Study on Detection Mechanism of Tin Contamination Layer on the EUV Collector Mirror Surfaces Based on Secondary Electrons

Tin contamination on extreme ultraviolet (EUV) collector mirrors significantly degrades mirror reflectivity. Hydrogen-based plasma cleaning is currently the standard method for removing the tin layer. However, to prevent substrate damage from over-cleaning, real-time monitoring of the tin layer thickness is critical. It has been established that the secondary electron yield (SEY) induced by high-energy primary electron bombardment correlates with the tin layer thickness. Thus, SEY can serve as a thickness indicator to determine the optimal cleaning endpoint. In this study, the evolution of secondary electrons during the cleaning process is simulated using a Particle-in-Cell (PIC) model combined with the Monte Carlo method, and the relationship between SEY and tin layer thickness is established. The simulation results indicate that under the specified conditions, H3+ is the dominant ionic species generated. Primary electrons account for nearly 24% of the incident particles, with an average energy of approximately 47 eV. Most secondary electrons possess energies below 30 eV, and their yield increases monotonically with the tin layer thickness, ranging from 0.60 to 1.05. These findings provide a novel approach for in situ detection of tin contamination layer evolution on EUV collector mirrors.