Defects of Nb/AlOx/Nb Josephson Junctions Caused by Underneath Fine Particles

Almost all defects in Nb/AlOx/Nb Josephson junctions (JJs) give rise to JJs with irregularly large critical current (I_c) compared with normal I_c distribution. We experimentally demonstrated the cause of the leakage JJs is fine particles underneath Nb/AlOx/Nb tri-layer. Since their size and density are too small and low to observe using ordinary methods such as microscopes, we think the particles are a previously unknown origin of JJ defects. The particles are included both superconductor and dielectric films. They are generated during their depositions. We expected that scales of superconducting integrated circuits can be improved to a few hundred thousand or a few million JJs by reducing the particles.