A typical example of the x-ray aligner is illustrated in FIG. 1 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 toward the palladium target 2, and 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 exposure space (not shown) for partitioning and has a thickness of, for example, 50 microns. The target chamber 6 is maintained in a vacuum condition, and, on the other hand, the exposure space is in the atmospheric ambient, so that the beryllium window 5 is subjected to a substantial amount of force due to the difference in pressure between the target chamber 6 and the exposure space. If the beryllium window 5 is too thin to support the force, the beryllium window 5 is broken. On the other hand, when the beryllium window 5 is too thick, the x-ray is partially absorbed by the beryllium window 5, and, accordingly, a relatively small amount of the x-ray participates in the pattern transfer. Thus, there is a trade-off between the mechanical strength and the pattern transferring capability. In general, the larger the pattern transferring capability, the higher is the performance of the x-ray aligner system. Thus, research and development efforts have been directed towards fabricating a beryllium plate member having large mechanical strength.
One of the prior art processes is classified as powder metallurgy, and another is a vacuum evaporation technique. The former process sequence starts with melting down beryllium ingot to produce beryllium bulk. The beryllium bulk is pulverized into powder which in turn is subjected to a cold working or pressing. After the cold pressing, a hot vacuum pressing is carried out, followed by a hot rolling.
The later process firstly places a copper plate in a vacuum chamber, and a beryllium alloy is, then, deposited on the copper plate to coat with the thin film. After the deposition, the copper plate is removed by using, for example, nitric acid, so that the thin plate of the beryllium alloy is left thereafter.
However, a problem is encountered with respect to the former process with respect mechanical strength. This is because of the fact that beryllium particles are oxidized during the pulverization by using, for example, a ball mill and, accordingly, covered with undesirable oxide films. This results in a reduction in purity, which is causative of lower mechanical strength. The later process also has a problem with respect to mechanical strength. Since a plurality of bubbles or hollow spaces are much liable to be produced in the thin beryllium film during the vacuum evaporation stage, the thin film is relatively small in density and, accordingly, in mechanical strength.