Catadioptric optical reduction system with high numerical aperture

Abstract: A catadioptric optical reduction system for use in the photolithographic manufacture of semiconductors having a concave mirror operating near unit magnification, or close to a concentric condition. A lens group before the mirror provides only enough power to image the entrance pupil at infinity to the aperture stop at or near the concave mirror. A lens group after the mirror provides a larger proportion of reduction from object to image size, as well as projecting the aperture stop to an infinite exit pupil. An aspheric concave mirror is used to further reduce high order aberrations. The catadioptric optical reduction system provides a relatively high numerical aperture of 0.7 capable of patterning features smaller than 0.35 microns over a 26 x 5 millimeter field. The optical reduction system is thereby well adapted to a step and scan microlithographic exposure tool as used in semiconductor manufacturing. Several other embodiments combine glasses of different refracting power to widen the spectral bandwidth which can be achieved.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates generally to optical systems used in semiconductor manufacturing, and more particularly to an unobscured catadioptric optical reduction system having a relatively high numerical aperture. In particular, the invention relates to a catadioptric optical reduction system having a relatively high numerical aperture, from the long conjugate end to the short conjugate end, comprising: first lens means; a beamsplitter; a concave mirror; and second lens means for providing a positive power, wherein the first lens means is for providing only enough power to image an entrance pupil at infinity to an aperture stop at or near said mirror; and wherein the second lens means images the exit pupil to infinity. Such a system is known from DE-A-4 203 464. Description of Related Art Semiconductors are usually manufactured using various photolithographic techniques. The circuitry used in a semiconductor is reproduced from a reticle onto a semiconductor chip. This reproduction is often accomplished with the use of optical systems. The design of these optical systems is often complex, and it is difficult to obtain the desired resolution necessary for reproducing the ever-decreasing size of components being placed on a semiconductor chip. Therefore, there has been much effort expended to develop an optical reduction system capable of reproducing very fine component features, less than 0.35 microns. In addition to the need to develop an optical system capable of reproducing very fine component features, there is also the need to augment system performance by increasing numerical aperture. Increasing numerical aperture provides for a higher resolution optical system. An optical system similar to that of the present invention is disclosed in U.S. Patent 4,953,960 entitled "Optical Reduction System" issuing September 4, 1990 to Williamson. Therein disclosed is an optical reduction system operating in the range of 248 nanometers and having a numerical aperture of 0.45. Another similar optical system is disclosed in U.S. Patent 5,089,913 entitled "High Resolution Reduction Catadioptric Relay Lens" issuing February 18, 1992 to Singh et al. Therein disclosed is an optical system having a restricted spectral waveband at 248 nanometers, and having a numerical aperture of 0.6. While these prior optical systems perform adequately for their intended purpose, there is an ever increasing need to improve system performance by increasing numerical aperture. Therefore, there is a need for an optical system having a relatively high numerical aperture capable of acceptable system performance over a relatively large spectral waveband. SUMMARY OF THE INVENTION The present invention comprises a catadioptric optical reduction system of the above mentioned type which is characterised in that the positive power of the second lens means provides most of the power of the system. In particular, the present invention comprises a catadioptric reduction system having, from the object or long conjugate end to the reduced image or short conjugate end, a first lens group, a second lens group, a beamsplitter cube, a substantially or nearly concentric concave mirror, and a third lens group. The concave mirror operates substantially near unit magnification. This reduces the aberrations introduced by the mirror and the diameter of radiation entering the beamsplitter cube. The first and second lens groups before the concave mirror provide only enough power to image the entrance pupil at infinity at the aperture stop at or near the concave mirror. The third lens group after the concave mirror provides a substantial portion of the reduction from object to image of the optical system, as well as projecting the aperture stop to an infinite exit pupil. High-order aberrations are reduced by using an aspheric concave mirror. Accordingly, it is an object of the present invention to provide an optical system having a relatively high numerical aperture. It is another