Measurement of F-, O- and CF3- densities in 60 and 100 MHz asymmetric capacitively coupled plasma discharge produced in an Ar/O2/C4F8 gas mixture

The behaviour of absolute electron density and density of F-, O- and CF3- is studied in asymmetric capacitively coupled plasma discharge produced in an Ar/O-2/C4F8 ( 80: 10: 10) gas mixture excited at 60 and 100 MHz. The measurements are performed using a hairpin probe and laser photo-detachment at 532 nm and 355 nm laser wavelengths. For both 60 and 100 MHz driving frequencies, the results show that the electrons and F- density increases almost linearly with the increase in rf power. On the other hand, the O- density increases in 60 MHz and decreases in 100 MHz with a rise in rf power. For a fixed rf power in the 60 MHz discharge, the O- density increases and electron density decreases with gas pressure. The corresponding F- density first increases, reaching a maximum value, and then decreases with a further increase in gas pressure. A similar trend in electron, F- and O- density versus gas pressure is observed in 100 MHz suggesting that the initial increase in densities is dominated by the ionization and dissociative attachment, whereas, at a higher gas pressure, electron-ion recombination, ion-neutral recombination and ion-ion neutralization play a significant role in the losses of charged particles. The ratio of F- density to O- density is similar to 80:20 and similar to 95:5 in 60 MHz and 100 MHz respectively. The density of both electrons and F- is higher in 100 MHz when compared to 60 MHz discharge. The observed trend is explained on the basis of productions and loss mechanisms for electrons and negative ions. It is concluded that the higher F- density in 100 MHz in comparison to 60 MHz is mainly due to higher electron density and dissociation degree.