Chemistry studies of SF6/CF4, SF6/O2 and CF4/O2 gas phase during hollow cathode reactive ion etching plasma

Abstract

In this work, mass spectrometry and optical emission spectroscopy techniques were used to monitor the molecular and atomic neutral species during SF6/CF4, SF6/O2 and CF4/O2 plasmas generated in a radio-frequency Hollow Cathode Reactive Ion Etching (HCRIE) reactor keeping constant the following operational conditions: total gas flow rate, gas pressure, and discharge power. The investigations were aimed to understand the chemistry behavior of plasmas generated with SF6 and CF4 mixtures or mixed separately with O2. The neutral mass spectrometry analysis showed a high concentration of gas species namely SF5+ (parent specie = SF6), SF3+ (SF4), CF3+ (CF4) and HF+ (HF) in SF6/CF4 plasmas, SF5+, SF3+, O2+ (O2), F+ (F) and HF+ in SF6/O2 plasmas and CF3+, O2+, CO2+ (CO2) and HF+ in CF4/O2 plasmas. The presence of other species was observed in quantities lower than 1% of total gas pressure. It was used the actinometry method to monitor the atomic fluorine (F) concentration during discharge operation. Higher density of F was observed in all experiments, varying from (2.8–9.5) × 1019 m−3 in SF6/CF4 plasmas, (0.2–1.7) × 1020 m−3 in SF6/O2 plasmas and (0.06–1.17) × 1020 m−3 in CF4/O2 plasmas. The addition of CF4 in SF6 plasma reduces monotonically the F concentration when compared with the SF6/O2 and CF4/O2 plasmas that promotes an increase of F for low O2 concentrations. This effect shows the importance of oxygen species in the dissociative processes of the fluorine-based plasma also for this type of plasma reactor. Moreover, it is highlighted the higher concentrations of HF in gas phase that promotes reactions paths that decrease the F species in gas/plasma phase.