Structure and thermal stability of Mo/Al multilayers for soft x-ray mirrors

The structure and thermal stability of Mo/Al multilayers prepared by a four facing target sputtering system have been studied. Cross-sectional high-transmission electron microscopy reveals that Mo and Al layers are polycrystalline and that the thickness of the mixed layers at Mo-on-Al interfaces is larger than that at the Al-on-Mo interfaces. Annealing treatment up to 450 deg. C leads to the gradual period expansion and appearance of the 5th and 6th order Bragg peak, which can be attributed to grain growth and sharpness of the interfaces and/or improvement of the correlations of the roughness at different interfaces through annealing, respectively. Thermally induced interdiffusion or reaction at grain boundaries with high density restricts the further period expansion in spite of the increasing temperature. At 550 deg. C, the modulation structure is still well maintained though the amorphous Al-Mo alloy layer replaces the Al layer. On the base of effective heat of formation, Mo3Al8 is suggested to be the first phase formed at the interfaces. Further annealing to 600 deg. C would partly destroy the layered modulation structure