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

摘要

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-1)O~+ (1≤ n ≤ 6) positive ions and the C_nF_(2n)O (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_(2n)O (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.