QUADRUPOLE MASS-SPECTROMETRIC STUDY OF POSITIVE-IONS FROM RF PLASMAS OF PURE CH4, CH4/H2, AND CH4/AR SYSTEMS

Quadrupole mass spectrometry has been employed to characterize the ionic species in the discharges of pure CH4, CH4/H2, and CH4/Ar systems. For pure methane, the major positive ions in the discharge at low pressure (e.g., 0.15 torr) are CH3+, C2H3+, CH2+, C2H2+, CH4+, C2H4+, and C2H5+; at high pressure (e.g., 0.5 torr) the major ions are CH3+, C2H3+, C2H5+, C3H3+, C3H5+, C3H7+, C4H7+, C5H7+, C6H5+, and C7H7+. The relative abundances of C1 ions decrease with increasing pressure, whereas those of higher-order ions increase with pressure. For 5% CH4+ 95% H-2 mixture, in addition to those sampling from the pure methane plasma at the lower pressure, H(n)+ ions have also been detected. For 5% CH4 + 95% Ar mixture, the principal ions are CH3+, CH2+, CH+, CH5+, Ar+, and ArH+; the ions containing more than two carbon atoms are negligible. In these discharges, the CH3+ and C2H3+ are the most important positive ions in C1 and C2 ions, respectively. The ions detected are believed to come from the sheat between the electrode and the luminous plasma, and have high kinetic energy. An ion-molecule reaction mechanism is proposed which can well explain the observed main features of ionic products.