Effect of radio-frequency power levels on electron density in a confined two-frequency capacitively-coupled plasma processing tool

The plasma electron density ne in a symmetric confined capacitive-coupled plasma processing tool containing Ar∕O2∕C4F8 gas mixtures is studied as a function of two, combined radio frequency (2MHz+27MHz) powers. For measuring ne we have used a floating hairpin resonance probe. The results show a linear increase in ne with 27MHz power. Also the density is higher with an increase in 2MHz power, in contrast with published particle-in-cell simulation results in argon where the plasma density decreased with increases in low frequency voltage, for fixed high frequency current [P. C. Boyle et al., J. Phys. D 37, 697 (2004)]. Analyzing the relative phase between radio frequency current and voltage, we observe slightly lower 2MHz phase shifts at higher 2MHz voltage, which is attributed to an increase in the real component of the current through the sheath. This is possible due to the increase in secondary electron emissions arising from ion bombardment, which is favored by an increase in 2MHz voltage. We therefore conclude that the secondary electrons could play an important role in the discharge process.