Etching of silicon in alkaline solutions: a critical look at the {1 1 1} minimum

Anisotropic wet-chemical etching of silicon in alkaline solutions is a key technology in the fabrication of sensors and actuators. In this technology, etching through masks is used for fast and reproducible shaping of microme chanical structures. The etch rates R-{hkl} depend mainly on composition an d temperature of the etchant. In a plot of etch rate versus orientation, th ere is always a deep, cusped minimum for the { 1 1 1} orientations. We have investigated the height of the {1 1 1} etch-rate minimum, as well as the e tching mechanisms that determine it. We found that in situations where mask s are involved, the height of the {1 1 1} minimum can be influenced by nucl eation at a silicon/mask-junction. A junction which influences etch or grow th rates in this way can be recognized as a velocity sour ce, a mathematica l concept developed by us that is also applicable to dislocations and grain boundaries. The activity of a velocity source depends on the angle between the relevant {1 1 1} plane and the mask, and can thus have different value s at opposite { 1 1 1} sides of a thin wall etched in a micromechanical str ucture. This observation explains the little understood spread in published data on etch rate of {1 1 1} and the anisotropy factor (often defined as R -100/R-111). (C) 1999 Elsevier Science B.V. All rights reserved.