Terahertz (THz) frequencies have been proving quite useful for research in materials science and medical analysis. The challenge is in generating and directing signals at such high frequencies without excessive loss. By using micromachining techniques on silicon wafers, researchers at the Jet Propulsion Laboratory (JPL) of the California Institute of Technology were able to fabricate two antennas for use at 1.9 THz based on a leaky-wave waveguide feed and silicon microlens. That particular frequency is of interest for the heterodyne detection of the spectral line of ionized C+ fine structure transitions, as applied in galactic studies of dark clouds. The researchers worked under a contract from NASA, with the support of the Submillimeter-Wave Advanced Technology Group of JPL.
In constructing these antennas, the scientists were faced with the tradeoff between size and directivity: Longer horns are needed for higher directivity. The two antennas were a 2.6-mm-diameter microlens antenna with directivity of 33.2 dB and a 6.35-mm-diameter microlens antenna with directivity of 41.2 dB. The antennas were machined using deep reactive ion etching (DRIE) to form multiple-depth features with high aspect ratios on silicon wafers. The approach provides features with high precision and also enables integration of a large of a THz heterodyne receiver on a silicon wafer.
The two lenses were fabricated on different wafers, since they required different photoresist thicknesses and different silicon etch rates due to the different heights. The surfaces of the lenses were scanned to determine the accuracy of the fabrication process; they were found to achieve root-mean-square (RMS) errors in the neighborhood of 1.4 μm for the smaller lens and 9.9 μm for the larger lens. Measurements on the two antennas were made using a 1.9-THz transmitter chain starting with a 17-GHz frequency synthesizer feeding a commercial multiplier to reach 105.5 GHz.
The multiplier’s output signals were boosted by a WR-10 power amplifier and then doubled to produce signals at 211.1 GHz. A tripler transformed those signals to output signals at 633.3 GHz and 2 mW, which was fed to a tripler on a silicon wafer stack to yield 800 nW at 1.9 THz. A bolometer was used for the power measurements. The researchers were encouraged by the test results, which included radiation efficiencies of 70 and 60%, respectively, for the smaller and larger lens antennas. Future work will target the development of larger antennas with less surface defects and more precise curvature.
See “Development of Silicon Micromachined Microlens Antennas at 1.9 THz,” IEEE Transactions on Terahertz Science and Technology, Vol. 7, No. 2, March, 2017, p. 191.