(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2003/0047694 A1
`(43) Pub. Date:
`Mar. 13, 2003
`Van Der Laan
`
`US 20030047694A1
`
`(54) METHOD OF MEASURING ABERRATION
`OF A PROJECTION SYSTEM OF A
`LITHOGRAPHIC APPARATUS, DEVICE
`MANUFACTURING METHOD, AND DEVICE
`MANUFACTURED THEREBY
`
`(30)
`
`Foreign Application Priority Data
`
`Aug. 23, 2001 (EP) ...................................... .. 012031886
`
`Publication Classi?cation
`
`(75) Inventor: Hans Van Der Laan, Veldhoven (NL)
`
`(51) Int. Cl? ................................................... ..G01N 21/86
`(52) US. Cl. ............................................................ ..250/548
`
`Correspondence Address:
`PILLSBURY WINTHROP, LLP
`PO. BOX 10500
`MCLEAN, VA 22102 (US)
`
`(73) Assignee: ASML NETHERLANDS B.V., De Run
`1110, Veldhoven 5503 (NL)
`
`(21) Appl. No.:
`
`10/225,458
`
`(22) Filed:
`
`Aug. 22, 2002
`
`(57)
`
`ABSTRACT
`
`A method of determining aberration of a projection system
`according to one embodiment of the invention includes
`using a test pattern to pattern a projection beam of radiation,
`using the projection system to project the patterned beam,
`and directly measuring an aerial image of the test pattern as
`formed by the projection system. The test pattern includes a
`two-dimensional lattice comprising a plurality of unit cells,
`each unit cell including at least three isolated areas. At least
`one of a transmissivity, a re?ectivity, and a phase-shifting
`property of the isolated areas is substantially different from
`that of a remainder of the area of the unit cell.
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`Patent Application Publication Mar. 13, 2003 Sheet 4 0f 8
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`Patent Application Publication Mar. 13, 2003 Sheet 5 0f 8
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`Patent Application Publication
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`Patent Application Publication Mar. 13, 2003 Sheet 7 0f 8
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`

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`US 2003/0047694 A1
`
`Mar. 13, 2003
`
`METHOD OF MEASURING ABERRATION OFA
`PROJECTION SYSTEM OF A LITHOGRAPHIC
`APPARATUS, DEVICE MANUFACTURING
`METHOD, AND DEVICE MANUFACTURED
`THEREBY
`This application claims priority to European Patent Appli
`cation EP 012031886 ?led Aug. 23, 2001, Which document
`is herein incorporated by reference.
`
`FIELD OF THE INVENTION
`
`[0001] The present invention relates to lithographic pro
`jection apparatus and methods.
`
`BACKGROUND
`
`[0002] The term “patterning structure” as here employed
`should be broadly interpreted as referring to any structure or
`?eld that may be used to endoW an incoming radiation beam
`With a patterned cross-section, corresponding to a pattern
`that is to be created in a target portion of a substrate; the term
`“light valve” can also be used in this context. Generally,
`such a 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:
`
`[0003] 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. Placement 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.
`
`[0004] 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 apparatus is that (for example)
`addressed areas of the re?ective surface re?ect incident light
`as diffracted light, Whereas unaddressed areas re?ect inci
`dent light as undiffracted light. Using an appropriate ?lter,
`the 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-addressable surface. An
`alternative embodiment of a programmable mirror array
`employs a matrix arrangement of very small (possibly
`microscopic) mirrors, each of Which can be individually
`tilted about an axis by applying a suitable localiZed electric
`?eld, or by employing pieZoelectric actuation means. For
`example, the mirrors may be 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 addressing
`pattern of the matrix-addressable 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 program
`mable mirror arrays. More information on mirror arrays as
`here referred to can be gleaned, for example, from US. Pat.
`
`No. 5,296,891 and No. 5,523,193, Which are incorporated
`herein by reference, and PCT patent applications WO
`98/38597 and WO 98/33096, Which are incorporated herein
`by reference. 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.
