Fabrication of Submicrometer High Current Density ${\hbox{ Nb/Al-AlN}}_{\rm x}/{\hbox{ Nb}}$ Junctions

We have developed a sub-mum Nb/Al-AlN
_x
/Nb Josephson junction and integrated circuit fabrication process using deep-UV lithography and inductively coupled plasma etch tools. The baseline process consists of 11 masking steps including ground plane, PdAu resistor, Nb/Al-AlN
_x
/Nb trilayer, and two additional Nb wiring layers. The AlN
_x
tunnel barriers are grown with plasma nitridation. These junctions exhibit low subgap leakage even at current densities exceeding 100 kA/cm
²
. The critical current spread of a series array of 50- kA/cm
²
, 0.6 mum diameter junctions is under 3%. For very high current density applications, these junctions are a good candidate to replace Nb/Al-AlO
_x
/Nb junctions particularly in future generations of very high speed, rapid single flux quantum logic circuits. In this paper we discuss our baseline fabrication process and device characterization including junction capacitance extraction from direct measurements of the Josephson plasma frequency.