Ulllted States Patent [19]
`Taniguchi et al.
`
`US006078380A
`[11] Patent Number:
`[45] Date of Patent:
`
`6,078,380
`Jun. 20, 2000
`
`[54] PROJECTION EXPOSURE APPARATUS AND
`METHOD INVOLVING VARIATION AND
`CORRECTION OF LIGHT INTENSITY
`
`4,629,313 12/1986 Tanimoto ................................ .. 355/53
`4,650,983
`3/1987 Suwa ......... ..
`..
`ihlmlilu e191
`- 356/358
`
`,
`
`,
`
`ana a .................................. ..
`
`ggggggggqgggfggm AND
`
`6. a1 ....................... ..
`
`CHARACTERISTICS’ AND CONTROL OF
`EXPOSURE
`
`[75] Inventors: Tetsuo Taniguchi, Yokohama;
`Nobutaka Magome; Naomasa
`
`4,780,913 11/1988 Williams . . . . . . . . .
`
`1/1989 Ishizaka et al.
`4,801,977
`8/1989 Kamiya et al. .
`4,853,745
`4,871,237 10/1989 AnZai et al.
`4,931,830
`6/1990 Suwa et al. ..
`4,947,030
`8/1990 Takahashi
`
`Shiraishi, both of Kawasaki, all of
`
`4,988,188
`
`1/1991 Ohta . . . . . . . . . . . .
`
`. . . . .. 4/217
`
`355/30
`355/43
`350/419
`..... .. 355/71
`350/469 X
`
`. . . .. 353/122
`
`Japan
`
`[73] Assigneez Nikon Corporation, Tokyo, Japan
`
`[21] Appl. No.: 09/338,535
`
`[22]
`
`Filed:
`
`Jun. 23, 1999
`
`362/259
`5/1991 Tanaka et al.
`5,016,149
`355/53 x
`4/1992 Ohta etal.
`5,105,075
`355/53
`5/1992 Shiraishi et al.
`5,117,255
`5,160,962 11/1992 Miura et al. ............................ .. 355/53
`
`(List continued on next page.)
`
`FOREIGN PATENT DOCUMENTS
`
`-
`-
`Related US. Application Data
`
`62-50811
`4/1982 Japan .
`2234411 9/1990 Japan '
`
`[63] Continuation of application No. 08/872,385, Jun. 10, 1997,
`abandoned, which is a continuation of application No.
`08/418,125, Apr. 6, 1995, abandoned, which is a continua-
`tion of application No. 08/207,723, Mar. 9, 1994, aban-
`doned, which is a continuation-in-part of application No.
`08/086,913, Jul. 7, 1993, abandoned, which is a continua
`tion-in-part of application No. 07/956,908, Oct. 5, 1992,
`
`abandoned.
`
`""""""""""""""" "
`
`3_26O766
`
`FOI‘EigIl APPIiCHtiOII PI‘iOI‘itY Data
`[30]
`Oct 8 1991
`[JP] M an
`Oct: 8’ 1991
`[JP]
`Jagan
`Dec 19: 1991
`[JP]
`Japan
`Feb, 14, 1992
`[JP]
`Japan
`Mar. 4, 1992
`[JP]
`Japan
`--
`Jul. 7, 1992
`[JP]
`Japan
`Mar. 11,
`Japan .................................. ..
`[51] Int Cl 7
`G03B 27/68
`[52] U Ci """""""""""""""""
`/53_ 356/400
`’
`_'
`'
`' """"""""""""""""" "
`’
`53, 67,
`Fleld of Search ........... ................... ..
`355/71’ 353/97’ 101’ 356/400’ 401
`_
`References Clted
`Us‘ PATENT DOCUMENTS
`
`[56]
`
`WO 92/03842 3/1992 WIPO '
`Primary Examiner_pred LBraun
`d P
`S
`A”
`A
`F-
`V
`S t
`Orney’ gem’ Or lrm— orys’ a er’ eymour an
`ease
`LLP
`
`[57]
`
`ABSTRACT
`
`_
`
`_
`
`_
`
`_
`
`In a projection exposure apparatus and method, the Intensity
`distribution of illumination light for detecting an imaging
`characteristic of a projection optical system is set substan
`tially equal to the intensity distribution of exposure illumi
`nation light. The intensity distribution of a secondary light
`source in an exposure illumination optical system is changed
`in accordance With a pattern of a mask. Magni?cation and
`aberration of the projection optical system are adjusted in
`accordance
`the changed intensity distribution of the
`secondary light source. Asubstrate stage is moved along an
`optical axis of the projection optical system to compensate
`movement of the image plane of the projection optical
`System Caused
`a Change in the intensity distribution of the
`secondary light source. An exposure operation is interrupted
`When a light amount distribution is changed. Exposure
`control is also responsive to thermal accumulation in the
`projection optical system.
`
`4,558,949 12/1985 Uehara et al. ........................ .. 356/152
`
`56 Claims, 32 Drawing Sheets
`
`53
`34
`CONT
`PORTION
`
`WS
`
`Nikon Exhibit 1022 Page 1
`
`

