Abstract:
A Wynne-Dyson projection lens for use in an ultraviolet optical lithography system is disclosed, wherein the projection lens is configured to have reduced susceptibility to damage from ultraviolet radiation. The projection lens utilizes lens elements that are made of optical glasses that are resistant to damage from ultraviolet radiation, but that also provide sufficient degrees of freedom to correct aberrations. The glass types used for the lens elements are selected from the group of optical glasses consisting of: fused silica, S-FPL51Y, S-FSL5Y, BSM51Y and BAL15Y.

Description:
FIELD 
       [0001]    The present disclosure relates to projection lenses, and in particular to Wynne-Dyson projection lenses used in photolithography performed at ultraviolet (UV) and deep UV (DUV) wavelengths, and more particularly to such projection lens that have reduced susceptibility to damage from UV radiation. 
         [0002]    The entire disclosure of any publication or patent document mentioned herein is incorporated by reference, including the following references: J. Dyson, “Unit Magnification Optical System without Seidel Aberrations,”  Journal of the Optical Society of America , July 1959; U.S. Pat. No. 3,748,015; U.S. Pat. No. 6,879,383; U.S. Pat. No. 7,166,496; U.S. Pat. No. 7,573,655; U.S. Pre-Grant Publication No. US2014/0049978; European Patent No. 0779558 A2; European Patent No. EP 206707 A2; and European Patent No. EP 0960074 A1. 
       BACKGROUND 
       [0003]    Modern-day semiconductor manufacturing utilizes optical lithography projectors that image mask patterns onto semiconductor wafers as part of the process of making integrated circuits. The optical lithography projectors include illumination systems (“illuminators”) that irradiate the mask with UV light because the smaller wavelength of UV light (as compared to visible light) allows for higher-resolution imaging. The optical lithography projectors utilize projection lenses (also called microlithographic lenses or optical lithography lenses), most of which include at least some glass lens elements. Because of the high demands placed on the imaging resolution over the image field, the projection lenses are designed for high performance. 
         [0004]    A portion of the UV radiation (light) that passes through a given glass lens element interacts with the optical material. This includes light absorption whereby the absorbed light energy is converted to heat. The heat raises the local glass temperature, which changes the local index of refraction, thereby adversely changing the imaging performance of the projection lens. This effect is reversible, i.e., when the illumination terminates, the glass cools down and the imaging performance is restored. Thus, turning the illumination on (or off) gives rise to a thermal transient effect whereby the projection lens heats up (or cools down) until the temperature stabilizes. During this time, there is a thermally induced variation in imaging performance. 
         [0005]    A second type of interaction between the UV light and the glass material changes the glass atomic structure and makes the structure denser, thereby locally increasing the glass refractive index and degrading the projection lens imaging performance. This effect is called “radiation compaction.” It is not reversible, and the attendant refractive index increase is proportional to the irradiance of the exposure light as well as the time-accumulated irradiance of the exposure light. 
         [0006]    A third type of interaction is called solarization, which also changes the glass atomic structure and reduces the glass transmission. Like radiation compaction, solarization is not reversible and is also proportional to the light irradiance within the glass and the time-accumulated irradiance. 
         [0007]    Consequently, when a projection lens is exposed to high irradiance UV light over an extended period of time, compaction and solarization effects can permanently damage the glass elements and result in significantly degraded imaging performance. These adverse effects are becoming more problematic as increasingly greater amounts of UV radiation and increasing large imaging fields are being employed, which lead to greater irradiance levels traveling through the glass elements. Further, increasingly higher degrees of imaging performance limit the amount of performance degradation that is acceptable. 
       SUMMARY 
