3D IC Copper Bonding via Metal Adhesion Layer
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Solution Overview
Problem
In three-dimensional integrated circuits, direct Cu-to-Cu bonding often results in the formation of CuO at the bonding surfaces, increasing resistance and reducing adhesion reliability.
Innovation Solution
A method involving the formation of a metal adhesion layer on Cu surfaces, followed by bonding at a temperature below 400°C, where the adhesion layer incorporates oxygen from the Cu surfaces to form a metal oxide bonding layer, thereby eliminating CuO formation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If direct Cu-to-Cu bonding is performed at high temperature and pressure, then bonding strength is improved, but CuO forms at the bonding surfaces increasing resistance and reducing reliability
Solution Approach 1:
A metal adhesion layer (such as Ti, Co, or Ni) is introduced as an intermediary between the two Cu surfaces. This adhesion layer serves as a mediator that prevents direct Cu-to-Cu oxidation while enabling reliable bonding. The adhesion layer forms a metal oxide bonding layer that actually facilitates the bonding process, resolving the contradiction by providing both strong bonding and preventing harmful CuO formation at the Cu surfaces.
Solution Approach 2:
The bonding temperature is reduced to below 400°C, which is a significant parameter change from conventional high-temperature bonding. This temperature reduction prevents excessive CuO formation while the metal adhesion layer compensates for the lower thermal energy, maintaining bonding strength despite the lower temperature.
2Strength
If thermal compression bonding is applied to Cu surfaces, then bonding is achieved, but CuO formation increases resistance at the bonded interface
Solution Approach 1:
The metal adhesion layer acts as a protective intermediary that prevents Cu oxidation during thermal compression bonding. By placing this layer between the Cu surfaces, the process enables bonding to occur while the adhesion layer consumes oxygen that would otherwise form resistive CuO at the Cu bonding interfaces, thus maintaining low surface resistance.
Solution Approach 2:
The oxygen that would normally harm the Cu bonding surfaces by forming resistive CuO is instead captured by the metal adhesion layer. The adhesion layer's oxidation reaction converts the harmful oxygen into a beneficial metal oxide bonding layer that actually facilitates bonding while protecting the Cu surfaces from oxidation.
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 enhances Cu-to-Cu bonding without increasing surface resistance, improving the reliability and adhesion of the bonded structure.
Implementation Method 1
the at least one metal adhesion layer gets oxygen atoms from the two Cu surfaces and forms at least one metal oxide bonding layer between the Cu surfaces
Implementation Method 2
The bonding includes heating the devices wafers to a temperature of less than 400° C.
Data Source
AI summary
At least one metal adhesion layer is formed on at least a Cu surface of a first device wafer. A second device wafer having another Cu surface is positioned atop the Cu surface of the first device wafer and on the at least one metal adhesion layer. The first and second device wafers are then bonded together. The bonding includes heating the devices wafers to a temperature of less than 400° C., with or without, application of an external applied pressure. During the heating, the two Cu surfaces are bonded together and the at least one metal adhesion layer gets oxygen atoms from the two Cu surfaces and forms at least one metal oxide bonding layer between the Cu surfaces.