`

`6,078,380
`Page 2
`
`US. PATENT DOCUMENTS
`
`5,335,044
`
`8/1994 Shiraishi ................................. .. 355/53
`
`5243 377 9/1993 Umatate et a1. ........................ .. 355/53
`5:286:963
`2/1994 Torigoe ~~~~ n
`" 250/2012
`
`5337997 8/1994 Suzuki et a1‘ """"""""""""" " 353/101
`5,345,292
`9/1994 ShioZaWa et a1. ...................... .. 355/67
`
`
`
`573007967 5,300,971
`
`
`
`4/1994 Kamon 4/1994 Kudo _____ __
`
`
`
`~~~~~ ~~ 355/53 X
`
`
`
`5,363,172 11/1994 Tokuda .................................... .. 5,367,404 11/1994 Hayata .................................. .. 359/558
`
`5,305,054
`
`4/1994 Suzuki et a1, ___________________________ __ 355/53
`
`5,379,090
`
`1/1995 Shiraishi ................................. .. 355/67
`
`Nikon Exhibit 1022 Page 2
`
`

`

`US. Patent
`
`Jun. 20, 2000
`
`Sheet 1 0f 32
`
`6,078,380
`
`.nJWTIHUUmim
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`5N
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`n_
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`0—Nm8038Eli...Eom!l-mNE
`t
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`
`m
`
`Nikon
`
`Exhibit 1022 Page 3
`
`Nikon Exhibit 1022 Page 3
`
`

`

`U.S. Patent
`
`Jun. 20, 2000
`Sheet 2 0f 32
`FIG. 2
`
`6,078,380
`
`Nikon Exhibit 1022 Page 4
`
`

`

`U.S. Patent
`
`Jun. 20,2000
`
`Sheet 3 0f 32
`
`6,078,380
`
`FIG. 3
`
`F\G. 5
`
`Asme ASA
`
`Nikon Exhibit 1022 Page 5
`
`

`

`U.S. Patent
`
`Jun. 20, 2000
`
`Sheet 4 0f 32
`
`6,078,380
`
`FIG. 4A
`
`H6. 40
`
`RA
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`Nikon Exhibit 1022 Page 6
`
`

`

`U.S. Patent
`
`Jun. 20,2000
`
`Sheet 5 0f 32
`
`6,078,380
`
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`
`Nikon Exhibit 1022 Page 7
`
`

`

`U.S. Patent
`
`Jun. 20,2000
`
`Sheet 6 0f 32
`
`6,078,380
`
`1
`
`90 90a 90b 90c
`
`42
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`
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`
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`
`FIG. 7A
`
`Nikon Exhibit 1022 Page 8
`
`

`

`U.S. Patent
`
`Jun. 20, 2000
`
`Sheet 7 0f 32
`
`6,078,380
`
`41
`
`12
`
`R
`
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`
`+151 ORDER
`
`‘
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`
`Nikon Exhibit 1022 Page 9
`
`

`

`U.S. Patent
`
`Jun. 20,2000
`
`Sheet 8 0f 32
`
`6,078,380
`
`FIG. 9A
`/42
`
`12
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`
`Nikon Exhibit 1022 Page 10
`
`

`

`U.S. Patent
`
`Jun. 20, 2000
`
`Sheet 9 0f 32
`
`6,078,380
`
`FIG. 10A
`
`+151 ORDER”,
`
`FIG. 10B
`
`Nikon Exhibit 1022 Page 11
`
`

`

`US. Patent
`
`Jun. 20, 2000
`
`Sheet 10 0f 32
`
`6,078,380
`
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`Nikon
`
`Exhibit1022
`
`Page 12
`
`Nikon Exhibit 1022 Page 12
`
`

`

`U.S. Patent
`
`Jun. 20,2000
`
`Sheet 11 0f 32
`
`6,078,380
`
`FIG. 12
`
`102
`
`62A 100
`
`104
`
`64C
`
`Nikon Exhibit 1022 Page 13
`
`

`

`U.S. Patent
`
`Jun. 20, 2000
`
`Sheet 12 0f 32
`
`6,078,380
`
`FIG. 13A
`
`42
`/
`
`12
`
`+1stORDE\R
`
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`
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`
`FIG. 13B
`
`Nikon Exhibit 1022 Page 14
`
`

