US 20050036213Al
`
`(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2005/0036213 A1
`Mann et al.
`_(43) Pub. Date:
`Feb. 17, 2005
`
`(54) PROJECTION OBJECTIVES INCLUDING A
`PLURAIJTY ()F MIRRORS WITH LENSES
`AHEAD OF MIRROR M3
`
`(76)
`
`Inventors: Hans-Jurgen Mann, Oberkoehen (DE);
`Russell Hudyma, San Ramon, CA
`(US); Alexander Epple, Aalen (DE)
`
`Correspondence Address:
`DARBY & DARBY P.C.
`P. 0. BOX 5257
`NEW YORK, NY 10150-5257 (US)
`
`(21) Appl. No:
`
`10/639,780
`
`(22)
`
`Filed:
`
`Aug. 12, 2003
`
`Publication Classification
`
`(51)
`
`Int. Cl? ..................................................... G02B 17/00
`
`(52) US. Cl.
`
`.............................................................. 359/726
`
`(57)
`
`ABSTRACT
`
`According to one exemplary embodiment, a projection
`objective is provided and includes at least two non-planar
`(curved) mirrors, wherein an axial distance between a next
`to last non-planar mirror and a last non—planar mirror, as
`defined along a light path, is greater than an axial distance
`between the last non-planar mirror and a first refracting
`surface of lenses following in the light path. In one exem-
`plary embodiment, the first refracting surface is associated
`with a single pass type lens. The present objectives form
`images with numerical apertures of at least about 0.80 or
`higher, e.g., 0.95. Preferably, the objective does not include
`folding mirrors and there is no intermediate image between
`the two mirrors, as well as the pupil of the objective being
`free of obscuration.
`
`
`
`ZEISS 1110
`
`

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`Patent Application Publication Feb. 17, 2005 Sheet 1 0f 2
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`US 2005/0036213 A1
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`

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`Patent Application Publication Feb. 17, 2005 Sheet 2 0f 2
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`US 2005/0036213 A1
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`US 2005/0036213 A1
`
`Feb. 17, 2005
`
`PROJECTION OBJECTIVES INCLUDING A
`PLURALITY OF MIRRORS WITH LENSES AHEAD
`OF MIRROR M3
`
`TECIINICAL FIELD
`
`[0001] The present invention relates to an optical system,
`such as projection lithography and more particularly, relates
`to an optical system with at least two mirrors, preferably at
`least 4 mirrors, with at least one lens element spatially ahead
`of mirror M3.
`
`BACKGROUND
`
`In the manufacture of semiconductor devices, pho-
`[0002]
`tolitbography is often used, especially in view of the circuit
`patterns of semiconductors being increasingly miniaturized
`in recent years. Projection optics are used to image a mask
`or reticle onto a wafer and as circuit patterns have become
`increasingly sm aller, there is an increased demand for higher
`resolving power in exposure apparatuses that print these
`patterns. To satisfy this demand, the wavelength of the light
`source must be made shorter and the NA (numerical aper-
`ture) of the optical system (i.e., the projection lens) must be
`made larger.
`
`[0003] Optical systems having a refractive group have
`achieved satisfactory resolutions operating with illumination
`sources having wavelengths of 248 or 193 nanometers. At
`these wavelengths, no or only slight correction of chromatic
`aberration is needed. As the element or feature size of
`semiconductor devices becomes smaller, the need for optical
`projection systems capable of providing enhanced resolution
`increases. In order to decrease the feature size which the
`optical projection systems used in photolithography can
`resolve, shorter wavelengths of electromagnetic radiation
`must be used to project the image of a reticle or mask onto
`a photosensitive substrate, such as a semiconductor wafer.
