(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2002/0145717 A1
`Baselmans et al.
`(43) Pub. Date:
`Oct. 10, 2002
`
`US 20020145717A1
`
`(54) LITHOGRAPHIC PROJECTION APPARATUS,
`A GRATING MODULE, A SENSOR MODULE,
`A METHOD OF MEASURING WAVE FRONT
`ABERRATIONS
`
`(76) Inventors: Johannes Jacobus Matheus
`Baselmans, Oirschot (NL); Marco
`Hugo Petrus Moers, Eindhoven (NL);
`Hans Van Der Laan, Veldhoven (NL);
`Robert Wilhelm Willekers, Oirschot
`(NL); Wilhelmus Petrus De Boei,
`Veldhoven (NL); Marcus Adrianus
`Ven De Kerkhof, Helmond (NL)
`
`Correspondence Address:
`PILLSBURY WINTHROP, LLP
`PO. BOX 10500
`MCLEAN, VA 22102 (US)
`
`(21)
`
`Appl. No.:
`
`10/073,119
`
`(22) Filed:
`
`Feb. 12, 2002
`
`(30)
`
`Foreign Application Priority Data
`
`Feb. 13, 2001 (EP) ...................................... .. 013012836
`
`Publication Classi?cation
`
`(51) Int. Cl? ................................................... .. G03B 27/52
`
`(52) US. Cl. ............. ..355/55; 355/53; 355/52; 356/399;
`356/400
`
`(57)
`
`ABSTRACT
`
`A lithographic projection apparatus comprising an illumi
`nation system; a support structure for holding a mask; a
`substrate table for holding a substrate; a projection system
`for projecting a pattern onto a target portion of the substrate;
`and an interferometric measurement system for measuring
`Wave front aberrations of the projection system, character
`iZed in that the interferometric measurement system com
`prises: a grating, featuring a grating pattern in a grating
`plane, said grating being movable into and out of the
`projection beam, such that the grating plane is substantially
`coincident With said object plane; a pinhole, featuring a
`pinhole pattern in a pinhole plane and arranged in a pinhole
`plate, said pinhole being movable into and out of the
`projection beam, such that the pinhole plane is substantially
`coincident With a plane downstream of the projection system
`and optically conjugate to said object plane, and a detector
`With a detector surface substantially coincident With a detec
`tion plane, said detection plane located downstream of the
`pinhole at a location Where a spatial distribution of the
`electric ?eld amplitude of the projection beam is substan
`tially a Fourier transformation of a spatial distribution of the
`electric ?eld amplitude of the projection beam in the pinhole
`plane.
`
`Nikon Exhibit 1007 Page 1
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`Patent Application Publication Oct. 10, 2002 Sheet 1 0f 7
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`US 2002/0145717 A1
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`Fig.1.
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`Nikon Exhibit 1007 Page 2
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`Patent Application Publication Oct. 10, 2002 Sheet 2 0f 7
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`US 2002/0145717 A1
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`Nikon Exhibit 1007 Page 3
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`Patent Application Publication Oct. 10, 2002 Sheet 3 0f 7
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`US 2002/0145717 Al
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`Nikon Exhibit 1007 Page 4
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`Patent Application Publication Oct. 10, 2002 Sheet 4 0f 7
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`US 2002/0145717 A1
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`Nikon Exhibit 1007 Page 5
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`Patent Application Publication Oct. 10, 2002 Sheet 5 0f 7
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`US 2002/0145717 A1
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`Nikon Exhibit 1007 Page 6
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`Patent Application Publication
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`Oct. 10, 2002 Sheet 6 0f 7
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`US 2002/0145717 A1
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`Fig.5A.
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`Nikon Exhibit 1007 Page 7
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`Patent Application Publication Oct. 10, 2002 Sheet 7 0f 7
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`US 2002/0145717 A1
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`Fig.5C. PW32
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`Nikon Exhibit 1007 Page 8
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`

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`US 2002/0145717 A1
`
`Oct. 10, 2002
`
`LITHOGRAPHIC PROJECTION APPARATUS, A
`GRATING MODULE, A SENSOR MODULE, A
`METHOD OF MEASURING WAVE FRONT
`ABERRATIONS
`
`BACKGROUND OF THE INVENTION
`
`from
`priority
`claims
`application
`[0001] This
`EP01301283.6 ?led Feb. 13, 2001, herein incorporated by
`reference.
