Anisotropic Etching of Silicon

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Abstract
A ternary liquid etchant solution for silicon, consisting of hydrazine, iso‐2‐propyl alcohol (IPA), and water has been studied in some detail. In common with other anisotropic etchants reported in the literature, marked dependence of dissolution rate on the crystal orientation of the etched surface has been observed with a low rate of attack on (111) planes. In contrast to other formulations, this etch attacks neither silicon dioxide nor aluminium vacuum‐deposited films, either of which may be used as a very convenient in‐contact masking medium for the engraving of intricate shapes on an appropriately oriented silicon substrate. The experimental study indicates that the etch velocity is a maximum for (211) planes, which accounts for the rounding of convex corners frequently observed when engraving regular shapes in silicon with anisotropic etches. The optimum etch solution is an equimolar mixture of NH2 and H2O.

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