Membrane based quasi-optical superconducting HEB mixers at Terahertz frequencies

Superconducting Hot Electron Bolometer mixers offer the highest sensitivity for heterodyne detections at frequencies above 1 THz. Important efforts have been made these recent years to further increase the HEB mixers' sensitivity and working frequency and also to design multi-pixel configuration. We present in this paper the developments of a non-standard quasi-optical membrane based HEB mixer where the commonly used focusing element, the silicon lens, is replaced by a micro-mirror and a membrane-based back-short. This configuration offers many advantages: easier processing for circuits at very high frequencies, better noise temperature brought by lower RF coupling loss and higher gain of the antenna. This design is also considered very attractive for multi-pixel receivers. The devices are made of phonon-cooled NbN HEB mixers processed on 1.4 mu m thick stress-less Si3N4/SiO2 membrane. Quasi-optical designs have been made for frequencies at 600 GHz and 2.5 THz. The design and the device fabrication process will be discussed and both DC and RF measurements at 600 GHz will be presented.