The fabrication and characterization of nano-SQUIDs based on Nb thin films

• We developed a nano-SQUID fabrication process starting from a high-quality thin film.
• The fabricated nano-SQUIDs exhibited flux modulation depth up to 10.3% at 4.6 K.
• The measured data agreed with the Ginzburg–Landau simulation.
• We found that a small critical current <50 μA is important for a deep flux modulation.
• The suggestions in improving the nano-SQUID’s performance were discussed.

SQUIDs with nano-junctions (or nano-SQUIDs) are able to be miniaturized into nanoscale to measure a single Bohr magneton. Here, we reported the development of a fabrication process for Nb (niobium) nano-SQUIDs using the thin film deposition and the electron-beam lithography technology. The developed process started from a high-quality superconducting thin film so that it is compatible with a variety of film growing techniques. The as-fabricated nano-SQUIDs exhibited functional flux modulation depth up to 10.3% at 4.6 K, in agreement with the numerical simulation based on the Ginzburg–Landau equation. By further comparing the results from both experiments and simulations, we found that a small critical current below ∼50 μA played a leading role in order to obtain a decent flux-modulation depth for Nb nano-SQUIDs.