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⁺, C H₃H₇⁺, 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% H₂ 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 sheath 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.Died June 1, 1991.