Superconducting Qubit Solder Bumps via Isolated Seed-Layer Plating

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Solution Overview

Problem

Existing methods for forming solder bumps on superconducting qubits are costly, limited to large bump dimensions, and not scalable for high-density arrays on larger wafers, and electrodeposition processes can damage qubit structures or require non-superconducting materials that affect conductivity and superconducting properties.

Innovation Solution

A method involving a protection layer, non-superconducting seed layer, and multiple mask layers to enable conformal electrodeposition of solder bumps, ensuring equal height and maintaining superconducting properties by isolating the seed layer from the bumps, using materials like copper for conductivity and indium for solder.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Productivity

If electrodeposition is used to form solder bumps, then productivity and cost-effectiveness are improved, but the process damages qubit structures or requires non-superconducting materials that affect conductivity

Engineering Contradiction:

Improveproduction efficiencyVSAvoidqubit structure integrity

Core Design Contradiction:

ProductivityVSReliability

Solution Approach 1:

The process segments the wafer surface into protected qubit areas and exposed contact pad areas using a patterned protection layer. This allows electrodeposition to proceed only on contact pads where solder bumps are needed, preventing damage to qubit structures while maintaining production efficiency

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

A non-superconducting seed layer (e.g., copper) is introduced as an intermediary between the superconducting contact pad and the solder bump material. This seed layer enables electrodeposition to proceed without requiring the contact pad material itself to be conductive at deposition temperatures, thus preserving the superconducting properties of the original contact pad

Inventive Principle:

Principle #24Intermediary (Mediator)

2Use of energy by moving object

If copper seed layer is used for electrodeposition, then electrical conductivity is improved, but superconducting properties are lost

Engineering Contradiction:

Improveelectrical conductivityVSAvoidsuperconducting properties

Core Design Contradiction:

Use of energy by moving objectVSStability of the object's composition

Solution Approach 1:

The copper seed layer is segmented to be present only in specific locations (contact pad areas) where electrodeposition is needed, while qubit areas maintain their superconducting materials. The patterned protection layer ensures copper is deposited only where required

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The copper seed layer serves as a localized intermediary that provides the necessary electrical conductivity for electrodeposition only at contact pad locations. The superconducting contact pad material remains intact underneath and continues to provide superconducting pathways to the qubits

Inventive Principle:

Principle #24Intermediary (Mediator)

3Reliability

If lift-off technique is used to form indium bumps, then superconducting properties are maintained, but manufacturing complexity and cost increase

Engineering Contradiction:

Improvesuperconducting propertiesVSAvoidprocess complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The mechanical lift-off process is replaced with an electrochemical electrodeposition process. Instead of depositing material and then mechanically removing the resist, the process uses electrical current to directly deposit indium onto the seed layer, simplifying the overall manufacturing process while maintaining superconducting properties

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

4Productivity

If electrodeposition is performed on large wafers, then productivity is improved, but bump height uniformity deteriorates due to position-dependent resistivity

Engineering Contradiction:

Improvewafer throughputVSAvoidbump height uniformity

Core Design Contradiction:

ProductivityVSManufacturing precision

Solution Approach 1:

The seed layer is designed with locally optimized properties - using highly conductive non-superconducting materials like copper specifically at contact pad locations where electrodeposition occurs. This local optimization ensures uniform current distribution and consistent bump height across the entire wafer surface

Inventive Principle:

Principle #3Local quality

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

Enables the formation of uniform, superconducting solder bumps across larger wafers without damaging qubit structures, allowing for high-density arrays and cost-effective production.

Implementation Method 1

producing a protection layer on the substrate and patterning said protection layer so that the qubits and/or qubit parts are covered by the protection layer, while the contact pads are at least partially exposed

Methodology Applied

Scientific EffectPhotolithography patterning:

Implementation Method 2

The seed layer conducts current towards the openings at the bottom of which the solder bumps are formed during the electrodeposition

Methodology Applied

Scientific EffectElectrodeposition: Electrodeposition

Implementation Method 3

producing a mask layer on the seed layer, and patterning the mask layer so as to form openings therein, the surface area of said openings lying within respective exposed areas of the contact pads

Methodology Applied

Scientific EffectPhotolithography patterning:

Implementation Method 4

removing the material of the seed layer from the bottom of said openings

Methodology Applied

Scientific EffectWet etching:

Implementation Method 5

depositing the superconducting solder material by electrodeposition on at least part of the bottom of the respective openings thereby forming the solder bumps

Methodology Applied

Scientific EffectElectrodeposition: Electrodeposition

Implementation Method 6

producing one or more thin layers conformally on the patterned protection layer, said thin layers comprising at least a non-superconducting layer suitable for acting as a seed layer

Methodology Applied

Scientific EffectConformal deposition: Deposition (physical)

Data Source

PatentEP4425542B1A method for producing solder bumps on a superconducting qubit substrate

Publication Date: 2026.01.14 INTERUNIVERSITAIR MICRO ELECTRONICS CENT (IMEC VZW)

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AI summary

According to the invention, superconducting solder bumps (17) are produced on a qubit substrate by electrodeposition. The substrate comprises qubit areas, and superconducting contact pads connected to the qubit areas. First a protection layer is formed on the substrate, and patterned so as to cover at least the qubit areas. Then one or more thin layers are deposited conformally on the patterned protection layer, said thin layers comprising at least a non-superconducting layer suitable for acting as a seed layer for the electrodeposition of the solder bumps. The seed layer is removed locally in areas which lie within the surface area of respective contact pads. This is done by producing and patterning a mask layer, so that openings are formed therein, and by removing the seed layer from the bottom of the openings. The solder bumps are formed by electrodeposition of the solder material on the bottom of said openings. After the formation of the solder bumps, the seed layer and the protection layer are removed.