`
`[0002] 1. Field of the Invention
`[0003] The present invention relates generally to litho
`graphic projection apparatus and more particularly to litho
`graphic projection apparatus including an interferometric
`measurement system.
`
`[0004] 2. Background of the Related Art
`[0005] A typical lithographic projection apparatus
`includes:
`
`[0006] a radiation system for providing a projection
`beam of radiation;
`
`[0007] a support structure for supporting patterning
`structure, the patterning structure serving to pattern
`the projection beam, according to a desired pattern,
`in an object plane traversed by the projection beam;
`[0008]
`0009
`p j
`'
`y
`d
`f
`'d bj
`a ro ect1on s stem oWnstream o sa1 o ect
`plane, for projecting the patterned beam onto a target
`portion of the substrate;
`
`a substrate table for holding a substrate;
`
`[0010] an interferometric measurement system for
`measuring Wave front aberrations of the projection
`system.
`
`[0011] The term “patterning structure” as here employed
`should be broadly interpreted as referring to structure that
`can be used to endoW an incoming radiation beam With a
`patterned cross-section in an object plane traversed by the
`projection beam, corresponding to a pattern that is to be
`created in a target portion of the substrate. Said target
`portion is, through the projection system, optically conjugate
`to the object plane. The projection system has a magni?ca
`tion factor M (generally <1) in relation to said object plane.
`The term “light valve” can also be used in this context.
`Generally, the said pattern Will correspond to a particular
`functional layer in a device being created in the target
`portion, such as an integrated circuit or other device (see
`beloW). Examples of such patterning structure include:
`
`[0012] A mask. The concept of a mask is Well knoWn
`in lithography, and it includes mask types such as
`binary, alternating phase-shift, and attenuated phase
`shift, as Well as various hybrid mask types. Place
`ment of such a mask in the radiation beam causes
`selective transmission (in the case of a transmissive
`mask) or re?ection (in the case of a re?ective mask)
`of the radiation impinging on the mask, according to
`the pattern on the mask. In the case of a mask, the
`support structure Will generally be a mask table,
`Which ensures that the mask can be held at a desired
`position in the incoming radiation beam, and that it
`can be moved relative to the beam if so desired.
`
`[0013] A programmable mirror array. One example
`of such a device is a matrix-addressable surface
`having a viscoelastic control layer and a re?ective
`surface. The basic principle behind such an appara
`tus is that (for example) addressed areas of the
`re?ective surface re?ect incident light as diffracted
`light, Whereas unaddressed areas re?ect incident
`light as undiffracted light. Using an appropriate ?lter,
`the said undiffracted light can be ?ltered out of the
`re?ected beam, leaving only the diffracted light
`behind; in this manner, the beam becomes patterned
`according to the addressing pattern of the matrix
`adressable surface. An alternative embodiment of a
`programmable mirror array employs a matrix
`arrangement of tiny mirrors, each of Which can be
`individually tilted about an axis by applying a suit
`able localiZed electric ?eld, or by employing pieZo
`electric actuation means. Once again, the mirrors are
`matrix-addressable, such that addressed mirrors Will
`re?ect an incoming radiation beam in a different
`direction to unaddressed mirrors; in this manner, the
`re?ected beam is patterned according to the address
`ing pattern of the matrix-adressable mirrors. The
`required matrix addressing can be performed using
`suitable electronic means. In both of the situations
`described hereabove, the patterning structure can
`comprise one or more programmable mirror arrays.
`More information on mirror arrays as here referred to
`can be gleaned, for example, from US. Pat. No.
`5,296,891 and US. Pat. No. 5,523,193, and PCT
`patent applications WO 98/38597 and WO
`98/33096, Which are incorporated herein by refer
`ence. In the case of a programmable mirror array, the
`said support structure may be embodied as a frame or
`table, for example, Which may be ?xed or movable
`as required.
`
`[0014] A programmable LCD array. An example of
`such a construction is given in Us. Pat. No. 5,229,
`872, Which is incorporated herein by reference. As
`above, the support structure in this case may be
`embodied as a frame or table, for example, Which
`may be ?xed or movable as required.
