Patent Number: 053533224
Section: summary

BACKGROUND A recognized way of reducing the feature size of circuit elements on microchips is to lithographically image them with radiation of a shorter wavelength. This is clear from the well-known relationship ##EQU1## where R is the resolution dimension representing feature size, K is a process-related constant of order unity, .lambda. is the wavelength of the radiation, and NA is the numerical aperture of the imaging system projecting the radiation onto a wafer. Shortening the wavelength .lambda. thus directly reduces the resolution dimension. Enlarging the numerical aperture, as another way of reducing the resolution dimension, also reduces the depth of focus (Df), by the relationship ##EQU2## For several reasons, including the practical flatness of wafers, depth of focus is preferably larger than about 1.0 micron, which in turn limits the resolution improvement achievable by enlarging the numerical aperture. This leaves shortening the wavelength of the radiation as the most desirable way of improving resolution, providing ways can be found for distortion-free imaging with shorter wavelengths. Moving down the electromagnetic spectrum to wavelengths shorter than UV radiation leads to the so-called "soft" X-ray radiation in the range of 2 to 20 nanometers wavelength. Radiation in the soft X-ray range cannot be focused refractively by passing through glass lenses, but can be focused by reflective mirrors having multilayer coated surfaces. This possibility has led to some work on soft X-ray imaging systems using mirrors in a projection imaging lens system. Examples of such work include: The basic problem is well explained by H. Kinoshita et al. in their paper "Soft x-ray reduction lithography using multilayer mirrors" (J. Vac. Sci. Technol. B 7 (6), November/December 1989, pages 1648-1651). PA1 One of the inventors of this application (J. H. Bruning) has contributed to a paper entitled "Reduction imaging at 14 nm using multilayer-coated optics: Printing of features smaller than 0.1 .mu.m" (J. Vac. Sci. Technol. B 8 (6), November/December 1990, pages 1509-1513). PA1 A workshop on this subject, High-Precision Soft X-ray Optics Workshop, held Oct. 5 and 6, 1989, was sponsored by the Air Force Office of Scientific Research and the National Institute of Standards and Technology. A notebook entitled "High-Precision Soft X-Ray Optics" from this workshop includes a section on "Optical Fabrication", pages 16-20. PA1 The Optical Society of America sponsored a topical meeting, Soft-X-Ray Projection Lithography Topical Meeting, on Apr. 10-12, 1991. A paper entitled "Design and Analysis of Multimirror Soft-X-Ray Projection Lithography Systems", by D. L. Shealy, C. Wang, and V. K. Viswanathan, was published in OSA Proceedings on Soft-X-Ray Projection Lithography, 1991, Vol. 12, Jeffrey Bokor (ed.), Optical Society of America, pages 22-26. PA1 Another paper on the subject, authored by R. H. Stulen and R. R. Freeman, and entitled "Optics Development for Soft X-Ray Projection Lithography Using a Laser Plasma Source", dated Nov. 15, 1990, was published in OSA Proceedings on Soft-X-Ray Projection Lithography, 1991, Vol. 12, Jeffrey Bokor (ed.), Optical Society of America, pages 54-57. PA1 A selection of overview papers from SPIE Proceedings-Summer/Fall 1990, SPIE Advent Technology Series, Volume AT 2, (ed., Western Washington University), SPIE Optical Engineering Press, includes, at page 320, a paper entitled "Design survey of x-ray/XUV projection lithography systems", by D. L. Shealy and V. K. Viswanathan. PA1 Another paper entitled "Reflective systems design study for soft x-ray projection lithography" is by T. E. Jewell, J. M. Rodgers, and K. P. Thompson, and appears in J. Vac. Sci. Technol. B 8 (6), November/December 1990, American Vacuum Society, pages 1519-1523. SUMMARY OF THE INVENTION In our research of the optical design of mirror-based imaging systems for X-ray projection lithography cameras, we have devised a way of representing all possible three-mirror lenses with only two parameters, allowing design solutions to be graphically displayed in a comprehensive and insightful manner. The two parameters, which are the magnification of a convex mirror and the magnification ratio of two concave mirrors, can be plotted on coordinate axes in a two-dimensional magnification plane where any specific three-mirror lenses are located as points. Then using the two-dimensional magnification display to help explore the characteristics of possible design solutions, we have located and identified a region of the magnification space where optimum design solutions exist. The solutions within our region are free of many problems encountered by solutions outside our region, and our solutions generally embody practical requirements that contribute to workability. Also, our simple way of two-dimensionally representing our region of optimum solutions enables a designer to proceed more rapidly to a first-order solution and to identify first-order solutions that present the best prospects for extremely low distortion lens designs using well-known computer optimization techniques. Although two- and four-mirror systems can be used in X-ray projection lithography cameras, we prefer three-mirror systems, with or without a fourth plane mirror for folding or turning the radiation. Three-mirror systems have been suggested for a soft X-ray projection lithography camera (published European Patent Application EP-252-734-A, entitled "X-ray Reduction Projection Exposure System of Reflection Type", of Canon KK), but the suggested systems all lie outside our region of optimum solutions. The shortcomings involved in this are explained below. Our work on three-mirror soft X-ray lithography lenses has led to discovery of a pair of unusual but especially effective lens systems offering several advantages. These lenses fall within our optimum design solution region of magnification space and are explained toward the end of this specification.