⚛️ quantum physics

Low-Loss, High-Coherence Airbridge Interconnects Fabricated by Single-Step Lithography

This paper presents a simplified single-step lithography process for fabricating low-loss, high-coherence nanoscale airbridges that enhance qubit dephasing times while maintaining robust mechanical stability for advanced quantum devices.

Original authors: Jibang Fu, Bo Ren, Jiandong Ouyang, Cong Li, Kechengqi Zhu, Yonggang Che, Xiang Fu, Shichuan Xue, Zhaohua Yang, Mingtang Deng, Junjie Wu

Published 2026-01-26

📖 4 min read🧠 Deep dive

CC BY 4.0

Original authors: Jibang Fu, Bo Ren, Jiandong Ouyang, Cong Li, Kechengqi Zhu, Yonggang Che, Xiang Fu, Shichuan Xue, Zhaohua Yang, Mingtang Deng, Junjie Wu

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine you are building a tiny, ultra-precise city for electrons. In this city, the "roads" (wires) need to cross over each other without touching, because if they touch, the delicate information they carry gets scrambled. In the world of quantum computers, this crossing is called an airbridge. Think of it like a suspension bridge for electricity: it spans over other wires, suspended in the air, so there is no physical contact to cause a short circuit or lose energy.

For a long time, building these microscopic bridges was like trying to build a suspension bridge using a complicated, multi-day construction project. You had to lay down layers, carve them out, align them perfectly, and then remove the scaffolding. This process was slow, prone to errors, and often left behind "construction debris" (residues) that could ruin the sensitive quantum signals.

The New "One-Step" Trick
The researchers in this paper found a way to build these bridges in a single step, like a master sculptor who can shape a complex statue with just one perfect pour of clay, rather than chiseling it away piece by piece.

Here is how they did it, using simple analogies:

The Special Clay (The Resist Stack): Instead of using just one type of material, they stacked three different layers of "clay" (a material called resist) on top of their chip.
The Three-Tiered Flashlight (Triple-Dose Exposure): Usually, when you shine a light on clay to shape it, you use one intensity. These researchers used a clever "three-tiered" flashlight approach:
- Brightest Flash: To carve out the deep foundation (the pedestal).
- Medium Flash: To shape the main arch of the bridge.
- Faintest Flash: To create a tiny, hidden undercut (a small overhang) underneath the middle layer. This is the secret sauce that allows the bridge to be lifted off cleanly later.
The Melting Trick (Thermal Reflow): After shaping the clay, they gently heated it. Imagine taking a rough, jagged piece of ice and warming it just enough so the edges melt into a perfectly smooth, rounded arch. This step ensures the metal bridge that goes on top will be incredibly smooth, which is vital for quantum computers.
The Metal Pour: They then poured liquid metal (aluminum) over this smooth, arched clay shape. Because of the hidden undercut they created earlier, the metal only stuck to the top and sides, forming a perfect bridge. When they washed away the clay, the metal bridge remained, suspended in mid-air.

Why This Matters: The "Toughness" Test
One of the biggest worries with these delicate bridges is that they might break during the cleaning process. In chip manufacturing, parts are often shaken in an ultrasonic cleaner (like a dishwasher for tiny chips) to remove dirt.

The Test: The team put 60 of these new bridges into a low-power ultrasonic cleaner.
The Result: All 60 survived perfectly intact. Even when they turned the power up high, most survived, with only about 30% breaking. This proves the bridges are strong and stable, unlike older methods that would crumble under similar cleaning.

The Quantum Result: A Quieter Room
To see if this new bridge actually helped quantum computers, they built a specific type of quantum bit (qubit) called an "8-mon" and put these airbridges in it. They compared it to a standard design (the "X-mon") and a version where they used a solid insulator instead of an airbridge.

The Comparison:
- The Solid Insulator: When they used a solid block of material (like glass) to cross the wires, the quantum signal died very quickly (in about 2 microseconds). It was like trying to have a whisper in a noisy, echoey room.
- The Old Standard (X-mon): The standard design worked well, keeping the signal alive for about 14 microseconds.
- The New Airbridge (8-mon): The new design with the airbridge was the clear winner. It kept the signal alive for about 36 microseconds—more than double the standard and vastly better than the solid insulator.

The Bottom Line
The paper claims that by using this single-step, "one-pour" method with a special heating trick, they created airbridges that are:

Smaller and Smoother: They can make bridges less than 200 nanometers wide with perfectly smooth edges.
Stronger: They can survive the cleaning steps required for making chips.
Quieter: They don't add any extra "noise" or loss to the quantum computer, actually helping the computer hold its information longer (specifically improving the "dephasing time" by 2.5 times compared to the standard design).

In short, they found a simpler, cleaner, and more robust way to build the suspension bridges that allow quantum computers to function at their best.

