Patent Number: 051611791
Section: summary

FIELD OF THE INVENTION This invention relates to a beryllium window incorporated in an x-ray radiation system and, more particularly, to the structure of the beryllium window less deformative against heat stress in a process of fabrication thereof. DESCRIPTION OF THE RELATED ART A typical example of the x-ray aligner is illustrated in FIG. 1 of the drawings and largely comprises an electron gun 1, a target holder used for retaining a palladium target 2 and a wafer holder where a semiconductor wafer 4 is mounted. When the electron gun 1 emits electron beams towards the palladium target 2, an x-ray radiates from the palladium target 2 through a beryllium window 5. If the wafer holder 3 is located under the beryllium window 5, the semiconductor wafer 4 is exposed to the x-ray, and a pattern is transferred from an x-ray mask (not shown) to the wafer 4. The beryllium window 5 is provided between a target chamber 6 and an exposed space (not shown) for partitioning, and a beryllium thin film is presently the most appropriate in view of the transmittance of x-ray and of the mechanical strength of the window. However, a thin beryllium plate can not be directly attached to the inner wall of the x-ray aligner, and a reinforcing frame (not shown) is used for the attachment. Namely, the thin beryllium plate is directly bonded, brazed or welded to the reinforcing frame, and the reinforcing frame is attached to the inner wall of the x-ray aligner. An adhesive compound is provided between the thin beryllium plate and the reinforcing frame, and an electron beam is usually used in the welding. The beryllium window thus fabricated is, thereafter, baked in vacuum at 200 degrees to 400 degrees in centigrade. Following problems are encountered in the prior art beryllium window depending upon the way of fixing. Namely, if the thin beryllium plate is bonded to the reinforcing frame with an adhesive compound, the beryllium window is less resistive against heat and, accordingly, damaged in the baking treatment in vacuum. The brazing exposes the thin beryllium plate to heat at 800 degrees to 1000 degrees in centigrade, and the heat deteriorates the mechanical strength of the thin beryllium plate. If the thickness is increased, the beryllium plate may well withstand. However, such a thick beryllium plate is less transmissible for the x-ray. Thus, there is a trade-off between the mechanical strength and the transmissibility of the beryllium plate brazed to the reinforcing frame. The welding with an electron-beam tends to deform the beryllium plate due to heat produced by the strong electron beam. If the beryllium plate is decreased in thickness so as to improve the transmissibility, the heat deformation becomes serious. SUMMARY OF THE INVENTION It is therefore an important object of the present invention to provide a beryllium window which is free from heat attack. It is also an important object of the present invention to provide a process of fabricating a beryllium window well withstanding heat. To accomplish these objects, the present invention proposes to weld a beryllium plate to a reinforcing member through a diffusion welding. In accordance with one aspect of the present invention, there is provided a beryllium window comprising a) a beryllium plate containing beryllium as an essential element, b) a reinforcing unit for retaining the beryllium plate, and c) a welding film merged into a part of the beryllium plate and a part of the reinforcing unit and formed of a substance having at least one element selected from the group consisting of silver, gold, nickel and copper, the reinforcing unit being formed of a substance allowing the at least one element to diffuse thereinto. In accordance with another aspect of the present invention, there is provided a process of fabricating a beryllium window comprising the steps of: a) preparing a beryllium plate containing beryllium as an essential element, a welding substance containing at least one element selected from the group consisting of silver, gold, nickel and copper, and a reinforcing unit formed of a substance allowing aforesaid at least one element to diffuse thereinto; b) forming a film of the welding substance on a part of the beryllium plate; and c) welding the beryllium plate to the reinforcing unit through a diffusion of the welding substance into the beryllium plate and the reinforcing unit. In accordance with still another aspect of the present invention, there is provided a process of fabricating a beryllium window comprising the steps of: a) preparing a beryllium plate containing beryllium as an essential element, a welding substance containing at least one element selected from the group consisting of silver, gold, nickel and copper, and a reinforcing unit formed of a substance allowing aforesaid at least one element to diffuse thereinto; b) forming a film of the welding substance on a part of the reinforcing unit; and c) welding the beryllium plate to the reinforcing unit through a diffusion of the welding substance into the beryllium plate and the reinforcing unit.