Covalently Bonded SAW Substrate for Low-Loss Broadband Operation
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Solution Overview
Problem
Existing surface acoustic wave (SAW) devices face challenges in achieving high-frequency and broadband operation with reduced propagation attenuation and temperature coefficient of frequency, while maintaining high yield due to issues with bulk wave radiation and adhesive bonding.
Innovation Solution
A bonded substrate comprising a quartz substrate covalently bonded with a piezoelectric substrate, optionally through an amorphous layer, where the piezoelectric substrate is oriented for optimal leaky surface acoustic wave propagation, and the bonding process involves ultraviolet light irradiation under reduced pressure followed by heating and pressurization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If an organic adhesive agent is used to bond the piezoelectric substrate to the supporting substrate, then the bonding process is simple and feasible, but the temperature characteristics deteriorate and the frequency variation increases
Solution Approach 1:
A silicon oxide layer is introduced as an intermediary bonding interface between the piezoelectric substrate and supporting substrate. This intermediate layer enables direct bonding without organic adhesives, achieving both manufacturing feasibility and excellent temperature characteristics with zero temperature coefficient of frequency
Solution Approach 2:
The organic adhesive bonding mechanism is replaced with direct bonding through a silicon oxide layer. This substitution eliminates the temperature-dependent properties of organic materials while maintaining bonding effectiveness, achieving frequency stability across temperature variations
2Loss of energy
If the piezoelectric substrate is made thinner to reduce propagation attenuation, then the LLSAW propagation loss decreases, but the yield of bonded substrates deteriorates
Solution Approach 1:
The thickness of the piezoelectric substrate is optimized to a specific range (0.5-5 μm) that balances propagation attenuation reduction with bonding yield improvement. The silicon oxide layer thickness is also controlled (50-500 nm) to facilitate reliable bonding of thin substrates, achieving both low loss and high yield
3Reliability
If a silicon oxide layer is introduced between the piezoelectric substrate and supporting substrate, then the temperature characteristics improve, but the device complexity increases
Solution Approach 1:
The silicon oxide layer is formed conformally on the bonding surfaces, creating a homogeneous bonding interface. This uniform structure simplifies the bonding process while achieving excellent temperature characteristics, without significantly increasing device complexity
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 solution achieves high phase velocity and electromechanical coupling factors, reducing propagation attenuation and temperature coefficient of frequency, thereby enabling high-speed and high-coupling SAW modes with improved temperature stability for next-generation mobile communication devices.
Implementation Method 1
the quartz substrate and the piezoelectric substrate are covalently bonded at an interface
Implementation Method 2
irradiating a bonding surface of the quartz substrate and a bonding surface of the piezoelectric substrate with ultraviolet light under a pressure lower than atmosphere pressure
Implementation Method 3
after the irradiation, bringing the bonding surface of the quartz substrate and the bonding surface of the piezoelectric substrate into contact with each other; and pressurizing the quartz substrate and the piezoelectric substrate in a thickness direction to bond the bonding surfaces
Data Source
AI summary
There is provided a bonded substrate including: a quartz substrate; and a piezoelectric substrate which is bonded on the quartz substrate and on which a surface acoustic wave propagates, wherein the quartz substrate and the piezoelectric substrate are bonded at a bonding interface through covalent bonding, and a surface acoustic wave element having a higher phase velocity and a higher electromechanical coupling factor than conventional one is obtained by disposing an interdigital electrode on a principal surface of the piezoelectric substrate.