Optical reduction system

Abstract: An unobscured catadioptric optical reduction system wherein all refractive elements are made of the same material. From its long conjugate end to its short conjugate end the system comprises a first lens group (12,13), a beamsplitter (17), a second lens group (14,15), a mirror and a third lens group (18,19) arranged so that radiation entering the system at its long conjugate end passes through the first lens group (12,13), the beamsplitter (17), the second lens group (14,15), and is reflected by the mirror (20) back through the second (14,15) lens group and is reflected by the beamsplitter (17) through the third lens group (18,19). Aberrations of the mirror are corrected by the refractive elements. Lateral color correction is obtained by balancing the power of the first lens group (12,13) against the power of the third lens group (18,19).

Description: BACKGROUND OF THE INVENTION The design of semiconductor manufacturing equipment is driven by the trend toward higher density integrated circuits. In an optical projection system decrease in feature size theoretically is limited only by the capability of the system to function at short wavelengths, i.e., the further into the U.V. region of the spectrum the projection light source extends, the smaller will be the feature size. In practice, however, feature size is limited by the resolution capabilities of the optical system, i.e., the capability of the optical system to transmit the object as an image substantially free of aberrations introduced by the optics. The problem of maintaining high resolution, i.e., an optical system substantially free of aberrations is exacerbated further where the projection system is designed to include the added advantage of reduction, i.e., reduction of the size of the object when it is imaged. BRIEF DESCRIPTION OF THE INVENTION The present invention is a major advance in optical projection systems and is capable of functioning in the deep U.V. with feature resolution capabilities of less than 0.35 microns. The present invention has the added important advantage of being compact, i.e., less than half a meter in length while incorporating a 0.45 numerical aperture and covering a relatively large image field size. The present invention is related to the optical system with unity magnification disclosed by C.G. Wynne in U.S. Patent No. 4,796,984, based on a system described by Dyson (ref: J. Dyson, "Unit Magnification Optical System Without Seidel Aberrations", J. Opt. Soc. Am. 49(7), 713-716 (1959)). A monocentric catadioptric optical reduction system is known from DE-A-1 957 628. This system contains a mirror operating at a 1:1 ratio, while the system's reduction is accomplished by monocentric lenses. It is the object of the present invenion to provide a catadioptric optical reduction system enabling an arbitrary reduction ratio other than 1:1. According to the invention, this object is solved with a catadioptric system according to claim 1. In an embodiment the present invention is a catadioptric optical reduction system comprising from the long conjugate end a first lens group, a thin tilted beamsplitter, a second lens group, a spherical mirror and a third lens group. The system is arranged to have object and image planes parallel and is telecentric at the short conjugate end. All the refractive elements are made of the same material - fused silica. Low order coma, sperical aberration and distortion of the reduction mirror are corrected by the second lens group. Astigmatism to all orders is corrected predominately by the second and third groups. The airspaces of the third group give higher order coma and distortion correction. Low order distortion of the first and third groups are balanced against each other. Residual axial colour is again balanced against the chromatic difference of field curvature. Lateral colour of the first group is balanced against that of the third group. In a further embodiment of the present invention according to claim 8 a catadioptric optical reduction system comprises from the long conjugate end to the short conjugate end a fourth lens group, a first lens group, a beamsplitter, a second lens group, a reduction mirror, and a third lens group. The system is arranged to have object and image planes parallel. The system is telecentric at both ends. All the refractive elements are made of the same material - fused silica. The beamsplitter is a cube, also of fused silica. Low order coma and spherical aberration of the reduction mirror are corrected by the second lens group. Astigmatism to all orders is corrected predominately by the third group, the airspaces of which also give high order coma and distortion correction. Low order distortion is primarily corrected by choosing an optimum power ratio between the first and third groups. Axial colour (chromatic difference of focus) is balanced against the chromatic difference of field curvature. Lateral colour (chromatic difference of magnification) of the first group is balanced against that of the third group. The present invention has general utility in projection microlithography and is useful in step-and-repeat, step-and-scan or full field scanning systems. SHORT DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic