Laser-Assisted Wafer Joining with a Heat-Absorbing Layer
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Solution Overview
Problem
Conventional joining techniques using eutectic reactions between metal layers on wafers face issues such as damage to heat-sensitive sensors, long processing times, thermal stress, and limited metal selection due to coefficient of linear expansion differences, and inefficient laser light absorption by metals with high reflectance and transmittance.
Innovation Solution
A method involving a heat absorbing layer and a joining material layer, where laser light is applied to the heat absorbing layer to indirectly heat the joining material layer, allowing efficient joining of targets using materials with low laser light absorbency, and a second method using titanium, chromium, or their oxides as major constituents in the joining material layer to absorb laser light efficiently.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Temperature
If the two wafers are heated entirely together with a sensor to generate eutectic reaction, then the joining temperature can be achieved, but the sensor may be damaged by heat
Solution Approach 1:
The patent applies local heating using laser light to the metal layers at the joining interface rather than heating the entire wafer assembly. This localized approach raises the temperature at the joining place to the eutectic reaction temperature while keeping the sensor area at a lower, non-damaging temperature, thus resolving the contradiction between achieving joining temperature and preventing sensor heat damage
Solution Approach 2:
The patent introduces a heat-absorbing layer as an intermediary between the laser light source and the metal layers. This layer absorbs the laser energy and converts it to heat, which is then transferred to the metal layers at the joining interface. This intermediary mechanism enables precise thermal control at the joining zone while protecting the sensor from direct thermal exposure
2Reliability
If the two wafers are heated entirely together to generate eutectic reaction, then the joining can be achieved, but the processing time becomes long
Solution Approach 1:
By concentrating laser energy locally at the joining interface through the heat-absorbing layer, the patent achieves rapid temperature rise at the metal layers without requiring heating of the entire wafer assembly. This localized energy delivery significantly reduces the time required to reach eutectic reaction temperature while maintaining reliable joining quality
Solution Approach 2:
The patent replaces conventional thermal conduction heating (which requires heating the entire wafer assembly through contact) with optical energy delivery using laser light. This substitution enables direct, rapid, and localized heating of the metal layers at the joining interface, dramatically reducing processing time while ensuring reliable eutectic bonding
3Adaptability or versatility
If metals with different coefficients of linear expansion are selected, then the degree of freedom in metal selection increases, but distortion occurs during temperature increase or decrease
Solution Approach 1:
The patent confines the thermal cycle to a very localized region at the joining interface using laser heating. Since only the metal layers at the interface undergo significant temperature changes while the bulk materials remain relatively stable, the thermal expansion differences between metals with different coefficients do not result in macroscopic distortion, enabling greater freedom in metal selection
Solution Approach 2:
The heat-absorbing layer acts as a thermal mediator that confines heat generation to the joining zone. This intermediary layer ensures that temperature gradients are localized primarily at the metal layer interface, minimizing differential thermal expansion effects across the entire structure and reducing distortion even when using metals with different expansion coefficients
4Use of energy by moving object
If laser light is applied directly to the metal layer to heat it, then the heating efficiency can be improved, but metals with high reflectance and transmittance cannot absorb laser light efficiently
Solution Approach 1:
The patent introduces a heat-absorbing layer as an intermediary between the laser light source and the metal layers. This layer is specifically selected to have high laser light absorption characteristics, allowing it to efficiently capture laser energy and convert it to heat. The heat is then transferred to the metal layers through thermal conduction, enabling effective heating of metals that would otherwise reflect or transmit laser light without adequate absorption
Solution Approach 2:
The patent changes the optical parameters of the joining system by introducing a layer with different optical properties (high absorption) between the laser source and the metal layers. This parameter change allows the system to overcome the inherent limitation of metal reflectance and transmittance, enabling efficient energy transfer and heating effectiveness for various metal combinations
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 efficient use of laser light for joining, reducing thermal stress and processing time, allowing for the use of metals with different coefficients of linear expansion and protecting heat-sensitive sensors.
Implementation Method 1
laser light is applied to the heat absorbing layer to heat the joining material layer with heat absorbed by the heat absorbing layer from the laser light
Implementation Method 2
a metal layer containing one of two types of metals to generate eutectic reaction as a major constituent and a metal layer containing the other metal as a major constituent are formed on respective joint surfaces of two wafers. Eutectic reaction is generated at a place of contact between the metal layers, thereby joining the two wafers to each other
Data Source
AI summary
A joining method comprises a stacking step and a joining process step. In the stacking step, a first joining target and a second joining target are stacked in such a manner as to interpose a heat absorbing layer and a joining material layer between respective joint surfaces of the first joining target and the second joining target. In the joining process step, laser light is applied to the heat absorbing layer to heat the joining material layer with heat absorbed by the heat absorbing layer from the laser light, thereby joining the first joining target and the second joining target to each other using the joining material layer.