Fabrication of NbN-HEB mixers with fluoride radical etching Process

Hot electron bolometers (HEBs) are expected to be used as low-noise heterodyne mixers for applications in radio astronomy and remote sensing in the terahertz frequency region. We fabricated and tested HEB mixers for these applications; however, those we made with the conventional process easily broke due to thermal stress and electrical surges. To improve their durability, a new process involving fluoride radical etching was developed. In this process, HEB mixers were fabricated using a relatively thick NbN strip (several tens of nm). Finally, the NbN strip was etched that was less than 4-nm thick enough to operate as HEB mixers by using the fluoride radical etching. We confirmed that fluoride radical etching caused no damage to the etched NbN strip. The dc properties of the HEB mixers fabricated with the new process did not change for fourteen 4.2-300 K thermal cycles for 50 days. The receiver noise temperature of the HEB mixer was evaluated and it was about 615 K at 780 GHz.