🔬 mesoscale physics

A scalable non-superconducting tunnel junction technology

This paper presents a scalable, CMOS-compatible non-superconducting tunnel junction technology based on TiW alloy and AlOx barriers, which overcomes the integration limitations of previous methods and demonstrates robust performance in cryogenic applications down to 20 mK.

Original authors: Juho Luomahaara, Kristupas Razas, Omid Sharifi Sedeh, Renan P. Loreto, Janne S. Lehtinen, Mingchi Xu, Armel A. Cotten, Aldo Tarascio, Peter Müller, Nikolai Yurttagül, Lassi Lehtisyrjä, Leif Grönberg

Published 2026-02-17

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CC BY 4.0

Original authors: Juho Luomahaara, Kristupas Razas, Omid Sharifi Sedeh, Renan P. Loreto, Janne S. Lehtinen, Mingchi Xu, Armel A. Cotten, Aldo Tarascio, Peter Müller, Nikolai Yurttagül, Lassi Lehtisyrjä, Leif Grönberg, Christian P. Scheller, Jonathan R. Prance, Michael D. Thompson, Richard P. Haley, Mika Prunnila, Dominik M. Zumbühl

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, microscopic city on a computer chip. In this city, electricity flows like traffic. Sometimes, you want the traffic to flow freely; other times, you need to put up a gate that only lets cars through one by one, or even stops them completely. These "gates" are called tunnel junctions, and they are the traffic controllers of the modern electronic world.

For decades, the best traffic controllers were made of Aluminum. But there was a catch: Aluminum has a "superpower" called superconductivity. Below a certain cold temperature (about -272°C), it stops acting like normal metal and becomes a super-highway where electricity flows with zero resistance.

While superconductivity is great for some things (like quantum computers), it's a disaster for others. If you are trying to build a precise thermometer or a specific type of memory, you don't want a super-highway; you want a normal, predictable road. To stop the Aluminum from using its superpower, scientists used to have to use "brute force" methods, like blasting the chip with strong magnetic fields or mixing in messy chemicals. These methods were like trying to stop a supercar by throwing a net over it—it worked, but it was messy, hard to scale up, and incompatible with the delicate circuits of modern chips.

The New Solution: The "TiW" Guard

In this paper, a team of scientists from Finland, Switzerland, and the UK invented a new way to build these gates without the superpower problem.

The Analogy: The "Anti-Super" Sandwich
Think of the old Aluminum junction as a delicious sandwich that accidentally turns into a magic, floating sandwich when it gets cold. The new invention is a TiW/Al-AlOx/TiW sandwich.

The Bread (TiW): They put a special type of metal alloy called TiW (Titanium-Tungsten) on the top and bottom. Think of TiW as a "bodyguard" or a "guard dog." It's a metal that never becomes a superconductor, no matter how cold it gets. It's also friendly with standard computer chip manufacturing (CMOS), meaning it plays well with other parts of the chip.
The Filling (Al-AlOx): In the middle, they keep the high-quality Aluminum and its oxide layer. This is the part that actually does the "tunneling" work.
The Magic Trick: The TiW bodyguards hug the Aluminum so tightly that they suppress its superpower. The Aluminum is still there, but it's forced to stay "normal" and behave like a regular metal, even at temperatures near absolute zero.

Why This Matters: The "Thermometer" Test

To prove their new "sandwich" works, the scientists built a Coulomb Blockade Thermometer (CBT).

What is a CBT? Imagine a row of toll booths (the tunnel junctions) where cars (electrons) have to pay a toll to pass. At very low temperatures, the toll is so high that the cars get stuck in a traffic jam unless they have just the right amount of energy. By measuring how many cars get stuck, you can calculate the exact temperature of the city.
The Problem: If the toll booths were made of superconducting Aluminum, the "traffic jam" would disappear, and the thermometer would break.
The Result: The scientists tested their new TiW-based thermometers at temperatures as low as 20 millikelvin (that's 0.02 degrees above absolute zero!).
- They worked perfectly.
- They didn't need any magnetic fields to stop the superconductivity.
- They worked consistently across a whole wafer (a large circle of silicon), proving they can be mass-produced.

The Big Picture: Why Should You Care?

Scalability: Because this new technology uses materials (TiW) that are already used in standard computer chip factories, we can now mass-produce these non-superconducting gates on a massive scale. It's like switching from hand-crafting a toy to putting it on a factory assembly line.
Versatility: This opens the door for new types of quantum devices, ultra-precise sensors, and advanced electronics that need to work at super-cold temperatures without the headache of fighting superconductivity.
Simplicity: No more need for giant magnets or complex doping. Just build the sandwich, and it works.

In summary: The scientists found a way to build a "traffic controller" for electrons that stays calm and predictable even in the coldest conditions, using a new "bodyguard" metal (TiW) to keep the old "superhero" metal (Aluminum) in check. This makes it possible to build better, more reliable, and mass-producible chips for the future of quantum computing and ultra-sensitive sensors.

1. Problem Statement

Tunnel junctions are fundamental components in microsystems ranging from classical microelectronics to quantum information processing. For decades, Aluminium (Al) and its native oxide (AlOx) have been the dominant materials for cryogenic tunnel junctions due to the high quality of the AlOx barrier and Al's superconducting properties below ~1.2 K.

