Patent Number: 055816053
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical element for uniformly irradiating X-rays in a broad area, a method for producing the optical element, an optical system using the optical element, and an optical apparatus provided with the illumination optical system. 2. Related Background Art As a semiconductor integrated circuit element becomes finer and finer these days, there has been developed a reduction projection lithography technology using X-rays of shorter wavelength than conventionally used ultraviolet rays in place thereof in order to improve the resolving power of optical system which is limited by the diffraction limit. An X-ray exposure apparatus used in this technology is composed mainly of an X-ray source, an illumination optical system, a mask, an imaging optical system (reduction projection optical system) and a wafer stage. A photon radiation or laser plasm X-ray source is used as the X-ray source. The illumination optical system is constituted mainly by an oblique incidence mirror for reflecting X-rays incident obliquely into a reflecting surface, a multi-layer film mirror having a reflecting surface formed of multi-layered films, and a filter reflecting or transmitting only X-rays of a predetermined wavelength, so that the system may illuminate a mask with X-rays of the desired wavelength. The mask may be a transmitting mask or a reflecting mask. The transmitting mask is constructed such that an X-ray-absorbing material is formed in a desired pattern on a thin membrane of a material which well transmits X-rays. The reflecting mask is constructed such that low-reflectivity portions are formed in a desired pattern on multi-layered films which reflect the X-rays for example. The thus formed pattern on the mask is focused on a wafer coated with a photoresist through the reduction projection optical system comprising a plurality of multi-layer film mirrors. Since the X-rays are attenuated by atmospheric absorption, all optical paths thereof are maintained at a predetermined degree of vacuum. Such an X-ray exposure apparatus must be so arranged that the illumination optical system irradiates (or illuminates) a broad area on the mask. For example, in case of an X-ray reduction projection exposure apparatus provided with a reduction projection optical system of 5:1 reduction ratio, the reduction projection optical system must illuminate a region of square with a side of 100 mm on the mask in order to effect exposure on a region of square with a side of 20 mm on the wafer. To obtain a desired resolving power of the diffraction limit in this case, it is desired to use the entire numerical aperture of reduction projection optical system. That is, it is desired for the rays transmitted or reflected by the mask to have an angular range which covers the angular range of entrance numerical aperture of imaging optical system (reduction projection optical system). To realize it, not only each point in the pattern surface on the mask is simply irradiated by the exposure light (X-rays), but the each point in the pattern surface on the mask must be irradiated with light having an angle of divergence corresponding to the entrance numerical aperture of exposure light of reduction projection optical system as well. Incidentally, in case the synchrotron radiation or laser plasma X-ray source is used as the X-ray source in the X-ray exposure apparatus, the size of portion as the source radiating the X-rays (the size of X-ray source) is very small. The size of X-ray source is determined by the diameter of electron beam in case of the photon radiation, while it is determined by the spot size of laser beam irradiated onto a target in case of the laser plasma X-ray source. The obtainable size of X-ray source is about 0.1 to 1 mm in diameter in either case, which is extremely small as compared with the region to be illuminated on the mask. Therefore, the above requirement cannot be satisfied by the conventional illumination optical system constructed of a curved-surface mirror such as the oblique incidence mirror or the multi-layer film mirror, or of a combination of such mirrors, as long as such a small X-ray source as described above is used. For example, as shown in FIG. 1, if a mask 4 is located near an image P of a light source 2 focused by an illumination optical system 3 (this arrangement is called as critical illumination), the illumination light B is irradiated with a sufficient angle of divergence on and in the vicinity of the optical axis A of exposure light B. However, the region irradiated by the exposure light B has only the size of the image of light source 2 in a very narrow region near the optical axis A. Although the image of light source 2 can be magnified to some extent by increasing the magnification of illumination optical system 3, the angle of divergence of exposure light is inevitably reduced in that case. On the other hand, as shown in FIG. 2, in case an image P of light source 2 is arranged to be focused by the illumination optical system 3 on the entrance pupil 50 of imaging optical system (this arrangement is called as Kohler illumination), the exposure light B can be irradiated in a considerably wide region on the mask 4. However, the divergence angle of light irradiating each point in the pattern surface on mask 4 becomes extremely small, which makes it difficult to obtain a resolving power of the diffraction limit of imaging optical system. Also, in case the mask 4 is located between the positions shown in FIG. 1 and in FIG. 2, that is, at a position between the image P of light source 2 and the entrance pupil of imaging optical system, there cannot exist together the sufficient region on the mask 4 irradiated by the exposure light B and the satisfactory angle of divergence of light irradiating each point in the pattern on mask 4. If the size of light source 2 is nearly