Studies on the effect of driving frequency in capacitively coupled plasma

Abstract

A modern trend of Very High Frequency (VHF) and Dual Frequency (DF) driven Capacitively Coupled Plasma (CCP) sources in industrial plasma processing stimulates a lot of studies concerning the effect of driving frequency in CCP. However, the physics associated with driving frequency are still not fully understood. The research presented here focuses on the driving frequency effect on the plasma parameters and plasma uniformity in CCP. To investigate the driving frequency effect, rf current-voltage monitoring system, rf-compensated Langmuir probe system, and magnetic fluctuation (B-dot) probe system are developed. Experiment has been performed in wide range of external parameters (i.e. various driving frequencies, gas pressures, and powers). The effect of driving frequency on the electrical characteristics and the plasma parameters (electron density and temperature) was measured using the rf current-voltage monitoring system and the rf-compensated Langmuir probe system under fixed discharge power and pressure conditions. Through the measurement, we found that as driving frequency increase (from 9 to 27.12 MHz), the discharge current increases while the discharge voltage decreases. And it was also found that the electron temperature increases and the electron density remains almost constant or decreases with driving frequency, over a wide range of discharge power and pressure conditions (50 - 300 W of discharge power and 50 - 400 mtorr of argon pressure). These results indicate that more power is dissipated for electron heating in the plasma at higher frequencies compared with lower frequencies, while the power dissipated for ion acceleration in the sheaths is reduced with driving frequency. In addition, the increased electron dissipation power at higher frequencies mainly influences the plasma for increasing average electron energy rather than increasing the electron density. The enhancement of electron temperature at higher driving frequencies can be exp...