`
`[0004] Because very few refractive optical materials are
`able to transmit significant electromagnetic radiation below
`a wavelength of 193 nanometers, it is necessary to reduce to
`a minimum or eliminate refractive elements in optical pro—
`jection systems operating at wavelengths below 193 nanom-
`eters. To date, no second optical material is known which
`allows for chromatic aberration correction at wavelengths
`below 160 nm or shorter with sufficient material properties
`(homogeneity property, availability). Consequently, one has
`to construct a catadioptric imaging system, such as the
`present one, in order to allow for correction of chromatic
`aberrations with the use of only one single material, espe-
`cially, SiO2 or Can,
`
`[0005] The desire to resolve ever smaller features makes
`necessary optical projection systems that operate at
`the
`extreme ultraviolet wavelengths, below 200 nm; and there—
`fore, as optical lithography extends into shorter wavelengths
`(e.g., deep ultraviolet (DUV) or very ultraviolet (VUV)), the
`requirements of the projection system become more difficult
`to satisfy. For example, at a wavelength of 157 nm, access
`to 65 nm design rules requires a projection system with a
`numerical aperture of at least 0.80. As optical lithography is
`extended to 157 nm, issues relating to resist, sources and
`more importantly calcium fluoride have caused substantial
`delays to the development of lithography tools that can
`perform satisfactorily at such wavelengths. In response to
`the technical issues relating to the source and the material,
`
`it is important that projection system development investi-
`gates and focuses on maximizing spectral bandwidth to an
`order of 1 pm, while simultaneously minimizing the defi-
`ciencies associated with the materials that are used, i.e., it is
`desirable to minimize the calcium fluoride blank mass.
`
`It has long been realized that catadioptric reduction
`[0006]
`optical systems (i.e., ones that combine a reflective system
`with a refractive system) have several advantages, especially
`in a step and scan configuration, and that catadioptric
`systems are particularly well-suited to satisfy the aforemen—
`tioned objectives. A number of parties have developed or
`proposed development of systems for wavelengths below
`365 nm. One catadioptric system concept relates to a Dyson-
`type arrangement used in conjunction with a beam splitter to
`provide ray clearance and unfold the path to provide for
`parallel scanning (e.g., US. Pat. Nos. 5,537,260; 5,742,436;
`and 5,805,357). However,
`these systems have a serious
`drawback since the size of the beam—splitting element
`becomes quite large as the numerical aperture is increased,
`thereby making the procurement of optical material with
`sufficient quality (in three dimensions) to make the cube
`beam splitter a high risk endeavor, especially at a wave-
`length of 157 nm.
`
`[0007] The difficulties associated with the cube beam
`splitter size are better managed by locating the cube beam
`splitter in the short conjugate of the system, preferably near
`the reticle or at
`its 1x conjugate if the design permits.
`Without too much effort, this beam splitter location shrinks
`the linear dimension of the cube by up to 50%, depending
`upon several factors. The advantages of this type of beam
`splitter placement are described in US. Pat. No. 5,052,763
`to Wilczynski. Further, U.S. Pat. No. 5,808,805 to Takahashi
`provides some different embodiments; however, the basic
`concept is the same as in Wilczynski.
`
`It is also possible to remove the cube beam splitter
`[0008]
`entirely from the catadioptric system. In one approach, an
`off-axis design is provided using a group with a numerical
`aperture of 0.70 operating at 248 nm.
`In US. Pat. Nos.
`6,195,213 and 6,362,926 to Omura et al. disclose other
`examples of this approach and US. Pat. No. 5,835,275 to
`Takahashi illustrates yet another example. Takahashi et a1.
`offer several similar examples of beam splitter free designs
`in European patent application EP 1168028.
`
`[0009] Most of these “cubeless” embodiments share a
`common theme, namely that the catadioptric group contains
`only a single mirror. Additional mirrors can possible be used
`to improve performance. However, pure reflective designs
`with multiple mirrors have been investigated but have
`largely failed because these designs have proven unable to
`achieve adequately high numerical apertures (e.g., US. Pat.
`Nos. 4,685,777; 5,323,263; 5,515,207; and 5,815,310).
`
`[0010] Another proposed solution is disclosed in US. Pat.
`No. 4,469,414 in which a restrictive off—axis field optical
`system is disclosed. The system disclosed in this reference
`does not include a doubly passed negative lens in a first
`partial objective. Further, the embodiments disclosed therein
`are of impractical geometry and of far too low numerical
`aperture to provide improved lithography performance in the
`ultraviolet wavelength region.
`
`In conventional practice, four mirror catadioptric
`[0011]
`configurations typically are limited in terms of their numeri—
`
`

`

`US 2005/0036213 A1
`
`Feb. 17, 2005
`
`cal apertures due to the location of the refractive lens part
`relative to the mirrors of the system.
