Metal Adhesion Layers for Low-Temperature Direct Bonding
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Solution Overview
Problem
Current direct bonding techniques for metal layers in microelectronic devices require high temperatures, pressures, or vacuum environments, which are not compatible with the production of certain heterostructures and can introduce bi-axial stresses due to differing thermal expansion coefficients, leading to mechanical instability and cracking.
Innovation Solution
A method involving the deposition of thin metal layers with controlled surface roughness, followed by venting and direct bonding at ambient temperature and pressure without additional polishing or thermal budgets, using a surface layer to prevent diffusion and allowing metallic bonds to form at room temperature.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If high temperature and pressure are applied to achieve strong copper-to-copper bonds, then bond strength is improved, but the process temperature exceeds 50°C which is incompatible with certain heterostructures
Solution Approach 1:
The invention changes the bonding parameters by using a metallic layer with controlled thickness (5-50 nm) and specific material properties (ductility, surface energy) to enable bonding at temperatures below 50°C. The layer thickness and material selection are optimized to achieve sufficient adhesion without requiring high thermal energy, thus resolving the contradiction between bond strength and temperature compatibility.
Solution Approach 2:
The metallic layer acts as an intermediary between the two substrates, providing a bonding interface that requires lower temperature and pressure than direct substrate-to-substrate bonding. This intermediate layer facilitates adhesion through its specific mechanical and surface properties, enabling bond formation under milder conditions while maintaining strength.
2Strength
If thermal budget is applied to reinforce the bond, then mechanical strength is improved, but materials with different thermal expansion coefficients generate biaxial stresses leading to cracking
Solution Approach 1:
The invention changes the bonding parameters by using a metallic layer with controlled thickness (5-50 nm) and specific material properties (ductility, surface energy) to enable bonding at temperatures below 50°C. The layer thickness and material selection are optimized to achieve sufficient adhesion without requiring high thermal energy, thus resolving the contradiction between bond strength and temperature compatibility.
Solution Approach 2:
The metallic layer serves as a cushioning element that accommodates thermal expansion differences between substrates with different coefficients of thermal expansion. By bonding at low temperature and using a ductile material, the layer can absorb subsequent thermal stresses without generating critical biaxial stresses that would cause cracking, thus protecting the heterostructure assembly.
3Temperature
If metallic surfaces are brought into direct contact under ultra-high vacuum, then bonding can be achieved at room temperature, but the process requires difficult-to-use vacuum environments with long desorption times
Solution Approach 1:
The metallic layer acts as an intermediary between the two substrates, providing a bonding interface that requires lower temperature and pressure than direct substrate-to-substrate bonding. This intermediate layer facilitates adhesion through its specific mechanical and surface properties, enabling bond formation under milder conditions.
Solution Approach 2:
The metallic layer's surface properties (high surface energy, ductility) enable it to self-adhere to the substrate and to the opposing metallic layer without requiring external vacuum or plasma treatment. The layer's inherent properties facilitate bonding at atmospheric pressure and room temperature, eliminating the need for complex vacuum equipment and long desorption processes.
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 robust bonding of substrates with different thermal expansion coefficients at low temperatures and atmospheric pressure, eliminating the need for vacuum environments and thermal budgets, while maintaining mechanical strength and avoiding substrate cracking.
Implementation Method 1
The metallic layers are deposited by physical vapor phase
Implementation Method 2
Atomic diffusion bonding of wafers with thin nanocrystalline metal films
Data Source
Figure 1a~1c
Figure 2a~2b
Figure 3a~3b
AI summary
The invention relates to a method for assembling a first substrate (1) and a second substrate (3) via metal adhesion layers (2, 4), said method comprising the steps of: depositing, on the surface of each of the first and second substrates (1, 3), a metal layer (2, 4) with a thickness controlled such as to limit the surface roughness of each of the deposited metal layers to below a roughness threshold; exposing the metal layers deposited on the surface of the first and second substrates to the air; directly adhering the first and second substrates by placing the deposited metal adhesion layers in contact, the surface roughness of the contacted layers being that obtained at the end of the depositing step. The adhesion can be carried out in the air, at atmospheric pressure and at room temperature, without applying pressure to the assembly of the first and second substrates resulting from directly contacting the deposited metal adhesion layers.