`patterned region arranged to receive at least a portion of the radiation and to forma plurality of
`
`diffraction beams, the diffraction beams being separated in a shearing direction;
`
`wherein the sensor apparatus comprises a radiation detector arranged to receive at least a
`
`portion of the diffraction beams,
`
`wherein the patterned region is arranged such that at least some of the diffraction beams form
`
`interference patterns on the radiation detector;
`
`whercin the scnsor apparatus comprisesa plurality of patterned regions, and
`
`wherein pitches of the patterned regions are different in adjacent patterned regions.
`
`17.
`
`The method of claim 16, the method further comprising:
`
`illuminating a patterning device with radiation, wherein the patterning device comprises a
`
`first patterned region arranged to receive at least a portion of the radiation and to form a plurality of
`
`first diffraction beams,the first diffraction beams being separated in the shearing direction;
`
`projecting, with the projection system,at least part of the plurality of first diffraction beams
`
`onto the sensor apparatus comprising:
`
`

`

`2020P0021 LEP
`
`39
`
`a patterning device;
`
`wherein the illumination system is arrangedto illuminate the patterning device with radiation,
`
`the patterning device comprising a first patterned region arranged to receive the radiation beam and to
`
`form a plurality of first diffraction beams, the first diffraction beams being separated in the shearing
`
`wi
`
`direction;
`
`wherein the patterned region of the sensor apparatus comprises a second patterned region;
`
`the projection system being configured to project the first diffraction beams onto the sensor
`
`apparatus, the second patterned region being arranged to receive thefirst diffraction beams from the
`
`projection system and to form a plurality of second diffraction beams from each ofthe first diffraction
`
`10
`beams suchthat the first and second patterned regions formaset;
`
`wherein the first and second patterned regions in the set are matched by matching the pitches
`
`of the first and second patterned regions in the shearing direction such that al least some of the second
`
`diffraction beams formed from at least one ofthe first diffraction beamsare spatially coherent with a
`second diffraction beam formed from at least one otherfirst diffraction beam to form interference
`
`patterns on the radiation detector;
`
`wherein the patterning device comprises a plurality of first patterned regions and the sensor
`
`apparatus comprises a plurality of second patterned regions such that there is a plurality of sets, each
`
`set comprising one ofthe plurality of first patterned regions and oneofthe plurality of second
`
`patterned regions, and
`
`wherein the pitches of the first patterned regions are different in adjacent sets and/or the
`
`pitches of the second patterned regions are different in adjacentscts.
`
`10.
`
`The measurement system of claim 9, wherein the pitches of the first patterned regions and the
`
`second patterned regions in at least one ofthe plurality of sets are the same.
`
`11.
`
`The measurement system of either of claims 9 or 10, further comprising a posilioning
`
`apparatus configured to moveat least one of the patterning device and the sensor apparatusin the
`
`shearing direction; and
`
`a controller configured to:
`
`30
`
`control the positioning apparatus so as to moveat least oneofthe first patterning
`
`device and the sensor apparatus in the shearing direction such that an intensity of radiation received
`
`by eachpart of the radiation detector varies as a function of the movement in the shearing direction so
`
`as to form oscillating signals corresponding to the different pitches of the first patterned regions in
`
`adjacent sets and/or the different pitches of the second patterned regions in adjacentsets;
`
`determine from the radiation detector phases of harmonicsof the oscillating signals at
`
`a plurality of positions on the radiation detector; and
`
`

