Wet Etching
Isotropic Wet Etching
Isotropic etchants etch in all crystallographic directions at the same rate; they
usually are acidic, such as ”HNA”, a mixture of hydrofluoric acid (HF), nitric acid
(HNO3), and acetic acid(CH3COOH), and lead to rounded isotropic features in
single crystalline silicon. The typical etch rate is several microns per minute. It is
widely used for removal of work-damaged surfaces, rounding of sharp anisotropically
etched corners to avoid stress concentration, removing of roughness and patterning.
Anisotropic Wet Etching
In anisotropic etching, the etch rate (ER) is much faster in certain direction than
in others, exposing the slowest etching crystal planes as etch time progresses, usually
the (111)planes of silicon. The ER dependence on orientation can be explained by
the crystal structure of silicon (see next chapter). The typical etch rate is about
1µm/min. Below is a scheme for both isotropic and anisotropic wet etching.