Abstract

Although zone plates have been used as focusing elements since the 1870s, recent advances in fabrication technology have made possible high resolution zone plates for x-ray microscopy use. The zone plate pattern can be either produced holographically by the interference of two coherent wavefronts or by a high resolution tool such as an electron beam lithography system where a small diameter electron beam, under computer control, writes the pattern. This paper concentrates on the latter. The diffraction-limited resolution of x-ray zone plates is approximately equal to the smallest zone width so small linewidth structures are required for high resolution. Producing small linewidth features is not sufficient, since each zone must be placed in the proper position to a tolerance which is a fraction of the outermost zone width. Also, practical working distances require long focal lengths. For a given resolution, the longer the focal length the larger the zone plate diameter. Therefore, the challenge for the lithographer is to produce a pattern over a relatively large area with small linewidth features that are precisely placed. To rest the result various moire techniques can be used as well as the actual x-ray optical performance. Experimental results are presented.

© 1989 Optical Society of America