`

`2020P0021 LEP
`
`38
`
`CLAIMS
`
`1.
`
`A measurement system, the measurement system comprising:
`
`a sensor apparatus;
`
`wi
`
`10
`
`an illumination systemarrangedto illuminate the sensor apparatus with radiation, the sensor
`
`apparatus comprising a patterned region arranged to receive a radiation beam andto form a plurality
`
`ofdiffraction beams, the diffraction beams being separated in a shearing direction;
`
`the sensor apparatus comprising a radiation detector;
`
`wherein the patterned region is arranged such that at least some ofthe diffraction beams form
`
`interference patterns on the radiation detector;
`
`wherein the sensor apparatus comprises a plurality of patterned regions, and
`
`wherein piiches of the patterned regions are different in adjacent patterned regions.
`
`2.
`
`The measurement system of claim 1, wherein the measurement system is arranged such that
`
`the interference patterns from adjacent patterned regions at least partially overlap at the radiation
`detector.
`
`3.
`
`The measurement system of either of claims | or 2, wherein the pitches of alternating
`
`patterned regions are the same.
`
`4.
`
`The measurement system of any preceding claim, wherein the pitches of the adjacent
`
`patterned regions are not even numberinteger multiples.
`
`5.
`
`The measurement system of claim 4, wherein the pitches of the adjacent patterned regions are
`
`not integer multiples.
`
`6.
`
`The measurement system of any preceding claim, wherein the plurality of patterned regions
`
`comprise thirtecn patterned regions.
`
`30
`
`7.
`
`The measurement system of any preceding claim, wherein the plurality of patterned regions
`
`are positioned at odd and evenfield point locations.
`
`8.
`
`The measurement system of any preceding claim, wherein the plurality of patterned regions
`
`extend in an x direction and in a second direction orthogonalto the x direction.
`
`9.
`
`The measurement system of any preceding claim, the measurement system further
`
`comprising:
`
`

`

`2020P0021 LEP
`
`37
`
`wi
`
`10
`
`[000169] Although specific reference may be madein this text to the use of a lithographic apparatus
`
`in the manufacture of ICs, it should be understood that the lithographic apparatus described herein may
`
`have other applications. Possible other applications include the manufacture of integrated optical
`
`systems, guidance and detection patterns for magnetic domain memories, flat-panel displays, liquid-
`
`crystal displays (LCDs), thin-film magnetic heads, etc.
`
`[000170] Although specific reference may be madein this text to embodiments of the invention in
`
`the context ofa lithographic apparatus, embodiments of the invention may be used in other apparatus.
`
`Embodiments of the invention may formpart 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 vacuumconditions or ambient (non-vacuum) conditions.
`
`[000171] Although specific reference may have been made above to the use of embodiments of the
`
`invention in the context of optical lithography, it will be appreciated that the invention, where the
`
`context allows, is not limited to optical lithography and maybe used in other applications, for example
`
`imprint lithography.
`
`[000172] 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 andthat
`
`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.
`
`[000173] While specific embodiments of the invention have been described above,
`
`it will be
`
`30
`
`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 madeto the invention as described without departing fromthe scope of the claims
`set out below.
`
`

`

`2020P0021 LEP
`
`36
`
`wi
`
`10
`
`[000163]
`
`For a third order polynomialfit, that may be used on the seven field point measurements of
`
`a measurement system, the thirteen field point measurements may improve the reproducibility of the
`
`measurement system by 1.5x. For overlapping spots potentially less improvement for higher order
`
`Zernikes may be achieved. Howeverfor a particular system control mainly lower order Zernikes may
`
`be of interest and thus the results for the higher order Zernikes may not be as important. The
`
`measurement may also improve overlay accuracy.
`
`[000164]
`
`In embodiments,
`
`the measurement system may be an alignment sensor rather than a
`
`measurement system for measuring aberrations.
`
`[000165]
`
`For an alignment sensor, generally, an alignment mark, such as a diffraction grating or
`
`another type of alignment mark, is provided on a substrate. The alignment sensor projects optical pulses
`
`onto the alignment mark (e.g. mask alignment marks M1, M2 or substrate alignment marks P1, P2) in
`
`order to be scattered by the alignment mark. An intensity of the scattered optical pulses is measured by
`
`a detector, and position information (expressing the position of the alignment mark in respect of for
`
`example the alignment sensor) is derived therefrom.
`
`[000166]
`
`In the present embodiment, patterned regions (i.e. diffraction gratings) of the sensor
`
`apparatus may be consideredto be the alignment marks. In a similar way as described above, adjacent
`
`diffraction gratings may have different pitches to allow overlapping radiation from the adjacent
`
`diffraction gratings detected by a radiation detector to be disentangled. For example, thirteen field
`
`points, instead of seven field points may be measured using the measurement system to measure
`
`alignment of e.g. a substrate.
`
`[000167]
`
`In cmbodiments, the alignment sensor may have a sensor apparatus comprising a radiation
`
`detector. The alignment sensor may comprise an illumination system arranged to illuminate the sensor
`
`apparatus with radiation. The sensor apparatus may comprise a plurality of patterned regions arranged
`
`to receive a radiation beam and to form a plurality of diffraction beams, the diffraction beams being
`
`separated in a shearing direction. The patterned regions may comprise a diffraction grating, e.g. in the
`
`form of a checkerboard with a 50% duty cycle. In the alignment sensor, the patterning device may not
`
`be needed and thus there may not befirst patterned regions and second patterned regions, although the
`
`patterned regions of the alignment sensor may be considered to be in the same position as the second
`
`patterned regions of the embodiment of Figure 12.
`
`30
`
`[000168]
`
`In the alignment sensor, in a similar way as with the second patterned regions ofthe
`
`embodiment of Figure 13B, the pitches of the patterned regions are different in adjacent patterned
`
`regions. The pitches of the adjacent patterned regions being different allows overlapping radiation at
`
`the radiation detector to he disentangled to differentiate the signals from adjacent patterned regions.
`
`This allows an increased numberof field points to be measured in a single measurement. This allows
`
`an increased amount of data to be used in measuring alignment and may lead to a more accurate
`
`alignment measurement.
`
`

