A Self-Flux-Biased NanoSQUID with Four NbN-TiN-NbN Nanobridge Josephson Junctions

Abstract

We report the development of a planar 4-Josephson-junction nanoscale superconducting quantum interference device (nanoSQUID) that is self-biased for optimal sensitivity without the application of a magnetic flux of Φ0/4. The nanoSQUID contains novel NbN-TiN-NbN nanobridge Josephson junctions (nJJs) with NbN current leads and electrodes of the nanoSQUID body connected by TiN nanobridges. The optimal superconducting transition temperature of ~4.8 K, superconducting coherence length of ~100 nm, and corrosion resistance of the TiN films ensure the hysteresis-free, reproducible, and long-term stability of nJJ and nanoSQUID operation at 4.2 K, while the corrosion-resistant NbN has a relatively high superconducting transition temperature of ~15 K and a correspondingly large energy gap. FIB patterning of the TiN films and nanoscale sculpturing of the tip area of the nanoSQUID’s cantilevers are performed using amorphous Al films as sacrificial layers due to their high chemical reactivity to alkalis. A cantilever is realized with a distance between the nanoSQUID and the substrate corner of ~300 nm. The nJJs and nanoSQUID are characterized using Quantum Design measurement systems at 4.2 K. The technology is expected to be of interest for the fabrication of durable nanoSQUID sensors for low temperature magnetic microscopy, as well as for the realization of more complex circuits for superconducting nanobridge electronics.