A computationally assisted spectroscopic technique to measure secondary electron emission coefficients in technological RF plasmas

摘要

Summary form only given. A Computationally Assisted Spectroscopic Technique to measure secondary electron emission coefficients (γ-CAST) in capacitively coupled radio-frequency plasmas is proposed. This non-intrusive, sensitive diagnostic is based on a combination of Phase Resolved Optical Emission Spectroscopy and particle-based kinetic simulations. In such plasmas (under most conditions in electropositive gases) the spatio-temporally resolved electron-impact excitation rate features two distinct maxima adjacent to each electrode at different times within each RF period. While one maximum is the consequence of the energy gain of the electrons due to sheath expansion, the second maximum is produced by secondary electrons accelerated towards the plasma bulk by the sheath electric field at the time of maximum voltage drop across the adjacent sheath. Due to the different excitation mechanisms the ratio of the intensities of these maxima is very sensitive to γ, which allows for its determination via comparing the experimentally measured excitation profiles with corresponding simulation data obtained with various γ-coefficients. This diagnostic is tested here in a geometrically symmetric reactor, for stainless steel electrodes and argon gas. It yields an effective secondary electron emission coefficient of γ=0.067±0.010 in excellent agreement with previous results obtained by particle beam experiment using clean metal surfaces.