HIGH-RESOLUTION FRESNEL ZONE PLATES FOR SOFT X-RAYS

We developed a complete system for fabrication of Zone Plates (ZPs) for soft x-rays covering all steps from design to absorber growth and to automatic dimensional inspection and final tests. The design phase is carried out by a custom software optimized for high resolution ZPs. The substrates used are 100 mn thick silicon nitride membranes, coated with a 5 nm chromium adhesion layer, a 5 nm gold electroplating base and a 200 nm thick layer of 950 K PMMA resist. The writing phase is accomplished by a Leica EBMF 10 cs/120 electron beam lithography machine operating at 30 kV. The absorber material is 120 nm electroplated gold. 100 mu m diameter soft x-rays ZPs having line-to-space ratios of l:1 (according to the theory) and 3:1 for 2(nd) order, double resolution microscopy were fabricated. A second custom software was exploited to extract metrological information on ZPs by using the EBL machine used in metrological mode. The ZPs have been tested at NSLS, Brookhaven Laboratory. First order ZPs showed a resolution of approximately 120 nm, while for second order it was approx. 60 nm. For some samples the first order efficiency was found to be 9%, close to the maximum theoretical value.