Patent Number: 051596210
Section: summary

FIELD OF THE INVENTION AND RELATED ART This invention relates generally to X-ray lithography or other X-ray technology and, more particularly, to an X-ray transmitting window with an X-ray transmitting film, which serves as a vacuum partition wall, allowing transmission therethrough of X-rays from a vacuum ambience to a non-vacuum ambience. In another aspect, the invention is concerned with a method of mounting such an X-ray transmitting window. FIG. 8 shows a known example of an X-ray transmitting window, wherein an X-ray transmitting film 81 made from a beryllium sheet, for example, is gas-tightly fixed to a connecting member 82 which in turn is gas-tightly fixed to an inside cylindrical surface of a ring frame member 83. For the connection, silver brazing, electron beam welding, diffusion welding or the like is usable. This X-ray transmitting film 81 can serve as a vacuum partition wall, when fixed by bolts at bores 84 formed in the ring frame member 83. SUMMARY OF THE INVENTION With this example, however, it is necessary to use a specific large ring frame member in addition to ordinary vacuum flange means, resulting in increased cost and heavy weight. On the other hand, in order to avoid oxidation and resultant damage of an X-ray transmitting film, it is desirable to keep the X-ray transmitting film in a gas-tight casing. With the structure shown in FIG. 8, however, because of the large size of the ring frame member, only a limited number of X-ray transmitting films can be accommodated in the casing of a particular size. Further, in such case, there is a high possibility of breakage of the connection between the X-ray transmitting film and the connecting member or the connection between the connecting member and the ring frame member. This causes an inconvenience of small vacuum leakage or damage of the X-ray transmitting film. It is accordingly an object of the present invention to provide an X-ray transmitting widow structure which is light in weight and small in size, but which assures gas-tightness positively. It is another object of the present invention to provide a method of mounting such an X-ray transmitting window structure. In accordance with an aspect of the present invention, there is provided an X-ray transmitting window for use in X-ray lithography, for allowing transmission therethrough of X-rays from a vacuum ambience to a different ambience, wherein the window includes an X-ray transmitting film; and a gasket material gas-tightly provided on at least one of opposite surfaces in a peripheral portion of the X-ray transmitting film, the gasket material having a Brinell hardness smaller than that of the X-ray transmitting film. The formed X-ray transmitting window is able to be sandwiched and fastened between a pair of flanges gas-tightly. In accordance with another aspect of the present invention, there is provided an X-ray transmitting window structure having a function as a vacuum partition wall device, for allowing transmission therethrough of X-rays from a vacuum ambience to a different ambience, wherein said structure comprises an X-ray transmitting film; an outer frame member for gastightly covering peripheral edge portions of opposite sides of said X-ray transmitting film; and a pair of flange means for sandwiching and fastening therebetween said outer frame member, said flange means being made of a material having a Brinell hardness greater than that of said outer frame member. An X-ray transmitting window to which the present invention pertains should have a higher X-ray transmissivity. This means that an X-ray transmitting film should be thinner, in this respect. On the other hand, since the X-ray transmitting window serves as a vacuum partition wall between a vacuum ambience and a different, non-vacuum ambience, there exists a pressure difference across the X-ray transmitting film. This applies a tensile stress to the X-ray transmitting film. If the tensile stress becomes larger than the breaking stress of the film, the film is broken. In this respect, the X-ray film should have a thickness sufficient for avoiding breakage by the pressure difference. Where a differential pressure p is applied to the opposite surfaces of a very thin film made of a material having a Young's modulus E and a large flexure is caused thereby, if the thickness of the film is h, then the tensile stress .sigma. at the center of the film of a circular shape is expressed as follows: ##EQU1## where S is the area of the film and S=.pi.a.sup.2 (wherein a is the radius). Also, the tensile stress at the center of a film of a square shape is given by: ##EQU2## The tensile stress of the film at the center of a film where it has an elliptical shape or a oblong shape, is slightly smaller than that of a circular or square film. From the above two equations, it is seen that a smaller tensile stress is attainable by increasing the film thickness. Also, it is attainable by reducing the area of the film. In consideration thereof, an X-ray transmitting film of the X-ray transmitting window of the present invention should desirably have a size (area) small enough but sufficiently large for attaining the function as an X-ray transmitting window. With regard to the film thickness, the film should desirably be as thin as possible but of a thickness sufficiently large to prevent overcoming of the tensile stress beyond the breaking stress. As an example, the present invention is suitably applicable to an X-ray transmitting window structure in an X-ray lithographic exposure apparatus. Such an X-ray exposure apparatus is suited for manufacture of integrated circuit devices of 64 megabit DRAMs having an expected chip size of 10.times.20 (mm.sup.2). Since an X-ray transmitting window is disposed closer to a light source side than a mask is and, generally, X-rays from the light source are divergent, there is a necessity that at the X-ray transmitting window position an X-ray transmission area slightly smaller than the area of 10.times.20 (mm.sup.2) is defined. Further, in future X-ray lithography for a 4 gigabit DRAM having an expected chip size of 30.times.60 (mm.sup.2), a required X-ray transmission area at the X-ray transmitting window position will be slightly smaller than the chip size. On the other hand, in X-ray lithography, there is a possibility that the X-ray transmitting film is scanningly moved along a one-dimensional direction, for effective expansion of the X-ray transmission area. In such case, in order to assure X-ray irradiation of the whole area of the chip size corresponding to the X-ray irradiation region, an X-ray transmitting window having an X-ray transmission area with a length at either side sufficiently covering only one of the longitudinal side and the transverse side of the chip size, may be prepared and such X-ray transmitting window may be scanningly moved along the longitudinal side or the transverse side by a distance corresponding to the length thereof. In such occasion, by making the scanning distance shorter than the length of the one side of the chip size, it is possible to provide an X-ray transmitting window of increased strength. Since the one side of the chip size has a length in a range of 10-60 mm, the longitudinal length of the X-ray transmitting area should be not less than 10 mm. On the other hand, if the X-ray transmitting area has a size larger than a required, the strength of the X-ray transmitting window decreases and, in this case, the thickness of the X-ray transmitting film may be made larger to assure a sufficient strength resistive to breakage. However, making the film thickness so large is not desirable. It is therefore preferable that the longitudinal length of the X-ray transmitting area is made not greater than 60 mm. Where an X-ray transmitting window is not scanningly moved, clearly it is necessary to define at the window position an X-ray transmitting area of a size allowing transmission of X-rays for exposure of the chip size. Thus, the longitudinal length should be in a range of 20-60 mm. In summary, it can be stated that an X-ray transmitting window should preferably have an X-ray transmitting area of a size ranging from 10 mm to 60 mm. In an X-ray transmitting window structure according to one aspect of the present invention, an outer frame member adjoining to an X-ray transmitting film is sandwiched and fastened between a pair of flanges each having a Brinell hardness larger than that of the outer frame member. Thus, when fastened between the flanges, there occurs plastic deformation of the outer frame member, causing intimate contact of the outer frame member with the flanges. Thus, high gas-tightness is ensured. In another aspect of the invention, each flange is provided with a seal edge (sealing projection) which bites into an opposed outer frame member when the latter is sandwiched and fastened between the flanges. This effectively improves the sealing property. Further, where the material of the outer frame member has a Brinell hardness smaller than that of the X-ray transmitting film, the fastening of the outer frame member between the flanges with resultant deformation of the outer frame member or with resultant biting of the seal edge of the flange into the outer frame member, does not cause deformation of the X-ray transmitting film itself which otherwise results in damage of the strength thereof. 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.