Effects of driving frequency on plasma density in Ar and H2/Ar capacitively coupled plasmas at Torr-order pressure

Abstract

Effects of driving frequency on the plasma density in capacitively coupled plasmas (CCPs) have been investigated at Torr-order pressures which are commonly used in film deposition processes (1 Torr = 133.322 Pa). Using a particle-in-cell, Monte Carlo collision model, frequency trends of the electron density have been simulated in Ar and H2/Ar CCPs from 300 kHz to 27 MHz. The results show a local maximum of the electron density at 450 kHz and a local minimum around 4 MHz in both the Ar and H2/Ar plasmas. In particular, in the H2/Ar plasma, 450 kHz is more effective to produce higher density plasma than 13.56 MHz at Torr-order pressure although it is generally believed that higher density plasma can be obtained as the driving frequency increases. The ion density starts to oscillate between electrodes around 4 MHz since it is close to the ion plasma frequency. Thus, the plasma bulk collapses resulting in the low plasma density. On the other hand, at 450 kHz, in addition to the gamma process, the re-formed plasma bulk contributes to high density plasma.