`
`[0012] US. patent application publication No. 2002/
`0024741 discloses various projection optical
`systems
`including one in which a lens element is positioned spatially
`in front of mirror M3; however, in this embodiment, the lens
`element that is positioned in front of mirror M3 is a double
`pass type lens element. The use of a double pass lens
`element complicates the system design because the use of a
`double pass lens between mirrors M3 and M4 requires the
`double pass lens to be close to mirror M4 and therefore it is
`difficult to mount.
`
`In addition, there are a number of other differences
`[0013]
`between the system disclosed in this published application
`and the present system. For example, FIG. 26 of the 2002/
`0024741 publication discloses a single pass lens element
`optically disposed between the very first mirror and the very
`last mirror of the whole system. Unfortunately, this element
`is very large in diameter and therefore difficult to manufac-
`ture. The disadvantage of such a single pass lens element is
`that it either requires a lateral separation of beam bundles
`traveling between the various mirrors, resulting in a large
`diameter of the lens or that it has to be physically disposed
`between the backside of mirror #1 and the backside of mirror
`#4 as shown in each of the FIGS. 23 to 28, again leading to
`a large diameter of the lens. As will be described in greater
`detail hereinafter, the present embodiments do not suffer
`from this disadvantage since the present lens elements are
`not required to be very large in diameter. With respect to the
`location of the aperture stop, the embodiments in the pub-
`lication (as shown in FIGS. 20-28) have an aperture stop
`located in front of the refractive group Gr2 with the aperture
`stop separating it from the field mirror group Grf.
`
`[0014] U.S. Pat. No. 5,323,263 to Schoenmakers discloses
`an embodiment in which there are multiple mirrors used in
`which a number of lens elements are disposed between the
`most optically forward mirror and the second most optically
`forward mirror. The lens elements between these two mir-
`rors are all single mirrors.
`
`[0015] What has heretofore not been available is a cata-
`dioptric projection system, especially a four mirror design,
`that has particular utility in 157 nm lithography and pro-
`duces an image with a numerical aperture of at least 0.80 and
`includes other desirable performance characteristics.
`
`SUMMARY
`
`[0016] Various photolithographic reduction projection
`objectives according to a number of embodiments are pro-
`vided herein. An exemplary projection objective includes at
`least
`two non-planar mirrors, wherein an axial distance
`between a next
`to last non-planar mirror and a last non—
`planar mirror, as defined along a light path (optical axis), is
`greater than an axial distance between the last non-planar
`mirror and a first refracting surface of lenses following in the
`light path. In one embodiment, the first refracting surface is
`associated with a single pass type lens. The present objec—
`tives form images with numerical aperture of at least about
`0.80 or higher, e.g., 0.95.
`
`In one aspect of the present invention, each of the
`[0017]
`present objectives consists of two parts, namely a catadiop-
`tric and a refractive part. According to the present designs,
`
`each of the refractive lens parts is advanced towards the
`front and thus begins already in front of the third mirror M3.
`This is in contrast to conventional designs that include at
`least four mirrors and by moving the refractive part forward,
`high numerical aperture is achieved in a four mirror con-
`figuration. Moreover, the present system is configured so
`that a pupil thereof is free of obscuration, thereby resulting
`in an improved image.
`
`[0018] An exemplary objective includes one more of the
`following features: (1) at least one of four aspherical lens
`surfaces and four aspherical mirror surfaces; (2) at least four
`non-planar mirrors; and (3) includes at
`least one light-
`dispersing mirror and at least two light-collecting mirrors. In
`addition, the objective does not have folding mirrors and
`also does not have an intermediate image between the two
`mirrors and in at least one embodiment, there is only one
`lens element (single pass type) between the next to last and
`last mirrors.
`
`[0019] Other features and advantages of the present inven-
`tion will be apparent from the following detailed description
`when read in conjunction with the accompanying drawings.
`
`BRIEF DESCRIPTION OF TIIE DRAWING
`FIGURES.
`
`[0020] The foregoing and other features of the present
`invention will be more readily apparent from the following
`detailed description and drawings figures of illustrative
`embodiments of the invention in which:
`
`schematically illustrates a microlitho—
`1
`[0021] FIG.
`graphic projection reduction objective according to a first
`embodiment; and
`
`[0022] FIG. 2 schematically illustrates a microlitho—
`graphic projection reduction objective according to a second
`embodiment.
`
`DETAILED DESCRIPTION OF PREFERRED
`EMBODIMENTS
`
`In order to provide the above advantages and to
`[0023]
`solve problems discussed above with respect to the related
`art systems, catadioptric projection systems according to a
`number of different embodiments are provided. The present
`systems achieve numerical apertures up to and in excess of
`0.80 while overcoming the disadvantages associated with
`the prior art.