`

`US. Patent
`
`Jun. 20, 2000
`
`Sheet 13 0f 32
`
`6,078,380
`
`#22
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`Exhibit1022 Page15
`
`Nikon Exhibit 1022 Page 15
`
`
`
`

`

`U.S. Patent
`
`Jun. 20, 2000
`
`Sheet 14 0f 32
`
`6,078,380
`
`FIG. 15
`
`EL
`
`112 111
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`EL Hoe 118
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`
`110
`
`93
`
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`
`Nikon Exhibit 1022 Page 16
`
`

`

`U.S. Patent
`
`Jun. 20, 2000
`
`Sheet 15 0f 32
`
`6,078,380
`
`FIG. 17
`
`Nikon Exhibit 1022 Page 17
`
`

`

`U.S. Patent
`
`Jun. 20,2000
`
`Sheet 16 0f 32
`
`6,078,380
`
`FIG. 18A
`
`FIG. 18B
`
`H6186
`
`//////// /
`’//////////
`/ /////// /
`
`/ ////////
`
`//////////'
`
`///////// ,
`
`’//////// /
`
`Nikon Exhibit 1022 Page 18
`
`

`

`U.S. Patent
`
`Jun. 20, 2000
`
`Sheet 17 0f 32
`
`6,078,380
`
`25k.
`
`MIA-IVA.
`in
`imlowP J % mm
`zQEom
`
`mp
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`
`Nikon Exhibit 1022 Page 19
`
`

`

`U.S. Patent
`
`Jun. 20,2000
`
`Sheet 18 0f 32
`
`6,078,380
`
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`
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`
`<6 m5
`
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`
`Nikon Exhibit 1022 Page 20
`
`

`

`US. Patent
`
`Jun. 20, 2000
`
`Sheet 19 0f 32
`
`6,078,380
`
`FIG. 21A
`
`FIG. 210
`
`FIG. 218
`
`FIG. 21D
`
`[gm
`
`
`AMXU-e“) IRRADIATING
`
`ENERGY
`
`Nikon
`
`Exhibit1022
`
`Page 21
`
`Nikon Exhibit 1022 Page 21
`
`

`

`US. Patent
`
`Jun. 20, 2000
`
`Sheet 20 0f 32
`
`6,078,380
`
`w02<IomoF2305?
`
`zofiomuaomn.z_
`
`:23zo_._.<o_u_zo<_2
`
`FIG. 23
`
`FIG. 24
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`
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`
` :23
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`
`Nikon
`
`Exhibit 1022
`
`Page 22
`
`Nikon Exhibit 1022 Page 22
`
`
`

`

`US. Patent
`
`Jun. 20,2000
`
`Sheet 21 0f 32
`
`6,078,380
`
`FIG.25
`
`
`
`
` VALUE Us
`CHANGED ?
`
`
`10
`
`STOP ALM
`
` 9
`
`
`_
`
`110
`
`SET NEW
`
`PARAM
`
`Nikon
`
`Exhibit1022
`
`Page 23
`
`Nikon Exhibit 1022 Page 23
`
`

`

`US. Patent
`
`Jun. 20, 2000
`
`Sheet 22 0f 32
`
`6,078,380
`
`FIG. 26A
`
`FIG. 268
`
`32
`
`FIG. 26C
`
`32
`
`Nikon
`
`Exhibit1022
`
`Page 24
`
`Nikon Exhibit 1022 Page 24
`
`

`

`US. Patent
`
`Jun. 20, 2000
`
`Sheet 23 0f 32
`
`6,078,380
`
`FIG. 27
`
`
`
`Nikon
`
`Exhibit1022
`
`Page 25
`
`Nikon Exhibit 1022 Page 25
`
`

`

`US. Patent
`
`Jun. 20, 2000
`
`Sheet 24 0f 32
`
`6,078,380
`
`FIG. 29
`
`START
`
`RTCL
`
`EXCHANGED?
`
`YES
`
`CONFIRM SET CONDI
`
`READ BARCODE a LIT
`
`CONDI
`
`SET LIT CONDI
`
`DETMN IMG CHARA 8:
`
`SET CALC PARAM
`
`SET LIMIT IMG CHARA
`
`LIT CONDI
`CHANGED?
`
`YES
`
`HISTORY
`AINTAINED?
`
`
`
`YES
`WAIT
`
`
`
`
`
`
`
`214
`
`EXCHANGE WAFER
`
`
`
`
`
`ZISN
`
`Y
`
`211
`
`EXPO
`
`TERMINATED?
`
`END
`
`Nikon
`
`Exhibit1022
`
`Page 26
`
`Nikon Exhibit 1022 Page 26
`
`

