Patent Number: 051270293
Section: summary

FIELD OF THE INVENTION AND RELATED ART This invention relates to an exposure apparatus using X-rays and, more particularly, to an X-ray exposure apparatus wherein an X-ray beam from a radiation source such as a synchrotron orbit radiation source (SOR source), for example, is made divergent and thus is expanded for exposure of a surface to be exposed. SOR source is a radiation source which emits sheet-like electromagnetic waves (X-rays and the like) having a large divergent angle in a horizontal direction but having a small divergent angle in a vertical direction. Because of small divergent angle in the vertical direction, if an X-ray beam from the SOR source is projected directly to a surface to be exposed, only a limited range of the surface with respect to the vertical direction can be illuminated. For this reason, in an X-ray exposure apparatus which uses a SOR source, it is necessary to take some measure for expanding the X-ray beam emitted from the SOR source in the vertical direction. As an example, a method has been proposed by R. P. Haelbick et al, "J. of Vac. Sci. Technol." B1(4), Oct.-Dec., 1983, pages 1262-1266, according to which a mirror is disposed between a SOR source and a surface to be exposed and the mirror is oscillated with an angle of a few miliradians to scan the whole surface to be exposed, with a slit-like X-ray beam from the SOR source. With this method, however, at a moment only a part of the surface to be exposed can be irradiated with the X-rays. This results in a possibility of local expansion of the surface to be exposed (e.g. a mask) which leads to distortion of a pattern of the mask to be transferred and thus causes a transfer error. Although such a problem may be solved if the period of oscillation of the mirror is made sufficiently short, in order to assure this it is necessary to use a large drive power. This is inconvenient. As another example of expanding the X-ray beam in the vertical direction, a method has been proposed by Warren D. Grobman, "Handbook on Synchrotron Radiation", Vol. 1, Chapter 13, page 1135, North-Holland Publishing Co., 1983, according to which a convex mirror is disposed between a SOR source and a surface to be exposed, so that with the reflection by the convex surface of the mirror the angle of divergence of the X-ray beam in the vertical direction is expanded. This method is free from the problem of local distortion of the pattern as described hereinbefore. However, there is another problem. That is, of the X-ray beam emitted from a SOR source, in a horizontal orbit plane (horizontal section), the X-rays having different emission angles have an intensity distribution which can be considered substantially uniform. However, in a plane (vertical section) perpendicular to the orbit plane, they have an intensity distribution like a Gaussian distribution. This means that there is a difference in intensity between a central part and a peripheral part of the X-ray beam. Therefore, if the X-ray beam is used for the exposure as it is, non-uniform exposure results. Thus, it can not properly be used in an exposure apparatus. SUMMARY OF THE INVENTION It is accordingly an object of the present invention to provide an X-ray exposure apparatus having an improved mirror reflection system. In accordance with an aspect of the present invention, to achieve the above object, there is provided an X-ray exposure apparatus, comprising: an X-ray source; and directing means for directing an X-ray beam from said X-ray source to a surface to be exposed, said directing means including a mirror having a reflection surface of a curvature radius R with respect to a predetermined sectional plane, for reflecting the X-ray beam and for expanding the diameter thereof with respect to said sectional plane; wherein said mirror satisfies the following conditions: EQU R=(2d.sub.1 d.sub.2 .sigma.')/{[.DELTA.-(d.sub.1 +d.sub.2).sigma.'].multidot..alpha.} where d.sub.1 : the distance from the emission point of said X-ray source to the center of effective X-ray beam diameter on said reflection surface; PA0 d.sub.2 : the distance from the center of effective X-ray beam diameter on said reflection surface to the center of effective X-ray beam diameter on the surface to be exposed; PA0 .alpha.: the angle defined at the center of effective X-ray beam diameter on said reflection surface, between the X-ray beam and said reflection surface; PA0 .sigma.': a standard deviation of a distribution of intensities of X-rays having different emission angles at said sectional plane, at the gravity center wavelength of the X-ray beam from said X-ray source; PA0 .DELTA.: 0.43a.ltoreq.4.0a; and PA0 a: the length of the surface to be exposed, with respect to said sectional plane. PA0 d.sub.1 : the distance from the emission point of said X-ray source to the center of effective X-ray beam diameter on said reflection surface; PA0 d.sub.2 : the distance from the center of