Sub-μm 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-mu m JJs is challenging due to non-flat 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-&-match process, combining electron-beam lithography (EBL) and conventional photolithography, we can fabricate JJs with lateral dimensions down to 0.023 mu m^2. We propose this refined process as an alternative to the commonly used chemical-mechanical polishing (CMP) procedure. We present transport measurements of JJs at 4.2 K that yield critical-current densities in the range from 50 to 10^4 A/cm^2. Our JJ process yields excellent quality parameters, Rsg/Rn up to ~50 and Vgap up to 2.81 mV, and also allows the fabrication of high-quality sub-mu m 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-mu m JJs for quantum devices such as SQUIDs, qubits and SIS mixers.