Effect of impurities in vacuum vessels on the plasma parameters in inductive discharges

Abstract

The mechanisms affecting plasma by external and internal inputs must be understood for both semiconductor processing and controlled nuclear fusion, which require stable and steady-state plasmas. Among various factors that can change the state of plasma, detecting and controlling the plasma–wall interactions inside the reactor are challenging. In particular, although water is a commonly encountered impurity in most vacuum systems including process chambers, studies on the direct evidence of water effect on plasma are limited. Herein, we show that there is a clear negative correlation between the impurity density and the plasma density. A small change in water impurity could cause an immediate change in the plasma density because impurities contribute to increasing the total energy loss in the steady-state plasma sustainment. To observe the impurity effect distinctly, we conducted experiments under various conditions to analyze the effect of vessel impurities on plasma parameters, such as applied power, wall contamination, and wall materials. In the experiments, it is found that the plasma density is significantly influenced by the impurity density. The results were compared with the global model calculation and they are in agreement on the relative change of the electron density.