Optical MEMS Microphone with Diffraction Grating for Low Noise Sensing
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Solution Overview
Problem
Conventional capacitive microphones face limitations such as viscous damping, reduced signal-to-noise ratio due to thermal noise, and diaphragm collapse issues due to bias voltage, which hinder their performance in miniature hearing aid applications.
Innovation Solution
A MEMS differential microphone utilizing optical interferometry with a diffraction device integrated into the diaphragm, eliminating the need for a bulky beam splitter, and employing a miniature VCSEL for low-power, low-voltage operation, enabling high sensitivity and directional response.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If capacitive sensing is used to detect diaphragm motion, then electrical sensitivity can be achieved, but viscous damping and thermal noise reduce the signal-to-noise ratio
Solution Approach 1:
The patent replaces the electrical capacitive sensing system with an optical interferometric sensing system. The optical system uses interference patterns of light to detect diaphragm displacement, eliminating the need for electrical fields and capacitive measurements that suffer from viscous damping and thermal noise. This substitution of measurement methodology directly resolves the contradiction by removing the source of harmful effects while maintaining measurement capability.
2Measurement precision
If bias voltage is increased to improve electrical sensitivity, then sensitivity increases, but diaphragm collapse against the back plate occurs
Solution Approach 1:
The patent eliminates the need for bias voltage by replacing electrical capacitive sensing with optical interferometric sensing. The optical system measures diaphragm position through light interference patterns without requiring electrical fields, thereby completely removing the problematic bias voltage that causes diaphragm collapse while maintaining high sensitivity measurement capability.
Solution Approach 2:
The patent changes the measurement parameter from electrical capacitance to optical path length. By measuring diaphragm displacement through optical interference fringes rather than electrical capacitance changes, the system achieves high sensitivity without requiring high bias voltages that would compromise diaphragm structural integrity.
3Strength
If gap spacing is increased to avoid diaphragm collapse, then collapse voltage increases, but microphone capacitance decreases
Solution Approach 1:
The patent replaces electrical capacitive sensing with optical interferometric sensing, eliminating the fundamental relationship between gap spacing and capacitance. The optical system's sensitivity depends on optical path length changes rather than electrical capacitance, allowing the use of smaller gap spacings to improve acoustic performance without worrying about capacitance magnitude or collapse voltage constraints.
4Quantity of substance
If small gap spacing is used to increase capacitance, then capacitance increases, but viscous damping from air flow increases
Solution Approach 1:
The patent replaces electrical capacitive sensing with optical interferometric sensing, making the measurement independent of the gap spacing-induced viscous damping. The optical system can accurately measure diaphragm displacement regardless of the small gap spacing required for high capacitance, thereby resolving the contradiction by decoupling measurement sensitivity from viscous damping effects.
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 solution achieves low noise and high sensitivity, reducing minimum detectable pressure levels and power consumption, making it suitable for hearing aids and other miniature devices.
Implementation Method 1
A micromachined differential microphone having optical means for converting sound-induced motion of the diaphragm into an electronic signal
Implementation Method 2
A diffraction device (e.g., a diffraction grating or, in alternate embodiments, inter-digitated fingers) is integrated with the microphone diaphragm to implement an optical interferometer
Data Source
AI summary
A microphone having an optical component for converting the sound-induced motion of the diaphragm into an electronic signal using a diffraction grating. The microphone with inter-digitated fingers is fabricated on a silicon substrate using a combination of surface and bulk micromachining techniques. A 1 mm×2 mm microphone diaphragm, made of polysilicon, has stiffeners and hinge supports to ensure that it responds like a rigid body on flexible hinges. The diaphragm is designed to respond to pressure gradients, giving it a first order directional response to incident sound. This mechanical structure is integrated with a compact optoelectronic readout system that displays results based on optical interferometry.