(12) INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT)
`
`(ARYA AUAA
`
`(10) International Publication Number
`WO 2019/149467 Al
`
`= =
`
`WIPO! PCT
`
`(19) World Intellectual Property
`Organization
`International Bureau
`
`(43) International Publication Date
`08 August 2019 (08.08.2019)
`
`(6)
`
`International Patent Classification:
`GOIM 11/02 (2006.01)
`GO3F 7/20 (2006.01)
`G02B 5/18 (2006.01)
`G02B 5/00 (2006.01)
`G02B 27/42 (2006.01)
`
`(74)
`
`(31)
`
`(21)
`
`International Application Number:
`
`PCT/EP2019/050132
`
`(22)
`
`International Filing Date:
`
`(25)
`
`Filing Language:
`
`Publication Language:
`
`04 January 2019 (04.01.2019)
`
`English
`
`English
`
`(26)
`
`(30)
`
`(71)
`
`(72)
`
`Priority Data:
`18154475.0
`
`31 January 2018 (31.01.2018)
`
`EP
`
`(84)
`
`Applicant: ASML NETHERLANDS B.V.[NL/NL]; P.O.
`Box 324, 5500 AH Veldhoven (NL).
`
`Inventors: DE GROOT,Pieter, Cristiaan; P.O. Box 324,
`5500 AH Veldhoven (NL). BASELMANS, Johannes, Ja-
`cobus, Matheus; P.O. Box 324, 5500 AH Veldhoven (NL).
`CHONG,Derick, Yun, Chek, P.O. Box 324, 5500 AH
`Veldhoven (NL). CHOWDHURY,Yassin; P.O. Box 324,
`5500 AH Veldhoven (NL).
`
`(54) Title: TWO-DIMENSIONAL DIFFRACTION GRATING
`
`Agent: SLENDERS,Peter; P.O. Box 324, 5500 AH Veld-
`hoven (NL).
`
`Designated States (unless otherwise indicated, for every
`kind of national protection available): AE, AG, AL, AM,
`AO, AT, AU, AZ, BA, BB, BG, BH, BN, BR, BW,BY, BZ,
`CA, CH, CL, CN, CO, CR, CU, CZ, DE, DJ, DK, DM, DO,
`DZ, EC, EE, EG, ES, FI, GB, GD, GE, GH, GM, GT, HN,
`HR, HU, ID, IL, IN,IR, IS, JO, JP, KE, KG, KH, KN, KP,
`KR, KW, KZ, LA, LC, LK, LR, LS, LU, LY, MA, MD, ME,
`MG, MK, MN, MW, MX, MY, MZ, NA, NG, NI NO, NZ,
`OM,PA, PE, PG, PH, PL, PT, QA, RO, RS, RU, RW, SA,
`SC, SD, SE, SG, SK, SL, SM, ST, SV, SY, TH, TJ, TM, TN,
`TR, TT, TZ, UA, UG, US, UZ, VC, VN, ZA, ZM, ZW.
`
`Designated States (unless otherwise indicated, for every
`kind of regional protection available); ARIPO (BW, GH,
`GM,KE, LR, LS, MW, MZ, NA, RW,SD, SL, ST, SZ, TZ,
`UG, ZM, ZW), Eurasian (AM, AZ, BY, KG, KZ, RU, TJ,
`TM), European (AL, AT, BE, BG, CH, CY, CZ, DE, DK,
`EE, ES, FI, FR, GB, GR, HR, HU,IE, IS, IT, LT, LU, LV,
`MC, MK, MT, NL, NO, PL, PT, RO, RS, SE, ST, SK, SM,
`TR), OAPI (BF, BJ, CF, CG, CI, CM, GA, GN, GQ, GW,
`KM,ML, MR,NE, SN, TD, TG).
