Plasma process-induced damage to Josephson tunnel junctions in superconducting integrated circuits

Abstract

It has been found that the critical current of Josephson junctions in superconducting integrated circuits may depend on the environment surrounding the junctions and on how a particular junction is connected (wired) to other junctions and circuit elements. This may cause large, pattern-dependent deviations of the junctions' critical currents from design values and ultimately limit the yield and performance of superconducting digital integrated circuits. In particular, we have found a difference in the critical current of grounded and floating junctions, and a dependence of the critical current on the size of metal structures connected to the junction—the 'antenna' effect. Experimental data were obtained for Nb/AlOx/Nb Josephson junctions fabricated on 150 mm wafers by an 11-layer process for superconducting integrated circuits. The results are explained by plasma process-induced damage to ultra-thin tunnel barriers. The most damaging plasma processing fabrication steps are discussed.