Abstract:
A 1× projection optical system for deep ultra-violet (DUV) photolithography is disclosed. The optical system is a modified Dyson system capable of imaging a relatively large field at high numerical apertures at DUV wavelengths. The optical system includes a lens group having first and second prisms and four lenses having a positive-negative-positive negative arrangement as arranged in order from the prisms toward the mirror. A projection photolithography system that employs the projection optical system of the invention is also disclosed.

Description:
This application is a continuation-in-part application of the application having Ser. No, 10/447,128 filed on May 27, 2003, now U.S. Pat. No. 6,863,403 issued Mar. 8, 2005. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to optical projection systems, and in particular to deep ultra-violet, large-field unit-magnification projection optical systems. 
   2. Description of the Prior Art 
   Photolithography is presently employed not only in sub-micron resolution integrated circuit (IC) manufacturing, but also to an increasing degree in advanced wafer-level IC packaging as well as in semiconductor, microelectromechanical systems (MEMS), nanotechnology (i.e., forming nanoscale structures and devices), and other applications. 
   The present invention, as described in the Detailed Description of the Invention section below, is related to the optical system described in U.S. Pat. No. 4,391,494 (hereinafter, “the &#39;494 patent”) issued on Jul. 5, 1983 to Ronald S. Hershel and assigned to General Signal Corporation, which patent is hereby incorporated by reference. In addition, the present invention as described below is also related to the optical system described in U.S. Pat. No. 5,031,977 (“the &#39;977 patent”), issued on Jul. 16, 1991 to John A. Gibson and assigned to General Signal Corporation, which patent is hereby incorporated by reference. 
     FIG. 1  is a cross-sectional diagram of an example prior art optical system  8  according to the &#39;494 patent. The optical system described in the &#39;494 patent and illustrated in  FIG. 1  is a unit-magnification, catadioptric, achromatic and anastigmatic, optical projection system that uses both reflective and refractive elements in a complementary fashion to achieve large field sizes and high numerical apertures (NAs). The system is basically symmetrical relative to an aperture stop located at the mirror, thus eliminating odd order aberrations such as coma, distortion and lateral color. All of the spherical surfaces are nearly concentric, with the centers of curvature located close to where the focal plane would be located were the system not folded. Thus, the resultant system is essentially independent of the index of refraction of the air in the lens, making pressure compensation unnecessary. 
   With continuing reference to  FIG. 1 , optical system  8  includes a concave spherical mirror  10 , an aperture stop  11  located at the mirror, and a composite, achromatic plano-convex doublet lens-prism assembly  12 . Mirror  10  and assembly  12  are disposed symmetrically about an optical axis  14 . Optical system  8  is essentially symmetrical relative to aperture stop  11  so that the system is initially corrected for coma, distortion, and lateral color. All of the spherical surfaces in optical system  8  are nearly concentric. 
   In optical system  8 , doublet-prism assembly  12  includes a meniscus lens  13 A, a plano-convex lens  13 B and symmetric fold prisms  15 A and  15 B located on opposite sides of optical axis  14 . In conjunction with mirror  10 , assembly  12  corrects the remaining optical aberrations, which include axial color, astigmatism, petzval, and spherical aberration. Symmetric fold prisms  15 A and  15 B are used to attain sufficient working space for movement of a reticle  16  and a wafer  18 . The cost of this gain in working space is the reduction of available field size to about 25% to 35% of the total potential field. In the past, this reduction in field size has not been critical since it has been possible to obtain both acceptable field size and the resolution required for the state-of-the-art circuits. However, today this field size reduction is problematic. 
     FIG. 2  is a cross-sectional diagram of an example prior art optical system  50  according to the &#39;977 patent. System  50  includes a first mirror  52  and a meniscus lens  54  which is desirably of fused silica. System  50  also includes a plano-convex lens  56 , desirably of lithium fluoride, and a pair of prisms  60 - 1 ,  60 - 2  made of calcium fluoride. System  50  includes an optical axis  64 . Operation of optical system  50  with a source of light exposure (desirably in the ultraviolet range) is analogous to that described in the &#39;494 patent. System  50  has a numerical aperture (NA) of 0.350 and design wavelengths of 249.8 nanometers and 243.8 nanometers. The air-lens and lens-lens transitions in optical system  50  are labeled  1 ,  2 ,  3 ,  4  and  5  across the bottom of  FIG. 2 . 
   Unfortunately, for larger NA applications (i.e., NA≧0.435), both the &#39;494 and the &#39;977 systems of a reasonable size cannot achieve high quality imagery over field sizes having a field height larger than 23 mm in the DUV (Deep Ultra-violet) spctrum. 
   SUMMARY OF THE INVENTION 
   A first aspect of the invention is a projection optical system. The system includes along an optical axis a mirror having a concave surface, and an aperture stop located at the mirror that determines a numerical aperture (NA) of the system. The system also includes a lens group with positive refracting power arranged adjacent the mirror and spaced apart therefrom. The lens group comprises in order towards the mirror: a) first and second prisms arranged on opposite sides of the optical axis and each having a planar surface, wherein the planar surfaces are arranged adjacent object and image planes, respectively; and b) a first positive lens, a second negative lens, a third positive lens and a fourth negative lens, wherein the lenses of the lens group have surfaces that are non-concentric with respect to the mirror surface. 
   A second aspect of the invention is a photolithography system that includes the projection optical system of the present invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional diagram of an example prior art unit-magnification projection optical system according to the &#39;494 patent; 
       FIG. 2  is a cross-sectional diagram of an example prior art unit-magnification projection optical system according to the &#39;977 patent; 
       FIG. 3  is cross-sectional diagram of a generalized embodiment of the unit-magnification projection optical system of the present invention; and 
       FIG. 4  is a schematic diagram of a photolithography system employing the unit-magnification projection optical system of the present invention. 
   