object of the present invention to provide an optical system having a spectral bandwidth substantially wider than could have previously been obtained at such a high numerical aperture. It is an advantage of the present invention that the concave mirror operates substantially closer to unit magnification. It is a feature of the present invention that the lens groups before the concave mirror provide only enough power to image the entrance pupil at infinity at the aperture stop at or near the concave mirror. It is yet another feature of the present invention that the lens group after the concave mirror provides most of the reduction from object to image of the system. These and other objects, advantages, and features will become readily apparent in view of the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic illustration of one embodiment of the present invention using a single refracting material. Fig. 2 is another embodiment of the present invention using two different refracting materials. Fig. 3 is yet another embodiment of the present invention using more than two different refracting materials. Fig. 4 illustrates a first example which is useful for understanding the optical reduction system of the present invention. Fig. 5 illustrates a second example which is useful for understanding the optical reduction system of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 illustrates one embodiment of the optical reduction system of the present invention. From its long conjugant end, it comprises an object or reticle plane 10, a first lens group LG1, a folding mirror 20, a second lens group LG2, a beamsplitter cube 30, a first quarter-waveplate 32, a concave mirror 34, a second quarter-waveplate 38, and a third lens group LG3. The image is formed at image or wafer plane 50. The first lens group LG1 comprises a shell 12, a spaced doublet including positive lens 14 and negative lens 16, and positive lens 18. The shell 12 is an almost zero power lens. The second lens group LG2 comprises a positive lens 22, a spaced doublet including a negative lens 24 and a positive lens 26, and negative lens 28. The third lens group LG3 comprises two positive lenses 40 and 42, which are strongly positive, shell 44, and two positive lenses 46 and 48, which are weakly positive. The folding mirror 20 is not essential to the operation of the present invention. However, the folding mirror permits the object and image planes to be parallel which is convenient for the intended application of the optical system of the present invention, which is the manufacture of semiconductor devices using photolithography with a step and scan system. Radiation enters the system at the reticle or long conjugate end and passes through the first lens group LG1, is reflected by the folding mirror 20, and passes through the second lens group LG2. The radiation enters the beamsplitter cube 30 and is reflected from surface 36 passing through quarter-waveplate 32 and reflected by concave mirror 34. The radiation then passes back through the quarter-waveplate 32, the beamsplitter cube 30, the quarter-waveplate 38, lens group LG3, and is focused at the image or wafer plane 50. The lens groups before the mirror, LG1 and LG2, provide only enough power to image the entrance pupil at infinity to the aperture stop at or near the concave mirror 34. The combined power of lens groups LG1 and LG2 is slightly negative. The shell 12 and air spaced doublet 14 and 16 assist in aberration corrections including astigmatism, field curvature, and distortion. The lens group LG3, after the concave mirror 34, provides most of the reduction from object to image size, as well as projecting the aperture stop to an infinite exit pupil. The two strongly positive lenses 40 and 42 provide a high numerical aperture at the image and exit pupils and infinity. The shell 44 has almost no power. The two weakly positive lenses 46 and 48 help correct high order aberrations. The negative lens 24 in the second lens group LG2 provides a strongly diverging beam directed at the beamsplitter cube 30 and concave mirror 34. The strongly positive lens 22 provides lateral color correction. The air space doublet comprising lenses 24 and 26 helps to correct spherical aberrations and coma. Concave mirror 34 is preferably aspheric, therefore helping further reduce high order aberrations. The transmission losses introduced by the beamsplitter cube 30 are minimized by illuminating the object or reticle with plane polarized light and including a true quarter-waveplate 32 between the beamsplitter cube 30 and the concave mirror 34. By true quarter-waveplate is meant a thickness of birefringent material which introduces a quarter of a wave phase retardance between the S and P polarization states. This is in contrast to an integral number of half plus quarter waves or two thicknesses of material whose phase retardance differs by a quarter wave. The deleterious effects of large angle of incidence variations are thereby minimized at the high numerical aperture by the use of such true zero order waveplates, and by restricting the field size in the