The sensors and models needed for the control of a high-density, multipolar, electron cyclotron resonance (ECR) plasma source have been studied for an argon soft-sputter clean process for semiconductor wafers. The ECR plasma source studied had a discharge diameter of 12.5 cm, a processing chamber diameter of 40 cm, and an excitation frequency of 2.45 GHz. The implementation of control on this plasma machine has been investigated by characterizing the machine using plasma diagnostic measurements, by implementing and testing sensors, and by developing both analytical and statistical models of the machine operation. The real-time usage of sensor data together with input parameter adjustments has been used to obtain predictable soft-sputter/etch rates in the process.