Zone plates are patterned areas with imaging properties. When printed on masks and wafers and suitably illuminated, they form high contrast images which are useful for alignment in microlithography. As with holograms, defects in the pattern arising from variations in processing and linewidth control and poor edge definition degrade the images only marginally. This paper describes the use of zone plates for automatic alignment in X-ray proximity printing. The best signal to noise ratios (important in automatic alignment) are obtained by using the zone plates to focus laser beams to nearly diffraction limited point sources. With zone plates approximately 100 microns in diameter and 300 microns focal length, we have obtained signal to noise ratios greater than 100 to 1 and position accuracies better than ±0.1 micron. In addition our geometry is chosen so that magnification errors in the mask or wafer up to ~1 micron are automatically compensated. A microprocessor based system is described which performs the alignment automatically.
A. D. White,
D. L. White,
"Application Of Zone Plates To Alignment In X-Ray Lithography", Proc. SPIE 0333, Submicron Lithography I, (30 June 1982); doi: 10.1117/12.933423; https://doi.org/10.1117/12.933423