Fabrication of reliable via conductors for niobium SFQ devices

The via-wiring process for niobium large-scale single-flux-quantum (SFQ) circuits was studied from two viewpoints: niobium deposition and inter-level-dielectric (ILD) SiO2 deposition. It was found that superconductive critical current density (J(c)) strongly depends on the target-to-wafer distance (TS) during niobium deposition. Transmission-electron-microscope observation revealed that the niobium film sputtered under a long-TS condition has enough step coverage, but less crystallinity and lower density. Moreover, deposition at a lower incident angle at the via walls was found to cause the degradation. Grain growth at an inclined crystal orientation during the deposition is thought to produce high porosity in the film, resulting in poor Superconductive characteristics. Vias formed in the SiO2 deposited without substrate biasing were also found to degrade niobium film crystallinity. The most probable cause of this niobium degradation is the influence from water (moisture) absorbed in the SiO2 vias. These results show that reliable vias can be fabricated by niobium sputtering with optimized TS and by SiO2 formation with substrate biasing. (c) 2005 Elsevier B.V. All rights reserved.