Ideally, the desired output of any process should be monitored directly during the process for closed loop feedback control. Unfortunately, for most processes in the semiconductor industry the output is difficult to monitor. Plasma etching is one such process with the additional complexity of being a nonlinear, multiple input, multiple output system. A phenomenological model can predict process behavior over a wide range of operating conditions and hence can be used for the design and control of plasma reactors. By contrast, empirical models are highly unreliable outside the range of operating conditions over which they are parameterized. A phenomenological process model for the etch rate of SiO2 in CF4 and CF4/O2 plasmas is presented. Direct process measurements including gas flow, chamber pressure, wafer temperature, RF power, optical spectroscopy and charge density are incorporated into the model to predict the etch rate. The model is used in conjunction with a previously developed, generalized, PC-based, controller to provide control of the process.