Evaluation of polycrystalline silicon membranes on fused silica for x-ray lithography masks

Abstract

Single crystalline Si films, heavily doped with boron for etch selectivity and often counterdoped with Ge to reduce stresses, are now commonly used as membranes in x-ray masks. We have investigated preparation and properties of much simpler membrane structures, consisting of undoped polycrystalline Si films deposited either on oxidized Si wafers or on fused silica substrates. The latter case is particularly simple since concentrated HF that is used for etching the fused silica substrate, does not attack Si. Therefore, after coating both sides of a fused silica substrate with polycrystalline Si, a window is opened in Si on one side by simple lithography, and the wafer is immersed in HF to complete the process. Stress of the membranes can be tuned by adjusting deposition conditions, annealing temperature, and choice of high temperature glass instead of silica for the substrate. Data from three polycrystalline membranes indicate a higher fracture strength and a larger effective elastic modulus than in equivalent single crystalline Si structures. However, optical transparency is lower because of light scattering by grain boundaries.