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-μm-long linear BFLs, we were able to fabricate 8-mm-long linear BFLs with 0.5-μm 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 (∼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 μm) 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 μm, comparable to the geometrically demagnified source size of ∼1 μm. Lens efficiency was estimated to be ∼30%. Work is underway to incorporate the BFL-microprobe in x-ray microdiffraction and fluorescence microscopy experiments to study spatially confined complex materials.