Mg/B 4 C EUV multilayer by introducing Co as barrier layer

Mg/B 4 C multilayer provides very high theoretical reflectivity in extreme ultraviolet range near 30.4nm wavelength, while the interface between Mg and B 4 C layer is very poor. In this paper, Co was introduced into the interface between Mg and B 4 C as a barrier layer. Mg/B 4 C and Co/Mg/Co/B 4 C multilayers were designed, fabricated, and measured for the wavelength of 30.4nm. The thickness of Co barrier layer was optimized. Grazing incidence x-ray reflectance measurements show that the structural quality of Co/Mg/Co/B 4 C multilayer is improved significantly after Co barrier layer inserting, and the optimum thickness of the barrier layer is 1.5nm. Keywords: multilayer; magnetron sputtering; barrier layer; Co; EUV 1. INTRODUCTION Solar radiation lines in the 25-40nm interval, Fe-XV (28.4nm), He-II (30.4nm) and Fe-XVI (33.5nm) included, are of great interest for coronal imaging [1,2]. To detect and image these extreme ultraviolet (EUV) spectrum lines at normal incidence, multilayer mirrors are widely used. Mo/Si material combination multilayer has very high reflectance and excellent stability in the EUV range just above Si L-edge ( =12.4nm). The technology on Mo/Si multilayer becomes extremely developed especially under the motivation of integrated circuit industry, which has been widely used in EUV and astronomic observation [3,4]. However, the reflectivity of Mo/Si and other Si-based multilayers gets lower and the bandwidth becomes wider as the wavelength grows. The most serious problem is that the second Bragg peak intensity is much higher than the reflective intensity at the working wavelength of 30.4nm. Thus, much effort should be focused on the new multilayer combination working at the wavelength of 30.4nm. Magnesium (Mg) is a hopeful candidate for spacer layer at wavelengths longer than Mg L-edge ( =25.2nm). Measured reflectivities at near normal incidence of Mg/SiC and Mg/Co multilayer are 44% at 31.2nm and 40.3% at 30.5nm, respectively [6]. The theoretical reflectivity of Mg/B