Anisotropic KOH Etching
KOH is one the most commonly used silicon etch chemistry for micromachining silicon wafers.
1. Anisotropic KOH Etching Rates vs. Orientation
The KOH etch rate is strongly effected by the crystallographic orientation of the silicon (anisotropic). Table 1 relates silicon orientation-dependent etch rates (μm min-1) of KOH to crystal orientation with an etching temperature of 70°C. Table 1 is taken directly from [1]. In parentheses are normalized values relative to (110).
The (110) plane is the fastest etching primary surface. The ideal (110) surface has a more corrugated atomic structure than the (100) and (111) primary surfaces. The (111) plane is an extremely slow etching plane that is tightly packed, has a single dangling-bond per atom, and is overall atomically flat. As shown above, the strongly stepped and vicinal surfaces to the primary planes are typically fast etching surfaces.