Real-time feedback control of electron density in inductively coupled plasmas

The real-time feedback control of electron density was performed in an indu ctively coupled plasma (ICP). A 36 GHz heterodyne interferometer was adopte d as the sensor for electron density measurement. The actuator was rf power which drove the ICP antenna. The results show that the electron density in an ICP system is a type 0 system. Therefore, a proportional-integral contr oller is necessary to eliminate steady-state error. A prefilter was designe d to smooth the desired step change of electron density so that the variati on of rf power became mild. A feedforward compensator was added to reduce t he disturbance effect resulting from pressure change. The experimental resu lts showed that the control system could quickly track the desired electron density and compensate the electron density variation resulting from press ure disturbance. (C) 2001 American Vacuum Society.