Detection of carbon contamination in EUV multilayer mirrors based on secondary electrons

Abstract

The issue of carbon contamination on the surface of extreme ultraviolet (EUV) optical elements needs to be addressed urgently. In the process of removing carbon contamination, it is necessary to detect the thickness of the carbon contaminated layer in order to avoid damage to EUV multilayer mirrors (MLMs) by overcleaning. The yield of secondary electrons (SEY) generated by the excitation of EUV photons and primary electrons has been proved to be correlated with the thickness of the carbon layer, and thus the cutoff point for carbon layer cleaning can be found by detecting the SEY. In this study, the spatial distribution and motion state of secondary electrons (SEs) in the vacuum cavity of the EUV system during the cleaning process are described, utilizing the Particle-in-cell model and Monte Carlo method, and the variation relationship between SEY and the thickness of the carbon layer is obtained. The results show that under the set conditions, the SEY generated by EUV photons is very low, but it will generate primary electrons that are 1.05 times as numerous as itself, of which 24% bombarded to the carbon layer, resulting in SEs which amount to 16% of primary electrons generation. Moreover, as the carbon layer thickness decreases to 2 nm or below, SEY increases significantly, ranging from 0.55 to 0.71. The present work is of great significance in realizing the accurate detection of the carbon layer thickness of EUV multilayer film mirrors and the recovery of the reflectivity of MLMs.