BAW Resonator Electrode Bragg Stack for Spurious Wave Suppression

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Solution Overview

Problem

Film bulk acoustic wave resonators suffer from spurious acoustic waves due to transverse acoustic waves generated perpendicular to the main acoustic wave, degrading frequency response, and high electrical resistance at higher frequencies leading to insertion loss deterioration and reduced quality factor.

Innovation Solution

Implementing a multilayer stack of metals for the electrodes forming a Bragg reflector that confines acoustic energy to the piezoelectric material layer, using metals with large acoustic impedance mismatches and low resistivity, such as ruthenium and tungsten for high impedance layers and aluminum or titanium for low impedance layers.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Device complexity

If a single-layer electrode is used in film bulk acoustic wave resonators, then the device structure is simple, but the electrical resistance increases at higher frequencies leading to insertion loss deterioration and reduced quality factor

Engineering Contradiction:

Improveelectrode structureVSAvoidquality factor

Core Design Contradiction:

Device complexityVSReliability

Solution Approach 1:

The electrode is segmented into multiple alternating layers of metals with different acoustic impedances (e.g., tungsten/ruthenium and aluminum/titanium), creating a Bragg reflector structure. This segmentation reduces electrical resistance while maintaining structural functionality, directly resolving the contradiction between structural simplicity and quality factor at high frequencies.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The electrode uses composite material structure with alternating layers of high-acoustic-impedance metals (tungsten, ruthenium) and low-acoustic-impedance metals (aluminum, titanium). This composite approach optimizes both electrical conductivity and acoustic wave confinement, improving quality factor without excessive structural complexity.

Inventive Principle:

Principle #40Composite materials

2Productivity

If transverse acoustic waves are generated perpendicular to the main acoustic wave, then the resonator operates, but spurious acoustic waves are generated degrading frequency response

Engineering Contradiction:

Improveacoustic wave generationVSAvoidspurious acoustic waves

Core Design Contradiction:

ProductivityVSObject-generated harmful factors

Solution Approach 1:

The Bragg reflector structure converts the potentially harmful transverse acoustic waves by reflecting them back into the piezoelectric layer through acoustic impedance mismatch. This transforms the harmful spurious waves into useful acoustic energy that reinforces the main resonant mode, improving frequency response while maintaining productivity.

Inventive Principle:

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

Solution Approach 2:

By carefully selecting metal layers with specific acoustic impedance parameters (high impedance metals like tungsten and ruthenium alternating with low impedance metals like aluminum and titanium), the Bragg reflector creates optimal acoustic wave confinement. This parameter optimization suppresses spurious waves while maintaining efficient acoustic wave generation.

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 solution significantly reduces electrode resistance, enhances conductivity, improves power handling, and maintains resonator quality factor, while suppressing spurious acoustic waves, thereby improving frequency response.

Implementation Method 1

one of a top electrode disposed on top of the layer of piezoelectric material or a bottom electrode disposed on a bottom of the layer of piezoelectric material, the one of the top electrode or the bottom electrode including a Bragg pair having alternating layers of a first metal and a second metal

Methodology Applied

Scientific EffectBragg reflection: Bragg Diffraction

Implementation Method 2

a layer of piezoelectric material

Methodology Applied

Scientific EffectPiezoelectric effect: Piezoelectric Effect

Data Source

PatentUS20250219615A1Bulk acoustic wave resonator with bragg grating embedded in electrode

Publication Date: 2025.07.03 SKYWORKS GLOBAL PTE LTD

AI summary

Aspects and embodiments disclosed herein include a bulk acoustic wave resonator comprising a layer of piezoelectric material and one of a top electrode disposed on top of the layer of piezoelectric material or a bottom electrode disposed on a bottom of the layer of piezoelectric material, the one of the top electrode or the bottom electrode including a Bragg pair having alternating layers of a first metal and a second metal.