Abstract
Currently, superconductor digital integrated circuits fabricated atHYPRES, Inc. can operate at clock frequencies approaching 40 GHz. Thecircuits present multilayered structures containing tens of thousands ofNb/Al/AlOx/Nb Josephson junctions (JJs) of various sizes interconnected by four Nb wiring layers,resistors, and other circuit elements. In order to be fully operational, the integratedcircuits should be fabricated such that the critical currents of the JJs are within thetight design margins and the proper relationships between the critical currents ofJJs of different sizes are preserved. We present experimental data and discussmechanisms of process-induced variations of the critical current and energy gap ofNb/Al/AlOx/Nb JJs in integrated circuits. We demonstrate that the Josephson critical current may dependon the type and area of circuit elements connected to the junction, on the circuit pattern,and on the step in the fabrication process at which the connection is made. In particular,we discuss the influence of (a) the junction base electrode connection to the groundplane, (b) the junction counter electrode connection to the ground plane, and(c) the counter electrode connection to the Ti/Au or Ti/Pd/Au contact padsby Nb wiring. We show that the process-induced changes of the properties ofNb/Al/AlOx/Nb junctions are caused by migration of impurity atoms (hydrogen) between the differentlayers comprising the integrated circuits.
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