Spectroscopic Studies of Non-Equilibrium Low Temperature Argon Plasmas

A low-temperature capacitively-coupled plasma (CCP) cell has been built for fundamental studies in argon plasmas. Such systems are widely used in plasma physics research and in semiconductor manufacturing. The device consists of two cylindrical plate electrodes powered by a 13.56 MHz radio-frequency source, forming a diffuse low pressure plasma between the electrodes inside a 10 cm diameter glass cylinder. The device operates at pressures from 0.1-10 Torr and electrode voltages up to 300 V, producing plasmas in which the electron temperature ranges up to several electron-volts while the background gas temperature remains close to ambient (~295 K). The emission lines in the visible and near infra-red (NIR) correspond primarily to transitions from 4p states to metastable and resonant 4s argon states. Absolute radiance calibrated Optical Emission Spectroscopy (OES) measurements on these transitions are used to determine population densities of argon 4p states. These densities are presented in combination with electron density and energy distribution function measurements performed with a Langmuir probe. These measurements enable us to assess the degree of non-equilibrium in the plasma as a function of pressure and electrode voltage. Tunable laser absorption spectroscopy will also be explored to measure the argon 4s state populations. These results will be used to enhance our understanding of non-equilibrium plasmas and to validate kinetic models of excitation and ionization processes in plasmas.