Abstract:
A catadioptric projection lens comprising a first lens group (G 1 ) disposed along a first optical axis ( 16   a ), a mirror ( 18 ) which creates a second optical axis ( 16   b ), a beam splitter ( 20 ) which creates a third optical axis ( 16   c ), a second lens group (G 2 ) including a concave mirror (L 22 ) disposed along the third optical axis on one side of the beam splitter, and a third lens group (G 3 ) disposed along the third optical axis on the side of the beam splitter. The first and third optical axes are parallel, a configuration which reduces aberrations arising from gravitational deformation of the lens elements, when the first and third axes are aligned with the direction of gravity.

Description:
FIELD OF THE INVENTION 
     The present invention relates to projection lenses, and more particularly to high-resolution, ultraviolet projection lenses for projection optical systems. 
     BACKGROUND OF THE INVENTION 
     The process of manufacturing certain electronic devices, such as semiconductor integrated circuits, liquid crystal displays and the like, requires the use of high-resolution projection optical systems. In such systems, a pattern on a photomask or “reticle” is illuminated with light from a source of illumination. The light passing therethrough is imaged onto a workpiece, such as a light-sensitive substrate (e.g., as a silicon wafer coated with photoresist) by a projection lens. Due to the increasing degree of integration of electronic devices, the resolution demanded by the projection optical system has steadily increased. To meet this demand, it is necessary for the projection lens of the projection optical system to operate at a shorter wavelength light and/or to have a larger numerical aperture (NA). 
     Shortening the operating wavelength of the projection optical system to the ultraviolet (UV) region of the electromagnetic spectrum has the consequence that a limited number of optical materials are available for use. For example, for wavelengths of light under 300 nanometers (nm), synthetic quartz and fluorite (calcium fluoride) are the only glass types that have suitable transmission properties. Unfortunately, the Abbe number of these glasses are close to one another, so it is difficult to compensate for the various aberrations in the projection lens, including chromatic aberration, for lens designs that employ only these glass types. 
     In contrast, reflective optical systems have no chromatic aberration. Thus, various projection lenses have been proposed which have both reflective and refractive lens elements (i.e., “catadioptric” lenses). Certain proposed high-resolution catadioptric projection lenses include a beam splitter for folding the optical path and have been disclosed in Japanese patent application Kokoku No. Hei 7-117648, Japanese patent application Kokai No. Hei 6-300973, Japanese patent application Kokai No. Hei 5-88089, Japanese patent application Kokai No. Hei 3-282527, PCT/EP95/01719, and U.S. Pat. No. 5,241,423. 
     The projection lenses disclosed in the above-identified patent applications have optical axes associated with the lens elements before to (i.e., upstream of) the beam splitter element and after (i.e., downstream of) the beam splitter element that are not parallel. However, the recent demands for higher NA, as well as larger field size, require that the size of the refractive elements and the reflective element be increased. Unfortunately, increasing the size of these lens elements is problematic because of the deformation effects of gravity when the projection lens is mounted in the projection optical system. When the optical axes of the refractive elements upstream and downstream of the beam splitter are not parallel, asymmetric deformations arise in the lens elements due to gravity. These deformations induce aberrations which unacceptably reduce the resolution of the projection lens. Unfortunately, these aberrations are such that they cannot be readily corrected during manufacturing. 
     SUMMARY OF THE INVENTION 
     The present invention relates to projection lenses, and more particularly to high-resolution, ultraviolet projection lenses for projection optical systems. 
     One aspect of the present invention is a catadioptric projection lens having an object plane and an image plane. The lens comprises, in order from the object plane to image plane, a first lens group having one or more refractive lens elements disposed along a first optical axis. Next is a first mirror, which creates a second optical axis that is not parallel to the first optical axis. Next to the first mirror, along the second optical axis, is a beam splitter, which creates a third optical axis that is parallel to the first optical axis. Next, disposed along the third axis, on the side of the beam splitter opposite the image plane, is a second lens group having one or more refractive lens elements, and which includes a concave mirror. Next, immediately adjacent the image plane and disposed along the third optical axis, is a third lens group having one or more refractive lens elements. 
     In another aspect of the invention, an aperture stop is included between the beam splitter and the third lens group. 