   The various elements depicted in the drawings are merely representational and are not necessarily drawn to scale. Certain proportions thereof may be exaggerated, while others may be minimized. The drawings are intended to illustrate various implementations of the invention, which can be understood and appropriately carried out by those of ordinary skill in the art. 
   DETAILED DESCRIPTION OF THE INVENTION 
   The unit-magnification projection optical system of the present invention is an improvement over the prior art optical system of the &#39;494 patent and the &#39;977 patent, embodiments of which are described briefly above and illustrated in  FIGS. 1 and 2 . 
   The projection optical system of the present invention provides an optical design configuration that forms the basis of the unit magnification projection optical system suitable for application in exposure apparatus utilizing illumination systems with excimer laser radiation sources such as a KrF laser (248nm), an ArF laser (193 nm) and an F2 laser (157nm). Moreover, the present invention provides a common lens design configuration with refractive optical components (prism and tens elements) manufacturable using low refractive index optical materials (such as fused silica, calcium fluoride, barium fluoride, strontium fluoride, etc.), that transmit radiation having the above-mentioned DUV laser wavelengths. 
   The projection optical system of the present invention as described in detail below has very good image quality (i.e., Strehl ratios greater than 0.96). 
     FIG. 3  is a cross-sectional diagram of a generalized embodiment of a DUV unit-magnification projection optical system  100  according to the present invention. Projection optical system  100  includes, along an axis OA, a concave spherical mirror M. In an example embodiment, mirror M includes an aperture AP on the optical axis. Aperture AP may be used, for example, to introduce light into the optical system for performing functions other than direct imaging with optical system  100 , such as for aligning an object (e.g., a mask) with its image, or inspecting the object. 
   Optical system  100  further includes an aperture stop AS 1  located at mirror M. In an example embodiment, aperture stop AS 1  is variable and may include any one of the known forms for varying the size of an aperture in an optical system, such as an adjustable iris. In an example embodiment, the size of variable aperture stop AS 1  is manually set. In another example embodiment, variable aperture stop AS 1  is operatively connected via a line  101  (e.g., a wire) to a controller  102  that allows for automatically setting the size of the aperture stop. Aperture stop AS 1  defines the numerical aperture NA of the system, which in example embodiments of the present invention is in the range of between 0.3 and 0.5 (inclusive). 
   Optical system  100  further includes a prism/lens group G (hereinafter, simply “lens group G”) with positive refractive power arranged along axis OA adjacent to, and spaced apart from, mirror M. Lens group G includes two prisms PA and PB farthest from mirror M and located on opposite sides of optical axis OA. Prism PA has a planar surface S 1 A, and prism PB has a planar surface S 1 B. Surface S 1 A faces an object plane OP 1  and surface S 1 B faces an image plane IP 1 . The object plane OP 1  and the image plane IP 1  are spaced apart from respective planar surfaces S 1 A and S 1 B by respective gaps WDA and WDB representing working distances. In example embodiments where there is complete symmetry with respect to aperture stop AS 1 , WDA=WDB. Since WDA and WDB are equal to each other, in the accompanying Tables 1–7 those distances are referred collectively to as WD. 
   Prisms PA and PB play a role in the aberration correction, including chromatic aberration correction. Prisms PA and PB also serve to separate object plane OP 1  from image plane IP 1  (without prisms PA and PB, the object and image planes would be co-planar). 