plane of incidence. Additionally, by increasing the numerical aperture in lens group LG3, after the concave mirror 34 and beamsplitter cube 30, the greatest angular range is not seen in these elements. However, the use of plane polarized light at numerical apertures greater than about 0.5 introduces small but noticeable asymmetries in the imaging. In the present invention, this can effectively be removed by introducing a second quarter-waveplate 38 after the final passage through the beamsplitter cube 30, thereby converting the plane polarized light into circularly polarized light. This circularly polarized light is indistinguishable from unpolarized light in its imaging behavior. The optical system illustrated in Fig. 1 is designed to operate at a reduction ratio of 4 to 1. Therefore, the numerical aperture in the image space is reduced from 0.7 by a factor of 4 to 0.175 at the object or reticle. Upon leaving the first lens group LG1 the numerical aperture is reduced to 0.12, a consequence of the positive power needed in lens group LG1 to image the entrance pupil at infinity to the aperture stop of the system close to the concave mirror 34. The numerical aperture after leaving the second lens group LG2 and entering the beamsplitter is 0.19. This is very similar to the object space numerical aperture, which is 0.175, due to the overall negative power of the second lens group LG2. This is contrary to prior art systems where the numerical aperture entering a beamsplitter cube is typically close to zero or almost collimated. The concave mirror 34 being almost concentric, the numerical aperture of the radiation reflected from it is increased only slightly from 0.19 to 0.35. The third lens group LG3 effectively doubles the numerical aperture to its final value of 0.7 at the wafer or image plane 50. The present invention achieves its relatively high numerical aperture without obstruction by the edges of the beamsplitter cube by means of the negative second group LG2 and the strongly positive third lens group LG3. The use of the beamsplitter cube 30 rather than a plate beamsplitter is important in the present invention because at numerical apertures greater than about 0.45 a beamsplitter cube will provide better performance. There is a reduction of the numerical aperture within the cube by the refractive index of the glass, as well as the absence of aberrations that would be introduced by a tilted plate beamsplitter in the non-collimated beam entering the beamsplitter. The construction data for the lens system illustrated in Fig. 1 according to the present invention is given in Table 1 below. Element Number Radius of Curvature (mm) Thickness (mm) Aperture Diameter (mm) Glass Front Back Front Back 10 Infinite 63.3853 12 -158.7745 -177.8880 15.0000 124.0478 131.7725 Silica Space 36.1130 14 -556.6911 -202.0072 22.2126 148.3881 152.5669 Silica Space 38.7188 16 -183.7199 -558.8803 15.0000 156.5546 166.5750 Silica Space 10.0674 18 427.2527 -612.2450 28.8010 177.4010 179.0292 Silica Space 132.3320 20 Infinite -74.0000 184.6402 Reflection 22 -240.4810 2050.9592 -33.3135 188.4055 185.3395 Silica Space -29.3434 24 421.7829 -145.6176 -12.0000 175.5823 169.0234 Silica Space -4.2326 26 -150.4759 472.0653 -46.5091 171.4244 169.9587 Silica Space -2.0000 28 -1472.2790 -138.2223 -15.0000 165.3586 154.8084 Silica Space -27.2060 30 Infinite Infinite -91.8186 155.6662 253.0917 Silica 36 Infinite 253.0917 Reflection 30 Infinite Infinite 91.8186 253.0917 253.0917 Silica Space 2.0000 32 Infinite Infinite 6.0000 185.8693 186.8401 Silica Space 17.9918 Stop 188.0655 34 Aspheric -17.9918 188.0655 Reflection 32 Infinite Infinite -6.0000 183.5471 180.1419 Silica Space -2.0000 30 Infinite Infinite -91.8186 178.3346 149.2832 Silica 30 Infinite Infinite -70.000 149.2832 128.8604 Silica Space -2.0000 38 Infinite Infinite -4.5000 127.9681 126.6552 Silica Space -0.7500 40 -175.1330 1737.4442 -17.7754 121.4715 118.2689 Silica Space -0.7500 42 -108.8178 -580.1370 -18.2407 104.5228 97.7967 Silica Space -0.7500 44 -202.2637 -86.6025 -31.1216 91.7061 57.4968 Silica Space -2.3507 46 -122.1235 -488.7122 -17.9476 56.4818 41.1675 Silica Space -0.2000 48 -160.8506 -360.1907 -6.1500 39.4528 33.5764 Silica Space -4.000 50 Infinite 26.5019 Concave mirror 34 has an aspheric reflective surface according to the following equation. Z = ( CURV ) Y 2 1+(1-(1+ k ) ( CURV ) 2 Y 2 ) 1/2 +( A ) Y 4 +( B ) Y 6 + ( D ) Y 10 + ( E ) Y 12 + ( F ) Y 14 wherein the constants are as follows, CURV = -0.00289051 K = 0.000000 A = 6.08975 X 10 -11 B = 2.64378 X 10 -14 C = 9.82237 X 10 -19 D = 7.98056 X 10 -23 E = -5.96805 X 10 -27 F = 4.85179 X 10 -31 The lens according to the construction in Table 1 is optimized for radiation centered on 248.4 nanometers. The single refracting material of fused silica and the large portion of refracting power restricts the spectral bandwidth of the embodiment illustrated in Fig. 1 to about 10 picometers or 0.01 nanometers. This spectral bandwidth is more than adequate for a line narrowed krypton fluoride excimer laser light source. The embodiment illustrated in Fig. 1 can be optimized for any