`
`[0005] Aprogrammable 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.
`
`[0006] 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.
`
`[0007] 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 (eg a Wafer of
`silicon or other semiconductor material) that has been coated
`With a layer of radiation-sensitive material (resist). In gen
`eral, a single Wafer Will contain a Whole netWork of adjacent
`target portions that are successively irradiated via the pro
`jection system (eg one at a time).
`
`[0008] Among current apparatus that employ 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 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 progressively 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 projection system Will have a magni
`?cation factor M (generally<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. A projection beam in a
`scanning type of apparatus may have the form of a slit With
`a slit Width in the scanning direction. More information With
`regard to lithographic devices as here described can be
`gleaned, for example, from US. Pat. No. 6,046,792, Which
`is incorporated herein by reference.
`
`[0009] 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
`
`Nikon Exhibit 1005 Page 10
`
`

`

`US 2003/0047694 A1
`
`Mar. 13, 2003
`
`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.
`
`[0010] The term “projection system” should be broadly
`interpreted as encompassing various types of projection
`system, including refractive optics, re?ective optics, and
`catadioptric systems, for example. For the sake of simplicity,
`the projection system may hereinafter be referred to as the
`“lens”. The radiation system as Well as the projection system
`may include components operating according to any of these
`design types for directing, shaping, reducing, enlarging,
`patterning, and/or otherWise controlling the projection beam
`of radiation, and such components may also be referred to
`beloW, collectively or singularly, as a “lens”. In particular,
`the projection system Will generally comprise means to set
`the numerical aperture (commonly referred to as the “NA”)
`of the projection system, and the radiation system typically
`comprises adjusting means for setting the outer and/or inner
`radial extent (commonly referred to as o-outer and o-inner,
`respectively) of the intensity distribution upstream of the
`patterning means (in a pupil of the radiation system).
`
`[0011] 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 PCT Application No. WO 98/40791, Which
`documents are incorporated herein by reference
`
`[0012] Generally, in order to realiZe integration of an
`increasing number of electronic components in an IC, it is
`necessary to increase the surface area of an IC and/or to
`decrease the siZe of the components. For the projection
`system, it is desirable in particular to increase the resolution
`so that increasingly smaller details, or line Widths, can be
`imaged in a Well-de?ned Way onto a target portion. Such a
`projection system must comply With very stringent quality
`requirements.
`[0013] A projection system may exhibit residual aberra
`tion. In practice, the projection system is not an ideal
`(diffraction-limited) system; generally the projection system
`is an aberration-limited system. Such aberration may be due,
`for example, to manufacturing tolerances and generic lens
`design limitations. Residual aberration may comprise loW
`order aberrations (e.g. third-order distortion, third-order x
`astigmatism, third-order 45° astigmatism, third-order x
`coma, third-order y coma and third order spherical aberra
`tion) as Well as higher-order aberrations (e.g. ?fth-order and
`seventh-order distortion, x and 45° astigmatism, x and y
`coma, and x and y three-Wave aberration). For more infor
`
`mation about aberrations mentioned above, see, for
`example, the paper entitled “ToWards a comprehensive
`control of full-?eld image quality in optical photolithogra
`phy”, authored by D. Flagello et al., Proc. SPIE 3051, pp.
`672-685, 1997, Which document is incorporated herein by
`reference.
`[0014] The in?uence of residual aberration becomes
`increasingly signi?cant With the application of neWer tech
`niques, such as phase-shift masks or off-axis illumination, to
`enhance the resolving poWer of a lithographic projection
`apparatus. Moreover, the loW- and higher-order aberrations
`are not constant in time. Such variation may be due, for
`example, to changing environmental conditions, reversible
`changes as caused by lens heating, and/or ageing of com
`ponents of the projection system as caused by interaction of
`the radiation of the projection beam With the material of said
`components. In order to minimiZe the residual aberration
`(e.g. intermittently during a manufacturing process), modern
`projection lithography apparatus generally comprise means
`to measure loW-order and/or higher-order aberrations con
`tributing to said residual aberration, means to adjust said
`aberrations (eg through adjustments of the position of one
`or more movable lens elements of the projection system, or
`of the support structure), and means to calculate and apply
`the required adjustments. For a description of a method to
`substantially minimiZe residual aberration, see, for example,
`European Patent Application 013030366, Which document
`is incorporated herein by reference.
