Surface and interface dynamics in multilayered systems

In the broad scientific field of thin films, applications have rapidly expanded since the second half of the 20th century, as has been described in the valorization chapter. This thesis describes research at the interface between surface chemistry and physics. The characterization of d-metal surface nanolayer caps in chapter 3 has revealed that they can actually promote oxidation of the Mo layer on which they are deposited. Vise versa, Ni and Co caps themselves oxidize, but in monolayer quantities already prevent Mo oxidation. We attribute these observations to sacrificial e- donation, and in the case of Au and Cu, also to roughening of the surface. Chapter 4 shows that all cap oxide and part of the MoO3 can be reduced by atomic H exposure, which also removes part of the carbonaceous contaminants that absorb on the surface both in ambient air and during use in lithography tools. The carbonaceous contaminants are hydrogenated to volatile hydrocarbons that desorp from the surface. Atomic O exposure removes remaining C and leaves a clean metal oxide surface without erosion via volatile metal oxides. With subsequent atomic H treatment, this offers an attractive technique for non-destructive surface cleaning of the optical elements in lithography. The in-depth chemical and vacancy mediated diffusion have further been investigated in chapter 5 and 6 with XPS, AES, CS-EELS, HAADF-STEM and EDX. Several capping layers and boron are observed to diffuse into and through the underlying Mo, B4C and Si layers without significant chemical interaction, towards the subsurface Si-on-Mo interface, where d-metal silicide formation and agglomeration occurs. Rather than in the mid-Si layer, kinetically favorable silicides and borides are formed at the Si-on-Mo interface front, notably RuSix and MoBx. The MoBx appeared to be an attractive barrier material and has been patented. Reversed “substrate-on-adlayer” interfaces can yield entirely suppressed reactivity and diffusion, stressing the influence of surface free energy and the supply of atoms to the interface via segregation on thin layer growth. Chemical diffusion and LaB6 and LaC2 interlayer formation in La/B4C multilayers for reflection of wavelengths just above 6.65 nm have been studied in chapter 7 and 8. The research in chapter 8 reveals surfactant mediated growth and chemical stabilization of the La/B4C interface by nitridation. We have shown in chapter 8 that B4C is more readily nitridated by N2 + bombardment during or after growth than La, which apparently yields an equilibrium that involves dinitrogen complexes. The loosely bound N or N2 in the La/B4C interface substrate partially diffuses into the adlayer, resulting in surfactant mediated growth. Subsequent nitridation of the adlayer is observed, yielding nitridated interfaces that are chemically inactive to LaB6 and LaC2 interlayer formation. These processes occur without significantly affecting interface diffuseness when N2 + bombardment is applied after individual layer growth, yielding local BN and more diffuse LaN formation. Complete interdiffusion is observed when N2 + bombardment is applied during B4C growth. Nitridation of the interfaces has greatly improved the reflective properties of La/B4C multilayers. This makes application in reflective optics highly advantageous for short-wavelength light sources, e.g. the FLASH free electron laser, next generation photolithography beyond EUVL, soft x-ray spectroscopy, Rontgen fluorescence analysis and imaging.