of a first embodiment of the optical system of the present invention; and Fig. 2 is a schematic of a second embodiment of the optical system of the present invention. DESCRIPTION Fig. 1 illustrates a version of the optical reduction system 10 of the present invention. From its long conjugate end it comprises object or reticle plane 11, a fourth lens group comprising shell 12 and positive lens 13, a first lens group comprising negative lens 14 and positive lens 15. A folding mirror 16 is disposed between the fourth and first lens group, i.e., between lens pair 12 and 13 and lens pair 14 and 15. As can be seen, folding mirror 16 changes the direction of radiation 90° and its purpose is to cause the object and image planes to be parallel as well as to shorten the overall length of the optical system. Next there is beamsplitter cube 17 followed by a second lens group comprising positive lens 18 and negative lens 19. This lens group arrangement is sometimes referred to as an air-spaced doublet. Next is the concave spherical mirror 20. Finally, a third lens group with overall positive power comprises positive lens 21, shell 22 which is slightly negative and positive lenses 23 and 24. Radiation entering the system at the reticle or long conjugate end passes through the fourth group, i.e., lenses 12 and 13, is reflected by folding mirror 16 and passes through the first lens group, i.e., lenses 14 and 15, beamsplitter 17, the second lens group, i.e., lenses 18 and 19, is reflected by mirror 20 passes back through lens 18 and 19 and is reflected by beamsplitter 17 through the third lens group which comprises lenses 21, 22, 23, and 24 and is focussed at image or wafer plane 25. Lenses 12 and 13 of the fourth lens group and lenses 21, 22, 23 and 24 of the third lens group provide telecentricity at the object and image planes 11 and 25, respectively. As in the related Wynne-Dyson optical system, the field curvature and astigmatism of the spherical mirror 20 is balanced against the total power of the refracting elements. However, unlike the Wynne-Dyson system, mirror 20 is operating at a reduction ratio other than 1:1. It therefore introduces spherical aberration and coma - these are corrected predominately by the air-spaced doublet (lenses 18, 19 - the second group) with substantial assistance from the beamsplitter cube in the converging beam between the second and third groups. The other problem arising from a reduction ratio other than 1:1 is that distortion has to be corrected - this is achieved primarily by the optimum choice of power and location between the first and third groups, with some assistance from the fourth group and beamsplitter cube. A problem with the original Dyson system is that there is negligible clearance between the last optical surface and the image plane. This is overcome in the Wynne system by the use of a second refracting material of different refractive index. In the present invention only one material is used and the airspaces in the third group (lenses 21-24) achieve the same goal. In addition, they provide higher order coma and distortion correction that again is required by the reduction ratio being other than 1:1. Performance of the Wynne-Dyson system is limited by high order astigmatism. In the present invention this is substantially corrected by the fourth and first groups, particularly the shell 12 and weak negative lens 14. The predominant low order astigmatism correction comes from the power of the third group. As the majority of the reduction is provided by spherical mirror 20, chromatic aberrations are inherently small. Residual axial color (chromatic difference of focus) is not corrected, but rather balanced against chromatic variation of the field curvature, which limits the spectral bandwidth of the system to the range 240-256 nanometers. This is more than adequate for an Excimer laser source, and also allows the use of a filtered Mercury Arc. Over this range, Secondary Spectrum is negligible - a benefit of using only one refracting material. Lateral color (chromatic difference of magnification) is corrected by the balance of power between the first and third groups. Spherochromatism (chromatic variation of spherical aberration) is minimized by the second group. The embodiment of Fig. 1 is designed to operate with a 0.45 numerical aperture at a 4 to 1 reduction ratio with a 30 mm diameter image field. Table I is an example indicating the construction data of the optical system of Fig. 1, given with the system unfolded. The embodiment of Fig. 2 shows a catadioptric optical reduction system 30. This embodiment is similar to that of Fig. 1 but differs in that the fourth lens group is eliminated and the beamsplitter cube is replaced by a thin tilted beamsplitter. Elimination of the fourth lens group does away with telecentricity at the object or reticle plane, which is not a necessity. Also, such removal of several refractive elements as well as use of a thin beamsplitter permits use of the optical system of Fig. 2 deeper in the U.V. since absorption is lessened. The embodiment of Fig. 2 is designed to function with a numerical aperture of 0.45 at a 4:1 reduction ratio with a light source centered at 193 nm ± 1 nm, and has an image field of 20 x 5 mm. From the