Dual-frequency capacitive discharges: Effect of low-frequency current on electron distribution function

Abstract

In low-pressure dual-frequency capacitive discharges, the effect of the low-frequency current on the electron distribution function (EDF) was investigated through the particle-in-cell simulation with Monte Carlo collision model. As the low-frequency (2MHz) current increases for the fixed high-frequency (27MHz) current, the EDF changes from Druyvesteyn to bi-Maxwellian (in the absence of secondary electron emission) or Maxwellian type (in case with secondary electron emission), along with the significant drop in the effective electron temperature. When the role of secondary electron emission is negligible, the EDF transition is attributed to the transition from collisional to collisionless property (but not stochastic heating) of the low-energy electrons. The Ramsauer minimum which makes low-energy electrons less collisional plays an important role in making this transition as well as in determining the spatial electric field structure. When the role of secondary electron emission is significant, the transition is attributed to the α-γ transition.