Comments on mechanisms for self-E → H and inverse H → E mode transitions in radio frequency inductively coupled plasmas

Abstract

The previously observed self-E → H and inverse H→E mode transitions were explained by unrelated mechanisms. The argument in this Brief Communication finds that both mode transitions can be interpreted via wall-heating that affects electron density by changing neutral density. In the self-E → H mode transition, the shift of the preset E-mode discharge to the E → H mode-transition point is caused by the increasing neutral density in the cooling down process of the chamber wall overheated in the preceding high-power H-mode discharge. The requirement for the inverse H →E mode transitions is a small-sized cylindrical radio frequency (RF) inductively coupled plasma source powered by multi-turn coil under which the strong wall-heating in the ramping-up phase of RF power or RF coil current leads to a reduction of neutral density in the subsequent ramping-down phase. The wall-heating can affect all processes in gas discharges but is most remarkable near a mode transition. The wall-heating effect on discharge mode transition has not been intensively investigated but was only suggested to explain the unknown mode transition in an electron cyclotron resonance plasma source [Jarnyk et al., Appl. Phys. Lett. 62, 2039 (1993)].