wavelength for which fused silica transmits adequately. A wider spectral bandwidth can be achieved by the use of two optical materials with different dispersions. A second embodiment of the present invention is illustrated in Fig. 2. From its long conjugant end, it comprises an object or reticle plane 10, a lens group LG4, a folding mirror 122, a lens group LG5, a beamsplitter cube 132 having surface 138, a first quarter-waveplate 134, a concave mirror 136, a second quarter-waveplate 140, and lens group LG6. The image is formed at image or wafer plane 50. The lens group LG4 comprises a spaced doublet including negative lens 112 and positive lens 114, a weak positive lens 116, positive lens 118, and shell 120. The lens group LG5 comprises a positive lens 124, a negative lens 126, a positive lens 128, and a negative lens 130. The lens group LG6 comprises two positive lenses 142, 'cemented' doublet including positive lens 144 and negative lens 146, positive lens 148, and 'cemented' doublet including shell 150 and positive lens 152. This second embodiment uses calcium fluoride in one of the individual positive lenses of the lens group LG4, negative lenses of the lens group LG5, and two of the positive lenses of the lens group LG6. The construction data of the second embodiment illustrated in Fig. 2 of the present invention is given in Table 2 below. Element Number Radius of Curvature (mm) Thickness (mm) Aperture Diameter (mm) Glass Front Back Front Back 10 Infinite 60.4852 112 -205.5158 -539.1791 15.2158 124.0926 137.3346 Silica Space 8.8054 114 -2080.9700 -210.6539 32.4984 142.6149 151.7878 Silica Space 1.2676 116 -310.4463 -700.3748 40.7304 162.4908 165.2126 CaFl Space 0.5000 118 634.1820 -798.8523 27.5892 165.4595 166.4747 Silica Space 0.5000 120 1480.0597 1312.1247 25.4322 168.7516 164.7651 Silica 122 Infinite -74.0000 161.9590 Reflection 124 -761.9176 1088.9351 -19.2150 160.3165 159.2384 Silica Space -19.9465 126 648.8361 -202.5872 -12.0000 155.1711 153.0635 CaFl Space -7.6304 128 -400.4276 458.5060 -25.8769 153.0635 153.8055 Silica Space -2.0000 130 -818.0922 -168.5034 -27.5927 152.6663 147.5200 CaFl Space -20.5014 132 Infinite Infinite -91.7553 148.6158 252.7349 Silica 138 Infinite 252.7349 Reflection 132 Infinite Infinite 91.7553 252.7349 252.7349 Silica Space 2.0000 134 Infinite Infinite 6.0000 185.8070 187.0026 Silica Space 18.1636 Stop 188.5681 136 Aspheric -18.1636 188.5681 Reflection 134 Infinite Infinite -6.0000 184.2566 181.1084 Silica Space -2.0000 132 Infinite Infinite -91.7553 179.3838 151.7747 Silica 132 Infinite Infinite -70.0000 151.7747 133.3985 Silica Space -2.0000 140 Infinite Infinite -4.5000 132.5690 131.3876 Silica Space -0.5000 142 -112.0665 -597.6805 -21.4866 123.4895 119.2442 Silica Space -0.5000 144 -116.3137 282.3140 -24.0940 107.8451 101.2412 CaFI 146 282.3140 -66.5293 -13.7306 101.2412 72.6862 Silica Space -2.6346 148 -77.2627 -374.4800 -17.9594 72.0749 62.7659 Silica Space -0.5452 150 -130.1381 -57.1295 -20.8147 58.9696 37.4889 Silica 152 -57.1295 -7305.8777 -6.1425 37.4889 34.3156 CaFI Space -4.0000 Image Infinite 26.4992 wherein the constants for the aspheric mirror 134 used in the equation after Table 1 are as follows, CURV = -0.00286744 K = 0.000000 A = -1.92013 X 10 -09 B = -3.50840 X 10 -14 C = 2.95934 X 10 -19 D = -1.10495 X 10 -22 E = 9.03439 X 10 -27 F = -1.39494 X 10 -31 This second embodiment is optimized for radiation centered on 193.3 nanometers and has a spectral bandwidth of about 200 picometers or 0.2 nanometers. A slightly line narrowed argon fluoride excimer laser is an adequate light source. Additionally, the design can be optimized for any wavelength for which both refractive materials transmit adequately. The bandwidth will generally increase for longer wavelengths, as the material dispersions decrease. For example, around 248.4 nanometers such a two-material design will operate over at least a 400 picometers, 0.4 nanometers bandwidth. At wavelengths longer than 360 nanometers, a wider range of optical glasses begin to have adequate transmission. A third embodiment illustrated in Fig. 3 takes advantage of this wider selection of glasses and further reduced dispersion. From its long conjugant end, it comprises an object or reticle plane 10, a lens group LG7, a folding mirror 222, a lens group LG8, a beamsplitter cube 232 having a surface 238, a first quarter-waveplate 234, a concave mirror 236, a second quarter-waveplate 240, and lens group LG9. The image is formed at image or wafer plane 50. The lens group LG7 comprises a spaced doublet comprising negative lens 212 and positive lens 214, spaced doublet including positive lens 216 and negative lens 218, and positive lens 220. The lens group LG8 comprises a positive lens 224, a negative lens 226, a positive lens 228, and a negative lens 230. The lens group LG9 comprises a positive lenses 242, cemented doublet including positive lens 244 and negative lens 246, positive lens 248, and cemented doublet including shell 250 and positive lens 252. The construction data of the third embodiment illustrated in Fig. 3 is given in Table 3 below. Element Number