(12) United States Patent
`van der Laan et al.
`
`(10) Patent N0.:
`(45) Date of Patent:
`
`US 6,646,729 B2
`Nov. 11,2003
`
`US006646729B2
`
`(54) METHOD OF MEASURING ABERRATION IN
`AN OPTICAL IMAGING SYSTEM
`
`(75) Inventors: Hans van der Laan, Veldhoven (NL);
`Marco H Moers, Eindhoven (NL)
`
`(73) Assignee: ASML Netherlands B.V., Veldhoven
`(NL)
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`USC 154(b) by 240 days.
`
`( * ) Notice:
`
`(21) Appl. No.: 09/788,478
`(22) Filed:
`Feb. 21, 2001
`(65)
`Prior Publication Data
`
`US 2002/0008869 A1 Jan. 24, 2002
`Foreign Application Priority Data
`
`(30)
`
`Feb. 23, 2000
`
`(EP) .......................................... .. 00301420
`
`(51) Int. Cl.7 ................................................ .. G01B 9/00
`
`(52) US. Cl. ..................................................... .. 356/124
`(58) Field of Search ....................... .. 356/124, 388—401;
`382/141, 145, 151, 152, 181, 190, 207,
`211, 286
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`
`4,943,733 A * 7/1990 Mori et al. ............... .. 250/548
`5,666,206 A * 9/1997 Uchiyama ....... ..
`356/401
`5,754,299 A * 5/1998 Sugaya et al. ............ .. 356/401
`5,821,014 A 10/1998 Chen et 211.
`5,828,455 A 10/1998 Smith et 211.
`6,078,380 A * 6/2000 Taniguchi et al. .......... .. 355/52
`6,356,345 B1
`3/2002 McArthur et al.
`356/121
`6,360,012 B1 * 3/2002 KreuZer .................... .. 382/211
`
`FOREIGN PATENT DOCUMENTS
`
`EP
`TW
`WO
`
`0 849 638 A2
`357262
`WO00/31592
`
`6/1998
`5/1999
`6/2000
`
`OTHER PUBLICATIONS
`
`Impact of Lens Aberrations on Optical Lithography, T.A.
`Brunner, IBM HopeWell Junction, NeW York, Conference
`San Diego, CA Oct. 27—29, 1996.
`Novel Aberration Monitor for Optical Lithography, P. Dirk
`sen et al., Part of the SPIE Conference on Optical Microli
`thography XII, Santa Clara, CA, Mar. 1999, SPIE vol. 3679,
`pp. 77—86.
`A Practical Technology Path to Sub—0.10 Micron Process
`Generations via Enhanced Optical Lithography, J. Fung
`Cheng et al., ASML Mask Tools, Inc., 19th Annjual Sym
`posium on Photomask Technology, Monterey, CA, Sep.
`15—17, 1999.
`0.35 pm Lithography Using Off—Axis Illumination, Paul
`Luehrmann et al., SPIE vol. 1927, pp. 103—124, Optical/
`Laser Microlighography VI (1993)/ 103.
`Optical Proximity Correction for Intermediate—Pitch Fea
`tures Using Sub—Resolution Scattering Bars, J. Fung Cheng
`et al., J. Vac. Sci. Technol. B 15(6), Nov./Dec. 1997, pp.
`2426—2433.
`Aberration Evaluation and Tolerancing of 193nm Litho
`graphic Objective Lenses, Bruce W. Smith et al., SPIE vol.
`3334/269, pp. 269—280, Optical Microlithography XI, Santa
`Clara, CA, Feb. 25—27, 1998.
`Measurement of Lens Aberration Using an In—Situ Interfer
`ometer Reticle, Nigel Farrar et al., Advanced Reticle Sym
`posium 1999, Optical Microlithography XIII, Santa Clara,
`CA, Mar. 1—3, 2000.
`
`* cited by examiner
`
`Primary Examiner—Michael P. Sta?ra
`(74) Attorney, Agent, or Firm—Pillsbury Winthrop LLP
`(57)
`ABSTRACT
`
`A method of determining aberration of an optical imaging
`system comprises measuring at least one parameter, such as
`position of best-focus and/or lateral position, of an image
`formed by the imaging system. This is repeated for a
`plurality of different illumination settings of the imaging
`system, and from these measurements at least one
`coef?cient, representative of aberration of said imaging
`system, is calculated.
`
`17 Claims, 8 Drawing Sheets
`
`Nikon Exhibit 1004 Page 1
`
`

