PMUT Stopper Design for Resonant Frequency Stability

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Solution Overview

Problem

Micromachined ultrasonic transducers (MUTs), particularly piezoelectric micromachined ultrasonic transducers (PMUTs), face variations in resonant frequency due to inconsistencies in membrane size across and between wafers, affecting their acoustic performance.

Innovation Solution

A PMUT design featuring a substrate with a cavity and a stopper made of the same single-crystalline material, where the stopper protrudes from the substrate to surround the cavity edge, allowing precise control of membrane size and position, thereby stabilizing the resonant frequency through etching and sacrificial layer management.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If the cavity is formed by etching the backside of the substrate, then the acoustic resonator performance is improved, but the membrane size varies significantly across the wafer and from wafer-to-wafer, causing variation in resonant frequency

Engineering Contradiction:

Improveacoustic resonator performanceVSAvoidmembrane size uniformity

Core Design Contradiction:

ReliabilityVSManufacturing precision

Solution Approach 1:

The invention divides the membrane structure into two parts: the membrane layer itself and the stopper structure. The stopper is formed separately by etching the substrate to create a protruding structure that defines the membrane boundary. This segmentation allows independent control of the membrane size through the stopper's dimensions, resolving the contradiction between acoustic performance and manufacturing precision.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The stopper structure is formed beforehand by etching the substrate before depositing the membrane layer. This preliminary action establishes the precise membrane boundaries before the membrane is deposited, ensuring uniform membrane size across the wafer. The stopper acts as a pre-defined template that guides the membrane formation process.

Inventive Principle:

Principle #10Preliminary action

2Ease of manufacture

If the membrane size is controlled by the cavity opening on the front side, then the membrane positioning is simplified, but the resonant frequency varies significantly across the wafer

Engineering Contradiction:

Improvemembrane positioningVSAvoidresonant frequency uniformity

Core Design Contradiction:

Ease of manufactureVSManufacturing precision

Solution Approach 1:

The stopper structure serves as an intermediary element between the substrate and the membrane layer. It provides a physical reference structure that defines the membrane boundaries with high precision. The stopper mediates the relationship between the cavity opening and the membrane, ensuring that the membrane size is controlled by the stopper's dimensions rather than the cavity opening, thereby achieving both ease of manufacture and manufacturing precision.

Inventive Principle:

Principle #24Intermediary (Mediator)

3Reliability

If the stopper is formed by etching the substrate, then the stopper is tightly attached to the substrate with vertical sidewall, but the fabrication process becomes more complex

Engineering Contradiction:

Improvestopper attachment reliabilityVSAvoidfabrication process complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The stopper structure is merged with the substrate through a single etching process. The stopper is formed by etching the substrate to create a protruding structure, combining the stopper formation and substrate processing into one integrated step. This merging approach ensures tight attachment with vertical sidewalls while avoiding the need for separate attachment processes, thus maintaining fabrication simplicity.

Inventive Principle:

Principle #5Merging (Combining)

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 approach enhances the uniformity of resonant frequencies, improves reliability, and maintains electrical performance by ensuring the stopper is tightly attached to the substrate with vertical sidewalls, reducing variations in membrane size and position.

Implementation Method 1

piezoelectric micromachined ultrasonic transducer (PMUT)... piezoelectric material... an ultrasound wave generated by vibrating the membrane

Methodology Applied

Scientific EffectPiezoelectric effect: Piezoelectric Effect

Implementation Method 2

PMUTs typically operate at the membrane's flexural resonant frequency... an ultrasound wave generated by vibrating the membrane

Methodology Applied

Scientific EffectFlexural resonance: Resonance

Data Source

PatentUS11498097B2Piezoelectric micromachined ultrasonic transducer and method of fabricating the same

Publication Date: 2022.11.15 VANGUARD INTERNATIONAL SEMICONDUCTOR CORPORATION

AI summary

A piezoelectric micromachined ultrasonic transducer (PMUT) includes a substrate, a stopper, and a membrane, where the substrate and the stopper are composed of same single-crystalline material. The substrate has a cavity penetrating the substrate, and the stopper protrudes from a top surface of the substrate and surrounds the edge of the cavity. The membrane is disposed over the cavity and attached to the stopper.