SUMMARY
The invention introduces a method for fabricating low-loss niobium Josephson junctions which enhance quantum device performance by using niobium superconductors that are separated by an aluminum oxide barrier and are encapsulated with aluminum layers to prevent chemical diffusion and minimize loss.
The Unmet Need: Superconducting logic circuits operating at high temperatures and high frequencies
Superconducting Josephson junctions, formed by two primary superconductors separated by a thin (< 2 nm thick) barrier, have enabled a wide range of possibilities from superconducting logic circuits to quantum sensing, quantum computation, and the exploration of emergent phenomena in quantum circuits. While the original Josephson junction technologies relied on higher temperature superconductors, such as lead or niobium, the demonstration of low-loss aluminum junctions led to their widespread adoption for sensitive quantum circuitry, where it is crucial to minimize sources of decoherence.
Due to the lower superconducting critical current, however, modern aluminum Josephson junction-based quantum circuits are still limited to operation at extremely low temperatures (< 0.1 Kelvin) and lower frequencies (< 70 GHz). There are numerous publications and patents detailing various fabrication methods of niobium Josephson junctions, but these processes typically target larger junctions for use in superconducting integrated circuits, metrology, or single-flux-quantum logic, rather than low-loss quantum circuits. Many of these methods rely on anodization or the presence of an insulating structure embedded in the final product, which are known channels for dielectric loss. Additionally, methods involving a variety of primary superconductors (e.g., NbN) and barriers (e.g., Nb-Ox, AlN) often result in at least one primary superconductor making direct contact with the barrier, which may lead to chemical diffusion and the emergence of losses near the barrier.
These limitations of current technology underscore the need for improved methods that minimize loss and decoherence in Josephson junctions for higher temperature and higher frequency quantum devices.
The Proposed Solution: Vertical material stacking in a vacuum based fabrication process
The disclosed fabrication method for low-loss niobium Josephson junctions aims to enhance quantum device performance. These junctions enhance quantum device performance by using niobium superconductors separated by an aluminum oxide barrier. The barrier is encapsulated by thin aluminum layers to prevent chemical diffusion. The fabrication process involves vertical material stacking in a vacuum, patterning with plasma reactive ion etching, and silicon dielectric growth. A second niobium layer is added, patterned, and chemically treated to remove residual dielectric material, achieving low-loss comparable to modern aluminum junctions.
Additionally, the junction current density can be tuned through annealing, providing flexibility in device parameters. This technology is differentiated by its ability to significantly reduce loss and decoherence in niobium Josephson junctions, bringing their performance closer to that of aluminum-based junctions. The key improvement lies in the encapsulation of the aluminum oxide barrier with aluminum layers, which isolates the barrier chemically from the niobium and prevents diffusion-related losses. The use of the self-aligned side-wall passivation spacer method minimizes dielectric material in the wiring layer, further reducing potential sources of decoherence. The vertical growth process in a vacuum ensures a clean substrate, and optimized plasma RIE patterning reduces contamination. The ability to tune the junction current density through annealing offers a wider range of operational parameters, making this technology adaptable to various quantum device requirements. This approach opens the door for higher temperature and higher frequency superconducting quantum devices, setting it apart from traditional junction fabrication methods.
FIGURE
ADVANTAGES
Advantages
- Improved quantum device performance with low-loss niobium Josephson junctions
- Higher temperature and higher frequency operation compared to aluminum junctions
- Minimized chemical diffusion and loss with aluminum oxide barrier encapsulated by thin aluminum layers
- Vertical material stack growth in vacuum to avoid contamination
- Optimized plasma reactive ion etching (RIE) for patterning and optimally reducing contamination
- Flexible tuning of junction current density through annealing
- Potential extension to various superconducting material stacks
- Reduced decoherence and loss using a self-aligned side-wall passivation spacer method
- High-quality dielectric formation through plasma chemical vapor deposition
- Final chemical treatment to remove residual dielectric material and clean metal surfaces
Applications
- Quantum computing devices
- Quantum sensing equipment
- Superconducting logic circuits
- High-frequency quantum circuits
PUBLICATIONS
INTELLECTUAL PROPERTY
- Pending PCT (PCT/US2023/035153)
- Publication link: https://patents.google.com/patent/WO2024086077A1/
December 5, 2024
Proof of concept
PCT Pending
Licensing,Co-development
David Schuster