   Lens group G further includes, in order from prisms PA and PB toward mirror M, lens elements L 1 , L 2 , L 3 , and L 4  disposed symmetrically about axis OA. The refractive powers of the lens elements are such that L 1  is positive, L 2  is negative, L 3  is positive and L 4  is negative. The planar surface S 2  of each of prisms PA and PB is opposite the ulanar surface S 3  of lens L 1 , the convex surface S 4  of lens L 1  is opposite the concave surface S 5  of lens L 2  the convex surface S 6  of lens L 2  is opposite the high radius concave surface S 7  of lens L 3 , the high radius convex surface S 8  of lens L 3  is opposite the high radius concave surface S 9  of lens L 4  and the surface S 10  of lens L 4  is facing and spaced auart from the concave surface S 11  of mirror M. The optical system is also basically symmetrical relative to aperture stop AS 1  and thus initially corrected for coma, distortion,, and lateral color. Moreover, the lens group G, in conjunction with the prisms PA and PB, and the mirror M, corrects the remaining optical aberrations, which include axial color, astigmatism, petzval, and spherical aberration. The chromatic variations of the optical aberrations are reduced also by the +−+−lens element geometry and by the alternating optical materials choice. Together, these two features greatly help to boost the optical performance of optical system  100  in achieving a sufficiently high quality imagery over a large field and with a high numerical aperture in a 1×, DUV exposure system. In particular, L 3  and L 4 , improve the overall correction of astigmatism and petzval curvature in optical system  100  helping to provide a fiat field. Mirror M, when aspherized corrects higher order spherical aberrations, and also improves the overall residual aberration balance in system  100 . 
   The respective working distances WDA and WDB provide sufficient mechanical clearances and spaces for positioning a large wafer W and a large reticle R in image plane IP 1  and object plane OP 1 , respectively. 
   Example Designs 
   While the projection optical system of the present invention is described in conjunction with the optical design layout shown in  FIG. 3 , it will be understood that it is not intended to limit the invention to this design form, but also intended to cover alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined and described in connection with particular design examples having the optical prescriptions shown in Tables 1–7, and as set forth in the claims. Each of the design examples in Table 1–7, has a design form based on the general design configuration illustrated in  FIG. 3 . 
   Since projection optical system  100  of the present invention is completely symmetric with respect to aperture stop AS 1  at mirror M, the optical prescriptions in accompanying Tables 1–7 include only values of the optical specifications from object plane OP 1  to the concave mirror M. 
   In Tables 1–7, a positive radius indicates the center of curvature is to the right of the surface, and a negative radius indicates the center of curvature is to the left. The thickness is the axial distance to the next surface. All dimensions are in millimeters. All of the example embodiments basically preserve the system symmetry relative to the aperture stop located at the concave mirror thus inherently eliminating the odd order aberrations such as coma, distortion, and lateral color. There are no lens elements with concentric surfaces in lens group G, nor are there any lens surfaces that are concentric with mirror M. 
   Further, “S#” stands for surface number, e.g. as labeled across the bottom of the lens system in  FIG. 3 , “T or S” stands for “thickness or separation”, and “STOP” stands for “aperture stop AS 1 ”. Also, “CC” stands for “concave” and “CX” stands for “convex.” 
   Further, under the heading “surface shape”, an aspheric surface is denoted by “ASP”, a planar (flat) surface by “FLT” and a spherical surface by “SPH”. 
   The aspheric equation describing an aspherical surface is given by: 
           Z   =           (   CURV   )     ⁢     Y   2         1   +       (     1   -       (     1   +   K     )     ⁢       (   CURV   )     2     ⁢     Y   2         )       1   /   2           +       (   A   )     ⁢     Y   4       +       (   B   )     ⁢     Y   6       +       (   C   )     ⁢     Y   8       +       (   D   )     ⁢     Y   10               
wherein “CURV” is the spherical curvature of the surface, K is the conic constant, and A, B, C, and D are the aspheric coefficients. In the Tables, “E” denotes exponential notation (powers of 10).
 
