Temperature-dependent nanocrystal formation in Mo/Si multilayers

We investigated the nanocrystallinity of Mo/Si multilayers as a function of the Mo:Si ratio in the period using grazing incidence and wide angle x-ray diffraction, both for as-deposited samples and after thermal annealing up to 800 degrees C under UHV conditions. The research was performed on multilayers, as applied for extreme UV lithography with period thickness of approximately 7 nm. The as-deposited multilayer nanostructure was found to depend on the Mo to Si layer thickness ratio. For intermediate Mo fractions in the multilayer period, a four layer system is formed, with amorphous Si and polycrystalline Mo layers separated by silicide interfaces, while for low and high Mo fractions, a two component system is formed, respectively, consisting of a pure Mo layer (in the case of a high Mo fraction) or pure Si layer (low Mo fraction) separated by a single silicide interface. Using the crystallographic properties of the multilayer during annealing, we describe the continuous development of the multilayer structure and growth of the silicide interfaces. Our study has led to an explanatory model which is based on the total free energy minimization of the multilayer system. Finally, a phase transition to a crystalline silicide is observed at T > 300 degrees C. This phase transition can also be explained by minimization of the total free energy.