Park Yeong-Shin, Lee Yuna, Chung Kyoung-Jae, Hwang Y S
Department of Nuclear Engineering, Seoul National University, Seoul 151-742, South Korea.
Rev Sci Instrum. 2011 Dec;82(12):123303. doi: 10.1063/1.3664616.
The operating conditions of a rf plasma ion source utilizing a positively biased electrode have been investigated to develop a stably operating, high-current ion source. Ion beam characteristics such as currents and energies are measured and compared with bias currents by varying the bias voltages on the electrode immersed in the ambient rf plasma. Current-voltage curves of the bias electrode and photographs confirm that a small and dense plasma, so-called anode spot, is formed near an extraction aperture and plays a key role to enhance the performance of the plasma ion source. The ion beam currents from the anode spot are observed to be maximized at the optimum bias voltage near the knee of the characteristic current-voltage curve of the anode spot. Increased potential barrier to obstruct beam extraction is the reason for the reduction of the ion beam current in spite of the increased bias current indicating the density of the anode spot. The optimum bias voltage is measured to be lower at higher operating pressure, which is favorable for stable operation without severe sputtering damage on the electrode. The ion beam current can be further enhanced by increasing the power for the ambient plasma without increasing the bias voltage. In the same manner, noble gases with higher atomic number as a feedstock gas are preferable for extracting higher beam current more stably. Therefore, performance of the plasma ion source with a positively biased electrode can be enhanced by controlling the operating conditions of the anode spot in various manners.
为了开发一种稳定运行的高电流离子源，对使用正偏置电极的射频等离子体离子源的运行条件进行了研究。通过改变浸没在周围射频等离子体中的电极上的偏置电压，测量离子束的电流和能量等特性，并与偏置电流进行比较。偏置电极的电流-电压曲线和照片证实，在引出孔附近形成了一个小而密集的等离子体，即所谓的阳极斑，它对提高等离子体离子源的性能起着关键作用。观察到来自阳极斑的离子束电流在阳极斑特征电流-电压曲线拐点附近的最佳偏置电压下达到最大值。尽管偏置电流增加表明阳极斑密度增加，但阻碍束流引出的势垒增加是离子束电流降低的原因。测量发现，在较高的工作压力下，最佳偏置电压较低，这有利于稳定运行，且不会对电极造成严重的溅射损伤。在不增加偏置电压的情况下，通过增加周围等离子体的功率可以进一步提高离子束电流。同样，使用原子序数较高的稀有气体作为原料气体更有利于更稳定地引出更高的束流。因此，通过以各种方式控制阳极斑的运行条件，可以提高带有正偏置电极的等离子体离子源的性能。
