In the above figure dimension 'x' shows the degree of anisotropy, 'y' shows the lack of selectivity with respect to the underlying layer and 'z' shows the lack of selectivity with respect to the masking layer.

• A poorly selective etch removes the desired layer, but also attacks the underlying material.


• A highly selective etch leaves the underlying material unharmed.


• A perfectly isotropic etch produces round sidewalls.


• A perfectly anisotropic etch produces vertical sidewalls

The wet etching processes used liquid-phase ("wet") etchants. The wafer is immersed in a bath of etchant, which must be agitated to achieve good process control. For instance, buffered hydrofluoric acid (HF) was used commonly to etch silicon dioxide over a silicon substrate. Wet etchants are usually isotropic, which leads to undercuts when etching thick films. They also require the disposal of large amounts of toxic waste. For these reasons, they are seldom used in state-of-the-art processes.