Zone Plate Replication by Contact X-ray Lithography, and Its Application to Scanning X-ray Microscopy

Abstract

The resolution obtainable with an x-ray microscope which uses a Fresnel zone plate as the focusing element is primarily dependent on the width of the finest ring of the zone plate and the accuracy of the zone plate pattern (Simpson, 1984). High accuracy zone plates with outermost zone widths of only a few hundred angstroms have been made by carbon contamination e-beam lithography at Kings College London (Buckley et al., 1985). Whilst these zone plates have the finest zone widths yet produced, the fabrication of one zone plate takes ten hours and the carbon material from which the absorbing zones are formed is semitransparent to soft x-rays. This lack of opacity results in a reduction of focusing efficiency of about a factor of ten compared to the theoretical figure. The reduced efficiency increases the length of time required to form an image in a scanning x-ray microscope (Kenney et al., this volume). The time taken to form an image in an x-ray microscope should be as short as possible to minimize specimen deterioration and maximize throughput. For this reason, the replication of carbon zone plates in a higher contrast material, to improve efficiency and minimize imaging times, is highly desirable.