Abstract
Real-time feedback control of inductively coupled plasmas was studied using a 36 GHz hetrodyne interferometer as sensor. The dependence of electron density on controlled plasma characteristics like dc bias voltage, pressure and fluorine concentration was studied. A prefilter and a feedforward compensator were used to reduce the transient variations. The proportional-integral-derivative (PID) controller sucessfully regulated the electron density when the gas pressure was varied. A larger error was noted for the plasma processing tools having smaller aspect ratio.