   In the projection optical system  100  as set forth in Table 1, prisms PA and PB, and lenses L 1 –L 4  are all formed from fused silica and are spherical lenses. The NA is 0.435, the field height is 23.2 mm. The operating wavelength range is 248.39 nm (±0.1 nm), which makes the lens suitable for use with a DUV laser radiation source. When employed with a narrowed or ultra-line narrowed DUV laser source, optical system  100  yields reasonably high quality imagery. 
   
     
       
             
           
             
             
             
             
           
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
           
         
             
               TABLE 1 
             
           
           
             
                 
             
             
               NA = 0.435 Field Height (mm) = 23.2 
             
             
               Design Wavelengths (nm) 248.39 ± 0.1 
             
           
        
         
             
                 
               SURFACE 
                 
               ELEMENT 
             
             
                 
               DESCRIPTION 
                 
               DE- 
             
           
        
         
             
               S# 
               RADIUS 
                 
               SHAPE 
               T or S 
               MATERIAL 
               SCRIPTION 
             
             
                 
             
           
        
         
             
               0 
               INF 
                 
               FLT 
               0.0000 
                 
                 
             
             
                 
                 
                 
                 
               3.5021 
                 
               Working 
             
             
                 
                 
                 
                 
                 
                 
               distance WD 
             
             
               1 
               INF 
                 
               FLT 
               34.0000 
               Fused Silica 
               Prism A/Prism 
             
             
                 
                 
                 
                 
                 
                 
               B glass path 
             
             
               2 
               INF 
                 
               FLT 
               0.0000 
             
             
               3 
               INF 
                 
               FLT 
               43.9936 
               Fused Silica 
               L1 
             
             
               4 
               −140.088 
               CX 
               SPH 
               4.6554 
             
             
               5 
               −115.435 
               CC 
               SPH 
               77.8000 
               Fused Silica 
               L2 
             
             
               6 
               −158.457 
               CX 
               SPH 
               1.5000 
             
             
               7 
               −2198.913 
               CC 
               SPH 
               38.0000 
               Fused Silica 
               L3 
             
             
               8 
               −411.153 
               CX 
               SPH 
               2.7870 
             
             
               9 
               −1423.533 
               CC 
               SPH 
               38.0000 
               Fused Silica 
               L4 
             
             
               10 
               3458.549 
               CC 
               SPH 
               305.7620 
             
             
               11 
               −543.979 
               CC 
               SPH 
               −305.7620 
               REFL 
               Mirror M 
             
             
                 
                 
                 
                 
                 
               (STOP) 
             
             
                 
             
           
        
       
     
   
   In the projection optical system  100  as set forth in Table 2, prisms PA and PB and lenses L 1  and L 3  are formed from calcium fluoride, and lenses L 2  and L 4  are formed from fused silica. All the lenses are spherical lenses. In addition, mirror M has an aspherical surface. The NA is 0.435, the field height is 23.2 mm. The operating wavelength range is 248.34 nm (±0.5 nm), which makes the lens suitable for use with a DUV laser radiation source. The combination of calcium fluoride and fused silica materials for the lens group G, i.e. calcium fluoride for the positive lens elements and fused silica for the negative elements, corrects axial color and the chromatic variations of residual aberrations. This enables optical system  100  to operate with a broader line width DUV laser source. The aspheric mirror corrects high order spherical aberration and thus improves overall system performance. 
   
     
       
             
           
             
             
             
             
           
             
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
           
         
             
               TABLE 2 
             
             
                 
             
             
               NA = 0.435 Field Height (mm) = 23.2 Design Wavelengths (nm) = 248.34 ± 0.5 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                 
               SURFACE DESCRIPTION 
                 
               ELEMENT 
             
           
        
         
             
                 
               S# 
               RADIUS 
                 
               SHAPE 
               T or S 
               MATERIAL 
               DESCRIPTION 
             
             
                 
                 
             
             
                 
               0 
               INF 
                 
               FLT 
               0.0000 
             
             
                 
                 
                 
                 
                 
               5.4824 
                 
               Working distance 
             
             
                 
                 
                 
                 
                 
                 
                 
               WD 
             
             
                 
               1 
               INF 
                 
               FLT 
               34.0000 
               Calcium Fluoride 
               Prism A/Prism B 
             
             
                 
                 
                 
                 
                 
                 
                 
               glass path 
             
             
                 
               2 
               INF 
                 
               FLT 
               0.0000 
             
             
                 
               3 
               INF 
                 
               FLT 
               28.3056 
               Calcium Fluoride 
               L1 
             
             
                 
