Pressure dependent mode transition in an electron cyclotron resonance plasma discharge

Despite the wide range of applications of the electron cyclotron resonance (ECR) plasma sources, the ECR plasma processing control is tricky at certain operation parameters. There are several reports of regimes where abrupt changes and instabilities in plasma parameters occur. In the present work we report extensive probe measurements of plasma potential, electron temperature, ion beam energy, and velocity as well as plasma density over a mode change appearing when the neutral argon gas pressure is changed. The parameters were measured over the entire pressure range from 0.15 to 7 mTorr. We found a large drop in electron temperature and plasma potential when the pressure increased from 0.15 to 0.4 mTorr. At 0.4 mTorr the temperature reached a minimum and the density a local maximum, while at 1 mTorr the density reached a minimum. When increasing the pressure above 1.2 mTorr the temperature decreased and the density increased rapidly. While the plasma appearance at low and high pressure can be explained by a global conservation model, the behavior in the intermediate pressure between 0.4 and 1.2 mTorr needs a deeper investigation. We discuss the possibility that the mode change is connected to the competition between stepwise and direct ionization of neutral argon and excitations to metastable argon atoms.