A planar array of microscopic diffractive optical elements that resemble macroscopic Fresnel lenses can be fabricated as an array of continuous relief patterns on a GaAs substrate by a procedure that includes the transfer of a corresponding array of patterns formed on a surface layer of poly(methyl methacrylate) (PMMA) by electron-beam lithography. The diffractive optical elements could be, for example, microlenses on back-side-illuminated imaging arrays of GaAs-based quantum-well infrared photodetectors (these microlenses are used to concentrate incident infrared light into sub-pixel-size active device areas). The fabrication procedure is not limited to arrays of microlenses; it can also be adapted to making other optical elements (for example, holograms or diffractive macrolenses) in GaAs.

A GaAs substrate to be patterned is first coated with PMMA to a suitable thickness (e.g., 2 µm). A pattern corresponding to an approximation of the desired surface relief pattern on GaAs is written in the PMMA by a scanning electron-beam apparatus, with local electron-beam dosage pixelized on a grid of suitably high resolution (e.g., 1-µm-square cells). The local electron-beam dosage to the PMMA must be varied to obtain the desired final local variation of surface height of the GaAs substrate, taking account of the effects of subsequent processing steps, including such complicating effects as nonlinear dose-vs.-depth relationships and back-scattering of the electron beam from previously written nearby areas. The exposed PMMA is then developed by spinning the coated substrate and spraying acetone down onto it, yielding an intermediate surface relief pattern on the PMMA. The total development time is usually about 10 seconds, and depths are usually accurate to within ±5 percent.

The surface relief pattern on the PMMA is transferred to the underlying GaAs by plasma etching. The PMMA-coated substrate is placed in an electron-cyclotron-resonance system, wherein it is cooled to 10 °C and etched, using Ar and BCl3as process gases. The chosen combination of process gases and physical processing conditions yields an advantageously high GaAs/PMMA etch ratio.

This work was done by Frederick Pool, Daniel Wilson, Richard Muller, and Paul Maker 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

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Fabrication of diffractive GaAs microlenses

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This article first appeared in the October, 1998 issue of Photonics Tech Briefs Magazine.

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