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

Effect of a negative direct current (DC) bias,|Vdc|, on the electrical parameters and discharge mode is investigated experimentally in a radio frequency (RF) capacitively coupled Ar plasma operated at different RF voltage amplitudes and different gas pressures. The electron density is measured by using a hairpin probe and the spatio-temporal distribution of electron-impact excitation rate is determined by phase-resolved optical emission spectroscopy, and the electrical parameters are obtained based on the waveforms of the electrode voltage and the plasma current measured by a voltage and current probe. It was found that at a low |Vdc|, i.e., in α mode, the electron density and the RF current decline with increasing |Vdc|, and meanwhile, the plasma impedance becomes more capacitive, due to a widened sheath. So, the RF power deposition is suppressed. When |Vdc| exceeds a certain value, the plasma turns into 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 the RF power deposition is enhanced with |Vdc|. Besides, we found that the electrical parameters show similar dependence on |Vdc| at different RF voltages, and the α-γ mode transition occurs at a lower |Vdc| at a higher RF voltage. By increasing the pressure, the plasma impedance becomes more resistive, so the RF power deposition and the electron density are enhanced. Especially, the α-γ mode transition tends to occur at a lower |Vdc| with the increase of the pressure.