Microfabricated Waveguide Terminations for Wideband and Low-power THz Applications

Journal article, 2025

This paper presents the design, simulation, microfabrication, and characterization of broadband waveguide terminations intended as drop-in components in waveguide blocks. Two types of terminations were developed, fabricated and characterized. The first type employs a quartz-based E-probe to couple to a waveguide, integrating an on-substrate titanium nitride (Ti-N) alloy resistive absorber with broadband tuning circuitry. This design achieves a return loss of better than 20 dB over the 260-375 GHz frequency range. The second type features a finline to slotline transition, constructed from a 30 μm thick Si membrane covered with high resistivity Ti-N alloy. This load demonstrates a return loss of better than 20 dB across the 210-380 GHz band. In order to ensure the required performance of the loads at cryogenic temperatures, the sheet resistance of the employed Ti-N resistive film was characterized from room temperature to 4K employing a closed-cycle cryostat and a 4-probe measurement system. For comparative purposes, the room temperature performance of terminations employing a traditional EccosorbTM material was measured and compared with the proposed waveguide terminations. Furthermore, the sideband rejection ratio SBRR of a 2SB SIS mixer was evaluated at cryogenic temperatures using the proposed finline-based terminations for LO directional couplers and 90° RF hybrids in comparison with the EccosorbTM made terminations. The measurements showed that the performance of the proposed terminations outperforms those achieved with Eccosorb™ absorbers at room temperature and is comparable at cryogenic temperatures.