The use of titanium and titanium dioxide as masks for deep silicon etching

Abstract

The possibility of using sputtered metals as mask materials for deep anisotropic chemical etching in silicon was investigated. Sputtered films of chrome, nickel and tungsten were all found to be chemically resistant to potassium hydroxide (KOH) and tetramethyl ammonium hydroxide (TMAOH). However as expected, these metals had poor adhesion to the silicon substrate. By comparison sputtered titanium was found to have excellent adhesion properties, and was chemically resistant to TMAOH but not to KOH. Resistance to KOH was achieved by thermal oxidation of the titanium film, at temperatures between 600 and 900° C. Following oxidation, etch depths more than 200μm were readily achieved in KOH etching. This makes sputtered titanium a potential alternative to the conventional mask material, silicon nitride, for the application of deep anisotropic etching. The reduction in etch rates due to a galvanic effect of conductive metal masks on silicon-on-insulator wafers was also investigated. It was observed that this effect was also overcome by thermal oxidation of the titanium mask.