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Chinese Physics B

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

Yi Zhao, Wei Xin, Zhenchuan Zhang, Yipeng Ouyang, Yuting Sun, Xiaoyu Xia, Jie Zhao, Wen Zheng, Yang Yu

Published Sep 15, 2026• 0 citations
Publisher

Abstract

Abstract Surface dielectric loss remains a primary bottleneck limiting the energy relaxation time ( T 1 ) 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 T 1 improving from 67.4 μs in the standard 0° process to 98.7 μs, and the average quality factor ( Q ) increasing from 2.0×10 6 to 2.5 × 10 6 . This work offers a practical nanofabrication strategy to suppress junction-region processing loss, providing a viable pathway for high-coherence silicon-based quantum processors.

DOI
10.1088/1674-1056/aea75f

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Abstract Surface dielectric loss remains a primary bottleneck limiting the energy relaxation time ( T 1 ) 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 T 1 improving from 67.4 μs in the standard 0° process to 98.7 μs, and the average quality factor ( Q ) increasing from 2.0×10 6 to 2.5 × 10 6 . This work offers a practical nanofabrication strategy to suppress junction-region processing loss, providing a viable pathway for high-coherence silicon-based quantum processors.
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Abstract Shadow evaporation is commonly used to micro-fabricate the key element of superconducting qubits—the Josephson junction. However, in conventional two-angle deposition circuit topology, unwanted stray Josephson junctions are created which contribute to dielectric loss. So far, this could be avoided by shorting the stray junctions with a so-called bandage layer deposited in an additional lithography step, which may further contaminate the chip surface. Here, we present an improved shadow evaporation technique allowing one to fabricate sub-micrometer-sized Josephson junctions together with bandage layers in a single lithography step. We also show that junction aging is significantly reduced when junction electrodes are passivated in an oxygen atmosphere directly after deposition.