Mass spectrometric investigation and formation mechanisms of high-mass species in the downstream region of Ar/CF4/O2 plasmas

Abstract

Mass analysis has been conducted on the positive ions and neutral species in the downstream region of Ar/CF₄/O₂ plasmas. The neutral species have been ionized by Li⁺ attachment before mass analysis. The CF₂O⁺, C₂F₅O⁺ and C_nF_2n−1O⁺ (1 ≤n≤ 6) positive ions and the C_nF_2nO (1 ≤n≤ 7) neutral species have been found as the species composed of C, F and O. The intensity of C₂F₄O observed via the Li⁺-attachment mass spectra has been exceptionally weak in comparison to the intensities of CF₂O and C₃F₆O. In addition, neither C₂F₄ nor C₃F₆ have been observed, although C_nF_2n (n≥ 4) have been observed as the species composed only of C and F. These findings suggest that C_nF_2nO (n≥ 3) are produced mainly through the following reactions: CF₃(CF₂)_mCF = CF₂ + O(³P) → CF₂(³B₁) + CF₃(CF₂)_mCFO (m≥ 1) and CF₃(CF₂)_mCF = CF(CF₂)_nCF₃ + O(³P) → CF₃(CF₂)_mCF + CF₃(CF₂)_nCFO (m, n≥ 1), where the CF₃(CF₂)_n– group might have side chains, as in (CF₃)₂CF(CF₂)_n−2–. With the help of quantum chemistry calculations of reaction enthalpies and transition states, the formation mechanisms of the observed species have been discussed in detail.