     One of the main benefits of this configuration is that when the first and third optical axes are made parallel to the direction of gravity, aberrations due to the deformation of the lens elements due to gravity are alleviated. This allows for a high NA (e.g., 0.6 and above) and short-wavelength (193 nm) design for achieving high resolution (e.g., 0.25 um or less). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic optical diagram of Working Example 1 of the present invention; 
     FIG. 2 a - 2   c  are plots of the transverse aberrations for five wavelengths and three image heights for Working Example 1; 
     FIG. 3 is a schematic optical diagram of Working Example 2 of the present invention; and 
     FIG. 4 a - 4   c  are plots of the transverse aberrations for five wavelengths and three image heights for Working Example 2. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to projection lenses, and more particularly to high-resolution, ultraviolet projection lenses for projection optical systems. The projection lenses of the present invention are designed to alleviate the deformation effects of the lens elements due of gravity, which induce aberrations that are not readily correctable during manufacturing. To this end, the projection lens of the present invention includes a plurality of refractive lens elements arranged in three lens groups, one of which includes at least one concave mirror. In addition, the optical axes passing through the refractive lens elements are parallel. This configuration allows for the direction of the force of gravity to be in the same direction for each lens group. A preferred embodiment of the present invention is a narrow-band projection lens using quartz and/or calcium fluoride refractive lens elements and having high-resolution, such as sub-quarter micron, and a high NA, such as 0.6 or greater. 
     In a preferred embodiment, a reticle is disposed in a reticle holder capable of holding the reticle at or near the object plane of the projection lens of the present invention. In addition, a workpiece (e.g., a silicon wafer coated with photoresist) is disposed in a workpiece holder capable of holding the workpiece at or near the image plane of the projection lens. In the present invention, the object planes and image planes are substantially parallel to one another. The reticle and wafer are then scanned in parallel planes (typically, the object and image planes, respectively), which are at right angles to the optical axes of the lens groups upstream and downstream from the beam splitter. 
     With reference to FIGS. 1 and 3, which show representative catadioptric projection lenses  10  and  20 , the catadioptric projection lens of the present invention includes, in order from object plane  12  to image plane  14  along optical axes  16   a - 16   c , a first lens group G 1 , a plane mirror  18 , a beam splitter  20 , a second lens group G 2 , which includes a concave mirror L 22  disposed in the optical path R the reflection optical path of light ray bundles  24   a  and  24   b  reflected by beam splitter  20  in the direction opposite image plane  14 , and a third lens group disposed in the optical path T the transmission optical path, immediately adjacent beam splitter  20  on the image-plane side. 
     Lens group G 1 , disposed along optical axis  16   a , comprises a negative meniscus lens element L 11  having an objectwise concave surface, a biconvex lens element L 12 , a biconvex lens element L 13 , a biconcave lens element L 14 , a biconvex lens element L 15 , a negative meniscus lens element  116  having an objectwise concave surface, a positive meniscus lens element L 17  having an objectwise convex surface, a biconvex lens element L 18 , a positive meniscus lens clement L 19  having an objectwise convex surface, a negative meniscus lens clement L 110  having an objectwise convex surface, and a positive meniscus lens element L 111  having an objectwise convex surface. 
     Lens group G 2 , disposed along optical axis  16   c , comprises a negative meniscus lens element L 21  having an objectwise concave surface, and a concave mirror L 22 . 
     Lens group G 3 , disposed along optical axis  16   c , comprises a positive meniscus lens element L 31  having an objectwise concave surface, a biconcave lens element L 32 , a biconvex lens element L 33 , a biconvex lens element L 34 , a positive meniscus lens element L 35  having a objectwise convex surface, a biconcave lens element L 36 , a biconvex lens element L 37 , and a positive meniscus lens element L 38  having an objectwise convex surface. An aperture stop AS is disposed between lens groups G 2  and G 3 , or within lens group G 3 . 
     In a preferred embodiment of the present invention, beam splitter  20  is formed at the interface of a pair of right triangular prisms. This arrangement avoids the introduction of coma and astigmatism that accompanies the use of a plate-type beam splitter. Beam splitter  20  may be plate-type, prism-type, polarizing plate-type or polarizing prism-type. 
     Also, in preferred embodiment, lens group G 1  satisfies the design condition 
     