`

`2020P0021 LEP
`
`35
`
`fit B points to provide a decomposition A line and a decomposition B line respectively. As can be seen
`
`from the graph, decomposition A has a period whichis three times the period of decomposition B (i.e.
`
`three full sine waves are shown for decomposition B for the one sine wave of decomposition A) and
`
`thus, decomposition A maps onto the even field points, which, in this case, have gratings with pitches
`
`which are three times less than that of the gratings at the odd field points.
`
`[000158] More particularly, since the period is known (from the pitches of the gratings) the phase
`
`may be determined at each pixel location and a phase map for each ofthe pixels may be provided. In
`
`this case a total of nine phase steps are usedto illustrate the situation. However, it will be appreciated
`
`that, in other embodiments, a different number of phase steps may be used. Tofit the sine wave for the
`
`gratings with a different pitch, then a minimum of 4 phase steps would be required. This is because
`
`wi
`
`10
`
`there are four unknowns, two phases and two amplitudes.
`
`[000159]
`
`The phase map may then be linearly fitted to the Zernike coefficients to provide the
`
`aberration map. The aberration map provides the aberrations for the projection system PS’.
`
`[000160]
`
`Figure 16 shows a graph indicating simulated measurementreproducibility (repro (nm)) for
`
`eight phase steps with one grating having a 3x smaller pitch than the adjacent grating. The
`
`reproducibility per field point may be on par with the reproducibility per field point achieved currently
`
`(straight black line in Figure 16, top). This means that by introducing the additional field points (e.g.
`
`at the even field point locations) the amount of data collected in a single measurement may be increased
`
`from seven field points to thirteen field points with the same or similar reproducibility per field point.
`
`The reproducibility may be considered to be an assessmentof the noise of the measurement.
`
`[000161]
`
`Figure 17 shows a graph indicating simulated expected position dependencefor cight phase
`
`steps and one grating having a 3x smaller pitch than the adjacent grating. Misalignment (nm) may
`
`indicate what happens whenthefirst and second patterned regions are not optimally aligned. That is,
`
`when the patterning device is not at the position expected.
`
`It may be desired to minimize expected
`
`position dependence. This may be donein two ways, increasing the numberof phase steps and choosing
`
`different piiches.
`
`Increasing the number of phase steps has a disadvantage of increasing time of
`
`measurementso it may be desired to optimize these variables for a particular implementation.
`
`[000162]
`
`The reproducibility and expected position dependency may be simulated with a particular
`
`camera non-linearity. With this camera non-linearity, for example, eight phase steps may be needed.
`
`30
`
`With a camera chip having less nonlinearity, less than eight phase steps may be used. Increasing the
`
`number of phase steps may reduce error in measurements but will increase time for the measurements.
`
`With more than 5 phasesteps the actual gain in cycle is small when comparing the two measurements
`
`of the seven field points with the one measurement of the thirteen field points: 2 (separate odd and even
`
`field points measurement) x 2 (u, v directions) x 5 (phase steps) is not that much slower than | (both
`
`odd and even field points in one measurement) x 2 (u, v directions) x 8 (phase steps). In embodiments,
`
`the numberof phase steps used maybe e.g. 4, 5, 6, 7, 8 or 9.
`
`