Radius of Curvature (mm) Thickness (mm) Aperture Diameter (mm) Glass Front Back Front Back 10 Infinite 59.2960 212 -620.7809 361.8305 20.2974 125.9406 134.7227 PBM2Y Space 2.6174 214 515.7935 -455.1015 39.8858 135.3384 145.6015 PBM2Y Space 14.7197 216 431.3189 -239.4002 36.9329 155.6269 157.3014 BSL7Y Space 0.5000 218 -259.6013 685.3286 26.3534 156.9363 162.2451 PBM2Y Space 1.4303 220 361.5709 -1853.2955 23.3934 168.7516 165.1801 BAL15Y Space 131.8538 222 Infinite -77.8469 169.9390 Reflection 224 -429.2950 455.4247 -32.3086 173.0235 171.1102 PBL6Y Space -27.6206 226 401.0363 -180.0031 -12.0000 159.3555 154.7155 BSL7Y Space -5.6227 228 -258.4722 1301.3764 -26.1321 154.7155 154.1517 PBM8Y Space -2.0000 230 -1282.8931 -180.2226 -12.0000 153.1461 149.4794 BSL7Y Space -19.7282 232 Infinite Infinite -91.7349 150.4585 252.6772 Silica 238 Infinite 252.6772 Reflection 232 Infinite Infinite 91.7349 252.6772 252.6772 Silica Space 2.0000 234 Infinite Infinite 6.0000 185.6435 186.7758 Silica Space 18.2715 Stop 188.1745 236 Aspheric -18.2715 188.1745 Reflection 234 Infinite Infinite -6.0000 183.6393 180.1377 Silica Space -2.0000 232 Infinite Infinite -91.7349 178.3236 147.9888 Silica 232 Infinite Infinite -70.0000 147.9888 126.9282 Silica Space -2.0000 240 Infinite Infinite -4.5000 126.0289 124.6750 Silica Space -0.5000 242 -119,8912 -610.6840 -18.6508 117.5305 113.4233 BSM51Y Space -0.5000 244 -114.1327 384.9135 -21.1139 102.6172 96.4137 BSL7Y 246 384.9135 -70.2077 -13.0576 96.4137 71.1691 PBL26Y Space -2.8552 248 -85.7858 -400.3240 -16.9147 70.5182 61.2633 BSM51Y Space -0.8180 250 -151.5235 -54.0114 -19.5810 57.6234 37.3909 BSM51Y 252 -54.0114 -2011.1057 -6.3947 37.3909 34.2119 PBL6Y Space -4.0000 Image Infinite 26.5002 wherein the constants for the aspheric mirror 234 used in the equation after Table 1 are as follows, CURV = -0.00291648 K = 0.000000 A = -1.27285 X 10 -9 B = -1.92865 X 10 -14 C = 6.21813 X 10 -19 D -6.80975 X 10 -23 E = 6.04233 X 10 -27 F = 3.64479 X 10 -32 This third embodiment operates over a spectral bandwidth of 8 nanometers centered on 365.5 nanometers. A radiation of this spectral bandwidth can be provided by a filtered mercury arc lamp at the I-line waveband. The optical glasses other than fused silica used in this third embodiment are commonly known as I-line glasses. These optical glasses have the least absorption or solarization effects at the mercury I-line wavelength. These glasses may be found in a commonly available glass catalog provided by O'Hara Corporation, 50 Columbia Road, Branchburg Township, Somerville, New Jersey 08876-3519, USA. Figure 4 illustrates a first example which is useful for understanding the optical reduction system of the present invention. This example has a numerical aperture of 0.63 and can operate at a spectral bandwidth of 300 picometers, and preferably of 100 picometers, centered on 248.4 nanometers. From the long conjugate end, it includes an object or reticle plane 410, a first lens group LG1, a folding mirror 420, a second lens group LG2, a beamsplitter cube 430, a first quarter-waveplate 432, a concave mirror 434, a second quarter-waveplate 438, and a third lens group LG3. The image is formed at the image or wafer plane 450. The first lens group LG1 comprises a shell 412, a spaced doublet including a positive lens 414 and a negative lens 416, and a positive lens 418. The second lens group LG2 comprises a positive lens 422, a spaced doublet including a negative lens 424 and a positive lens 426, and a negative lens 428. The third lens group LG3 comprises two positive lenses 440 and 442, a shell 444, and two positive lenses 446 and 448. Again, as in the embodiment illustrated in Fig. 1, the folding mirror 420 of Fig. 4 is not essential to the operation of the invention, but nevertheless permits the object 410 and image plane 450 to be parallel to each other which is convenient for the manufacture of semiconductor devices using photolithography. The construction data of the first example illustrated in Fig. 4 is given in Table 4 below. Element Number Radius of Curvature (mm) Thickness (mm) Aperture Diameter (mm) Glass Front Back Front Back 410 Infinite 63.3853 412 -183.5661 CC -215.7867 CX 17.0000 122.8436 130.6579 Silica Space 46.6205 414 -601.1535 CC -230.9702 CX 21.4839 149.1476 153.3103 Silica Space 68.8075 416 -195.1255 CC -345.4510 CX 15.0000 161.6789 170.1025 Silica Space 3.0000 418 435.8058 CX -1045.1785 CX 24.9351 177.4520 178.2672 Silica Space 130.0000 Decenter(1) 420 Infinite -64.5000 180.3457 Reflection 422 -210.7910 CX 380.1625 CX -43.1418 181.6672 178.0170 Silica Space -15.8065 424 300.1724 CC -123.4555 CC -12.0000 166.7278 153.3103 Silica Space -3.8871 426 -126.8951 CX 972.6391 CX -41.3263 154.8530 151.8327 Silica Space -1.5000 428 -626.4905 CX -116.6456 CC -12.0000 147.6711 136.1163 Silica Space -31.8384 430 Infinite Infinite -74.0000 137.2448 200.1127 Silica Decenter(2) 436 Infinite 200.1127 Reflection 430 Infinite Infinite 74.0000 200.1127 200.1127 Silica Space 2.0000 432 Infinite Infinite 6.0000 148.6188 149.0707 Silica Space 14.4638 Stop 149.6392 434 Aspheric -14.4638 149.6392 Reflection 