Submillimeter-Wave Image Sensor

NASA's Jet Propulsion Laboratory, Pasadena, California

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.

Figure 1. In a Submillimeter-Wavelength Camera, incident radiation would be focused onto the image sensor, which would contain a grid of closely spaced dipole antennas and associated detector circuitry.

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.

Figure 2. Antennas Would Be Arrayed on a GaAs membrane grid, along with other circuit elements (which are omitted from this drawing for clarity). The indicated dimensions are for a design operating frequency of 2.5 THz. In principle, an array could be made larger than the 5 by 10 pixels shown here; the practical upper limit on size (several square millimeters) depends on the strength of the membrane grid.

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