Patent Number: 050349724
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS According to FIGS. 1A and 1B, the substrate 1 is composed of an extremely planar and smooth wafer, preferably silicon, having a thickness of 0.5 mm and a diameter of 100 mm. As shown in FIGS. 2A and 2B a release layer 2, preferably of carbon, is sputtered or vapor-deposited onto one surface side of this wafer 1 to a thickness of about 100 nm in a manner such that surface region (an annular surface region) of a width of about 1 mm along the edge of the substrate is left free of carbon. Thereafter, as shown in FIGS. 3A and 3B, a sheet 3 of a desired material, e.g. titanium, is sputtered onto the side of wafer 1 coated with the carbon layer 2 to a thickness of about 2 to 3 .mu.m so that the sheet 3 also covers the carbon-free annular surface 1a along the edge of the wafer 1. Thus the sheet material 3 adheres to the silicon wafer 1 in the edge region 1a where the wafer surface is free of release material (carbon) 2 and thus retains its internal tensioned state as defined by the manufacturing conditions (particularly temperatures, type and pressure of the operating gas, power of the HF field, tempering temperature and duration). The thus tensioned sheet 3 is connected, for example by gluing, with a stable frame 4, e.g. of Pyrex and preferably in the form of a ring as shown in FIGS. 4A and 4B. When seen in cross section, frame 4 lies within the region occupied by the release layer 2. By scoring or etching, sheet 3 is severed around the periphery of frame 4 so that the sheet portion 3a stretched over frame 4 rests only on the carbon release layer 2, as can be seen in FIGS. 5A and 5B. Since the carbon of the release layer 2 adheres only slightly to the surface of the silicon wafer 1, sheet portion 3a together with frame 4 can be mechanically released from the silicon wafer 1 as shown in FIGS. 6A and 6B. Any remainder of carbon from release layer 2 still on sheet 3a can be removed by means of an oxygen plasma so that a sheet or membrane 3a results which is freely stretched over the frame 4 and has an extremely planar and smooth surface with a low density of defects as shown in FIG. 7. Since the carbon can be easily separated from the substrate 1 as well as from the sheet 3, the above mentioned prior art etching method is no longer required so that the material of sheet 3 can be freely selected within wide limits. The thus produced framed sheet of FIG. 7, also called a mask blank, can now be further processed into an X-ray mask according to know methods by providing X-ray absorber structures on the surface of the sheet 3a within the area defined by the frame 4. However, these methods of producing the absorber structure can also be integrated into the method steps of the invention in that, as shown in FIG. 8, the absorber structures 5 required for the X-ray mask are produced on the surface of sheet 3, for example by known lithographic-galvanoplastic processes, immediately after application of the sheet material layer 3 to the surface of the substrate wafer 1 coated with the release layer 2. That is, the absorber structures are applied to the surface of the sheet 3 within the area to be surrounded by the frame 4 prior to the attachment of the frame 4 as shown in FIGS. 4A and 4B. For the production of several smaller mask blanks on a joint large-area substrate, it is also possible to leave, for example, a honeycomb area of surfaces that are free of release layer material which are then jointly covered by the sheet material. As a sheet material not only metals like titanium or aluminum but also compounds like aluminum nitride may be used. For the substrate material instead of silicon a glass plate may be used. The invention now being fully described, it will be apparent to one of ordinary skill in the art that any changes and modifications can be made thereto without departing from the spirit or scope of the invention as set forth herein.