Micromachining on (111)-oriented silicon

Abstract

Micromachining on (111)-oriented silicon in various alkaline solutions was studied. By means of spoke and donut-like etching patterns, the experiment shows that 〈110〉 directions have the highest etch rate and the final emergent periphery is hexagon bounded on the bottom surface plane. The six sidewalls are defined by {111} planes, which can be derived from its crystal geometry, having three facets with inclining angles of 70.5° and another three with declining angles of 109.5° respect to the (111) surface plane. SEM pictures show that aqueous KOH solution results in smooth surface morphology due to its higher etch rate of residual oxide existed in the silicon, while the other etchants such as hydrazine (N 2H 4) and tetramethyl ammonium hydroxide (TMAH) induce seriously wavy roughness. Moreover, various concentrations of KOH solution have been studied to determine the (110)/(111) etch ratio, and the ratio more than 100 were demonstrated. Using this method, suspended single-crystal silicon (c-Si) microstructures were fabricated for some potential uses such as thermopile, silicon bolometer, mass flow transducer and other force microsensors. As an application example, shallow-gap μ-Pirani vacuum sensor demonstrated its high pressure measuring capability as high as 10 3 Torr.