Experimental study on the ignition process of a pulsed capacitively coupled RF plasma with a trenched electrode

Abstract The ignition process of a pulse-modulated capacitively coupled plasma with a trenched-electrode driven at 12.5 MHz is investigated via electrical and optical diagnostics. The electrical characteristics are derived by analyzing the waveforms measured by a voltage and current probe. Time resolved optical emission spectroscopy is utilized to determine the evolution of the optical emission intensity (OEI) of Ar atoms at 750.4 nm and the spatio-temporal distribution of the electron-impact excitation rate at some selected RF periods since the beginning of each pulse. It is found that the OEI in the system with the trenched electrode overshoots more significantly compared to that without the trenched electrode, and the power deposition evolves with time in the same manner as the OEI before the overshoot moment of the two parameters. In the trenched-electrode case, during the very beginning of each pulse, the OEI is slightly stronger above the planar part due to a stronger axial electric field, and several RF periods later the OEI is enhanced at the opening of the trench edges. In particular, the trench structure generates a finite two-dimensional boundary, leading to a distinct enhancement of the excitation rate, attributed to the hollow cathode effect (HCE) during the sheath expansion phases, and a significant abnormal excitation enhancement because of the ‘focus effect’ when the sheath collapses. These phenomena evolve continuously from the onset of the pulse until the steady state. At relatively low pressure, the excitation maximum inside the trench moves upward and out of the trench as time goes by, since the HCE is gradually suppressed by increased negative bias on the trenched-electrode. While at higher pressure, the maximum of the OEI splits into two at the edges and the corners of the trench, suggesting a completely suppressed HCE, due to very local electron kinetic.