Hydrogen-Inclusion-Induced Critical Current Deviation of Nb/AlOx/Nb Josephson Junctions in Superconducting Integrated Circuits

Abstract

Josephson junctions with niobium electrodes connected with palladium, which is employed in the bump metallization, have about 20% larger critical current density than those with electrodes not connected with palladium. This increase in the critical current density coincides with the desorption of hydrogen from the niobium electrodes. Hydrogen incorporates during the fabrication process and desorbs in an atmosphere when the niobium surface is covered with palladium. The decrease in hydrogen concentration in niobium electrodes causes an increase in the critical current density of the junctions. Most of the change can be attributed to the niobium work function change, with some of it due to the change in superconductivity of niobium. Elastic deformation can also be added as a cause. As hydrogen diffuses fast in niobium and stops in aluminum, hydrogen concentration differences arise within a circuit, which result in a critical current deviation beyond statistical scattering. This mechanism explains why the junctions at both ends of a serial array of circuits exhibit abnormal critical current about 20% larger than the average.

Aging- and annealing-induced variations in Nb/Al–AlOx/Nb tunnel junction properties

“…The normal state resistance of the junctions decays, which agrees with results reported in Refs. [8][9][10]. Furthermore, we have observed the junction size effect on R n aging behavior, similar to that, reported in Ref.…”

“…[8][9][10]. Unusually, this R n reduction was accompanied with an increase in the subgap resistance, R j .…”

Origin of hydrogen-inclusion-induced critical current deviation in Nb/AlOx/Al/Nb Josephson junctions

“…We have recently found that hydrogen inclusions in Nb electrodes induce a large I c deviation. 2,3 An identical report followed confirming the influence of hydrogen inclusion in Nb films on the I c values. 4 We stated in our previous report 3 that I c changes coincide with the inclusion/exclusion of hydrogen into/from Nb electrodes, and that they are gradual and reversible.…”

“…However, a part of the Pd remained in the via holes of the CTL layer in the way we explained in our previous reports. 2,3 The Pd areas for single junction TEGs were formed on and within the Nb pads using the lift-off method. The electrical measurements could be performed without Pd removal.…”

“…3 The purpose of this paper is to further investigate and clarify the mechanism underlying the I c variation caused by hydrogen.…”