Patterned PMUT Actuating Structure for Stress-Resonance Trade-off

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Solution Overview

Problem

Piezoelectric micromachined ultrasonic transducers (pMUTs) face challenges with residual stress in the piezoelectric layer, leading to frequency variations and poor acoustic performance, especially when operated at resonance.

Innovation Solution

A pMUT design featuring a patterned actuating structure with a central portion and radially projecting rib portions, which reduces residual stress and maintains flexural rigidity, thereby stabilizing the resonance frequency and improving acoustic performance.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Power

If a central electrode structure is used for actuation, then electromechanical coupling is improved, but frequency variation increases due to stress variations across the wafer

Engineering Contradiction:

Improveelectromechanical couplingVSAvoidfrequency matching

Core Design Contradiction:

PowerVSManufacturing precision

Solution Approach 1:

The actuating structure is segmented into multiple radially distributed rib portions instead of a continuous central electrode. This segmentation distributes the piezoelectric material and stress across multiple locations, reducing the impact of local stress variations while maintaining overall electromechanical coupling effectiveness.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The actuating structure has non-uniform local properties with rib portions positioned at specific radial distances from the center. This local quality variation optimizes the balance between electromechanical coupling at different locations and stress distribution across the membrane, reducing frequency sensitivity.

Inventive Principle:

Principle #3Local quality

2Manufacturing precision

If a ring electrode structure is used to reduce residual stress, then frequency stability is improved, but electromechanical coupling decreases

Engineering Contradiction:

Improvefrequency stabilityVSAvoidelectromechanical coupling

Core Design Contradiction:

Manufacturing precisionVSPower

Solution Approach 1:

The actuating structure is divided into multiple discrete rib portions arranged radially, combining the stress-reducing benefits of a ring structure with the high coupling of a central structure. The segmented rib configuration provides frequency stability while maintaining adequate electromechanical coupling through distributed piezoelectric actuation.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The invention merges the stress-reducing geometric features of a ring electrode with the high coupling characteristics of a central electrode by positioning rib portions radially from the center. This hybrid configuration integrates the advantages of both structures while avoiding their respective disadvantages.

Inventive Principle:

Principle #5Merging (Combining)

3Manufacturing precision

If the membrane is released from the substrate to reduce stress sensitivity, then frequency sensitivity to stress is reduced, but acoustic performance deteriorates

Engineering Contradiction:

Improvefrequency sensitivity to stressVSAvoidacoustic performance

Core Design Contradiction:

Manufacturing precisionVSReliability

Solution Approach 1:

The membrane is segmented into multiple identical tapered cantilevers that are released from the substrate, reducing stress sensitivity while maintaining acoustic performance. The segmented configuration allows the membrane to flexurally vibrate with reduced stress coupling, improving frequency stability without sacrificing acoustic output.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The membrane structure is designed with dynamic flexural vibration characteristics, allowing it to operate as a cantilevered structure that naturally accommodates stress variations. The tapered cantilever geometry provides dynamic response that reduces stress sensitivity while maintaining adequate acoustic performance through controlled flexural modes.

Inventive Principle:

Principle #15Dynamics

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 proposed design achieves a stable single resonance mode with reduced sensitivity to stress, enhancing the acoustic performance and manufacturability of pMUTs.

Implementation Method 1

Piezoelectric micromachined ultrasonic transducers (pMUTs) are MUTs that use a piezoelectric layer for electro-mechanical transduction

Methodology Applied

Scientific EffectPiezoelectric effect: Piezoelectric Effect

Implementation Method 2

A typical pMUT is a multilayer membrane structure that is excited into flexural vibration using piezoelectric actuation

Methodology Applied

Scientific EffectFlexural vibration: Vibration

Implementation Method 3

Sound is emitted from the tube when the membrane vibrates, and the tube may be designed as an acoustic resonator to improve acoustic performance of the pMUT

Methodology Applied

Scientific EffectAcoustic resonance: Resonance

Data Source

PatentUS12284920B2Piezoelectric micromachined ultrasonic transducer with a patterned membrane structure

Publication Date: 2025.04.22 INVENSENSE INC

AI summary

A piezoelectric micromachined ultrasonic transducer (PMUT) device includes a substrate having an opening therethrough and a membrane attached to the substrate over the opening. An actuating structure layer on a surface of the membrane includes a piezoelectric layer sandwiched between the membrane and an upper electrode layer. The actuating structure layer is patterned to selectively remove portions of the actuating structure from portions of the membrane to form in a central portion proximate a center of the open cavity and three or more rib portions projecting radially outward from the central portion.