If you are interested in intermixing between the layers you can try specular and non-specular low angle x-ray scattering techniques, as well as high angle x-ray diffraction. See for example N. Zotov et al. J. Appl. Phys. 100 (2006), 073517.

If one film is somewhat transparent and you want a quick method you can try ellipsometry or spectrophotometry. Provided you have a decent optical model (i.e. you know n&k and thickness) of the films themselves, you can get an idea of the abruptness of the interface from optical data.

As pointed above, you can also use x-ray reflectivity (XRR). You can also have a look at some related discussions on similar topics on RG, making possible use of XRR (or other techniques)

https://www.researchgate.net/post/Is_it_possible_to_find_the_interface_diffusion_in_multilayer_thin_films_by_XRR?_tpcectx=home_feed

or

https://www.researchgate.net/post/How_can_measure_thin_film_thickness_by_XRD2?_tpcectx=home_feed

Regards

    cristian

In addition to the good comments you already got, SIMS can be useful to track diffusion. You'll need to design a careful, comparative experiment because SIMS suffers from many artifacts and quantification is a serious challenge; however, when you do it right, SIMS can offer some of the best depth resolutions and elemental sensitivities out there.

You also could use Rutherford backscattering,

with best regards

Depth profile by Rutherford Backscattering (RBS) could be very useful.