`

`US. Patent
`
`Jun. 20, 2000
`
`Sheet 25 0f 32
`
`6,078,380
`
`
`
`Nikon
`
`Exhibit 1022
`
`Page 27
`
`Nikon Exhibit 1022 Page 27
`
`

`

`US. Patent
`
`Jun. 20, 2000
`
`Sheet 26 0f 32
`
`6,078,380
`
`FIG. 31A
`
`FIG.31B
`
` 15A
`
`88
`
`AFS
`(2)
`
`FIG. 32A
`
`FIG. 328
`
`R
`
`PL
`
`15
`
`15A
`
`Nikon
`
`Exhibit1022
`
`Page 28
`
`Nikon Exhibit 1022 Page 28
`
`

`

`US. Patent
`
`Jun. 20, 2000
`
`Sheet 27 0f 32
`
`6,078,380
`
`FIG. 33
`
`
`
`Nikon
`
`Exhibit1022
`
`Page 29
`
`Nikon Exhibit 1022 Page 29
`
`

`

`US. Patent
`
`Jun. 20, 2000
`
`Sheet 28 0f 32
`
`6,078,380
`
`F I G.34A
`
`m5
`
`m3
`7/
`
`m8
`
`R FIG.34B
`PA @
`
`FIG.34C
`
`—~\IV—
`
` X(Y)
`
`(AM)
`
`MP'
`
`MF’
`
`Nikon
`
`Exhibit1022
`
`Page 30
`
`Nikon Exhibit 1022 Page 30
`
`

`

`US. Patent
`
`Jun. 20, 2000
`
`Sheet 29 0f 32
`
`6,078,380
`
`FIG.35
`
`
`
`Nikon
`
`Exhibit1022
`
`Page 31
`
`Nikon Exhibit 1022 Page 31
`
`

`

`US. Patent
`
`Jun. 20, 2000
`
`Sheet 30 0f 32
`
`6,078,380
`
`FIG.36
`
`302
`
`READ CONDIOF RTCL
`
`CHG ILM SYS
`
`YES
`
`PARA
`
`303
`
`305
`
`308
`
`309
`
`ACT MEAS a EXPO
`
`REGlayERED
`NO
`
`304
`
`READ NORM PARA
`
`306
`
`COR BY CALC,
`
`ACT MEAS a EXPO
`
`
`
`
`
`
`PRE CONDI
`
`PRE CONDI
`INFLUENCED
`INFLUENCED
`
`
`7
`
`
`
`COR BY CALC, ACT MEAS,
`MEMO MEAS VAL,
`ILM HYST 8 EXPO
`
`
`
`314
`
`LOT
`
`
`
`
`TERMINATED
`
`CALC 8 REG NEW PARA
`
`Nikon
`
`Exhibit1022
`
`Page 32
`
`Nikon Exhibit 1022 Page 32
`
`

`

`US. Patent
`
`Jun. 20, 2000
`
`Sheet 31 0f 32
`
`6,078,380
`
`A““
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`
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`
`Nikon
`
`Exhibit 1022
`
`Page 33
`
`Nikon Exhibit 1022 Page 33
`
`

`

`US. Patent
`
`Jun. 20, 2000
`
`Sheet 32 0f 32
`
`6,078,380
`
`FIG.38A
`
`OUTPUT OF PD
`
`CLOSE
`
`OPEN
`
`F l G. 588
`
`ILLUMINANCE
`
`DISTRIBUTION
`
`o
`
`RADIUS
`
`F l G. 39
`
`DEFLECTION
`VALUE
`
`0
`
`RADNS
`
`Nikon
`
`Exhibit1022
`
`Page 34
`
`Nikon Exhibit 1022 Page 34
`
`