`

`U.S. Patent
`
`Nov. 11,2003
`
`Sheet 1 of 8
`
`US 6,646,729 B2
`
`-c0 PB MT
`
`N PL
`
`(3
`
`LA H EX Y .
`
`Fig.1.
`
`Nikon Exhibit 1004 Page 2
`
`

`

`U.S. Patent
`
`Nov. 11,2003
`
`Sheet 2 of 8
`
`US 6,646,729 B2
`
`Fig.2.
`
`Aberration
`
`Low Order Zernikes
`
`Higher Orders
`
`m
`
`0
`
`1
`
`Name
`
`Spherical
`
`X-Coma
`
`Function
`
`Term
`
`Term
`
`6r4- 6r2 + 1
`
`Z9 Z16, Z25, Z36, Z37
`
`(3r3 - 2r)cos(9) Z7
`
`Z14, Z23, Z34
`
`Y-Coma
`1
`Astigmatism
`2
`450 Astigmatism
`2
`3 X-Three Point Aberration
`3 Y-Three Point Aberration
`
`(3r3 - 2r)sin(6) Z8
`r2 cos(26)
`Z5
`r2 SW29)
`Z6
`r3 005(36)
`Z10
`r3 sin(36)
`Z11
`
`Z15, Z24, Z35
`Z12, Z21, Z32
`Z13, Z22, 233
`Z19, Z30
`Z20, Z31
`
`Fig.3(a)
`BFZ9=1nm [nm] BFZ16=1nm [nm]
`6
`NA
`0.63 0.82-0.62
`-4.4
`-12.8
`0.63 0.29-0.09
`-5.6
`-9.5
`0.4 0.5-0.2
`-16.0
`0.0
`0.48 0.5-0.2
`-12.1
`-7.6
`0.55 0.5-0.2
`-7.9
`-12.4
`0.63 0.72-0.52
`0.0
`-13.0
`
`F|g.3(b)
`BFZ9-1nm [nm] BFZ16=1nm [nm]
`'5
`NA
`0.7 0.58-0.88
`-1.7
`-8.5
`0.7 0.11-0.32
`-4.8
`-6.2
`0.5 0.2-0.5
`'11.8
`0.0
`0.55 02-05
`-9.5
`-4.1
`0.6 0.2-0.5
`-6.7
`-7.4
`0.7 0.42-0.72
`0.0
`-9.0
`
`Nikon Exhibit 1004 Page 3
`
`

`

`U.S. Patent
`
`Nov. 11,2003
`
`Sheet 3 of 8
`
`US 6,646,729 B2
`
`Fig.4. 0
`
`R2=0.972
`
`'
`
`4B. O |
`
`do 0 |
`
`1-
`
`y=O.80x - 1.52
`R2=O.86 I
`
`,
`
`Nikon Exhibit 1004 Page 4
`
`

`

`U.S. Patent
`
`Nov. 11,2003
`
`Sheet 4 of 8
`
`US 6,646,729 B2
`
`Nikon Exhibit 1004 Page 5
`
`

`

`U.S. Patent
`
`Nov. 11,2003
`
`Sheet 5 0f 8
`
`US 6,646,729 B2
`
`Fig.8(a)
`
`07
`
`0.65
`
`BF [um]
`
`El 0.12-0.14
`El 0.10-0.12
`0.08-0.10
`0.06-0.08
`
`0-6
`
`0.55
`
`0.3 0.4 0.5 0.6 0.7 0.8 0.9‘
`
`Fig.8(b)
`
`BF [um]
`I: 0.00-0.02
`-0.02-0.00
`-0.04--0.02
`-0.06--0.04
`-0.08--0.06
`-0.10--0.08
`
`Contrast
`[I 0.90-0.95
`III 0.85-0.90
`0.80-0.85
`0.75-0.80
`0.70-0.75
`
`E 0.60-0.65
`80.55080
`
`0.3 0.4 0.5 0.6 0.7 0.8 0.9
`
`Fig.9.
`
`0.5
`~
`0.3 0.4 0.5 0.6 0.7 0.8 0.9
`
`Nikon Exhibit 1004 Page 6
`
`

