Bragg-Fresnel optics for hard x-ray microscopy: Development of fabrication process and x-ray characterization at the Advanced Photon Source

Results are presented on development of processes for fabricating linear and circular Bragg-Fresnel lenses (BFLs) on Si and III-V compound material substrates, and on x-ray characterization of linear BFLs at the Advanced Photon Source (APS). Processes were developed for fabricating long (zone length >5 mm) linear BFLs on Si with enhanced capability for focusing high-energy x rays. By stitching together 20 sequentially exposed 400-mu m-long linear BFLs, we were able to fabricate 8-mm-long linear BFLs with 0.5-mu m finest zone width. BFLs were also fabricated on III-V compound semiconductor substrates GaAs and InP, with improved process control due to the substantially reduced zone thickness required (similar to 50% less than Si). Reduction of the zone aspect ratio (thickness/width) lessens the demand on the process technology, and may lead to higher lens resolution and pattern transfer accuracy. A process was explored to fabricate BFLs on a GaAs/AlGaAs heterostructure incorporating a built-in "etch stop" layer to ensure uniform zone thickness. Experimental characterization of the focusing properties of a field-stitched 8-mm-long linear BFL on Si (zone aperture=150 mu m) was conducted at APS undulator beamline 1-ID-C using 10-keV x rays. Based on measured focal plane intensity distribution, the focus was estimated to be 1.2 mu m, comparable to the geometrically demagnified source size of similar to 1 mu m. Lens efficiency was estimated to be similar to 30%. Work is underway to incorporate the BFL-microprobe in x-ray microdiffraction and fluorescence microscopy experiments to study spatially confined complex materials. (C) 1998 American American of Physics.