`

`6,078,380
`
`1
`PROJECTION EXPOSURE APPARATUS AND
`METHOD INVOLVING VARIATION AND
`CORRECTION OF LIGHT INTENSITY
`DISTRIBUTIONS, DETECTION AND
`CONTROL OF IMAGING
`CHARACTERISTICS, AND CONTROL OF
`EXPOSURE
`
`CROSS-REFERENCE TO RELATED
`APPLICATIONS:
`
`This application is a continuation of application Ser. No.
`08/872,385 filed Jun. 10, 1997, now abandoned, which is a
`continuation of application Ser. No. 08/418,125 filed Apr. 6,
`1995, now abandoned, which is a continuation of application
`Ser. No. 08/207,723 filed Mar. 9, 1994, now abandoned,
`which is a continuation-in-part of application Ser. No.
`08/086,913 filed Jul. 7, 1993, now abandoned, which is a
`continuation-in-part of application Ser. No. 07/956,908 filed
`Oct. 5, 1992, now abandoned.
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`
`The present invention relates to projection exposure appa-
`ratus method, used in a lithographic process in the manu-
`facture of semiconductor integrated circuits and liquid crys-
`tal devices and, more particularly,
`to maintenance and
`adjustment of imaging performance of a projection optical
`system.
`2. Related Background Art
`In a photolithographic process for forming circuit patterns
`as of semiconductor elements, there is employed a method
`of transferring a pattern formed on a reticle (mask) onto a
`substrate (e.g., a semiconductor wafer or glass plate). A
`photoresist having photosensitive properties is applied to the
`substrate, and a circuit pattern is transferred to the photo-
`resist in accordance with an irradiating optical image, i.e., a
`pattern shape of a transparent portion of a reticle pattern. In
`a projection exposure apparatus (e.g., a stepper), a reticle
`pattern image is imaged and projected on the wafer through
`a projection optical system.
`In an apparatus of this type, illumination light is limited
`to have an almost circular (rectangular) shape centered on
`the optical axis of the illumination optical system to illumi-
`nate the reticle on an illumination optical system plane (to be
`referred to as a pupil plane of an illumination optical system
`hereinafter) serving as a Fourier transform plane for a
`surface on which a reticle pattern is present or a plane near
`the pupil plane of the illumination optical system (this
`illumination scheme is called normal
`illumination
`
`hereinafter). For this reason, the illumination light is inci-
`dent on the reticle at almost a right angle. A circuit pattern
`having a transparent portion (i.e., the naked surface of the
`substrate) having a transmittance of 100% for the illumina-
`tion light and a light-shielding portion (chromium or the
`like) having a transmittance of almost 0% is formed on a
`reticle (i.e., a glass substrate as of quartz) used in this
`apparatus.
`The illumination light radiated on the reticle, as described
`above, is diffracted by the reticle pattern, and 0-th-order and
`=1st-order diffracted light components emerge from the
`pattern. These diffracted light components are focused by
`the projection optical system to form interference fringes,
`i.e., reticle pattern images on the wafer. An angle defined by
`the 0-th-order diffracted light component and each of the
`=1st-order diffracted light components is defined as sin0=
`
`2
`
`)L/P where )L is the wavelength (am) of the exposure light,
`and NA is the numerical aperture of the projection optical
`system on the reticle side.
`the sine value is
`When a pattern pitch is decreased,
`increased. When the sine value is larger than the numerical
`aperture NA of the projection optical system on the reticle
`side, the =1st-order diffracted light components are limited
`by the effective diameter of the projection optical system
`plane (to be referred to as a pupil plane of the projection
`optical system hereinafter) serving as the Fourier transform
`plane of the reticle pattern and cannot pass through the
`projection optical system. That is, only the 0-th-order dif-
`fracted light component can reach the wafer, and the inter-
`ference fringes (pattern image) are not formed.
`In the
`conventional exposure method described above, when the
`reticle having only the transparent and light-shielding por-
`tions (to be referred to as a normal reticle hereinafter) is
`used, the degree of micropatterning of the reticle pattern
`(minimum pattern pitch) P which can be resolved on the
`wafer is given as Pal/NA since sin0 =NA. The minimum
`pattern size is 1/2 the pitch P, and the minimum pattern size
`is given as about 0.5><MNA. In a practical photolithographic
`process, however, a given depth of focus is required due to
`warping of the wafer, influences of steps on the wafer during
`the process, and the thickness of the photoresist itself. For
`these reasons, the practical minimum resolution pattern size
`is represented as kxk/NA where k is the process coefficient
`which generally falls within the range of about 0.6 to 0.8.
`In order to expose and transfer a fine pattern in accordance
`with the conventional exposure method, an exposure light
`source which emits light having a shorter wavelength or a
`projection optical system having a larger numerical aperture
`must be used.
`
`However, it is difficult to arrange an exposure light source
`which emits light having a shorter wavelength (e.g., 200 nm
`or less) than that of the existing exposure light source at
`present because an optical material suitably used as a
`light-transmitting optical member is not available and a
`stable light source capable of emitting a large amount of
`light
`is not available either.
