The design, fabrication and characterization of broadband multilayer mirrors for EUV optics at normal incidence

摘要

A method of designign multilayers with broadband wavelength responses for use as coatings in EUV optics at normal incidence is presented in this paper. The method is based on the well-known Fresnel equations and recursive calculation combined with a defined merit function, with random variation of the thickness of each layer. This allows the design of multilayers for specific requirements. The method was used to design Mo/Si multilayers with broadband wavelength responses for the EUV region, 18-20nm. Such mirrors were made by mangetron sputtering in 99.99percent pure argon. The deposition rates, after calibration, were 0.12 nm s~(-1) for molybdenum and 0.07 nm s~(-1) for silicon. The broadband multilayers were deposited on 30 mm diameter K9 glass substrates with rms surface roughnesses less than 0.8 nm. For comparison, reference periodic multilayers with working wavelength of 19 nm were also deposited. X-ray reflectivity and diffraction measurements were made in theta-2theta geometry using a D/Max-B X-ray diffractometer and Cu Kalpha radiation. The results show that broadband and periodic multilayer mirrors have different eprformances which matched with simulaiton calculations. In order to improve the making technique, the combination of Rutherford backscattering spectroscopy with X-ray low-angle diffraction were used. We have also developed the equipment that can measure reflectivity qualitatively using a transmission grating spectrograph because fo no reflectometer. The results showed that the wavelength responses of broadband multilayers were broader and the integrated reflectivities were higher than for periodic multilayers, but that the peak reflectivity was slightly lower.