Modes and the alpha-gamma transition in rf capacitive discharges in N ₂O at different rf frequencies

This paper reports current-voltage characteristics and pressure-voltage transition curves from the weak-current α -mode to the strong-current γ -mode for rf capacitive discharges in N2 O at frequencies of 2 MHz, 13.56 MHz, and 27.12 MHz. At 2 MHz the rf discharge is mostly resistive whereas at 13.56 MHz and 27.12 MHz it is mostly capacitive. The weak-current α -mode was found to exist only above a certain minimum gas pressure for all frequencies studied. N. Yatsenko [Sov. Phys. Tech. Phys. 26, 678 (1981)] previously proposed that the α-γ transition corresponds to breakdown of the sheaths. However, we show that this is the case only for sufficiently high gas pressures. At lower pressure there is a smooth transition from the weak-current α -mode to a strong-current γ -mode, in which the sheaths produce fast electrons but the sheath has not undergone breakdown.