Etching of silicon in alkaline solutions : a critical look at the M 1 1 1 N 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 RMhklN 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 M1 1 1N orientations. We have investigated the height of the M1 1 1N 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 M1 1 1N 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 M1 1 1N plane and the mask, and can thus have different values at opposite M1 1 1N sides of a thin wall etched in a micromechanical structure. This observation explains the little understood spread in published data on etch rate of M1 1 1N and the anisotropy factor (often defined as R 100 /R 111 ). ( 1999 Elsevier Science B.V. All rights reserved. PACS: 81.65.Cf; 61.72.Bb; 81.10.Aj