Etching is a commonly used process in microelectronics. It is used to partly or fully remove materials in order to generate desired patterns on a wafer surface during device manufacturing. The two main properties of the etching process are its selectivity and isotropy. Selectivity refers to the etchant's ability to remove a certain material while leaving another material in tact to serve as an etchant mask. Isotropy refers to the etchant's ability to remove materials in multiple directions. When a mask is used, isotropy determines how much underetch occurs below the mask.
The common etching processes are wet, which use liquids which chemically react with the materials in question and dry, which use a plasma process where ions and neutral radicals are accelerated toward the surface required to be etched and the physical interaction between the accelerated ions and neutral radicals cause the removal of a material. Wet chemical processes are frequently isotropic, etching in all directions with the same rate. However, they can also be anisotropic by causing an etch rate which varies with different material crystalline orientation. Underetch frequently occurs during chemical etching processes. A plasma process is more anisotropic, where the etch rate is mainly in the direction of the accelerating ions. However the neutral radicals which are natively isotropic also play a role in the etching process causing some isotropy in plasma etching as well.