Lateral-Mode MEMS Microphone Eliminates 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 noise levels, especially when the movable membrane has a large surface area to gap length ratio.
Innovation Solution
The development of a lateral-mode capacitive microphone design where the movable membrane does not move into the fixed backplate, and the configuration of two capacitors 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 microphone where the movable membrane moves perpendicular to the backplate (one-dimensional motion) to a lateral mode design where the movable membrane moves parallel to the backplate (lateral/different dimensional motion). This dimensional change allows the membrane to remain at a constant distance from the backplate, maintaining capacitance sensitivity while eliminating squeeze film damping that occurs when the membrane moves toward and away from the backplate.
2Measurement precision
If the movable membrane has a large surface area to gap length ratio to increase sensitivity, then the acoustic pressure sensitivity is improved, but squeeze film damping becomes more significant increasing mechanical noise
Solution Approach 1:
The lateral mode capacitive microphone enables the movable membrane to have a large surface area while maintaining a constant gap distance from the backplate. The membrane moves laterally (side-to-side) rather than perpendicular to the backplate, so the large surface area increases acoustic pressure sensitivity without causing squeeze film damping, as the gap length remains constant throughout the motion range.
3Power
If the movable membrane moves close to the fixed backplate during operation, then the capacitance change is maximized improving signal output, but air flow resistance increases causing damping
Solution Approach 1:
The patent changes the motion dimension from perpendicular (vertical) to parallel (lateral) relative to the backplate. This allows the movable membrane to achieve maximum capacitance change through lateral displacement while maintaining a constant gap distance, thereby maximizing signal output without the energy loss associated with air flow resistance and squeeze film damping that occurs in perpendicular motion 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 design significantly reduces mechanical noise, enhancing the quality of the audio signal by minimizing squeeze film damping and maintaining effective acoustic pressure sensitivity.
Implementation Method 1
a movable membrane that is movable relative to the substrate along a primary direction that is perpendicular to the planar surface... the mutual capacitance can be varied by an acoustic pressure impacting upon the movable single conductor and/or the stationary composite conductor
Implementation Method 2
Fabricating the first capacitor may include fabricating a first electrical conductor ECA1, fabricating a second electrical conductor ECA2, and configuring conductors ECA1 and ECA2 in a lateral mode. By 'later mode,' it is intended to mean that conductors ECA1 and ECA2 have a mutual capacitance therebetween. The mutual capacitance can be varied by an acoustic pressure impacting upon ECA1 and/or ECA2
Implementation Method 3
Squeeze film damping occurs when the moving component is moving perpendicular and in close proximity to the surface of the fixed component (e.g., between approximately 2 and 50 micrometers). The squeezed film effect results from compressing and expanding the fluid (e.g., a gas or liquid) trapped in the space between the moving plate and the solid surface. The fluid has a high resistance, and it damps the motion of the moving component
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.