United States Patent [19]
`Phillips, Jr. et al.
`
`US005650877A
`[11] Patent Number:
`[45] Date of Patent:
`
`5,650,877
`Jul. 22, 1997
`
`[54] IMAGING SYSTEM FOR DEEP
`ULTRAVIOLET LITHOGRAPHY
`
`FOREIGN PATENT DOCUMENTS
`4203464 8/1992 Germany I
`
`[75] Inventors: Anthony R. Phillips, J r.. Fairport; Paul
`F. Michaloski. Rochester. both of NY.
`
`[73] Assigneei Tl‘opel col'pol‘a?on~ FaJ'IPML N-Y
`
`[21] Appl. No.: 514,614
`
`Aug‘ 14’ 1995
`[22] Filed:
`[51] Int. 01.6 ................................................... .. G02B 17/00
`[52] US. Cl. ........................................... .. 359/732; 359/731
`[58] Field of Search ................................... .. 359/732. 731,
`359/73O_ 727, 726
`
`[56]
`
`References Cited
`
`us PATENT DOCUMENTS
`4,953,960 9/1990 wmmon _
`5,031,976
`7/1991 Shafer'
`5,031,977
`7/1991 Gibson .................................. .. 359/732
`5,206,515
`4/1993 Elliott et al. .
`5,212,593
`5/1993 wllliamsoll 6t 31- -
`5,220,454 6/1993 Ichihara et a1. .
`5,241,423
`8/1993 Chill et al. .
`5,251,070 10/1993 Hashimoto et al. .
`5,289,312 2/1994 Hashimoto et a1. .
`5,402,267
`3/1995 Ftirter et a1. .......................... .. 359/732
`5,461,456 10/1995 Michaloski .
`
`OTHER PUBLICATIONS
`
`“A New Series of Microscope Objectives: I. Catadioptric
`Newtonian Systems” by David S. Grey and Paul H. Lee.
`Journal of the Optical Society of America. vol. 39. No. 9.
`Sep. 1949. pp. 719-728.
`
`Primary Examiner-Scott J. Sugannan
`Mame» Agent, or Fi'mr-Eugene StcPhtIls & Asmiates
`[57]
`ABSTRACT
`
`A catadioptric reduction system operating in the deep ultra
`violet range projects a reduced image of a mask on a
`substrate. A reducing optic made of a material transmissive
`to deep ultraviolet light has a concave front face covered by
`a partially re?ective surface and a convex back face covered
`by a concave re?ective surface surrounding a central aper
`ture. The partially re?ective surface transmits a portion of
`the light passing through the mask to the concave re?ecting
`surface. Which returns a portion of the transmitted light to
`the partially re?ective surface. Aportion of the returned light
`is re?ected by the partially re?ective surface on a converging
`path through said central aperture for producing a reduced
`image of the mask on the substrate.
`
`30 Claims, 2 Drawing Sheets
`
`1
`
`ZEISS 1031
`Zeiss v. Nikon
`IPR2013-00362
`
`

`

`US. Patent
`US. Patent
`
`Jul. 22, 1997
`Jul. 22, 1997
`
`Sheet 1 of 2
`Sheet 1 0f 2
`
`5,650,877
`5,650,877
`
`N A KJ
`
`FIG.I
`
`36
`
`2
`
`
`
`

`

`U.S. Patent
`
`Jul. 22, 1997
`
`Sheet 2 of 2
`
`5,650,877
`
`(2
`
`/_?_____ \ .
`
`i
`
`42
`
`4 72
`
`4O
`
`44
`
`46
`
`72L
`48
`
`54
`
`52
`
`64
`
`66
`
`56
`
`FIG. 2
`
`62
`
`36
`
`3
`
`
`
`