`[0015] International Patent Application WO 00/31592,
`Which document is incorporated herein by reference, dis
`closes methodology for the determination of aberration in an
`optical projection system. In particular, this WO application
`describes the Aberration Ring Test (“AR ”). This technique
`employs a series of ring-like features on a special test reticle,
`Which are imaged through an optical projection system onto
`a photosensitive substrate. The images of the ring-like
`features on the substrate are then inspected, using a tech
`nique such as SEM (scanning electron microscopy). A
`comparison of the measured images With the corresponding
`original features on the reticle may reveal the type(s) of
`aberration that the optical projection system has introduced
`into the images.
`[0016] The same WO application also describes a re?ne
`ment of the ART technique knoWn as ARTEMIS (ART
`Extended to Multiple Illumination Settings). This re?nement
`makes use of the fact that each kind of aberration can be
`mathematically expressed as a speci?c Fourier harmonic
`that is a combination of a number of so-called Zernike
`polynomials, each With an associated Zernike aberration
`coef?cient and Weighting factor. In order to determine a
`number N of such Zernike aberration coef?cients, the ART
`technique is performed at a plurality N of different groups of
`settings of o-outer, o-inner and NA. For simplicity, a group
`of settings of o-outer, o-inner and NA Will be referred to
`hereinafter as a o-NA setting. In this Way, one is able to
`measure the same Fourier harmonic for each of the plurality
`N of o-NA settings. Using a simulation program, reference
`values can be obtained for the above-mentioned Weighting
`factors. In combination, this technique alloWs the desired set
`of Zernike aberration coef?cients to be calculated, thus
`alloWing quanti?cation of the aberration concerned.
`[0017] An alternative method to measure aberrations of a
`lithographic projection system is described in European
`
`Nikon Exhibit 1005 Page 11
`
`

`

`US 2003/0047694 A1
`
`Mar. 13, 2003
`
`Patent Application 013015714, Which document is incor
`porated herein by reference. It concerns an in situ measure
`ment of aberrations that is performed fast enough such as to
`not substantially impair the number of substrates that can be
`processed per unit of time. According to this method, the
`projection beam is patterned into a desired test pattern, and
`the intensity distribution of the projected aerial image of the
`test pattern is detected in situ using detection means incor
`porated in the substrate table. The position of best-focus
`(along the optical axis of the projection system) as Well as
`the lateral position (in mutually orthogonal directions per
`pendicular to the optical axis of the projection system) of the
`projected aerial image of the test pattern are measured for a
`plurality of different o-NA settings. Based on the results of
`said best focus and lateral position measurements, coef?
`cients representative of one or more aberrations of the
`projection system may be calculated. The method is referred
`to hereinafter by TAMIS (Transmission image sensing At
`Multiple Illumination Settings).
`[0018] The test pattern is typically a segment of a periodic
`grating comprising lines and spaces (respectively substan
`tially blocking and transmitting projection beam radiation),
`for example. Segments of such gratings Wherein the Width of
`the spaces is large compared to the Width of the lines may
`also be used as test patterns. Typically, tWo test patterns With
`the lines and spaces arranged parallel to tWo corresponding,
`mutually orthogonal, directions (in the plane comprising the
`pattern) are used to enable measurement of aberrations such
`as, for example, X coma and y coma.
