Determination of rotational and vibrational temperatures of CH in CH4 plasmas

Abstract

Understanding fundamental plasma–surface interactions remains an important component of increasing the utility of plasma systems in a range of applications including plasma-assisted catalysis (PAC). A key element in realizing the potential of these applications is understanding the foundational data that control the overall process. Here, fundamental gas-phase and film chemistry trends in methane inductively coupled plasmas are studied to model PAC systems for decomposition of volatile organic compounds. Specifically, energy partitioning trends are presented in conjunction with surface characterization data to elucidate processes occurring at the plasma–surface interface. Optical emission spectroscopy yields rotational and vibrational distribution measurements that can be converted to temperatures (TR and TV) for CH in 100% CH4 and CH4/Ar plasmas as a function of pressure (50–200 mTorr) and applied rf power (25–150 W). Under these conditions, TV ranges from ∼3000 to ∼5000 K, whereas TR has values of 1500–2500 K. These are considerably above room temperature, even at pressures of 200 mTorr and Ar content as high as 25% of total precursor feed. Surface characterization of plasma-treated substrates yields minimal changes in chemical composition but more significant variations in film morphology as functions of plasma pressure and applied rf power. Collectively, these data aim to unravel the complex chemistry of plasma systems for PAC.