equal to that of mask 4 as shown in FIG. 3, rays (exposure light B) are incident in various directions even at a point distant from the optical axis A on the mask 4 and therefore a resolving power of the diffraction limit can be obtained in the region covering the entire pattern surface on the mask 4. FIG. 3 shows the case of Kohler illumination, but the illumination region is also enlarged even in the case of critical illumination, because the image of large light source is formed on the mask. It has been, however, difficult to obtain an X-ray source of such large size in actual. SUMMARY OF THE INVENTION It is an object of the present invention to achieve uniform irradiation of electromagnetic waves in a wide area. The above and other objects will be apparent from the following description. The present invention provides an optical element comprising: a substrate having a plurality of concave surfaces or convex surfaces with a substantially same curvature; and multilayer films for reflecting X-rays, formed on the concave surfaces or convex surfaces of said substrate and composed of thin films; wherein when X-rays are incident into the multilayer films on said concave surfaces or convex surfaces, the X-rays are reflected with a predetermined diverging angle on the multilayer films and as a result a plurality of secondary X-ray sources having the diverging angle are formed on a same plane located a predetermined distance apart from said concave surfaces or convex surfaces. It is preferred that the shape of concave surfaces or convex surfaces is a part of spherical surface or a toroidal surface for example. Also, the present invention provides a method for producing an optical element, comprising: a first step of forming a substrate having convex surfaces or concave surfaces on a surface thereof and being a ground for multilayer films for reflecting X-rays; and a second step of forming the multilayer films for reflecting said X-rays on said substrate. The first step is preferably either one of a method (first method) of applying a photosensitive material onto a substrate and processing the photosensitive material layer by the photolithography process to form fine convex portions, a method (second method) of screen-printing a heat-resistant material on a substrate to form fine convex portions, and a method (third method) of etching a substrate to form fine concave portions. The photosensitive material used in the first method may be a photoresist employed in lithography or a photoreactive polyimide. Since polyimide is superior in heat resistance, it is advantageous in case of strong X-rays being irradiated. Ordinary photoresists or photoreactive polyimides will form columns in a rectangular cross section after development. To smooth the rectangular section, it is preferable to utilize unsharpness of image due to diffraction in the arrangement with a gap between the substrate and the mask. Also, an alternative method may be a method of overlaying a coating on the pattern of rectangular section or a method of post-bake at high temperature (baking the resist after development). The heat-resistant material used in the second method is preferably a heat-resistant resin such as polyimide for example. In this case, concave portions of spherical surface are naturally formed by the surface tension of resin. The etching step in the third method may be either dry etching or wet etching, but it is preferable to properly select a method to maintain smoothness sufficient on the etched surface. The second step for forming the multilayer films on the fine convex portions or concave portions formed in the above step may be one of the ordinary methods for forming multilayer films. For example, a preferable method is the method for producing a thin film, such as the sputtering, the vacuum vapor deposition and the CVD (Chemical Vapor Deposition). The materials for the multilayer films may be a combination of Mo (molybdenum) and Si (silicon), but are not limited to the combination. Also, the present invention provides an optical system for illuminating an illuminated surface in an arc shape. The optical system comprises: an optical reflector having an X-ray reflecting surface forming a part of a parabola of revolution obtained by revolving an arbitrary parabola about a base axis passing through a vertex and a focus of the parabola and inclined at a predetermined angle relative to a normal line to said illuminated surface, said optical reflector reflecting X-rays by said X-ray reflecting surface to irradiate said illuminated surface; and an optical element for reflecting X-rays incident thereinto substantially in parallel with said base axis to irradiate said X-rays onto the X-ray reflecting surface of said optical reflector; wherein said optical reflector and said optical element are rotated in a united manner about a rotation axis passing through said optical element and being parallel to the normal line to said illuminated surface; wherein said optical element comprises: a substrate having a plurality of concave surfaces or convex surfaces with a substantially same curvature; and multilayer films for reflecting X-rays, formed on the concave surfaces or convex surfaces of said substrate and composed of thin films; wherein when X-rays are incident into the multilayer films on said concave surfaces or convex surfaces, the X-rays are reflected with a predetermined diverging angle on the multilayer films and as a result a plurality of secondary X-ray sources having the diverging angle are formed on a same plane located a predetermined distance apart from said concave surfaces or convex surfaces. Also, the present invention provides an optical apparatus for illuminating a predetermined area. The optical apparatus comprises: an optical reflector having an X-ray reflecting surface forming a part of a parabola of revolution obtained by revolving an