Energy-variable X-ray diffraction technique is established as a novel method for depth-resolved measurements of lattice d-spacings and residual strains in layered polycrystalline materials. Achieving high depth resolution in X-ray diffraction measurements is still a problem because of rather large penetration lengths. In single-crystalline structures, an advantage is taken from the coherent interaction between X-rays and crystal lattice. In polycrystalline materials, the phase correlations exist within individual grains only and the coherence is lost on larger length scales. As a result, the obtained diffraction profiles are free of interference features and cannot be used for analysis of structural parameters with depth resolution.

In our method, the depth sensitivity is achieved by the controlled changing the X-ray energy and, hence, the X-ray penetration into the sample. Theoretical analysis of the problem accomplished for bulk materials [1] and thin films [2] showed that X-ray quanta scattered at different depths will be registered with different probabilities in the detector's system. Based on this fact, a characteristic depth is analytically derived, which provides the maximum of the detected X-ray intensity. An analysis of this expression allowed us to conclude that a sub-micrometer range for the in-depth steps is easily achievable.

The developed technique is applied to characterize the metal-metal, ceramic-metal, and ceramic-ceramic multilayers produced by different methods. We show a capability to characterize the strain changes at interfaces buried a 30-50 microns beneath the sample surface, to detect strain variations on a 100 nm scale and to collect experimental data in steps of 10 nm, when working not far away from absorption edges.

[1] E. Zolotoyabko, B. Pokroy, J. P. Quintana. J. Synchrotron Rad. 11, 309-313 (2004).

[2] E. Zolotoyabko, B. Pokroy, T. Cohen-Hyams, J. P. Quintana. Nucl. Instr. & Meth. Phys. Res. B 246, 244-248 (2006).

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