Structure and interface properties of Mo/B4C/Si multilayers deposited by rf-magnetron sputtering

Mo/Si multilayer mirrors with reflectance higher than 70% at 13 nm have been developed in last years by the introduction of diffusion barriers, but many physical aspects concerning their growth and interface formation need to be investigated. As a matter of fact knowing the growth kinetic, a tight control of the process can lead to a multilayer structure with sharp and smooth interfaces. The aim of the present work is to investigate the effects of the ion energy and flux on the growing surfaces during sputtering deposition of Mo/B4C/Si multilayers. The multilayers presented in this paper have been produced by static deposition in a UHV chamber equipped with three magnetron sputtering sources driven by rf power supplies. Ion energy has been varied by dc biasing the sample holder. Plasma diagnostic has been performed by Langmuir probe measurements in order to determine the Ar ion flux and energy impinging the growing film. The ion energy and flux effects on crystal structure have been investigated by XRD and m-XRD. The effects on roughness and interfaces have been observed by XRR measurements. Clear dependence of Mo nano-crystal size, texture and interface smoothness on the deposition parameters is pointed out. XRD analyses have been used for a Mo nano-crystal nucleation study in presence of the carbide buffer layer. Interface composition and structure is characterised by Rutherford backscattering (RBS) and nuclear reaction analysis (NRA).