Abstract: A method for controlling the plasma etching of semiconductor wafers determines the impedance of a plasma chamber using values representing voltage, current, and the phase angle between them, as provided by a sensor. All or less than all of the data during a first time period may be used to calculate a model. During a second time period, real time data is used to calculate a version of the instant impedance of the chamber. This version of impendence is compared to a time-projected version of the model. The method determines that etching should be stopped when the received data deviates from the extrapolated model by a certain amount. In some embodiments a rolling average is used in the second time period, the rolling average compared to the model to determine the end point condition.