甚高频激发的容性耦合Ar+O_2等离子体电负特性研究

Electronegativity of capacitively coupled Ar+O_2 plasma excited at very high frequency

利用探针辅助的脉冲激光诱导负离子剥离诊断技术对掺入5%O_2的容性耦合Ar等离子体电负特性进行了诊断研究.首先详细解析了脉冲激光剥离后探针的电信号,分析了探针偏压在低于或高于空间电位下的探针收集信号特征;根据探针偏压与探针收集信号之间的依赖关系,用来描述Ar+O_2等离子体电负特性的等离子体电负度被定义为脉冲激光剥离出的电子电流与偏压高于空间电位的探针收集到的背景电子电流的饱和比值,并对等离子体电负度随放电气压、射频功率以及轴向位置的变化进行了诊断测量.实验结果表明等离子体的电负度随着射频功率的增加而减小、随着放电气压的上升而变大;由于非对称电极的分布特性,在轴向方向上靠近功率电极时等离子体电负度有升高的趋势,这种趋势可能与鞘层边界附近二次电子的动力学行为以及负离子的产生与消失过程有关.

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＋O2plasma 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.