Diffusion phenomena in chemically stabilized multilayer structures

Multilayered thin film structures are widely applied as reflective coatings for optical elements in the extreme ultraviolet wavelength regime. In this thesis we investigate the structural and chemical changes that occur in Mo/Si based multilayers as a result of radiation induced thermal loads and other heating schemes. This thesis addresses thermally induced diffusion in such multilayers, focussing on reaction mechanisms at the interfaces and how these are modified in the presence of a diffusion barrier layer. To allow these studies, a new, in-situ X-ray diffraction method is introduced to analyse diffusion induced interface growth and measure diffusion speeds in Mo/Si multilayers during thermal annealing. This method can determine the change in the interface thickness at picometer accuracy. Because of this high accuracy it is possible to study diffusion at relatively low temperatures. A diffusion-reaction model was developed to describe the interface growth. The diffusion in multilayers is shown to be dependent on the structure of all the layers as well as on the chemical interactions with barrier layer materials. In particular, the crystallinity of the Mo layer (crystalline or quasi-amorphous) has a large influence on the diffusion speed. The structure and density of the B4C diffusion barrier layers have a large influence on the diffusion coefficient. Furthermore it is shown that B4C also forms molybdenum boride compounds during annealing, which reduce the diffusion rate. In conclusion, the diffusion properties of thin film multilayer structures are determined by both the structure and the chemical interactions of the individual (barrier-)layers. The damage mechanisms of these layered structures by intense femtosecond pulses are also investigated, to find a possible difference in damage mechanism between continuous heat loads and ultrafast pulsed heat loads. This investigation was performed on MoN/SiN multilayers. We find that the damage mechanisms for annealing and pulsed irradiation are fundamentally the same. In both cases, the MoN layer dissociates and N2 gas is released, which subsequently forms bubbles in the MoN layer which may lead to delamination.