Dual Membrane MEMS Microphone Acoustic Stiffness
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Solution Overview
Problem
MEMS microphones face issues with particle contamination in the airgap between the membrane and backplate, leading to reduced sensitivity and increased harmonic distortion, and acoustic stiffness due to the need for many pillars to maintain a low pressure region, which results in high noise levels.
Innovation Solution
A dual-membrane MEMS microphone design where the membranes are mechanically coupled only in the center area, with a backplate having electrically isolated electrodes on both surfaces, allowing for differential signal readout at four output ports, reducing noise and enhancing signal-to-noise ratio by minimizing particle entry through smaller holes and independent bias voltage adjustment.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If large holes (5-20 μm) are made in the backplate to reduce acoustic resistance, then acoustic performance is improved, but particles can enter the airgap and contaminate the membrane
Solution Approach 1:
The single large hole in the backplate is segmented into multiple small holes (smaller than 0.5 μm). This segmentation maintains acoustic performance by providing sufficient open area while preventing particle entry, as particles larger than the small holes cannot penetrate into the airgap between membrane and backplate.
Solution Approach 2:
Different regions of the backplate are given different properties: the sound port region has small holes for particle protection, while the electrode regions maintain electrical isolation. The small holes are strategically positioned to balance acoustic resistance reduction with particle contamination prevention.
2Object-affected harmful factors
If two membranes are hermetically sealed to maintain low pressure region, then acoustic stiffness is reduced, but many pillars are needed to couple membranes and maintain distance
Solution Approach 1:
The low pressure region is extracted from a hermetically sealed configuration. Instead of sealing both membranes to maintain vacuum, only the first membrane is sealed to create the low pressure region, while the second membrane remains accessible. This eliminates the need for numerous pillars to maintain structural integrity under vacuum.
Solution Approach 2:
Instead of using pillars to create and maintain the low pressure region (as in prior art), the invention inverts the approach by using a sealed first membrane to define the low pressure region, with the second membrane floating freely. The coupling is achieved through minimal central support rather than multiple pillars.
3Reliability
If membranes are mechanically coupled in the center area only, then correlation coefficient is reduced and noise is lowered, but structural support is minimized
Solution Approach 1:
Mechanical coupling is localized to the center area of the membranes only, rather than edge-to-edge coupling. This local coupling provides sufficient structural support and maintains spacing where needed, while allowing the majority of the membrane surfaces to move independently, reducing correlation coefficient and noise.
Solution Approach 2:
The membrane coupling is segmented into a small central region rather than continuous edge coupling. This segmentation allows independent motion of the membrane perimeters, reducing correlated noise while maintaining structural stability through the central support point.
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 design reduces noise levels, enhances signal-to-noise ratio, and increases durability by preventing particle contamination and allowing independent bias voltage tuning, thereby improving the microphone's sensitivity and total harmonic distortion performance.
Implementation Method 1
the flexible membrane may oscillate relative to the backplate changing the capacitance over time. A bias voltage (Vb) applied to the backplate (or the membrane) facilitates measuring sound pressure induced deflections of the membrane as a capacitance change results in a voltage change
Implementation Method 2
If acoustic pressure waves impinge the microphone the flexible membrane may oscillate relative to the backplate
Data Source
AI summary
A dual membrane microphone is disclosed. In an embodiment, a MEMS microphone includes a first membrane, a backplate with a separated central area including a first backplate electrode on a lower portion of the backplate, a second backplate electrode on a upper portion of the backplate and a backplate insulation layer galvanically isolating the first and the second backplate electrodes, a second membrane and a coupling central portion, wherein the first membrane couples mechanically to the separated central area of the backplate in an electrically isolating manner and the separated central area of the backplate couples to the second membrane in an electrically isolating manner.