Lateral-Mode Capacitive Microphone with Sandwich Backplate
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Solution Overview
Problem
Capacitive microphones suffer from significant mechanical noise due to squeeze film damping, which is a major limiting factor in their performance, especially when the movable membrane moves towards the fixed backplate, leading to systemic noise issues.
Innovation Solution
A lateral-mode capacitive microphone design is fabricated with a sandwich structure where the movable membrane does not move towards the fixed backplate, reducing squeeze film damping by using a lateral-mode configuration and air flow restrictors to minimize air leakage, thereby canceling systemic noise. This design involves fabricating electrical conductors side by side over a substrate, dividing one into sub-conductors, and sandwiching an electrical insulator between them, allowing for maximum variation in mutual capacitance along a primary direction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the movable membrane is positioned close to the fixed backplate to increase capacitance sensitivity, then the capacitance change is improved, but squeeze film damping increases causing mechanical noise
Solution Approach 1:
The patent transitions from a conventional parallel-plate configuration where the movable membrane moves perpendicular to the backplate toward a lateral-mode configuration where the movable membrane moves laterally relative to the backplate. This dimensional change eliminates squeeze film damping while preserving capacitance sensitivity, as the lateral motion avoids compressing the air film between surfaces.
2Measurement precision
If the gap between movable membrane and fixed backplate is reduced to enhance sensitivity, then measurement precision is improved, but air leakage increases reducing performance
Solution Approach 1:
By changing from perpendicular to lateral motion mode, the patent allows the use of smaller gaps between the movable membrane and fixed backplate without suffering from air leakage issues. The lateral configuration prevents air from being trapped and compressed, eliminating the air leakage problem that plagues conventional designs with small gaps.
3Object-generated harmful factors
If a lateral-mode configuration is used to reduce squeeze film damping, then mechanical noise is reduced, but device complexity increases
Solution Approach 1:
The patent divides the capacitor structure into two separate capacitors (first capacitor and second capacitor) with distinct movable membranes and fixed backplates. This segmentation allows each capacitor to be optimized independently for lateral-mode operation, reducing the complexity burden on any single component while achieving the overall noise reduction goal.
Solution Approach 2:
The patent combines two capacitors in parallel to form the complete microphone structure. By merging the output signals from both capacitors, the design achieves noise cancellation through differential signaling while maintaining the benefits of lateral-mode configuration in both sections.
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 effectively reduces mechanical noise and enhances the frequency response by minimizing air leakage and maintaining sensitivity across a broader frequency range, improving the overall performance of the microphone.
Implementation Method 1
mutual capacitance between the two electrical conductors consists of two sub-capacitances generated between another one of the two electrical conductors and the two sub-conductors
Implementation Method 2
squeeze film damping occurs when the moving component is moving perpendicular and in close proximity to the surface of the fixed component
Data Source
AI summary
The present invention provides a process for fabricating a capacitive microphone such as a MEMS microphone. In the microphone, a movable or deflectable membrane/diaphragm may be so fabricated that it moves in a lateral manner relative to a fixed backplate, instead of moving toward/from the fixed backplate. The fixed backplate may be so fabricated that it includes an electrical insulator sandwiched between two sub-conductors to cancel systematic/background noise. The squeeze film damping is substantially avoided, and the performance, such as signal to noise ratio, of the fabricated microphone is significantly improved.