`
`[0015] For purposes of simplicity, the rest of this text may,
`at certain locations, speci?cally direct itself to examples
`involving a mask and mask table; hoWever, the general
`principles discussed in such instances should be seen in the
`broader context of the patterning structure as hereabove set
`forth.
`
`[0016] Lithographic projection apparatus can be used, for
`example, in the manufacture of integrated circuits (ICs). In
`such a case, the patterning structure may generate a circuit
`pattern corresponding to an individual layer of the IC, and
`this pattern can be imaged onto a target portion (e.g.
`comprising one or more dies) on a substrate (silicon Wafer)
`that has been coated With a layer of radiation-sensitive
`material (resist). In general, a single Wafer Will contain a
`Whole netWork of adjacent target portions that are succes
`sively irradiated via the projection system, one at a time. In
`current apparatus, employing patterning by a mask on a
`mask table, a distinction can be made betWeen tWo different
`types of machine. In one type of lithographic projection
`apparatus, each target portion is irradiated by exposing the
`entire mask pattern onto the target portion at once; such an
`
`Nikon Exhibit 1007 Page 9
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`

`

`US 2002/0145717 A1
`
`Oct. 10, 2002
`
`apparatus is commonly referred to as a Wafer stepper. In an
`alternative apparatus—commonly referred to as a step-and
`scan apparatus—each target portion is irradiated by progres
`sively scanning the mask pattern under the projection beam
`in a given reference direction (the “scanning” direction)
`While synchronously scanning the substrate table parallel or
`anti-parallel to this direction; since, in general, the projec
`tion system Will have a magni?cation factor M (With M <1)
`the speed V at Which the substrate table is scanned Will be
`a factor M times that at Which the mask table is scanned.
`More information With regard to lithographic devices as here
`described can be gleaned, for example, from US. Pat. No.
`6,046,792, incorporated herein by reference.
`[0017] In a manufacturing process using a lithographic
`projection apparatus, a pattern (eg in a mask) is imaged
`onto a substrate that is at least partially covered by a layer
`of radiation-sensitive material (resist). Prior to this imaging
`step, the substrate may undergo various procedures, such as
`priming, resist coating and a soft bake. After exposure, the
`substrate may be subjected to other procedures, such as a
`post-exposure bake (PEB), development, a hard bake and
`measurement/inspection of the imaged features. This array
`of procedures is used as a basis to pattern an individual layer
`of a device, eg an IC. Such a patterned layer may then
`undergo various processes such as etching, ion-implantation
`(doping), metalliZation, oxidation, chemo-mechanical pol
`ishing, etc., all intended to ?nish off an individual layer. If
`several layers are required, then the Whole procedure, or a
`variant thereof, Will have to be repeated for each neW layer.
`Eventually, an array of devices Will be present on the
`substrate (Wafer). These devices are then separated from one
`another by a technique such as dicing or saWing, Whence the
`individual devices can be mounted on a carrier, connected to
`pins, etc. Further information regarding such processes can
`be obtained, for example, from the book “Microchip Fab
`rication: A Practical Guide to Semiconductor Processing”,
`Third Edition, by Peter van Zant, McGraW Hill Publishing
`Co., 1997, ISBN 0-07-067250-4, incorporated herein by
`reference.
`[0018] For the sake of simplicity, the projection system
`may hereinafter be referred to as the “lens”; hoWever, this
`term should be broadly interpreted as encompassing various
`types of projection system, including refractive optics,
`re?ective optics, and catadioptric systems, for example. The
`radiation system may also include components operating
`according to any of these design types for directing, shaping
`or controlling the projection beam of radiation, and such
`components may also be referred to beloW, collectively or
`singularly, as a “lens”. The position of a second element
`traversed by the projection beam relative to a ?rst element
`traversed by the projection beam Will for simplicity herein
`after be referred to as “downstream” of or “upstream” of said
`?rst element. In this context, the expression “downstream”
`indicates that a displacement from the ?rst element to the
`second element is a displacement along the direction of
`propagation of the projection beam; similarly, “upstream”
`indicates that a displacement from the ?rst element to the
`second element is a displacement opposite to the direction of
`propagation of the projection beam. Further, the lithographic
`apparatus may be of a type having tWo or more substrate
`tables (and/or tWo or more mask tables). In such “multiple
`stage” devices the additional tables may be used in parallel,
`or preparatory steps may be carried out on one or more
`tables While one or more other tables are being used for
`
`exposures. Dual stage lithographic apparatus are described,
`for example, in US. Pat. No. 5,969,441 and WO 98/40791,
`incorporated herein by reference.
