Improving Josephson junction reproducibility for superconducting quantum circuits: shadow evaporation and oxidation

The most commonly used physical realization of superconducting qubits for quantum circuits is a transmon. There are a number of superconducting quantum circuits applications, where Josephson junction critical current reproducibility over a chip is crucial. Here, we report on a robust chip scale Al/AlO_x/Al junctions fabrication method due to comprehensive study of shadow evaporation and oxidation steps. We experimentally demonstrate the evidence of optimal Josephson junction electrodes thickness, deposition rate and deposition angle, which ensure minimal electrode surface and line edge roughness. The influence of oxidation method, pressure and time on critical current reproducibility is determined. With the proposed method we demonstrate Al/AlO_x/Al junction fabrication with the critical current variation $σ/I_c$ less than 3.9% from $150times200$ to $150times600$ $nm²$ area and 7.7% for $100times100$ $nm²$ area over 20times20 mm² chip. Finally, we fabricate separately three 5times10 mm² chips with 18 transmon qubits (near 4.3 GHz frequency) showing less than 1.9% frequency variation between qubit on different chips. The proposed approach and optimization criteria can be utilized for a robust wafer-scale superconducting qubit circuits fabrication.