Experimental investigation of the electron impact excitation behavior in pulse-modulated radio frequency Ar/O2 inductively coupled plasma

In inductively coupled plasmas (ICPs), mode transition between capacitive coupling (E mode) and inductive coupling (H mode) is a key issue. Using an intensified charge-coupled device camera, the mode transition-related behaviors of the electron impact excitation of Ar(2p1) are investigated under different discharge conditions in pulse-modulated radio-frequency (rf) Ar/O2 ICPs. The initiation time of the E-H mode transition at the initial stage of a pulse period is examined under nanosecond time-resolution for the first time. It is found that the initiation time increases with increasing the applied power (300-600 W), while it decreases with raising the duty cycle (50%-80%) or gas pressure (20-80 mTorr). Besides, we also examined the spatial-temporal electron impact excitation rate over the whole pulse period (microsecond time-resolution), especially in the H mode when the discharge is operated at the steady state. We found that as the O2 content/pressure increases, the electron impact excitation axially concentrates closer to the quartz window, and the bimodal structure becomes more prominent in the H mode. However, the excitation gets farther away from the window at higher power. In addition, the maximum value of the excitation rate appears earlier at the initial stage of a pulse period at higher pressure/O2 content.