Lateral Mode Capacitive Microphone Reducing Squeeze Film Damping
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Solution Overview
Problem
Conventional capacitive microphones suffer from mechanical noise due to squeeze film damping, leading to inaccurate sound detection, especially when subjected to vibrations like those experienced in a moving automobile, resulting in a low Signal to Acceleration Ratio (SAR) that misinterprets device vibrations as sound signals.
Innovation Solution
A lateral mode capacitive microphone design where the movable membrane does not move towards a fixed backplate, instead gliding parallel to it, reducing squeeze film damping and incorporating a motional sensor with air ventilation holes to differentiate between acoustic pressure and acceleration-induced movements, allowing for signal subtraction to improve accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the movable membrane moves perpendicular to the fixed backplate in conventional capacitive microphones, then the capacitance changes with sound pressure, but squeeze film damping occurs causing mechanical noise and low signal to acceleration ratio
Solution Approach 1:
The patent transitions from conventional perpendicular membrane movement (one dimension) to lateral membrane movement parallel to the backplate (another dimension). This dimensional change eliminates the membrane from approaching or receding from the backplate, thereby avoiding squeeze film damping between the membrane and backplate while still enabling capacitance changes through lateral displacement.
Solution Approach 2:
Instead of the membrane moving toward and away from the backplate as in conventional designs, the patent inverts the movement direction so the membrane glides laterally parallel to the backplate. This inversion of the movement direction eliminates the harmful squeeze film effect while maintaining the capacitive sensing function.
2Object-generated harmful factors
If the movable membrane glides parallel to the fixed backplate in lateral mode, then squeeze film damping is reduced, but the device complexity increases due to additional motional sensor and signal processing requirements
Solution Approach 1:
The patent implements a multi-functional device where a single lateral mode capacitive microphone structure serves both as the primary acoustic sensor and as a motional sensor. The same membrane and capacitor structure that detects sound pressure also detects acceleration and vibration, eliminating the need for separate sensor structures and reducing overall device complexity.
Solution Approach 2:
The patent merges the acoustic sensing function and motion sensing function into a single integrated capacitive structure. The lateral mode membrane simultaneously performs both acoustic pressure detection and acceleration detection, combining multiple functions into one unified structure rather than requiring separate sensors.
3Measurement precision
If the movable membrane has large surface area to gap length ratio, then the capacitance sensitivity increases, but squeeze film damping becomes more significant causing mechanical noise
Solution Approach 1:
The patent changes the membrane movement from perpendicular (changing gap length) to lateral (changing overlap area). This allows the membrane to have a large surface area for high capacitance sensitivity without creating squeeze film damping, because the gap length remains constant during lateral movement, eliminating the pressure buildup that causes mechanical noise.
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
The design enhances sound detection accuracy by minimizing mechanical noise and correctly distinguishing between sound signals and device vibrations, thereby improving the Signal to Acceleration Ratio and reducing misinterpretation of vibrations as sound.
Implementation Method 1
conductor 202 laterally moves over, or 'glides' over, conductor 201, producing changes in the overlapped area between 201 and 202, and therefore varying the mutual capacitance therebetween
Implementation Method 2
The flow viscosity of air, therefore, gives rise to a force that resists the motion of moving membrane/diaphragm 101. Squeeze film damping is significant when membrane/diaphragm 101 has a large surface area to gap length ratio.
Data Source
AI summary
The present invention provides a process of fabricating a capacitive microphone such as a MEMS microphone. In the process, one electrically conductive layer is deposited on a removable layer, and then divided or cut into two divided layers, both of which remain in contact with the removable layer as they were. One of the two divided layers will become or include a movable or deflectable membrane/diaphragm that moves in a lateral manner relative to another layer, instead of moving toward/from another layer. A motional sensor is optionally fabricated within the microphone to estimate the noise introduced from acceleration or vibration of the microphone for the purpose of compensating the microphone output through a signal subtraction operation.