Measurement of spatially resolved gas-phase plasma temperatures by optical emission and laser-induced fluorescence spectroscopy

Abstract

Knowledge of the energy distributions of particles in glow discharges is crucial to the understanding and modeling of plasma reactors used in microelectronic manufacturing. Reaction rates, available product channels, and transport phenomena all depend upon the partitioning of energy in the discharge. Because of the nonequilibrium nature of glow discharges, however, the distribution of energy among different species and among different degrees of freedom cannot be characterized simply by one temperature. The extent to which different temperatures are needed for each degree of freedom and for each species is not known completely. How plasma operating conditions affect these energy distributions is also an unanswered question. We have investigated the temperatures of radicals, ions, and neutrals in CCl₄, CCl₄/N₂ (2%), and N₂ discharges. In the CCl₄ systems, we probed the CCl rotational and vibrational energy distributions by laser‐induced fluorescence spectroscopy. The rotational distribution always appeared to be thermal but under identical operating conditions was found to be ≊400 K colder than the vibrational distribution. The rotational temperature at any point in the discharge was strongly dependent upon both applied power and surface temperature. Thermal gradients as large as 10² K mm⁻¹ were observed near electrode surfaces but the bulk plasmas were isothermal. When 2% N₂ was added to a CCl₄ discharge, N₂ second positive emission was observed and used to estimate the N₂ rotational temperature. The results suggest that emission from molecular actinometers can be used to measure plasma temperatures, providing such measurements are not made in close proximity to surfaces.