432 Infinite Infinite -6.0000 144.8563 141.2737 Silica Space -2.0000 430 Infinite Infinite -74.0000 139.3606 117.3979 Silica Decenter(3) 430 Infinite Infinite -61.0000 117.3979 100.5074 Silica Space -2.0000 438 Infinite Infinite -4.5000 99.6617 98.4157 Silica Space -1.2000 440 -157.8776 CX 2282.2178 CX -13.7501 94.8267 91.8775 Silica Space -1.2000 442 -94.0059 CX -466.6659 CC -13.4850 82.8663 78.1418 Silica Space -1.2000 444 -147.2485 CX -77.8924 CC -22.2075 72.7262 50.6555 Silica Space -3.2091 446 -159.2880 CX -519.4850 CC -13.8321 49.5648 39.0473 Silica Space -0.2000 448 -129.3683 CX -426.7350 CC -6.1500 37.3816 32.4880 Silica Space Image Distance = -4.0000 450 Image Infinite 26.5003 The constants for the aspheric mirror 434 used in equation (1) located after Table 1 are as follows: CURV = -0.00332614 K = 0.000000 A = -4.32261E-10 B = 3.50228E-14 C = 7.13264E-19 D = 2.73587E-22 This first example is optimized for radiation centered on 248.4 nm. The single refracting material of fused silica and the large portion of refracting power restricts the spectral bandwidth of the a first example depicted in Fig. 4. However, because the a first example has a maximum numerical aperture of 0.63 rather than of 0.7 as in the three embodiments, the first example provides acceptable imaging over a spectral full-width-half-maximum bandwidth of 300 picometers, or preferably of 100 picometers. Thus, in the former, an unnarrowed, or, in the latter, a narrowed excimer laser can be employed for the illumination source. The first example differs from the three embodiments in that the net power of LG1 and LG2 of the first example is weakly positive rather than weakly negative as in the three embodiments. In addition, this illustrates that the overall focal power of LG1 plus LG2 can be either positive or negative and still permit an infinitely distant entrance pupil to be imaged at or near the concave mirror 434. Figure 5 illustrates a second example which is useful for understanding the optical reduction system of the present invention. Preferably, this second example has a numerical aperture of 0.60 and operates at a spectral bandwidth of 300 picometers centered on 248.4 nanometers. From the long conjugate end, it includes an object or reticle plane 510, a first lens group LG1, a folding mirror 520, a second lens group LG2, a beamsplitter cube 530, a first quarter-waveplate 532, a concave mirror 534, a second quarter-waveplate 538, and a third lens group LG3. The image is formed at the image or wafer plane. The first lens group LG1 comprises a shell 512, a spaced doublet including a positive lens 514 and a negative lens 516, and a positive lens 518. The second lens group LG2 comprises a positive lens 522, a spaced doublet including a negative lens 524 and a positive lens 526, and a negative lens 528. The third lens group LG3 comprises two positive lenses 540 and 542, a shell 544, and two positive lenses 546 and 548. Again, as in the embodiment illustrated in Fig. 1, the folding mirror 520 of Fig. 5 is not essential to the operation of the invention, but nevertheless permits the object and image planes to be parallel to each other which is convenient for the manufacture of semiconductor devices using photolithography. The construction data of the second example illustrated in Fig. 5 is given in Table 5 below. Element Radius of Curvature (mm) Thickness Aperture Diameter (mm) Number Front Back (mm) Front Back Glass 510 Infinite 62.7514 512 -136.1154 CC -152.5295 CX 16.8300 120.7552 129.4354 Silica Space 4.5206 514 -270.1396 CC -191.8742 CX 20.5341 132.9152 139.0377 Silica Space 90.8476 516 -188.9000 CC -284.7476 CX 17.5000 156.1938 165.6567 Silica Space 2.9700 518 433.8174 CX -841.5599 CX 25.8293 173.8279 174.8334 Silica Space 149.4549 Decenter(1) 520 Infinite -61.0000 177.2183 Reflection 522 -190.3251 CX -8413.4836 CC -34.4584 178.5071 174.2260 Silica Space -51.5487 524 690.5706 CC -146.4997 CC -11.8800 150.4109 141.8021 Silica Space -10.6267 526 -265.9886 CX 1773.5314 CX -24.1851 142.1592 141.2400 Silica Space -1.5000 528 -244.9899 CX -142.8558 CC -11.8800 139.3290 133.8967 Silica Space -21.6411 530 Infinite Infinite -71.2800 134.3115 189.7826 Silica Decenter(2) 536 Infinite 189.7826 Reflection 530 Infinite Infinite 71.2800 189.7826 189.7826 Silica Space 1.9800 532 Infinite Infinite 5.9400 142.3429 142.6707 Silica Space 18.5263 Stop 143.5034 534 Aspheric -18.5263 143.5034 Reflection 532 Infinite Infinite -5.9400 134.2788 130.9398 Silica Space -1.9800 530 Infinite Infinite -71.2800 130.1221 111.7247 Silica Decenter(3) 530 Infinite Infinite -60.4000 111.7247 96.1353 Silica Space -1.9800 538 Infinite Infinite -4.4550 95.3562 94.2064 Silica Space -1.1880 540 -127.4561 CX -1398.8019 CC -13.0104 90.4737 87.7002 Silica Space -1.1880 542 -98.8795 CX -424.1302 CC -12.2874 80.7016 76.3270 Silica Space -1.1880 544 -132.0104 CX -70.9574 CC -17.8706 71.0789 53.4306 Silica Space -3.1246 546 -123.1071 CX -585.4471 CC -19.9496 52.6417 38.2256 Silica Space -0.1980 548 -137.8349 CX -292.6179 CC -6.0885 36.7251 31.8484 Silica Space Image Distance = -4.0000 550 Image Infinite 26.5000 The