Reflectance change of Si- and Ru-capped Mo/Si multilayer mirrors caused by intense EUV irradiation

Abstract

An EUV irradiation and reflectance measurement system using intense EUV radiation emitted from a long undulator at the NewSUBARU synchrotron radiation (SR) facility was developed. The system can measure the real-time reflectance drop during intense EUV irradiation and reflectance mapping as well as the photoemission current after irradiation at a fixed energy for atom absorption. The irradiated EUV beam was very intense, and the power density was about 400 mW/mm². The reflectances of Si- and Ru-capped Mo/Si multilayer mirrors (MLMs) were measured under several conditions of EUV power, i.e., 120, 15, and 5 mW/mm² for Si-capped MLMs, or of water vapor, i.e., 6.6x10^-5 and 1.3x10^-2 Pa for Ru-capped MLMs. Each reflectance was reduced as the dose was increased. The reflectance was significantly reduced at the higher partial pressure of water vapor. When the intensity of the beam flux was reduced using ND filters, the reflectance was significantly reduced under the same conditions of atmosphere and dose. Carbon cleaning and oxidation were progressed in the beam center although carbon deposition was much progressed in the beam fringe for Si-capped MLM. Ru-capped MLM was more resistant to radiation damage than Si-capped MLM at each partial pressure of water vapor. The results of X-ray photoelectron spectroscopy (XPS) for Ru-capped MLM showed that deposited and desorbed carbons were balanced at the beam center and carbon deposition occurred on the fringe of the beam.