Sub- Josephson Junctions for Superconducting Quantum Devices

For high-performance superconducting quantum devices based on Josephson junctions (JJs), decreasing lateral sizes is of great importance. Fabrication of sub-μm JJs is challenging, due to nonflat surfaces with step heights of up to several 100 nm generated during the fabrication process. We have refined a fabrication process with significantly decreased film thicknesses, resulting in almost flat surfaces at intermediate steps during the JJ definition. In combination with a mix-and-match process, combining electron-beam lithography and conventional photolithography, we can fabricate JJs with lateral dimensions down to 0.023 μm². We propose this refined process as an alternative to the commonly used chemical-mechanical polishing procedure. Transport measurements of JJs, having critical-current densities ranging from 50 to 10⁴ A/cm², are presented at 4.2 K. Our JJ process yields excellent quality parameters, R_sg/R_N up to ~ 50, V_m from 15 to 80 mV and V_gap up to 2.81 mV, and also allows the fabrication of high-quality, sub-μm wide, long JJs (LJJs) for the study of Josephson vortex behavior. The developed technique can also be used for similar multilayer processes and is very promising for fabricating sub- μm JJs for quantum devices such as SQUIDs, qubits, and SIS mixers.