However, many advanced applications (e.g., primary thermometers, noise sources, and hybrid circuits) require non-superconducting (normal-metal) tunnel junctions. Existing methods to suppress Al superconductivity to achieve a normal state include:

Applying strong magnetic fields (brute force, lacks universality).
Doping with magnetic impurities (e.g., Mn).
Using proximity effects with non-superconducting metals (e.g., Cu).

Limitations of current approaches: These alternative materials often suffer from incompatibility with standard nanoelectronic circuits (CMOS), detrimental diffusion behavior, magnetic interference, and poor scalability. There is a critical need for a scalable, CMOS-compatible technology that maintains a normal-metal state down to deep cryogenic temperatures without external magnetic fields.

2. Methodology

The authors developed a novel TiW/Al-AlOx/TiW trilayer tunnel junction technology.

Material Stack: The core innovation is the use of a Titanium-Tungsten (TiW) alloy as the electrode material. TiW is a standard CMOS-compatible diffusion barrier and adhesion layer. The stack consists of:
1. Bottom TiW layer (100 nm).
2. Al layer (~7 nm) which is thermally oxidized in-situ to form the high-quality AlOx tunnel barrier.
3. Top TiW layer (100 nm).
Suppression Mechanism: The proximate TiW layers act as "pair-breaking" metals, suppressing the superconductivity of the thin Al layer, thereby keeping the junction in a normal-metal state even at zero magnetic field and very low temperatures.
Fabrication Process:
- Utilizes a modified version of a previously established process for superconducting Josephson junctions.
- Involves in-situ sputtering without breaking vacuum to ensure barrier quality.
- Features sidewall passivation (SiO2 spacer) to prevent short circuits and enable compact cross-type junction definitions.
- Fabricated on 150 mm wafers using UV lithography and reactive ion etching (RIE).
Validation Platform: The junctions were integrated into Coulomb Blockade Thermometers (CBTs). CBTs are ideal for validation because they function only if the electrodes are normal-conducting; any superconductivity would distort the characteristic conductance dip.
- Device structure: $N \times M$ arrays (61 junctions in series, 20 chains in parallel) to reduce impedance and offset charge noise.
- Testing range: Room temperature probing and cryogenic characterization down to 20 mK in a dilution refrigerator.

3. Key Contributions

Novel Material Architecture: Introduction of a TiW/Al-AlOx/TiW stack that leverages the superior barrier quality of AlOx while utilizing TiW to suppress superconductivity.
Scalability: Demonstration of wafer-scale fabrication (150 mm) with high yield, proving the process is compatible with industrial semiconductor manufacturing standards.
Zero-Field Operation: The technology operates as a robust normal-metal junction at zero magnetic field, eliminating the need for external magnets that complicate integration with other quantum or electronic components.
CMOS Compatibility: The use of TiW ensures the technology can be integrated alongside standard normal-metal and superconducting circuits without magnetic interference or diffusion issues.

4. Key Results

Room Temperature Characterization:
- Fabricated junctions across three wafers showed excellent agreement between measured resistance and theoretical models ( $R = \rho_t / L^2$ ).
- Specific resistivities ( $\rho_t$ ) were successfully tuned (ranging from 2,100 to 21,000 $\Omega\mu m^2$ ) by controlling oxidation parameters.
- Line-width reduction (LWR) was consistent and predictable, confirming process scalability down to sub-micron dimensions.
Cryogenic Performance (CBT Validation):
- Normal-State Confirmation: Conductance measurements showed no signatures of superconductivity (no zero-bias conductance suppression or gap overshoot) down to 20 mK.
- Thermometry Accuracy: The TiW-based CBTs functioned as primary thermometers. The electron temperature ( $T_{CBT}$ ) tracked the phonon temperature ( $T_p$ ) with <1% deviation across the 20 mK to 1 K range.
- Magnetic Field Robustness: Measurements under parallel magnetic fields (up to 1 T) showed negligible deviation (<3%) in temperature readings, confirming the junctions are insensitive to external fields and remain in the normal state.
- Low-Temperature Limit: The devices operated successfully down to 20 mK, with the slight deviation observed below 30 mK attributed to electron-phonon cooling limits rather than junction failure.

5. Significance and Impact

Enabling Hybrid Systems: This technology bridges the gap between superconducting quantum circuits (which require Al) and normal-metal components (required for thermometry, noise sources, and control electronics) on the same chip without magnetic interference.
Scalable Quantum Infrastructure: By providing a wafer-scale, CMOS-compatible normal-metal junction, this work removes a major bottleneck for scaling up quantum processors and cryogenic control electronics.
Improved Thermometry: It enables the use of Coulomb Blockade Thermometers in experiments where magnetic fields are either undesirable or already occupied by other components (e.g., spin qubits), facilitating easier and more convenient low-temperature experiments.
Future Outlook: The authors note that ongoing work aims to extend the normal-conducting regime into the microkelvin range and explore hybrid junctions for subgap spectroscopy and advanced cooling applications.

In summary, the paper presents a breakthrough in cryogenic device fabrication, replacing the "blessing and curse" of Al superconductivity with a robust, scalable, and CMOS-compatible TiW-based architecture that delivers high-performance normal-metal tunnel junctions for the next generation of microsystems.