The E–H mode transition affected by RF bias in inductively coupled Cl 2 plasma

Abstract This study systematically investigates the influence of bias power on E–H mode transition behavior and hysteresis characteristics in Cl 2 plasma. Experiments were performed using a planar-coil inductively coupled plasma (ICP) system equipped with a 13.56 MHz radio-frequency (RF) coil and a 2 MHz RF bias source. A VI probe monitored real-time power transfer between the coil and bias circuit, while high-voltage and current probes measured the voltage, current, and power absorption on the bias electrode. The electron density was diagnosed using a cutoff probe, and the optical emission intensity was measured with a spectrometer. With increasing bias power, the threshold coil power required for the E–H transition decreased, and the hysteresis loop narrowed until it disappeared. In the E mode, the coil power alone was insufficient to raise the electron density to the transition threshold, whereas the applied bias power enhanced electron density and reduced the required coil power. For the first time in Cl 2 plasma, the bias power was observed to initiate the transition from the bistable E mode to H mode. Once the discharge entered H mode, the plasma remained in this state even when the bias power was reduced or turned off. These results indicate that ICP hysteresis originates from the limited capability of the coil to increase plasma density in the E mode, which necessitates higher coil currents for the transition. Consequently, the hysteresis phenomenon in the ICP arises from the difference in the ionization efficiency of the coil between the two modes. Moreover, the bias power transfer efficiency differed markedly between the E and H modes, due to variations in electron density and impedance shifting toward resistive behavior, which enhanced bias power deposition in the H mode plasma.