Shadow-Masked Angled Ion Milling for Junction Integration in Silicon-based Tantalum Superconducting Qubits

Surface dielectric loss remains a primary bottleneck limiting the energy relaxation time (T₁) of super-conducting qubits. In particular, the amorphization damage introduced by in-situ argon ion milling on silicon (Si) substrates during junction fabrication constitutes a critical local loss channel. Addressing the vulnerability of the junction substrate in the conventional Manhattan process, we propose a bandage-free, shadow-masked angled ion milling technique that is fully compatible with existing mask layouts to enhance the coherence of Ta-on-Si transmons. This process leverages the geometric masking effect of a high-aspect-ratio bilayer resist, combined with a 45° azimuthal ion beam alignment, to isolate the sensitive cross-junction region from direct ion bombardment while ensuring reliable superconducting interconnections under gentle milling conditions. Cryogenic characterization across three independent cooldowns demonstrates that the 45° angled milling effectively enhances qubit performance, with the sample-average T₁ improving from 67.4 μs in the standard 0° process to 98.7 μs, and the average quality factor (Q) increasing from 2.0×10⁶ to 2.5 × 10⁶. This work offers a practical nanofabrication strategy to suppress junction-region processing loss, providing a viable pathway for high-coherence silicon-based quantum processors.