constants for the aspheric mirror 534 used in equation (1) located after Table 1 are as follows: CURV = -0.00325995 K = 0.000000 A = -6.91799E-10 B = 5.26952E-15 C = 6.10046E-19 D = 1.59429E-22 This second example is optimized for radiation centered on 248.4 nm. The single refracting material of fused silica and the large portion of refracting power restricts the spectral bandwidth of the second example depicted in Fig. 5. However, because the second example has a maximum numerical aperture of 0.6 rather than of 0.7 as in the three embodiments, the second example provides acceptable imaging over a spectral full-width-half-maximum bandwidth of 300 picometers. Thus, an unnarrowed excimer laser can be employed for an illumination source. The second example differs from the three embodiments in that the net power of LG1 and LG2 of the second example is weakly positive rather than weakly negative as in the three embodiments. In addition, this illustrates that the overall focal power of LG1 plus LG2 can be either positive or negative and still permit an infinitely distant entrance pupil to be imaged at or near the concave mirror 534. Although the preferred embodiments have been illustrated and described, it will be obvious to those skilled in the art that various modifications may be made without departing from the scope of this invention which is defined by the appended claims.

Claims: A catadioptric optical reduction system having a relatively high numerical aperture, from the long conjugate end to the short conjugate end, comprising: first lens means (LG1, LG2; LG4, LG5; LG7, LG8); a beamsplitter (30; 132; 232); a concave mirror (34; 136; 236); and second lens means (LG3; LG6; LG9) for providing a positive power, wherein said first lens means images an entrance pupil at infinity to an aperture stop at or near said concave mirror; and wherein said second lens means images an exit pupil to infinity, characterized in that the positive power of the second lens means provides most of the power of the system. An optical reduction system as in claim 1 further comprising: a first quarter-wave plate (32; 134; 234) placed between said beamsplitter and said concave mirror. An optical reduction system as in claim 2 further comprising: a second quarter-wave plate (38; 140; 240) placed between said beamsplitter and said second lens means. A catadioptric optical reduction system as in claim 2, wherein the first lens means comprises a first lens group of positive power (LG1; LG4; LG7) and a second lens group of negative power (LG2; LG5; LG8); the second lens means comprises a third lens group of positive power (LG3; LG6; LG9); and the positive power of said first lens group provides enough power to image an entrance pupil at infinity through said second lens group to an aperture stop at or near said mirror, the negative power of said second lens group provides the necessary conjugates for said concave mirror, and the positive power of said third lens group provides the remainder of the total system power and images the exit pupil to infinity. An optical reduction system as in claim 4 wherein: said first lens group includes refractive elements made from at least two different materials; and said second lens group includes refractive elements made from at least two different materials. An optical reduction system as in claim 4 wherein said first lens group comprises: at least one lens of positive power (18; 116; 118; 220); a first lens of substantially zero power (12; 120; 216, 218); and a first doublet (14; 16; 112, 114; 212, 214), whereby said at least one lens of positive power images an entrance pupil at infinity to an aperture stop near said mirror, and said first lens of substantially zero power and said first doublet help correct aberrations such as astigmatism, field curvature, and distortion. An optical reduction system as in claim 6 wherein: said first lens (216, 218) is an air spaced doublet. An optical reduction system as in claim 6 wherein said second lens group comprises: at least one lens of negative power (28; 130; 230) a positive lens (22; 124; 224); and a second doublet (24, 26; 126, 128; 226, 228); whereby said at least one lens of negative power provides a diverging beam for said beamsplitter and said mirror, said positive lens provides lateral color correction, and said second doublet helps to correct for spherical aberration and coma. An optical reduction system as in claim 8 wherein said third lens group comprises: a first at least one lens of positive power (40, 42; 142, 148; 242, 248); a second lens of substantially zero power (44; 150, 152; 250, 252); and a second at least two lenses of positive power (46, 48; 144, 146; 244, 246), weaker than said first at least one lens of positive power, whereby said first at least one lens of positive power provides a high numerical aperture to image an exit pupil at infinity, and said second lens of substantially zero power and said second at least two lenses of positive power provide high-order aberration correction. An optical reduction system as in claim 9 wherein: said second lens is a cemented doublet (150, 