`

`5,650,877
`
`1
`IMAGING SYSTEM FOR DEEP
`ULTRAVIOLET LITHOGRAPHY
`
`TECHNICAL FIELD
`
`The invention relates to catadioptric reduction systems for
`projecting images with low aberrations and to exposure
`systems for microlithographic manufacture with deep ultra
`violet light.
`
`10
`
`BACKGROUND
`Microelectronics. including semiconductors, storage
`devices. and ?at panel displays. are generally fabricated in
`successive layers using photolithographic techniques for
`patterning surface features. A reticle or mask having a
`predetermined pattern is evenly illuminated and projected
`onto a layer of photoresist on the surface of the microelec
`tronic substrate. Exposed portions of the photoresist are
`chemically altered. rendering them more or less soluble to a
`developer that removes the soluble portions leaving a posi
`tive or negative image of the mask.
`High resolution of the surface features is, of course.
`important; and improved resolution is continually sought for
`making the surface features smaller and more closely spaced
`so the resulting electronics can be made smaller, faster. and
`25
`cheaper. Aresolution dimension “R” representing minimum
`feature size is related to light wavelength “7t”, numerical
`aperture “NA”, and a process related constant “K1” as
`follows:
`
`20
`
`“mm
`
`Feature size “R” can be reduced by reducing the wave
`length “?t” or the process constant “K1” or by increasing the
`numerical aperture “NA”. In production environments. pro
`cess constants “K1” equal to 0.7 to 0.8 are typical, whereas
`constants “K” as low as 0.5 can be achieved in laboratory
`settings. Numerical aperture “NA” and wavelength “A” are
`also related to depth of focus ‘D?’ as follows:
`
`35
`
`40
`
`2»
`Df:
`NA2
`
`2
`Laser light sources operating within the ultraviolet and
`deep ultraviolet ranges emit light within narrow bands of
`Wavelengths. However. even narrow bands of wavelength
`cause signi?cant chromatic aberrations in single-material
`lenses with ?nite focal lengths. On the other hand, limiting
`laser output to a single wavelength is ine?icient.
`Accordingly. catadioptric imaging systems have evolved
`which use re?ective optics (mirrors) to reduce image size in
`combination with refractive optics (lenses) to compensate
`for symmetrical aberrations of the re?ective optics.
`Beamsplitters or partially re?ective mirrors are used to
`separate light traveling to and from the re?ective optics.
`Bearnsplitters and partially re?ective mirrors, particularly
`when subjected to angularly diverging beams. introduce
`additional aberrations requiring correction. The beamsplit
`ters also add to the complexity of the imaging systems by
`misaligning the object and image planes.
`A typical catadioptric optical reduction system used for
`microlithographic projections is disclosed in US. Pat. No.
`5.241,423 to Chiu et al. Aconcave spherical mirror provides
`a four to ?ve times reduction in the projected image size
`with respect to a mask. and a beam-splitting cube separates
`light beams traveling to and from the mirror. Groups of
`refractive optical elements located on opposite sides of the
`beam-splitting cube toward both the reticle (mask) and the
`substrate correct for aberrations of the mirror and beam
`splitting cube.
`Chiu et al.’s reduction system is intended for operation at
`wavelengths of about 248 NM produced by a KrF excimer
`laser. However. the large number of refractive elements and
`the bulky two prism construction of the beam-splitting cube
`limit usefulness of this system at shorter wavelengths. The
`transmission of light through fused quartz or ?uorite dimin
`ishes with shortening wavelengths, so the number and bulk
`of refractive optics must be limited to utilize wavelengths
`within the deep ultraviolet spectrum at less than 200 NM
`length.
`US. Pat. Nos. 5.251.070 and 5289312 to Hashimoto et
`al. also use a concave mirror to provide most of the reducing
`power but use a semi-transparent mirror on a plane parallel
`plate instead of a beam-splitting cube to separate light beams
`traveling to and from a concave mirror. The former patent of
`Hashimoto et al. incorporates plane parallel retracting plates
`to correct aberrations caused by the semi-transparent mirror.
`The latter patent of Hashimoto et al. uses high-power
`refractive optics to collimate the beam transmitted through
`the semi-transparent mirror. This reduces aberrations from
`the semi-transparent mirror but still requires other refractive
`optics to counteract aberrations introduced by the high
`power refractive optics.
`
`SUMlVIARY OF INVENTION
`Our invention extends microlithographic manufacture
`into the deep ultraviolet spectrum (e.g.. less than 200 NM
`Wavelength) for further reducing the minimum feature size
`of projected images to less than 0.2 microns. A practical size
`reticle (mask) is maintained by achieving the feature size
`with a highly reduced image of the reticle. The number of
`corrective refractive optics is held to a minimum, and the
`con?guration of optical elements is simpli?ed by maintain
`ing object and image planes of a reducing system both
`parallel to each other and aligned with a common optical
`axrs.
`A lens group conditions a beam of light after passing
`through the reticle. A reducing optic having specially con
`?gured front and back faces projects a reduced image of the
`
`A depth of focus ‘D?’ of at least a fraction micron (e.g.,
`0.5 microns) is needed to accommodate ?atness variations
`of the microelectronic substrates and their successive layers.
`Since numerical aperture “NA” is raised to a higher power
`than wavelength “X” in the above expression for depth of
`focus “Df’. resolution improvements achievable by enlarg
`ing numerical aperture “NA” are much more limited than
`those achievable by shortening the wavelength “7t”.
`Wavelengths less than 300 nanometers (NM) can be
`practically transmitted by only a few optical materials such
`as fused (synthetic) quartz and ?uorite (calcium ?uoride).
`The transmissivity of even these materials deteriorates at
`wavelengths in the deep ultraviolet range less than 200 NM
`so a minimum number of optical elements is desirable.
`Although it is advantageous to minimize feature size of
`the images projected onto the microelectronic substrates, the
`feature size of the masks should remain large enough to
`manufacture e?iciently and to avoid errors from mild levels
`of contamination. For example. it is important that small
`specks of contamination do not bridge features of the masks.
`Mask size can be maintained by optically reducing the
`projected image of the mask with respect to the mask itself.
`
`45
`
`50
`
`55
`
`65
`
`4
`
`
`
`