`
`FiG.4
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`[Continued on nextpage]
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`
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`wo2019/149467AdITNTRITTTAAA
`
`(57) Abstract: A two-dimensional diffraction grating for a phase-stepping measurement
`system for determining an aberration map for a projection system comprises a substrate
`provided with a square array of through- apertures, wherein the diffractiongrating is self-
`supporting. It will be appreciated that for a substrate provided with a square array of
`through-apertures to be self-supporting at least some substrate material is provided be-
`tween eachthrough-aperture and the adjacent throughapertures. A method of designing a
`two-dimensional diffraction grating for a phase-stepping measurement system for deter-
`mining an aberration map for a projection system comprises: selecting a general geometry
`for the iwo-dimensional diffraction grating, the general geometry having at least one pa-
`rameter; and selecting values for the least one parameterthat result in a grating efficiency
`mapfor the two-dimensional diffraction grating so as to control the contributionstoafirst
`harmonic of a phase stepping signal.
`
`30 ~,
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`

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`WO 2019/149467 AQF [IMMLTIUITTNN TAIT LTT TACCAT ATAAAA
`
`Published:
`
`—_with international search report (Art. 21(3))
`— before the expiration of the time limit for amending the
`claims and to be republished in the event of receipt of
`amendments (Rule 48. 2(h))
`
`

`

`WO 2019/149467
`
`PCT/EP2019/050132
`
`Two-dimensional diffraction grating
`
`
`CROSS-REFERENCE TO RELATED APPLICATIONS
`
`[0001]
`
`This application claims priority of EP application 18154475.0 which wasfiled on January
`
`31, 2018 and which is incorporated herein in its entirety by reference.
`
`TIELD
`
`[0002]
`
`The present invention relates to a two-dimensionaldiffraction grating for a phase-stepping
`
`measurement system for determining an aberration map for a projection system and to methods for
`
`designing such a two-dimensional diffraction grating.
`
`In particular, the present invention relates to a
`
`two-dimensional diffraction grating for a shearing phase-stepping interferometric measurement system.
`
`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 whichuses, 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 cxample a diffraction limited image
`
`of the patterning device).
`
`[0006]
`
`It may be desirable to provide methods and apparatus for accurately determining such
`
`aberrations caused by a projection system such that these aberrations can be better controlled.
`
`SUMMARY
`
`[0007]
`
`According to a first aspect of the invention there is provided a two-dimensional diffraction
`
`grating for a phase-stepping measurement system for determining an aberration map for a projection
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`PCT/EP2019/050132
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`system, the diffraction grating comprising a substrate provided with a square array of through-apertures,
`
`wherein the diffraction grating is self-supporting.
`
`[0008]
`
`It will be appreciated that for a substrate provided with a square array of through-apertures
`
`to be self-supporting at least some substrate material is provided between each through-aperture and the
`
`adjacent through apertures.
`
`[0009]
`
`Since the two-dimensional diffraction grating is self-supporting it does not need, for
`
`example a transmissive supporting layer. Therefore the first aspect of the invention is particularly
`
`beneficial for use in a phase-stepping measurement system for determining an aberration map for a
`
`projection system that uscs EUV radiation since the use of such a transmissive supporting layer would
`
`significanUy reduce the amount of EUV radiationthat is transmitted by the two-dimensional diffraction
`
`grating.
`
`[00010]
`
`The substrate may comprise: a support layer; and a radiation absorbing layer, and the
`
`through-apertures may extend through both the support layer and the radiation absorbing layer.
`
`[00011]
`
`The support layer may, for example, be formed from SiN. The radiation absorbing layer
`
`may, for example, be formed from a metal such as, for example, chromium (Cr), nickel (Ni) or cobalt
`
`(Co).
`
`[00012]
`
`The geometry of the two-dimensional diffraction grating may be arranged to result in a
`
`grating efficiency map that reduces the number of contributions above a threshold to a harmonic of a
`
`phase stepping signal assuming that the two-dimensional diffraction grating will be used with a first
`
`patterned region that comprises a one-dimensional diffraction grating with a 50% duty cycle.
`
`[00013]
`
`The harmonic maybe the first harmonic of the phase stepping signal.
`
`[0001 4)
`
`The geometry of the two-dimensional diffraction grating may be arranged to result in a
`
`grating efficiency map that reduces the number of contributions above a threshold to a harmonic of a
`
`phase stepping signal assuming that the two-dimensional diffraction grating will be used with a first
`
`patterned region that comprises a two-dimensional checkerboard diffraction grating with a 50% duty
`
`cycle.
`
`[00015]
`
`The harmonic maybe the first harmonic of the phase stepping signal.
