Acoustic Wave Filter Dielectric Film Layout for TCF Synchronization
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Solution Overview
Problem
Existing acoustic wave filters exhibit degraded attenuation and temperature instability due to a positive temperature coefficient of frequency (TCF) for resonant frequencies, leading to a lower slope at the edge of the band, especially at low temperatures.
Innovation Solution
The implementation of acoustic filter devices with series and parallel resonators, where some resonators are covered with negative or positive TCF materials and others are uncovered, and interdigital transducers with varying duty factors, to synchronize the TCF values of resonant and anti-resonant frequencies, thereby improving temperature stability and rejection performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a positive TCF material is used for the piezoelectric substrate, then the resonant frequency has a positive temperature coefficient, but the anti-resonant frequency has near zero TCF, resulting in degraded attenuation and temperature instability
Solution Approach 1:
The patent applies local quality by depositing dielectric material selectively on specific resonators (series or shunt) rather than uniformly across all resonators. This creates local variations in capacitance and TCF for different resonators, allowing the resonant and anti-resonant frequencies to be synchronized with near-zero TCF across the temperature range.
Solution Approach 2:
The patent changes physical parameters by controlling the dielectric constant, thickness, and coverage area of the deposited material. By adjusting these parameters, the capacitance of specific resonators is modified to compensate for the positive TCF of the piezoelectric substrate, achieving near-zero overall TCF for both resonant and anti-resonant frequencies.
2Manufacturing precision
If all resonators are covered with dielectric material, then the TCF adjustment becomes uniform, but the ability to independently tune resonant and anti-resonant frequencies is reduced
Solution Approach 1:
The patent segments the dielectric coverage by applying material to only certain resonators (either series or shunt resonators, but not both uniformly). This segmentation allows independent tuning of different resonator groups, providing the precision needed to synchronize resonant and anti-resonant frequencies while maintaining a relatively simple manufacturing process.
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 temperature stability and rejection performance of acoustic wave filters by synchronizing the TCF values of resonant and anti-resonant frequencies, resulting in improved attenuation and reduced temperature-dependent variations.
Implementation Method 1
A layer of negative temperature coefficient of frequency dielectric material is disposed over one or more of the second plurality of acoustic wave resonators
Implementation Method 2
a piezoelectric layer over the substrate. A first plurality of acoustic wave resonators is disposed over the piezoelectric layer
Implementation Method 3
each of the first plurality of acoustic wave resonators comprising an interdigital transducer electrode interposed between a pair of reflectors
Data Source
AI summary
An acoustic wave filter includes a substrate and a piezoelectric layer over the substrate. First acoustic wave resonators are disposed over the piezoelectric layer and arranged in series along a first branch, and second acoustic wave resonators are disposed over the piezoelectric layer, arranged in parallel, and connected to the first branch and to ground. The first and second acoustic wave resonators include an interdigital transducer electrode interposed between a pair of reflectors. A layer of negative temperature coefficient of frequency dielectric material is disposed over one or more of the second plurality of acoustic wave resonators to control the temperature coefficient of frequency and improve temperature stability of the acoustic wave filter.