Patterned pMUT Membrane Structure for Stable Resonance Frequency

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Solution Overview

Problem

Piezoelectric micromachined ultrasonic transducers (pMUTs) face challenges in achieving stable resonance frequencies due to residual stress in the piezoelectric layer, leading to variations in acoustic performance and phase/amplitude errors in ultrasound signals.

Innovation Solution

A pMUT design featuring a patterned actuating structure with a central portion and radially projecting ribs, reducing residual stress while maintaining flexural rigidity, and incorporating a passivation layer to encapsulate the actuating structure.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Power

If a central electrode structure is used to improve electromechanical coupling, then coupling efficiency is improved, but frequency variation increases due to stress sensitivity

Engineering Contradiction:

Improveelectromechanical couplingVSAvoidfrequency variation

Core Design Contradiction:

PowerVSManufacturing precision

Solution Approach 1:

The actuating structure is segmented into a central electrode region and multiple rib electrodes radiating outward, allowing different regions to serve different functions while reducing overall stress sensitivity

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

Different regions of the actuating structure have different properties: the central region provides strong coupling while the rib regions provide stress relief, creating local quality variations that resolve the contradiction

Inventive Principle:

Principle #3Local quality

2Power

If piezo material is placed over the entire membrane surface to improve actuation, then electromechanical coupling is improved, but stress sensitivity increases causing frequency variation

Engineering Contradiction:

Improveactuation efficiencyVSAvoidresonance frequency stability

Core Design Contradiction:

PowerVSReliability

Solution Approach 1:

The piezoelectric actuating structure is divided into discrete segments (central electrode and rib electrodes) rather than continuous coverage, reducing the total piezoelectric material stress while maintaining actuation effectiveness

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent converts the harmful effect of piezoelectric stress by strategically placing rib electrodes that radiate stress outward from the center, transforming stress concentration into stress distribution that stabilizes resonance frequency

Inventive Principle:

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

3Manufacturing precision

If released cantilevers or flexurally-suspended membranes are used to reduce stress sensitivity, then frequency stability is improved, but acoustic performance and manufacturability deteriorate

Engineering Contradiction:

Improvefrequency matchingVSAvoidacoustic performance

Core Design Contradiction:

Manufacturing precisionVSProductivity

Solution Approach 1:

The membrane maintains clamped boundary conditions at the edges (good for acoustic performance) while the patterned actuating structure creates local stress relief zones (good for frequency stability), combining advantages of different approaches

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 design achieves stable resonance modes with reduced sensitivity to stress variations, enhancing acoustic performance and manufacturability by minimizing the impact of residual stress on membrane stiffness.

Implementation Method 1

Piezoelectric micromachined ultrasonic transducers (pMUTs) are MUTs that use a piezoelectric layer for electro-mechanical transduction. A typical pMUT is a multilayer membrane structure that is excited into flexural vibration using piezoelectric actuation.

Methodology Applied

Scientific EffectPiezoelectric actuation: Piezoelectric Effect

Implementation Method 2

The membrane structure is often formed by etching through a silicon wafer to remove the material beneath the membrane, thereby allowing it to vibrate. Sound is emitted from the tube when the membrane vibrates

Methodology Applied

Scientific EffectFlexural vibration: Vibration

Implementation Method 3

the tube may be designed as an acoustic resonator to improve acoustic performance of the pMUT

Methodology Applied

Scientific EffectAcoustic resonance: Resonance

Data Source

PatentUS11844282B2Piezoelectric micromachined ultrasonic transducer with a patterned membrane structure

Publication Date: 2023.12.12 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 a central portion proximate a center of the open cavity and three or more rib portions projecting radially outward from the central portion.