X-ray reflectvity uses total external reflection from surfaces and interfaces. It is useful for layer thickness between 5 and 350 Angstrom. It can give accurate layer thickness, density and roughness. X-ray reflectvity measurements can be performed using a conventional power diffractometer .
XRR Scan of Thin ge Film on SI Substrate
The postions of the fringes are related to the film thickness by the modified bragg equation 
 Thin-film thickness and density determination from X-ray reflectivity data using a conventional power diffractometer HUANG T. C. ; GILLES R. ; WILL G. ;