       [0008]    An aspect of the disclosure is a Wynne-Dyson projection lens for use in an ultraviolet optical lithography system, wherein the projection lens is configured to have reduced susceptibility to damage from ultraviolet radiation is disclosed. The projection lens utilizes lens elements that are made of optical glasses that are resistant to damage from ultraviolet radiation, but that also provide sufficient degrees of freedom to correct aberrations. The glass types used for the lens elements are selected from the group of optical glasses consisting of: fused silica, S-FPL51Y, S-FSL5Y, BSM51Y and BAL15Y. 
         [0009]    Another aspect of the disclosure is a Wynne-Dyson projection lens having object and image planes and an image field, and that has reduced susceptibility to damage from ultraviolet radiation. The lens includes along an optical axis: a concave mirror having a concave surface; a positive lens group spaced apart from the concave surface of the concave mirror; first and second prisms made of either fused silica or BSM51Y and disposed adjacent the positive lens group opposite the concave mirror and on respective sides of the optical axis, with the first and second prisms having respective surfaces that reside adjacent the object and image planes. The positive lens group consists of: a first lens group having a cemented doublet consisting of a plano-convex lens element made of either S-FPL51Y, S-FSL5Y or fused silica and having a planar surface in contact with the prisms, and a negative meniscus lens made of either BAL15Y or BSM51Y; and a second lens group spaced apart from the first lens group and consisting of either one or two lens elements each made of either S-FSL5Y, S-FPL51Y or fused silica. The projection lens has a numerical aperture NA in the range from 0.16 to 0.2, a substantially unit magnification, is substantially doubly telecentric and a Strehl ratio of &gt;0.95 over the image field for a spectral bandwidth that includes ghi radiation. 
         [0010]    Another aspect of the disclosure is an optical lithography system for imaging a reticle onto a wafer coated with photoresist. The system includes the Wynne-Dyson projection lens as described above, wherein the reticle resides at the object plane and the wafer resides at the image plane. The system also includes an illuminator arranged to operably illuminate the reticle with the ultraviolet radiation. 
         [0011]    Another aspect of the disclosure is a Wynne-Dyson projection lens having object and image planes and an image field, and that has reduced susceptibility to damage from ultraviolet radiation. The lens includes along an optical axis: a concave mirror having a concave surface; a positive lens group spaced apart from the concave surface of the concave mirror; first and second prisms made of either fused silica or BSM51Y and disposed adjacent the positive lens group opposite the concave mirror and on respective sides of the optical axis, with the first and second prisms having respective surfaces that reside adjacent the object and image planes. The positive lens group includes: a first lens element L 1  made of S-FPL51Y, S-FSL5Y or fused silica; a second lens element L 2  made of BSM51Y or BAL15Y; and a third lens element L 3  made of S-FPL51Y, S-FSL5Y or fused silica. The projection lens has: a numerical aperture NA in the range from 0.16 to 0.2; a substantially unit magnification; doubly telecentricity; and a Strehl ratio of &gt;0.95 over the image field for a spectral bandwidth that includes ghi radiation. 
         [0012]    Another aspect of the disclosure is a Wynne-Dyson projection lens having object and image planes and an image field, and that has reduced susceptibility to damage from ultraviolet radiation. The lens includes along an optical axis: a concave mirror having a concave surface; a positive lens group spaced apart from the concave surface of the concave mirror; first and second prisms made of either fused silica or BSM51Y and disposed adjacent the positive lens group opposite the concave mirror and on respective sides of the optical axis, with the first and second prisms having respective surfaces that reside adjacent the object and image planes; first and second lens elements that define a cemented doublet, wherein the first lens element has a planar surface in contact with the prisms and that is made of either S-FPL51Y or S-FSL5Y, and wherein the second lens element is made of either BAL15Y or BSM51Y; a third lens element spaced apart from the second lens element and made of either S-FSL5Y or S-FPL51Y. The projection lens has: a numerical aperture NA of 0.16; the image field is 40×20 mm; a substantially unit magnification; doubly telecentricity; and a Strehl ratio of &gt;0.95 over the image field for a spectral bandwidth that includes ghi radiation. 
