Acoustic Wave Electrode Structure for Compact High-Steepness Filters
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Solution Overview
Problem
Acoustic wave devices used in filters, particularly those employing thickness-shear mode bulk waves, face challenges in achieving miniaturization while maintaining good filter characteristics, as increasing electrostatic capacitance typically requires larger device sizes.
Innovation Solution
The acoustic wave device incorporates a piezoelectric layer made of lithium niobate with a specific configuration of comb-shaped electrodes and a reference potential electrode, where the center-to-center distance between adjacent electrode fingers is constant in the comb-shaped electrodes but not in the reference potential electrode, allowing for miniaturization without degrading filter characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the acoustic wave resonator is increased in size to increase electrostatic capacitance, then the electrostatic capacitance is improved, but the device size increases
Solution Approach 1:
The patent applies local quality by creating non-uniform electrode finger spacing in specific regions. The first and second electrode fingers have different spacing configurations: in the first region, the spacing is optimized for one direction, while in the second region, the spacing is optimized for another direction. This local variation in electrode geometry allows different parts of the device to contribute differently to capacitance, achieving higher overall capacitance without proportionally increasing device area.
Solution Approach 2:
The patent employs asymmetry by designing the electrode structure with non-symmetric finger spacing. The first electrode fingers and second electrode fingers are arranged with different center-to-center distances in different regions, creating an asymmetric pattern that maximizes the electrostatic field distribution. This asymmetric configuration increases the effective capacitance area without requiring a symmetric increase in overall device dimensions.
2Reliability
If a reference potential electrode is added between input and output electrodes, then filter characteristics are improved, but device complexity increases
Solution Approach 1:
The reference potential electrode structure serves multiple functions simultaneously: it provides the reference potential for filter operation, creates additional capacitive coupling regions, and defines multiple electrode spacing regions (first and second regions) within a single integrated structure. This multi-functional design improves filter characteristics without adding separate independent components, thereby limiting the increase in device complexity.
3Manufacturing precision
If the center-to-center distance between electrode fingers is made constant, then manufacturing precision is improved, but filter characteristics degrade
Solution Approach 1:
The patent segments the electrode spacing into distinct regions: a first region with one spacing configuration and a second region with another spacing configuration. Within each segment, the spacing is uniform and precise, satisfying manufacturing requirements. However, the variation between segments creates the non-uniform overall pattern needed for optimal filter characteristics, thus resolving the contradiction between manufacturing precision and filter performance.
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 enables the achievement of miniaturized filter devices with improved filter characteristics, including increased steepness on the low-frequency side of the pass band, while maintaining or enhancing the frequency response without significant degradation.
Implementation Method 1
a piezoelectric layer made of lithium niobate... An AC voltage is applied between the electrodes to excite bulk waves in the thickness-shear mode
Implementation Method 2
A pair of electrodes are provided on the piezoelectric layer... connected to different potentials. An AC voltage is applied between the electrodes to excite bulk waves
Data Source
AI summary
An acoustic wave device includes a piezoelectric layer, a first comb-shaped electrode on the piezoelectric layer, including a first busbar and first electrode fingers, and connected to an input potential, a second comb-shaped electrode on the piezoelectric layer, including a second busbar and second electrode fingers interdigitated with the first electrode fingers, and connected to an output potential, and a reference potential electrode connected to a reference potential and including third electrode fingers on the piezoelectric layer aligned with the first and second electrode fingers, and a connection electrode connecting adjacent third electrode fingers. An order in which a first electrode finger, a second electrode finger, and a third electrode finger are arranged is such that, starting from the first electrode finger, the first electrode finger, the third electrode finger, the second electrode finger, and the third electrode finger define one period.