Patent Number: 050349724
Section: summary

REFERENCE TO RELATED APPLICATIONS This application claims the priority of Federal Republic of Germany application Serial No. P 39 20 788.9 of June 24th, 1989, which is incorporated herein by reference. BACKGROUND OF THE INVENTION The present invention relates to a method of producing thin sheets which are stretched over a frame and have a planar and smooth surface with a low defect density and to the use of such sheets for the production of X-ray masks as defined in the preamble of claim 1. The periodical Solid State Technology (SSTEAP 27 (9)), September 1984, pages 192-199, illustrates and describes, on page 194, in an article by A. R. Shimkunas, entitled "Advances in X-Ray Mask Technology," a method of manufacturing an X-ray mask blank of boron nitride. In this method, a silicon wafer is initially coated with boron nitride on all sides. Then the central region of the rear side is etched away until the silicon is exposed and the wafer is connected with a Pyrex ring. After being coated with polyimide and metallization of the frontal face, the silicon is etched away from the exposed rear side so that a mask blank results in which a membrane is stretched which can be processed further into an X-ray mask. However, this method can be employed only for such sheet or membrane materials which are not attacked by the etching medium. Titanium, for example, is also etched away by conventional etching media for silicon. The periodical, Journal of the Electrochemical Society, Volume 128, No. 5, May, 1981, pages 1116-1120, discloses in an article by W. D. Buckley, J. F. Nesler and H. Windischmann, entitled "X-Ray Lithography Mask Technology," on pages 1117 and 1118, a method of producing a titanium membrane in which absorber structures of gold are applied to the membrane by galvanoplastic means. To accomplish this, a glass substrate is initially provided with a thin layer of gold onto which the titanium is then applied. The titanium membrane is connected with a ring and is mechanically separated from the glass substrate. Then, the membrane is stretched and connected with a supporting ring. The free stretching of the membrane, however, is a difficultly managed procedure. Moreover, in this method, the gold layer remains adhering to the titanium membrane so that the X-ray contrast with reference to the absorber structures is reduced. A similar method is disclosed in European published application EP-B1-0,104,685 corresponding to U.S. Pat. No. 4,701,391. SUMMARY OF THE INVENTION It is an object of the present invention to modify a method of the above type so that the sheet retains the stretched state impressed during its production for the subsequent method steps, with the sheet material being freely selectable within wide limits. The above object is generally achieved according to the present invention by a method of producing a thin sheet or membrane of material which is stretched over a frame and which has a planar and smooth surface with a low defect density, which method is of the type wherein the sheet material is applied to a substrate through the intermediary of a release layer, whereupon the sheet is connected with the frame and thereafter is separated from the substrate; and wherein: (a) during the application of the release layer to the substrate, an area along the edge of the substrate is left free of the release layer material but is covered by the subsequently applied sheet material; PA1 (b) the frame is connected with the sheet within a region occupied by the release layer, and the sheet is severed around the periphery of the frame; and PA1 (c) the sheet with the attached frame are released from the substrate and any material of the release layer remaining on the sheet is removed. Preferably, carbon is employed as the material for the release layer since it is easily released from the substrate and any remaining carbon residues on the sheet can be simply and cleanly removed in an oxygen plasma without adversely affecting the sheet surface since the sheet surface need not come in contact with any etching medium. It is further proposed to produce X-ray masks by establishing the absorber structures on the sheet after step (a) and then to perform steps (b) and (c). This facilitates manipulation since the thin sheet is supported by the relatively stable substrate during this process. One embodiment of the invention will now be described below with reference to the drawing.