`
`to FIG. 1, a catadioptric multi—
`[0024] Referring first
`mirror projection reduction objective 100 according to a first
`embodiment is illustrated. FIG. 1 is a schematic optical
`diagram of the system 100 illustrating the system 100 in an
`manner to generally show the arrangement of the elements.
`The system 100 includes a plurality of mirrors and a
`plurality of lens elements that are arranged in distinct groups
`and in predetermined locations relative to the mirrors.
`
`[0025] For purpose of the present application, the term
`“optically in front oi" refers to a situation where light rays
`impinge upon a first element prior to a second element, thus
`making the first element optically in front of the second
`element. The term “optically behind” refers to the converse
`situation and therefore,
`in the above example, the second
`element is optically behind the first element since the light
`rays first encounter the first element. The terms “physically
`
`5
`
`

`

`US 2005/0036213 A1
`
`Feb. 17, 2005
`
`b)
`
`in front of” or “spatially in front of” and “physically behind”
`or “spatially behind” define spatial relationships between the
`surface vertexes of two elements irrespective of the path of
`the light rays and only with reference to a point of reference.
`
`the system 100
`In the illustrated embodiment,
`[0026]
`includes a reticle (object) 110 and contains more positive
`lens elements than negative lens elements and more specifi»
`cally and as detailed below, one exemplary system 100
`includes 15 lens elements with 10 being positive lens
`elements and 5 being negative lens elements.
`
`[0027] The system 100 includes the reticle 110 and a wafer
`120 on which a reduced image is formed based on the reticle
`110 as is commonly known. Beginning from the least image
`forward element and ending with the most image forward
`element along the optical path of the system 100, the system
`100 includes a first lens element E1 that is disposed between
`the reticle 110 and a mirror M2. The first lens element E1 is
`a positive lens through which the light rays pass from one or
`more points of the reticle 110 toward the mirror M2. The
`mirror M2 is preferably a curved mirror (e.g., spherical or
`aspherical mirror) and in the illustrated embodiment,
`the
`mirror M2 does not include a continuous reflective surface
`but rather the mirror M2 has one or more regions where an
`opening 112 or the like is formed to permit free passage of
`light rays therethrough without being influenced at all by the
`mirror M2. The one or more openings 112 are formed in the
`mirror M2 at locations at are optically aligned with locations
`where the light rays pass through the first lens element E1.
`Alternatively, it will be appreciated that the mirror M2 can
`be constructed and arranged as an oil axis mirror so that the
`mirror M2 is positioned off axis at locations that permit the
`light rays to pass by the mirror M2.
`
`[0028] Optically and physically behind the mirror M2 is a
`pair of second and third lens elements E2 and E3, respec-
`tively. Each of the lens elements E2 and E3 is a negative lens
`and because of the arrangement between mirror M2 and
`mirror M1 and the physical construction of the lens elements
`E2 and E3, the lens elements E2 and E3 act as double pass
`lenses. More specifically, the lens elements E2 and E3 are
`disposed between mirror M2 and mirror M1 and similar to
`the mirror M2, the lens elements E2 and E3 do not have an
`entirely continues surface but rather one or more openings
`114 are formed in each lens element to permit light rays to
`freely pass therethrongh without being influenced by the
`lens elements. The one or more openings 114 are optically
`aligned with the one or more openings 112 formed in the
`mirror M2 so that
`the light rays that pass through the
`openings or cut outs 112 in the mirror M2 before then
`passing through the opening or cut outs 114 in the lens
`elements E2 and E3 before contacting the reflective surface
`of mirror M1. Once again and instead of having a physical
`opening formed therein, the lens elements E2 and E3 can be
`constructed so that they are formed and arranged so that the
`light rays passing by the mirror M2 also pass by and not
`through the lens elements E2 and E3.
`'lhe mirror M1 is
`preferably a curved mirror (e.g., spherical 0r aspherical
`mirror). One will appreciate that the mirrors M1 and M2 are
`both light collecting mirrors.
`
`It will be appreciated that mirror M1 is the first
`[0029]
`mirror in the optical path since the light rays initially pass
`through openings 112 formed in mirror M2 and only later
`strike the reflective surface of mirror M2 after having
`
`contacted and been reflected by the surface of mirror M1.
