X‐Ray Microprobe for Fluorescence and Diffraction Analysis

Abstract X‐ray diffraction and x‐ray‐excited fluorescence analysis are powerful techniques for the nondestructive measurement of crystal structure and chemical composition. X‐ray fluorescence analysis is inherently nondestructive, with orders‐of‐magnitude lower power deposited for the same detectable limit as with fluorescence excited by charged particle probes. X‐ray diffraction analysis is sensitive to crystal structure with orders‐of‐magnitude greater sensitivity to crystallographic strain than electron probes. When a small‐area x‐ray microbeam is used as the probe, chemical composition, crystal structure, crystalline texture, and crystalline strain distributions can be determined. These distributions can be studied both at the surface of the sample and deep within the sample. Current state‐of‐the‐art can achieve an ∼1‐μm diameter x‐ray microprobe and an ∼0.1 μm diameter x‐ray microprobe has been demonstrated. Despite their great chemical and crystallographic sensitivities, x‐ray microprobe techniques have until recently been restricted by inefficient x‐ray focusing optics and weak x‐ray sources; x‐ray microbeam analysis was largely superseded by electron techniques in the 1950s. However, interest in x‐ray microprobe techniques has now been revived by the development of efficient x‐ray focusing optics and ultra‐high‐intensity synchrotron x‐ray sources. These advances have increased the achievable microbeam flux by more than 12 orders of magnitude; the flux in a tunable 1 μm‐diameter beam on a so‐called third‐generation synchrotron source such as the Advanced Photon Source (APS) can exceed the flux in a fixed‐energy mm2beam on a conventional source. These advances place x‐ray microfluorescence and x‐ray microdiffraction analysis techniques among the most powerful techniques available for the nondestructive measurement of chemical and crystallographic distributions in materials. This article reviews the physics, advantages, and scientific applications of hard x‐ray (E> 3 keV) microfluorescence and x‐ray microdiffraction analysis. Because practical x‐ray microbeam instruments are extremely rare, a special emphasis will be placed on instrumentation, accessibility, and experimental needs which justify the use of x‐ray microbeam analysis.