               4 
               −114.898 
               CX 
               SPH 
               4.4701 
             
             
                 
               5 
               −101.751 
               CC 
               SPH 
               77.8000 
               Fused Silica 
               L2 
             
             
                 
               6 
               −145.446 
               CX 
               SPH 
               1.6688 
             
             
                 
               7 
               −647.352 
               CC 
               SPH 
               38.0000 
               Calcium Fluoride 
               L3 
             
             
                 
               8 
               −447.517 
               CX 
               SPH 
               1.5000 
             
             
                 
               9 
               −1817.472 
               CC 
               SPH 
               38.0000 
               Fused Silica 
               L4 
             
             
                 
               10 
               −5113.783 
               CX 
               SPH 
               310.7731 
             
             
                 
               11 
               −531.928 
               CC 
               ASP 
               −310.7731 
               REFL (STOP) 
               Mirror M 
             
             
                 
                 
             
           
        
         
             
               ASPHERIC 
                 
                 
                 
                 
                 
                 
             
             
               S# 
               CURV 
               K 
               A 
               B 
               C 
               D 
             
             
                 
             
             
               S11 
               −0.00187995 
               0.000000 
               5.90655E−12 
               8.61447E−17 
               5.86675E−22 
               8.42714E−27 
             
             
                 
             
           
        
       
     
   
   In the projection optical system  100  as set forth in Table 3, prisms PA and PB and lenses L 1 –L 4  are all formed from fused silica. All the lenses have spherical surfaces. In addition, the mirror has an aspherical surface. The NA is 0.435, the field height is 23.2 mm. The operating wavelength range is 193.3 nm (±0.1 nm), which makes the lens suitable for use with a DUV line narrowed or ultra-line narrowed laser radiation source. 
   
     
       
             
           
             
             
             
             
           
             
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
           
         
             
               TABLE 3 
             
             
                 
             
             
               NA = 0.435 Field Height (mm) = 23.2 Design Wavelengths (nm) = 93.3 ± 0.1 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                 
               SURFACE DESCRIPTION 
                 
               ELEMENT 
             
           
        
         
             
                 
               S# 
               RADIUS 
                 
               SHAPE 
               T or S 
               MATERIAL 
               DESCRIPTION 
             
             
                 
                 
             
             
                 
               0 
               INF 
                 
               FLT 
               0.0000 
             
             
                 
                 
                 
                 
                 
               4.0000 
                 
               Working distance 
             
             
                 
                 
                 
                 
                 
                 
                 
               WD 
             
             
                 
               1 
               INF 
                 
               FLT 
               34.0000 
               Fused Silica 
               Prism A/Prism 
             
             
                 
                 
                 
                 
                 
                 
                 
               B glass path 
             
             
                 
               2 
               INF 
                 
               FLT 
               0.0000 
             
             
                 
               3 
               INF 
                 
               FLT 
               43.9936 
               Fused Silica 
               L1 
             
             
                 
               4 
               −137.305 
               CX 
               SPH 
               4.3078 
             
             
                 
               5 
               −116.108 
               CC 
               SPH 
               77.8000 
               Fused Silica 
               L2 
             
             
                 
               6 
               −161.314 
               CX 
               SPH 
               1.5000 
             
             
                 
               7 
               −481.652 
               CC 
               SPH 
               38.0000 
               Fused Silica 
               L3 
             
             
                 
               8 
               −395.742 
               CX 
               SPH 
               1.5002 
             
             
                 
               9 
               −1012.080 
               CC 
               SPH 
               38.0000 
               Fused Silica 
               L4 
             
             
                 
               10 
               −1266.111 
               CX 
               SPH 
               306.8983 
             
             
                 
               11 
               −544.016 
               CC 
               ASP 
               −306.8983 
               REFL (STOP) 
               Mirror M 
             
             
                 
                 
             
           
        
         
             
               ASPHERIC 
                 
                 
                 
                 
                 
                 
             
             
               S# 
               CURV 
               K 
               A 
               B 
               C 
               D 
             
             
                 
             
             
               S11 
               −0.00183818 
               0.000000 
               2.58962E−11 
               1.98197E−16 
               8.56012E−22 
               1.50805E−26 
             
             
                 
             
           
        
       
     
   
   In the projection optical system  100  as set forth in Table 4, prisms PA and PB and lenses L 1  and L 3  are formed from calcium fluoride, and lenses L 2  and L 4  are formed from fused silica. All the lenses have spherical surfaces, and mirror M has a spherical surface. The NA is 0.435, and the field height is 23.2 mm. The operating wavelength range is 193.3 nm (±0.1 nm), which makes the lens suitable for use with a DUV laser radiation source. Using calcium fluoride substrate material for positive lens elements and fused silica substrate material for negative elements enhances the correction of axial chromatic aberration as well as reduces the chromatic variation of field aberrations. 
   
