Thickness inhomogeneity during silicon X-ray mask membrane fabrication: generation and prevention

Highly boron doped silicon membranes fabricated by a deposition on silicon wafers and etching of the bulk well accepted in X-ray mask fabrication. Membrane inhomogeneities caused by the fabrication process can occur and must be minimized for successful application as mask blanks. The inhomogeneity is mainly influenced by the deposition process, the trenching effect, selectivity of the used etchant (KOH/IPA), and by the influence of different illumination during the etching. Besides a careful deposition process, the inhomogeneity can be reduced by working in conditions of high anisotropy (low temperature), high selectivity, accurate adjustment of patterns to crystal planes, and excluding influence of light. Under optimal conditions, an inhomogeneity of less than 100 nm can be reached.