Mode Transition (Α - Γ) and Hysteresis in Microwave-Driven Low-Temperature Plasmas

Abstract

We discovered hysteresis in microwave-driven low-pressure Ar plasmas across the transition of discharge modes [1] , [2] (α and γ). The hysteresis manifests as two different pressures of mode transition depending on the direction of the pressure change. The discharge mode transition is identified by a rapid change in the plasma shape as well as ion population and electron temperature. As a corollary, the discharge characteristics at given operation parameters depend on the direction of pressure change, suggesting the possibility of bistable states in processing plasmas. The details of the hysteresis are investigated by measuring the rotational and vibrational temperatures from OH (A-X) line emissions. We show that the hysteresis during mode transition is mainly due to the fast gas heating [3] in the γ-mode leading to a smaller neutral density than that of the α-mode. Thus, the mode transition occurs at a higher pressure for the γ-to-α transition than the α-to-γ transition. This interpretation is confirmed by replacing the abscissa from pressure to normalized neutral density, which leads to the elimination of hysteresis.