`

`U.S. Patent
`
`Nov. 11,2003
`
`Sheet 6 6f 8
`
`US 6,646,729 B2
`
`F|g.10.
`
`No.
`1
`2
`3
`4
`5
`6
`7
`8
`
`NA
`0.7
`0.5
`0.66
`0.5
`0.66
`0.7
`0.55
`0.7
`
`0
`0.33-0.11
`0.6-0.2
`0.65-0.22
`0.6-0.3
`0.7-0.4
`0.88-0.3
`0.5-0.2
`0.88-0.58
`
`AXZ7=1nm[nm]
`1.6
`1.8
`0.9
`1.7
`0.5
`-0.4
`1.8
`-1.1
`
`Nikon Exhibit 1004 Page 7
`
`

`

`U.S. Patent
`
`Nov. 11,2003
`
`Sheet 7 of 8
`
`US 6,646,729 B2
`
`Fig.12(a)
`
`0.7
`
`.65
`
`0X
`III 1.5-2.0
`1.0-1.5
`- 0.5-1.0
`0.0-0.5
`-0.5-0.0
`-1.0--0.5
`-1.5--1.0
`
`50.5
`V
`0.3 0.4 0.5 0.6 0.7 0.8 0.9
`
`Fig.12(b)
`
`0.7
`
`0.65
`
`0.6
`
`0.55
`
`.............................. ..
`
`.
`
`05
`
`0.3 0.4 0.5 0.6 0.7 0.8 0.9
`
`Nikon Exhibit 1004 Page 8
`
`

`

`U.S. Patent
`
`Nov. 11,2003
`
`Sheet 8 of 8
`
`US 6,646,729 B2
`
`Fig.13(a). 2°
`
`15
`
`1O
`
`Fig.13(b). 3
`
`Fig.14. 2O
`
`15
`
`1O_
`
`5_
`
`0
`
`|
`
`|
`
`|
`
`|
`
`|
`
`+DAMIS
`-e--s||=
`
`Nikon Exhibit 1004 Page 9
`
`

`

`US 6,646,729 B2
`
`1
`METHOD OF MEASURING ABERRATION IN
`AN OPTICAL IMAGING SYSTEM
`
`BACKGROUND
`1. Field of Invention
`The present invention relates to a method of measuring
`aberration in an optical imaging system, such as a litho
`graphic projection apparatus.
`2. Discussion of Related Art
`The patterning means here referred to should be broadly
`interpreted as referring to means 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 corre
`spond 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
`means include:
`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 trans
`mission (in the case of a transmissive mask) or re?ec
`tion (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.
`programmable mirror array. An example of such a
`A
`device is a matrix-addressable surface having a vis
`coelastic 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 unad
`dressed 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. The required matrix
`addressing can be performed using suitable electronic
`means. More information on such mirror arrays can be
`gleaned, for example, from US. Pat. No. 5,296,891 and
`US. Pat. No. 5,523,193, 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.
`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.
`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 means as hereabove set
`forth.
`Lithographic projection apparatuses can be used, for
`example, in the manufacture of integrated circuits (ICs). In
`
`10
`
`15
`
`25
`
`35
`
`45
`
`55
`
`65
`
`2
`such a case, the patterning means 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 in one go; 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 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 (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.
`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
`polishing, 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.
`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 control
`ling 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. TWin stage lithographic
`
`Nikon Exhibit 1004 Page 10
`
`