`In addition,
`the numerical
`aperture of the state-of-the-art projection optical system is
`almost a theoretical limit, and the numerical aperture is
`assumed not to drastically increase. Even if the numerical
`aperture can be larger than that currently used, the depth of
`focus determined by fit/NA2 abruptly decreases with an
`increase in numerical aperture, and the depth of focus used
`in practice further decreases, resulting in inconvenience.
`There is also proposed a phase shifting reticle having a
`phase shifter (e.g., a dielectric thin film) for shifting light
`transmitted through a specific one of transparent portions of
`the reticle circuit patterns from light transmitted through
`another transparent portion by at (rad). A phase shifting
`reticle is disclosed in Japanese Patent Publication No.
`62-50811. When this phase shifting reticle is used, a finer
`pattern can be transferred as compared with use of the
`normal reticle. That is, a resolving power can be increased.
`In order to use this phase shifting reticle, the numerical
`aperture (coherence factor 0) of the illumination optical
`system must be optimized. Various schemes are proposed for
`the phase shifting reticle, and typical examples are a spatial
`frequency modulation scheme, a shifter light-shielding
`scheme, and an edge emphasis scheme.
`In recent years, various attempts have been made to allow
`transfer of micropatterns in accordance with optimization of
`illumination conditions (LIT CONDI) or implementations of
`an exposure method. As described in US. Pat. No. 4,931,
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`6,078,380
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`830, there is provided a method of increasing the resolving
`power and the depth of focus for patterns having specific line
`widths, in such a manner that a combination of an optimal
`numerical aperture (value 0) of the illumination optical
`system and an optimal numerical aperture (N.A.) of the
`projection optical system is selected every pattern line
`width. In addition, an annular illumination method is also
`proposed in which a light amount distribution of illumina-
`tion light on or near the pupil plane of the illumination
`optical system is defined to have an annular shape, and a
`reticle pattern is irradiated with the annular illumination
`light. There is also proposed an oblique illumination method
`in which the light amount distribution of the illumination
`light on or near the pupil plane of the illumination optical
`system is set
`to have maximal values at a plurality of
`positions eccentric from the optical axis of the illumination
`optical system, and illumination light is inclined at a pre-
`determined angle in correspondence with the periodicity of
`a reticle pattern and is incident on the reticle pattern from a
`specific direction, as described in PCT/JP91/01103 (Aug.
`19, 1991) and Ser. No. 791,138 (Nov. 13, 1991). Any one of
`the methods described above is not effective for all reticle
`
`patterns, i.e., line widths and shapes of the patterns. Optimal
`illumination method and conditions must be selected every
`reticle or patterns thereof. The projection exposure appara-
`tus must have a structure in which illumination conditions
`
`(e.g., the value a) in the illumination optical system must be
`set variable.
`
`In the manufacture of semiconductor integrated circuits,
`although a high resolving power is required, a large depth of
`focus (focus margin) also serves as an important factor.
`A method of increasing the depth of focus by giving a
`specific aberration (particularly, a spherical aberration) to a
`projection optical system without using the modified light
`source and the like described above is proposed in Japanese
`Laid-Open Patent Application No. 2-234411. This method
`utilizes the thickness (about 1 gm) of a photosensitive
`material (photoresist) on the wafer. The depth of focus can
`be increased although the contrast
`level
`is slightly
`decreased.
`
`In a recent projection exposure apparatus, imaging char-
`acteristics of the projection optical system are required to be
`constant with high precision. Various methods of adjusting
`the imaging characteristics are proposed and put into prac-
`tice. Among them all, a method of correcting variations in
`imaging characteristics which are caused by exposure light
`absorption in the projection optical system is disclosed in
`US. Pat. No. 4,666,273. In this method, an energy amount
`(heat) accumulated in the projection optical system upon
`incidence of exposure light on the projection optical system
`is sequentially calculated, an amount of change in each
`imaging characteristic caused by the accumulated energy
`amount is obtained, and each imaging characteristic is finely
`adjusted by a predetermined correction mechanism. This
`correction mechanism can be arranged in accordance with,
`e.g., a scheme for sealing a space defined by two of a
`plurality of lens elements constituting the projection optical
`system and adjusting the pressure of the sealed space.
`The imaging characteristics corrected as described above
`are a projection magnification,
`the focal position,
`the
`distortions, and the like. In a projection optical apparatus
`which requires high-precision control of imaging
`characteristics, other imaging characteristics such as the
`curvature of field, which characteristics are not convention-
`ally corrected because the correction is difficult and varia-
`tions in accumulated energy amount (thermal accumulation
`amount) are small, are also taken into consideration as
