Performance Test and Evaluation of Multilevel Fresnel Zone Plate with Three-Step Profile Fabricated with Electron-Beam Lithography

A multilevel Fresnel zone plate (FZP) designed as an approximation of a kinoform profile has been developed. The FZP is made by electron-beam lithography and reactive ion etching. The zone structure consists of three levels made from tantalum with a total thickness of 4 μm. The half pitch of the outermost zone structure is 0.6 μm. The theoretical efficiency of the 1st-order diffraction is 0.49 at an X-ray energy of 9.85 keV. A performance test is carried out at the beamline 20XU of SPring-8. The focused beam width is measured to be 0.6 μm. The measured efficiency of the 1st-order diffraction is 0.39 at 9.85 keV. Although the measured efficiency is less than the theoretical value, it is superior to the efficiency of an optimized tantalum binary FZP. The diffraction efficiencies of the 0th, $-1$st, and $\pm 2$nd orders are also measured. Using the measured data, the structure of the fabricated zone is evaluated.