a method and apparatus for monitoring , measuring and / or controlling the etch rate in a dry etch semiconductor wafer processing system . the wafer processing system has a monitoring assembly which comprises an electromagnetic radiation source and detector which interferometrically measures the etch rate . the source , preferably a uv light source , is directed at a portion of the wafer surface where the etching is taking place . a first portion of the uv light reflects back from the surface at one phase because it is reflected from the surface of the features of the wafer and a second portion of the uv light reflects from the bottom of the features at a slightly different phase . the differences in these phases when properly filtered set up interference patterns which are more intense where the differently phased first and second portions of the light combine or interfere and less intense where they cancel . the interference pattern by its change in wave length is proportional or representative of the rate of change in the etch pattern . the actual rate of change of the etch as it progresses is measured by this technique and is compared to a model of a desired rate of change in a controller . the error between the actual rate of change and the desired rate of change is then used to vary at least one of the process parameters of the system in a direction tending to null the difference .