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`correction targets. There is proposed a method in which the
`limit value (reference value) of the thermal accumulation
`amount
`is determined in advance so that variations in
`
`imaging characteristics (caused by exposure light
`absorption) corresponding to the thermal accumulation
`amounts of the projection optical system do not exceed
`predetermined amounts. According to this method,
`in
`sequential exposure of wafer with a reticle pattern in accor-
`dance with a step-and-repeat scheme, when an actual ther-
`mal accumulation amount of the projection optical system
`exceeds the reference value,
`the exposure operation is
`stopped, and the exposure operation is inhibited (kept
`stopped) until
`the actual
`thermal accumulation amount
`becomes below the reference value. More specifically, in the
`same manner as in the technique disclosed in the prior art
`described above,
`the thermal accumulation amount of
`energy accumulated in a projection optical system upon
`incidence of exposure light
`thereon is sequentially
`calculated, and the calculated thermal accumulation amount
`is compared with a predetermined reference value every shot
`in exposure or every exchange of the wafer to determine
`whether the exposure operation for the next shot is per-
`formed.
`
`Since the method disclosed in US. Pat. No. 4,666,273
`does not directly detect the imaging characteristics of the
`projection optical system, it is necessary to directly detect
`the imaging characteristics of the projection optical system
`by using a focal position detection system, the patent appli-
`cation of which is filed as Ser. No. 830,213 (Jan. 30, 1992).
`This focal position detection system will be described with
`reference to FIGS. 30 to 33.
`
`Referring to FIG. 30, a reticle R on which circuit patterns
`are drawn is held in a reticle holder 14. The reticle R is
`
`illuminated at a uniform illuminance with exposure light
`such as bright lines from a mercury lamp or an excimer laser
`beam. A wafer W is held on a wafer holder 16 placed on a
`wafer stage WS. In a normal exposure transfer mode, the
`pattern on the reticle R is imaged on the wafer W through a
`projection optical system PL.
`The wafer stage WS is constituted by an XY stage
`movable within a plane (to be referred to as an XY plane
`hereinafter) perpendicular to the optical axis of the projec-
`tion optical system PL and a Z stage movable above the XY
`stage in the Z direction parallel to the optical axis of the
`projection optical system PL. The wafer holder 16 also
`serves as a 0 stage which can be slightly rotated within the
`XY plane. Coordinates of each exposure point correspond-
`ing to the optical axis of the projection optical system PL are
`measured by a biaxial laser interferometer (not shown). Z
`coordinates of the wafer W on the exposure surface upon
`movement of the Z stage in the Z direction can also be
`measured by a measuring mechanism (not shown). When the
`wafer stage WS is moved within the plane perpendicular to
`the optical axis of the projection optical system PL, the
`pattern on the reticle R is exposed on the wafer W in
`accordance with the step-and-repeat scheme. When the
`wafer stage WS is also slightly moved in the axial direction
`of the projection optical system PL, the wafer W can be
`matched with the focal position of the projection optical
`system PL.
`A focal position detection pattern plate (reference
`member) 15 is disposed on the wafer stage WS. An aperture
`pattern (multislit pattern) 15A constituted by light-shielding
`and transparent portions is formed on the upper surface of
`the pattern plate 15, as shown in FIG. 31A. This aperture
`pattern 15A is constituted such that four amplification type
`diffraction gratings each consisting of lines and spaces at a
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`6,078,380
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`5
`predetermined pitch are rotated through every 90°. The
`pattern plate 15 is fixed on the wafer stage WS such that the
`surface (i.e., the surface on which the aperture pattern 15A
`is formed) of the wafer stage WS is located at almost the
`same level as that of the exposure surface (upper surface) of
`the wafer W with respect to the Z direction (i.e., the axial
`direction of the projection optical system PL). A detection
`illumination optical system is arranged below the lower
`surface (i.e., inside the wafer stage WS) of the pattern plate
`15.
`
`In the detection illumination optical system, illumination
`light EL having a wavelength range which is the same as or
`close to that of exposure light for illuminating the reticle R
`is incident on one branch end 110a of a two-split fiber
`bundle 110. The illumination light EL is light obtained by
`splitting part of exposure light IL by means of a beam splitter
`or the like. The illumination light EL is guided from the
`branch end 110a to a merged end 110C and is supplied to the
`inside of the wafer stage WS. The illumination light EL
`illuminates the aperture pattern 15A of the pattern plate 15
`upward through an output lens 111, a field stop 113, a relay
`lens 114, a mirror 115, and a condenser lens 116. The beam
`passing through the pattern plate 15 forms an image of the
`aperture pattern 15A of the pattern plate 15 on the lower
`surface (pattern surface) of the reticle R through the pro-
`jection optical system PL. Light reflected by the pattern
`surface of the reticle R returns again to the inside of the
`wafer stage WS through the projection optical system PL
