B₄C/Mo/Si multilayers for 20-40 nm wavelengths: Application to broadband mirrors

Theoretically, periodic three component multilayers as B₄C/Mo/Si allow an improvement of the reflectivity in the 25 nm-40 nm range as compared to two component multilayers as B₄C/Si. Optimized B ₄C/Mo/Si and B₄C/Si multilayers have been deposited by magnetron sputtering and ion beam sputtering. The multilayers have been characterized by x-ray grazing reflectometry (λ = 0.154 nm) and synchrotron radiation measurements at near normal incidence. The maximum experimental reflectivity for B₄C/Si is 25% at 32 nm and 23% at 38 nm. For B₄C/Mo/Si multilayers, we obtained an experimental reflectivity of 34% at 32 nm and 29% at 39.5 nm. Moreover the width of the Bragg peak is larger for B₄C/Mo/Si than for B₄C/Si. We have used these multilayers in a non periodical structure in order to produce broadband mirrors. It consists of the superposition of two periodic B ₄C/Mo/Si multilayers with different period thickness. Theoretical optimization of such structure by simulation is presented. Preliminary experimental results demonstrate the interest of such structure : two broadband mirrors have been deposited and measured over a wide wavelength range (12 nm to 45 nm). The first mirror presents a broadband spectrum centered at 32 nm with peak reflectivity of 22% and bandwidth larger than 9 nm. The second mirror has been optimized to produce theoretically an average reflectivity of 19% from 25 nm to 35 nm.