Abstract

Fresnel zone plate lenses for focussing and imaging radiation in the 10-50 Å wavelength region have been fabricated with outer zone widths as small as 400 Å. The zone plates were fabricated by electron beam lithography using a high resolution vector scan system designed specifically for nanometer pattern writing. A special circular pattern generation technique, utilizing high speed polar/cartesian conversion was utilized. Field size was calibrated for squareness and orthogonality using electron beam lithography calibration marks and laser interferometry. Patterns were written in a 180-200 nm thick PMMA resist on a 30 nm gold electroplating base. The substrate was a 100 nm silicon nitride film. The pattern received a dose of 250 μC/cm² at 25 KeV. Special patterns requiring a thick central stop region, received a doubled dose, followed by a double development/double plating technique.

A method utilizing the phenomenon of form birefringence was used for monitoring the development of the fine features. In the optical microscope with polarized light and crossed analyzer, a distinct change of the reflected intensity is observed during the final stages of development.

Text patterns imaged at 45 Å wavelength demonstrate a spatial resolution approaching 500 Å. Measurements of zone plate efficiency are also described.

© 1988 Optical Society of America