An in situ method using a Y-type optical fiber for measuring the thickness of the carbon contamination layer on the surface of an extreme ultraviolet mirror

Abstract

A new method for in situ measurement of the thickness of the carbon contamination layer on the surface of an extreme ultraviolet (EUV) mirror is proposed. This measurement is important in order to determine the most effective timing with which the mirror should be cleaned. The method we propose uses a Y-type optical fiber to measure the reflectivity profile over the wavelength range from 200 nm to 800 nm from the surface of the mirror; the reflectivity profile is normalized by the reflectivity at 800 nm wavelength. This is because the change in reflectivity is more sensitive to the carbon layer thickness in the short wavelength region rather than in the long wavelength region. The method was demonstrated using carbon/ruthenium bilayer samples deposited on commercial Si wafers. The results show that the proposed method can successfully estimate the thickness of the carbon layer and thus has the potential to measure the thickness of a thin carbon layer on an EUV mirror.