`After being reflected by the mirror M1, the light rays are
`directed toward the mirror M2 and first pass through lens
`element E2 and E3 before striking the reflective surface of
`mirror M2. As previously mentioned, the lens elements F2
`and E3 are double pass types lenses since the light rays
`travel first through the lens elements as they travel from
`mirror M1 to mirror M2 and then pass through the lens
`elements a second time as the light rays are rellected by
`mirror M2 toward a mirror M3 which is preferably a curved
`mirror (e.g., spherical or aspherical mirror). Mirror M3 is a
`light collecting mirror.
`
`[0030] Mirror M1 is also constructed so that it includes
`one or more openings or cut outs 116 to permit passage of
`the light rays from mirror M2 to a reflective surface of
`mirror M3. As will be appreciated by the illustration of FIG.
`1, the mirror M1 has one or more active regions that reflect
`light rays as well as containing the one or more openings or
`cut outs 116 to permit the lights rays to travel along the
`optical path of the system 100, e.g., from mirror M2 to
`mirror M3, which are both optically behind mirror M1.
`Again, mirror M1 can be constructed as an off axis mirror
`that permits the light rays to pass thereby from the mirror
`M2 to the mirror M3.
`
`[0031] Amirror M4 is positioned physically in front of but
`optically behind the mirror M3 such that a reflective surface
`of mirror M3 faces a reflective surface of mirror M4. Mirror
`M4 is preferably a curved mirror (e.g., spherical or aspheri-
`cal mirror). As will be described in greater detail hereinafter,
`mirror M3 includes an opening or cut out 118 formed therein
`to accommodate a lens element. Mirror M4 is a light
`dispersing mirror. Mirror M3 can similarly be formed as an
`off axis mirror that is constructed and designed to permit the
`light rays to pass thereby after the light rays are reflected by
`the mirror M4.
`
`[0032] According to one aspect of the present invention, a
`lens element, namely a fourth lens element E4, is disposed
`between the mirror M3 and mirror M4 such that the lens
`element E4 is physically in front of mirror M3 but optically
`behind both mirrors M3 and M4. The lens element E4 is a
`negative lens that is positioned proximate the mirror M3 and
`as will be appreciated by viewing FIG. 1, the lens element
`E4 acts as a single pass lens element in that the light rays
`only pass through the lens element E4 once as they travel
`according to the optical path of the system 100. This is a
`difference between other lens designs where a lens element
`that may be placed between mirror M3 and M4 is positioned
`close to mirror M4 such that
`the lens element acts as a
`double pass lens element since the light rays pass once
`therethrough to the reflective surface of M4 before then
`passing through the lens element a second time as the light
`rays travel in a direction toward M3.
`
`[0033] The system 100 includes a fifth lens element E5
`that is disposed within the opening or cut out 118 formed in
`the mirror M3. The lens elements E4 and E5 are disposed
`along the optical axis so that light rays that are reflected from
`mirror M4 pass through the lens element E4 and then
`through lens element E5 which is contained within the
`mirror M3. In other words, the mirrors M3 and M4 are both
`optically in front of the lens elements E4 and E5. In the
`illustrated embodiment, the lens element E5 is a positive
`
`

`

`US 2005/0036213 A1
`
`Feb. 17, 2005
`
`lens element that at least partially extends beyond one face
`of the mirror M3 as well as an opposite face of the mirror
`M3.
`
`[0034] The system 100 is designed so that there are a
`group of lens elements that are both physically and optically
`behind the mirror M3 and the lens elements E4 and E5.
`According to one exemplary embodiment, there are ten lens
`elements that are disposed optically behind the lens element
`E5 and more specifically,
`lens elements E6 to E15 are
`disposed along the optical axis and optically behind the lens
`element E5 and optically and physically in front of the wafer
`120. Lens elements E6—E8 are positive lenses, lens element
`E9 is a negative lens, lens elements ElO-EIZ are positive
`lenses, lens element E13 is a negative lens; and lens ele-
`ments E14 and E15 are positive lenses.
`
`[0035] As will be appreciated with reference to FIG. 1, an
`intermediate image is produced in the light path at the axial
`location of M4. In order to ensure that no Vignetting occurs,
`the light path has to be routed so that it completely by—passes
`mirror M4 and it has to be ensured that the light rays do not
`fall into the central opening (bore) 118 of mirror M3. This
`requires large deflection angles and thus large refractive
`powers in the mirrors M3 and M4 as well as high incidence
`angles, especially on mirror M4.