Technical Summary: Low-Loss, High-Coherence Airbridge Interconnects Fabricated by Single-Step Lithography

Problem Statement
The miniaturization and performance enhancement of modern integrated circuits, photonic devices, and quantum systems require complex three-dimensional architectures. A critical bottleneck in these fields is the creation of high-performance, low-parasitic interconnects (airbridges) that span over underlying components without introducing detrimental electrical, optical, or mechanical interference. In quantum information processing, specifically for superconducting qubits, airbridges are vital for low-loss microwave resonators, qubit coupling, and grounding strategies that suppress spurious electromagnetic modes.

Conventional airbridge fabrication relies on multi-layer resist stacks and multiple lithography steps involving selective etching of sacrificial layers. These methods present significant drawbacks: they require complex alignment across exposures, risk introducing organic residues or process-induced defects that degrade quantum coherence, and hinder further miniaturization and high-density integration. While alternative single-step methods exist (e.g., gradient exposure or two-photon lithography), they often struggle to scale to sub-micron dimensions or produce arched profiles with sufficient smoothness.

Methodology
The authors report a streamlined fabrication process for nanoscale airbridges using only a single electron-beam lithography (EBL) step. The core of the methodology involves a carefully optimized multilayer resist stack and a specific processing sequence:

Resist Stack Design: A three-layer stack is spin-coated onto the substrate:
- A 300-nm bottom layer of PMMA (950k MW) to define the electrical contact region.
- A 400-nm middle layer of MMA (EL9).
- A 400-nm top blended layer (PMMA:MMA in a 2:3 ratio) offering higher sensitivity for profile formation while maintaining stability during reflow.
Triple-Dose Exposure: A critical innovation is the application of three distinct, calibrated exposure doses during EBL:
- High Dose (>1000 µC/cm²): Fully exposes the entire stack to define the airbridge pedestal.
- Intermediate Dose (300–500 µC/cm²): Exposes the MMA and blend layers to pattern the suspended bridge profile.
- Low Dose (<150 µC/cm²): Creates a controlled undercut within the middle MMA layer, crucial for successful lift-off after reflow.
Thermal Reflow: After development in MIBK:IPA, the sample undergoes thermal reflow at 125°C (the glass transition temperature of PMMA) for 45 seconds. This step transforms the resist profile into a smooth, arched shape essential for continuous metal deposition.
Metallization and Lift-off: The process concludes with argon ion milling to clean contact areas, electron-beam evaporation of a 150-nm aluminum film, and lift-off in an NMP bath with low-power ultrasonic agitation to remove edge residues.

Key Contributions and Experimental Validation
The paper demonstrates that this single-lithography airbridge (SL-airbridge) process yields smooth, suspended metallic bridges with sub-200-nm features and lengths exceeding 5 µm.

Process Optimization: Control experiments confirmed the necessity of both the thermal reflow and the triple-dose scheme. Without reflow, metal edges were rough and connections discontinuous. Without the triple-dose scheme (using only double-dose), the undercut was insufficient after reflow, leading to unstable, raised metal walls. The optimized triple-dose approach preserved the necessary undercut while ensuring smooth edges.
Mechanical Robustness: The fabricated bridges demonstrated resilience to low-power ultrasonic treatment (a common step in superconducting device fabrication). In tests involving 60 samples, all airbridges survived low-power cleaning intact, whereas high-power cleaning damaged approximately 30%, establishing a practical operating limit.
Quantum Coherence Performance: The method was validated by integrating SL-airbridges into a "8-mon" transmon qubit design featuring a doubly connected gradiometric SQUID.
- Relaxation Time ( $T_1$ ): The SL-airbridge introduced no measurable additional loss. The 8-mon qubit achieved a $T_1$ of 38.65 µs, comparable to conventional X-mon qubits.
- Dephasing Time ( $T_2^*$ ): The 8-mon qubit achieved a $T_2^*$ of 35.98 µs, representing a 2.5-fold enhancement over the conventional X-mon qubit ( $T_2^* = 14.22$ µs). This confirms the efficacy of the gradiometric design in mitigating dephasing noise without the penalty of the interconnect.
- Dielectric Comparison: A control sample using a dielectric (SiO $_2$ ) crossover instead of an airbridge resulted in severe coherence degradation ( $T_1$ and $T_2^*$ reduced to ~2 µs), highlighting the superior low-loss nature of the airbridge approach.

Significance and Claims
The authors claim that this efficient method offers a practical route for integrating high-performance three-dimensional interconnects in advanced quantum and nano-electronic devices. By eliminating the need for multi-step lithography and complex alignment, the process reduces the risk of process-induced defects and organic residues that degrade quantum coherence. The resulting SL-airbridges provide robust mechanical stability and full compatibility with standard fabrication workflows, enabling greater design freedom for complex quantum circuit geometries while maintaining low parasitic interactions. The work suggests that this approach is accessible for advancing performance and integration density in next-generation quantum circuits, photonic devices, and nano-electronics.

Technical Summary: Low-Loss, High-Coherence Airbridge Interconnects Fabricated by Single-Step Lithography

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