`

`US 6,646,729 B2
`
`3
`apparatus are described, for example, in Us. Pat. No.
`5,969,441 and WO 98/40791, incorporated herein by refer
`ence.
`There is a desire to integrate an ever-increasing number of
`electronic components in an IC. To realize this, it is neces
`sary to increase the surface area of an IC and/or to decrease
`the siZe of the components. For the projection system, this
`means that both the image ?eld and/or the resolution must be
`increased, so that increasingly smaller details, or line Widths,
`can be imaged in a Well-de?ned Way in an increasingly large
`image ?eld. This requires a projection system that must
`comply With very stringent quality requirements. Despite the
`great care With Which such a projection system is designed
`and the very high accuracy With Which the system is
`manufactured, such a system may still exhibit aberrations,
`such as spherical aberration, coma and astigmatism. In
`practice, the projection system (“lens”) is thus not an ideal,
`diffraction-limited system, but an aberration-limited system.
`The aberrations are dependent on position in the image ?eld
`and are an important source of variations in the imaged line
`Widths occurring across the image ?eld, as Well as in?uenc
`ing the focus, exposure latitude and so on. They also cause
`?eld-dependent overlay errors betWeen different mask struc
`tures and/or different illumination settings. The in?uence of
`aberrations becomes increasingly signi?cant With the appli
`cation of neWer techniques, such as phase-shift masks or
`off-axis illumination, to enhance the resolving poWer of a
`lithographic projection apparatus.
`Afurther problem is that the aberrations are not constant
`in modern lithographic projection systems. In order to
`minimise loW-order aberrations, such as distortion, curva
`ture of ?eld, astigmatism, coma and spherical aberration,
`these projection systems generally comprise one or more
`movable elements. The Wavelengths of the projection beam
`or the position of the mask table may be adjustable for the
`same purpose. When these adjusting facilities are used,
`other, smaller aberrations may be introduced. Moreover,
`since the intensity of the projection beam must be as large
`as possible, the components of the projection system are
`subject to ageing so that the aberrations may change during
`the lifetime of the apparatus. Moreover, reversible changes,
`eg as caused by lens heating, may temporarily change the
`aberrations.
`Consequently there is a further problem of being able to
`measure the aberration reliably and accurately.
`
`SUMMARY
`It is an object of the present invention to provide an
`improved method and apparatus for determining aberration
`of the projection system.
`Accordingly, the present invention provides a method of
`determining aberration of an optical imaging system com
`prising:
`a radiation system for supplying a projection beam of
`radiation;
`a support structure for supporting patterning means, the
`patterning means 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 substrate. Observations are made as
`a function of parameters of the projection apparatus,
`and from these the presence of different types of
`aberration can be quanti?ed. The method comprises the
`step of:
`patterning the projection beam With said patterning
`means; and
`
`15
`
`25
`
`35
`
`45
`
`55
`
`65
`
`4
`characteriZed by the steps of:
`measuring at least one parameter of an image formed
`by the projection system, for a plurality of differ
`ent settings of said radiation system and/or said
`projection system; and
`calculating at least one coefficient, representative of
`aberration of said imaging system, on the basis of
`said at least one parameter measured at said plu
`rality of settings.
`Preferably, said plurality of different settings comprise
`different numerical aperture settings and/or sigma settings,
`illumination modes or telecentricity modes; furthermore,
`one may use various types and siZes of test structures on, for
`instance, one or more masks, to create different diffraction
`effects in the projection system. All such variation should be
`interpreted as falling Within the meaning of the phrase
`“different illumination settings” as used in this text. The
`term “sigma (0) setting” refers to the radial extent of the
`intensity distribution in the beam at a pupil in the imaging
`system through Which the radiation passes, normaliZed With
`respect to the maximum radius of the pupil. Thus, a sigma
`value of 1 represents an illumination intensity distribution
`With a radius at the pupil equal to the maximum radius of the
`pupil. The term “illumination mode” denotes the spatial
`distribution of the radiation at the pupil, Which may be, for
`example, disc-shaped, annular (Which Would be character
`iZed by sigma inner and sigma outer settings), quadrupolar,
`dipolar, soft-multipolar (including some radiation ?ux in
`betWeen the poles), etc. The term “telecentricity modes”
`encompasses con?guring the imaging system telecentrically
`and/or With varying degrees of non-telecentricity, for
`example by the use of prisms on top of a mask to tilt the
`illumination pro?le. These different settings can be selected
`conveniently in a lithographic projection apparatus.
`The measured parameter can be one or more of: the
`position of best-focus of said image; the lateral position of
`said image; the deformation of said image; and other prop
`erties of exposing said image lithographically, such as line
`Width and shape, and distance betWeen adjacent structures.
`Preferably, the plurality of different settings are selected
`such that the variation in the or each at least one measured
`parameter is substantially maximiZed. In this Way the accu
`racy of the determined coefficient(s) can be improved.
`Preferably, the plurality of different settings are selected
`such that the variation in said at least one measured param
`eter resulting from aberration represented by one or more of
`said coefficients is substantially Zero, Whilst the variation in
`said at least one parameter as the function of a coef?cient
`that is to be determined, is non-Zero. This technique enables
`different aberration coef?cients, such as Zernike coef?cients,
`to be obtained independently of each other.
`The invention also provides a lithographic projection
`apparatus for projecting a patterned beam of radiation onto
`a substrate provided With a radiation-sensitive layer, the
`apparatus comprising:
`a radiation system for providing a projection beam of
`radiation;
`a support structure for supporting patterning means, the
`patterning means serving to pattern the projection beam
`according to a desired pattern;
`a substrate table for holding a substrate;
`a projection system for projecting the patterned beam onto
`a target portion of the substrate; and
`illumination setting means for providing a plurality of
`different illumination settings of said radiation system
`and/or said projection system;
`
`Nikon Exhibit 1004 Page 11
`
`

