X-ray techniques have grown in popularity over the past decade. Owing to the high penetrating power and non-destructive capabilities of X-ray radiation, these techniques have opened new frontiers for material characterization. In spite of these advantages, however, conventional laboratory X-ray instrumentation is intrinsically limited by the brightness of the illuminating x-rays, which, in turn, limits the detection sensitivity and spatial resolution of commercial X-ray fluorescence spectrometers (XRFs). Conventional laboratory micro-XRFs are typically limited to detection sensitivities in the ppm range, with analytical spot sizes on the order of 10s to 100s of microns. For higher sensitivities or finer spatial resolutions, researchers typically must select alternative techniques, such as LA-ICP-MS or EDS; however, these techniques have the disadvantage of either being destructive (as in the case of LA-ICP-MS) or limited in sensitivity and to surface features only (as for EDS).
In designing a new generation of laboratory micro-XRFs, we began by redesigning the X-ray source. Using a fine array of micro-drilled targets embedded in diamond, the heat dissipation has been significantly enhanced, producing an X-ray source that is over 50x brighter than conventional micro-XRF laboratory sources. This novel source technology is then paired with a twin paraboloidal X-ray mirror lens, fabricated with minimal slope errors and smoothness on the order of single-digit angstroms, providing achromatic focusing with long working distances (~30 mm) and enabling high-spatial resolutions to be achieved. In order to maximize the x-ray fluorescence flux and detection sensitivity, the source is typically customized with the most appropriate X-ray target material(s), may be selected and tuned for each characterization study, varying the x-ray fluorescence cross section by several orders of magnitude.
Combining this high brightness X-ray source with the precision X-ray optics has produced a unique approach to laboratory micro-XRF, providing spatial resolutions on the order of single-digit microns and detection sensitivities in the sub-femtogram (attogram) regime. This system is also designed with a variety of X-ray detectors, enabling correlative in situ X-ray radiography and spectroscopy, in order to quickly identify regions of interest and analyze them with micro-XRF. Here, we will review the design principles of this novel micro-XRF, describing in more detail how the paired X-ray source and twin paraboloidal optics work together to provide enhanced detection sensitivities and spatial resolutions. We will then demonstrate the application of this technique to a range of studies, including advanced material characterization, microelectronics inspection, and environmental contamination, as time permits.