       
         −1&lt;1/β 1 &lt;1  (1) 
       
     
     wherein β 1  is the magnification of lens group G 1 . If the expression 1/β 1  exceeds the upper limit of condition ( 1 ), it becomes difficult accommodate mirror  18  for folding the optical path, and beam splitter  20 . If the expression 1/β 1  falls below the lower limit of condition ( 1 ), the size of projection lens  10  increases, and the correction of off-axis aberrations becomes difficult. In a preferred embodiment, the lower limit of condition ( 1 ) is set at 0 or even 0.4, and the upper limit at 0.7 
     In a preferred embodiment of the present invention, aperture stop AS is variable in size, thereby allowing for adjustment of resolution and depth of focus. Also, by making the projection lens image-side telecentric, distortion (i.e., image-placement error) as a function of defocus can be made negligible. Furthermore, to prevent undue light absorption and scattering, in a preferred embodiment, a polarizing beam splitter is used in combination with a ¼-wave plate disposed between beam splitter  20  (which, in the instant case would be a polarizing beam splitter) and concave mirror L 22 . 
     WORKING EXAMPLES 
     Working Examples 1 and 2, corresponding to projection lenses  10  and  20 , respectively, are set forth in Tables 1 and 2, below, and in FIGS. 1 and 3, respectively, along with their corresponding aberration plots (FIGS. 2 a-c  and  4   a-c , respectively). In the aberration plots, the various line types correspond to five different wavelengths ranging from 192.295 nm to 193.305 nm, as indicated. The Working Examples considered herein are reduction projection lenses used for printing by scanning a reticle (not shown) in object plane  12 , while simultaneously scanning a wafer (not shown) in image plane  14 , as discussed above (see FIG. 1 or FIG.  3 ). The scanning speed of the reticle and the wafer are synchronized based on the reduction magnification. In Tables 1 and 2, the “exposure region” is the field size at the reticle plane. The exposure region in Working Examples 1 and 2 is a rectangular slit having a long side of dimension “a” in the direction orthogonal to the scanning direction, and a short side “b” along the scanning direction. The exposure region is centered on optical axis  16   a . Note that mirror  18  and beam splitter  20  bend the optical path by 90 degrees so that optical axis  16   a  and optical axis  16   c  are parallel. Also, aperture stop AS is disposed between beam splitter  20  and lens group G 3 . 
     In Tables 1 and 2, “S” is the surface number, “r”=is the radius of curvature, which is positive when the lens surface precedes the center of curvature in relation to the incident light (however, the sign of the curvature reverses at each reflection), “d” is the spacing between adjacent surfaces (which sign reverses at each reflection), “Glass Type” is the type of glass of the particular lens element, and “Lens Group” identifies the lens group to which the particular lens element belongs. Also, Working Examples 1 and 2 have a plurality of lenses configured as described above, and have a plate-type beam splitter. Note, however, that a prism-type beam splitter may also be used. 
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Exposure wavelength = 193.3 nm ± 5 pm 
               
               
                 Quartz (synthetic quartz): refractive index = 1.56033 
               
               
                 CaF 2  (fluorite): refractive index = 1.50146 
               
               
                 Image-side NA = 0.650 
               
               
                 Exposure region: a = 25 mm b = 6 mm 
               
               
                 1/β 1  = 0.57 
               
             
          
           
               
                 S 
                 r 
                 d 
                 Glass Type 
                 Group 
               
               
                   
               
               
                  0 
                 (reticle) 
                 50.000 
                   
                   
               
               
                  1 
                 −188.0568 
                 20.000 
                 Quartz 
                 G1 
               
               
                  2 
                 −281.5034 
                 0.500 
               
               
                  3 
                 431.1247 
                 21.392 
                 Quartz 
                 G1 
               
               
                  4 
                 −288.5924 
                 6.668 
               
               
                  5 
                 236.2574 
                 20.000 
                 Quartz 
                 G1 
               
               
                  6 
                 1882.3987 
                 36.559 
               
               
                  7 
                 −357.7902 
                 20.000 
                 Quartz 
                 G1 
               
               
                  8 
                 167.6051 
                 23.376 
               
               
                  9 
                 −175.6436 
                 20.000 
                 CaF 2   
                 G1 
               
               
                 10 
                 −1470.5670 
                 12.623 
               
               
                 11 
                 −186.9077 
                 20.000 
                 Quartz 
                 G1 
               
               
                 12 
                 −1274.9545 
                 23.974 
               
               
                 13 
                 −286.1706 
                 20.000 
                 Quartz 
                 G1 
               
