Mode transition induced by low-frequency current in dual-frequency capacitive discharges

The mode transition induced by varying the low-frequency current in low-pressure dual-frequency discharges in argon is found through particle-in-cell or Monte Carlo simulations. As the low-frequency (2 MHz) current increases for the fixed high-frequency (27 MHz) current, the electron distribution function (EDF) changes from Druyvesteyn to bi-Maxwellian (in alpha mode) or Maxwellian-type (in gamma mode), along with the significant drop in the effective electron temperature. It is shown that this EDF evolution is attributed to the transition from collisional to collisionless property (but not stochastic heating) of the low-energy electrons as well as the alpha-gamma transition.