Sheath electric field oscillation and ion kinetics in radio-frequency discharges

Time resolved maps of the electric field magnitude in the sheath of discharges in pure H₂ have been determined as a function of pressure (0.6 and 1 Torr) and excitation frequency (30 kHz, 1, and 2.78 MHz). The electric field was determined from the Stark splitting of the n = 6 level of atomic hydrogen using 2+1 photon laser-induced fluorescence of ground state hydrogen atoms. The sheath electric field oscillations throughout the rf cycle were observed for different values of the rf driving frequency, corresponding to different regimes. We observe a "low" frequency regime at 30 kHz, a "high" frequency regime at 2.78 MHz, and an intermediate regime at 1 MHz, depending on whether the ion density profile is modulated during the rf cycle. This is the first observation of an intermediate regime where the ion transit time across the sheath is comparable to the rf period. Fluid model simulations were carried out and are in good agreement with our experimental observations.