`
`[0036] The large ray angles after M4 have the effect that
`the main rays converge rapidly towards the optical axis.
`Thus, the diaphragm position is pulled closer to mirror M4.
`However, in order to be able to effectively correct chromatic
`variation of the magnification, it is necessary to use refrac-
`tive power in front of the aperture diaphragm. This is
`possible only by pulling the lenses of the refractive portion
`spatially (physically) ahead of M3.
`
`[0037] This (the need for pulling the lenses of the refrac-
`tive portion spatially ahead) is considered a direct conse-
`quence of the high aperture of this design arrangement. In
`other words,
`the arrangement of the lens elements and
`mirrors as illustrated in FIG. 1 permit it possible to achieve
`high numerical aperture values in designs of this kind. For
`example, the system 100 has a numerical aperture of at least
`about 0.80.
`
`In addition, the present system 100 has a conjugate
`[0038]
`aperture Stop between mirrors M1 and M2, while the aper-
`ture stop is located in a back part of the lens arrangement
`which acts as the refractive part. More specifically,
`the
`aperture stop is indicated by marker 101 in FIG. 1 (which
`is a position immediately spatially and optically in front of
`lens element E8).
`
`[0039] One of the advantages of the present system 100 is
`that there is no obscuration of the pupil. In other Words, there
`is no central obscuration in the present system and therefore,
`all of the disadvantages associated with having a central
`obscuration are eliminated in the present design. As a result
`of the present lens system being free of a central obscuration
`of the pupil, the performance of the system is not jeopar-
`dized and the quality of the image is similarly not jeopar-
`dized or reduced since the presence of a central obscuration
`will prevent all of the light rays from passing through to
`form the image.
`
`[0040] Aeomplete optical description is found in Table 1,
`describing the optical surfaces of the system 100.
`
`[0041] Now referring to FIG. 2 in which a catadioptric
`multi—mirror projection reduction objective 200 according to
`a second embodiment is illustrated. FIG. 2 is a schematic
`optical diagram of the system 200 illustrating the system 200
`in an manner to generally show the arrangement of the
`elements. The system 200 includes a plurality of mirrors and
`a plurality of lens elements that are arranged in distinct
`groups and in predetermined locations relative to the mir-
`FOI'S.
`
`the system 200
`In the illustrated embodiment,
`[0042]
`includes a reticle 210 and contains more positive lens
`elements than negative lens elements and more specifically
`and as detailed below, one exemplary system 200 includes
`20 lens elements with 13 being positive lens elements and 7
`being negative lens elements.
`
`[0043] The system 200 includes the reticle 210 and a
`wafer 220 on which a reduced image is formed based on the
`reticle 210 as is commonly known. Beginning from the least
`image forward element and ending with the most
`image
`forward clement along the optical path of the system 200,
`the system 200 includes a first lens element E1 and a second
`lens element E2 that are disposed between the reticle 210
`and a mirror M2. The first and second lens elements E1 and
`E2 are positive lenses through which the light rays pass from
`one or more points of the reticle 210 toward the mirror M2.
`The mirror M2 is preferably a Curved mirror (e.g., spherical
`or aspherical mirror) and in the illustrated embodiment, the
`mirror M2 does not include a continuous reflective surface
`but rather the mirror M2 has one or more regions where an
`opening 212 or cut out or the like is formed to permit free
`passage of light rays therethrough without being influenced
`at all by the mirror M2. The one or more openings 212 are
`formed in the mirror M2 at
`locations that are optically
`aligned with locations where the light rays pass through the
`first and second lens elements E1 and E2 so that the light
`rays travel
`through the E1 and E2 and then through the
`mirror M2. As previously mentioned, mirror M2 does not
`necessarily have to have a physical opening formed there-
`through but instead it can be constructed as an off axis mirror
`that is positioned so that the light rays travel thereby from
`lens elements E1 and E2 to lens element E3.
`
`[0044] The system 200 includes a third lens element E3
`that is disposed physically (spatially) and optically behind
`the mirror M2. The light rays traveling through openings
`212 formed in the mirror M2 subsequently travel through the
`lens element E3. In the illustrated embodiment, the third lens
`element E3 is a negative lens and is characterized as a triple
`pass lens element since the lights rays pass through this lens
`element three distinct times as the light rays travel along the
`optical path of the system 200.