`

`2020P0021 LEP
`
`34
`
`regions or the second patterned regions in adjacent sets maybe different in adjacentsets, i.e. either the
`
`first or the second patterned regions may have pitches that are the samefor adjacent gratings.
`
`It may
`
`be preferable for the pitches of the second patterned regions in adjacent sets to be different rather than
`
`the pitches of the first patterned regions in adjacentsets to be different.
`
`wi
`[000152]=Tn embodiments, the pitches of alternating patterned regions may be the same. That is, the
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`pitch of every other diffraction grating may be the same. For example, the pitches of each of the
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`additional diffraction gratings 20a-20b may be the same and the pitches of each ofthe diffraction
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`gratings 19a-19c may be the same (with the pitch of the adjacent diffraction gratings still being
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`different). This may simply require calculations to disentangle the signals. However, in embodiments,
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`10
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`the pitches of the alternating patterned regions may be different as well as the adjacent patterned regions
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`being different.
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`[000153]
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`Figure 14 showsa spatial iniensily plot of measurements taken by the measurement system
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`11 with an example thirteen field points being measured. The x andy axes showthespatial position in
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`bits with the intensity bar on the right measuring up to 1000. It may be seen that the intensity is highest
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`in the centre of the field points but that there is some overlap between field points. The field points
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`have enough separation to allowthirteen field points onto the sensor apparatus 22 without saturating
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`the radiation detector 24 (e.g. a camera). The thirteen field points may require thirteen sets of first and
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`second patterned regions.
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`[000154]
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`Figure 15 showsthe different phase curves measured on different positions on the camera.
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`More particularly, Figure 15 shows a front graph of phase (pi) for the camera and a rear graph indicating
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`the spatial position for the camera for the field points. For the back graph, the x and y axes again show
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`the spatial position in bits.
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`[000155]
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`In this embodiment, the pitches of the additional patterned regions 16a-16b are three times
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`less than that of patterned regions 15a-15c and the pitches ofthe additional diffraction gratings 20a-20b
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`are three times less than that of the diffraction gratings 19a-19c. That is, the pitches of the gratings in
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`the even field point locations are three times less than the pitches in the odd field point locations. For
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`clarity, Figure 15 only showsthree of the thirteen field points, but it will appreciated that the remaining
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`ten could be shownsimilarly. The even and odd ficld points may more gencrally be considered to be
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`adjacent field points.
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`[000156] As can be seen from the position and intensity graph in Figure 15, there are overlapping
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`regions between the detected radiation for adjacent field points. Each field point may have a plurality
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`of diffraction beams which interfere (i.e. diffraction beams with different orders). Some of these
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`diffraction beams may overlap as shown.
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`[000157]
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`In the phase and intensity graph of Figure 15, the fit of the measurements taken by the
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`measurement system 11 is shown (i.e. the measured line) indicating intensity peaks for the field points
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`locations. The phase and corresponding intensity can be fitted to distinguish between the signals from
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`the odd field points and the signals from the even field points. Lines may be fitted to fit A points and
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`2020P0021 LEP
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`at even field point locations, e.g. at positions 2, 4, 6, 8, 10 and 12 in a thirteen field point line. However,
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`due to the diffraction gratings being closer together, pixels on the radiation detector 24 may receive
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`signals from more than one diffraction grating (i.e. more than one measurement beam.) Thus, in the
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`embodiment of Figures 13A and 13B, the interference patterns formedon the radiation detector 24 may
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`overlap to a certain extent. This is not specifically shown in Figure 12 for clarity reasons.
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`[000147]
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`In orderto distinguish the signals from the pixels which have overlapping radiation incident
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`thereon from more than one measurement beam from adjacent diffraction gratings,
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`the adjacent
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`diffraction gratings have different pitches. This introduces encoding for the information from adjacent
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`diffraction gratings.
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`[000148]
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`In the embodiment of Figures 13A and 13B thepitches of the additional patterned regions
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`16a-16b are shown being two times that of patterned regions 15a-15c. Thatis, the additional patterned
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`regions 16a-16b are shown having approximately two times the numberof grating lines as that of the
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`patterned regions 15a-15c.
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`In addition, the pitches of the additional diffraction gratings 20a-20b are
