Wafer Bonding via Selective Oxide Removal and Mechanical Shock
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Solution Overview
Problem
Existing wafer bonding methods, such as those involving pre-treatment for removing oxide films, can alter the oscillation characteristics of piezo-resonators and cause mechanical damage to wafers, leading to unreliable bonding and potential cracking.
Innovation Solution
A method where a first wafer with a first metal layer of lower rigidity modulus and a second wafer with a second metal layer of higher rigidity modulus are bonded, with only the oxide film on the second metal layer removed, using a mechanical shock under controlled load to break the oxide film without pre-treating the first metal layer, thereby achieving eutectic bonding while minimizing adverse effects on the wafers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If pre-treatment for removing oxide films is applied to both wafers, then bonding reliability is improved, but oscillation characteristics of piezo-resonators are altered and mechanical damage may occur
Solution Approach 1:
The patent divides the treatment approach into segments: only the second wafer (without piezo-resonator) receives oxide film removal treatment, while the first wafer (with piezo-resonator) is treated only by mechanical shock during bonding. This segmentation allows each wafer to receive appropriate treatment without harmful effects on sensitive components.
Solution Approach 2:
Different treatment methods are applied to different wafers based on their local characteristics. The second wafer undergoes pre-treatment for oxide film removal, while the first wafer with piezo-resonator is protected from such treatment and relies on the mechanical shock effect during bonding to break oxide films locally at the bonding interface.
2Ease of manufacture
If high-load mechanical shock is applied to break oxide films, then oxide film removal is achieved, but wafers may be cracked
Solution Approach 1:
The patent optimizes the bonding parameters including load magnitude, heating temperature, and holding time to achieve effective oxide film breakdown through mechanical shock without exceeding the fracture strength of the wafers. By carefully controlling these parameters, the process removes oxide films while maintaining wafer integrity.
Solution Approach 2:
The bonding process incorporates gradual loading and temperature ramping to cushion the mechanical shock effect, preventing sudden stress concentrations that could cause cracking. The controlled application of load during heating allows oxide films to break down progressively without damaging the wafer structure.
3Strength
If no pre-treatment is applied, then wafer integrity is maintained, but oxide films interfere with bonding
Solution Approach 1:
The second wafer receives pre-treatment for oxide film removal before bonding, preparing the bonding surface in advance. This preliminary action on the second wafer compensates for the lack of pre-treatment on the first wafer, ensuring that at least one bonding surface is free of oxide interference while maintaining the integrity of both wafers.
Solution Approach 2:
The patent converts the potential harm of oxide films into a benefit by using the controlled mechanical shock during bonding to break down oxide films on the first wafer. The heating and loading process, which could potentially damage the wafer, is instead used beneficially to remove oxide barriers at the bonding interface through controlled mechanical shock.
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 allows for reliable wafer bonding with reduced risk of cracking and minimal disruption to the oscillation characteristics of piezo-resonators, using a lower load and avoiding the need for pre-treatment, thus enhancing the bonding process's reliability and efficiency.
Implementation Method 1
a relatively large load acts on a pair of wafers stacked on each other so as to apply a mechanical shock to the bonded surfaces of both the metal layers, and thus mechanically break the oxide films
Implementation Method 2
eutectic bonding is carried out. The bonded surfaces of metal layers of the wafers are oxidized in the atmosphere, thereby forming oxide films at the bonded surfaces
Data Source
AI summary
Provided is a method for bonding wafers, which can bond the wafers to each other with high reliability while reducing the influence on the wafers. The method for bonding wafers includes the steps of: preparing a first wafer that has, on the surface thereof, a first metal layer with a first rigidity modulus, and a second wafer that has, on the surface thereof, a second metal layer with a second rigidity modulus higher than the first rigidity modulus; removing an oxide film at the surface of the second metal layer while an oxide film at the surface of the first metal layer is not removed; and bonding the surface of the first wafer to the surface of the second wafer.