Composite Substrate Bonding With Silica Interlayer for Thermal Stability
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Solution Overview
Problem
The challenge is to create a composite substrate with a piezoelectric single crystal layer bonded to a support substrate using an inorganic intermediate layer that minimizes peeling during repeated heating and cooling processes, while addressing issues of temperature stability and thermal expansion coefficient differences between materials.
Innovation Solution
A composite substrate is designed with a piezoelectric single crystal substrate bonded to a support substrate using an intermediate layer that includes thermally synthesized silica, which absorbs gases and reduces peeling, and additional inorganic materials like SiOx, Al2O3, AlN, SiN, and Ta2O5, deposited at low temperatures to manage thermal stress and expansion coefficients.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Stability of the object's composition
If a thin piezoelectric single crystal film is bonded to a support substrate to reduce thermal expansion, then temperature stability is improved, but noise (spuriousness or ripples) increases due to reflection from the interface
Solution Approach 1:
An intermediate layer is introduced between the piezoelectric single crystal film and the support substrate. This intermediate layer serves as a mediator that reduces the acoustic impedance mismatch at the interface, thereby minimizing reflection and noise while maintaining the thermal expansion compensation function.
Solution Approach 2:
The bonding structure uses a composite configuration combining the piezoelectric single crystal film, an intermediate layer, and a support substrate. This composite structure optimizes both thermal properties and acoustic properties by selecting materials with appropriate thermal expansion coefficients and acoustic impedances.
2Ease of manufacture
If an organic adhesive is used to bond the piezoelectric crystal to the substrate, then bonding is achieved, but reliability deteriorates due to peeling or cracking during heating processes
Solution Approach 1:
The organic adhesive layer is designed as a temporary bonding medium that is removed during subsequent processing. This allows easy initial bonding while eliminating the source of reliability problems (adhesive degradation at high temperatures) in the final product.
Solution Approach 2:
The organic adhesive is extracted (removed) after serving its bonding purpose during assembly. This extraction eliminates the harmful effects of organic material degradation during high-temperature processing while maintaining the benefits of easy bonding during manufacturing.
3Reliability
If an inorganic intermediate layer is formed by CVD or PVD and heat treated at high temperature to eliminate out gas, then peeling is reduced, but the piezoelectric crystal film may be damaged due to high temperature
Solution Approach 1:
The intermediate layer is pre-formed and pre heat-treated to eliminate out gas before bonding the piezoelectric crystal film. This preliminary action removes harmful gases that would cause peeling during subsequent high-temperature processing, while the piezoelectric crystal is only exposed to temperatures below its damage threshold.
Solution Approach 2:
The heat treatment process is segmented into two distinct stages: first, the intermediate layer is heat-treated at high temperature to eliminate out gas; second, the piezoelectric crystal is bonded at lower temperatures that preserve its integrity. This segmentation allows each component to be processed at its optimal temperature range.
4Stability of the object's composition
If the piezoelectric single crystal thickness is reduced to a few μm to a few tens μm, then thermal expansion is reduced and temperature characteristics are improved, but spuriousness increases due to interface reflection
Solution Approach 1:
An intermediate layer is introduced between the thin piezoelectric single crystal film and the support substrate. This intermediate layer serves as a mediator that reduces the acoustic impedance mismatch at the interface, thereby minimizing reflection and noise while maintaining the thermal expansion compensation function.
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 configuration enhances the thermal stability and reliability of the composite substrate, reducing peeling and cracking, and allows for the production of high-performance, small-sized surface acoustic wave devices with improved Q-values and reduced spuriousness.
Implementation Method 1
the intermediate layer is a film formed of an inorganic material, and at least a part of the film is thermally synthesized silica... thermally synthesized silica absorbs an out gas possibly generated in heating and cooling in the post-processes
Implementation Method 2
a material having a small expansion coefficient, such as sapphire and silicon (Si), is bonded to a piezoelectric single crystal... The thermal expansion of the piezoelectric single crystal is reduced by bonding the material having a small expansion coefficient
Data Source
AI summary
A method for manufacturing a composite substrate includes: forming a first intermediate layer including thermally synthesized silica on a surface of a support substrate; forming a second intermediate layer including an inorganic material on a surface of a piezoelectric single crystal substrate; flattening a surface of the second intermediate layer; and bonding a surface of the first intermediate layer to the flattened surface of the second intermediate layer.