Reactive Ion Etch is a plasma etching technique in which a substrate is placed on a RF powered electrode. The wafer takes on a potential, which accelerates etching species extracted from the plasma toward the etched surface. Introducing different gases can create a chemical reaction, and the etch profiles can range from isotropic to anisotropic by changing the process conditions.
A DC bias measurement can be taken from the chuck which can give a good indication of the amount of etching that is occurring.