`
`[0045] The system 200 includes fourth and fifth lens
`elements E4 and E5 that are spatially and optically behind
`the lens element E3. In the illustrated embodiment, the lens
`elements E4 and E5 are negative lenses. The lens elements
`E4 and E5 are disposed spatially proximate to the mirror M1
`such that light rays that pass through the lens element E3
`pass through the lens elements E4 and E5 before contacting
`the reflective surface of the mirror M1. It will be appreciated
`that the lens elements E4 and E5 are double pass type lens
`elements since the lights rays pass through this lens element
`
`

`

`US 2005/0036213 A1
`
`Feb. 17, 2005
`
`two distinct times as the light rays travel along the optical
`path of the system 200. More specifically, after the light rays
`contact the mirror M1, the light rays pass back through the
`lens elements E4 and E5 as the rays travel toward the mirror
`M2. Before the light rays contact the minor M2, the light
`rays pass through the lens element E3 for a second time and
`then after the light rays contact the mirror M2, the light rays
`are reflected back through the lens element E3 for a third
`time (resulting in E3 being a triple pass lens). Preferably, the
`openings 212 of mirror M2 are formed in one region while
`the mirror M2 has another region that acts as a reflective
`mirror surface for reflecting the light rays from mirror M1.
`Mirror M1 is a light collecting mirror, while mirror M2 is a
`light dispersing mirror. Alternatively, the mirror M2 can be
`an otf axis mirror without a physical opening 212 formed
`therethrough.
`
`[0046] After the light rays pass through the lens element
`E3 for a third time, the light rays are directed to a mirror M3.
`The mirror M3, like mirrors M1 and M2, is a curved mirror
`(e.g., spherical or aspherical mirror).
`In the illustrated
`embodiment, the mirror M3 is centered off of the optical axis
`so that it includes an off axis region, generally indicated at
`220, that receives the light rays from the mirror M2 as shown
`in FIG. 2. In other words, the mirror M3 is eoaxially aligned
`but only used at an off-axis region.
`
`is
`[0047] The system 200 includes a mirror M4 that
`spatially in front of but optically behind the mirror M3, As
`shown, the mirrors M1 and M4 are positioned back-to-back
`relative to one another with no lens elements being disposed
`therebetween. Mirror M3 is a light collecting mirror, while
`mirror M4 is a light dispersing mirror.
`
`[0048] According to one aspect of the present invention
`and similar to the above described first embodiment, a lens
`element, namely a sixth lens element E6,
`is disposed
`between the mirror M3 and mirror M4 such that the lens
`element E6 is physically in front of mirror M3 but optically
`behind both mirrors M3 and M4. The lens element E6 is a
`positive lens that is positioned proximate the mirror M3 and
`as will be appreciated by viewing FIG. 2, the lens element
`E6 acts as a single pass lens element in that the light rays
`only pass through the lens element E6 once as they travel
`according to the optical path of the system 200 in contrast to
`other lens designs (e.g., see US. Patent Application publi-
`cation No. 2002/0024741) in which the lens element acts as
`a double pass lens element.
`
`to the
`[0049] The lens element E6 is positioned next
`mirror M3 in a region where the mirror M3 has one or more
`openings or cut outs 214 formed therethrough to permit the
`light rays that travel through the lens element E6 to then
`subsequently pass through the one or more openings 214 as
`the light travels according to the light path of the system 200,
`In other words, the lens element E6 preferably extends at
`least partially through the opening 214. Again, mirror M3
`can be an off axis mirror that is positioned to permit the light
`rays to travel along the optical path from mirror M4.
`
`there are a
`[0050] The system 200 is designed so that
`group of lens elements that are both physically and optically
`behind the mirror M3 and the lens elements E6. According
`to one exemplary embodiment,
`there are fourteen lens
`elements that are disposed optically behind the lens element
`E6 and more specifically,
`lens elements E7 to E20 are
`disposed along the optical axis and optically behind the lens
`
`8
`
`element E6 and optically and physically in front of the wafer
`120. Lens element E7 is a negative lens, lens elements 138
`and E9 are positive lenses, lens element E10 is a negative
`lens, lens element E11 is a positive lens, lens element E12
`i