Weighted Reflectors in Acoustic Wave Filters for Passband Attenuation

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Solution Overview

Problem

Longitudinally coupled resonator acoustic wave filters face challenges in maintaining consistent attenuation characteristics near the lower side of the pass band due to variations in the width of IDT electrode fingers, particularly influenced by the planarity of the piezoelectric film, which affects the Q factor and attenuation performance.

Innovation Solution

The implementation of weighted reflectors with varying lengths of reflective electrode fingers and specific busbar configurations in longitudinally coupled resonator acoustic wave filters, which include a piezoelectric substrate with a high acoustic velocity material layer and a low acoustic velocity film, helps to reduce or prevent variations in attenuation near the lower side of the pass band by dispersing the response outside the stop band and confining acoustic wave energy effectively.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If a high acoustic velocity member and low acoustic velocity film are laminated to increase Q factor, then the Q factor is improved, but variations in electrode finger width due to piezoelectric film planarity issues cause increased attenuation variations near the lower side of the pass band

Engineering Contradiction:

ImproveQ factorVSAvoidelectrode finger width variation

Core Design Contradiction:

ReliabilityVSManufacturing precision

Solution Approach 1:

The reflector electrode fingers are designed with non-uniform lengths, where each finger has a different length corresponding to its position. This local variation in geometry compensates for the uniform width variations caused by piezoelectric film planarity issues, maintaining consistent attenuation characteristics across the reflector structure while preserving the high Q factor benefits of the laminated acoustic velocity structure.

Inventive Principle:

Principle #3Local quality

2Ease of manufacture

If IDT electrodes are patterned on the piezoelectric film, then the filter functionality is achieved, but variations in electrode finger width induce attenuation variations particularly near the lower side of the pass band

Engineering Contradiction:

Improvefilter structure implementationVSAvoidattenuation characteristic control

Core Design Contradiction:

Ease of manufactureVSManufacturing precision

Solution Approach 1:

The reflector electrode finger lengths are intentionally varied as a design parameter to compensate for manufacturing variations in the IDT electrode fingers. By changing the length parameter of reflector fingers rather than attempting to control the width parameter of IDT fingers, the design achieves better attenuation control despite manufacturing limitations in maintaining uniform electrode dimensions.

Inventive Principle:

Principle #35Parameter changes

3Manufacturing precision

If uniform electrode finger widths are maintained, then attenuation characteristics are improved, but this requires high precision manufacturing that is difficult to achieve due to piezoelectric film planarity variations

Engineering Contradiction:

Improveelectrode finger width consistencyVSAvoidmanufacturing process requirements

Core Design Contradiction:

Manufacturing precisionVSDevice complexity

Solution Approach 1:

Instead of attempting to make all electrode fingers uniform in width (the conventional approach), the invention inverts the strategy by making the reflector electrode fingers non-uniform in length. This inversion transforms the problem from trying to control width variations to utilizing length variations as a compensatory mechanism, thereby achieving consistent attenuation characteristics without requiring high-precision uniform manufacturing.

Inventive Principle:

Principle #13The other way round (Inversion)

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 Q factor while minimizing attenuation variations near the lower side of the pass band, resulting in improved frequency response and reduced ripple in the filter's attenuation characteristics.

Implementation Method 1

a piezoelectric substrate, a plurality of IDT electrodes on the piezoelectric substrate

Methodology Applied

Scientific EffectPiezoelectric effect: Piezoelectric Effect

Implementation Method 2

Longitudinally coupled resonator acoustic wave filter

Methodology Applied

Scientific EffectResonance: Resonance

Data Source

PatentUS11984867B2Longitudinally coupled resonator acoustic wave filter and filter device

Publication Date: 2024.05.14 MURATA MFG CO LTD

AI summary

A longitudinally coupled resonator acoustic wave filter includes a piezoelectric substrate, IDT electrodes on the piezoelectric substrate along an acoustic wave propagation direction, and a pair of reflectors on the piezoelectric substrate on both sides of the IDT electrodes in the acoustic wave propagation direction. Each of the reflectors includes first and second reflector busbars, and first reflective electrode fingers connected to at least one of the first reflector busbar and the second reflector busbar. The reflector includes a first portion in which lengths of the first reflective electrode fingers change in the acoustic wave propagation direction.