`
`[0019] There is a desire to integrate an ever-increasing
`number of electronic components in an IC. To realiZe this it
`is necessary to decrease the siZe of the components and
`therefore to increase the resolution of the projection system,
`so that increasingly smaller details, or line Widths, can be
`projected on a target portion of the substrate. For the
`projection system this means that the projection system and
`the lens elements used in the projection system must comply
`With very stringent quality requirements. Despite the great
`care taken during the manufacturing of lens elements and the
`projection system they both may still suffer from Wave front
`aberrations, such as, for example, displacement, defocus,
`astigmatism, coma and spherical aberration across an image
`?eld projected With the projection system onto a target
`portion of the substrate. Said aberrations are important
`sources of variations of the imaged line Widths occurring
`across the image ?eld. It is important that the imaged line
`Widths at different points Within the image ?eld are constant.
`If the line Width variation is large, the substrate on Which the
`image ?eld is projected may be rejected during a quality
`inspection of the substrate. Using techniques such as phase
`shifting masks, or off-axis illumination, the in?uence of
`Wave front aberrations on the imaged line Widths may
`further increase.
`
`[0020] During manufacture of a lens element it is advan
`tageous to measure the Wave front aberrations of the lens
`element and to use the measured results to tune the aberra
`tions in this element or even to reject this element if the
`quality is not suf?cient. When lens elements are put together
`to form the projection system it may again be necessary to
`measure the Wave front aberrations of the projection system.
`These measurements may be used to adjust the position of
`certain lens elements in the projection system in order to
`minimiZe Wave front aberrations of the projection system.
`
`[0021] After the projection system has been built into a
`lithographic projection apparatus, the Wave front aberrations
`may be measured again. Moreover, since Wave front aber
`rations are variable in time in a projection system, for
`instance, due to deterioration of the lens material or lens
`heating effects local heating of the lens material), it may be
`necessary to measure the aberrations at certain instants in
`time during operation of the apparatus and to adjust certain
`movable lens elements accordingly to minimiZe Wave front
`aberrations. The short time scale, on Which lens-heating
`effects may occur, may require measuring the Wave front
`aberrations frequently.
`
`[0022] The use of an interferometric measurement system
`for in-situ measurement of Wave front aberrations of the
`projection system of a lithographic projection apparatus is
`described in P. Venkataraman, et al., “Aberrations of step
`pers using Phase Shifting Point Diffraction Interferometry”,
`in Optical Microlithography XIII, J. Progler, Editor, Pro
`ceedings of SPIE Vol. 4000, 1245-1249 (2000). A Phase
`Shifting Point Diffraction Interferometry method and a
`corresponding system are disclosed in P. P. Naulleau et al.,
`US. Pat. No. 6,100,978, issued Aug. 8, 2000, incorporated
`herein by reference. The Phase Shifting Point Diffraction
`Interferometry measurement method and corresponding
`measurement system Will be referred to hereinafter as the
`
`Nikon Exhibit 1007 Page 10
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`

`

`US 2002/0145717 A1
`
`Oct. 10, 2002
`
`PSPDI method and PSPDI system, respectively. The dis
`closed PSPDI systems feature the following elements, men
`tioned here in the order Wherein these elements are traversed
`by the projection beam: a ?rst pinhole in an object plane; a
`grating (With a one dimensional periodic structure of lines
`and spaces) betWeen the object plane and the projection
`system, for generating by diffraction a test beam and a
`reference beam; the projection system, and a set of tWo
`pinholes comprising a WindoW pinhole (traversed by the test
`beam) and a reference pinhole (traversed by the reference
`beam, and acting as a spatial ?lter for generating an unab
`errated reference beam) in the plane that is optically conju
`gate to the object plane. The test beam and the reference
`beam generate an interference fringe pattern on a detector
`surface doWnstream of the set of tWo pinholes. This inter
`ference fringe pattern carries information on Wave front
`aberrations. The grating, generally embodied as a grating
`pattern on a plane surface of a carrier substrate, acts as a
`beamsplitter; the grating shall be located doWnstream of said
`object plane such as to provide suf?cient lateral separation
`of the areas traversed by the reference beam and the test
`beam in the plane that is optically conjugate to the object
`plane. Further, the grating is movable in a direction perpen
`dicular to the direction of propagation of the projection
`beam such as to provide “phase shifting” (as explained
`beloW) of the interference fringe pattern With respect to a
`coordinate system associated With the detector surface, as
`needed for measuring aberrations.
