Investigation of absolute atomic fluorine density in a capacitively coupled SF6/O2/Ar and SF6/Ar discharge

Abstract

Detection of fluorine atoms in a capacitively coupled SF6/O2/Ar discharge has been achieved using appearance potential mass spectrometry (APMS). Atomic fluorine density, [F], variation with power in the range of 50–300 W was found to saturate. The dissociation fraction for a fixed gas pressure of 40 mTorr and a feedstock mix of SF6/O2/Ar (85/10/5%) ranged from 3.0 × 10−4–2.4 × 10−3. The addition of O2 to SF6 plasma was shown to have a significant effect on [F]. The dissociation fraction in SF6/Ar (95/5%) plasma at 20 mTorr was over a factor of 22 lower than SF6/O2/Ar (85/10/5%) plasma. For a fixed gas pressure and power of 60 mTorr and 500 W the F+ signal showed a variation of up to a factor of 60 depending if O2 was in the feedstock gas which indicates a marked enhancement of [F] with O2 addition. Fluorine actinometry was also used to monitor relative fluorine atom concentrations. A comparison of actinometry with APMS showed that a classical actinometric approach for measuring fluorine atoms was unreliable for the conditions examined in this work. Over the power range investigated the absolute fluorine concentration increased by a factor of 10 whereas the actinometric signal only showed a two fold increase.