Etching of silicon in alkaline solutions: A critical look at the {111} 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 micromechanical structures. The etch rates R{hkl} depend mainly on composition and temperature of the etchant. In a plot of etch rate versus orientation, there is always a deep, cusped minimum for the {111} orientations. We have investigated the height of the {111} etch-rate minimum, as well as the etching mechanisms that determine it. We found that in situations where masks are involved, the height of the {111} minimum can be influenced by nucleation at a silicon/mask-junction. A junction which influences etch or growth rates in this way can be recognized as a velocity source, a mathematical 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 {111} plane and the mask, and can thus have different values at opposite {111} sides of a thin wall etched in a micromechanical structure. This observation explains the little understood spread in published data on etch rate of {111} and the anisotropy factor (often defined as R100/R111).