(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2002/0167651 A1
`(43) Pub. Date:
`NOV. 14, 2002
`Boonman et al.
`
`US 20020167651A1
`
`(54) LITHOGRAPHIC APPARATUS, DEVICE
`MANUFACTURING METHOD, AND DEVICE
`MANUFACTURED THEREBY
`
`(75) Inventors: Marcus Emile J oannes Boonman,
`Veldhoven (NL); Johannes Catharinus
`Hubertus Mulkens, Maastricht (NL);
`Hans Butler, Best (NL)
`
`Correspondence Address:
`PILLSBURY WINTHROP, LLP
`P.O. BOX 10500
`MCLEAN, VA 22102 (US)
`
`(30)
`
`Foreign Application Priority Data
`
`Feb. 8, 2001 (EP) ...................................... .. 013011168
`
`Publication Classi?cation
`
`(51) Int. c1.7 ................................................... .. G03B 27/54
`(52) Us. 01. ............................ .. 355/67; 355/53; 356/399;
`356/400
`
`(57)
`
`ABSTRACT
`
`(73) Assignee: ASML NETHERLANDS B.V., Veld
`hoven (NL)
`
`(21) Appl. No.:
`
`10/066,784
`
`(22) Filed:
`
`Feb. 6, 2002
`
`In a lithographic apparatus the shape of the focal plane is
`adjusted using available manipulators in the projection lens
`system so that it is in closer conformity to the shape of the
`Wafer surface in the exposure area. The control of the focal
`plane shape can be integrated With the leveling control
`Which determines the height and tilt of the Wafer surface.
`
`Nikon Exhibit 1017 Page 1
`
`

`

`Patent Application Publication Nov. 14, 2002 Sheet 1 0f 4
`
`US 2002/0167651 A1
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`Nikon Exhibit 1017 Page 2
`
`

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`Patent Application Publication Nov. 14, 2002 Sheet 2 0f 4
`
`US 2002/0167651 A1
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`Nikon Exhibit 1017 Page 3
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`

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`Patent Application Publication Nov. 14, 2002 Sheet 3 0f 4
`
`US 2002/0167651 A1
`
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`Nikon Exhibit 1017 Page 4
`
`

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`Patent Application Publication Nov. 14, 2002 Sheet 4 0f 4
`
`US 2002/0167651 A1
`
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`Nikon Exhibit 1017 Page 5
`
`

`

`US 2002/0167651 A1
`
`Nov. 14, 2002
`
`LITHOGRAPHIC APPARATUS, DEVICE
`MANUFACTURING METHOD, AND DEVICE
`MANUFACTURED THEREBY
`
`BACKGROUND OF THE INVENTION
`
`[0001] 1. Field of the Invention
`[0002] This application claims priority to EPC application
`No. 013011168 ?led Feb. 8, 2001, herein incorporated by
`reference.
`
`[0003] 2. Background of the Related Art
`
`[0004] The present invention relates generally to litho
`graphic projection apparatus and more particularly to litho
`graphic projection apparatus including a controller to adjust
`a shape of a focal plane of the projection system.
`[0005] In general, a lithographic projection apparatus in
`accordance With embodiments of the present invention
`includes a radiation system for supplying a projection beam
`of radiation; a support structure for supporting patterning
`structure, the patterning structure serving to pattern the
`projection beam according to a desired pattern; a substrate
`table for holding a substrate; and a projection system for
`projecting the patterned beam onto a target portion of the
`substrate, said projection system having a focal plane and
`comprising at least one adjustable element capable of chang
`ing the shape of the focal plane.
`
`[0006] 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, corresponding to a pattern that is to
`be created in a target portion of the substrate; 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:
`
`[0007] 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.
`
`[0008] 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 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-addressable 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
`suitable localiZed electric ?eld, or by employing pieZoelec
`tric 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 unad
`dressed mirrors; in this manner, the re?ected beam is pat
`terned 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 programmable mirror arrays. More
`information on mirror arrays as here referred to can be
`gleaned, for eXample, from US. Pat. Nos. 5,296,891 and
`5,523,193, and PCP 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.
`
`[0009] 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.
`
`[0010] 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.
`
`[0011] 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 (eg com
`prising 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 successively
`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 apparatus is
`commonly referred to as a Wafer stepper. In an alternative
`apparatus—commonly referred to as a step-and-scan appa
`ratus—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.
`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.
`
`[0012] 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
`
`Nikon Exhibit 1017 Page 6
`
`

