Interface diffusion kinetics and lifetime scaling in multilayer Bragg optics

The internal structure of Mo/Si multilayers is investigated during and after thermal annealing. Multilayer period compaction is shown to result from diffusion induced MoSi2 interlayer growth, reducing optical contrast and changing the reflected wavelength. We focus on early-stage interface growth observed at relatively low temperatures (100 °C - 300 °C), determining diffusion constants from parabolic interface growth laws. Diffusion constants obey Arrhenius-type behavior, enabling temperature scaling laws. Using the methods developed, we compare results on Mo/Si based multilayers designed for enhanced thermal stability and discuss their relevant diffusion behavior. Arrhenius-type behavior can be observed in all multilayers studied here, and demonstrates reduction of diffusion rates over several orders of magnitude. The method described here is of general interest for any multilayer application that is subjected to enhanced thermal loads and demonstrates the enormous technology gain that this type of optics has experienced the last decade.