XBAR Buried Oxide Strip Structure for High-Frequency RF Filtering

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Solution Overview

Problem

Current RF filters, particularly those using acoustic wave resonators, are not well-suited for higher frequencies and wider bandwidths required in future communication networks, such as the 5G NR standard, which includes bands like n77, n79, and millimeter wave frequencies, leading to performance limitations in terms of insertion loss, rejection, and bandwidth handling.

Innovation Solution

The development of transversely-excited film bulk acoustic resonators (XBARs) with oxide strip acoustic confinement structures, which enhance electromechanical coupling and frequency capability by incorporating dielectric strips to improve Bode Q factors and filter performance, particularly for frequencies above 3 GHz.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Speed

If conventional acoustic wave resonators are used, then the filter structure is simple, but the frequency capability and bandwidth are limited for future communication networks

Engineering Contradiction:

Improvefrequency capabilityVSAvoidresonator structure complexity

Core Design Contradiction:

SpeedVSDevice complexity

Solution Approach 1:

The resonator structure is segmented into distinct functional layers: piezoelectric film, acoustic confinement layers (oxide strips), and substrate. This segmentation allows each layer to be optimized independently for high-frequency operation while maintaining overall structural integrity and managing complexity.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The resonator employs composite material structures combining piezoelectric films with acoustic confinement materials (oxide strips or nitride layers). This composite approach enables enhanced frequency capability by leveraging the complementary properties of different materials to achieve both high-frequency operation and effective acoustic confinement.

Inventive Principle:

Principle #40Composite materials

2Reliability

If acoustic confinement structures are added to improve Bode Q factors, then filter performance improves, but manufacturing complexity increases

Engineering Contradiction:

Improvefilter performanceVSAvoidmanufacturing complexity

Core Design Contradiction:

ReliabilityVSEase of manufacture

Solution Approach 1:

Acoustic confinement layers are formed preliminarily during the resonator fabrication process, before final electrode patterning. This preliminary action integrates the confinement structures into the manufacturing flow, improving filter performance while minimizing additional manufacturing complexity by combining multiple functions in a unified process sequence.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The acoustic confinement structures serve as intermediary elements between the piezoelectric film and the substrate, mediating acoustic energy confinement without requiring direct modification of the piezoelectric material itself. This intermediary approach improves Bode Q factors while maintaining manufacturing simplicity through standard thin-film deposition techniques.

Inventive Principle:

Principle #24Intermediary (Mediator)

3Adaptability or versatility

If wider bandwidth handling is implemented, then the filter meets future communication requirements, but insertion loss and rejection performance deteriorate

Engineering Contradiction:

Improvebandwidth handlingVSAvoidinsertion loss

Core Design Contradiction:

Adaptability or versatilityVSLoss of energy

Solution Approach 1:

The resonator design employs parameter optimization including piezoelectric film thickness, acoustic confinement layer thickness and material composition, and electrode geometry. These parameter changes enable the filter to handle wider bandwidths while maintaining low insertion loss by tuning the acoustic resonance characteristics to match future communication network requirements.

Inventive Principle:

Principle #35Parameter changes

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

The XBARs with oxide strip acoustic confinement structures demonstrate improved Bode Q factors and filter performance, enabling better handling of high-frequency bands and wider bandwidths, thus addressing the limitations of existing RF filters in future communication networks.

Implementation Method 1

oxide strip acoustic confinement structures which enhance electromechanical coupling and frequency capability

Methodology Applied

Scientific EffectAcoustic confinement:

Implementation Method 2

transversely-excited film bulk acoustic resonators (XBARs) with oxide strip acoustic confinement structures

Methodology Applied

Scientific EffectPiezoelectric effect: Piezoelectric Effect

Data Source

PatentUS12255607B2Transversely-excited film bulk acoustic resonator with buried oxide strip acoustic confinement structures

Publication Date: 2025.03.18 MURATA MFG CO LTD

AI summary

Acoustic resonators, filters, and methods. An acoustic resonator includes a substrate, a piezoelectric plate, and a diaphragm including a portion of the piezoelectric plate spanning a cavity in a substrate. An interdigital transducer (IDT) on a front surface of the piezoelectric plate includes first and second sets of interleaved interdigital transducer (IDT) fingers extending from first and second busbars respectively. The interleaved IDT fingers extend onto the diaphragm. Overlapping portions of the interleaved IDT fingers define an aperture of the acoustic resonator. First and second dielectric strips are on the front surface of the piezoelectric plate. Each dielectric strip has a first portion under the IDT fingers in a respective margin of the aperture and a second portion extending into a gap between the respective margin and the respective busbar.