Patent Number: 056235291
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exposure apparatus which uses synchrotron radiation for, for example, manufacturing devices. 2. Description of the Related Art An X-ray exposure apparatus has been proposed in which a mask pattern is transferred onto a wafer in close proximity thereto by exposure using X-rays as illumination light in the manufacture of semiconductor integrated circuits having a high degree of integration. In the manufacture of semiconductor integrated circuits, the exposure process is performed in a plurality of lines in order to increase the productivity, and a plurality of the same X-ray masks are prepared. The pattern to be formed on a mask is generally formed by a method of drawing by an electron beam drawing apparatus. However, the time required for drawing a circuit pattern having a degree of integration on a 256 Mbit-DRAM scale is several minutes, and sometimes reaches several hours, and much time is thus required for forming a plurality of masks. On the other hand, there is known a method of duplicating an original pattern by proximity X-ray exposure, or life-size exposure, using an original mask formed by the electron beam drawing apparatus to form a plurality of X-ray masks. The X-ray exposure apparatus can employ a wide exposure region, and exhibits a shorter time required for exposure than that in electron beam drawing. Thus, this method can significantly improve the productivity of masks. In order to absorb X-rays, an absorber pattern made of gold or the like and having a thickness of about 0.7 .mu.m is formed on the X-ray mask by plating or etching. Although the electron beam drawing method uses a multi-layer photosensitive material for obtaining a thick absorber layer, the proximity X-ray exposure method can expose a photosensitive material having a sufficient thickness because a resolvable depth is large, and can employ the exposure process using a single layer of photosensitive material. The X-ray exposure method can therefore sometimes form X-ray masks at a cost lower than that in the electron beam drawing method. Considering the above advantageous conditions, attention is paid to the method of forming X-ray masks for wafer exposure by using the X-ray exposure apparatus. SUMMARY OF THE INVENTION In proximity exposure, the parallelism of illumination light significantly affects the distortion caused in transfer. Namely, the illumination light is applied to portions of an exposure region at different incident angles unless the illumination light is not completely parallel, and a deviation between a mask image and a transferred image is represented by a value obtained by multiplying the incident angle by a gap dimension between a mask and a substance to be exposed. If the gap dimension is not constant, therefore, a deviation between the mask image and the transferred image is not constant. In the X-ray exposure apparatus which uses a synchrotron radiation source as an illumination light source, for example, when the mask is disposed at a distance of about 10 m from the light source in order to ensure an exposure region of about 30 mm, illumination light having a divergence angle of 3 mrad can be obtained. In this case, if the setting precision of the gap is, for example, 3 .mu.m, a deviation of 9 nm occurs between the mask image and the transferred image. When the wafer is exposed, for example, since the superposition precision required for exposure with a resolution of 0.2 .mu.m is about 50 nm, a deviation of 9 nm between images is permissible. However, the precision is a problem when a mask is duplicated. Although the sensitivity of a photosensitive material to X-rays is several tens of mJ/c, the illumination light in an amount of about 2 times the sensitivity of the photosensitive material is applied to the masks in view of the energy of illumination light absorbed by a mask substrate. Most of the illumination light absorbed by the mask is converted into thermal energy and transmitted to the wafer through gases present between the mask and the wafer. The thermal energy transmitted from the mask to the wafer and the thermal energy generated by the illumination light absorbed by the photosensitive material and the wafer are dissipated to a wafer chuck which adheres to the wafer. However, with a low heat transfer rate of such a heat transfer passage or a high intensity of illumination light, thermal distortion occurs due to a temperature rise of the wafer and the mask during exposure, thereby causing deterioration in resolving power. In the X-ray exposure apparatus, therefore, means such as cooling means or the like is provided on the wafer chuck, and the intensity of the illumination light is increased within a permissible range of resolving power so that the productivity is increased. When a mask is duplicated by using such an X-ray exposure apparatus, there are not only the demand for resolving power higher than that for exposure of the wafer but also structural conditions more disadvantageous than those in exposure of the wafer. A mask substrate to be exposed has a portion which is previously back-etched for forming a pattern therein. Since this portion of the substrate generally has a thickness of about 1 to 2 .mu.m, the back of the portion is difficult to adhere directly to the chuck. The heat transfer rate of the heat transfer passage to the chuck is thus lower than that in exposure of the wafer. This increases the thermal distortion with the same intensity of the illumination light, and makes it difficult to obtain higher resolving power than that for exposure of the wafer. It is an object of the present invention to solve the above problems, an object which is achieved by providing an SOR exposure system and exposure apparatus which are suitable for duplicating X-ray masks and which enable formation of masks with high precision and at low cost. In accordance with a preferred embodiment of the present invention, an SOR exposure system comprises a plurality of X-ray exposure apparatus connected to beam lines extended from a common SOR light source, wherein an exposure apparatus which can be used for duplicating masks is connected to at least one beam line. In accordance with another preferred embodiment of the present invention, a duplicate mask duplicated by transferring a pattern by proximity exposure using an original mask has a pattern having a higher aspect ratio than that of the original mask. In accordance with a further preferred embodiment of the present invention, an exposure apparatus for manufacturing duplicate masks comprises a lock mechanism for connecting first holding means for an original mask and second holding means for a duplicate mask substrate so as to prevent relative movement between the original mask and the duplicate mask substrate during exposure transfer. Other objects and embodiments of the present invention will be made clear in the description of the preferred embodiments below.