3D x-ray fluorescence microscopy with 1.7 μm resolution using lithographically fabricated micro-channel arrays

We report the fabrication and characterization of lithographically-fabricated arrays of micron-scale collimating channels, arranged like spokes around a single source position, for use in 3D, or confocal x-ray fluorescence microscopy. An early energy-independent depth resolution of 1.7 +/- 0.1 mu m has been achieved from 4.5-10 keV, degrading to 3 +/- 0.5 mu m at 1.7 keV. This represents an order-of-magnitude improvement over prior results obtained using state-of-the-art, commercial polycapillaries as the collection optic. Due to their limited solid angle, the total collection efficiency of these optics is approximately 10x less than that obtained with polycapillaries. Three designs have been tested, with 1,2, and 5-mu m-wide channels ranging from 30-50 mu m in depth and 2 mm in length. In addition to characterizing the devices in confocal geometry, the transmission behavior of individual channels was characterized using a small, highly collimated incident beam. These measurements reveal that, despite taking no particular steps to create smooth channel walls, they exhibit close to 100% reflectivity up to the critical angle for total external reflection. Most of this reflected power is spread into a diffuse angular region around the specular reflection condition. These results significantly impact future designs of such collimating channels, since transmission through the channels via side-wall reflection limits their collimating power, and hence device resolution. Ray-tracing simulations, designed specifically for modeling the behavior of channel arrays, successfully account for the transmission behavior of the optics, and provide a useful tool for future optic design.