     
       
             
           
             
             
             
             
           
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
           
         
             
               TABLE 4 
             
           
           
             
                 
             
             
               NA = 0.435 Field Height (mm) = 23.2 
             
             
               Design Wavelengths (nm) = 193.3 ± 0.1 
             
           
        
         
             
                 
               SURFACE 
                 
               ELEMENT 
             
             
                 
               DESCRIPTION 
                 
               DE- 
             
           
        
         
             
               S# 
               RADIUS 
                 
               SHAPE 
               T or S 
               MATERIAL 
               SCRIPTION 
             
             
                 
             
           
        
         
             
               0 
               INF 
                 
               FLT 
               0.0000 
                 
                 
             
             
                 
                 
                 
                 
               3.5000 
                 
               Working 
             
             
                 
                 
                 
                 
                 
                 
               distance WD 
             
             
               1 
               INF 
                 
               FLT 
               34.0000 
               Calcium 
               Prism A/Prism 
             
             
                 
                 
                 
                 
                 
               Fluoride 
               B glass path 
             
             
               2 
               INF 
                 
               FLT 
               0.0000 
             
             
               3 
               INF 
                 
               FLT 
               43.9936 
               Calcium 
               L1 
             
             
                 
                 
                 
                 
                 
               Fluoride 
             
             
               4 
               −137.984 
               CX 
               SPH 
               5.7288 
             
             
               5 
               −116.193 
               CC 
               SPH 
               77.8000 
               Fused Silica 
               L2 
             
             
               6 
               −158.614 
               CX 
               SPH 
               13.8279 
             
             
               7 
               −1852.115 
               CC 
               SPH 
               38.0000 
               Calcium 
               L3 
             
             
                 
                 
                 
                 
                 
               Fluoride 
             
             
               8 
               −515.570 
               CX 
               SPH 
               1.5000 
             
             
               9 
               −3516.377 
               CC 
               SPH 
               38.0000 
               Fused Silica 
               L4 
             
             
               10 
               3090.388 
               CC 
               SPH 
               293.6497 
             
             
               11 
               −543.130 
               CC 
               SPH 
               −293.6497 
               REFL 
               Mirror M 
             
             
                 
                 
                 
                 
                 
               (STOP) 
             
             
                 
             
           
        
       
     
   
   In the projection optical system  100  as set forth in Table 5, prisms PA and PB and lenses L 1  and L 3  are formed from calcium fluoride, and lenses L 2  and L 4  are formed from fused silica. All the lenses have spherical surfaces, and mirror M has an aspherical surface. The NA is 0.435, the field height is 23.2 mm. The operating wavelength range is 193.3 nm (±0.1 nm), which makes the lens suitable for use with a DUV laser radiation source. As in the embodiment of Table 4, the embodiment of Table 5 has well-corrected chromatic aberrations and chromatic variations of residual field aberrations. The aspheric mirror M provides correction of higher order spherical aberrations and overall balance of residual aberrations. 
   
     
       
             
           
             
             
             
             
           
             
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
           
         
             
               TABLE 5 
             
             
                 
             
             
               NA = 0.435 Field Height (mm) = 23.2 Design Wavelengths (nm) = 193.3 ± 0.1 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                 
               SURFACE DESCRIPTION 
                 
               ELEMENT 
             
           
        
         
             
                 
               S# 
               RADIUS 
                 
               SHAPE 
               T or S 
               MATERIAL 
               DESCRIPTION 
             
             
                 
                 
             
             
                 
               0 
               INF 
                 
               FLT 
               0.0000 
             
             
                 
                 
                 
                 
                 
               3.5000 
                 
               Working distance 
             
             
                 
                 
                 
                 
                 
                 
                 
               WD 
             
             
                 
               1 
               INF 
                 
               FLT 
               34.0000 
               Calcium Fluoride 
               Prism A/Prism 
             
             
                 
                 
                 
                 
                 
                 
                 
               B glass path 
             
             
                 