`

`3
`reticle onto a substrate. Both the lens group and the reducing
`optic are made from materials that transmit deep ultraviolet
`light. The back face of the reducing optic has a central
`aperture surrounded by a concave re?ective surface. The
`front face of the reducing optic has a partially re?ective
`surface that transmits a portion of the light beam toward the
`concave re?ecting surface and re?ects a portion of the
`remaining light beam returned by the concave re?ective
`surface on a converging path through the central aperture.
`The substrate is aligned with the aperture for receiving the
`reduced image of the reticle.
`The re?ective surfaces of the reducing optic provide the
`reducing power. which is preferably a 10-fold reduction in
`the mask size. The re?active elements of the lens group and
`reducing optic exhibit little or no combined reducing power
`to avoid chromatic aberrations. Instead, the lens group
`corrects at least some of the nonchromatic aberrations
`generated by the reducing optic. A substantially plane par
`allel plate is preferably incorporated into the lens group and
`modi?ed to include an aspheric surface to correct spherical
`aberrations. The concave re?ective surface of the reducing
`optic can also be modi?ed to include an aspheric surface to
`correct spherical aberrations at an even higher rate.
`A central obscuration blocks a portion of the beam of
`light. which would not be re?ected by the concave re?ective
`surface. from passing through the central aperture.
`Preferably. the central obscuration is limited in size to block
`no more than 15 percent of projected image. The central
`obscuration can be conveniently the beam diameter within
`the lens group. More than 15 percent blockage can cause
`signi?cant degradation in contrast of the formed as a stop on
`the plane parallel plate.
`
`DRAWINGS
`
`FIG. 1 is a schematic layout of a microlithographic
`projection system arranged according to our invention.
`FIG. 2 is an enlarged diagram of our new catadioptric
`reducing system for completing the projection of an illumi
`nated mask on a substrate.
`
`DETAILED DESCRIPTION
`
`According to a preferred embodiment of our invention
`illustrated in the drawing ?gures, a laser light source 10 is
`an Argon-Fluoride excimer laser that produces a collimated
`beam 12 of ultraviolet light having a wavelength bandwidth
`between 192.6 and 194 NM. A series of three folding mirrors
`14, 16. and 18 convey the collimated beam 12 to an
`illuminator 20.
`Within the illuminator 20, the collimated beam 12 is
`attenuated and dispersed by a pair of diifusers 22 and 24
`before entering a square re?ecting tunnel 26. The diffuser 22
`is adjustable along an optical axis 28 both to control the
`amount of light entering the re?ecting tunnel 26 through the
`di?’user 24 and to more uniformly disperse the entering light
`over an area of the diffuser 24 in common with an entrance
`25 of the re?ecting tunnel 26. The amount of separation
`between the diffuser 22 and the tunnel entrance 25 controls
`the amount of excess light that is scattered beyond the tunnel
`entrance 25. Together. the two di?users 22 and 24 produce
`a wider angle of uniformly dispersed light entering the
`re?ecting tunnel 26.
`The re?ecting tunnel 26 functions as a “uniformizer” by
`dividing the diffused beam 12 into segments and arranging
`the segments into a contiguous array. Unlike most
`uniformizers. which are made from solid optical materials
`
`45
`
`55
`
`65
`
`5,650,877
`
`15
`
`30
`
`35
`
`4
`such as polyhedral rods or ?y’s eye lenses, the re?ecting
`tunnel 26 is hollow with re?ective sides to avoid excessive
