1 April 1983 Application Of Zone Plates To Alignment In X-Ray Lithography
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Fresnel zone plate patterns, when printed on masks and wafers and suitably illuminated, form images that are useful for alignment in microlithography. These images are observed to be largely unaffected by poor edge definition or by pattern defects arising from variations in processing and linewidth control. This paper describes how these zone plates are used for automatic alignment in x-ray lithography. To obtain high contrast bright images with good signal-to-noise ratio (important in automatic alignment), the zone plates are illuminated with a collimated HeNe laser beam. The images formed at the focus are found to be nearly diffraction limited with diameters determined by the zone plate aperture ratio. With zone plates approximately 100 um in diameter and 300 um focal length, we have observed signal-to-noise ratios better than 20 dB and have been able to obtain position accuracies better than ±0.1 um. Our geometry is chosen so that magnification errors in the mask or wafer up to ~1 um are automatically compensated. A microprocessor-based system that performs all the alignment functions automatically is described.
M. Feldman, M. Feldman, A. D. White, A. D. White, D. L. White, D. L. White, } "Application Of Zone Plates To Alignment In X-Ray Lithography," Optical Engineering 22(2), 222203 (1 April 1983). https://doi.org/10.1117/12.7973083 . Submission:

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