Shaped Electrode MEMS Microphone Noise Reduction

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Solution Overview

Problem

Smaller microphones in devices face higher acoustic noise and lower signal-to-noise ratios due to increased acoustic and thermal losses, which are not effectively addressed by conventional designs.

Innovation Solution

The electrode in capacitive microphone assemblies is positioned away from the region of maximum diaphragm deflection, reducing maximum diaphragm deflection without significantly impacting sensitivity, and incorporating openings in the electrode to increase the gap between the back plate and diaphragm, thereby reducing squeeze film damping.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Volume of moving object

If the microphone size is reduced to fit smaller devices, then the device compactness is improved, but the acoustic noise increases and signal-to-noise ratio deteriorates due to higher acoustic and thermal losses

Engineering Contradiction:

Improvemicrophone sizeVSAvoidacoustic noise

Core Design Contradiction:

Volume of moving objectVSObject-affected harmful factors

Solution Approach 1:

The electrode is positioned at a specific location on the back plate that corresponds to a region of relatively lower diaphragm deflection, rather than uniformly across the entire back plate. This localized positioning creates a region of interest with different mechanical properties (lower deflection) that reduces acoustic noise while maintaining sensitivity in other regions

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The electrode positioning is designed in advance to anticipate and prevent the problem of acoustic noise. By pre-positioning the electrode in a region of lower deflection, the design proactively reduces acoustic noise before it becomes a performance limitation, rather than attempting to correct it after the fact

Inventive Principle:

Principle #10Preliminary action

2Measurement precision

If the electrode is positioned at the center of the diaphragm to maximize sensitivity, then the signal-to-noise ratio is improved, but the maximum diaphragm deflection increases causing higher acoustic noise

Engineering Contradiction:

Improvesignal-to-noise ratioVSAvoidacoustic noise

Core Design Contradiction:

Measurement precisionVSObject-affected harmful factors

Solution Approach 1:

The back plate is designed with regions of different deflection characteristics. The electrode is positioned in a localized region of lower deflection, while other regions of the diaphragm maintain their natural deflection patterns. This creates spatial variation in mechanical properties that simultaneously reduces noise and preserves sensitivity

Inventive Principle:

Principle #3Local quality

Solution Approach 2:

The back plate and diaphragm system is effectively segmented into different functional zones: a central region with the electrode experiencing lower deflection (noise reduction zone) and surrounding regions that maintain higher deflection for sensitivity. This segmentation allows independent optimization of different performance aspects

Inventive Principle:

Principle #1Segmentation

3Measurement precision

If the gap between the back plate and diaphragm is reduced to improve sensitivity, then the acoustic sensitivity increases, but the squeeze film damping increases causing higher acoustic noise

Engineering Contradiction:

Improveacoustic sensitivityVSAvoidsqueeze film damping

Core Design Contradiction:

Measurement precisionVSLoss of energy

Solution Approach 1:

The electrode positioning creates a localized measurement zone with optimized gap characteristics. By positioning the electrode in a region of lower deflection, the effective gap varies spatially, reducing squeeze film damping in the electrode region while maintaining sensitivity through the capacitive coupling mechanism

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

This approach enhances the signal-to-noise ratio and acoustic sensitivity while minimizing noise, allowing for improved performance in compact microphone designs.

Implementation Method 1

The back plate and the diaphragm each include an electrode, which are used to convert the acoustic energy into an electrical signal

Methodology Applied

Scientific EffectCapacitance: Capacitance

Implementation Method 2

the diaphragm, which moves relative to the back plate in response to acoustic energy (e.g., pressure fluctuations)

Methodology Applied

Scientific EffectAcoustic energy conversion: Acoustic Radiation Pressure

Implementation Method 3

incorporating openings in the electrode to increase the gap between the back plate and diaphragm, thereby reducing squeeze film damping

Methodology Applied

Scientific EffectSqueeze film damping: Viscous Damping

Data Source

PatentUS11463817B2Capacitive microphone with shaped electrode

Publication Date: 2022.10.04 KNOWLES ELECTRONICS LLC

AI summary

A microelectromechanical systems (MEMS) die includes a substrate, a back plate, and a diaphragm. The back plate is coupled to the substrate and includes a dielectric layer and an electrode. The electrode is coupled to the dielectric layer and defines an opening that exposes a central portion of the dielectric layer. The diaphragm is oriented parallel to the back plate and is spaced apart from the back plate. In one implementation, a diameter of the opening is greater than or equal to 1/10 of the diameter of the diaphragm.