`
`[00016]
`
`It will be appreciated that a phase-stepping measurement system (or a phase-stepping
`
`lateral shearing interferometric measurement system) for determining an aberration map for a projection
`
`system generally comprises a first grating, or patterned region, disposed in an object plane of the
`
`projection system and a scnsor apparatus comprising a sccond grating or patterned region disposed in
`
`an image plane of the projection system. Diffraction beams generated by the first grating may be
`
`referred to as first diffraction beams and diffraction beams generated by the second grating may be
`
`referred to as second diffraction beams.
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`[00017]
`
`The pitches of the first and second patterned regions are matched in a shearing direction,
`
`taking into account any reduction factor applied by the projection system, such that the pitch of the
`
`second patterned region (which may be in accordance with the first aspect) in said shearing direction is
`
`an integer multiple of the pitch of the first patterned region in said shearing direction or, alternatively,
`
`the pitch of the first patterned regionin said shearing direction should be an integer multiple of the pitch
`
`of the second patterned region in said shearing direction.
`
`[00018]
`
`Atleast one of the first and second patterned regions are movedin a shearing direction such
`
`that an intensity of radiation received by each part of the radiation detector varies as a function of the
`
`movement in the shearing direction so as to form an oscillating signal (also knownas a phasc stepping
`
`10
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`signal).
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`
`[00019]
`
`For example, the first harmonic of such an oscillating phase-stepping signal only depends
`
`on contributions that arise from the interference betweenspatially coherent diffraction beams (of the
`
`two-dimensional diffraction grating) that originate from diffraction beamsof the first patterned region
`
`that differ in order by +1.
`
`[00020]
`The geometry of the two-dimensional diffraction grating may be arranged to result in a
`grating efficiency map that suppresses grating efficiencies of the (n, m)"diffraction orders whereeither
`n or mis a non-zero even number.
`
`[00021]
`
`Such a grating geometry is suitable for use with a first patterned region with the shearing
`
`and non-shearing directions defined by said first patterned region being disposed at 45° to a unit cell of
`
`the two-dimensionalgrating, the first patterned region having a pitch (taking into accountany reduction
`
`factor applied by the projection system) that is half that of a pitch of the two-dimensional diffraction
`
`grating in said shearing direction (which may be referred to as a pseudo-pitch of the two-dimensional
`
`grating) so as to limit the numberof significant contributions to the first harmonic the oscillating phase-
`
`stepping signal.
`
`[00022]
`
`Again assuming a first patterned region comprising a one-dimensional diffraction grating
`
`with a 50% duty cycle, the interference strengths for all pairs of second diffraction beams that contribute
`
`to the first harmonic of the oscillating phase-stepping signal can be determined by overlaying a second
`
`copy of the scattering efficiency plot for the two-dimensional diffraction grating weighted by the
`
`scattering efficiency for the +1* order diffraction beamsofthefirst patterned region with the scattering
`
`efficiency plot for the second patterned region. Again, this copy is shifted in the shearing direction by
`
`1 diffraction order of the first diffraction grating, which is equal to 2 pscudo-diffraction orders (in the
`
`shearing direction, the pseudo-diffraction orders being defined using the pseudo pitch) of the two-
`
`dimensionaldiffraction grating.
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`WO 2019/149467
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`PCT/EP2019/050132
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`[00023]
`
`In the limit that the grating efficiencies of the (n, m)" diffraction orders are zero where
`
`either n or m is a non-zero even number there are only four interference contributions to the first
`
`harmonicof the oscillating phase-stepping signal.
`
`[00024]
`
`The through-apertures may be are square apertures having a length that is half the distance
`
`betweenthe centres of adjacent (hrough-apertures, the sides of the square apertures being parallel to the
`
`axes of the square array of through-apertures.
`
`This grating geometry may be referred to as a Gingham pattern. Such a geometry results
`[00025]
`in the grating efficiencies of the (n, m)" diffraction orders being zero where either n or m is a non-zero
`
`even number. When the unit ccll of such a two-dimensional grating is disposed at 45° to a shearing
`
`direction defined by a first patterned region, the first patterned region having a pitch (taking into account
`
`any reduction factor applied by the projection system) thatis half that of a pitch of the two-dimensional
`
`diffraction grating in said shearing direction, there are only four interference contributions to the first
`
`harmonicof the oscillating phase-stepping signal.
