Lateral Mode Capacitive Microphone Reducing Squeeze Film Damping
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Solution Overview
Problem
Capacitive microphones suffer from significant mechanical noise due to squeeze film damping, which affects their performance and signal quality, particularly when the movable membrane has a large surface area to gap length ratio.
Innovation Solution
A lateral mode capacitive microphone design is implemented, where the movable membrane does not move into the fixed backplate, and the capacitors are configured such that one signal output is the additive inverse of the other, reducing noise by minimizing squeeze film damping through a specific spatial relationship and air flow restrictors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the movable membrane is placed in close proximity to the fixed backplate to form a capacitive structure, then the capacitance sensitivity is improved, but squeeze film damping occurs causing mechanical noise
Solution Approach 1:
The patent transitions from a conventional parallel-plate capacitive structure (one-dimensional separation) to a lateral mode capacitive structure where the movable membrane and fixed backplate are separated by a lateral distance rather than being closely spaced in the vertical direction. This dimensional change eliminates squeeze film damping while preserving capacitance sensitivity through lateral electrode arrangements.
Solution Approach 2:
The invention extracts the harmful squeeze film damping effect by removing the close proximity arrangement that causes it. The movable membrane is positioned laterally offset from the fixed backplate, extracting the problematic air gap compression mechanism while retaining the essential capacitive sensing function through lateral electrode configurations.
2Measurement precision
If the movable membrane has a large surface area to improve sensitivity, then the signal output is enhanced, but squeeze film damping increases causing more mechanical noise
Solution Approach 1:
The patent enables large surface area membranes to be used without increased mechanical noise by implementing lateral mode operation. The movable membrane can have extensive lateral dimensions while maintaining a larger lateral separation from the fixed backplate, allowing high signal output through increased membrane area without compromising the noise-free lateral displacement mechanism.
3Ease of manufacture
If air is allowed to flow freely between the movable membrane and fixed backplate, then manufacturing is simplified, but mechanical noise increases due to air leakage
Solution Approach 1:
The lateral mode structure inherently provides improved acoustic isolation without complex manufacturing. The lateral offset arrangement creates natural acoustic barriers and flow paths that reduce air leakage noise while maintaining manufacturing simplicity. The structure allows air to flow more freely during manufacturing while the lateral configuration naturally suppresses acoustic noise during operation.
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 significantly reduces mechanical noise, enhancing the signal quality and sensitivity of the microphone by minimizing air leakage and optimizing the acoustic resistance within the microphone structure.
Implementation Method 1
The mutual capacitance can be varied by an acoustic pressure impacting upon ECA1 and/or ECA2 along a range of impacting directions in 3D space
Implementation Method 2
Capacitive microphones suffer from significant mechanical noise due to squeeze film damping, which affects their performance and signal quality
Data Source
AI summary
The present invention provides a process of fabricating a capacitive microphone such as a MEMS microphone with two capacitors. The two capacitors may be so fabricated that the signal output from the first capacitor is additive inverse of that from the second capacitor, and a total signal output is a difference between the two outputs. In at least one of the two capacitors, a movable or deflectable membrane/diaphragm moves in a lateral manner relative to the fixed capacitor plate, instead of moving toward/from the fixed plate. The squeeze film damping, and the noise are substantially avoided, and the performances of the microphone are significantly improved.