         [0013]    Another aspect of the disclosure is the Wynne-Dyson projection lens as described above, wherein the positive lens group further includes a fourth lens element spaced apart from the third lens element towards the concave mirror and made of either S-FSL5Y or S-FPL51Y. 
         [0014]    Another aspect of the disclosure is an optical lithography system for imaging a reticle onto a wafer coated with photoresist. The system includes the Wynne-Dyson projection lens as described above, wherein the reticle resides at the object plane and the wafer resides at the image plane, and an illuminator arranged to illuminate the reticle with the ultraviolet radiation. 
         [0015]    Additional features and advantages are set forth in the Detailed Description that follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the embodiments as described in the written description and claims hereof, as well as the appended drawings. It is to be understood that both the foregoing general description and the following Detailed Description are merely exemplary, and are intended to provide an overview or framework to understand the nature and character of the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The accompanying drawings are included to provide a further understanding, and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiment(s), and together with the Detailed Description serve to explain principles and operation of the various embodiments. As such, the disclosure will become more fully understood from the following Detailed Description, taken in conjunction with the accompanying Figures, in which: 
           [0017]      FIGS. 1 through 4  are optical diagrams of first, second third and fourth examples, respectively, of a Wynne-Dyson projection lens configured to have reduced susceptibility to UV damage according to the disclosure; 
           [0018]      FIG. 5  is a schematic diagram of an example optical lithography system that employs the Wynn-Dyson projections lens having reduced susceptibility to UV damage as disclosed herein; 
           [0019]      FIG. 6A  is a schematic front-on view of an example reticle showing the reticle pattern that defines the reticle field and also showing an example of a scanned illumination field; 
           [0020]      FIG. 6B  is a schematic diagram of an example scanning illumination field and an example exposure field associated with the scanned illumination field; 
           [0021]      FIG. 6C  is similar to  FIG. 6A  and illustrates an example of a full-field illumination field; 
           [0022]      FIG. 6D  is similar to  FIG. 6B  and illustrates an example of a full-field image field that corresponds in size to the corresponding exposure field; and 
           [0023]      FIG. 7  is a plan view of a semiconductor wafer with exposure fields formed thereon by the photolithography system of  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    Reference is now made in detail to various embodiments of the disclosure, examples of which are illustrated in the accompanying drawings. Whenever possible, the same or like reference numbers and symbols are used throughout the drawings to refer to the same or like parts. The drawings are not necessarily to scale, and one skilled in the art will recognize where the drawings have been simplified to illustrate the key aspects of the disclosure. 
         [0025]    The claims as set forth below are incorporated into and constitute part of this Detailed Description. 
         [0026]    Cartesian coordinates are shown in some of the Figures for the sake of reference and are not intended to be limiting as to direction or orientation. 
         [0027]    Reference is made below to the g, h and i lines of radiation, which nominally correspond to 365 nm, 405 nm, and 436 nm, respectively. UV light using these wavelengths is referred to below as “ghi light” or “ghi radiation,” or just “ghi” for short. Optical lithography using these wavelengths is referred to as “ghi optical lithography.” A projection lens designed for ghi optical lithography is referred to as a “ghi projection lens.” In an example, the reduced susceptibility of the projection lens to UV radiation refers to ghi radiation. 
       Design Considerations 
       [0028]    Most of the standard optical glasses from the well-known commercial optical glass manufacturers are not suitable for high-quality projection lenses for semiconductor manufacturing because of their low transmission for ghi radiation, and especially for i-line radiation. Three optical glasses that have good transmission at ghi wavelengths and that have a low susceptibility to UV-induced damage are fused silica, S-FPL51Y and S-FSL5Y, which are available from Ohara Corporation, of Kanagawa, Japan. 