`

`US 2002/0167651 A1
`
`Nov. 14, 2002
`
`prining, 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.
`
`[0013] 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”. 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.
`
`[0014] To correctly image the mask pattern onto the
`substrate it is necessary to position the Wafer accurately in
`the focal plane of the projection lens. The position of the
`focal plane can vary according to the position of the mask,
`illumination and imaging settings in the illumination and
`projection systems and due to, for example, temperature
`and/or pressure variations in the apparatus, during an expo
`sure or series of exposures. To deal With these variations in
`focal plane position, it is knoWn to measure the vertical
`position of the focal plane using a sensor such as a trans
`mission image sensor (TIS) or a re?ection image sensor
`(RIS) and then position the Wafer surface in the focal plane.
`This can be done using so-called “on-the-?y” leveling
`Whereby a level sensor measures the vertical position of the
`Wafer surface during the exposure and adjusts the height
`and/or tilt of the Wafer table to optimiZe the imaging
`performance. Alternatively, so-called “off-axis” leveling can
`be used. In this method, a height map of (a part of) the Wafer
`surface is taken, eg in a multi-stage apparatus, in advance
`of the exposure and height and tilt set points for the
`exposure, or series of exposures, to optimiZe the focus
`according to de?ned criteria, are calculated in advance.
`Methods and a system for such off-axis leveling are
`described in European Patent Application EP-A-1 037 117.
`
`In the off-axis method, it is proposed that the exact shape and
`position of the Wafer surface be measured and the Wafer
`height and tilt positions for the exposure can then be
`optimiZed to minimiZe defocus predicted relative to that
`measured Wafer surface. Since the focal plane of the pro
`jection system Will generally be ?at and the Wafer surface
`Will generally not be ?at, there Will alWays be some residual
`defocus Which cannot be compensated for by leveling pro
`cedures.
`
`SUMMARY OF THE INVENTION
`
`[0015] One aspect of embodiments of the present inven
`tion includes a system and a method for controlling a
`lithographic projection apparatus to further improve focus
`across the entire exposure area.
`
`[0016] This and other aspects are achieved according to
`embodiments of the invention in a lithographic apparatus as
`speci?ed in the opening paragraph, characteriZed by:
`
`[0017] a controller, operative during an exposure for imag
`ing the irradiated portion, to control said adjustable element
`to change the shape of said focal plane to more closely
`conform to the surface contour of said exposure area.
`
`[0018] As discussed above, certain methods in Which the
`focal plane is generally arranged to be as ?at as possible and
`the substrate height and/or tilt are controlled to minimiZe
`defocus, inevitably leave some residual defocus as the Wafer
`surface is generally not exactly ?at. According to the present
`invention, rather than attempting to make the focal plane
`exactly ?at, its shape is deliberately changed to make it
`conform more closely to the measured surface contour of the
`substrate in the exposure area to be exposed. Control of the
`Wafer height and tilt is integrated With control of the shape
`of the focal plane. Then, loW order (height and tilt) correc
`tions can be effected by positioning the substrate and high
`order corrections can be effected by adjustments to the shape
`of the focal plane. Also, loW order effects of high order
`adjustments to the shape of the focal plane can be compen
`sated for in positioning of the substrate.
`
`[0019] Embodiments of the present invention can there
`fore provide improved imaging by reducing defocus across
`the entire exposure area. This improves imaging quality on
`all exposure areas and also makes possible focusing on
`exposure areas having curved surfaces that Would previously
`have exceeded defocus units.
`
`[0020] Embodiments of the present invention can make
`use of all available manipulators in the projection system to
`adjust elements that affect the shape of the focal plane. Such
`manipulators are provided With suitable actuators, e.g.
`motors, pieZoelectric actuators, solenoids, etc., to enable the
`a controller to adjust the elements to Which the manipulators
`are connected. The adjustable elements may include ele
`ments speci?cally provided for the present invention or
`provided for other purposes such as correcting ?eld curva
`ture introduced by changes in magni?cation, or correcting
`astigmatisms in the lenses. The adjustable elements may
`have their position and/or orientation in any of the six
`degrees of freedom changed by the manipulators. Addition
`ally, it is possible to adjust the shape of the element, eg
`Where the element is a re?ector provided With pieZoelectric
`elements for adjusting its surface ?gure.
`
`Nikon Exhibit 1017 Page 7
`
`

