Process Control Monitoring for Fabrication Technology of Superconducting Integrated Circuits

We have developed a 6-kA/cm² superconducting integrated circuit fabrication process toward large-scale single flux quantum digital circuits. To evaluate the process reliability and controllability, a set of process control monitors (PCMs) for monitoring fabrication and design parameters was designed and tested. Fabrication parameters, for example, film thicknesses and etched depths of the layers, were measured by testing specific patterns during fabrication. Design parameters such as the critical current density
J
_c
, normal-state resistance
R
_n
and actual sizes of Josephson junctions, the sheet resistance of Mo resistor
R
_sh
, and the penetration depth of Nb films were evaluated by electrically characterizing PCM circuits after fabrication. We also monitored superconducting critical currents for vias and Nb lines, and insulation properties between the metal layers systematically.