         [0029]    For a Wynne-Dyson type unit magnification projection lens that operates over a relatively wide range of UV wavelengths such as the ghi wavelengths, chromatic aberrations need to be corrected. Because of the system&#39;s symmetry, lateral color is generally not a concern. On the other hand, axial color needs to be corrected, and this requires optical glass having a relatively high dispersion. However, the aforementioned S-FPL51Y, S-FSL5Y and fused silica optical glasses have relatively low dispersion. Thus, in example embodiments of the projection lenses disclosed herein, the optical glasses BSM51Y and BAL15Y from Ohara Corporation are utilized because they have the necessary dispersion to correct axial color while also having a relatively low susceptibility to UV-induced damage. 
         [0030]    More specifically, the optical glasses S-FPL51Y, S-FSL5Y, fused silica, BSM51Y and BAL15Y have respective Abbe numbers of 81.14, 70.35, 67.82, 60.65, and 58.68. Thus, S-FPL51Y can serve as the low dispersion glass, fused silica and/or S-FSL5Y can serve as the medium dispersion glass, and BAL15Y and/or BSM51Y serve as the high dispersion glass. Using these glasses provides enough degrees of freedoms to adequately correct for the secondary spectrum as well as other aberrations over the ghi wavelengths. 
       Example Projection Lenses 
       [0031]      FIGS. 1 through 3  are optical system diagrams that respectively illustrate three different example embodiments of a Wynne-Dyson projection lens (“projection lens”)  10 . The projection lens  10  disclosed herein is substantially unit magnification, substantially doubly telecentric, and is designed for use with ghi radiation, i.e., it is well-corrected over a spectral bandwidth that includes ghi radiation. 
         [0032]    Projection lens  10  includes an optical axis A 1 , a concave primary mirror M with a surface MS, and prisms PA and PB spaced apart from the primary mirror and arranged on opposite sides of the optical axis. In an example, prisms PA and PB are isosceles prisms. Prisms PA and PB have respective surfaces SR and SW that respectively reside adjacent a reticle plane RP and a wafer plane WP. Prisms PA and PB also each have surfaces S TIR  that serve at total-internal reflection surfaces, thereby allowing for the reticle and wafer planes RP and WP to be spatially separated. Reticle plane RP resides at an object plane OP while wafer plane WP resides at an image plane IP. 
         [0033]    Projection lens  10  includes a refractive lens group G with overall positive refractive power (i.e., a “positive” lens group) that resides between the primary mirror M and prisms PA and PB. The refractive lens group G can be divided into a first lens group G 1  that includes a cemented doublet made up of a plano-convex lens element L 1  and a (nominally) plano-concave lens element L 2 , and a second lens group G 2  axially spaced apart from the first lens group and that includes either one lens element L 3  or two lens elements L 3  and L 4 . In an example, prisms PA and PB can be considered part of first lens group G 1 . Primary mirror M defines an aperture stop AS for projection lens  10 . 
         [0034]    Because projection lens  10  is symmetric about the stop aperture AS, the distortion and lateral color are generally well-corrected. Primary mirror M generates mainly astigmatism and field curvature without any color aberrations. So, the other refractive elements in projection lens  10  are designed to compensate for the two types of aberrations generated by the mirror, as well as aberrations that they would otherwise generate, such as spherical aberration and coma. Prisms PA and PB have no optical power, which allows for relatively loose mounting tolerances for these refractive elements. 
         [0035]    In an example, even when projection lens  10  is substantially defocused (e.g., by +/−5 microns for Examples 1 and 3, and by +/−2.5 microns for Example 2), its imaging performance (e.g., as reflected by the Strehl ratio) remains substantially diffraction limited everywhere across the image field. The ability to maintain imaging performance over a relatively large range of focus positions is important characteristic since projection lens  10  is typically used to form images in photoresist, as described below in connection with optical lithography system  200  of  FIG. 5 . 
         [0036]    For the example of projection lens  10  having three refractive lens elements L 1  through L 3 , each lens can be made of the following optical glasses: 
         [0000]    Lens element L 1 : S-FPL51Y, S-FSL5Y or fused silica;
 
