Ag-Sn Substrate Bonding for MEMS Thermal Stress Reduction
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Current wafer bonding techniques for MEMS devices face challenges such as high eutectic temperatures causing thermal stress and deformation, high costs, and limitations in miniaturization due to the use of materials like Al—Ge and Au—Si, while Ag—Sn eutectic bonding suffers from low mechanical strength and melting point issues.
Innovation Solution
A substrate bonding method using a structure with a first and second substrate coated with Ag layers and a Sn layer, forming Ag3Sn intermetallic compounds at a lower temperature (250-350°C) to enhance bonding strength and reduce thermal stress, with optional annealing to convert pure Sn into Ag3Sn, improving mechanical strength and hermetic sealing.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Strength
If Al-Ge or Au-Si eutectic bonding is used to achieve strong bonding, then bonding strength is improved, but process temperature increases to 363-450°C causing thermal stress and deformation
Solution Approach 1:
The patent changes the material composition parameters by using Ag-Sn eutectic bonding instead of Al-Ge or Au-Si systems, which lowers the eutectic temperature from 363-450°C to 221°C. This parameter change in material selection directly reduces the process temperature while maintaining bonding strength through the formation of Ag3Sn intermetallic compounds.
Solution Approach 2:
The patent employs composite material structure by forming Ag3Sn intermetallic compounds within a Sn matrix through controlled diffusion during bonding. This composite structure combines the low melting point advantage of Sn with the strength benefits of Ag3Sn intermetallic phase, achieving strong bonding at lower temperatures.
2Temperature
If Ag-Sn eutectic bonding is used to reduce temperature, then process temperature decreases to 221°C, but mechanical strength is reduced due to low melting point and brittleness of pure Sn
Solution Approach 1:
The patent resolves this contradiction by creating a composite material structure where Ag3Sn intermetallic compounds are formed within a Sn matrix. The Ag3Sn phase provides high mechanical strength and thermal stability, while the Sn matrix maintains the low melting point advantage, achieving both low temperature processing and high mechanical strength.
Solution Approach 2:
The patent applies local quality by creating distinct phases with different properties within the bonding interface: the Ag3Sn intermetallic compounds provide localized strength enhancement at the bonding interface, while the Sn matrix maintains overall ductility and low melting point characteristics. This spatial differentiation of material properties resolves the strength-temperature contradiction.
3Ease of manufacture
If screen printing is used to form bond rings with glass frit, then bonding is achieved, but bond ring width cannot be less than 100-200 μm limiting miniaturization
Solution Approach 1:
The patent replaces the mechanical screen printing process with a vapor deposition or sputtering process that can deposit metallic layers (Ag and Sn) with much finer control. This substitution of manufacturing method enables bond ring widths significantly smaller than the 100-200 μm limitation of screen printing, facilitating chip miniaturization while maintaining ease of manufacture through standardized semiconductor fabrication processes.
4Length of moving object
If polymers such as BCB or photoresist are used for bonding, then bond ring width can be reduced significantly through photolithography, but outgassing at high temperature and weak bonding strength reduce reliability
Solution Approach 1:
The patent replaces long-lived polymer materials (BCB, photoresist) with metallic Ag-Sn layers that form stable Ag3Sn intermetallic compounds. While metals require precise deposition, they eliminate the reliability issues of polymers (outgassing, weak bonding) and provide permanent, thermally stable bonding suitable for high-temperature applications and long-term reliability.
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
The method achieves stronger bonding and improved hermetic sealing by forming Ag3Sn intermetallic compounds within a Sn matrix, reducing thermal stress and enabling miniaturization, while being cost-effective and compatible with semiconductor manufacturing processes.
Implementation Method 1
heating them to a temperature higher than eutectic temperature and keeping the temperature for a while, these two wafers would be bonded together
Implementation Method 2
In an eutectic bonding process by bringing specific metals into contact under a relatively low temperature to form eutectic phase
Implementation Method 3
annealing to convert pure Sn into Ag3Sn, improving mechanical strength
Data Source
AI summary
A substrate bonding method comprises the following steps. Firstly, a first substrate and a second substrate are provided, wherein a surface of the first substrate is covered by a first Ag layer and a surface of the second substrate is covered by a second Ag layer and a metallic layer from bottom to top, wherein the metallic layer comprises a first Sn layer. Secondly, a bonding process is performed by aligning the first and second substrates followed by bringing the metallic layer into contact with the first Ag layer followed by applying a load while heating to a predetermined temperature in order to form Ag3Sn intermetallic compounds. Finally, cool down and remove the load to complete the bonding process.