               
                 14 
                 −211.3700 
                 0.500 
               
               
                 15 
                 701.0264 
                 38.078 
                 Quartz 
                 G1 
               
               
                 16 
                 −261.5668 
                 0.500 
               
               
                 17 
                 −598.9669 
                 20.647 
                 Quartz 
                 G1 
               
               
                 18 
                 −271.9048 
                 5.943 
               
               
                 19 
                 −230.3536 
                 20.000 
                 Quartz 
                 G1 
               
               
                 20 
                 −404.2724 
                 0.500 
               
               
                 21 
                 −893.4980 
                 20.000 
                 Quartz 
                 G1 
               
               
                 22 
                 −524.1623 
                 100.000 
               
               
                 23 
                 ∞ 
                 −220.000 
                 Plane Mirror 
                 M 
               
               
                 24 
                 ∞ 
                 −115.000 
                 Quartz 
               
               
                 25 
                 ∞ 
                 115.000 
                 Quartz 
                 BS 
               
               
                   
                   
                   
                   
                 (reflective) 
               
               
                 26 
                 ∞ 
                 42.590 
               
               
                 27 
                 −205.0171 
                 20.000 
                 Quartz 
                 G2 
               
               
                 28 
                 −362.2973 
                 0.500 
               
               
                 29 
                 −584.5154 
                 −0.500 
                 Concave Mirror 
                 G2 
               
               
                 30 
                 −362.2973 
                 −20.000 
                 Quartz 
                 G2 
               
               
                 31 
                 −205.0171 
                 −42.590 
               
               
                 32 
                 ∞ 
                 −115.000 
                 Quartz 
               
               
                 33 
                 ∞ 
                 −115.000 
                 Quartz 
                 BS 
               
               
                   
                   
                   
                   
                 (transmissive) 
               
               
                 34 
                 ∞ 
                 −10.000 
               
               
                 35 
                 (aperture stop) 
                 −5.000 
               
               
                 36 
                 −214.1175 
                 −45.000 
                 Quartz 
                 G3 
               
               
                 37 
                 −1561.0010 
                 −14.542 
               
               
                 38 
                 707.0832 
                 −35.000 
                 Quartz 
                 G3 
               
               
                 39 
                 −243.0109 
                 −7.154 
               
               
                 40 
                 −344.7902 
                 −35.000 
                 CaF 2   
                 G3 
               
               
                 41 
                 472.9348 
                 −0.500 
               
               
                 42 
                 −246.5665 
                 −35.000 
                 CaF 2   
                 G3 
               
               
                 43 
                 16237.2470 
                 −21.872 
               
               
                 44 
                 −115.5125 
                 −29.384 
                 CaF 2   
                 G3 
               
               
                 45 
                 −1618.6280 
                 −5.704 
               
               
                 46 
                 703.4519 
                 −20.000 
                 Quartz 
                 G3 
               
               
                 47 
                 −672.9060 
                 −0.500 
               
               
                 48 
                 −368.2245 
                 −40.350 
                 Quartz 
                 G3 
               
               
                 49 
                 1073.8114 
                 −0.500 
               
               
                 50 
                 −572.4918 
                 −20.000 
                 Quartz 
                 G3 
               
               
                 51 
                 −3000.0000 
                 −17.000 
               
               
                 52 
                 (wafer) 
               
               
                   
               
             
          
         
       
     
     
       
         
               
             
               
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 Exposure wavelength = 193.3 nm ± 5 pm 
               
               
                 Quartz (synthetic quartz): refractive index = 1.56033 
               
               
                 CaF 2  (fluorite): refractive index = 1.50146 
               
               
                 Image-side NA = 0.60 
               
               
                 Exposure region: a = 25 mm b = 6 mm 
               
               
                 1/β 1  = 0.51 
               
             
          
           
               
                 S 
                 r 
                 d 
                 Glass Type 
                 Group 
               
               
                   
               
               
                  0 
                 (reticle) 
                 50.000 
                   
                   
               
               
                  1 
                 −179.5776 
                 20.000 
                 Quartz 
                 G1 
               
               
                  2 
                 −282.5988 
                 0.500 
               
               
                  3 
                 454.1650 
                 22.901 
                 Quartz 
                 G1 
               
               
                  4 
                 −252.3208 
                 5.836 
               
               
                  5 
                 233.6302 
                 20.000 
                 Quartz 
                 G1 
               
