Table-top focused EUV optical system with high energy density and its application on EUV damage tests

Abstract

With the rapid development of extreme ultraviolet (EUV) light sources, such as plasma-based light source and Free Electron Laser (FEL), it provides unprecedented powerful ultra-short EUV radiations. These extremely high intense ultra-short pulses of radiation bring great challenges to the optical components utilized for steering these light beams, especially the radiation damage issues. However, more studies on the EUV damage mechanisms on optical materials are still quite desired because of limited beamtime provided by FEL facilities. In this study, we present a table-top focused EUV optical system built at the Institute of Precision Optical Engineering (IPOE) for performing EUV damage tests on optical materials. This setup consists of a laser-plasma light source, a modified Schwarzschild objective and an EUV energy attenuator. With a large numerical aperture of 0.44 and a demagnification of 11, the Schwarzschild objective is composed of two annular spherical mirrors coated with Mo/Si multilayers. By using the Zirconium filter and Mo/Si multilayers, this setup can generate the focused radiation with an energy density of 2.27 J/cm2 at the wavelength of 13.5 nm on the image plane of the objective with ~8.3 ns pulse duration. The EUV energy can be changed using a gas attenuator by varying the gas pressure of Helium or Nitrogen inside the chamber. The performance and potentials of this setup are demonstrated by the single-shot or multi-shot damage tests on some samples, such as Au thin film, CaF2 and Mo/Si multilayer mirror. The damage thresholds were determined and the possible damage mechanisms are discussed together with available experimental results.