Microwave microplasma parameters at extremely high driving frequencies

Abstract

The microplasma supported by high microwave frequencies is analyzed using a one-dimensional Particle-in-Cell Monte Carlo collision model. The results indicate that there are two regimes of the discharge operation, which are correlated with different regimes of the electron kinetics of the high-energy tail of the electron energy distribution. We obtain that the regimes of electron kinetics are separated by the frequency νcr, which is the frequency of collisions between the electrons having the energy equal to the excitation threshold of argon and neutrals. Below νcr, the tail of the electron energy distribution function is non-stationary and is defined by the instantaneous electric field. This results in very efficient heating of this group of electrons. As a consequence, the increase in the microwave frequency up to νcr results in the increase in the plasma density. For driving frequencies larger than νcr, the tail is stationary, implying less efficient plasma heating. Consequently, a decrease in plasma density is obtained for extremely high driving frequencies.