Fabrication and Characteristics of All-NbN SQUID Series Array

In this article, an all-NbN superconducting quantum interference device (SQUID) array was developed for future applications by taking advantage of the higher critical temperature of a NbN film compared with Nb. The NbN SQUID array was composed of 100 SQUIDs with equal loop areas connected in series. The array was fabricated on a single-crystal MgO (100) substrate using 3-μm-diameter NbN/AlN/NbN Josephson junctions. The hysteresis and shielding parameters of the SQUID were 0.67 and 0.15, respectively. The SQUIDs in the array were coherently modulated by injecting current in a common flux control line, which resulted in a modulation voltage of 3.54 mV with consistent periodicity and a flux-to-voltage transfer coefficient of 23 mV/Φ0. Using a flux locked loop circuit, the fabricated NbN SQUID array exhibited excellent performance with a low white flux noise of 1.4 μΦ
₀
/√Hz, which included the preamplifier noise.