Gas-phase reactions of CF3 and CF2 with hydrogen atoms: Their significance in plasma processing

The use of hydrogen-rich CF₄ plasmas leads to the selective etching of SiO₂ in the presence of Si. The mechanisms which control this selectivity are poorly understood, and much more needs to be known about the fundamental chemical processes. In this work the reactions $$begin{gathered} CF_3 + H to CF_2 + HF hfill CF_2 + H to CF + HF hfill end{gathered} $$ have been studied at 295 K using a gas-flow reactor sampled by a mass spectrometer. The rate coefficients obtained are (8.9±1.8)×10^?11 cm³ s^?1 and (1.65±0.40)×10^?13 cm³ s^?1 for reactions (3) and (4) respectively. These values establish reaction (3) as a major source of CF₂ while reaction (4) would be expected to be only a minor loss process in the etching environment.