BAW Resonator Frame Recess for Spurious Mode Suppression

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Solution Overview

Problem

Achieving high quality factor (Q) and suppressing spurious modes in bulk acoustic wave (BAW) devices while meeting performance specifications is challenging, particularly due to issues with frame geometries that cause raised frame modes and degrade electromechanical coupling coefficients.

Innovation Solution

Incorporating a recess in the piezoelectric layer outside the active region of BAW devices, along with a frame structure positioned on opposite sides of the piezoelectric layer, to suppress asymmetric and symmetric modes, and using different thicknesses in various regions to enhance energy confinement and maintain electromechanical coupling.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If a frame structure is added to suppress spurious modes, then the quality factor (Q) is improved, but the device complexity increases

Engineering Contradiction:

Improvequality factorVSAvoiddevice complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The piezoelectric layer is segmented by creating a recess that divides it into an active region and a passive region. This segmentation allows the passive region to act as a frame structure for suppressing spurious modes while maintaining the active region for electromechanical coupling, thus improving quality factor without adding external complex structures.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

Different regions of the piezoelectric layer are given different properties: the active region maintains full thickness for high electromechanical coupling, while the passive region has reduced thickness to form a frame structure for suppressing spurious modes. This local differentiation resolves the contradiction by achieving both goals in different locations.

Inventive Principle:

Principle #3Local quality

2Reliability

If the piezoelectric layer thickness is reduced outside the active region to form a recess, then spurious modes are suppressed and quality factor is improved, but the electromechanical coupling coefficient may be degraded

Engineering Contradiction:

Improvequality factorVSAvoidelectromechanical coupling coefficient

Core Design Contradiction:

ReliabilityVSManufacturing precision

Solution Approach 1:

The piezoelectric layer is designed with local quality differentiation: the active region maintains its full thickness to preserve electromechanical coupling coefficient, while only the passive region outside the active electrodes has reduced thickness to suppress spurious modes. This ensures that the electromechanical coupling is not degraded while achieving high quality factor.

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The piezoelectric layer is segmented into active and passive regions with different thicknesses. The active region under the electrodes maintains full thickness for optimal coupling, while the passive region forms a recessed frame for spurious mode suppression, thus resolving the contradiction between quality factor and coupling coefficient.

Inventive Principle:

Principle #1Segmentation

3Reliability

If a recess is created in the piezoelectric layer to suppress asymmetric modes, then the quality factor is improved, but the manufacturing precision requirements increase

Engineering Contradiction:

Improvequality factorVSAvoidmanufacturing precision

Core Design Contradiction:

ReliabilityVSManufacturing precision

Solution Approach 1:

The recess is created only in the passive region outside the active electrodes, leaving the active region with full thickness and simple geometry. This local approach reduces manufacturing precision requirements compared to creating a recess throughout the entire piezoelectric layer, while still achieving spurious mode suppression and high quality factor.

Inventive Principle:

Principle #3Local quality

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 increases the quality factor (Q) and attenuates spurious modes, maintaining a stable electromechanical coupling coefficient (kt2), thereby improving BAW device performance and reducing energy losses.

Implementation Method 1

In BAW resonators, acoustic waves propagate in the bulk of a piezoelectric layer

Methodology Applied

Scientific EffectPiezoelectric effect: Piezoelectric Effect

Implementation Method 2

The piezoelectric layer can have a flat surface on a side facing the acoustic reflector

Methodology Applied

Scientific EffectAcoustic reflection: Reflection

Data Source

PatentUS20250260381A1Bulk acoustic wave device including frame outside of active region and flat bottom piezoelectric layer

Publication Date: 2025.08.14 SKYWORKS GLOBAL PTE LTD

AI summary

Aspects of this disclosure relate to bulk acoustic wave devices that include an acoustic reflector, a frame structure over the acoustic reflector, electrodes including a first electrode and a second electrode, and a piezoelectric layer having a surface facing the acoustic reflector that is flat (a) over an entirety of the active region and (b) beyond the first electrode over the acoustic reflector, The first electrode and the second electrode overlap and are on opposing sides of the piezoelectric layer in the active region. The first electrode is positioned between the piezoelectric layer and the acoustic reflector in the active region. Related acoustic wave filters, multiplexers, radio frequency modules, radio frequency systems, wireless communication devices, methods of manufacture, and methods of filtering are disclosed.