Composite SAW Substrate Bonding for High-Temperature Peel Resistance
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Solution Overview
Problem
Existing methods for producing composite substrates for surface acoustic wave devices face issues with temperature stability and spurious noise, particularly due to the use of lithium tantalate or lithium niobate, which have high thermal expansion coefficients, and the reliability of organic adhesives used in bonding processes, leading to peeling and cracking of piezoelectric single crystal films during heat treatment.
Innovation Solution
A method involving the formation of a composite substrate by sandwiching an inorganic material film between a piezoelectric single crystal substrate and a support substrate, with an uneven structure on at least one surface to reduce spurious noise, and using surface activation treatments and specific deposition methods to enhance bonding, such as chemical vapor deposition or magnetron sputtering, to prevent peeling at high temperatures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If organic adhesive is used to bond piezoelectric single crystal substrate to support substrate, then bonding is achieved, but peeling and cracking occur during heat treatment at 250-400°C
Solution Approach 1:
The invention changes the bonding method from using organic adhesive to using direct bonding without adhesive, eliminating the temperature limitation imposed by organic material decomposition. This allows heat treatment at 250-400°C without peeling or cracking.
Solution Approach 2:
The invention eliminates the organic adhesive layer entirely, replacing it with direct bonding between substrates. This removes the component that degrades at high temperatures, allowing the bonded structure to withstand heat treatment without failure.
2Adaptability or versatility
If lithium tantalate or lithium niobate is used as piezoelectric material, then large electromechanical coupling coefficient and broad bandwidth are achieved, but temperature stability deteriorates due to high thermal expansion coefficient
Solution Approach 1:
The invention creates a composite substrate structure by bonding piezoelectric single crystal (LT or LN) to a support substrate with low thermal expansion coefficient. This composite structure combines the high electromechanical coupling of LT/LN with the thermal stability of the support substrate, achieving both broad bandwidth and temperature stability.
Solution Approach 2:
The invention applies local quality by using different materials for different functions: the piezoelectric single crystal provides high electromechanical coupling in the active region, while the support substrate provides thermal expansion compensation. This functional differentiation resolves the contradiction between coupling coefficient and temperature stability.
3Stability of the object's composition
If LT or LN film is laminated on support substrate to reduce thermal expansion, then temperature characteristics are improved, but spurious noise occurs in antiresonant frequency band
Solution Approach 1:
The invention changes the thickness parameter of the piezoelectric single crystal to several μm to several tens μm, which optimizes the acoustic wave propagation characteristics and reduces spurious noise generation while maintaining temperature compensation benefits.
Solution Approach 2:
The invention creates functional differentiation within the substrate structure: the piezoelectric single crystal layer provides temperature compensation through its bonded connection to the low thermal expansion support, while the specific thickness and material properties of this layer are optimized to minimize spurious noise generation in the acoustic wave path.
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 effectively prevents peeling of the piezoelectric single crystal film during heat treatment up to 400°C and reduces spurious noise by using an inorganic material film instead of organic adhesives, ensuring reliable bonding and improved temperature stability.
Implementation Method 1
forming a film made of an inorganic material on at least one of the piezoelectric single crystal substrate and the support substrate
Implementation Method 2
using surface activation treatments and specific deposition methods to enhance bonding, such as chemical vapor deposition or magnetron sputtering
Data Source
AI summary
Provided is a composite substrate for surface acoustic wave device which does not cause peeling of an entire surface of a piezoelectric single crystal film even when heating the film to 400° C. or higher in a step after bonding. The composite substrate is formed by providing a piezoelectric single crystal substrate and a support substrate, forming a film made of an inorganic material on at least one of the piezoelectric single crystal substrate and the support substrate, and joining the piezoelectric single crystal substrate with the support substrate so as to sandwich the film made of the inorganic material.