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`shown being two times that of the diffraction gratings 19a-19c. That is, the additional diffraction
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`gratings 20a-20b are shown having approximately two times the numher of checkerboard boxes as that
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`of the diffraction gratings 19a-19c. However, this is just an example, and in other embodiments, the
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`pitches of the additional patterned regions 16a-16b maybe different multiples of the patterned regions
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`15a-15c and the pitches of the additional diffraction gratings 20a-20b maybe different multiples of the
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`diffraction gratings 19a-19c, e.g. 3 times larger.
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`[000149]
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`For example, the pitches of the of the additional patterned regions 16a-16b may be non-
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`integer multiples of the patterned regions 15a-15c¢ and the pitches of the additional diffraction gratings
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`20a-20b may be non-integer multiples of the diffraction gratings 19a-19c. Software (e.g. algorithms)
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`may be programmedto disentangle the signals from adjacent gratings without requiring them to have
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`pitches ofinteger multiples. Furthermore, having non-integer multiples may provide more freedom to
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`choose the particular difference in pitch,
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`i.e.
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`there may be more choice to obtain the desired
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`performance.
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`It may be preferable that the pitches of adjacent diffraction gratings (e.g. additional
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`diffraction grating 20a and diffraction grating 19a) are not even numberinteger multiples as, since the
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`gratings may have a 50% duty cycle (i.c. checkerboard design), then even diffraction orders, c.g. factor
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`2,4, 6 etc. may be difficult, or not possible, to distinguish between adjacent gratings.
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`[000150]
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`In embodiments, the first and second patterned regions only extend in the x-direction(i.e.
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`a single direction) so they are 1D but, in other embodiments, the first and second patterned regions may
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`extend in both the x-direction and the y-direction (i.e. two orthogonal directions) so they are 2D. In the
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`2D case, adjacent patterned regions may have different pitches in both directions.
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`[000151]
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`In embodiments, the pitches of the first patterned regions (patterned regions 15a-15c and
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`additional patterned regions 16a-16b) may be different in adjacent sets and the pitches of the second
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`patterned regions (diffraction gratings 19a-19c¢ and additional diffraction gratings 20a-20b) may be
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`different in adjacent sets. However, in other embodiments, the pitches of either the first patterned
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`2020P0021 LEP
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`may control an adjusting means PA’ for adjusting components of the projection system PS’. For
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`example, the adjusting means PA’ may adjust optical elements of the projection system PS’ so as to
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`correct for aberrations which are caused by the projection system PS’ and which are determined by the
`controller CN’.
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`[600143] Determining aberrations (which may be caused by the projection system PS or by
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`placement errors of the patterning device MA or the substrate W) may comprise fitting the
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`measurements which are made by the sensor apparatus 22 to Zernike polynomials in order to obtain
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`Zernike coefficients. Different Zernike coefficients may provide information about different forms of
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`aberration which are caused by the projection system PS’. Zernike coefficients may be determined
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`independently at different positions in the x and/or the y-directions. For example, in the embodiment
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`which is shown in Figures 12, 13A and 13B, Zernike coefficients may be determined for each
`measurement beam 17a-17c and for each additional measurement beam 18a-18b.
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`[000144]
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`In this embodiment, the measurement patterning device MA’’ comprises five patterned
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`regions I5a-15c, 16a-16b, and the sensor apparatus 22 comprises five detector regions 25a-25c, 26a-
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`26b and five measurement beams 17a-17c, 18a-18b are formed. This allows the Zernike coefficients
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`to be determined at more positions (i.e. more field points) than in the example of Figures 2, 3A and 3B.
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`In some embodiments the measurement patterning device MA’’ may comprise more than five patterned
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`regions, the sensor apparatus 22 may comprise more than five detector regions and more than five
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`measurement beams may be formed.
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`In some embodiments the patterned regions and the detector
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`regions may be distributed at different positions in both the x and y-directions. This may allow the
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`Zernike coefficients to be determined at positions which are separated in both the x and the y-directions.
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`[000145]
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`If more field points are measured,
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`then there is more potential for correction of the
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`aberrations. For example, by inserting an additional lens to manipulate the wavefront. This may result
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`in more degrees of freedom and thus a mo