Electronegativity of capacitively coupled Ar+O2 plasma excited at very high frequency

Abstract

By using pulsed laser induced detachment technique assisted with a Langmuir probe, the electronegative characteristics of the capacitively coupled Ar plasma doped with 5% O2 are studied in this paper. We first focus on the electrical signal of the probe after laser pulse has induced negative ion detachment, and then analyze characteristics of the probe signal with the probe bias below or above the plasma space potential. When the bias is set to be lower than the plasma potential, the probe signal usually shows a downward surge signal. As the bias is higher than the plasma potential, the main characteristics of the signal takes on an upward wide wave packet. The evolution behavior of the probe signal with bias from the downward surge valley to the upward wide wave packet might be due to the potential difference between the plasma space potential and the probe bias voltage. Furthermore, it shows that the position of the upward peak appears later than that of the downward surge valley, which may be related to the changing of the rate of the electron diffusion flux and the electric field drift flux. According to the dependence of probe collection signal on bias, the electronegativity describing the Ar+O2 plasma electronegative property is defined as saturation ratio of electron current after pulsed laser radiation to that of collection probe at a potential above plasma spatial potential. Plasma electronegativity is diagnosed with discharge pressure, radio-frequency (RF) input power and axial position. The experimental results show that the electronegativity of plasma decreases with input RF power increasing. As the gas pressure is kept at 12.0 Pa, the plasma electronegativity decreases from 5.05 to 0.98 with RF input power increasing from 50 to 300 W. It also shows an increasing trend of electronegativity with plasma discharge pressure increasing. Due to asymmetrical distribution of electrodes, the plasma electronegativity also takes on asymmetric one with respect to the axial position. In our experiments, the electronegativity near the power electrode shows about 1-4 times higher than that near the ground electrode, the lowest point of the plasma electronegativity seems to be located in the center of the plasma discharge. This may be related to the dynamics of the secondary electrons emitted from electrode and the competition processes between negative ion production in collisional dissociation of oxygen molecules and the losses of high energy electron and negative ion in collisional detachment of negative ion with oxygen molecule.