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Currently, superconductor digital integrated circuits fabricated at HYPRES,Inc. can operate at clock frequencies of ~ 40 GHz. The circuits presentmultilayered structures containing tens of thousands of Nb/Al/AlOx/Nb Josephsonjunctions (JJs) of various sizes interconnected by four Nb wiring layers,resistors, and other circuit elements. In order to be operational, theintegrated circuits should be fabricated such that the critical currents ofJosephson junctions remain within the tight design margins and the properrelationships between the critical currents of JJs of different sizes arepreserved. We present experimental data and discuss mechanisms ofprocess-induced variations of the critical current and energy gap ofNb/Al/AlOx/Nb Josephson junctions in integrated circuits. We demonstrate thatthe Josephson critical current may depend on the type and area of circuitelements connected to the junction, on the circuit pattern, and on the step inthe fabrication process at which the connection is made. In particular, wediscuss the influence of a) junction base electrode connection to ground plane,b) junction counter electrode connection to ground plane, and c) counterelectrode connection to Ti/Au or Ti/Pd/Au contact pads by Nb wiring. We showthat the process-induced changes of the properties of Nb/Al/AlOx/Nb junctionsare caused by migration of impurity atoms (hydrogen) between different layerscomprising the integrated circuits.

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