In situ x-ray reflectivity study of swift heavy ion induced interface modification in a W/Si multilayer x-ray mirror

Intermixing induced in W/Si multilayers by irradiation with 150 MeV Ag+ ions has been studied. In situ x-ray reflectivity measurements during heavy ion irradiation allows one to monitor the evolution of interface mixing as well as internal stresses as a function of time during irradiation. The technique of x-ray reflectivity is sensitive enough to measure diffusion length down to 0.1 nm. A finite intermixing occurs in the system, in spite of it having a positive heat of mixing. High accuracy of the technique allows one to discern subtle effects of factors like stress or film thickness, on swift heavy ion irradiation induced intermixing. The measurements provide clear evidence of an enhanced intermixing in the presence of tensile stress in the film. Mixing efficiency decreases with increasing film thickness. The film thickness dependence of mixing efficiency can be understood in terms of the thermal spike model by taking into consideration the changes in electron mobility due to scattering from surface and interfaces.