Effect of a negative DC bias on a capacitively coupled Ar plasma operated at different radiofrequency voltages and gas pressures

The effect of a negative DC bias, |V_dc|, on the electrical parameters and discharge mode is investigated experimentally in a radiofrequency (RF) capacitively coupled Ar plasma operated at different RF voltage amplitudes and gas pressures. The electron density is measured using a hairpin probe and the spatio-temporal distribution of the electron-impact excitation rate is determined by phase-resolved optical emission spectroscopy. The electrical parameters are obtained based on the waveforms of the electrode voltage and plasma current measured by a voltage probe and a current probe. It was found that at a low |V_dc|, i.e. in α-mode, the electron density and RF current decline with increasing |V_dc|; meanwhile, the plasma impedance becomes more capacitive due to a widened sheath. Therefore, RF power deposition is suppressed. When |V_dc| exceeds a certain value, the plasma changes to α–γ hybrid mode (or the discharge becomes dominated by the γ-mode), manifesting a drastically growing electron density and a moderately increasing RF current. Meanwhile, the plasma impedance becomes more resistive, so RF power deposition is enhanced with |V_dc|. We also found that the electrical parameters show similar dependence on |V_dc| at different RF voltages, and α–γ mode transition occurs at a lower |V_dc| at a higher RF voltage. By increasing the pressure, plasma impedance becomes more resistive, so RF power deposition and electron density are enhanced. In particular, the α–γ mode transition tends to occur at a lower |V_dc| with increase in pressure.