arbitrary parabola about a base axis passing through a vertex and a focus of the parabola and inclined at a predetermined angle relative to a normal line to said illuminated surface, said optical reflector reflecting X-rays by said X-ray reflecting surface to irradiate said illuminated surface; an optical element for reflecting X-rays incident thereinto substantially in parallel with said base axis to irradiate said X-rays onto the X-ray reflecting surface of said optical reflector; an X-ray source for emitting X-rays toward said optical element; and rotation driving means for rotating said optical reflector and said optical element in a united manner about a rotation axis passing through said optical element and being parallel to the normal line to said illuminated surface; wherein said optical reflector and said optical element are rotated in a united manner by said rotation driving means to irradiate the X-rays emitted from said X-ray source onto said illuminated surface in an arc shape; wherein said optical element comprises: a substrate having a plurality of concave surfaces or convex surfaces with a substantially same curvature; and multilayer films for reflecting X-rays, formed on the concave surfaces or convex surfaces of said substrate and composed of thin films; wherein when X-rays are incident into the multilayer films on said concave surfaces or convex surfaces, the X-rays are reflected with a predetermined diverging angle on the multilayer films and as a result a plurality of secondary X-ray sources having the diverging angle are formed on a same plane located a predetermined distance apart from said concave surfaces or convex surfaces. Also, the present invention provides another optical system for illuminating an illuminated surface in an arc shape. The optical system comprises: an optical reflector having a reflecting surface forming a part of a parabola of revolution obtained by revolving an arbitrary parabola about a base axis passing through a focus of the parabola and inclined at a predetermined angle relative to a normal line to said illuminated surface, said optical reflector reflecting light by said reflecting surface to irradiate said illuminated surface; a fly's eye lens comprising a plurality of lens elements and forming a plurality of secondary light source images on an exit side thereof when light is incident thereinto; and a first light deflecting element for reflecting light emergent from said fly eye lens substantially in parallel with said base axis to irradiate said light onto the reflecting surface of said optical reflector; wherein said optical reflector and said first light deflecting element are rotated in a united manner about a rotation axis passing through said first light deflecting element and being parallel to the normal line to said illuminated surface. Also, the present invention provides another optical apparatus for illuminating a predetermined area. The optical apparatus comprises: an optical reflector having a reflecting surface forming a part of a parabola of revolution obtained by revolving an arbitrary parabola about a base axis passing through a focus of the parabola and inclined at a predetermined angle relative to a normal line to an illuminated surface, said optical reflector reflecting light by said reflecting surface to irradiate said illuminated surface; a fly's eye lens comprising a plurality of lens elements and forming a plurality of secondary light source images on an exit side thereof when light is incident thereinto; a first light deflecting element for reflecting light emergent from said fly eye lens substantially in parallel with said base axis to irradiate said light onto the reflecting surface of said optical reflector; light source means for emitting light toward said fly's eye lens; and rotation driving means for rotating said optical reflector and said first light deflecting element in a united manner about a rotation axis passing through said first light deflecting means and being parallel to the normal line to said illuminated surface; wherein said optical reflector and said first light deflecting element are rotated in a united manner by said rotation driving means to irradiate the light emitted from said light source means onto said illuminated surface in an arc shape. Also, the present invention provides an optical apparatus for illuminating an illuminated surface in an arcuate shape with electromagnetic waves emitted from a light source. The optical apparatus comprises: a first reflective element having a reflective surface with a plurality of concave surfaces or convex surfaces having a substantially same curvature, said reflective surface reflecting the electromagnetic waves incident from said light source to form a plurality of secondary light source images; and an optical reflector for reflecting the electromagnetic waves from said secondary light source images to illuminate said illuminated surface, said optical reflector having a reflective surface forming a part of a parabolic toric surface obtained by revolving an arbitrary parabola about a second axis passing a point located on a first axis passing a vertex of the parabola and a focus of the parabola, said point being opposite to a directrix of the parabola with respect to said focus, said second axis being parallel to the directrix; wherein said first reflective element reflects the electromagnetic waves at a predetermined angle of divergence by the reflective surface thereof when the electromagnetic waves are incident onto the reflective surface of said first reflective element, so that said plurality of secondary light source images with the angle of divergence are formed on a same plane located at a place apart from the reflective surface of said first reflective element. The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention. Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.