Master thesis defense by Filip Manu-Marin

Fabrication of Josephson junction for superconducting Quantum Computing Processors

The development of scalable, fault-tolerant superconducting quantum processors relies on achieving high reproducibility of qubit parameters. Signal crosstalk imposes constraints on the frequency separation between neighboring qubits. Obtaining high uniformity in transmon qubit frequency depends fundamentally on the reproducibility of the normal-state resistance of their underlying Josephson junctions. This apparent limitation is attributed to variations in both junction area and tunnel barrier thickness that arise from the fabrication process.

In this study, resistance measurements from a statistically significant number of junctions across twelve chips demonstrate intra-chip relative deviations of 6.04-1.37% and chip-to-chip variations of 9.06-3.99%. These results correspond to four distinct design areas within a range of 0.0225-0.2025 μm2 and translate to 91-51 MHz deviations (i.e. 3.36-0.71%) in qubit frequency. This was achieved by optimizing the beam kinetics and source geometry of the electron-beam evaporation system. Furthermore, the effect of different design and fabrication parameters was systematically analyzed by evaluating electrode thickness ratios (15/90 nm and 40/80 nm) alongside oxidation environments (120 mBar at 10 min and 40 mBar at 30 min). The higher-pressure, thicker bottom-electrode process achieved the best overall baseline reproducibility. These results were further extended to wafer-scale for 2025 junctions spread across a 66.22 x 66.22 mm2 area yielding a 2.54% relative deviation in resistance. The set fabricated at a lower oxidation pressure resulted in a thinner and less uniform tunnel barrier, which degraded reproducibility. Ultimately, an asymmetric 15/90 nm electrode ratio processed at high pressure is found to be uniquely suited for highly coherent transmon qubits. This configuration balances good scaling uniformity with the capacity to suppress quasiparticle tunneling in large quantum processors for operational qubit frequencies up to 7 GHz.