The impact of energetic ions during fabrication of W/Si multilayers may result in interface layers with a gradually changing concentration, notably in the W-on-Si interface layer. This process may be employed to deliberately form a multilayer system with a graded refractive index within each period, in principle allowing an adjustment of the optical, wavelength-selective properties of the multilayer system. We also have given a first demonstration of a new method based on a combination of high-resolution TEM image analysis and grazing incidence X-ray reflectivity analysis to determine the in-depth density profile of a multilayer structure.
Nucl. Instrum. Methods Phys. Res. B

Kessels, M. J. H., Verhoeven, J., Yakshin, A. E., Tichelaar, F. D., & Bijkerk, F. (2004). Ion beam induced intermixing of interface structures in W/Si multilayers. Nucl. Instrum. Methods Phys. Res., Sect B, 222, 484–490. doi:10.1016/j.nimb.2004.03.006