Two modes of capacitively coupled rf discharge in CF4

Capacitively coupled rf discharge in pure CF4 was studied using a one-dimensional self-consistent particle-in-cell Monte Carlo model. Two different discharge modes are observed depending on the discharge conditions: the regime of electronegative plasma with high-electron temperatures in the bulk, and the regime of electropositive plasma with abnormally low electron temperatures in the bulk. The characteristic features of the two discharge modes are considered. A sharp transition from the former to the latter mode is observed with an increase in applied voltage. The dependence of the transition voltage on gas pressure is analyzed. In the studied range of gas pressures, the existence of a high-temperature mode in an electronegative gas like CF4 is suggested by the balance between the ionization rate and attachment rate in the bulk region. As a result, the transition voltage increases with gas pressure because of the increased relative role of electron attachment. It is shown that the differences in the used electron cross-section sets may noticeably affect the simulation results and the discharge properties. Three different electron cross-section sets for CF4 are considered. In particular, the transition voltage between the two discharge modes differs essentially for different cross-sections used. In order to analyze the fundamental causes of this difference, a detailed comparison of three cross-section sets was done on the basis of the Monte Carlo calculation of swarm parameters in constant electric fields.