Photodetachment Study of Capacitively-Coupled RF C4F8Plasma

Behaviors of electrons and negative ions in a capacitively coupled RF (13.56 MHz) plasma ofc-C4F8(perfluorocycrobutane) were studied using a laser-induced photodetachment technique in combination with a microwave cavity resonance technique. The results indicate that the negative ion formation process strongly depends on the RF power because of the dissociation ofc-C4F8even in the low power range (<100 W) studied in this work. The negative ion density estimated on the assumption that all negative ion species have the same photodetachment efficiency as that for F-is larger by a factor of ∼50 than the electron density in the typical condition in this work (100 mTorr C4F8, 10 W RF power). However, the electron attachment rate and ion recombination rate suggest that the actual negative ion density should be significantly higher than that derived on the above assumption; this discrepancy could be attributed to the low photodetachment efficiency for large molecular negative ions which may exist abundantly in the low RF power region.