Abstract:
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 { 1 1 1} orientations. We have investigated
the height of the {1 1 1} 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 {1 1 1} 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 sour ce, 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
{1 1 1} plane and the mask, and can thus have different values at opposite
{ 1 1 1} sides of a thin wall etched in a micromechanical structure. 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).