`absorption of the deep ultraviolet light. Such excessive
`absorption limits control over the amount of light that can be
`transmitted through the illuminator 20 and reduces the
`useful life of the illuminator 20 by degrading the optical
`materials.
`A lens group 30 magni?es and projects an image of a
`plane at an exit 27 of the re?ecting tunnel 26 onto a plane
`of a reticle 34, which functions as a mask for microlitho
`graphic manufacture of a substrate 36. In addition. the lens
`group 30 images a plane at the entrance 25 of the re?ecting
`tunnel 26 onto a plane at a variable aperture stop 38 within
`the lens group 30. The tunnel entrance 25 is imaged at the
`variable aperture stop 38 as an array of closely knit re?ec
`tions produced by the re?ecting tunnel 26. The variable
`aperture stop 38 functions as a mask by excluding portions
`of the beam 12 to enhance the diifractive effects of the reticle
`34. For example. the aperture stop 38 can take the form of
`an annular ring or a series of holes which transmit only
`selected portions of the beam 12.
`The combined effect of the ditfusers 22 and 24. which
`provide a wider angular dispersion of light entering the
`re?ecting tunnel 26. improves spatial uniformity of the
`distribution of light energy throughout the array of re?ec
`tions within the aperture stop 38. The dispersion of light
`produced by the adjustable diffuser 22 on the diffuser 24 also
`improves the spatial uniformity of the distribution of light
`within each re?ection of the tunnel entrance 25 that com
`prises the array. The improved spatial uniformity of the
`beam 12 at the aperture stop 38 enhances the masking effect
`of the aperture stop 38.
`Although the uniformizing effects of the diffusers 22 and
`24 are most evident at the aperture stop 38. spatial unifor
`mity is also improved at the tunnel exit 27. which is imaged
`at the reticle 34. Thus. the light beam 12 impinges on the
`reticle 34 with a uniform spatial distribution of light energy,
`while the angular distribution of the impinging light is
`controlled by the aperture stop 38 to enhance the contrast of
`the reticle’s image on the substrate 36.
`Our catadioptric reducing system 40. shown in more
`detail by FIG. 2, projects a greatly reduced image of the
`illuminated reticle 34 onto a surface of the substrate 36. The
`reticle 34 and the substrate 36 are oriented parallel to each
`other and are aligned together with our catadioptric reducing
`system 40 along the optical axis 28.
`A lens group 42. comprising transmissive optical ele
`ments 44. 46. 48. and 50, conditions the beam 12 for entry
`into reducing optic 52 having a concave front face 54 and a
`convex back face 56. The concave front face 54 of the
`reducing optic 52 is coated to form a partially re?ective
`surface 58 that provides partial transmission uniformly
`throughout its aperture. The convex back face 56 is coated
`in an annular pattern to form a concave re?ective surface 60
`surrounding a central aperture 62.
`A portion 64 of the beam 12 is transmitted through the
`partially re?ective surface 58 to the concave re?ective
`surface 60. which returns a converging beam 66. A portion
`68 of the returning beam 66 is re?ected by the partially
`re?ective surface 58 on a converging path through the
`central aperture 62 to a point of focus on the substrate 36.
`The reducing optic 52 focuses a reduced image of the reticle
`34 on the substrate 36.
`The lens groups 30 and 42. along with the reducing optic
`52, are preferably made of fused silica for transmitting the
`beam 12 of deep ultraviolet light. However. ?uorite could
`
`5
`
`
`
`

