Highly tunable NbTiN Josephson junctions fabricated with focused helium ion beam

We demonstrate a direct writing method for the fabrication of planar Josephson junctions from high-quality superconducting niobium titanium nitride (NbTiN) thin films, by creating local disorder using focused He ion beam irradiation. We control the suppression of superconductivity in NbTiN as a function of the helium ion beam fluence, enabling us to successfully fabricate Josephson junctions with highly tunable weak links ranging from the metallic to the insulating phase, utilizing the continuous nature of the disorder-induced superconductor-insulator transition. We demonstrate the successful fabrication of both superconductor-normal metal-superconductor (SNS) and superconductor-insulator-superconductor (SIS) type of devices, and show that we can achieve an exceptionally wide range ( <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:mo>∼</a:mo> <a:mn>5</a:mn> </a:mrow> </a:math> orders of magnitude) of critical current densities and junction resistances. The SNS-type junctions follow closely the ideal resistively and capacitively shunted junction behavior, have high characteristic voltages up to <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:mrow> <b:mo>∼</b:mo> <b:mn>1.5</b:mn> <b:mspace width="0.16em"/> <b:mi>mV</b:mi> </b:mrow> </b:math> , and show Shapiro steps up to very high orders, while the SIS junctions also follow established theories well. The results suggest that the junctions are suitable for a wide range of applications in superconducting electronics because of the excellent mechanical, electrical, and microwave properties of NbTiN. In particular, as NbTiN has previously been used to fabricate high-quality factor microwave resonators, we see the method as a promising and simple way to realize high-frequency superconducting qubits and other quantum devices using only a single superconducting film.