               2 
               INF 
                 
               FLT 
               0.0000 
             
             
                 
               3 
               INF 
                 
               FLT 
               43.9936 
               Calcium Fluoride 
               L1 
             
             
                 
               4 
               −138.821 
               CX 
               SPH 
               6.2692 
             
             
                 
               5 
               −116.466 
               CC 
               SPH 
               77.8000 
               Fused Silica 
               L2 
             
             
                 
               6 
               −158.824 
               CX 
               SPH 
               2.0521 
             
             
                 
               7 
               −1391.235 
               CC 
               SPH 
               38.0000 
               Calcium Fluoride 
               L3 
             
             
                 
               8 
               −457.136 
               CX 
               SPH 
               1.5000 
             
             
                 
               9 
               −2126.796 
               CC 
               SPH 
               38.0000 
               Fused Silica 
               L4 
             
             
                 
               10 
               5607.105 
               CC 
               SPH 
               304.8851 
             
             
                 
               11 
               −543.641 
               CC 
               ASP 
               −304.8851 
               REFL (STOP) 
               Mirror M 
             
             
                 
                 
             
           
        
         
             
               ASPHERIC 
                 
                 
                 
                 
                 
                 
             
             
               S# 
               CURV 
               K 
               A 
               B 
               C 
               D 
             
             
                 
             
             
               S11 
               −0.00183945 
               0.000000 
               1.14297E−12 
               1.51380E−17 
               1.57864E−23 
               5.81245E−27 
             
             
                 
             
           
        
       
     
   
   In the projection optical system  100  as set forth in Table 6, prisms PA and PB and lenses L 1 –L 4  are all formed from calcium fluoride. All the lenses have spherical surfaces. In addition, the mirror has an aspherical surface. The NA is 0.435, the field height is 23.2 mm. The operating wavelength range is 157.631 nm (±0.0008 nm), which makes the lens suitable for use with a DUV line narrowed or ultra-line narrowed laser radiation source. 
   
     
       
             
           
             
             
             
             
           
             
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
           
         
             
               TABLE 6 
             
             
                 
             
             
               NA = 0.435 Field Height (mm) = 23.2 Design Wavelengths (nm) = 157.631 ± 0.0008 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                 
               SURFACE DESCRIPTION 
                 
               ELEMENT 
             
           
        
         
             
                 
               S# 
               RADIUS 
                 
               SHAPE 
               T or S 
               MATERIAL 
               DESCRIPTION 
             
             
                 
                 
             
             
                 
               0 
               INF 
                 
               FLT 
               0.0000 
             
             
                 
                 
                 
                 
                 
               4.0000 
                 
               Working distance 
             
             
                 
                 
                 
                 
                 
                 
                 
               WD 
             
             
                 
               1 
               INF 
                 
               FLT 
               34.0000 
               Calcium Fluoride 
               Prism A/Prism B 
             
             
                 
                 
                 
                 
                 
                 
                 
               glass path 
             
             
                 
               2 
               INF 
                 
               FLT 
               0.0000 
             
             
                 
               3 
               INF 
                 
               FLT 
               47.5563 
               Calcium Fluoride 
               L1 
             
             
                 
               4 
               −140.686 
               CX 
               SPH 
               4.4547 
             
             
                 
               5 
               −117.667 
               CC 
               SPH 
               77.8000 
               Calcium Fluoride 
               L2 
             
             
                 
               6 
               −162.562 
               CX 
               SPH 
               1.5000 
             
             
                 
               7 
               −528.689 
               CC 
               SPH 
               38.0000 
               Calcium Fluoride 
               L3 
             
             
                 
               8 
               −401.134 
               CX 
               SPH 
               1.5000 
             
             
                 
               9 
               −1563.422 
               CC 
               SPH 
               38.0000 
               Calcium Fluoride 
               L4 
             
             
                 
               10 
               −2463.529 
               CX 
               SPH 
               303.1890 
             
             
                 
               11 
               −544.406 
               CC 
               ASP 
               −303.1890 
               REFL (STOP) 
               Mirror M 
             
             
                 
                 
             
           
        
         
             
               ASPHERIC 
                 
                 
                 
                 
                 
                 
             
             
               S# 
               CURV 
               K 
               A 
               B 
               C 
               D 
             
             
                 
             
             
               S11 
               −0.00183686 
               0.000000 
               1.96149E−11 
               1.55956E−16 
               8.12495E−22 
               1.21311E−26 
             
             
                 
             
           
        
       
