Submillimeter-wave radar systems operating at beyond 300 GHz are effective solutions for imaging concealed weapons at security checkpoints. These fine wavelengths can detect metal objects through most clothing without the health risks posed by x-ray imaging systems.
In pursuit of a practical submillimeter-wave front end, researchers at the California Institute of Technology’s Jet Propulsion Laboratory (JPL; Pasadena, Calif.), under contract with the National Aeronautics and Space Administration (NASA), developed an eight-pixel transceiver array for use in a 340-GHz imaging radar. The array was fabricated by silicon micromachining for relatively low cost and with high circuit density, with 12-mm pixel spacing in a vertically integrated waveguide configuration.
This work builds on active imaging techniques developed at JPL in the development of a frequency-modulated, continuous-wave (FMCW) radar that measures the time of flight between the system and a target by transmitting chirped tones, then demodulating received signals for determining the range to the point of focus on a target. Because the scanning speed of JPL’s radar system was limited by a mechanical scanning mechanism, the authors sought an eight-pixel transceiver array capable of faster scanning frame rates.
The researchers’ experiments produced a transceiver array capable of operating from 324 to 354 GHz. The transceiver provides 0.5 mW transmit power per pixel with conversion loss of 8 dB. Performance is ultimately limited by combining the receiver and transmitter paths in a 3-dB waveguide hybrid coupler with about 28 dB isolation. High isolation is required of the hybrid coupler to achieve good system sensitivity because of transmitter phase noise leakage that degrades receiver performance. The High Frequency Structure Simulator (HFSS) electromagnetic (EM) simulation software from ANSYS was used to simulate and optimize the hybrid coupler.
Silicon waveguide structures for the transceiver array were fabricated at JPL’s Micro-Devices Lab using a multi-etch-depth deep reactive ion etching (DRIE) silicon micromachining process. A tiered hard mask was employed to define all the circuit patterns prior to silicon etching, thus avoiding spinning photoresist across a silicon wafer, as well as minimizing pits and over-etched channels in the wafer.
Deep waveguide trenches were formed in an 800-μm-thick silicon wafer, with through-wafer waveguides etched from both sides of the wafer to minimize surface roughness and loss. Experiments using a single pixel of the array have provided effective imaging of concealed weapons, even when concealed beneath thick leather jackets. See “A Silicon Micromachined Eight-Pixel Transceiver Array for Submillimeter-Wave Radar,” IEEE Transactions on Terahertz Science and Technology, Vol. 5, No. 2, March, 2015, p. 197.
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