Electron-beam (e-beam) lithography has shown promise as a technique for fabricating diffractive optical elements on nonflat substrates. Such optical elements could include convex or concave diffraction gratings with curved grating lines, for use in imaging spectrometers or other scientific instruments operating at wavelengths from ultraviolet through midinfrared.

Heretofore, diffractive optical elements made, variously, by diamond ruling and optical holography have been available commercially on flat substrates only. The lines in these gratings have been straight or else have had modest, regular curvatures at most. In contrast, diffractive optical elements made by electron-beam lithography can have arbitrary line shapes and/or arbitrary phase functions.

A Substrate To Be Patterned by electron-beam lithography is partitioned into zones of different focal depth, in essentially the same manner in which elevation contours are formed on a topographical map. An electron-beam subpattern exposure is then performed in each zone.

The present electron-beam-lithographic technique is an extension of another, recently developed electron-beam-lithographic technique for writing phase holograms into thin films of poly(methyl-methacrylate) on flat substrates. By patterning and otherwise controlling the electron-beam exposure and monitoring the development/etching process until precise depths are achieved, one can adjust optical phase delays to a precision of less than 1/50th of a wavelength, within 0.5-µm-square regions. Devices produced by use of this technique on flat substrates include Fresnel lenses, arrays of Fresnel lenslets, gratings with both straight and curved grooves, holograms that yield gray-scale images, and patterns for free-space optical interconnections.

Application of the technique to a concave or convex substrate (see figure) involves the following sequence of steps:

  1. Establish a grid of points on the substrate.
  2. For each grid point, determine the electron-beam-apparatus focus, rotation, and deflection calibration values.
  3. From the values obtained in step 2, determine the depth of focus over which patterning errors can be considered negligible, and use the depth-of-focus information to define depth zones.
  4. Partition the exposure pattern into subpatterns — one subpattern for each depth zone.
  5. Using the electron-beam apparatus, expose each depth zone according to its subpattern. Readjust the apparatus, as needed, when proceeding to the next subpattern.

The technique has been demonstrated by using it to form a small prototype diffraction grating on a convex spherical substrate. In a test, the grating exhibited a first-order-diffraction efficiency of 88 percent. There was no evidence of degradation of the grating by curvature of the substrate. The prototype grating was small. Continuing development efforts are directed toward increasing the patterned area and decreasing the amount of light scattered (as distinguished from diffracted) by gratings of this type.

This work was done by Paul Maker, Richard Muller, and Daniel Wilson of Caltech for NASA's Jet Propulsion Laboratory. In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to:

Technology Reporting Office, JPL, Mail Stop 122-116, 4800 Oak Grove Drive, Pasadena, CA 91109; (818) 354-2240.

Refer to NPO-20296

Photonics Tech Briefs Magazine

This article first appeared in the May, 1999 issue of Photonics Tech Briefs Magazine.

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