Investigating Millimeter-Wave Thin-Film Superconducting Resonators: A Study Using Tunnel Junction Detectors

Investigations into the propagation characteristics, specifically loss and wave velocity, of superconducting coplanar waveguides and microstrip lines were conducted at a 2 mm wavelength. This was achieved through the measurement of on-chip half-wavelength resonators, employing superconductor-insulator-superconductor tunnel junctions as detectors. A continuous wave millimeter wave probe signal was introduced to the chip via a silicon membrane-based orthomode transducer. This setup not only facilitated the injection of the probe signal but also provided a reference path essential for differential measurements. The observed resonance frequencies aligned closely with theoretical predictions, exhibiting a discrepancy of only several percent. However, the measured losses significantly exceeded those anticipated from quasi-particle loss mechanisms, suggesting the presence of additional loss factors. Notably, the measurement results revealed that the tangential loss attributable to the dielectric layer, specifically silicon dioxide, was approximately <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\text{7}\pm \text{2} \times \text{10}^{-\text{3}}}$</tex-math></inline-formula>. This factor emerged as the dominant contributor to overall loss at temperatures around <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\text{4}\,{\text{K}}$</tex-math></inline-formula>.