     
   
   In the projection optical system  100  as set forth in Table 7, prisms PA and PB and lenses L 1 –L 4  are all formed from calcium fluoride. All the lenses have spherical surfaces. In addition, the mirror has an aspherical surface. The NA is 0.50, and the field height is 23.2 mm. The operating wavelength range is 157.631 nm (±0.0008 nm), which makes the lens suitable for use with a DUV line narrowed or ultra line narrowed laser radiation source. A broader DUV laser source may be used if the two optical materials are used in the embodiments shown in Tables 6 and 7, such as calcium fluoride for the positive lens elements and barium fluoride for the negative lens elements. 
                                                                                                     TABLE 7               NA = 0.50 Field Height (mm) = 23.2 Design Wavelengths (nm) = 157.631 ± 0.0008                                    SURFACE DESCRIPTION       ELEMENT                S#   RADIUS       SHAPE   T or S   MATERIAL   DESCRIPTION                       0   INF       FLT   0.0000                           4.0000       Working distance                                   WD           1   INF       FLT   34.0000   Calcium Fluoride   Prism A/Prism                                   B glass path           2   INF       FLT   0.0000           3   INF       FLT   47.5563   Calcium Fluoride   L1           4   −146.835   CX   SPH   4.9586           5   −120.689   CC   SPH   77.8000   Calcium Fluoride   L2           6   −163.592   CX   SPH   1.5000           7   −546.710   CC   SPH   38.0000   Calcium Fluoride   L3           8   −403.614   CX   SPH   1.5000           9   −2316.156   CC   SPH   38.0000   Calcium Fluoride   LE           10   −3758.411   CX   SPH   302.6851           11   −545.373   CC   ASP   −302.6851   REFL (STOP)   Mirror M                        ASPHERIC                               S#   CURV   K   A   B   C   D               S11   −0.00183361   0.000000   2.61844E−11   1.93771E−16   8.38751E−22   1.46130E−26                    
Photolithography System
 
     FIG. 4  is a schematic diagram of a photolithography system  200  employing the unit-magnification projection optical system  100  of the present invention. System  200  has an optical axis A 2  and includes along the optical axis a mask stage  210  adapted to support a mask  220  at object plane OP 1 . Mask  220  has a pattern  224  formed on a mask surface  226 . An illuminator  230  is arranged adjacent mask stage  210  opposite optical system  100  and is adapted to illuminate mask (reticle)  220 . 
   System  200  also includes a wafer stage  240  adapted to movably support a wafer  246  at image plane IP 1 . In an example embodiment, wafer  246  is coated with a photosensitive layer  250  that is activated by one or more wavelengths of radiation from the illuminator. Such radiation is referred to in the art as “actinic radiation”. In an example embodiment, the one or more wavelengths of radiation include 248 nm, 193 nm and 157 nm. 
   In operation, illuminator  230  illuminates mask  220  while stage  240  positions wafer  250  to align the image with previously produced patterns so that pattern  224  is imaged at wafer  246  by optical system  100 , thereby forming a pattern in photoresist layer  250 . The result is an exposure field EF that occupies a portion of the wafer surface. Wafer stage  240  then moves (“steps”) wafer  246  in a given direction  260  (e.g., the x-direction) by a given increment (e.g., the size of one exposure field EF), and the exposure process is repeated. This step-and-repeat exposure process is continued (hence the name “step-and-repeat” until a desired number of scanned exposure fields EF are formed on wafer  246 . 
   Wafer  246  is then removed from system  200  (e.g., using a wafer handling system, not shown) and processed (e.g., developed, baked, etched, etc.) to transfer the pattern formed in the photoresist in each exposure field EF to the underlying layer(s) on the wafer. Once the pattern is transferred the resist is typically stripped, a new layer of material is added with a deposition process, and the wafer is again coated with resist. Repeating the photolithography process with different masks allows for three-dimensional structures to be formed in the wafer to create operational devices, such as ICs. 
   In the foregoing Detailed Description, various features are grouped together in various example embodiments for ease of understanding. The many features and advantages of the present invention are apparent from the detailed specification, and, thus, it is intended by the appended claims to cover all such features and advantages of the described apparatus that follow the true spirit and scope of the invention. Furthermore, since numerous modifications and changes will readily occur to those of skill in the art, the invention is not to be limited to the exact construction and operation described herein. Accordingly, other embodiments are within the scope of the appended claims and the invention is only limited by the scope of the appended claims.