Wet etching is the removal of material by immersing the wafer in a liquid bath of chemical etchant. There are two kinds of wet etching etchants, isotropic etchants and anisotropic etchants:

• Isotropic etchants attack the material being etched at the same rate in all directions.

• Anisotropic etchants attack the silicon wafer at different rates in different directions. On wafers the most used etchant is KOH.

Anisotropic etching does not cause undercutting, and is preferred in applications where straight side walls are essential.

Anisotropic etching requires a substrate with a well defined crystalline structure such as silicon. The etch is directional and proceeds along the exposed plane in the crystal lattice. As atoms are removed from the crystal lattice, different planes are exposed to the etchant. Since the density of atoms on the planes varies, the etch rate varies significantly.

In silicon, a popular anisotropic wet etch substrate, the initial (0 degree) surface is called 100 and etches a hundred times faster than the 110 (45 degree) and 111 (54 degree) surfaces. This results in a consistent wedge shaped etch pattern.

Wet etching is also used for the characterization of the quality of a wafer. The etch pit density is a measure for the number of dislocations per area.

Contrary to popular belief, glass lacks long-range order, making it noncrystalline. Thus, it cannot be anisotropically etched.

Semiconductor device fabrication

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