甚高频电容耦合氢等离子体特性研究

The study of capacitively-coupled hydrogen plasma at very high frequency

采用高H2稀释的SiH_4等离子体放电,特别是甚高频等离子体增强化学气相沉积技术是当前高速制备优质微晶硅薄膜的主流方法.尽管在实验上取得了很大的突破,但其沉积机理一直是研究的热点和难点.本文通过建立二维时变的轴对称模型,在75 MHz放电频率下,对与微晶硅沉积非常相关的甚高频电容耦合氢等离子体放电进行了数值模拟,研究了沉积参数对等离子体特性的影响,并与光发射谱(OES)在线监测结果进行了比较.结果表明:电子浓度n_e在等离子体体层中间区域最大,而电子温度T_e及H_α与H_β的数密度在体层和鞘层界面附近取极大值;当气压从1 Torr(1Torr=133.322 Pa)增大至5 Torr时,等离子体电势单调降低,在体层中间区域n_e先快速增大然后逐渐减小,T_e先下降后趋于稳定;随着放电功率从30 W增大到70 W,电子浓度n_e及H_α与H_β的数密度均线性增大,而电子温度T_e基本保持不变;OES在线分析结果与模拟结果符合得很好.

In the high rate deposition of device grade microcrystalline silicon films and their solar cells, plasma enhanced chemical vapor deposition excited using very high frequency （VHF） has become a mainstream method. Although, great breakthroughs in the experiment are achieved, the depositional mechanism is still a research hot spot and difficulty point. In this paper, the capacitively-coupled hydrogen plasma discharge at VHF is simulated. A two-dimensional, time-dependent axial symmetry model is adopted at a frequency of 75 MHz, and the influences of pressure and plasma power on hydrogen plasma characteristic are simulated. At the same time, the hydrogen plasma is monitored on-line using the optical emission spectrometry in experiment. The results show that the value of the electronic concentration ne takes a maximum in the middle of the plasma bulk, while the electron temperature Te and the number densities of Ha and Hβ each have a maximal value at the place near the sheath and plasma bulk; the potential decreases with pressure increasing from 1 Torr to 5 Torr, the electron concentration in the plasma bulk first increases with the increase of pressure, then decreases with the further increase of pressure, but the electron temperature first decreases and then keeps stable in plasma bulk; the electron concentrations, Ha and Hβ increase linely with power increasing from 30 W to 70 W, but the electron temperature keeps stable. The experimental results and simulation results are in good agreement.