Patent Number: 048624900
Section: summary

BACKGROUND OF THE INVENTION This invention relates to x-ray machines, and more particularly, to vacuum windows for x-ray machines. DESCRIPTION OF THE PRIOR ART X-rays can be generated by the bombardment of a metal target by a beam of electrons. By necessity, the target and electron beam are contained within an evacuated chamber for the proper generation and acceleration of the electron beam. X-rays comprise electromagnetic radiation of extremely short wavelength. "Hard" x-rays are generally defined as x-rays with wavelengths shorter than a few Angstroms, while "soft" x-rays have wavelengths of tens of Angstroms or more. For example, carbon K-alpha x-rays have wavelengths of approximately 44 Angstroms, and, thus, are soft x-rays. There is a class of analytical machines which utilize x-rays to determine the composition and structure of substances. These machines direct a beam of x-rays towards a sample of the substance, and then detect the resultant scattering, reflection, and absorption of the beam with a number of x-ray detectors surrounding the sample. Since different samples have different x-ray scattering, reflecting, and absorbing characteristics, the chemical nature and structure of the sample can be determined by an analysis of the data gathered by the x-ray detectors. Hard x-rays can be used to analyze the composition and structure of matter having relatively high atomic mass. The hard x-rays are formed within the evacuated chamber and are then beamed out of the chamber through a "vacuum window" and into the sample to be tested. The vacuum window must, therefore, be capable of withstanding continuous x-ray bombardment and a pressure differential of approximately one atmosphere. These prior art, hard x-ray vacuum windows are typically made from a thin, metal foil approximately 50 micrometers thick and having an atomic number (Z) less than 14. Light elements such as hydrogen or oxygen cannot be detected with hard x-rays because they tend to ionize and otherwise react with the x-rays. Therefore, lower energy, soft x-rays would have to be used to detect light elements. Unfortunately, soft x-rays are not sufficiently energetic to adequately penetrate prior art vacuum windows. For example, a prior art vacuum window which can pass a significant percentage of incident hard x-rays may only pass a fraction of a percent of incident soft x-rays. One theoretical solution to this problem is to place the sample within the evacuated chamber where the soft x-rays are generated. Unfortunately, this immediately eliminates the possibility of analyzing gaseous samples, since the presence of the gas would destroy the vacuum within the chamber. This solution would, therefore, be limited to the analysis of non-volatile, solid samples which could be safely placed within an evacuated chamber. Even so, this arrangement would be expensive and cumbersome, since it would require an enlarged vacuum chamber, special high-vacuum detectors, portholes, larger vacuum systems, etc. As noted above, most prior art hard x-ray windows are made from thin, metal foil. Another type of hard x-ray window is described by Smith, et al. in "Prospects for X-Ray Fabrication of Si IC Devices", Journal of Vacuum Science Technology, Vol. 12, No. 6, November/December, 1975. In their paper, Smith, et al, describe a unitary vacuum window structure made from a silicon wafer which includes an annular perimeter, a number of parallel ribs, and a thin silicon membrane supported by the perimeter and reinforced by the ribs. While the vacuum window of Smith, et al would appear to be satisfactory for use in hard x-ray applications, it would not be possible to make the membrane portion of the window structure thin enough to pass a significant proportion of soft x-rays and still hold one atmosphere of pressure. This is due, in part, to the physical limitations of silicon for this purpose, and is also due to a weakening of the silicon membrane caused by the etching process, which tends to produce pits, grooves, and pinholes. It is also known from the prior art to make electron permeable vacuum windows from thin membranes of SiC, BN, B.sub.4 C, Si.sub.3 N.sub.4 and Al.sub.4 C.sub.3, see U.S. Pat. Nos. 4,468,282 and 4,494,036. SUMMARY OF THE PRESENT INVENTION An object of this invention is to produce a practical x-ray vacuum window which is relatively transparent to soft x-rays. Another object of this invention is to provide a method for producing a soft x-ray vacuum window. Briefly, the invention includes a support substrate provided with an aperture, and a membrane formed over the support substrate. The membrane includes a window portion aligned with the aperture having a number of thin pane sections which are relatively transparent to soft x-rays. The thin pane sections are supported and reinforced by a number of relatively thick rib sections attached to a perimeter portion of the membrane. The substrate should be made from a material having a low atomic number but high tensile strength. Three materials which are suitable for the formation of the membrane of the present invention are boron nitride, boron carbide and silicon carbide. The method in accordance with the present invention for making a soft x-ray vacuum window includes the steps of: growing a thick, boron nitride membrane on both sides of a silicon wafer; patterning the boron nitride on one side of the silicon wafer to form a window aperture pattern; patterning the boron nitride on the other side of the silicon wafer to form a number of pane openings; depositing a thin layer of boron nitride over the pane openings; and etching a window aperture into the back of the silicon wafer through the window aperture pattern. Since the pane sections are formed by deposition rather than by etching, they are virtually defect-free and have great structural integrity. An advantage of the present invention is that vacuum windows for x-ray machines can be produced which permit the transmission of soft x-rays. Another advantage of this invention is that light elements such as hydrogen and oxygen can be detected without placing them inside of a vacuum environment. These and other objects and advantages of the present invention will be apparent to those skilled in the art after reading the following descriptions and studying the various figures of the drawing.