In-situ non-destructive removal of tin particles by low-energy plasma for imitation of EUV optical mirrors self-cleaning

Abstract

In this study, the feasibility was explored of realizing in-situ and non-destructive self-cleaning of the multilayers mirror (MLM) by utilizing hydrogen plasma at low input power (<10 W). The hydrogen plasma generated by the electrode at low power was comprehensively diagnosed using the Langmuir probe and the retarding field ion energy analyzer (RFEA). Meanwhile, the ion concentration ratios and atomic density in hydrogen plasma were calculated by constructing a zero-dimensional plasma chemical model. These experimental results show that the plasma distribution at 1 W is more uniform than that at 10 W, and the peak ion energy at 1 W is lower than 22 eV. Cleaning rates of 0.3–0.75 nm min−1 are achieved to remove discontinuous tin particles at different powers. According to the experiment and calculation results, the chemical etching of ions (breaking the Sn–Sn bond) occurred at 1 W. The etching yield is 0.23–0.87 Sn/ion for 10–40 eV. The analysis of surface morphology and cross-sectional morphology shows that the removal of tin particles is realized by top-down cleaning with hydrogen plasma at low power. This study is of great significance for understanding the detailed mechanism of MLM self-discharge cleaning tin pollutants at low power. It has made a comprehensive calculation and diagnosis of plasma parameters.