`

`US 6,646,729 B2
`
`5
`characterized by further comprising:
`measuring means for measuring at least one parameter
`of a projected image formed by the projection sys
`tem;
`control means for selecting a plurality of different
`illumination settings at Which said measuring means
`takes measurements; and
`calculation means for calculating at least one
`coefficient, representative of aberration in said pro
`jection and/or radiation system, on the basis of said
`at least one parameter measured by said measuring
`means.
`According to a further aspect of the invention there is
`provided a device manufacturing method comprising the
`steps of:
`(a) providing a substrate that is at least partially covered
`by a layer of radiation sensitive material;
`(b) providing a projection beam of radiation using a
`radiation system;
`(c) using patterning means to endoW the projection beam
`With a pattern in its cross-section;
`(d) using a projection system to project the patterned
`beam of radiation onto a target portion of the layer of
`radiation-sensitive material, and characteriZed by the
`steps of:
`measuring, prior to step (d), at least one parameter of an
`image formed by the projection system, for a plu
`rality of different settings of said radiation system
`and/or said projection system;
`calculating at least one coef?cient, representative of
`aberration of said projection and/or radiation system,
`on the basis of said at least one parameter measured
`at said plurality of settings;
`correcting for said aberration on the basis of said at
`least one calculated coefficient, to reduce aberration
`of an image projected by said projection system.
`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 inte
`grated optical systems, guidance and detection patterns for
`magnetic domain memories, liquid-crystal display panels,
`thin-?lm magnetic heads, etc. The skilled artisan Will appre
`ciate that, in the context of such alternative applications, 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.
`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
`Embodiments of the invention Will noW be described, by
`Way of example only, With reference to the accompanying
`schematic draWings in Which:
`FIG. 1 depicts a lithographic projection apparatus accord
`ing to an embodiment of the invention;
`FIG. 2 is a table summariZing the relationship betWeen
`different loW-order aberrations and their respective Zernike
`coef?cients;
`FIGS. 3(a) and 3(b) are tables of computed best-focus
`positions (With respect to an aberration-free scenario) at
`
`10
`
`15
`
`25
`
`35
`
`45
`
`55
`
`65
`
`6
`different illumination settings as a function of aberration due
`to Zernike coefficients Z9 and Z16;
`FIG. 4 is a plot shoWing the correlation betWeen measured
`best-focus position and best-focus determined on the basis
`of Zernike aberration coef?cients Z9 and Z16 measured
`according to the present invention, Whereby each point in the
`plot corresponds to one of the illumination settings in FIG.
`3. The measured best-focus position is plotted along the
`vertical axis, in nm; the calculated best-focus position is
`plotted along the horiZontal axis, in nm;
`FIGS. 5(a) and 5(b) are graphs shoWing the correlation
`betWeen a Zernike coef?cient measured using the method
`according to the present invention (plotted along the vertical
`axis, in nm) and as determined using another method
`(plotted along the horiZontal axis, in nm), for Z9 and Z16
`respectively;
`FIGS. 6(a) and 6(b) shoW plots of Zernike coef?cients Z9
`and Z16, respectively, as a function of x position along the
`slit of the projection system in a step-and-scan apparatus (in
`Which the slit is scanned in the y-direction), as determined
`both by the method of the present invention and by another
`method. The vertical axis of both FIGS. 6(a) and 6(b)
`represents the value of the aberration coefficient, in nm. The
`horiZontal axes represent the x position along the slit, in mm;
`FIG. 7 is a graph of Z12 (shoWn along the vertical axis,
`in nm) as a function of x position along the slit of the
`projection system (shoWn along the horiZontal axis, in mm)
`both as measured according to the method of the present
`invention and as measured using another method;
`FIGS. 8(a) and 8(b) are contour plots of best-focus
`position as a function of the outer sigma setting (shoWn
`along the horiZontal axes) and numerical aperture (shoWn
`along the vertical axes) of the projection system as a result
`of aberration due to non-Zero Zernike coef?cients Z9 and
`Z16, respectively;
`FIG. 9 is a contour plot of image contrast as a function of
`outer sigma setting (shoWn along the horiZontal axis) and
`numeral aperture (shoWn along the vertical axis) of the
`projection system;
`FIG. 10 is a table of calculated lateral shift in image
`position resulting from comatic (coma) aberration (non-Zero
`Z7) as a function of illumination setting;
`FIG. 11 is a graph shoWing correlation betWeen measured
`x-shift (shoWn along the vertical axis, in nm) and calculated
`x-shift (shoWn along the horiZontal axis, in nm) due to
`comatic aberration, Whereby each point in the plot corre
`sponds to one of the illumination settings in FIG. 10;
`FIGS. 12(a) and 12(b) are contour plots of x-shift due to
`non-Zero Zernike coef?cients Z7 and Z14, respectively, as a
`function of outer sigma setting (shoWn along the horiZontal
`axes) and numerical aperture (shoWn along the vertical axes)
`of the imaging system;
`FIGS. 13(a) and 13(b) are plots of Zernike coma coeffi
`cients Z7 and Z8 (shoWn along the vertical axes, in nm),
`respectively, as functions of x position along the slit of the
`imaging system (shoWn along the horiZontal axes, in mm),
`both as measured according to the method of the present
`invention and as measured using another method; and
`FIG. 14 is a plot corresponding to that of FIG. 13(41), but
`With a Wavelength correction applied to the data of the
`present invention.
`In the Figures, corresponding reference symbols indicate
`corresponding parts.
`
`Nikon Exhibit 1004 Page 12
`
`

