Bonded Quartz Substrate Layout for Wideband Low-TCF SAW Resonators
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Solution Overview
Problem
Existing surface acoustic wave resonators with quartz-crystal substrates face challenges in achieving wide bandwidth and low temperature coefficient of frequency (TCF) while maintaining high electro-mechanical coupling and low propagation attenuation.
Innovation Solution
A bonded substrate configuration is employed, where the Euler angles of the quartz-crystal substrates are specifically set to optimize the cut angles and thickness relative to the wavelength of the surface acoustic wave, allowing for a combination of AT-cut and X-cut substrates to enhance phase velocity, reduce propagation attenuation, and achieve desired resonance characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a single quartz-crystal substrate is used, then the structure is simple, but the bandwidth is limited and TCF cannot be sufficiently reduced
Solution Approach 1:
The patent divides the substrate into two separate quartz-crystal substrates (first and second substrates) with different crystal orientations and cutting angles. This segmentation allows each substrate to contribute different properties: the first substrate provides temperature compensation while the second substrate enables wide bandwidth operation, thereby resolving the contradiction between performance improvement and structural simplicity.
Solution Approach 2:
The patent employs a composite substrate structure combining two quartz-crystal substrates with different crystallographic orientations (AT-cut and X-cut) and specific Euler angles. This composite approach leverages the complementary characteristics of different crystal cuts to achieve both low TCF and wide bandwidth simultaneously, overcoming the limitations of single-substrate designs.
2Reliability
If the thickness of the second quartz-crystal substrate is increased, then the Q-value improves, but propagation attenuation increases
Solution Approach 1:
The patent precisely controls the thickness of the second quartz-crystal substrate within the range of 0.17 to 0.19 times the wavelength of the surface acoustic wave. This parameter optimization balances the competing requirements: sufficient thickness to maintain high Q-value for reliability, while limiting thickness to control propagation attenuation and energy loss.
3Stability of the object's composition
If the Euler angles are optimized for low TCF, then temperature stability improves, but electro-mechanical coupling coefficient decreases
Solution Approach 1:
The patent assigns different functional roles to each substrate through segmentation. The first substrate (AT-cut with specific Euler angles) is optimized for temperature compensation and TCF reduction, while the second substrate (X-cut with different Euler angles) is optimized for electro-mechanical coupling and bandwidth. This functional segmentation resolves the contradiction by distributing different performance requirements to different components.
Solution Approach 2:
The composite substrate structure combines two quartz-crystal substrates with different crystallographic orientations and Euler angle configurations. This composite approach allows the system to simultaneously achieve low TCF (from the first substrate's optimization) and high electro-mechanical coupling coefficient (from the second substrate's optimization), overcoming the trade-off present in single-substrate designs.
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 results in a surface acoustic wave resonator with improved bandwidth, reduced TCF, and high Q-value, suitable for applications like band-pass filters and high-frequency sensors, by minimizing propagation attenuation and optimizing electro-mechanical coupling.
Implementation Method 1
a piezoelectric crystal substrate is bonded over a quartz-crystal substrate
Implementation Method 2
a thickness of the second quartz-crystal substrate is 0.17 to 0.19 times a wavelength of a surface acoustic wave
Data Source
AI summary
In a bonded substrate according to an embodiment, Euler angles (φ1, θ1, ψ1) of a first quartz-crystal substrate satisfy 0°≤φ1≤2°, 123°≤θ1≤128°, and 31°≤ψ1≤44°, Euler angles (φ2, θ2, ψ2) of a second quartz-crystal substrate bonded over the first quartz-crystal substrate satisfy 83°≤φ2≤95°, 82°≤θ2≤95°, and 159°≤ψ2≤161°, and a thickness of the second quartz-crystal substrate is 0.17 to 0.19 times a wavelength of a surface acoustic wave.