`

`_
`
`_
`
`_
`
`5
`also be used. The total refractive power of the lens group 42
`and reducing optic 52 of our catadioptric reducing system 40
`is minimized to avoid chromatic aberrations caused by
`retracting the diiferent wavelengths of the output band of the
`laser light source 10. The re?ective surface 60, along with 5
`the partially re?ective surface 58, provides the reducing
`power; and the lens group 42, along with the refractive
`interactions of the reducing optic 52, provides correction for
`the systematic aberrations of the re?ecting surfaces.
`One of the members of the lens group 42 is the substan- 10
`tially plane parallel plate 48 having a front face 66 and a
`back face 68. The front face 66 is planar, but the back face
`68 is modi?ed toinclude an aspheric surface that corrects for
`spherical aberrations. The back face 56 of the reducing opt1c
`52 is also modi?ed to include an aspheric surface to correct 15
`spherical aberrations at an even faster rate.
`A central obscuration 70. such as a re?ective coating, is
`applied to the front face 66 of the plate 48 to block portions
`of the beam 12 that would otherwise pass directly through
`the central aperture 62 without ?rst re?ecting from the 20
`re?ecting surface 60 of the reducing optic 52. The plate 48
`containing the central obscuration 70 is positioned close to
`an aperture stop 72 at which an image of the adjustable
`
`5,650,877
`
`6
`
`TABLE 1_c0ntinued
`
`Cm'vatm'e
`
`Aperture
`
`From
`
`Back
`
`From
`
`“705974
`
`A0)
`
`560455
`
`.
`Back mckms
`050000
`645635 257923
`
`2.5000
`
`Element
`
`Number
`Space
`52
`Image
`
`space
`
`An equation de?ning the aspheric Surfaces “A( 1)” and
`“AQY’ of the plate 48 and reduction Optic 52 is given below:
`
`gCURDYZ
`_
`Z- l+(1—(1+K)(CURV)2l'2)”2 +(A)Y4+(B)W+(C)YS+ (myw
`
`The coe?icient “K” is equal to zero. The coe?icient
`“CURV” and the coef?cients “A” through “D” are given in
`Table 2.
`
`TABLE 2
`
`CURV
`
`A
`
`B
`
`c
`
`D
`
`A(1)
`A(2)
`
`—1.5899E-04
`—1.4052E-02
`
`2.4964E~07
`—6.4218E-10
`
`—1.5511E-1O
`—8.7217E-13
`
`5.5612E~14
`—8.7864E-16
`
`—1.8490E-16
`7.1696E-19
`
`Of course. our invention can be practiced with a variety
`aperture 38 is formed The central obscuration 70 is rela-
`of other prescriptions operating at other reduction powers
`tively small and blocks only about 10 percent of the diameter
`of the surrounding aperture, whose outer diameter is con- 35 and sizes of scale. Numerical apertures of at least 0.4 are
`[rolled
`the aperture stop
`This converts to only ]_
`preferred. A single aspherical COITCCtiVC surface could be
`percent of the aperture area" Preferably, the central obscu-
`formed 011 One of the elemCntS including either th? back face
`ration is limited to no more than 15 percent of the aperture
`68 of the Plate