`

`US 2002/0167651 A1
`
`Nov. 14, 2002
`
`[0021] According to a further aspect of embodiments of
`the invention there is provided a device manufacturing
`method comprising the steps of:
`
`[0022] providing a substrate that is at least partially
`covered by a layer of radiation-sensitive material;
`
`[0023] providing a projection beam of radiation using
`a radiation system;
`
`atternin structure to endoW the ro
`0024 usin
`g P
`g
`P
`jection beam With a pattern in its cross-section; and
`
`[0025] projecting the patterned beam of radiation
`onto a target portion of the layer of radiation-sensi
`tive material using a projection system, said projec
`tion system having a focal plane and comprising at
`least one adjustable element capable of changing the
`shape of the focal plane;
`
`characteriZed by the step of:
`
`[0026]
`[0027] controlling said adjustable element during
`the step of imaging to change the shape of said
`focal plane to more closely conform to the surface
`contour of said exposure area.
`
`[0028] Although speci?c reference maybe 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.
`[0029] 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, eg. having a Wavelength in the range
`5-20 nm), as Well as particle beams, such as ion beams or
`electron beams.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`[0030] The invention and its attendant advantages Will be
`further described beloW With reference to exemplary
`embodiments and the accompanying schematic draWings, in
`Which:
`[0031] FIG. 1 depicts a lithographic projection apparatus
`according to a ?rst embodiment of the invention;
`
`[0032] FIG. 2 is a vieW shoWing hoW the Wafer height is
`determined from measurements by the level sensor and the
`Z-interferometer,
`[0033] FIGS. 3 to 6 are vieWs shoWing various steps of the
`focus control and leveling procedure according to the
`present invention;
`[0034] FIG. 7 is a plan vieW of a substrate table shoWing
`the sensors and ?ducials used in the focus control and
`leveling procedure according to the present invention;
`
`[0035] FIG. 8 is a vieW shoWing adjustable elements in
`the projection lens used in the present invention; and
`
`[0036] FIG. 9 is a diagram of a control system for putting
`the present invention into effect.
`[0037] In the draWings, like references indicate like parts.
`
`DETAILED DESCRIPTION
`
`[0038] FIG. 1 schematically depicts a lithographic pro
`jection apparatus according to a particular embodiment of
`the invention. The apparatus comprises:
`[0039] a radiation system LA, IL (EX, IN, CO), for
`supplying a projection beam PB of radiation (eg. UV
`or EUV radiation). In this particular case, the radia
`tion system also comprises a radiation source LA;
`[0040] a ?rst object table (mask table) MT provided
`With a mask holder for holding a mask MA (eg. a
`reticle), and connected to ?rst positioning means for
`accurately positioning the mask With respect to item
`
`a
`
`[0041] a second object table (substrate table) WTa
`provided With a substrate holder for holding a sub
`strate W (eg. a resist-coated silicon Wafer), and
`connected to second positioning means for accu
`rately positioning the substrate With respect to item
`PL;
`[0042] a third object table (substrate or Wafer table)
`WTb provided With a substrate holder for holding a
`substrate W (eg. a resist-coated silicon Wafer), and
`connected to third positioning means for accurately
`positioning the substrate With respect to item PL;
`[0043] a measurement system MS for performing
`measurement characterization) processes on a sub
`strate held on a substrate table WTa or WTh at a
`measurement station
`[0044] a projection system (“lens”) PL (eg. a refrac
`tive or catadioptric system, a mirror group or an
`array of ?eld de?ectors) for imaging an irradiated
`portion of the mask MA onto a target portion C (eg.
`comprising one or more dies) of the substrate W held
`on a substrate table WTa or Wmb at an exposure
`station.
`[0045] 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.
`
`[0046] The source LA (eg. a Hg lamp, an excimer laser, a
`laser-produced plasma source, a discharge plasma source, an
`undulator provided around the path of an electron beam in
`a storage ring or synchrotron, or an electron or ion beam
`source) produces a beam of radiation. This beam is fed into
`an illumination system (illuminator) IL, either directly or
`after having traversed conditioning means, such as a beam
`expander EX, for example. The illuminator IL may comprise
`adjusting means AM for setting the outer and/or inner radial
`extent (commonly referred to as o-outer and o-inner, respec
`tively) of the intensity distribution in the beam. In addition,
`it Will generally comprise various other components, such as
`an integrator IN and a condenser CO. In this Way, the beam
`
`Nikon Exhibit 1017 Page 8
`
`

