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

The possibility of using sputtered metals as mask materials for deep anisotropic alkaline 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 silicon. By comparison, sputtered titanium was found to have excellent adhesion properties, but was susceptible to chemical attack under some conditions. Titanium was found to be chemically resistant to TMAOH if the solution was kept below 80 °C, and to KOH solution only below 80 °C and with the pH reduced by the addition of a weak acid. An alternative was to thermally oxidize the titanium, in which case it was chemically resistant to KOH and TMAOH under all conditions. However, TiO 2 was found to be more difficult to pattern and remove than titanium.