Lateral Mode MEMS 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 dominates noise sources in these devices, affecting their performance in capturing high-quality audio signals.
Innovation Solution
The design incorporates a lateral mode configuration for the capacitors, where the movable membrane does not move into the fixed backplate, and the use of comb fingers reduces air resistance, minimizing squeeze film damping and enhancing signal quality by ensuring the movable membrane moves laterally over the stationary conductor, thereby varying mutual capacitance effectively.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If two layers are placed closely in parallel with small gap, then capacitance sensitivity is improved, but mechanical noise increases due to squeeze film damping
Solution Approach 1:
The patent transitions from a conventional parallel-plate capacitor geometry to a lateral mode capacitor geometry where the movable membrane moves laterally over the stationary conductor rather than vertically toward it. This dimensional change in motion direction eliminates the squeeze film damping effect that occurs when membranes move perpendicular to the backplate, thereby reducing mechanical noise while preserving capacitance sensitivity through the lateral variation of mutual capacitance.
2Measurement precision
If movable membrane moves perpendicular to backplate, then capacitance varies with sound pressure, but air resistance increases causing mechanical noise
Solution Approach 1:
The invention changes the direction of membrane motion from perpendicular (vertical) to lateral (horizontal) relative to the stationary conductor. In this lateral mode configuration, the membrane moves sideways over the conductor rather than toward or away from it, which eliminates the compression and expansion of air that causes squeeze film damping and mechanical noise, while still achieving capacitance variation through the changing lateral position.
3Measurement precision
If gap between layers is reduced to increase sensitivity, then signal detection improves, but squeeze film damping becomes dominant noise source
Solution Approach 1:
The patent eliminates the need for small vertical gaps by transitioning to lateral mode operation where the membrane moves horizontally over the stationary conductor. The gap between layers can be larger since the capacitance variation is achieved through lateral displacement rather than vertical compression, thereby avoiding the squeeze film damping that dominates noise in conventional small-gap designs.
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 configuration significantly reduces mechanical noise, improving the microphone's ability to produce high-quality audio signals by minimizing squeeze film damping and optimizing signal output through the additive inverse relationship between the two capacitors' signal outputs.
Implementation Method 1
ECA1 and ECA2 are configured in a lateral mode. 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
The design incorporates a lateral mode configuration for the capacitors, where the movable membrane does not move into the fixed backplate, and the use of comb fingers reduces air resistance, minimizing squeeze film damping
Implementation Method 3
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, generating the signal output S1 of the first capacitor
Data Source
AI summary
The present invention provides a capacitive microphone such as a MEMS microphone with two capacitors. 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 is significantly improved.