`
`[0019] HoWever, in spite of such measures, the intensity
`distribution of projected aerial images of any such grating
`segments may not yield substantially detectable information
`on the presence of speci?c higher-order aberrations such as,
`for example, three-Wave aberration. Consequently, there is
`the problem of providing test patterns suitable for reliably
`indicating and measuring the presence and magnitude of
`both loW-order and higher-order aberrations, Where the
`measurement can be done in situ such as to not substantially
`impair the number of substrates that can be processed per
`unit of time.
`
`SUMMARY
`[0020] Embodiments of the invention include a method of
`measuring aberration With improved sensitivity.
`[0021] Amethod of determining aberration of a projection
`system according to one embodiment of the invention
`includes supplying a projection beam of radiation, using a
`test pattern to pattern the projection beam, and using the
`projection system to project the patterned beam. Such a
`method also includes directly measuring an aerial image of
`the test pattern as formed by the projection system to obtain
`a corresponding value of each of at least one parameter.
`
`[0022] Based on the at least one corresponding value, at
`least one coef?cient relating to aberration of the projection
`system is calculated. In this case, the test pattern includes a
`tWo-dimensional lattice comprising a plurality of unit cells,
`each unit cell including at least three isolated areas. At least
`one of a transmissivity, a re?ectivity, and a phase-shifting
`property of the isolated areas is substantially different from
`that of a remainder of the area of the unit cell.
`
`shape. In these or other applications, the direct measurement
`may be performed at each of a plurality of different illumi
`nation settings (i.e. different numerical aperture settings;
`different settings of the outer and/or inner extent of the
`intensity distribution of the projection beam; different illu
`mination modes such as disc-shaped, annular, and quadru
`polar; etc.). The direct measurement may also be performed
`using radiation detection means including a plurality of
`radiation apertures.
`[0024] 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, liquid-crystal display pan
`els, thin-?lm magnetic heads, etc. The skilled artisan Will
`appreciate that, in the context of such alternative applica
`tions, any use of the terms “reticle”, “Wafer”, or “die” in this
`text should be considered as being replaced by the more
`general terms “mask”, “substrate”, and “target portion”,
`respectively.
`[0025] In the present document, the terms “radiation” and
`“beam” are used to encompass all types of electromagnetic
`radiation, including ultraviolet radiation (eg with a Wave
`length of 365, 248, 193, 157 or 126 nm) and EUV (extreme
`ultra-violet radiation, e.g. having a Wavelength in the range
`5-20 nm).
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`[0026] Embodiments of the invention Will noW be
`described, by Way of example only, With reference to the
`accompanying schematic draWings in Which:
`[0027] FIG. 1 depicts a lithographic projection apparatus
`according to an embodiment of the invention;
`
`[0028] FIG. 2 depicts a so-called “brick Wall” pattern and
`a corresponding lattice With hexagonal unit cells.
`
`[0029] FIG. 3 depicts an example of a layout of isolated
`areas in a hexagonal unit cell.
`
`[0030] FIG. 4 illustrates an intensity distribution of a
`projected image of a test pattern of isolated areas, corre
`sponding to a lattice of hexagonal unit cells (as illustrated in
`FIG. 3), in the absence of residual aberration. A plot of a
`local intensity distribution is shoWn; along the vertical axis
`is the intensity of the projection beam radiation as a function
`of position along a line in the x-direction, traversing a lattice
`point.
`[0031] FIG. 5 illustrates an intensity distribution of a
`projected image of a test pattern in the presence of three
`Wave aberration. The graph shoWs, along the vertical axis, a
`detected signal as measured With a slit-shaped radiation
`detector, and along the horiZontal axis a lateral position of
`the slit-shaped radiation detector.
`
`[0032] FIG. 6 shoWs a decomposition of the detected
`signal into a harmonic signal and a ?rst higher-order har
`monic signal. Along the vertical axis the signals are plotted,
`and along the horiZontal axis a lateral position of the
`slit-shaped radiation detector is plotted.
`
`[0023] In particular applications of such a method, each
`unit cell may have a triangular, quadrangular, or hexagonal
`
`[0033] FIG. 7 lists a table of Zernike coef?cients and
`polynomials.
