Dual-Step BAW Top Electrodes for Spurious Mode Suppression

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Solution Overview

Problem

Existing bulk acoustic wave (BAW) resonators face challenges in suppressing spurious modes, particularly below the series resonance frequency, which affect quality factors and introduce undesirable BO modes that can interfere with multiplexing applications.

Innovation Solution

The introduction of a top electrode with a dual-step border (BO) region and a dielectric spacer layer between the outer step and the piezoelectric layer, along with a passivation layer that can be inset from the peripheral edges, enhances acoustic impedance mismatch and reduces lateral energy leakage, thereby suppressing spurious modes and shifting BO modes away from the passband.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If a conventional top electrode structure is used, then the device complexity is low, but spurious modes are not suppressed and quality factor is reduced

Engineering Contradiction:

Improvequality factorVSAvoidelectrode structure complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The top electrode is segmented into multiple regions with different heights: a central region at the first height, an inner step at the second height, and an outer step at the third height. This segmentation creates distinct acoustic impedance zones that suppress spurious modes while maintaining structural functionality, thereby improving quality factor without excessive complexity

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

Different regions of the top electrode are assigned different local properties through varying heights. The central region, inner step, and outer step each have specific height configurations optimized for their local function in suppressing particular spurious modes, allowing targeted improvement of quality factor in specific frequency ranges

Inventive Principle:

Principle #3Local quality

2Reliability

If a dual-step border region with dielectric spacer layer is introduced, then spurious modes are suppressed and quality factor increases, but the device complexity and manufacturing complexity increase

Engineering Contradiction:

Improvequality factorVSAvoidmanufacturing complexity

Core Design Contradiction:

ReliabilityVSEase of manufacture

Solution Approach 1:

A dielectric spacer layer is introduced as an intermediary element between the outer step of the top electrode and the piezoelectric layer. This spacer layer creates additional acoustic impedance mismatch that enhances suppression of spurious modes and improves quality factor, while being manufacturable through standard thin-film deposition processes

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The structure combines multiple materials with different acoustic properties: the piezoelectric layer, the top electrode material, and the dielectric spacer layer (such as silicon nitride or silicon dioxide). This composite structure creates acoustic impedance mismatches that suppress spurious modes while utilizing materials and processes compatible with existing manufacturing workflows

Inventive Principle:

Principle #40Composite materials

3Loss of energy

If the passivation layer is extended to peripheral edges of the piezoelectric layer, then lateral energy leakage is reduced, but the device complexity increases

Engineering Contradiction:

Improvelateral energy leakageVSAvoidpassivation layer configuration

Core Design Contradiction:

Loss of energyVSDevice complexity

Solution Approach 1:

The passivation layer is selectively extended only to the peripheral edges of the piezoelectric layer where lateral energy leakage occurs, rather than covering the entire device area. This targeted approach reduces energy loss at the critical boundaries while minimizing the added complexity and material usage

Inventive Principle:

Principle #2Taking out (Extraction)

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 results in higher quality factors and suppressed spurious modes, improving the performance of BAW resonators by minimizing energy leakage and interference, making them suitable for wide bandwidth filtering and multiplexing applications.

Implementation Method 1

enhances acoustic impedance mismatch and reduces lateral energy leakage

Methodology Applied

Scientific EffectAcoustic impedance mismatch: Acoustics

Implementation Method 2

a piezoelectric layer that is arranged between top and bottom electrodes

Methodology Applied

Scientific EffectPiezoelectric effect: Piezoelectric Effect

Data Source

PatentUS12425002B2Top electrodes and dielectric spacer layers for bulk acoustic wave resonators

Publication Date: 2025.09.23 QORVO US INC

AI summary

Bulk acoustic wave (BAW) resonators and particularly top electrodes with step arrangements for BAW resonators are disclosed. Top electrodes on piezoelectric layers are disclosed that include a border (BO) region with a dual-step arrangement where an inner step and an outer step are formed with increasing heights toward peripheral edges of the top electrode. Dielectric spacer layers may be provided between the outer steps and the piezoelectric layer. Passivation layers are disclosed that extend over the top electrode either to peripheral edges of the piezoelectric layer or that are inset from peripheral edges of the piezoelectric layer. Piezoelectric layers may be arranged with reduced thickness portions in areas that are uncovered by top electrodes. BAW resonators as disclosed herein are provided with high quality factors and suppression of spurious modes while also providing weakened BO modes that are shifted farther away from passbands of such BAW resonators.