Abstract

Anisotropic etching of silicon is of such fundamental importance in silicon micromachining that it has been studied and continues to be studied by many researchers. Yet, even with all the attention that it has received, there remain many difficulties, such as toxicity of the most commonly-used etchants. This work is an extensive experimental investigation of anisotropic etching of silicon in the relatively non-toxic tetra-methyl-ammonium hydroxide (TMAH). Using a temperature-controlled etch apparatus, basic etch rate experiments on both Sid and Sin confirm and extend previous literature results. Extensive underetch experiments shows that the TMAH is qualitatively similar to other more common etchants such as KOH and EDP, but has several quantitative differences. The underetched planes are found to be a sensitive function of etchant composition. Under certain etchant conditions, {110) planes are found to be planes that etch slower than expected. The evolution of hillocks on {100) etched surface is characterized, as a function of etchant parameters. Hillocks are found to occur only at TMAH concentrations less than 20 wt.%. Re-etch experiments show that hillock-producing conditions at the etched surface are disturbed if the etch is interrupted. The effect of externally applied stress on the etch rate is investigated and found to be not significant. However very subtle stress-induced features are observed. The use of TMAH in CMOS-compatible micromachining is explored in certain device geometries.