Micromachined metallic substrates as a technological platform for superconductor-insulator-superconductor tunnel devices

Journal article, 2025

Millimeter (mm) and sub-mm superconducting structures, such as waveguides and mixers on substrate chips, are pivotal in sensing applications but often suffer from dielectric losses in the substrate at these wavelengths. To address this challenge, we introduce a method for fabricating Superconductor Insulator Superconductor (SIS) mixers that significantly mitigates theses losses by employing micromachined metallic substrates. This approach not only broadens the applicability of SIS devices, including THz mixers, but also demonstrates a novel broadband SIS mixer design for the band of 211-375 GHz. Our design features an electroplated Au substrate for effective waveguide-to-substrate transition and twin SIS junctions using Nb-Al/AlN-Nb technology We validate the viability and integrity of the SIS junction on metallic substrate with cryogenic transport measurements. Our results confirm the stability and quality of the junctions across junction sizes ranging from 0.5 µm² to 3.8 µm² highlighting the potential of our method to enhance the performances and reliability of SIS mixers for THz applications. This work proves the feasibility of our novel fabrication approach and lays the groundwork for future advancements in superconducting technology.