`
`Nikon Exhibit 1005 Page 12
`
`

`

`US 2003/0047694 A1
`
`Mar. 13, 2003
`
`[0034] FIG. 8 illustrates a correlation between measure
`ment results for aberrations as measured With ARTEMIS,
`With a shearing interferometer, and With the present method.
`Along the vertical axis, a root mean square value in nm of
`a difference betWeen measurement results is plotted.
`
`[0035] FIG. 9 shoWs a test pattern With isolated areas,
`corresponding to a lattice With quadrangular unit cells.
`
`DETAILED DESCRIPTION
`
`[0036] FIG. 1 schematically depicts a lithographic pro
`jection apparatus according to a particular embodiment of
`the invention. The apparatus comprises:
`[0037] A radiation system con?gured to supply (e.g. hav
`ing structure capable of supplying) a projection beam of
`radiation. In this particular example, the radiation system
`Ex, IL, for supplying a projection beam PB of radiation (e.g.
`UV radiation, or radiation With Wavelengths Within spectral
`Wavelength ranges substantially centered at 248 nm, 193
`nm, 157 nm, 126 nm, or 13.5 nm) also comprises a radiation
`source LA;
`
`[0038] A support structure con?gured to support a pattern
`ing structure capable of patterning the projection beam. In
`this example, a ?rst object table (mask table) MT is provided
`With a mask holder for holding a mask MA (eg a reticle),
`and is connected to a ?rst positioning structure for accu
`rately positioning the mask With respect to item PL;
`[0039] A second object table (substrate table) con?gured
`to hold a substrate. In this example, substrate table WT is
`provided With a substrate holder for holding a substrate W
`(eg a resist-coated silicon Wafer), and is connected to a
`second positioning structure for accurately positioning the
`substrate With respect to item PL; and
`[0040] A projection system (“lens”) con?gured to project
`the patterned beam. In this example, projection system PL
`(eg a lens group that may include lenses made of quartZ
`and/or CaF2, a catadioptric or catoptric system that may
`include lens elements made of such materials, and/or a
`mirror system) is con?gured to image an irradiated portion
`of the mask MA onto a target portion C (e. g. comprising one
`or more dies) of the substrate W.
`
`[0041] As here depicted, the apparatus is of a transmissive
`type (ie has a transmissive mask). HoWever, in general, it
`may also be of a re?ective type, for example (With a
`re?ective mask). Alternatively, the apparatus may employ
`another kind of patterning structure, such as a programmable
`mirror array of a type as referred to above.
`
`[0042] The source LA (eg a mercury lamp, a UV excimer
`laser, an electron gun, a laser-produced plasma source or
`discharge plasma source, or an undulator or Wiggler pro
`vided around the path of an electron beam in a storage ring
`or synchrotron) produces a beam of radiation. This beam is
`fed into an illumination system (illuminator) IL, either
`directly or after having traversed a conditioning structure or
`?eld, such as a beam expander Ex, for example. The
`illuminator IL may comprise an adjusting structure or ?eld
`AM for setting the outer and/or inner radial extent (com
`monly referred to as o-outer and o-inner, respectively) of the
`intensity distribution in the beam, Which may affect the
`angular distribution of the radiation energy delivered by the
`projection beam at, for example, the substrate. In addition,
`
`the apparatus Will generally comprise various other compo
`nents, such as an integrator IN and a condenser CO. In this
`Way, the beam PB impinging on the mask MA has a desired
`uniformity and intensity distribution in its cross-section.
`
`[0043] It should be noted With regard to FIG. 1 that the
`source LA may be Within the housing of the lithographic
`projection apparatus (as is often the case When the source LA
`is a mercury lamp, for example), but it may also be remote
`from the lithographic projection apparatus, the radiation
`beam Which it produces being led into the apparatus (eg
`with the aid of suitable direction mirrors). This latter sce
`nario is often the case When the source LA is an excimer
`laser. The current invention and claims encompass both of
`these scenarios.
