On hillocks generated during anisotropic etching of Si in TMAH

Hillocks on etched Si{100} surfaces produced by anisotropic etching are a common irritant in the creation of micromachined devices. Close inspection of typical pyramidal hillock shapes reveals that they are usually bounded by convex <101>-directed edges and {111} or near-{111} planes. Underetch experiments at varying TMAH etchant composition confirm that the etch rates of {101} planes and {100} planes vary with etchant conditions. Hillocks are suppressed when {101} etches faster than {100}, which occurs when the TMAH concentration is low. A simple model involving kinks and ledges is proposed and allows direct relation of hillock features to etch anisotropy. Hillocks are hypothesized to be stable due to a lower etch rate for <101> ledges adjacent to the etched surface. The apex of the pyramids may be protected by impurities or defects. Re-etch experiments indicate that hillock-producing conditions are quite sensitive to etchant conditions.