Lens element L 2 : BSM51Y or BAL15Y; and
 
Lens element L 3 : S-FPL51Y, S-FSL5Y or fused silica.
 
For the example of projection lens  10  having the additional lens element L 4 , the first three lens elements can be made of the above optical glasses while the fourth lens element L 4  can be made of S-FPL51Y, S-FSL5Y or fused silica.
 
       Example 1 
       [0037]      FIG. 1  is an optical diagram of Example 1 of projection lens  10 . The projection lens  10  of Example 1 has a designed field radius of 50 mm, an usable field of view of 68 mm by 28 mm, a numerical aperture (NA) of 0.16, and an overall length of 800 mm. Prisms PA and PB are made of fused silica, while lens elements L 1 , L 2 , L 3  and L 4  are respectively made of S-FPL51Y, BAL15Y, S-FSL5Y and fused silica. The optical glasses BSM51Y and BAL15Y have very similar refractive indices at the ghi wavelengths (and hence have similar dispersion), and also have low susceptibility to UV-induced damage. Thus, the optical glass BAL15Y used in the design for lens element L 2  can be replaced by optical glass BSM51Y with minor modification of the design while still obtaining similar image performance. 
         [0038]    Table 1 below sets forth the optical prescription for Example 1, wherein “S” stands for the surface number, “TH” is the thickness, and “CA/2” is half of the clear aperture, i.e., or semi-diameter of the given element. Also, “r” stands for radius, k is the conic constant, and c represents the aspheric coefficients that describe an aspheric surface using the standard equation for an aspheric surface as is known in the art. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 EXAMPLE 1 
               
             
          
           
               
                 S 
                 R (mm), k, c 
                 TH (mm) 
                 Material 
                 CA/2 (mm) 
               
               
                   
               
             
          
           
               
                 S0 
                 r = infinity 
                 5.182 
                 mm 
                   
               
               
                 S1 
                 r = infinity 
                 55.5 
                 mm 
                 Fused Silica 
               
             
          
           
               
                 S2 
                 r = infinity 
                 0 
                   
                   
               
               
                 S3 
                 r = infinity 
                 0 
                   
                 56.45 mm 
               
             
          
           
               
                 S4 
                 r = infinity 
                 26.372 
                 mm 
                 S-FPL51Y 
                 57.15 mm 
               
             
          
           
               
                 S5 
                 r = −92.4315 
                 0 
                   
                 57.15 mm 
               
             
          
           
               
                 S6 
                 r = −92.4315 
                 35.011 
                 mm 
                 BAL15Y 
                 64.44 mm 
               
               
                 S7 
                 r = infinity 
                 25.766 
                 mm 
                   
                 69.87 mm 
               
             
          
           
               
                 S8 
                 r = −6150 
                 80.001 
                 S-FSL5Y 
                 79.28 
               
               
                 S9 
                 r = −212.6136 
                 2.000 
                   
                 79.75 
               
               
                 S10 
                 r = 410.827 
                 38.989 
                 Fused Silica 
               
               
                 S11 
                 r = 428.8633 
                 529.210 
                   
                 77.80 
               
               
                   
                 k = −3.52295157 
               
               
                   
                 c 4th  = 6.4527E−9 
               
               
                   
                 c 6th  = −5.8530E−14 
               
               
                   
                 c 8th  = 6.0603E−18 
               
               
                   
                 c 10th  = −3.52E−22 
               
               
                 S12 
                 r = −800 
                   
                   
                 85.44 
               
               
                   
               
             
          
         
       
     
         [0039]    In Example, 1, the mirror-wise surface S 11  of lens element L 4  is aspheric and is the dominant contributor to compensating the astigmatism and field curvature from primary mirror M. Lens element L 4  has a meniscus shape and has very little power. Most of the optical power of projection lens  10  is controlled by the primary mirror and lens element L 2 . The asphere lens element L 4  is mainly used to control overall length and chief ray aberrations, in addition to higher-order aberrations. It should be noted that aspheric lens element L 4  is not limited to having its aspheric surface S 9  bend towards the primary mirror M. This surface could also bend away from the primary mirror. 
         [0040]    Example 1 of projection lens  10  includes the following two design considerations. The first involves locating aspheric surface S 9  as far away from the reticle and wafer planes RP and WP as possible. This is so that any high-frequency surface errors, which can occur during fabrication of the aspheric surface, have less effect on telecentricity and distortion. The second consideration is to avoid making the surface MS of primary mirror M an aspheric surface since any surface errors are multiplied by a factor of two due to the reflective nature of the mirror surface, and because testing of aspheric mirror surfaces tends to be more difficult and costly. 
       Example 2 
       [0041]      FIG. 2  is an optical diagram of an Example 2 of the projection lens  10  as disclosed herein. The projection lens  10  of Example 2 has four lens elements L 1  through L 4  of the same glass type as in Example 1. The Example 2 design has a field radius of 50 mm, an usable field of view of 68 mm by 28 mm, a NA of 0.2, and an overall length of 805 mm. As in Example 1, surface S 11  of lens element L 4  is aspheric and is the dominant contributor to correcting astigmatism and field curvature introduced by primary mirror M. Lens element L 4  has negative overall optical power, with its lens surfaces S 8  and S 9  bending away from primary mirror M. Lens element L 4  has the same main function as in Example 1, namely to finely control the overall length and telecentricity. 
         [0042]    In this Example 2 design, lens elements L 3  and L 4  are significantly thinner than those used in Example 1. This serves to reduce the amount of glass in the optical path, which reduces absorption and thus the aforementioned adverse thermal transient effects. 
         [0043]    Table 1 below sets forth the optical prescription for Example 2. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 EXAMPLE 2 
               