`
`[0044] The beam PB subsequently intercepts the mask
`MA, Which is held on a mask table MT. Having traversed
`(alternatively, having been selectively re?ected by) the mask
`MA, the beam PB passes through the lens PL, Which focuses
`the beam PB onto a target portion C of the substrate W. With
`the aid of the second positioning structure (and interfero
`metric measuring structure IF), the substrate table WT can
`be moved accurately, e.g. so as to position different target
`portions C in the path of the beam PB. Similarly, the ?rst
`positioning structure can be used to accurately position the
`mask MA With respect to the path of the beam PB, eg after
`mechanical retrieval of the mask MA from a mask library, or
`during a scan. In general, movement of the object tables MT,
`WT Will be realiZed With the aid of a long-stroke module
`(coarse positioning) and a short-stroke module (?ne posi
`tioning), Which are not explicitly depicted in FIG. 1. HoW
`ever, in the case of a Wafer stepper (as opposed to a
`step-and-scan apparatus) the mask table MT may just be
`connected to a short stroke actuator, or may be ?xed. Mask
`MA and substrate W may be aligned using mask alignment
`marks and substrate alignment marks.
`
`[0045] The depicted apparatus can be used in tWo different
`modes:
`
`[0046] 1. In step mode, the mask table MT is kept essen
`tially stationary, and an entire mask image is projected at
`once (ie in a single “?ash”) onto a target portion C. The
`substrate table WT is then shifted in the x and/or y directions
`so that a different target portion C can be irradiated by the
`beam PB;
`
`[0047] 2. In scan mode, essentially the same scenario
`applies, except that a given target portion C is not exposed
`in a single “?ash”. Instead, the mask table MT is movable in
`a given direction (the so-called “scan direction”, eg the y
`direction) With a speed v, so that the projection beam PB is
`caused to scan over a mask image. Concurrently, the sub
`strate table WT is simultaneously moved in the same or
`opposite direction at a speed V=Mv, in Which M is the
`magni?cation of the lens PL (typically, M=1A1 or
`In this
`manner, a relatively large target portion C can be exposed,
`Without having to compromise on resolution.
`
`[0048] To enable a measurement of aberrations, a particu
`lar mask may include test patterns. Generally, standard
`alignment marks having equal lines/spaces along the x- and
`y-directions (as shoWn in FIG. 1, eg With a line Width of
`8 pm for the imaged mark) and special, asymmetrically
`segmented alignment markers are used as test patterns. The
`lateral position (ie the position in the x,y plane shoWn in
`
`Nikon Exhibit 1005 Page 13
`
`

`

`US 2003/0047694 A1
`
`Mar. 13, 2003
`
`FIG. 1, also referred to as ‘the horizontal position’ herein
`after) and best focus position (ie the position along the Z
`direction in FIG. 1, also referred to as ‘the vertical position’
`hereinafter) of aerial images of test patterns can be measured
`With one or more transmission image sensors TIS. An
`example of a transmission image sensor is described in
`greater detail in US. Pat. No. 4,540,277, Which document is
`incorporated herein by reference.
`
`[0049] A transmission image sensor T15 is set into a
`physical reference surface associated With the substrate table
`WT. In one example, tWo sensors are mounted on a ?ducial
`plate that is mounted to the top surface of the substrate table
`WT, the sensors being located at diagonally opposite posi
`tions outside the area covered by the Wafer W. The ?ducial
`plate is made of a highly stable material With a very loW
`coef?cient of thermal expansion (e.g. Invar) and has a ?at
`re?ective upper surface that may carry markers used With
`another ?ducial in alignment processes.
`
`[0050] The transmission image sensor or sensors TIS are
`used to determine directly the vertical and horiZontal posi
`tion of the aerial image, as projected by the projection lens,
`of a test pattern on the mask. Such a sensor includes one or
`more apertures in the re?ective surface, and one or more
`photodetectors placed close behind the apertures tha