`

`US 2002/0167651 A1
`
`Nov. 14, 2002
`
`PB impinging on the mask MA has a desired uniformity and
`intensity distribution in its cross-section.
`
`[0047] It should be noted With regard to FIG. 1 that the
`source LA maybe Within the housing of the lithographic
`projection apparatus (as is often the case When the source LA
`is a mercury lamp, for example), but that it may also be
`remote from the lithographic projection apparatus, the radia
`tion beam Which it produces being led into the apparatus
`(eg. with the aid of suitable directing mirrors); this latter
`scenario is often the case When the source LA is an eXcimer
`laser. The current invention and Claims encompass both of
`these scenarios.
`
`[0048] The beam PB subsequently intercepts the mask
`MA, Which is held on a mask table MT. Having traversed 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 interferometric measuring means IF,
`the substrate tables WTa, WTb can be moved accurately by
`the second and third positioning means, eg. so as to position
`different target portions C in the path of the beam PB.
`Similarly, the ?rst positioning means can be used to accu
`rately-position the maskMA 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, WTa, WTh Will be realiZed With the
`aid of a long-stroke module (coarse positioning) and a
`short-stroke module (?ne positioning), Which are not eXplic
`itly depicted in FIG. 1. HoWever, 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.
`
`[0049] The second and third positioning means may be
`constructed so as to be able to position their respective
`substrate tables WTa, Wm over a range encompassing both
`the eXposure station under projection system PL and the
`measurement station under the measurement system MS.
`Alternatively, the second and third positioning means may
`be replaced by separate eXposure station and measurement
`station positioning systems for positioning a substrate table
`in the respective eXposure stations and a table eXchange
`means for exchanging the substrate tables betWeen the tWo
`positioning systems. Suitable positioning systems are
`described, inter alia, in WO 98/28665 and WO 98/40791
`mentioned above. It should be noted that a lithography
`apparatus may have multiple eXposure stations and/or mul
`tiple measurement stations and that the numbers of mea
`surement and eXposure stations may be different than each
`other and the total number of stations need not equal the
`number of substrate tables. Indeed, the principle of separate
`eXposure and measurement stations may be employed even
`With a single substrate table.
`
`[0050] The depicted apparatus can be used in tWo different
`modes:
`
`[0051] 1. In step mode, the mask table MT is kept essen
`tially stationary, and an entire mask image is projected at
`once (ie. a single “?ash”) onto a target portion C. The
`substrate table WTa or WTh is then shifted in the X and/or
`y directions so that a different target portion C can be
`irradiated by the beam PB;
`
`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 Wra or Wfb 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.
`
`[0053] An important factor in?uencing the imaging qual
`ity of a lithographic apparatus is the accuracy With Which the
`mask image is focused on the substrate. Wafers are generally
`polished to a very high degree of ?atness but nevertheless
`deviations of the Wafer surface from perfect ?atness
`(referred to as “un?atness”) of suf?cient magnitude notice
`ably to affect focus accuracy can occur. Un?atness may be
`caused, for eXample, by variations in Wafer thickness, dis
`tortion of the shape of the Wafer or contaminants on the
`substrate table. The presence of structures due to previous
`process steps also signi?cantly affects the Wafer height
`(?atness). In the present invention, the cause of un?atness is
`largely irrelevant; only the height of the top surface of the
`Wafer is considered. Unless the conteXt otherWise requires,
`references beloW to “the Wafer surface” refer to the top
`surface of the Wafer onto Which Will be projected the mask
`image.
`
`[0054] After loading a Wafer onto one of the substrate
`tables WTa, WTh, the height of the Wafer surface ZWafer
`relative to a physical reference surface of the substrate table
`is mapped. This process is carried out at the measurement
`station using a ?rst sensor, referred to as the level sensor,
`Which measures the vertical (Z) position of the physical
`reference surface and the vertical position of the Wafer
`surface, ZLS, at a plurality of points, and a second sensor,
`for eXample a Z-interferometer, Which simultaneously mea
`sures the vertical position of the substrate table, ZIF at the
`same points. As shoWn in FIG. 2, the Wafer surface height
`is determined as ZWafer=ZLS-ZIF. The substrate table
`canying the Wafer is then transferred to the eXposure station
`and the vertical position of the physical reference surface is
`again determined. The height map is then referred to in
`positioning the Wafer at the correct vertical position during
`the eXposure process. This procedure is described in more
`detail beloW With reference to FIGS. 3 to 6.
`
`[0055] As shoWn in FIG. 3, ?rst the substrate table is
`moved so that a physical reference surface ?Xed to the
`substrate table is underneath the level sensor LS. The
`physical reference surface may be any convenient surface
`Whose position in X, Y and Z on the substrate table Will not
`change during processing of a Wafer in the lithographic
`apparatus and, most importantly, in the transfer of the
`substrate table betWeen measurement and eXposure stations.
`The physical reference surface may be part of a ?ducial
`containing other alignment markers and the surface should
`have such properties as to alloW its vertical position to be
`measured by the same sensor as measures the vertical
`position of the Wafer surface. The physical reference surface
`may be a re?ective surface in a ?ducial in Which is inset a
`so-called transmission image sensor BIDS). The TIS is
`described further beloW.
`
`[0052] 2. In scan mode, essentially the same scenario
`applies, eXcept that a given target portion C is not eXposed
`
`[0056] The level sensor may be, for eXample, an optical
`sensor, alternatively, pneumatic or capacitive sensors (for
`
`Nikon Exhibit 1017 Page 9
`
`

