Microfabricated Waveguide Terminations for Wideband and Low-Power THz Applications

This article 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 four-probe measurement system. For comparative purposes, the room temperature performance of terminations employing a traditional Eccosorb material was measured and compared with the proposed waveguide terminations. Furthermore, the sideband rejection ratio of a 2SB superconductor-insulator-superconductor mixer was evaluated at cryogenic temperatures using the proposed finline-based terminations for LO directional couplers and 90° RF hybrids in comparison with the Eccosorb 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.