               
                  6 
                 1110.1931 
                 33.789 
               
               
                  7 
                 −303.6817 
                 20.000 
                 Quartz 
                 G1 
               
               
                  8 
                 181.8725 
                 19.204 
               
               
                  9 
                 −270.4288 
                 20.000 
                 CaF 2   
                 G1 
               
               
                 10 
                 21100.7147 
                 17.698 
               
               
                 11 
                 −148.2217 
                 20.000 
                 Quartz 
                 G1 
               
               
                 12 
                 −849.3763 
                 24.229 
               
               
                 13 
                 −286.5078 
                 20.000 
                 Quartz 
                 G1 
               
               
                 14 
                 −206.8107 
                 0.500 
               
               
                 15 
                 772.3575 
                 36.184 
                 Quartz 
                 G1 
               
               
                 16 
                 −275.2952 
                 0.500 
               
               
                 17 
                 −588.4319 
                 20.000 
                 Quartz 
                 G1 
               
               
                 18 
                 −280.6383 
                 5.486 
               
               
                 19 
                 −240.2882 
                 20.000 
                 Quartz 
                 G1 
               
               
                 20 
                 −450.1379 
                 0.500 
               
               
                 21 
                 −6350.6655 
                 20.000 
                 Quartz 
                 G1 
               
               
                 22 
                 −631.0481 
                 100.000 
               
               
                 23 
                 ∞ 
                 −270.000 
                 Plane Mirror 
                 M 
               
               
                 24 
                 ∞ 
                 −110.000 
                 Quartz 
               
               
                 25 
                 ∞ 
                 110.000 
                 Quartz 
                 BS 
               
               
                   
                   
                   
                   
                 (reflective) 
               
               
                 26 
                 ∞ 
                 37.334 
               
               
                 27 
                 −202.0637 
                 20.000 
                 Quartz 
                 G2 
               
               
                 28 
                 −353.9158 
                 0.500 
               
               
                 29 
                 −571.0735 
                 −0.500 
                 Concave Mirror 
                 G2 
               
               
                 30 
                 −353.9158 
                 −20.000 
                 Quartz 
                 G2 
               
               
                 31 
                 −202.0637 
                 −37.334 
               
               
                 32 
                 ∞ 
                 −110.000 
                 Quartz 
               
               
                 33 
                 ∞ 
                 −110.000 
                 Quartz 
                 BS 
               
               
                   
                   
                   
                   
                 (transmissive) 
               
               
                 34 
                 ∞ 
                 −10.000 
               
               
                 35 
                 (aperture stop) 
                 −5.000 
               
               
                 36 
                 −192.8373 
                 −31.424 
                 Quartz 
                 G3 
               
               
                 37 
                 −665.8447 
                 −12.739 
               
               
                 38 
                 1564.0875 
                 −35.000 
                 Quartz 
                 G3 
               
               
                 39 
                 −250.0795 
                 −19.121 
               
               
                 40 
                 −407.4761 
                 −35.000 
                 CaF 2   
                 G3 
               
               
                 41 
                 617.7314 
                 −0.500 
               
               
                 42 
                 −231.8625 
                 −35.000 
                 CaF 2   
                 G3 
               
               
                 43 
                 13701.5774 
                 −5.993 
               
               
                 44 
                 −127.4513 
                 −26.172 
                 CaF 2   
                 G3 
               
               
                 45 
                 −2284.2542 
                 −5.838 
               
               
                 46 
                 571.7610 
                 −23.983 
                 Quartz 
                 G3 
               
               
                 47 
                 −342.8918 
                 −0.500 
               
               
                 48 
                 −213.0073 
                 −45.000 
                 Quartz 
                 G3 
               
               
                 49 
                 1139.9867 
                 −3.123 
               
               
                 50 
                 −429.2310 
                 −20.000 
                 Quartz 
                 G3 
               
               
                 51 
                 −3000.0000 
                 −17.000 
               
               
                 52 
                 (wafer) 
               
               
                   
               
             
          
         
       
     
     As can be seen from the aberration plots in FIGS. 2 a-c  and  4   a-c , Working Examples 1 and 2 have excellent imaging performance. 
     While the present invention has been described in connection with preferred embodiments and Working Examples, it will be understood that it is not so limited. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined in the appended claims.