Tunnel Junction Fabrication Using ALD and Wetting Layer
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Solution Overview
Problem
Current tunnel junction fabrication methods, such as thermal oxidation, result in point defects in the tunnel barriers of superconducting junctions, limiting device performance, decoherence in SIS junctions, and subgap leakage in NIS junctions, which restricts the size and yield of superconducting tunnel junctions.
Innovation Solution
The use of Atomic Layer Deposition (ALD) to deposit high-quality, single atomic monolayers of Al2O3 as tunnel barriers, facilitated by an aluminum wetting layer, reduces defects and allows precise control of barrier thickness, enabling the fabrication of large-area, high-performance tunnel junctions on non-oxidizing metals like gold.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If thermal oxidation process is used to fabricate tunnel barriers, then the fabrication process is simple and well-established, but point defects are introduced in the tunnel barrier that affect junction quality and limit device performance
Solution Approach 1:
The patent changes the fundamental fabrication parameter from thermal oxidation to atomic layer deposition (ALD). This parameter change transforms the deposition mechanism from a diffusive process that creates point defects to a layer-by-layer atomic deposition process that produces defect-free barriers, thereby improving junction quality while maintaining fabrication feasibility
Solution Approach 2:
The patent substitutes the thermal oxidation mechanism with an alternative deposition mechanism (ALD). This replacement eliminates the harmful diffusive nature of thermal oxidation that causes point defects, while achieving the same functional outcome of creating insulating tunnel barriers with superior quality
2Power
If larger junction area is fabricated to improve cooling power, then refrigeration capability increases, but point defects and manufacturing variability limit the maximum achievable yield and performance
Solution Approach 1:
By changing the fabrication parameter from thermal oxidation to ALD, the patent achieves uniform, defect-free barriers across large areas. This parameter change removes the fundamental limitation that previously constrained junction size, allowing larger areas to be fabricated while maintaining high yield and performance consistency
Solution Approach 2:
The patent enables fabrication of larger junctions by eliminating point defects that previously acted as failure points in large-area devices. The improved barrier quality allows the junction area to be segmented into larger regions without compromising overall device reliability or performance
3Adaptability or versatility
If non-oxidizing metals like gold are used as electrodes, then device versatility and performance are improved, but traditional thermal oxidation cannot form adequate tunnel barriers on these materials
Solution Approach 1:
The patent substitutes thermal oxidation with ALD, which fundamentally changes how the barrier is formed. Unlike thermal oxidation that requires oxidizable metals, ALD uses sequential surface reactions that can deposit oxide layers on any substrate including non-oxidizing metals like gold, thereby expanding material compatibility
Solution Approach 2:
The patent introduces an aluminum wetting layer as an intermediary between the non-oxidizing metal electrode (e.g., gold) and the oxide barrier. This intermediary layer enables subsequent oxide deposition by providing a surface that can be oxidized, thereby facilitating barrier formation on otherwise incompatible substrates
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach eliminates point defects, enhances device performance, and allows for the creation of larger, high-quality tunnel junctions, improving refrigeration capabilities and enabling the use of non-oxidizing metals in superconducting quantum computer bits and cryogenic thermometers.
Implementation Method 1
The tunnel junction barriers are deposited as a single atomic monolayer one at a time using Atomic Layer Deposition (ALD). For example, ALD can deposit single atomic monolayers of Al2O3 on a sample surface.
Implementation Method 2
When a voltage is applied to a completed tunnel junction device, quantum tunneling moves the hottest electrons from one side of the tunnel junction barrier through the barrier into the other side of the tunnel junction
Implementation Method 3
These barriers are typically fabricated via thermal oxidation of Al or Al alloys using a controlled combination of temperature, partial pressure of oxygen, and time.
Data Source
AI summary
A method for fabricating a tunnel junction includes depositing a first electrode on a substrate, depositing a wetting layer having a thickness of less than 2 nm on the first electrode, using atomic layer deposition (ALD) to deposit an oxide layer on the wetting layer, and depositing a second electrode on the oxide layer. The wetting layer and the oxide layer form a tunnel barrier, and the second electrode includes a superconductor.