`
`[00026]
`The geometry of the two-dimensional diffraction grating may be arranged to result in a
`grating efficiency map that suppresses grating efficiencies of the (n, m)" diffraction orders wherein
`n+m is an even number except the (0, 0)diffraction order.
`
`A two-dimensional diffraction grating that comprises a checkerboard grating has a
`[00027]
`diffraction efficiency pattern wherein the grating efficiency of the (n, m)"diffraction orders are zero
`
`when n+m is an even number. Whenused with a first patterned region that comprises a one-dimensional
`
`diffraction grating with a 50% duty cycle, this results in a particularly advantageous phase-stepping
`
`measurementsystem wherein there are only two contributionsto the first harmonic of the phase stepping
`
`signal, the two contributions having the sameinterference strengths.
`
`[00028]
`
`It will be appreciated that, due to mechanical and thermal considerations,
`
`it may be
`
`desirable to provide an alternative general geometry for the two-dimensional diffraction grating.
`
`However, by selecting values for the least one parameter that minimise grating efficiencies for one or
`more(n, m)"diffraction orders wherein n+m is an even number, the grating efficiency of one or more
`
`diffraction orders that would be zero for a checkerboard grating are minimised.
`
`[00029]
`
`The through-apertures may be generally octagonal, being formed from a square that is
`
`orientated at 45° to the axes of the square array of through-apertures and having a diagonal dimension
`
`that matches a distance between the centres of adjacent through-apertures, each of the four corners of
`
`the square having been truncated so as to form a generally rectangular connecting portion of the
`
`substrate between each pair of adjacent through apertures.
`
`[00030]
`
`This provides an arrangement that is similar to a checkerboard grating but wherein
`
`connecting portions or side-bars are provided to ensure that the grating is self-supporting.
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`[00031]
`
`It will be appreciated that the dimensions of such connecting portions that are required so
`
`as to ensure that the grating is self-supporting may be dependent on the thickness of the substrate.
`
`[00032]
`
`A width of the generally rectangular connecting portion of the substrate between each pair
`
`of adjacent through apertures may be approximately 10% of the distance between the centres of adjacent
`
`through-apertures.
`
`[00033]
`
`For example, the width of the generally rectangular connecting portion of the substrate
`
`between each pair of adjacent through apertures may be between 5% and 15% of the distance between
`
`the centres of adjacent through-apertures, for example, between 8% and 12% of the distance between
`
`the centres of adjacent through-aperturcs.
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`[00034]
`
`The geometry of the two-dimensional diffraction grating may be arranged to result in a
`
`grating efficiency map that suppresses a grating efficiency of one or more diffraction orders, the one or
`more diffraction orders being the (n, m)" diffraction orders wherein nmis an even number.
`
`[00035]
`
`The geometry of the two-dimensional diffraction grating may be arranged to suppress the
`
`(+2, 0) and (0,42) diffraction orders. For example, the through apertures in the square array may be
`
`circular and a ratio of the radius of the circular apertures to the distance between the centres of adjacent
`
`apertures may be approximately 0.3.
`
`[00036]
`
`The geometry of the two-dimensional diffraction grating may be arranged to suppress the
`
`(+1, 41) diffraction orders. For example, the through apertures in the square array may be circular and
`
`a ratio of the radius of the circular apertures to the distance between the centres of adjacent apertures
`
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`may be approximately 0.43.
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`
`[00037]
`
`According to a second aspect of the invention there is provided a method of designing a
`
`two-dimensional diffraction grating for a phase-stepping measurement system for determining an
`
`aberration map for a projection system, the method comprising: selecting a general geometry for the
`
`two-dimensional diffraction grating, the general geometry having at least one parameter; and selecting
`
`values for the least one parameter that result in a grating efficiency map for the two-dimensional
`
`diffraction grating so as to control the contributions to a harmonic of a phase stepping signal.
