Josephson junctions are the foundation of modern superconducting devices, from SQUIDs, which serve as the most sensitive magnetic flux sensors, to qubits in quantum computers. The growth of these technologies demands mass production of junctions with uniform properties across the wafer. Traditional window junctions suffer from layer misalignment and parasitic capacitances, which limit sensitivity. Cross-type junctions solve these issues, but their fabrication typically involves a laborious lift-off step that leaves defects. This work joins a line of research aimed at improving Josephson junctions and qubits, where reproducibility and scalability are key challenges.
On a three-inch silicon wafer, the Nb/Al-AlOx/Nb trilayer was deposited by magnetron sputtering: a 125 nm bottom electrode, a 14 nm aluminum barrier oxidized at 33.3 mbar for 65 minutes, and a 200 nm top electrode. The trilayer was etched into a strip, then 500 nm of SiO2 was sputtered without removing the photoresist. Excess silicon dioxide was removed by chemical-mechanical polishing (CMP) on a GNP POLI-400L tool using a slurry of 80 nm SiO2 particles. The removal rate was about 128 nm/min, and polishing lasted 70 seconds. A Nb contact layer was sputtered on top perpendicular to the strip and etched together with the top electrode, defining the junction area by the intersection of the strips. Current-voltage characteristics and Fraunhofer patterns were measured at 4.2 K using a four-probe configuration.
The junction yield exceeded 90% on the wafer, and for junctions with a 200 nm top electrode, all tested samples were of high quality. The critical current depends linearly on the junction area, yielding a critical current density of 220±5 A/cm² for wafer A and 549±4 A/cm² for wafer B. The normal resistance is inversely proportional to the area, with a specific resistance of 743±26 Ω·µm² (wafer A) and 269±2 Ω·µm² (wafer B). The characteristic voltage Vc = IcRN was 1.39±0.02 mV and 1.47±0.02 mV, respectively, indicating barrier uniformity. The subgap-to-normal resistance ratio Rsg/RN reached 38.3±1.5 and 31.2±0.8, and the gap voltage was 2.84±0.03 mV and 2.79±0.01 mV, close to the theoretical value for niobium. Fraunhofer patterns showed a smooth critical current profile with an effective penetration depth of about 84 nm. For junctions smaller than 1×1 µm², the yield dropped due to slurry residues, but for sizes from 1 to 4.2 µm, the process is stable.
This work shows that chemical-mechanical polishing can replace lift-off in the fabrication of cross-type Josephson junctions without compromising quality. Unlike neighboring studies that applied polishing to other structures or with additional cleaning steps, here the process was accomplished without supports or post-polish cleaning, keeping the photoresist intact during polishing. This simplifies the process and makes it suitable for multilayer integration of superconducting circuits. The method scales to larger wafers and other barrier materials.
The next logical step is to adapt the process to submicron junctions, where abrasive particle residues currently interfere. Adding a post-polish cleaning step, such as a scrubber, could improve the yield for junctions smaller than 1 µm². Also promising is replacing magnetron sputtering of SiO2 with atomic layer deposition or plasma-enhanced chemical vapor deposition to avoid shadow effects and grooves around structures. This would enable denser junction arrays for quantum processors.
This technology will benefit manufacturers of SQUIDs, superconducting qubits, and rapid single-flux-quantum logic circuits, which require large arrays of uniform junctions.
Optimize wafer cleaning after polishing for submicron junctions and test reproducibility on larger wafers with different photoresists.
For superconducting electronics, the key unsolved problem is scalable fabrication of qubits with high coherence. This work addresses part of that challenge by providing a reproducible method for creating Josephson junctions with uniform parameters, which directly affects qubit frequency spread and the quality of quantum circuits.
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The authors fabricated Nb/Al-AlOx/Nb cross-type Josephson junctions, using chemical-mechanical polishing to embed the trilayer into SiO2 instead of lifting off the mask. They achieved a yield above 90% on a three-inch wafer, a linear dependence of critical current on area, and high barrier uniformity (Vc ≈ 1.4 mV, Rsg/RN > 30). Fraunhofer patterns confirmed the quality of the junctions.
Where its strength is
The CMP-based process eliminates the need for laborious lift-off and additional cleaning, preserving the photoresist, which simplifies fabrication and improves reproducibility. The achieved uniformity of critical current and resistance across the wafer provides a strong foundation for scaling.
Where it could go next
Worth trying
Why this may matter
The proposed method could accelerate the transition from laboratory samples to mass production of superconducting electronics, where reproducibility and low parasitic capacitance are critical for quantum computers and sensors.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.52, against a usual 0.64. It looks like few have drilled nearby.
The pressure during polishing is only 138 mbar, about the force of a gentle breeze on your palm, yet it is enough to remove 128 nanometers of silicon dioxide per minute.
In the TV series 'The Expanse', ships use superconducting magnetic fields for radiation shielding. Josephson junctions are key components in sensors for such fields, and their mass production brings science fiction closer to reality.