`

`US 2002/0167651 A1
`
`Nov. 14, 2002
`
`example) are conceivable. A presently preferred form of
`sensor making use of Moire patterns formed betWeen the
`image of a projection grating re?ected by the Wafer surface
`and a ?xed detection grating is described in European Patent
`Application EP-A-1 037 117. The level sensor should pref
`erably measure the vertical position of a plurality of posi
`tions on the Wafer surface simultaneously and for each
`position the sensor may measure the average height of a
`particular area, so averaging out un?atnesses of high spatial
`frequencies.
`
`[0057] Simultaneously With the measurement of the ver
`tical position of a physical reference surface by the level
`sensor LS, the vertical position of the substrate table is
`measured using the Z-interferometer, ZIF. The Z-interfer
`ometer may, for example, be part of a three, ?ve or six-axis
`interferometric metrology system such as that described in
`WO 99/28790 or WO 99/32940, Which documents are
`incorporated herein by reference. The Z-interferometer sys
`tem preferably measures the vertical position of the substrate
`table at a point having the same position in the XY plane as
`the calibrated measurement position of the level sensor LS.
`This may be done by measuring the vertical position of tWo
`opposite sides of the substrate table WT (WTa or WTh) at
`points in line With the measurement position of the level
`sensor and interpolating/modeling betWeen them. This
`ensures that, in the event that the substrate table is tilted out
`of the XY plane, the Z-interferometer measurement cor
`rectly indicates the vertical position of the substrate table
`under the level sensor.
`
`[0058] Preferably, this process is repeated With at least a
`second physical reference surface spaced apart, e.g. diago
`nally, from the ?rst physical reference surface. Height
`measurements from tWo or more positions can then be used
`to de?ne a reference plane.
`
`[0059] The simultaneous measurement of the vertical
`position of one or more physical reference surfaces and the
`vertical position of the substrate table establishes a point or
`points determining the reference plane relative to Which the
`Wafer height is to be mapped. A Z-interferometer of the type
`mentioned above is effectively a displacement sensor rather
`than an absolute sensor, and so requires Zeroing, but pro
`vides a highly linear position measurement over a Wide
`range. On the other hand, suitable level sensors, eg those
`mentioned above, may provide an absolute position mea
`surement With respect to an externally de?ned reference
`plane (i.e. nominal Zero) but over a smaller range. Where
`such sensors are used, it is convenient to move the substrate
`table vertically under the level sensor until the physical
`reference surface(s) is (are) positioned at a nominal Zero in
`the middle of the measurement range of the level sensor and
`to read out the current interferometer Z value. One or more
`of these measurements on physical reference surfaces Will
`establish the reference plane for the height mapping. The
`Z-interferometer is then Zeroed With reference to the refer
`ence plane. In this Way the reference plane is related to the
`physical surface on the substrate table and the ZWafer height
`map is made independent of the initial Zero position of the
`Z-interferometer at the measurement station and other local
`factors such as any un?atness in the base plate over Which
`the substrate table is moved. Additionally, the height map is
`made independent of any drift in the Zero position of the
`level sensor.
`
`[0060] As illustrated in FIG. 4, once the reference plane
`has been established, the subst