Patent Number: 059178795
Section: description

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT A preferred embodiment of this invention is described below. FIG. 1 illustrates an application example of the reflective reduction imaging optical system for X-ray lithography of this invention. FIG. 2 is a diagram illustrating its main portion. As can be seen from these figures, from the side of mask M to the side of wafer W, in order, the following mirrors are placed coaxially: first concave mirror G.sub.4, plane mirror G.sub.3, convex mirror G.sub.2, and second concave mirror G.sub.1. The concave mirrors G.sub.4 and G.sub.1, and the convex mirror G.sub.2 are formed in aspherical shapes. The convex mirror G.sub.2 is placed at the pupil plane, and the side of wafer W becomes telecentric. The parameters of this application example are shown in Table I, which follows. In the "overall parameters", NA represents the numerical aperture and RF represents the ring field. In the "parameters of the mirrors", the first column represents the identification of the reflective surface from the side of wafer W, the second column r represents the apex radius of curvature of the various reflective surfaces, the third column d represents the distance between apexes of the various reflective surfaces, the fourth column .kappa. represents the conical coefficient of each reflective surface, and the fifth column .PHI. represents the aperture. The conical coefficient .kappa. is defined by the following formula. Aspherical coefficient C.sub.n is set at 0 for each reflective surface. In this application example, the aspherical shapes are the so-called secondary aspherical surfaces. In particular, first concave mirror G.sub.4 is formed as an ellipsoidal surface. ##EQU1## wherein y is the height in the direction perpendicular to the optical axis; S(y) is the displacement in the direction of the optical axis at height y; PA1 r is the apex radius of curvature; PA1 .kappa. is the conical coefficient; and PA1 C.sub.n is the nth aspherical coefficient. Aperture .PHI. refers to the aperture, including the portion actually cut off for guaranteeing the optical path. TABLE I ______________________________________ "Overall parameters" Magnification: 1/4 Wafer-side NA: 0.06 (mask-side NA: 0.015) Wafer-side RF inner radius: 29.8 (mask-side RF inner radius: 119.2) Wafer-side RF outer radius: 30.0 (mask-side RF inner radius: ______________________________________ 120) "Parameters of mirrors" r d .kappa. .phi. ______________________________________ W -- 289.9966 G.sub.1 -353.8219 -176.50426 1.19513 73.8 G.sub.2 -266.80893 200 1.38210 21.3 G.sub.3 0 -271.76014 -- 107.2 G.sub.4 1069.69783 2008.57114 0.41521 221.2 M -- ______________________________________ FIG. 3 is a diagram illustrating the lateral aberration on the meridional plane in this application example. This diagram shows the aberration on the mask M surface when an X-ray of 13 nm is incident from the side of wafer W. Consequently, height Y.sub.0 of the object is the height on wafer W. It can be seen from this figure that good imaging performance is obtained in this application example. As explained above, in this application example, by scanning with mask M and wafer W in synchronization with each other in the ring field, an exposure apparatus having a wide field is obtained. In the ring field, an image with a high resolution and a small skew aberration can be obtained. Also, as the reduction side, that is, the wafer side, becomes the telecentric ring field, or the aperture stop position is on convex mirror G.sub.2, it is possible to obtain the same exposure condition everywhere in the ring field. As the optical path is reflected back by plane mirror G.sub.3, there is no mechanical interference by the wafer in the optical path from the mask. Also, since the incident angles (the angle from the normal to the reflective surface) of the light beam on reflective surfaces G.sub.1 -G.sub.4 are nearly 0.degree., it is possible to suppress the wavefront aberration caused by the phase shift by the various reflective surfaces. In particular, in this application example, since a plane mirror and ellipsoidal mirrors are used, when these mirrors are manufactured, it is easy to obtain surfaces with a high precision. This is an advantage. In this application example, among the secondary aspherical surfaces, ellipsoidal and oblate spheroidal surfaces are used. However, it is also possible to use parabolic or hyperbolic surfaces for the secondary aspherical surfaces. As described above, it is possible to obtain a reflective reduction imaging optical system for X-ray lithography with a good imaging performance and with a simple construction. While the invention has been described above with respect to certain embodiments thereof, it will be appreciated by a person skilled in the art that variations and modifications may be made without departing from the spirit and scope of the invention.