Patent Number: 060977906
Section: summary

FIELD OF THE INVENTION AND RELATED ART This invention relates to a pressure partition for use as an X-ray window in an exposure chamber of an X-ray exposure apparatus, for example, for mutually isolating two ambiences of different pressures, and also to an X-ray exposure apparatus using the same. In X-ray exposure apparatuses to be used with exposure light comprising synchrotron radiation such as synchrotron X-rays from a charged particle accumulation ring, for example, a high vacuum should be maintained in a beam duct extending from a light source to an exposure chamber in order to reduce attenuation of the X-rays by the atmosphere. To this end, an X-ray extracting window (pressure partition) is provided to mutually isolate two ambiences, that is, the high vacuum beam duct and the exposure chamber of an ambience approximately the same as the atmosphere or of a low vacuum or reduced pressure ambience of helium gas, for example. Such an X-ray window comprises a thin film of a material of a high X-ray transmissivity, such as beryllium, silicon nitride, silicon carbide, and diamond, for example. The thin film should have a high X-ray transmissivity in order to avoid loss of X-ray energy, and also it should have mechanical strength sufficient to bear the pressure difference between the beam duct and the exposure chamber. FIG. 6 shows a known type X-ray window E.sub.0 which comprises a beryllium film 111 of a few microns or several tens of microns in thickness and a flange 112 for supporting the outside periphery of the film. The outside periphery of the beryllium film 111 is fixed to the flange 112 by means of a bonding ring 113. The flange 112 is fixedly connected to a flange 102a of a beam duct 102 by means of an O-ring 114 and bolts 115. As described, the beryllium film 111 should have mechanical strength sufficient to bear a pressure difference .DELTA.P between a pressure P.sub.1 of the beam duct 102, being kept in a high vacuum, and a pressure P.sub.2 of the exposure chamber, being kept in a reduced pressure of helium gas. Additionally, it should have a high X-ray transmissivity. Thus, it is desirable to reduce the thickness of the beryllium film 111 as much as possible, within the limit of the required mechanical strength. Generally, when a pressure difference p is applied to a very thin film of a material having a Young's modulus E and a large deflection or flexure is produced, and if the thickness of the film is h, the tension stress .sigma.(r=0) at the center of the film of a circular shape with a radius a and the tension stress .sigma.(r=a) at the outside peripheral edge of that film can be calculated in accordance with the following equations: ##EQU1## Thus, usually, the thickness of the beryllium film 111 is determined so that the tension stress .sigma..sub.1 at the center of the film 111 as the flexure is produced in the film 111 by the applied pressure difference .DELTA.P, does not exceed the breaking stress. More specifically, if the breaking stress of the beryllium film 111 is .sigma..sub.0 and the tolerance for the pressure difference .DELTA.P related to the beryllium film 111, that is, the design pressure, is P.sub.0, then the necessary thickness T can be expressed from equation (1) as follows: EQU T&gt;0.275.multidot.P.sub.0 .multidot.a/.sigma..sub.0 .multidot.(E/.sigma..sub.0).sup.1/2 (3) In practical design of an X-ray window, a safety factor A is taken into account and the thickness T.sub.0 of the beryllium film 111 is determined in accordance with the following equation: EQU T.sub.0 =A.multidot.0.275.multidot.P.sub.0 .multidot.a/.sigma..sub.0 .multidot.(E/.sigma..sub.0).sup.1/2 (4) Actually, however, even if the thickness of a beryllium film is determined in accordance with equation (4), there is a possibility that the beryllium film is broken by a pressure difference lower than the design pressure P.sub.0. It is, therefore, necessary to design the thickness with a larger safety factor A. Breakage of a beryllium film with a pressure difference lower than the design pressure P.sub.0 may appear at its contact portion with the flange which fixes the outside periphery of the film. Such breakage cannot be prevented simply by rounding the corner of the flange. If the safety factor A is made large, the beryllium film should then have a larger thickness and, thus, the X-ray transmissivity cannot be large. This causes a large loss of energy and a reduction of productivity of semiconductor products. SUMMARY OF THE INVENTION It is accordingly an object of the present invention to provide a pressure partition which enables a design with sufficient mechanical strength and with a very small thickness. It is another object of the present invention to provide an X-ray exposure apparatus or a device manufacturing method which uses a pressure partition as described above. In accordance with an aspect of the present invention, there is provided a pressure partition, comprising: a thin film for dividing a predetermined space into two spatial zones; and supporting means for supporting said film at an outside peripheral portion thereof, said supporting means having a curvature support for producing curvature in the outside peripheral portion of said film, along a predetermined curvature plane. The curvature support may have a curvature radius which is larger than the value of the curvature radius to be defined when a tension stress .sigma.f to be produced at the outside peripheral portion of said film by flexure thereof along said curvature support becomes equal to a difference between tension stresses .sigma..sub.1 and .sigma..sub.2, to be produced at the center portion and the outside peripheral portion of the film, respectively, by deflection of the film due to a pressure difference between the two spatial zones. Said curvature support of said supporting means may have a curvature radius which increases toward the center of said film. The thickness of the thin film for bisecting the predetermined space may be designed so that, as deflection is produced due to a difference of pressures applied to both faces of the film, it bears the tension stress to be produced at the center of the film. The tension stress produced at the center of the film by flexure of the film due to the pressure difference is larger than the tension stress at the outside peripheral portion of the film. However, since a tension stress produced by bending or flexure of the film, caused by contact with the supporting means, is additionally applied to the outside peripheral portion of the film, there is a possibility that a tension stress larger than that at the center is applied to this portion. In consideration of this, a curvature support for supporting the outside peripheral portion of the film along a curved surface having a large curvature radius may be provided, to reduce the tension stress to be produced by flexure due to contact with the supporting means. If the tension stress to be produced at the outside peripheral portion of the film does not exceed the tension stress at the center thereof and if the largest value of tension stress of the whole film corresponds to the tension stress at the center thereof, it is riot necessary to use an unnecessarily large safety factor in the film thickness design as has been described above. This enables a further reduction of the film thickness and enlargement of the X-ray transmissivity. Where such a pressure partition is used as an X-ray window, the efficiency of X-ray utilization can be improved and the productivity of an X-ray exposure apparatus can be enlarged.