Investigation of transient sheath structures on AC-biased electrode using time-resolved laser-induced fluorescence spectroscopy

Sheaths are potential profiles formed between a plasma and solids to balance the charge losses from the plasma. The bulk plasma potential and the sheath potential profile depend on the potential of the solid surface. For instance, when a positive high voltage is applied to a solid, the plasma potential increases owing to the enhanced electron flux toward the surface, and the sheath potential profile also changes. While such phenomena are widely recognized, the transient dynamics of the plasma potential and the sheath potential profile have not been completely understood. In this study, we investigated the temporal evolution of the plasma potential and the sheath potential profile by applying various AC voltages (offset +40 V, amplitude 40 V, frequency 0.01~1 kHz) to a metal plate placed in an ECR plasma (Ar, 0.5 Pa). Time-resolved Langmuir probe (TR-LP) and laser-induced fluorescence spectroscopy (TR-LIF) measurements were performed. TR-LP revealed time constants for the plasma potential (~1 ms) and for the floating potential (~10 ms) after the applied AC voltage suddenly shifted. TR-LIF measurements of the velocity distribution of Ar metastable ions were conducted to analyze the potential profile in the presheath and sheath regions. The results showed that the sheath potential profile changed even after the plasma potential reached a steady state. We discuss the dynamics of charge transfer and potential profile based on these results.