Patent Number: 049892266
Section: summary

BACKGROUND OF THE INVENTION This invention relates to a method of producing curvature on the surface of a substrate through selective removal of material from the substrate and selective application of stress-producing materials to the substrate. The use of lenses and mirrors to focus and direct visible light and other electromagnetic radiation is well established. Conventional lenses and mirrors, however, are not effective for focusing or directing electromagnetic radiation having short wavelengths, such as x-rays and short-wave ultraviolet rays. It is known that short wavelength radiation is strongly reflected from reflective surfaces if the angle of incidence to the surfaces is low, for example, less than five degrees for one nanometer (nm) or shorter wavelength x-rays. Employing such so-called grazing incidence techniques, however, is only effective if suitably uniform and smooth reflective surfaces can be found, and this has proved difficult to do. Use of conventional polished or mirrored surfaces, prepared using known grinding and polishing techniques, generally does not provide the desired control and accuracy in reflecting short wave radiation. One approach to achieving better control and accuracy in reflecting x-rays, even at larger angles of incidence, is to use so-called Bragg reflection--reflection of radiation from planes of a crystal. The drawback of this has, until recently, been that only very short wavelength x-rays (less than one nanometer) could be reflected. Recently, however, multi-layered thin film structures have been employed for reflecting longer wavelength x-rays. Such reflection occurs at the interfaces of adjacent films having different refractive indices. The layers can be selected so that reflected waves add constructively to produce a strong total reflection. In order to focus electromagnetic radiation using reflection, it is necessary to provide an appropriate curved surface capable of reflecting the radiation. Currently used focusing devices are constructed either by lathing, grinding or otherwise abraiding a surface of a reflector element into a curved surface, or by casting a reflector element on a curved mandrel. Among the problems with these approaches are the difficulty of obtaining desired curvatures, the roughness of the resulting curved surface, and the time required to prepare the elements. Polishing, of course, would be performed to smooth out the surface, but remaining irregularities would preclude or discourage use of the surface for focusing shorter wave radiation such as x-rays. SUMMARY OF THE INVENTION It is an object of the invention to provide a method of producing a smooth curved surface of a desired shape suitable for, but not limited to, use as an optical focusing blank for short wavelengths such as x-rays. It is also an object of the invention to provide such a method which is relatively easy and inexpensive to carry out. It is a further object of the invention to provide such a method wherein the degree of curvature can be precisely controlled. It is another object of the invention to provide a precision focusing structure for radiant energy. The above and other objects are realized in a specific illustrative embodiment of a method of producing curvature on a working surface of a substrate. The substrate, which may be a crystalline or amorphous material, has a working side and surface, and a second side and oppositely facing surface from which substrate material is removed according to a predetermined pattern. The removal of material may be carried out by etching or even by cutting or abraiding with a cutting or abrasive tool. A stress producing film is applied to at least one of the surfaces to cause the substrate to bend and produce the desired curvature in the working surface. The amount and shape of the curvature is determined by the substrate thickness at the bottom of the removed material pattern, the shape of the areas from which material is removed, and the stresses (dependent in part on the thickness) of the film. The film may be selected to produce tensile stress or compressive stress on the substrate, and placed on either or both sides of the substrate to cause the desired bending. Although the above discussion concerned the use of the curved surface for focusing electromagnetic waves, the curved surface structure might also be used as molds for machinery, tools, etc., as specially shaped support structure for electrical circuitry, as electromagnetic radiation detectors, or as diffraction gratings.