`
`[00038]
`
`The method according to the second aspect of the invention allows the geometry of the
`
`two-dimensional diffraction grating to be varied so as to control the contributions to the harmonicof a
`
`phase stepping signal. The harmonic may be the first harmonic of the phase stepping signal. For
`
`example, for a given general geometry,
`
`it may be desirable to generally reduce the number of
`
`contributions to a harmonic (for cxample the first harmonic) of the phase stepping signal. Additionally
`
`or alternatively, it may be desirable to enhance certain contributions to a harmonic (for example the
`
`first harmonic) of a phase stepping signal and/or to suppress certain contributions to the first harmonic
`
`of a phase stepping signal.
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`WO 2019/149467
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`PCT/EP2019/050132
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`[00039]
`
`It will be appreciated that a phase-stepping measurement system for determining an
`
`aberration mapfor a projection system generally comprisesa first grating, or patterned region, disposed
`
`in an object plane of the projection system and a sensor apparatus comprising a second grating or
`
`patterned region disposed in an image plane of the projection system. Diffraction beams generated by
`
`the first grating may be referred to as first diffraction beams, and may be angularly separated in a
`
`shearing direction, and diffraction beams generated by the second grating may be referred to as second
`
`diffraction beams.
`
`[00040]
`
`Thepitches of the first and second patterned regions are matched, taking into account any
`
`reduction factor applicd by the projection system, such that the pitch of the second patterned region
`
`(which may be designed using a method according to the second aspect) in the shearing directionis an
`
`integer multiple of the pitch of the first patterned region in the shearing direction or, alternatively, the
`
`pitch of the first patterned region in the shearing direction should be an integer multiple of the pitch of
`
`the second patterned region in the shearing direction.
`
`[00041]
`
`Atleast one of the first and second patterned regions are movedin a shearing direction such
`
`that an intensity of radiation received by each part of the radiation detector varies as a function of the
`
`movement in the shearing direction so as to form an oscillating signal (also knownas a phase stepping
`
`signal).
`
`[00042]
`
`For example, the first harmonic of such an oscillating phase-stepping signal only depends
`
`on contributions that arise from the interference between spatially coherent diffraction beams (of the
`
`two-dimensional diffraction grating) that originate from diffraction beamsof the first patterned region
`
`that differ in order by +1. Therefore, it will be appreciated that the method according to the second
`
`aspect will, in general, take into account the geometry ofa first patterned region.
`
`[00043]
`
`The selection of values for the at least one parameterthat result in a grating efficiency map
`
`for the two-dimensional diffraction grating so as to control the contributions to a harmonic of a phase
`
`stepping signal may assume that the two-dimensional diffraction grating will be used with a first
`
`patterned region that comprises a one-dimensional diffraction grating with a 50% duty cycle.
`
`[00044] With sucha first patterned region, the efficiencies of the even diffraction orders (except the
`0"diffraction order) are zero. Therefore, the only twopairsoffirst diffraction beamsthat differ in order
`
`by +1 (and therefore contribute to the first harmonic of such an oscillating phase-stepping signal) are
`the 0order beam with either the +1“ order beams. Furthermore, with this geometry for the first
`
`patterned region, the scattcring cfficicncics are symmetric such that the cfficicncics of the +1* order
`
`diffraction beams are both the same. Therefore, the interference strengths y; for all pairs of second
`
`diffraction beams that contribute to the first harmonic of the oscillating phase-stepping signal can be
`
`determined as follows. A second copy of the scattering efficiency plot for the second patterned region
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`weighted by the scattering efficiency for the +1* order diffraction beamsof the first patterned region is
`
`overlaid with the scattering efficiency plot for the second patterned region but shifted in the shearing
`
`direction by 1 diffraction order (of the first diffraction grating). The product of the scattering
`
`efficiencies of these two overlaid scattering efficiencies plots is then determined.
`
`[00045]
`
`The method according to the second aspect of the invention may involve the selection of
`
`values for the least one parameter that result in a grating efficiency map for the two-dimensional
`
`diffraction grating so as to control these interference strengths. For example, the values for the least
`
`one parameter maybe selected to reduce the numberof interference strengths that are above a threshold
`
`valuc; to enhance (i.c. inercasc) certain interference strengths; and/or to suppress (i.c. reduce) certain
`
`10
`
`interference strengths.