long conjugate end, i.e., beginning at the object or reticle plane 31 the optical system of Fig. 2 comprises a folding mirror 32 which redirects light or radiation entering the system, thereby allowing the object and image planes to be parallel. Folding mirror 32 is succeeded by a first lens group comprising positive lens 33, negative lens 34, and positive lens 35, thin parallel plane beamsplitter 36, a second lens group comprising only negative lens 37, concave spherical mirror 38, a third lens group comprising positive lens 39, shell 40, positive lens 41, shell 42 and positive lens 43. This last group among other functions provides telecentricity at the image or wafer plane 44. The first lens 33 of the first lens group collimates the rays entering beamsplitter 36 which would otherwise cause some aberrations. As in the first embodiment, the field curvature and astigmatism of the spherical mirror 38 is balanced against the total power of the refracting elements. Mirror 38 is again operating at a reduction ratio other than 1:1. It therefore introduces spherical aberration and coma - these are corrected predominately by lens 37. Distortion is corrected primarily by the optimum choice of power and location between the first and third groups. The airspaces in the third group (lenses 39-43) allow finite clearance between the last optical surface and the image plane. In addition, they provide higher order coma and distortion correction that again is required by the reduction ratio being other than 1:1. The predominant low order astigmatism correction comes from the power of the third group. In contrast to the first embodiment, high order astigmatism correction is included in the third group - this is why the group has one more element than the third group of the first embodiment. Residual axial color (chromatic difference of focus) is again balanced against chromatic variation of field curvature, which limits the spectral bandwidth of the system to the range 192-194 nanometers. This spectral range is less than the first embodiment as the dispersion of fused silica increases rapidly at shorter wavelengths. It remains adequate for an Excimer laser source, but does not allow the use of a filtered Mercury Arc. The construction data of the embodiment of Fig. 2 is given in unfolded form in TABLE II. The chief difference between the two optical reduction systems above discussed is that one was designed to function at 248 nm and the other at 193 nm. It should be understood that the optical systems could be modified to function at other wavelengths. Both optical systems are unusually compact, e.g., the optical system of Fig. 1 has a length less than 0.5 meter. By making the object fields smaller the systems can be made more compact. These and other modifications are possible in light of the above description which should not be deemed as placing restrictions on the present invention other than those set forth in the claims which follow:

Claims: A catadioptric optical reduction system (30) comprising from the long conjugate end to the short conjugate end: a first lens group (33, 34, 35; 14, 15); a beamsplitter (36; 17); a second lens group (37; 18, 19); a mirror (38; 20); a third lens group (39-43; 21-24); arranged such that radiation entering said system passes through said first lens group (33, 34, 35; 14, 15), said beamsplitter (36; 17), said second lens group (37; 18, 19) and is reflected by said mirror (38; 20) back through said second lens group (37; 18, 19) and is reflected by said beamsplitter (36; 17) through said third lens group (39-43; 21-24), characterized in that said mirror (38; 20) is a reduction mirror having a reduction ratio other than 1:1, said mirror introducing spherical aberration, coma and distortion which are corrected by the second lens group, which is formed either by an air-spaced doublet (18, 19) or a negative lens (37), and by air spaces in the third lens group. An optical system according to claim 1 wherein the power of said first lens group (33, 34, 35; 14, 15) is balanced against the power of the third lens group (39-43; 21-24) for lateral color and further distortion correction. An optical system according to claim 1 or 2 wherein said first, second and third lens groups and said beamsplitter are composed of the same material. An optical system according to claim 3 wherein said material is fused silica. An optical system according to claim 3 or 4 wherein said beamsplitter is a cube. An optical system according to claim 3 or 4 wherein said beamsplitter (36; 17) is a tilted thin parallel plate. An optical system according to claim 6 wherein said first lens group (33, 34, 35) includes means (33) for collimating radiation before it enters said beamsplitter. An optical system according to claim 5 further including a fourth lens group (12, 13) disposed before said first lens group (14, 15) providing telecentricity at said long conjugate end. An optical system according to claim 7 or 8 wherein said third lens group (39-43; 21-24) provides telecentricity at said short conjugate end.

Inventors: WILLIAMSON DAVID M, WILLIAMSON, DAVID M.
Applicants: SVG LITHOGRAPHY SYSTEMS INC, SVG LITHOGRAPHY SYSTEMS, INC.
Application year: 1989
Main classification: G02B17
IPC codes: H01L21, G03F7, G02B17