Characterization of a Fresnel zone plate using higher-order diffraction

The performance of a Fresnel zone plate has been tested by observing the focusing property of higher-order diffraction. The Fresnel zone plate was fabricated by the electron-beam lithography technique. The zone material was made from 1 mum-thick tantalum and the outermost zone width was 0.25 mum. The third-order focused spot size measured by the knife-edge scan method was 0.1 mum full width at half-maximum at an X-ray energy of 8 keV, which is exactly equal to one-third of the first-order focal spot size.