`and the pattern plate 15. The light is then incident on the
`merged end 110C of the fiber bundle 110 along a path
`opposite to the incident path. This reflected light emerges
`from the other branch lens 110b of the fiber bundle 110 and
`
`is incident on a photoelectric sensor PD. The photoelectric
`sensor PD outputs a focus signal FS, i.e., a photoelectric
`signal corresponding to the amount of beam reflected by the
`pattern surface of the reticle R and passing through the
`aperture pattern 15A of the pattern plate 15, to an autofocus
`controller (AFC) 35.
`According to this scheme, when the focus signal FS
`output from the photoelectric sensor PD has a maximum
`magnitude,
`i.e., when the amount of light obtained by
`limiting the light reflected by the reticle R by means of the
`pattern plate 15,
`the corresponding Z coordinates are
`detected as a focal position. The principle of the maximum
`amount of light at the focal position will be described below
`with reference to FIGS. 32A to 32C. FIG. 32A shows an
`
`optical path diagram in which the aperture pattern formation
`surface of the pattern plate 15 is conjugate (imaging
`relationship) with the pattern surface of the reticle R with
`respect to the projection optical system PL, i.e., the upper
`surface of the pattern plate 15 is located at the focal position
`of the projection optical system PL. The beam passing
`through the transparent portion of the pattern plate 15
`toward the projection optical system PL forms the focused
`image of the aperture pattern 15A on the lower pattern
`surface of the reticle R, and its reflected light also forms a
`focused image on the pattern plate 15. For this reason, the
`aperture pattern 15A of the pattern plate 15 and the refo-
`cused aperture pattern image are perfectly superposed on
`each other. The image of the aperture pattern (i.e.,
`the
`imaging beam) directly passes through the pattern plate 15
`and is finally incident on the photoelectric sensor PD.
`On the other hand, FIG. 32B shows an optical path
`diagram in which the aperture pattern formation surface of
`the pattern plate 15 is not located at the focal position of the
`projection optical system PL. In this case, the light reflected
`by the lower surface of the reticle R cannot be entirely
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`transmitted through the aperture pattern 15A of the pattern
`plate 15, so that part of the reflected light is reflected by the
`light-shielding portion (hatched portion) of the aperture
`pattern 15A. The amount of light incident on the photoelec-
`tric sensor PD is reduced. In practice, since an interference
`phenomenon between the rays occurs, the focus signal FS
`corresponding to the amount of light obtained by limiting
`the light reflected by the reticle R by means of the pattern
`plate 15 has a waveform shown in FIG. 31B. Referring to
`FIG. 31B, an autofocus signal AFS detected by an indirect
`focus position detection system (30 and 31) (to be described
`later) is plotted along the abscissa. This signal AFS corre-
`sponds to the Z coordinates of the wafer stage WS.
`Referring to FIG. 30, a beam emitted from a light-
`emitting optical system 30 is incident obliquely on the
`pattern plate 15 with respect
`to the optical axis of the
`projection optical system PL. For example, a slit-like pattern
`is projected on the pattern plate 15. Light reflected by this
`pattern plate 15 is projected on the light-receiving element
`of a light-receiving optical system 31, and an image of the
`slit pattern formed on the pattern plate 15 is refocused on the
`light-receiving element of the light-receiving optical system
`31. When the pattern plate 15 is moved in the Z direction
`parallel to the optical axis of the projection optical system
`PL, the slit pattern image on the light-receiving element of
`the light-receiving optical system 31 is also moved. The
`position of the aperture pattern formation surface (or the
`exposure surface of the wafer W) in the Z direction can be
`detected from the slit pattern position.
`The light-receiving optical system 31 outputs the signal
`(to be referred to as an autofocus signal) AFS corresponding
`to the position of the slit pattern image. The autofocus signal
`AFS is supplied to the autofocus controller 35. The autofo-
`cus controller 35 also receives the focus signal FS of the
`direct scheme from the photoelectric sensor PD. The auto-
`focus controller 35 performs offset adjustment of the auto-
`focus signal AFS by using the focus signal FS of the direct
`scheme. A Z-axis drive signal ZS is supplied to a motor 17
`so that the autofocus signal AFS is set at a predetermined
`level, thereby driving the Z stage of the wafer stage W8. A
`technique for performing calibration (offset adjustment) of a
`focusing mechanism of an indirect scheme using the detec-
`tion result of the direct scheme is disclosed in US. Pat. No.
`4,650,983.
`Amethod of focusing the exposure surface of the wafer W
`with respect to the projection optical system PL by using the
`focal position detection system having the pattern plate 15
`will be briefly described below. As described above, the
`distance between the wafer W and the projection optical
`system PL is adjusted by a wafer position detection system
`comprising the light-emitting optical system 30 and the
`light-receiving optical system 31 during exposure of the
`wafer W. For this reason, the focal position obtained in the
`focal position detection system including the pattern plate 15
`upon movement of the pattern plate 15 to t