Interface study of a Co/Si/W/Si multilayer with enhanced thermal stability

The interface evolution in an amorphous Co/Si/W/Si multilayer (ML) of 150 Å nominal period processed by rapid thermal annealing (RTA) in the temperature range of 523-1273 K for 5-30 s was studied by X-ray reflectivity and diffuse scattering measurements at Cu Kα₁ wavelength. The results were simulated within the Fresnel theory and the distorted-wave Born approximation. The interdiffusion and intermixing induced by RTA starts at Co/Si interfaces and takes place via an interface shift without a considerable increase of the interface roughness up to the 873K/30s RTA. A strong conformality of the interface profiles, stretching originally over the entire ML thickness in the whole interval of the interface roughness frequencies covered by the experimental scans, is reduced to about 2 ML periods. After the 1023K/30s RTA, the interface morphology is controlled by the crystallization of Co and W silicide phases leading to interface roughening. The vertical interface correlation is sensititve to the interface roughness frequency. A fast diffusion along grain boundaries leads to a complete interdiffusion and collapse of the ML on 1273K/30s RTA. Compared with a W/Si ML studied previously, the incorporation of Co layers into the ML stack enhanced the thermal stability by about 250 K. Some consequences for optical applications are discussed.