Fabrication of high current density Nb integrated circuits using a self-aligned junction anodization process

We have developed a self-aligned Nb/Al-AlO_x/Nb junction anodization process that allows outside junction contacts and relaxed contact alignment. In this process, the junction, rather than the junction contact, becomes the minimum definable feature size. Junction size is limited only by the resolution of the lithography and etch tools, which is 0.65 μm in our foundry. The self-aligned junction anodization process allows a significant increase in circuit speed due to the decrease in minimum junction size and increase in junction critical current density without investment in new fabrication tools. This process requires only one additional, noncritical masking step and has no impact on existing design rules. We describe the fabrication and electrical characteristics of lightly anodized junctions and arrays at 8 kA/cm² and the development of new 5 Ω/sq. MoN_x and 0.15 Ω/sq. Mo/Al resistors. We also discuss the 300 GHz T flip-flop benchmark results from our new 8 kA/cm², 1.25 μm Nb integrated circuit process and compare these results to other Nb processes.