`[0023] Said phase shifting of the interference fringe pat
`tern involves shifting the interference fringe pattern With
`respect to said coordinate system. For an explanation of
`“phase shifting” in relation to interferometry see, for
`example, D. Malacara, “Optical Shop Testing”, John Wiley
`& Sons, Inc., NeW York, second edition. Movement of an
`optical element (such as, for example, a grating) to provide
`phase shifting Will be referred to hereinafter as “phase
`stepping”. A?nite movement of an optical element (such as,
`for example, a grating) to provide a ?nite phase shift of said
`interference fringe pattern Will be referred to hereinafter as
`a “phase step”.
`
`[0024] An embodiment of a PSPDI system in a litho
`graphic projection apparatus comprises, besides the support
`structure for supporting patterning structure and the sub
`strate table for holding a substrate, one or more dedicated,
`movable support structures for supporting the grating and/or
`for moving the grating into and out of the projection beam
`and/or for phase stepping the grating. Incorporation of these
`one or more dedicated support structures into the litho
`graphic projection apparatus leads to added mechanical
`complexity and increased costs of manufacturing the litho
`graphic projection apparatus. Further, as explained above, in
`a PSPDI system each individual beam (the test beam and the
`reference beam) impinging on the detector has traversed tWo
`pinholes, one pinhole upstream of the projection system, and
`one pinhole doWnstream of the projection system. This
`circumstance typical for a PSPDI system poses a limitation
`to the amount of radiation that may reach the detector, and
`hence, to the sensitivity of the measurement system.
`
`SUMMARY OF THE INVENTION
`
`[0025] One aspect of embodiments of the present inven
`tion provides a measurement system for measuring the Wave
`front aberrations in a lithographic projection apparatus While
`
`alleviating the problem of incorporating one or more dedi
`cated, movable support structures.
`
`[0026] This and other aspects are achieved according to
`the invention in a lithographic projection apparatus as speci
`?ed in the opening paragraph, characteriZed in that the
`interferometric measurement system comprises
`[0027] a grating, featuring a grating pattern in a
`grating plane, said grating being movable into and
`out of the projection beam, such that the grating
`plane is substantially coincident With said object
`plane;
`[0028] a pinhole, featuring a pinhole pattern in a
`pinhole plane and arranged in a pinhole plate, said
`pinhole being movable into and out of the projection
`beam, such that the pinhole plane is substantially
`coincident With a plane doWnstream of the projection
`system and optically conjugate to said object plane,
`and
`[0029] a detector With a detector surface substantially
`coincident With a detection plane, said detection
`plane located doWnstream of the pinhole at a location
`Where a spatial distribution of the electric ?eld
`amplitude of the projection beam is substantially a
`Fourier Transformation of a spatial distribution of
`the electric ?eld amplitude of the projection beam in
`the pinhole plane.
