Interface study of W / Si multilayers with increasing number of periods

The X-ray reflectivity and diffuse-scattering measurements at grazing incidence on W / Si multilayers with increasing number of periods N from 3 to 15 were performed using the Cu Kα1 radiation. The Fresnel optical computational code and distorted-wave Born approximation were used to evaluate the results. The multilayer Bragg interferences are observed from N = 6. The reflectivity on the 1 st interface roughness of 0.65 nm does not change with N. A model of vertically correlated interface profiles was successfully used to simulate the diffuse scattering results and confirmed a negligible cumulative interface roughness upwards the multilayers. The lateral correlation length of 10 nm is independent of N. An additional roughness component of very large correlation length (waviness) had to be included to obtain self-consistent simulations of the sample and detector scans for N ≥ 9. A gradual increase of the fractal parameter of the interface profiles from h = 0.15 for N = 3 to h = 0.75 for N = 15 is the most pronounced effect of increasing N and implies the loss of the fractal behaviour