`

`US 6,646,729 B2
`
`1O
`
`15
`
`25
`
`35
`
`7
`DETAILED DESCRIPTION
`First Embodiment
`FIG. 1 schematically depicts a lithographic projection
`apparatus according to a particular embodiment of the
`invention. The apparatus comprises:
`a radiation system Ex, IL, for supplying a projection beam
`PB of radiation (e.g. UV or EUV radiation). In this
`particular case, the radiation system also comprises a
`radiation source LA;
`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 PL;
`a second object table (substrate table) WT provided With
`a substrate holder for holding a substrate W (eg a
`resist-coated silicon Wafer), and connected to second
`positioning means for accurately positioning the sub
`strate With respect to item PL;
`a projection system (“lens”) PL (eg a refractive or
`catadioptric system or a mirror group) for imaging an
`irradiated portion of the mask MA onto a target portion
`C (e.g. comprising one or more dies) of the substrate W.
`As here depicted, the apparatus is of a transmissive type
`(i.e. 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 means, such as a programmable mirror
`array of a type as referred to above.
`The source LA (eg a Hg lamp or an excimer laser)
`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 o-outer and o-inner
`values 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
`PB impinging on the mask MA has a desired uniformity and
`intensity distribution in its cross-section.
`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 that 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 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 sce
`narios.
`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 second positioning means (and
`interferometric measuring means IF), the substrate table WT
`55
`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 means 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
`(course positioning) and a short-stroke module (?ne
`positioning), Which are not explicitly 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.
`
`45
`
`65
`
`8
`The depicted apparatus can be used in tWo different
`modes:
`1. In step mode, the mask table MT is kept essentially
`stationary, and an entire mask image is projected in one
`go (i.e. 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;
`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 substrate 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=1/4 or 1/5). In this manner, a relatively
`large target portion C can be exposed, Without having
`to compromise on resolution.
`Aberrations of the projection lens in particular are con
`sidered in the embodiments of this invention. The projection
`lens Wavefront aberrations can be Written as a series accord
`ing to their angular form:
`
`Where r and 6 are radial and angular co-ordinates,
`respectively, (r is normaliZed) and m is an index indicating
`the contribution of the mth aberration. R and R‘ are functions
`of r.
`The aberration can also be expressed in terms of the
`Zernike expansion:
`
`Where each Z is the Zernike coef?cient and each 6 is the
`corresponding Zernike polynomial. The fu