             
          
           
               
                 S 
                 r (mm), k, c 
                 TH (mm) 
                 Material 
                 CA/2 (mm) 
               
               
                   
               
             
          
           
               
                 S0 
                 r = infinity 
                 5.182 
                   
                   
               
               
                 S1 
                 r = infinity 
                 70.000 
                 Fused Silica 
               
               
                 S2 
                 r = infinity 
                 0 
               
               
                 S3 
                 r = infinity 
                 0 
               
               
                 S4 
                 r = infinity 
                 24.185 
                 S-FPL51Y 
                 60.21 
               
               
                 S5 
                 r = −101.4781 
                 0 
                   
                 60.75 
               
               
                 S6 
                 r = −101.4781 
                 20.000 
                 BAL15Y 
                 60.75 
               
               
                 S7 
                 r = −559.7193 
                 70.003 
                   
                 66.44 
               
               
                 S8 
                 r = −870.4049 
                 25.000 
                 S-FSL5Y 
                 79.01 
               
               
                 S9 
                 r = −212.9743 
                 24.381 
                   
                 80.99 
               
               
                 S10 
                 r = −322.6115 
                 20.000 
                 Fused Silica 
                 81.68 
               
               
                 S11 
                 r = 484.4477 
                 546.254 
                   
                 83.99 
               
               
                   
                 k = −1.59831712 
               
               
                   
                 c 4th  = 7.9861E−10 
               
               
                   
                 c 6th  = −2.1783E−14 
               
               
                   
                 c 8th  = −5.757E−20 
               
               
                   
                 c 10th  = 2.2703E−23 
               
               
                 S12 
                 r = −813.1166 
                   
                   
                 120.04 
               
               
                   
               
             
          
         
       
     
       Example 3 
       [0044]      FIG. 3  is an optical system diagram of Example 3 of projection lens  10  as disclosed herein. Example 3 of projection lens  10  has a design field radius of 36 mm and usable field of view of 40 mm by 20 mm, and an NA of 0.16. The overall length of projection lens  10  is 691 mm. As the field size is substantially smaller than Examples 1 and 2, the number of refractive lens elements is reduced by one relative to Examples 1 and 2. The prisms PA and PB are made of fused silica. Lenses L 1 , L 2  and L 3  are made from S-FPL51Y, BAL15Y and S-FSL5Y, respectively. Note that lens element L 2  can also be made from BSM51Y. Lens element L 3  has an aspheric surface S 9 , and surfaces S 8  and S 9  are configured to correct for astigmatism and field curvature introduced by primary mirror M. 
         [0045]    Table 3 sets for the optical prescription for Example 3 of projection lens  10 . 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                 EXAMPLE 3 
               
             
          
           
               
                 S 
                 r (mm), k, c 
                 TH (mm) 
                 Material 
                 CA/2 (mm) 
               
               
                   
               
             
          
           
               
                 S0 
                 r = infinity 
                 5.182 
                   
                   
               
               
                 S1 
                 r = infinity 
                 50.000 
                 Fused Silica 
               