`[0030] With the measurement system built into the litho
`graphic projection apparatus it is possible to measure in situ
`the Wave front aberration of the projection system. The term
`“grating pattern” in the context of the present invention
`should be interpreted throughout this text and in the claims
`to include any periodic structure. Also the term “pinhole
`pattern” in the context of the present invention should be
`interpreted throughout this text and in the claims to include
`one or more apertures of arbitrary shape such as, for
`example a circular shape, a slit shape, a rectangular shape,
`and a substantially square shape. Upon illumination of the
`grating, an intensity distribution featuring an interference
`fringe pattern of radiation representative of the Wave front
`aberration of the projection system is obtained in said
`detection plane doWnstream of the pinhole. In the absence of
`any Wave front aberration said interference fringe pattern is
`a substantially uniform intensity distribution. In the presence
`of Wave front aberrations said intensity distribution Will be
`non-uniform, and Will generally comprise interference
`fringes. In the context of the present invention, the term
`“interference fringe pattern” besides referring to the com
`mon concept of interference fringes, should also be inter
`preted as referring to a substantially uniform intensity dis
`tribution, the latter intensity distribution being typical for the
`absence of aberrations. Some of the physical principles
`exploited in the present invention are discussed, for
`example, in J. Braat et al., “Improved Ronchi test With
`extended source”, Journal of the Optical Society of America,
`Vol. 16, 131-139 (1999). Said detection plane may, for
`example, be located at a position doWnstream of the pinhole
`Where the “Fraunhofer Diffraction” approximation is appli
`cable to the calculation of the electric ?eld amplitude of
`radiation emerging from the pinhole. At such a location a
`spatial distribution of the electric ?eld amplitude of the
`projection beam is substantially a Fourier Transformation of
`a spatial distribution of the electric ?eld amplitude of the
`projection beam in the pinhole plane.
`
`Nikon Exhibit 1007 Page 11
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`

`

`US 2002/0145717 A1
`
`Oct. 10, 2002
`
`[0031] Phase shifting of the interference fringe pattern
`With respect to a coordinate system associated With the
`detector surface, as needed for measuring aberrations, can be
`provided by phase stepping either the grating or the pinhole.
`
`[0032] In case the patterning structure is a mask and the
`support structure is a mask table for holding the mask, the
`grating can be provided to a grating module that has the
`same outer eXtent as the mask, and the mask table can be
`used for holding the grating module. During measurement of
`the Wave front aberrations the grating module may be held
`at a location Where, during normal use of the projection
`apparatus, a mask is held. One advantage of this scenario is
`that there is no need to provide an additional support
`structure to hold the grating, and/or to move the grating in
`and out of the projection beam, and/or to phase step the
`grating. Another advantage is that, during projection, the
`mass of the grating is not added to the mass of the mask
`table, such that it is easier to accelerate and decelerate the
`mask table. As explained above, in a PSPDI system each
`individual beam (the test beam and/or the reference beam)
`impinging on the detector has traversed tWo pinholes, one
`pinhole upstream of the projection system, and one pinhole
`doWnstream of the projection system. These tWo pinholes
`are each embodied as a single pinhole aperture, and in
`contrast to the present invention do not feature a pinhole
`pattern that may comprise a plurality of apertures. This
`circumstance, typical for a PSPDI system, poses a limitation
`to the amount of radiation that may reach the detector.
`Another advantage of the present invention over the use of
`a PSPDI system is that said limitation can be relaXed by the
`use of pinhole patterns comprising a plurality of apertures,
`leading to improved sensitivity.
`[0033] Instead of providing the grating to a grating module
`that has the same outer eXtent as the mask, one can,
`alternatively, provide the grating to the support structure at
`a location aWay from the location Where the patterning
`structure is supported. Whenever it is necessary to measure
`the Wave front aberrations of the projection system the
`grating can be easily moved into the projection beam to
`perform a Wave front aberration measurement. After ?nish
`ing the measurement the patterning structure is moved into
`the projection beam and the apparatus can continue project
`ing the patterned beam onto target portions of the substrate.
`This method of intermittently measuring Wave front aber
`rations during operation of the lithographic projection appa
`ratus is time saving, and enables, for eXample, a frequent
`measuring of Wave front aberrations needed to compensate
`for short-time-scale lens-heating effects. In an alternative
`scenario, the patterning structure may also be used to pattern
`the projection beam With a grating pattern in its cross
`section. This is advantageous because no additional grating
`has to be provided to the apparatus.
`[0034] The detector for detecting the radiation traversing
`the pinhole may, for eXample, be provided to the substrate
`table. Said pinhole plate may also, for eXample, be provided
`to the substrate table. One could also provide the pinhole
`plate and the detector to a sensor module Which, during
`measurement of Wave front aberrations, may be provided to
`the substrate table at a location Where, during the projection
`of the patterned beam, the substrate is held. After measuring
`Wave front aberrations of the projection system the sensor
`module may then be replaced by the substrate, such that the
`projection of the patterned beam onto the target portions of
`
`the substrate may continue. An advantage is that When the
`sensor and the pinhole plate are built into a sensor module,
`the mass of the sensor module Will not add to the mass of the
`substrate table during normal operation of the lithographic
`projection apparatus, and a further advantage is that the
`sensor doesn’t occupy any space in the substrate table.
