Superconducting NbN Resonator Parametric Amplifiers for Millimetre Wavelengths

TL;DR

This paper presents a new NbN thin-film fabrication process for millimeter-wave parametric amplifiers, demonstrating high gain and reproducibility, with potential for broad frequency applications.

Contribution

Developed a reactive sputtering process for high-$T_c$ NbN films with high resistivity, enabling efficient millimeter-wave parametric amplification.

Findings

Achieved NbN films with $T_c$ of 10.5 K and resistivity of ~1000 μΩ·cm.

Fabricated resonator amplifiers with over 20 dB gain at 25 GHz.

Amplifiers showed artefact-free, reproducible profiles matching theoretical models.

Abstract

We report the development of a reactive sputtering process for high $T_\mathrm{c}$ NbN films with high normal-state resistivity, tailored for kinetic inductance parametric amplifiers. The process includes precise control to ensure full nitridation of the target prior to deposition. Under optimized conditions, the resulting NbN thin films exhibit a critical temperature of $10.5\,\mathrm{K}$ and a resistivity of $\sim1000\,\mathrm{\mu\Omega\,cm}$. The high $T_\mathrm{c}$ of the NbN thin-films suggests strong potential for application over the entire millimetre-wave frequency range from $24\,\mathrm{GHz}$ to $300\,\mathrm{GHz}$, whereas the high resistivity suggests a reduced power requirement for the pump tone to achieve high gain. Resonator parametric amplifiers have been fabricated from these films using coplanar waveguide geometry. The devices were able to produce high gain exceeding $20\,\mathrm{dB}$ at $25\,\mathrm{GHz}$, with artefact-free, reproducible amplification profiles in good agreement with theoretical models.