               
                 S2 
                 r = infinity 
                 0 
               
               
                 S3 
                 r = infinity 
                 0 
               
               
                 S4 
                 r = infinity 
                 25.000 
                 S-FPL51Y 
                 42.38 
               
               
                 S5 
                 r = −92.1029 
                 0 
                   
                 43.87 
               
               
                 S6 
                 r = −92.1029 
                 30.000 
                 BAL15Y 
                 43.87 
               
               
                 S7 
                 r = −311.7121 
                 79.930 
                   
                 48.49 
               
               
                 S8 
                 r = −519.4889 
                 30.000 
                 S-FSL5Y 
                 56.96 
               
               
                 S11 
                 r = −274.7936 
                 471.374 
                   
                 59.80 
               
               
                   
                 k = 1.075090621 
               
               
                   
                 c 4th  = 9.7906E−9 
               
               
                   
                 c 6th  = 2.0779E−13 
               
               
                   
                 c 8th  = 4.2347E−18 
               
               
                 S12 
                 r = −697.211955 
                   
                   
                 84.06 
               
               
                   
               
             
          
         
       
     
       Example 4 
       [0046]      FIG. 4  is an optical system diagram of an Example 4 of projection lens  10  as disclosed herein. Example 4 of projection lens  10  has a design field radius of 50 mm, a usable field of view of 68 mm by 26 mm, and an NA of 0.16. Surface S 11  is aspheric. Unlike the other examples, prisms PA and PB are made of BSM51Y, while lens element L 1  is made of S-FPL51Y, lens elements L 2  and L 4  are made of S-FSL5Y and lens element L 3  is made of fused silica. Thus, prisms PA and PB need not be only made of fused silica. 
         [0047]    Table 4 below sets for the optical prescription for Example 3 of projection lens  10 . 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                 EXAMPLE 4 
               
             
          
           
               
                 S 
                 r (mm), k, c 
                 TH (mm) 
                 Material 
                 CA/2 (mm) 
               
               
                   
               
             
          
           
               
                 S0 
                 r = infinity 
                 5.182 
                 mm 
                   
               
               
                 S1 
                 r = infinity 
                 55.5 
                 mm 
                 BSM51Y 
               
             
          
           
               
                 S2 
                 r = infinity 
                 0 
                   
                   
               
               
                 S3 
                 r = infinity 
                 0 
                   
                 56.45 mm 
               
             
          
           
               
                 S4 
                 r = infinity 
                 46.003 
                 mm 
                 S-FPL5Y 
                 57.15 mm 
               
             
          
           
               
                 S5 
                 r = −84.18704 mm 
                 0 
                   
                 57.15 mm 
               
             
          
           
               
                 S6 
                 r = −84.18704 mm 
                 60.001 
                 mm 
                 S-FSL5Y 
                 64.44 mm 
               
               
                 S7 
                 r = 930.18835 
                 18.711 
                 mm 
                   
                 69.87 mm 
               
               
                 S8 
                 r = 274.1023 mm 
                 73.661 
                 mm 
                 Fused Silica 
                 79.28 mm 
               
               
                 S9 
                 r = 254.1152 mm 
                 7.377 
                 mm 
                   
                 79.75 mm 
               
               
                 S10 
                 r = 363.3433 mm 
                 55.539 
                 mm 
                 S-FSL5Y 
               
               
                 S11 
                 r = −452.3749 mm 
                 480.041 
                 mm 
                   
                 77.80 mm 
               
               
                   
                 k = 9.230303384 
               
               
                   
                 c 4th  = 1.6900E−8 
               
               
                   
                 c 6th  = 3.6121E−13 
               
               
                   
                 c 8th  = 8.9935E−18 
               
               
                   
                 c 10th  = 6.5388E−22 
               
               
                 S12 
                 r = −800 mm 
                   
                   
                   
                 86.00 mm 
               
               
                   
               
             
          
         
       
     
