Relation between the ion flux and plasma density in an rf CCP discharge

摘要

Ion flux and plasma density are some of the most essential parameters for plasma-assisted process control. There is also a simple and well-known relation between them based on the Bohm criterion. This relation allows the avoidance of directly measuring both the ion flux and the plasma density simultaneously, but the question is, how accurate this estimation would be. This work represents the study of the sensitivity of the relation to plasma conditions, such as gas pressure, type of gas and plasma density. The experiments are carried out in an asymmetric rf dual-frequency CCP discharge in noble gases-Ar and Xe-and in molecular gas-N_2. The gas pressure is varied from 20 mTorr up to 200 mTorr and the plasma density range is 109 to 5.6 × 10~(10) cm~(-3). Analysis of the experimental data is made by using the 2D PIC MCC model of the ion current collection by a planar electrode. The results show that there is some sensitivity of the relation to the type of gas and pressure, but the dependence of the relation on plasma density turns out to be slight. Therefore, the pressure parameterization of the relation for a certain gas can be used to establish plasma density by measuring ion current to an electrode and vice versa.