Quadrupole mass spectrometric study of positive ions from RF plasmas of pure CH₄, CH₄/H₂, and CH₄/Ar systems

Abstract

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 CH ⁺₃ , C₂H ⁺₃ , CH ⁺₂ , C₂H ⁺₂ , CH ⁺₄ , C₂H ⁺₄ , and C₂H ⁺₅ at high pressure (e.g., 0.5 torr) the major ions are CH ⁺₃ , C₂H ⁺₃ , C₂H ⁺₅ , C₃H ⁺₃ , C H₃H ⁺₇ , C₄H ⁺₇ , C₅H ⁺₇ , C₆H ⁺₅ , and C₇H ⁺₇ . 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 CH ⁺₃ , CH ⁺₂ , CH⁺, CH ⁺₅ , Ar⁺, and ArH⁺; the ions containing more than two carbon atoms are negligible. In these discharges, the CH ⁺₃ and C₂H ⁺₃ 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.