`
`[00046]
`
`Alternatively, the selection of values for the least one parameter that result in a grating
`
`efficiency map for the two-dimensional diffraction grating so as to control the contributions to a
`
`harmonic of a phase stepping signal may assumethat the two-dimensional diffraction grating will be
`
`used with a first patterned region that comprises a two-dimensional checkerboard diffraction grating
`
`15
`
`with a 50% duty cycle.
`
`[00047]
`
`The selection of the general geometry for the two-dimensional diffraction grating may take
`
`into account mechanical and thermal considerations.
`
`[00048]
`
`The general geometry for the two-dimensional diffraction grating that is selected may be
`
`chosen suchthat the two-dimensional diffraction grating comprises a substrate provided with a square
`
`array of through-apertures and wherein the two-dimensional diffraction grating is self-supporting.
`
`[00049]
`
`Since the two-dimensional diffraction grating is self-supporting it does not need, for
`
`example a transmissive supporting layer. Such an arrangement may be beneficial for use in a phase-
`
`stepping measurement system for determining an aberration map for a projection system that uses EUV
`
`radiation since the use of such a transmissive supporting layer would reduce the amount of EUV
`
`radiation that is transmitted by the two-dimensionaldiffraction grating.
`
`[00050)
`
`It will be appreciated that for a substrate provided with a square array of through-apertures
`
`to be self-supporting at least some substrate material is provided between each through-aperture and the
`
`adjacent through apertures.
`
`[00051]
`
`In addition, the general geometry for the two-dimensionaldiffraction grating that is selected
`
`may be chosen such that the amount of substrate material provided between each through-aperture and
`
`the adjacent through apcrturcsis sufficiently large to allow a heat load expected during usc to be drained
`
`without damaging the two-dimensional diffraction grating.
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`[00052]
`
`The general geometry for the two-dimensional diffraction grating that is selected may be a
`
`square array of circular apertures and the at least one parameter comprises a ratio of the radius of the
`
`circular apertures to the distance between the centres of adjacent apertures.
`
`[00053]
`
`The step of selecting values for the least one parameter may involve selecting values for
`
`the least one parameter that minimises a grating efficiency of one or more diffraction orders, the one or
`morediffraction orders being the (n, m)" diffraction orders wherein n+m is an even number.
`
`A two-dimensional diffraction grating that comprises a checkerboard grating has a
`[00054]
`diffraction efficiency pattern wherein the grating efficiency of the (n, m)"diffraction orders are zero
`
`when n+m is an even number. When uscd withafirst patterned region that compriscs a one-dimensional
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`diffraction grating with a 50% duty cycle, this results in a particularly advantageous a phase-slepping
`
`measurementsystem wherein there are only two contributionsto the first harmonic of the phase stepping
`
`signal, the two contributions having the same interference strengths.
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`[00055]
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`It will be appreciated that, due to mechanical and thermal considerations,
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`it may be
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`desirable to provide an alternative general geometry for the two-dimensional diffraction grating.
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`However, by selecting values for the least one parameter that minimise grating efficiencies for one or
`more (n, m)" diffraction orders wherein n+m is an even number, the grating efficiency of one or more
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`diffraction orders that would be zero for a checkerboard grating are minimised.
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`[00056]
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`The step of selecting values for the least one parameter may involve selecting values for
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`the least one parameter that minimises a grating efficiency of the (+2, 0) and (0,42) diffraction orders.
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`For example, the step of selecting values for the least one parameter may involve selecting a value of
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`the ratio of the radius of the circular apertures to the distance between the centres of adjacent apertures
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`is approximately 0.3.
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`[00057]
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`The step of selecting values for the least one parameter may involve selecting values for
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`the least one parameter that minimises a grating efficiency of the (41, +1) diffraction orders. For
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`example, the step of selecting values for the least one parameter may involve selecting a value of the
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`ratio of the radius of the circular apertures to the distance between the centres of adjacent aperturesis
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`approximately 0.43.
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`[00058]
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`According to a third aspect of the invention there is provided a two-dimensionaldiffraction
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`grating designed according to the method the second aspect of the invention.
