XBAR Filter Dielectric Layer Division for Spurious Mode Reduction

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Solution Overview

Problem

Current RF filters using acoustic wave resonators are not well-suited for higher frequency communications networks, particularly those defined in the 5G NR standard, which requires bandpass filters capable of handling higher transmit power and wider communication channel bandwidths.

Innovation Solution

The development of transversely-excited film bulk acoustic resonators (XBARs) with divided frequency-setting dielectric layers, which include a front-side and back-side dielectric layer configuration to separate resonance frequencies and reduce spurious modes, enabling efficient excitation of primary shear acoustic modes and improving electromechanical coupling.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Manufacturing precision

If a single dielectric layer is used for frequency setting, then the structure is simple, but the frequency separation is insufficient and spurious modes are not reduced

Engineering Contradiction:

Improvefrequency separationVSAvoiddielectric layer structure

Core Design Contradiction:

Manufacturing precisionVSDevice complexity

Solution Approach 1:

The frequency-setting dielectric layer is divided into multiple separate dielectric layers with different thicknesses positioned at different locations on the resonator. This segmentation allows independent control of frequency settings at different regions, achieving better frequency separation and spurious mode reduction while maintaining manageable structural complexity

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The solution transitions from a single-layer dielectric structure to a multi-layer dielectric structure with varying thicknesses in the vertical dimension. This dimensional change enables differential frequency control across different layers, improving frequency separation capability without proportionally increasing overall device complexity

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

2Adaptability or versatility

If traditional acoustic wave resonators are used, then the structure is well-established, but they are not suitable for higher frequency communications networks with wider bandwidths

Engineering Contradiction:

Improvefrequency band adaptabilityVSAvoidperformance at higher frequencies

Core Design Contradiction:

Adaptability or versatilityVSReliability

Solution Approach 1:

The resonator design changes key parameters including the use of transversely-excited film bulk acoustic resonators with specific piezoelectric materials and divided dielectric layer configurations. These parameter changes enable the resonators to operate effectively at higher frequencies (e.g., 5G NR bands) with wider bandwidths while maintaining reliable performance

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The resonator employs composite structures combining piezoelectric materials with multiple dielectric materials of different properties. This composite approach allows optimization for higher frequency operation and wider bandwidth while maintaining structural integrity and performance reliability

Inventive Principle:

Principle #40Composite materials

3Reliability

If spurious modes are not mitigated, then the resonator structure remains simple, but the filter performance deteriorates with adverse effects

Engineering Contradiction:

Improvefilter performanceVSAvoidresonator structure

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The divided dielectric layer structure is specifically designed to suppress spurious modes that would otherwise degrade filter performance. By strategically positioning layers of different thicknesses, the design converts potential harmful spurious resonances into beneficial frequency-selective characteristics, improving filter performance while adding moderate structural complexity

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

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

XBARs with divided frequency-setting dielectric layers achieve improved frequency separation and reduced spurious modes, enhancing the performance of RF filters for higher frequency bands by increasing electromechanical coupling and reducing adverse effects of spurious modes, thus supporting wider communication channel bandwidths.

Implementation Method 1

a piezoelectric plate... conversion between electrical and acoustic energy in piezoelectric devices

Methodology Applied

Scientific EffectPiezoelectric effect: Piezoelectric Effect

Implementation Method 2

transversely-excited film bulk acoustic resonators... excitation of primary shear acoustic modes

Methodology Applied

Scientific EffectAcoustic wave resonance: Resonance

Data Source

PatentUS12132464B2Filter using transversely-excited film bulk acoustic resonators with divided frequency-setting dielectric layers

Publication Date: 2024.10.29 MURATA MFG CO LTD

AI summary

Acoustic filters and methods of fabrication are disclosed. A filter device includes a substrate and a single-crystal piezoelectric plate, a back surface of the piezoelectric plate attached to a surface of the substrate. The filter device includes a plurality of acoustic resonators including one or more shunt resonators and one or more series resonators. Each of the plurality of acoustic resonators includes an interdigital transducer (IDT) formed on the front surface of the piezoelectric plate, interleaved fingers of the IDT disposed on a respective diaphragm formed by a respective portion of the piezoelectric plate that spans a respective cavity in the substrate. A divided frequency setting layer is formed on at least some of the one or more shunt resonators but not on the one or more series resonators.