A proposed monolithic planar array of miniature dipole antennas, diodes, and associated input/output circuitry would serve as a prototype of image sensors for submillimeter-wavelength video cameras (see Figure 1). Sensors of this type could be designed to operate as either direct or heterodyne detectors of electromagnetic radiation at frequencies from 300 GHz to 3 THz; as such, they could offer new capabilities for such diverse uses as analysis of submillimeter radiation from far-infrared devices, measurements of the submillimeter-wavelength radiative properties of materials, molecular-line spec-troscopy of astronomical bodies and the upper atmosphere of the Earth, and perhaps imaging of biomaterials for medical applications.
The development of the proposed submillimeter-wave image sensor would extend the recent development of a single high-sensitivity, 2.5-THz heterodyne Schottky-diode mixer based on a monolithic membrane diode (MOMED) micromachined in GaAs. The dipole antennas, diodes, and other circuit elements would be formed on a 3-µm-thick, epitaxially grown GaAs membrane grid that would be suspended from a relatively thick GaAs frame (see Figure 2). The antennas would lie within a quarter wavelength of each other (the diffraction limit) in the y axis, and the rows of antennas would be staggered along the x axis for partial filling in of nonoverlapping pixels.
Associated with each antenna would be a Schottky diode for detection and/or down-conversion, plus a low-pass radio-frequency filter transmission line for supplying dc bias and removing and distributing the low-frequency products of detection and/or down-conversion. The filters would be closely coupled high-and-low-impedance transmission lines that would provide open circuits at the input signal frequency near the antenna terminals.
The frame, the membrane grid, and the antennas, diodes, and other circuit elements, would be fabricated simultaneously, all as parts of a monolithic unit, by micromachining from GaAs in a process similar to that used previously to
fabricate the single 2.5-THz mixer. An important feature of the membrane-grid design is that the antennas would be surrounded mostly by airgaps, which would serve to reduce circuit losses, provide better beam efficiency, and make it impossible for radiation to propagate undesirably in substrate modes.
The otherwise bidirectional dipole antennas would be rendered unidirectional by incorporation of a reflecting ground plane a quarter wavelength back from the membrane surface. Additional intermediate-frequency (IF) and/or dc circuitry could be incorporated on the back of this ground plane, GaAs frame, and/or on the GaAs membrane grid, whichever is more convenient. The circuitry on the GaAs chip could be connected to external dc and IF circuitry through contact pads on the GaAs frame.
This work was done by Peter Siegel of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Electronics & Computers category.
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-20718, volume and number of this NASA Tech Briefs issue, and the page number.
This Brief includes a Technical Support Package (TSP).
Submillimeter-Wave Image Sensor
(reference NPO-20718) is currently available for download from the TSP library.
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Overview
The document presents a technical support package detailing the development of a novel submillimeter-wave imaging system, designed by Peter H. Siegel at the Jet Propulsion Laboratory (JPL) under a NASA contract. This innovative imaging system is based on a unique, completely monolithic device and antenna architecture, which allows for high-resolution imaging at submillimeter wavelengths, a domain that has not been extensively explored with single-pixel sensors.
The proposed imaging system can operate in both direct detection and heterodyne modes at room temperature, making it versatile for various applications. The design incorporates a multipixel detector array utilizing submicron Schottky barrier diodes integrated with a two-dimensional grid of closely spaced antennas, all fabricated on a thin GaAs membrane. This approach aims to overcome limitations faced by previous designs, such as high dielectric losses and difficulties in coupling GaAs devices with silicon antenna structures.
The document outlines the initial design and fabrication details for the imaging array front-end, emphasizing the potential for diffraction-limited resolution. Although the complete system has yet to be tested, the individual components, including THz diodes and the planar antenna grid architecture, have been successfully demonstrated. The anticipated applications for this submillimeter-wave camera include remote sensing of naturally occurring submillimeter wave sources in Earth's and planetary atmospheres, as well as material property analysis through reflection and transmission measurements. There are also prospects for medical applications, contingent on achieving sufficient frequency agility and sensitivity.
The document highlights the flexibility of the array design, which can be scaled in size and adapted with different antenna architectures to meet specific impedance matching or beam formation requirements. It also discusses the integration of IF/DC circuitry, which can be incorporated on the backplane or directly on the array chip, depending on convenience.
In summary, this technical support package outlines a promising advancement in submillimeter-wave imaging technology, with the potential to significantly enhance imaging capabilities across various scientific and practical fields. The work is positioned at the intersection of cutting-edge materials science and engineering, paving the way for future innovations in imaging systems.
