1 1 Selective etch of n or p with added NaNO or H O

A wide variety of etching solutions are available for anisotropic etching. Two popular ones are KOH and tetramethylammonium hydroxide (TMAH). Table 2 gives some of the properties of these popular anisotropic etchants. Others include the hydroxides of other alkali metals, such as NaOH, CsOH, and RbOH, ammonium hydroxide, ethylenediamine pyrocatechol (EDP), hydrazine, and amine gallates. For each, a key feature is the fact that the <111> crystal directions are attacked at rates at least 10 times, and typically 20-100 times, lower than other crystalline directions. Anisotropic etching can also be further controlled by the introduction of dopants or by electrochemical modulation.