effective X-ray beam diameter on said reflection surface to the center of effective X-ray beam diameter on the surface to be exposed; PA0 .alpha.: the angle defined at the center of effective X-ray beam diameter on said reflection surface, between the X-ray beam and said reflection surface; PA0 .sigma.': a standard deviation of a distribution of intensities of X-rays having different angles of emission from said X-ray source, in a sectional plane perpendicular to a generating line of said mirror, at the gravity center wavelength of the X-ray beam from said X-ray source; PA0 .DELTA.: 0.43a.ltoreq..DELTA..ltoreq.4.0a; and PA0 a: the length of the surface to be exposed, with respect to a direction which is substantially perpendicular to the generating line of said mirror. PA0 d.sub.1 : the distance from the emission point of said X-ray source to the center of effective X-ray beam diameter on said reflection surface; PA0 d.sub.2 : the distance from the center of effective X-ray beam diameter on said reflection surface to the center of effective X-ray beam diameter on the mask; PA0 .alpha.: the angle defined at the center of effective X-ray beam diameter on said reflection surface, between the X-ray beam and said reflection surface; PA0 .sigma.': a standard deviation of a distribution of intensities of X-rays having different angles of emission from said X-ray source, in a sectional plane perpendicular to a generating line of said mirror, at the gravity center wavelength of the X-ray beam from said X-ray source; PA0 .DELTA.: 0.43a.ltoreq..DELTA..ltoreq.4.0a; and PA0 a: the length of an area for the pattern of the mask, with respect to a direction which is substantially perpendicular to the generating line of said mirror. In accordance with another aspect of the present invention, there is provided an X-ray exposure apparatus, comprising: an X-ray source; and directing means for directing an X-ray beam from said X-ray source to a surface to be exposed, said directing means including a mirror having a cylindrical surface of a curvature radius R, for reflecting the X-ray beam and for expanding the diameter thereof; wherein said mirror satisfies the following condition: EQU R=(2d.sub.1 d.sub.2 .sigma.')/{[.DELTA.-(d.sub.1 +d.sub.2).sigma.'].multidot..alpha.} where In accordance with a further aspect of the present invention, there is provided an X-ray exposure apparatus, comprising: means for supporting a mask; means for supporting a wafer; and directing means for directing an X-ray beam from an X-ray source to the wafer through the mask to thereby expose the wafer to a pattern of the mask, said directing means including a mirror having a cylindrical reflection surface of a curvature radius R, for reflecting the X-ray beam and for expanding the diameter thereof; wherein said mirror satisfies the following condition: R=(2d.sub.1 d.sub.2 .sigma.')/{[.DELTA.-(d.sub.1 +d.sub.2).sigma.'].multidot..alpha.} where In this Specification, the gravity center wavelength of an X-ray beam from an X-ray source means .lambda./ .sub. 0 which is given by the following equation: ##EQU1## where .lambda. is the wavelength, e(.lambda.) is the energy of the X-ray beam at the wavelength .lambda., and .lambda..sub.1 and .lambda..sub.2 are lower and upper limits of the wavelength region used for the exposure. In an X-ray exposure apparatus according to one preferred form of the present invention, the mirror of said directing means is set so as to satisfy the following condition: EQU R=(2d.sub.1 .multidot.d.sub.2)55 [.DELTA.'-(d.sub.1 +d.sub.2)].multidot..alpha.} where 4.3.times.10.sup.2 a.ltoreq..DELTA.'.ltoreq.4.0.times.10.sup.4 a. The reflection surface of a mirror usable in the present invention may have a well-known structure effective to reflect X-rays with a good efficiency. For example, it may have a multilayered film structure. The present invention can be applied effectively to an exposure apparatus for manufacture of semiconductor devices, for example, wherein the exposure is performed with X-rays from a SOR source. This is because: With the present invention, it is possible to reduce the non-uniformness in the intensity distribution of an X-ray beam from a SOR source. It is therefore possible to illuminate a mask having a semiconductor circuit pattern with X-rays of a desirable intensity distribution. As a result, the semiconductor circuit pattern of the mask can be transferred to a wafer very accurately. It is to be noted here that the present invention is applicable to various types of exposure apparatuses such as, for example, a contact type exposure apparatus wherein a mask is contacted to a wafer, a proximity type exposure apparatus wherein a mask is spaced from a wafer through a distance of a few microns to a few tens of microns, and a projection type exposure apparatus wherein a pattern of a mask is projected to a wafer through a projection system including mirrors. These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.