XRD analysis of small samples or areas, including mapping
X-ray diffraction (XRD) analysis performed on small samples or small areas of large samples is commonly referred to as microdiffraction. Considered the technique of choice when samples are too small for the optics and accuracy of conventional diffraction instrumentation, the method employs a micro X-ray beam so that diffraction characteristics can be mapped as a function of sample position. With the ability to accurately and precisely position a small X-ray beam on a sample surface, the information can be plotted as a diffraction function map (DFM). Diffraction data can contain information about compound identification, crystallite orientation (texture), stress, crystallinity, and crystallite size. The field of microdiffraction is rapidly growing in materials research and fabrication because smaller domains now affect product yield and reliability. Applications for microdiffraction analysis include: test pads on patterned wafers, compound libraries formed by combinatorial chemistry, inclusions on geological specimens, failure analysis of metal or plastic components, and quality control (QC) in manufacturing.
Conventional X-ray diffraction measures residual strain on bulk materials. The diffraction lines of interest are produced from the entire area illuminated by the X-ray beam, which is usually a line source approximately 2 mm by 12 mm. Penetration depth of the X-ray beam provides the third dimension for the volume being tested. When a point source with micro-optics is incorporated, a beam size as small as 50 μm in diameter can be precisely positioned on the sample, and the instrument can be aligned to within 10 μm accuracy. The illustration (right) shows the microbeam used as an X-ray probe to measure stress on a 304 stainless steel wire filament of 0.005" in diameter. A beam of appropriate size (100 μm pinhole) is selected for the experiment. The area illuminated by the X-ray beam as well as the penetration depth are labeled.