Sub-micrometre depth-gradient measurements of phase, strain and texture in polycrystalline thin films: a nano-pencil beam diffraction approach

A single-scan approach to evaluating chemical and structural gradients in polycrystalline thin films with a resolution of tens of nanometres is reported. Thinned samples are measured in cross section in a transmission geometry with a high-energy X-ray nano-pencil beam. Powder diffraction methods can be used because of the strongly asymmetric beam shape (i.e. the large number of diffracting grains), allowing the solution of structural phases within the film thickness. For each phase, microstructural gradients such as strain, stress, texture and grain size are deduced from two-dimensional diffraction patterns. Sample preparation (i.e. sample thinning), stress release and technique throughput are evaluated. The resolution, precision and limitations are discussed. The efficiency of this approach is demonstrated on ferritic thin films, where the phase ratio and stress gradient (in each phase) have been successfully quantified with a 150 nm depth resolution.