`er the hack faee _56 of the redhehlg °Phe
`diameter or a little more than 2 percent of the aperture area
`52- A1_th°ugh_1t 1S hhportaht to hmlt the alhouht Pf Opheal
`to minimize undesirable di?'ractive effects that reduce con- 40 mama} reqmred to transmlt the decp ‘il'mvlolet hght‘ more
`trast of the reticle pattern on the substrate 36.
`confccuve elerilents Could be use‘? Wm} a.larger aperture
`_
`.
`.
`.
`design to provrde a larger area of lllumrnatlon.
`Tables 1 and 2 prov1de prescription information on the
`WC claim:
`Preferred embodiment- Ah distances are measured in milli-
`1. A catadioptric reduction system for deep ultraviolet
`meters (mm), curvature is measured as a radius, but aperture
`lithography comprising;
`is measured as a diameter. The central aperture 62 has a 45
`a reducing optic having a main body made ofa transmis
`diameter of 6 mm- The reduction magni?cation is 10 fold
`sive material that conducts a beam of deep ultraviolet
`and the numerical aperture is 0.6. With a constant “K1”
`light;
`Icducing optic having a central
`assumed at
`the
`feature size that can bc
`a back face of
`on the substrate is reduced to 0.16 microns. Features less
`aPemn-e summnded by a concave re?ective surface;
`
`
`
`IlllCI'OIlS WOllld b6 POSSiblC under 1655 Stl‘illgCIlll 50 than Conditions (¢-g-~ With a constant “K1” at (l6)-
`
`
`
`
`
`
`
`reducing optic having a a front face of re?ective surface that transmits a portion of the beam to
`
`TABLE 1
`
`Element
`Number
`
`Cmvatme
`Front
`Back
`
`Apemhe
`From
`Back
`
`806
`
`Object
`451:!
`Space
`45
`space
`72
`Space
`4s
`Space
`50
`
`779574
`
`2975566
`
`61.4083
`
`60.1506
`
`491496
`
`333156
`
`573441
`
`52_Q793
`
`plane
`
`A(1)
`
`50-5998
`50.4728
`50.4321
`
`-37.1784
`
`~43 .9590
`
`50.3358
`
`55.6548
`
`55
`
`Thickmss
`
`.
`
`492 0402
`9.7738 60
`Q4000
`80000
`22.8357
`
`said concave re?ecting surface and re?ects a portion of
`the remaining beam returned by said concave re?ective
`surface on a path through said central aperture;
`a lens group also made from a transmissive material that
`conducts the beam of ultraviolet light for‘ correcting
`aberratlons generated by said reducing opt1c; and
`a central obscuration blocking a portion of the beam that
`1s not re?ected by said concave re?ective surface from
`passing through said aperture. .
`i
`.
`2. The reduction system Of ClEllm 1 1n WhlCh said re?ective
`and partially re?ective surfaces of said reducing optic pro
`vide substantially all reducing power of the system for
`lirnitin chromatic aberrations accom an in trans 'ss‘o
`100000
`g
`P y g
`ml 1 h
`5.0000
`164580 65 of a range of wavelengths.
`9.0000
`3. The reduction system of claim 2 in which said re?ective
`and partially re?ective surfaces provide a 10-fold reduction.
`
`'
`
`‘
`
`'
`
`6
`
`
`
`

