Film Stress Influence on Nb/Al-AlOx/Nb Josephson Junctions

Nb/Al-AlOx/Nb Josephson junctions are widely used in superconducting quantum interference devices and rapid single flux quantum (RSFQ) circuits. Very large-scale RSFQ integration often has a strict requirement on wafer-scale junction qualities. In particular, Nb film stress in Nb/Al-AlOx/Nb trilayer based Josephson junctions is a critical factor affecting their qualities. In this work, we develop a multi-step sputtering process to prepare Nb films with different combinations of stress and roughness to investigate the correlation between Nb film stress and junction quality. In particular, this method allows us to prepare Nb films with almost the same roughness but different stress, a key factor to study solely the influence of stress on junction qualities. Our measurements indicate that within a very tight range of film roughness (less than 2 nm), tensile film stress is found to decrease the quality of Nb/Al-AlOx/Nb Josephson junctions and larger film roughness has a positive effect on junction quality. By decreasing the total tensile stress and increasing the film roughness properly, we fabricated Nb/Al-AlOx/Nb Josephson junctions with a high quality factor characterized by R-sg/R-N (30 on average). The minimum size of junctions was 1 mu m.