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

Summary

The X-ray reflectivity and diffuse-scattering measurements at grazing incidence on W/Si multilayers with increasing number of periodsN from 3 to 15 were performed using the CuKα₁ radiation. The Fresnel optical computational code and distorted-wave Born approximation were used to evaluate the results. The multilayer Bragg interferences are observed fromN=6. The reflectivity on the 1st Bragg maximum increases from 20% to 55% on increasingN from 6 to 15. The r.m.s. interface roughness of 0.65 nm does not change withN. 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 ofN. 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 forN≥9. A gradual increase of the fractal parameter of the interface profiles fromh=0.15 forN=3 toh=0.75 forN=15 is the most pronounced effect of increasingN and implies the loss of the fractal behaviour.