152; 250, 252). An optical reduction system as in claim 4 further comprising: a folding mirror (20; 122; 222) placed between said first lens group and said second lens group. An optical reduction system as in claim 1 or 4 wherein: said mirror is aspheric. An optical reduction system as in claim 1 or 4 wherein: said first lens means and said second lens means include refractive elements made of at least two different materials. An catadioptric optical reduction system as in claim 3 comprising: a construction according to the construction data of Table 1. An optical reduction system as in any one of claims 1 to 14 wherein said beamsplitter is a cube. An optical reduction system as in any one of claims 1 to 14, wherein an aperture stop is provided between said beamsplitter and said concave mirror. An optical reduction system as in claim 16, wherein a variable aperture is placed between said beamsplitter and said concave mirror. An optical reduction system as in any one of claims 1 to 14, wherein said concave mirror includes a reduction ratio of between 1.6 and 2.7 times that of the total system reduction ratio. A catadioptric optical reduction system as in claim 3 wherein said first lens means comprises: a first doublet (112, 114); a first positive lens (116); a second positive lens (118); a shell (120); a third positive lens (124); a first negative lens (126); a fourth positive lens (128); a second negative lens (130); and said second lens means comprises: a fifth positive lens (142); a second doublet (144, 146); a sixth positive lens (148); and a third doublet (150, 152), arranged such that radiation entering the system passes through said first doublet, said first positive lens, said second positive lens, said shell, said third positive lens, said first negative lens, said second negative lens; said beamsplitter cube, said first quarter-wave plate, and is reflected by said concave mirror back through said first quarter-wave plate and said beamsplitter cube, and through said second quarter-wave plate, said fifth positive lens, said second doublet; said sixth positive lens, and said third doublet. An optical reduction system as in claim 19 further comprising: a folding mirror (122) placed between said shell (120) and said third positive lens (124). An optical reduction system as in claim 20 having: a construction according to the construction data of Table 2. A catadioptric optical reduction system as in claim 3 wherein said first lens means comprises: a first doublet (212, 214); a second doublet (216, 218); a first positive lens (220); a second positive lens (224); a first negative lens (226); a third positive lens (228); a second negative lens (230); and said second lens means comprises: a fourth positive lens (242); a third doublet (244; 246); a fifth positive lens (248); a shell (250); and a sixth positive lens (252), arranged such that radiation entering the system passes through said first doublet, said second doublet, said first positive lens, said second positive lens, said first negative lens, said third positive lens, said second negative lens, said beamsplitter cube, said first quarter-wave plate, and is reflected by said concave mirror back through said first quarter-wave plate and said beamsplitter cube, and through said second quarter-wave plate, said fourth positive lens, said third doublet, said fifth positive lens, said shell, and said sixth positive lens. An optical reduction system as in claim 22 further comprising: a folding mirror (222) placed between said first positive lens (220) and said second positive lens (224). An optical reduction system as in claim 23 having: a construction according to the construction data of Table 3. A catadioptric optical reduction system as in claim 3, wherein the first lens means comprises: a first lens group of positive power; a second lens group of negative power; the second lens means comprises: a third lens group of positive power; and the positive power of said first and second lens groups provide only enough power to image an entrance pupil at infinity to an aperture stop at or near said mirror, the negative power of said second lens group provides the correct conjugates for said concave mirror, and the positive power of said third lens group provides the remainder of the total system power and images the exit pupil to infinity. An optical reduction system as in claim 25, wherein the first quarter-wave plate and/or second quarter-wave plate are true zero order quarter wave plates. A catadioptric optical reduction system as in claim 1 wherein the first lens means comprises a first lens group of positive power and a second lens group of negative power; and being arranged such that radiation entering said system passes through said first lens group, said second lens group, said beamsplitter cube, and is reflected by said concave mirror back through said beamsplitter and through said third lens group.

Inventors: WILLIAMSON DAVID W, WILLIAMSON, DAVID W.
Applicants: SVG LITHOGRAPHY SYSTEMS INC, SVG LITHOGRAPHY SYSTEMS, INC.
Application year: 1993
Main classification: G02B17
IPC codes: G03F7, G02B17