Anisotropic Wet Etching Representation And Distrib

Some wet etchants etch crystalline materials at very different rates depending upon which crystal face is exposed. In single-crystal materials (e.g. silicon wafers), this effect can allow very high anisotropy, as shown in the figure.

Several anisotropic wet etchants are available for silicon. For instance, potassium hydroxide (KOH) can achieve selectivity of 400 between <100> and <111> planes. Another option is EDP (an aqueous solution of ethylene diamine and pyrocatechol), which also displays high selectivity for p-type doping. Neither of these etchants may be used on wafers that contain CMOS integrated circuits. Both of them etch aluminium, commonly used as a metallization (wiring) material. KOH introduces mobile potassium ions into silicon dioxide, and EDP is highly corrosive and carcinogenic. Tetramethylammonium hydroxide (TMAH) presents a safer alternative, although it has even worse selectivity between <100> and <111> planes in silicon than does EDP.