Fabrication-process-induced variations of Nb/Al/AlO_x/Nb Josephson junctions in superconductor integrated circuits

Abstract

Currently, superconductor digital integrated circuits fabricated at HYPRES,\nInc. can operate at clock frequencies of ~ 40 GHz. The circuits present\nmultilayered structures containing tens of thousands of Nb/Al/AlOx/Nb Josephson\njunctions (JJs) of various sizes interconnected by four Nb wiring layers,\nresistors, and other circuit elements. In order to be operational, the\nintegrated circuits should be fabricated such that the critical currents of\nJosephson junctions remain within the tight design margins and the proper\nrelationships between the critical currents of JJs of different sizes are\npreserved. We present experimental data and discuss mechanisms of\nprocess-induced variations of the critical current and energy gap of\nNb/Al/AlOx/Nb Josephson junctions in integrated circuits. We demonstrate that\nthe Josephson critical current may depend on the type and area of circuit\nelements connected to the junction, on the circuit pattern, and on the step in\nthe fabrication process at which the connection is made. In particular, we\ndiscuss the influence of a) junction base electrode connection to ground plane,\nb) junction counter electrode connection to ground plane, and c) counter\nelectrode connection to Ti/Au or Ti/Pd/Au contact pads by Nb wiring. We show\nthat the process-induced changes of the properties of Nb/Al/AlOx/Nb junctions\nare caused by migration of impurity atoms (hydrogen) between different layers\ncomprising the integrated circuits.\n