Trends and Uncertainty Quantification of Excited State Populations in Capacitively Coupled Argon Plasmas

A low-temperature capacitively-coupled plasma generator has been developed to study fundamental properties of argon plasmas. Such glow discharge systems are being used in plasma physics research and find use in the semiconductor manufacturing industry. The device consists of two cylindrical plate electrodes powered by a 13.56 MHz radio-frequency source. Our device operates at pressures from 0.1-10 Torr and up to 120 W input power, producing plasmas in which the electron temperature ranges from 0-3 electron-volts and the background gas temperature remains close to ambient at ~295 K. We perform simultaneous current and voltage measurements to determine the phase shift, the power deposited in the plasma and the DC bias. Absolute radiance calibrated Optical Emission Spectroscopy measurements in the visible and near infra-red are used to determine population densities of excited argon states. The emission lines observed correspond primarily to transitions from the 4p states to metastable argon states. We report on changes in the upper state population densities as a function of plasma pressure and power. A Monte-Carlo approach is implemented for uncertainty quantification (UQ). The UQ results are used to assess possible deviations from equilibrium population distributions.