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`[00059]
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`According to a fourth aspect of the invention there is provided a measurement system for
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`determining an aberration map for a projection system,
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`the measurement system comprising: a
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`patterning device; an illumination system arranged to illuminate the patterning device with radiation,
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`the patterning device comprising a first patterned region arranged to receive a radiation beam and to
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`form a plurality of first diffraction beams, the first diffraction beams being separated in a shearing
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`direction; a sensor apparatus comprising a second patterned region,
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`the second patterned region
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`comprising a two-dimensional diffraction grating according to the first aspect of the invention or the
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`third aspect of the invention, and a radiation detector; the projection system being configured to project
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`the first diffraction beams onto the sensor apparatus, the second patterned region being arranged to
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`receive the first diffraction beams from the projection system and to form a plurality of second
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`diffraction beams from each ofthe first diffraction beams; a positioning apparatus configured to move
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`at least one of the patterning device and the sensor apparatus in the shearing direction; and a controller
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`configured to: control the positioning apparatus so as to moveat least one of the first patterning device
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`and the sensor apparatus in the shearing dircction such that an intensity of radiation reccived by cach
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`part of the radiation detector varies as a function of the movement in the shearing direction so as to
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`form an oscillating signal; determine from the radiation detector a phase of a harmonic ofthe oscillating
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`signal at a plurality of positions on the radiation detector; and determine a set of coefficients that
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`characterize the aberration map of the projection system from the phase of a harmonic ofthe oscillating
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`signal at the plurality of positions on the radiation detector.
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`[00060]
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`The measurement system according to the fourth aspect of the invention is advantageous,
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`particularly for a projection system that uses EUV radiation, because the two-dimensional diffraction
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`grating is self-supporting (and therefore does not require a transmissive supporting layer that would
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`significantly attenuate the EUV radiation) and/or it provides better control the contributions to a
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`harmonic (for example the first harmonic) of a phase stepping signal.
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`[00061]
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`Therefore, the measurement system according to the fourth aspect of the invention can
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`provide a measurement system for determining aberrations for an EUV projection system which can
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`provide better control over contributions to the first harmonic of the phase stepping signal. Tn turn, this
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`can reduce errors in the determined set of coefficients that characterize the aberration map of the
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`projection system. Additionally or alternatively,
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`it may simplify the determination of the set of
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`coefficients that characterize the aberration map of the projection system from the phase of a first
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`harmonic of the oscillating signal at the plurality of positions on the radiation detector.
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`[00062]
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`According to a fifth aspect of the invention there 1s provided a lithographic apparatus
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`comprising the measurement system of the fourth aspect of the invention.
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`BRIEF DESCRIPTION OF THE DRAWINGS
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`[00063]
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`Embodiments of the invention will now be described, by way of cxample only, with
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`reference to the accompanying schematic drawings, in which:
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`Figure | depicts a lithographic system comprising a lithographic apparatus and a radiation
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`source;
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`10
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`-
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`Figure 2 is a schematic illustration of a measurement system according to an embodiment of
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`the invention;
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`-
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`Figures 3A and 3B are schematic illustrations of a patterning device and a sensor apparatus
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`which may form part of the measurement system of Figure 2;
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`-
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`Figure 4 is a schematic illustration of a measurement system according to an embodiment of
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`the invention, the measurement system comprising a first patterned region and a second patterned
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`region, the first patterned region arranged to receive
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`radiation and to formaplurality of first diffraction beams;
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`-
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`Figures 5A to 5C cach showsa different sct of second diffraction beams formed by the sccond
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`patterned region of the measurement system shownin Figure 4, that set of second diffraction beams
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`having been produced by a differentfirst diffraction beam formed bythe first patterned region;
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`-
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`Figure 6A showsthe scattering efficiency for one dimensional diffraction grating with a 50%
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`duty cycle and which may represent the first patterned region of the measurement system shown in
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`Figure 4;
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`-
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`Figure 6B showsthe scattering efficiency for two dimensional diffraction grating of the form
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`of a checkerboard with a 50% duty cycle and which may represent the second patterned region of the
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`measurement system shownin Figure4;
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`-
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`Figure 6C showsaninterference strength map for the measurement system shown in Figure 4
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`when employing the first patterned region shown in Figure 6A and the second patterned region shown
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`in Figure 6B, each of the interference strengths shown representing the second interference beams which
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`contribute to the first harmonic of the oscillating phase-stepping signal and which have a different
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`overlap, at the rad