There is an X-ray method to measure the thickness of a thin film, however it works best in the 1 to 100 nm range. It's called XRR (X ray reflectommetry), and is based on the interferences of an X-ray beam penatrationg at very low angle in a film and reflected and transmitted at the different interfaces.
The X-ray penetration into a material depends on the incident angle. So you may try low-angle diffraction. However it is too tedious way to go so I'd rather used some of the methods suggested by Mahdiyar.
There is an X-ray method to measure the thickness of a thin film, however it works best in the 1 to 100 nm range. It's called XRR (X ray reflectommetry), and is based on the interferences of an X-ray beam penatrationg at very low angle in a film and reflected and transmitted at the different interfaces.
You can estimate the thickness (at least for thin films) by the peak broadening of the XRD peaks using the Scherrer equation (http://en.wikipedia.org/wiki/Scherrer_equation). This may give a rough estimate, but could also just give you crystallite size. It does mean you don't have to take more measurements though. For accurate calibration you should use XRR though.