`
`[0035] According to a further aspect of the present inven
`tion, there is provided a method of measuring Wave front
`aberrations of a projection system in a lithographic projec
`tion apparatus comprising:
`
`[0036] a radiation system for providing a projection
`beam of radiation;
`[0037] a support structure for supporting patterning
`structure, the patterning structure serving to pattern
`the projection beam, according to a desired pattern,
`in an object plane traversed by the projection beam;
`[00%]
`0039 a ro'ection s stem doWnstream of said ob'ect
`P J
`y
`J
`plane, for projecting the patterned beam onto a target
`portion of the substrate;
`
`a substrate table for holding a substrate;
`
`[0040] an interferometric measurement system for
`measuring Wave front aberrations of the projection
`system,
`[0041] characteriZed in that the method comprises the
`folloWing steps:
`[0042] providing a grating, featuring a grating pattern
`in a grating plane, into the projection beam, such that
`the grating plane is substantially coincident With said
`object plane;
`[0043] providing a pinhole and a detector to the
`projection beam at a location doWnstream of the
`projection system, such that radiation traversing the
`pinhole is detectable by the detector, Whereby said
`pinhole is arranged in a pinhole plate and features a
`pinhole pattern in a pinhole plane, the pinhole plane
`being substantially coincident With a plane that is
`optically conjugate to said object plane, and Whereby
`said detector comprises a detector surface that is
`substantially coincident With a detection plane
`doWnstream of the pinhole, Whereby, in said detec
`tion plane, a spatial distribution of the electric ?eld
`amplitude of the projection beam is substantially a
`Fourier Transformation of a spatial distribution of
`the electric ?eld amplitude in the pinhole plane;
`[0044] illuminating the grating With the projection
`beam of radiation, and
`[0045] detecting an interference fringe pattern of
`radiation With said detector.
`[0046] By phase stepping the grating and/or the pinhole
`the interference fringe pattern Will move over the detector
`surface. Intensities, at a plurality of points on the detector
`surface, and detected as a function of phase step, can be used
`for calculating the Wave front aberration, as is discussed, for
`eXample, in D. Malacara, “Optical Shop Testing”, John
`Wiley & Sons, Inc., NeW York, second edition, chapter 14.
`This measurement has to be repeated at a particular mea
`surement position a plurality of times, Whereby the grating
`or the pinhole has to be moved over a distance equal to a
`
`Nikon Exhibit 1007 Page 12
`
`

`

`US 2002/0145717 A1
`
`Oct. 10, 2002
`
`phase step in one or more preselected directions along Which
`the grating pattern or the pinhole pattern is periodic. Aphase
`step of, for example, the grating should preferably be
`smaller or equal to 1/3 of the period (or 1/3 of the grating
`period plus an integer number of grating periods) of the
`grating pattern along said one or more preselected direc
`tions. The term period refers to the distance over Which a
`periodic structure along a preselected direction in the grating
`pattern is repeated. With this measurement one can measure
`Wave front aberrations at a particular measurement position
`and in a particular direction in the imaged ?eld. To obtain
`information on Wave front aberrations at a plurality of points
`in the ?eld one should measure the Wave front aberration at
`a corresponding plurality of measurement points in the
`imaged ?eld along at least tWo directions. Phase stepping
`can be provided by moving the grating and/or moving the
`pinhole. It is advantageous to move the grating because, due
`to the magni?cation M (With M<1) of the projection system,
`the accuracy requirements for moving the grating (located
`upstream of the projection system) by the mask table are
`loWer than for moving the pinhole (located doWnstream of
`the projection system).
`
`[0047] Although speci?c reference may be made in this
`text to the use of the apparatus according to the invention in
`the manufacture of ICs, it should be explicitly understood
`that such an apparatus has many other possible applications.
`For example, it may be employed in the manufacture of
`integrated optical systems, guidance and detection patterns
`for magnetic domain memories, liqui