Low Temperature Wafer Bonding via Solid-State Diffusion
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Solution Overview
Problem
Current wafer bonding techniques, such as eutectic bonding, require high temperatures above 400°C, which can cause defects in temperature-sensitive chips like MEMS and LEDs, and are costly; there is a need for a method to bond wafers at lower temperatures without compromising bond quality.
Innovation Solution
A method involving the use of a first and second wafer with exposed metal layers capable of forming a eutectic mixture, where a predetermined pressure is applied at a temperature below the eutectic melting point to form a solid-state diffusion bond, optionally with a cushioning layer for irregular surfaces, and potentially combining with transient liquid phase bonding for getter activation and multi-chip stacking.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If eutectic bonding is used to bond wafers, then bonding can be achieved at lower temperature compared to individual metal melting temperatures, but the bonding temperature is still above 400°C which causes defects in temperature-sensitive chips
Solution Approach 1:
The invention changes the bonding mechanism from eutectic bonding (relying on melting point depression) to solid-state diffusion bonding. This parameter change enables bonding at temperatures below 400°C (specifically 250-350°C), which is low enough to prevent defects in temperature-sensitive chips while still achieving reliable bonds through atomic diffusion across the interface.
Solution Approach 2:
The invention replaces the thermal-mechanical eutectic bonding process with a solid-state diffusion bonding process. Instead of relying on phase change (melting and solidification) to achieve bonding, the method uses solid-state atomic diffusion driven by temperature and pressure, eliminating the need for temperatures above the eutectic point while maintaining bonding effectiveness.
2Temperature
If eutectic bonding is used, then bonding temperature is reduced compared to individual metal melting points, but wafer production costs remain high
Solution Approach 1:
By changing from eutectic bonding to solid-state diffusion bonding, the process operates at lower temperatures (250-350°C vs. >400°C), which reduces energy consumption and allows for simpler, less expensive equipment. This parameter change directly lowers production costs while maintaining bonding quality.
3Strength
If high temperature bonding is used, then strong bond formation is achieved, but temperature-sensitive chips suffer from defects such as cracks, warpage, and performance degradation
Solution Approach 1:
The invention replaces high-temperature eutectic bonding with solid-state diffusion bonding that operates at lower temperatures. This substitution maintains bond strength through diffusion mechanisms while avoiding the thermal damage (cracks, warpage, performance degradation) that occurs with high-temperature processing of temperature-sensitive chips.
Solution Approach 2:
By changing the bonding temperature parameter to below 400°C (specifically 250-350°C) and using solid-state diffusion as the bonding mechanism, the process achieves strong bonds without exposing temperature-sensitive chips to harmful high temperatures, thereby preventing thermal damage while maintaining bond integrity.
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 bonding at temperatures below the eutectic melting point, suitable for temperature-sensitive devices, ensuring better bond quality and dicing yield, and applicable to wafers with high topography through the use of a cushioning layer, while maintaining low thermal damage and process costs.
Implementation Method 1
applying a predetermined pressure at a predetermined temperature to form a solid-state diffusion bond between the first layer and the second layer
Implementation Method 2
the soft metal is deformable under the predetermined pressure to diffuse through the solid-state diffusion bond and trace the contours of the irregular surface of the second wafer
Data Source
AI summary
A method for bonding wafers is provided. The method comprises the steps of providing a first wafer having an exposed first layer, the first layer comprising a first metal; and providing a second wafer having an exposed second layer, the second layer comprising a second metal, the first metal and the second metal capable of forming a eutectic mixture having a eutectic melting temperature. The method further comprises the steps of contacting the first layer with the second layer; and applying a predetermined pressure at a predetermined temperature to form a solid-state diffusion bond between the first layer and the second layer, wherein the predetermined temperature is below the eutectic melting temperature.