Highly tunable NbTiN Josephson junctions fabricated with focused helium ion beam

Abstract

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 in a helium ion microscope. We show that we can 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 metallic to insulating phase, due to the continuous nature of the disorder-induced superconductor-insulator transition. We demonstrate the successful fabrication of both SNS and SIS type of devices, and show that we can achieve exceptionally wide range ($\sim$5 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 $\sim$ 1.5\,mV and show Shapiro steps up to very high orders, while the SIS junctions also follow established theories well. The results suggest that junctions fabricated with this method from NbTiN 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 superconducting qubits and other quantum devices using only a single superconducting film.