`

`7
`4. The reduction system of claim 1 in which said reducing
`optic and said lens group are aligned with a common optical
`axrs.
`5. The reduction system of claim 1 further comprising a
`substantially plane parallel plate that is modi?ed to include
`an aspheric surface to correct for aberrations generated by
`said re?ective and partially re?ective surfaces.
`6. The reduction system of claim 1 in which said concave
`re?ective surface is modi?ed to include an aspheric surface
`to correct for aberrations generated by said re?ective and
`partially re?ective surfaces.
`7. The reduction system of claim 5 in which said central
`obscuration is formed as a stop on said plane parallel plate.
`8. The reduction system of claim 7 in which said central
`obscuration blocks no more than 15 percent of a diameter of
`the beam incident upon said plate.
`9. The reduction system of claim 8 in which said central
`obscuration blocks no more than 10 percent of the beam
`diameter.
`10. An optical projection system for projecting a reduced
`image of a ?rst surface onto a second surface comprising:
`a lens group that receives a beam of light passing through
`the ?rst surface;
`a partially re?ective surface for transmitting a portion of
`the beam;
`a concave re?ective surface surrounding a central aperture
`for re?ecting a portion of the transmitted beam;
`a central obscuration for blocking another portion of the
`beam of light from transmitting between said partially
`re?ective surface and said central aperture; and
`said partially re?ective surface being related to said
`central aperture and said concave re?ective surface for
`further re?ecting a portion of the re?ected beam from
`said re?ective surface on a path through said central
`aperture forming a reduced image of the ?rst surface on
`the second surface.
`11. The projection system of claim 10 in which the ?rst
`and second surfaces are parallel to each other and aligned
`with a common optical axis.
`12. The projection system of claim 11 in which said lens
`group. said partially re?ective surface, said concave re?ec
`tive surface, and said central obscuration are aligned with
`said common optical axis.
`13. The projection system of claim 12 in which said
`partially re?ective surface covers a front face of a transmis
`sive optic.
`14. The projection system of claim 13 in which said front
`face of the transmissive optic is a concave surface.
`15. The projection system of claim 14 in which said
`concave re?ective surface surrounds said central aperture on
`a back face of said transmissive optic.
`16. The projection system of claim 15 in which said back
`face of the transmissive optic is a convex surface.
`17. The projection system of claim 10 further comprising
`a substantially plane parallel plate that is modi?ed to include
`an aspheric surface to correct for aberrations generated by
`said re?ective and partially re?ective surfaces.
`18. The projection system of claim 17 in which said
`concave re?ective surface is also modi?ed to include an
`aspheric surface to correct for aberrations generated by said
`re?ective and partially re?ective surfaces.
`
`25
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`5,650,877
`
`10
`
`15
`
`8
`19. The projection system of claim 10 in which said
`central obscuration is surrounded by a given diameter aper
`ture of the beam, and said central obscuration blocks no
`more than 15 percent of the beam diameter.
`20. The projection system of claim 19 in which said
`central obscuration is formed as a stop on a plane parallel
`plate that is modi?ed to include an aspheric surface to
`correct for aberrations generated by said re?ective and
`partially re?ective surfaces.
`21. The projection system of claim 19 in which said
`central obscuration blocks no more than 10 percent of the
`beam diameter.
`_
`22. A method of projecting a reduced image of a mask on
`a substrate having a feature size less than 0.2 microns with
`a beam of deep ultraviolet light having a wavelength less
`than 200 NM comprising the steps of:
`emitting a beam of light having a band of wavelengths
`less than 200 NM;
`illuminating the mask with the beam;
`conditioning the beam with a lens group;
`transmitting a portion of the beam through a partially
`re?ective surface;
`re?ecting a portion of the transmitted beam with a con
`cave re?ective surface surrounding a central aperture;
`and
`further re?ecting a portion of the re?ected beam with the
`partially re?ective surface on a path through the central
`aperture forming a reduced image of the mask on the
`substrate having a feature size less than 0.2 microns.
`23. The method of claim 22 including the further step of
`orienting the reticle and the substrate parallel to each other
`and aligned with a common optical axis.
`24. The method of claim 23 including the further step of
`aligning the lens group, partially re?ective surface. and
`re?ective surface with the common optical axis.
`25. The method of claim 22 including the further step of
`blocking a portion of the beam that is not re?ected by the
`concave re?ective surface from transmitting between the
`partially re?ective surface and said central aperture.
`26. The method of claim 25 in which the beam has a given
`aperture diameter within the lens group and said step of
`blocking blocks no more than 15 percent of the beam
`diameter.
`27. The method of claim 26 in which said step of blocking
`blocks no more than 10 percent of the beam diameter.
`28. The method of claim 22 including the further step of
`adding a plane parallel plate to the lens group for correcting
`aberrations generated by the re?ective and partially re?ec
`tive surfaces.
`29. The method of claim 28 including positioning a
`central obscuration on the plane parallel plate for blocking
`a portion of the beam that is not re?ected by the concave
`re?ective surface from transmitting between the partially
`re?ective surface and said central aperture.
`30. The method of claim 22 including the step of forming
`the concave re?ective surface as an aspheric surface for
`correcting aberrations generated by the re?ective and par
`tially re?ective surfaces.
`
`* * * * =1'<
`
`7
`
`
`
`

`

`UNITED STATES PATENT AND TRADEMARK OFFICE
`CERTIFICATE OF CORRECTION
`5 , 650, 877
`
`PATENT NO. I
`
`‘DATED
`
`1
`
`Jul. 22, 1997
`
`lNvENToms) 1 Anthony R. Phillips , Jr. and Paul F. Michaloski
`
`It is certified that error appears in the above-identified patent and that said Letters Patent is hereby
`corrected as shown below:
`
`Column 3, lines 28-29 , delete "projected image. The
`central obscuration can be conveniently" and at line 31,
`after "of the" insert ——projected image. The central
`obscuration can be conveniently-—.
`
`Signed and Sealed this
`
`Ninth Day of December, 1997
`
`Am:
`
`60%
`
`BRUCE LEHMAN
`
`Arresting Officer
`
`Crmzmissimwr of Pmenm and TI'ut/L’Inarkx
`
`8
`
`
`
`