       Optical Lithography System 
       [0048]      FIG. 5  is a schematic diagram of an example optical lithography system  200  that employs the projection lens  10  as disclosed herein. The optical lithography system  200  includes, in order along an optical axis A 0 , an illuminator  202 , a reticle  210  (e.g., a patterned mask) supported by a reticle stage  220  at a reticle plane RP, a projection imaging lens  230 , and a wafer  240  supported by a wafer stage  250  at a wafer plane WP. The reticle  210  includes a patterned region  211  that includes pattern elements  212  and that defines a reticle field RF. The wafer  240  includes an outer edge  241  (see  FIG. 7 ). Note that the reticle and wafer planes RP and WP are shown parallel and on the same axis for ease of illustration.  FIG. 1A  shows the actual orientation of the reticle plane RP and wafer plane WP. 
         [0049]    The wafer  240  includes a photosensitive coating  242  (e.g., photoresist) on the wafer surface that is activated by light (i.e., “actinic light”)  320  generated by a light source  310  located in illuminator  210 . The actinic light  320  comprises ghi radiation. Illuminator  202  is shown as included an aperture stop  330 . 
         [0050]    The optical lithography system  200  also includes a controller  260  operably connected to illuminator  210 , reticle stage  220  and wafer stage  250 . The controller  260  is configured to control the operation of optical lithography system  200 . An example controller  260  includes a computer such as a personal computer or workstation. In an example, controller  260  includes apparatus-control software that includes instructions, embodied in a computer-readable medium, that control the various components of optical lithography system  200 . 
         [0051]    The illuminator  202  is configured to generate an illumination field ILF (see  FIG. 5A ) at object plane OP, which corresponds to reticle plane RP. The illumination field ILF includes uniformized actinic light  320 ′ and illuminates at least a portion of reticle field RF over a reticle field exposure time so that projection lens  10  forms a corresponding image field IF at wafer plane WP over a corresponding wafer exposure time. The wafer stage  250  is movable (e.g., via a control signal SCW from controller  260 ) so that image field IF can be placed at different parts of wafer  240  to form various exposure fields EF on the wafer and in particular in photoresist  242 . In an example, reticle stage  220  is movable via a control signal SCR from controller  260 . 
         [0052]    Thus, uniformized actinic light  320 ′ from illuminator  202  is used to illuminate at least a portion of patterned region  211  that defines reticle field RF. The illuminated portion of reticle field RF is then imaged onto photosensitive coating  242  of wafer  240  via projection imaging lens  230 . In an example embodiment, reticle  210  and wafer  240  are moved together in a manner that scans image field IF over the wafer as illumination field ILF scans over reticle field RF, as illustrated by arrows AR in  FIG. 1  and also as schematically illustrated in  FIGS. 6A and 6B . This operation forms a scanned exposure field EF that is larger than either illumination field ILF or image field IF. This process is then repeated for a different (unexposed) region of wafer  240 . This printing approach is referred to in the art as “step and scan.” 
         [0053]    In another example, illumination field ILF illuminates the entire reticle field RF at once, thereby forming, in a single exposure, one exposure field EF. The wafer  240  is then moved, and the single static exposure is repeated. This printing approach is called “step and repeat.”  FIG. 6C  is similar to  FIG. 5A  and illustrates an example of a full-field illumination field ILF used for step-and-repeat printing.  FIG. 6D  is similar to  FIG. 6B  and illustrates an example of a full-field image field IF that corresponds in size to the corresponding exposure field EF as used in step-and-repeat printing. 
         [0054]    With reference to  FIG. 7 , exposure fields EF formed on wafer  240  in photoresist  242  are in turn used to form integrated circuit chips via standard photolithographic and semiconductor processing techniques. 
         [0055]    It is estimated that lower susceptibility to UV-induced damage for the projection lens  10  as disclosed herein should allow the projection lens to last 7 to 10 times longer than a conventional projection lens used in the same optical lithography system under the same operating conditions. 
         [0056]    It will be apparent to those skilled in the art that various modifications to the preferred embodiments of the disclosure as described herein can be made without departing from the spirit or scope of the disclosure as defined in the appended claims. Thus, the disclosure covers the modifications and variations provided they come within the scope of the appended claims and the equivalents thereto.