Effect of the Secondary Electron Emission on the Decay of Temporal Sheath Perturbations in RF Plasmas

The probability of secondary electron emission (SEE), known as the secondary electron emission coefficient (SEEC), varies depending on the surface composition, cleanliness, microstructure, and type of particle impacting the boundary. We explore the potential of a simple in-situ method of SEEC characterization using Phase Resolved Optical Emission Spectroscopy to spatially image the time dependent dynamics of the Ar 750nm emission in the sheath resulting from a sheath perturbation. In this study, the perturbation is a laser induced pulse of photo-emission electrons from the surface. The electrons generated by this pulse in some cases can overwhelm the periodic steady state plasma production in the sheath. After the pulse, ions impact the surface generating electrons by SEE. An enhanced, but decaying gamma mode peak is observed for several RF cycles. We expect the rate of the perturbation decay to depend on the ion transit time and it is observed to be significantly longer than the RF period. Repeat measurements using surfaces with known SEEC differences determine the extent to which the perturbation decay is related to the SEEC.