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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Refer to NPO-20303
This Brief includes a Technical Support Package (TSP).
Fabrication of diffractive GaAs microlenses
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Overview
The document presents a technical support package from NASA detailing a novel method for fabricating diffractive GaAs microlenses, which are essential for enhancing the performance of infrared imaging systems. The process involves using electron-beam lithography to create continuous relief patterns on a poly(methyl methacrylate) (PMMA) layer, which is then transferred to a GaAs substrate through an electron cyclotron resonance (ECR) plasma etching technique.
The primary motivation behind this research is to overcome the limitations of traditional binary optics fabrication methods, which rely on discrete phase levels and require multiple lithography and etching steps. These conventional methods often lead to alignment errors and inefficiencies, significantly reducing the optical element's performance. In contrast, the proposed technique allows for the creation of a Fresnel lens in a single step, thereby eliminating alignment issues and enhancing the fidelity of the final optical design.
The document outlines the technical process in detail, starting with the sampling of a depth profile on a grid to form a pixelized profile for e-beam fabrication. The pixel depths are converted to e-beam doses using a nonlinear depth versus dose function specific to PMMA. The exposure is conducted using a JEOL JBX-5DII electron beam tool, and the resulting PMMA pattern is developed and transferred into the GaAs substrate through plasma etching. The etching process is carefully controlled to maintain the integrity of the PMMA and achieve a high GaAs/PMMA etch ratio, which is crucial for the successful fabrication of the microlenses.
The document emphasizes the advantages of this method, including the potential for achieving near-ideal efficiency in the optical elements, with fabrication errors likely reducing efficiency by less than 10%. The continuous relief structure produced by this technique is designed to focus incident plane-wave radiation to a diffraction-limited spot, making it highly effective for applications in back-side-illuminated imaging arrays, particularly in GaAs-based quantum-well infrared photodetectors.
Overall, this innovative approach represents a significant advancement in the field of diffractive optics, promising to enhance the efficiency and accuracy of optical elements used in various high-tech applications.
