MEMS Microphone DC Guard Structure for Leakage Current Reduction
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Solution Overview
Problem
Current MEMS microphones face challenges in miniaturization and cost-efficiency due to high noise levels caused by DC leakage currents and the need for high-cost substrates with high resistivity, which are prone to water absorption and degradation.
Innovation Solution
The design incorporates a MEMS transducer and IC chip on a substrate with a signal line that maintains a minimum distance from conducting structures, using a DC guard structure to eliminate DC leakage currents and an electromagnetic shielding structure to reduce noise, while employing a multilayer substrate for improved insulation and miniaturization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If high resistivity substrates are used to reduce DC leakage current, then noise level is reduced, but substrate cost increases and water absorption issues arise
Solution Approach 1:
A DC guard structure is introduced as an intermediary element between the signal line and surrounding conducting structures. This guard structure is held at the same DC potential as the signal line, intercepting leakage current paths and preventing DC leakage through the substrate insulation. This allows the use of lower cost substrates while maintaining low noise performance.
Solution Approach 2:
The DC leakage current path is extracted and redirected through the DC guard structure. By providing a dedicated path at the same potential, the harmful leakage current is separated from the signal path, preventing it from flowing through the substrate insulation and reducing noise without requiring high resistivity materials.
2Object-affected harmful factors
If minimum distance of 200-500 μm is maintained between signal line and conducting structures, then DC leakage current is reduced, but device area increases
Solution Approach 1:
The DC guard structure serves as a mediator that actively manages potential differences. By holding the guard structure at the same DC potential as the signal line, the potential difference driving leakage current is eliminated, allowing smaller spacing between conducting structures while maintaining low leakage current.
Solution Approach 2:
The DC guard structure is maintained at equipotential with the signal line by connecting it to the same DC voltage source. This eliminates the potential difference that drives DC leakage current through insulation resistance, enabling reduced spacing between conducting structures without increasing leakage.
3Object-affected harmful factors
If signal line is integrated on common chip with MEMS and amplifier, then signal line length is reduced and insulation quality is improved, but processing complexity and cost increase
Solution Approach 1:
The device is segmented into separate functional modules: MEMS transducer, IC chip with amplifier, and carrier substrate with signal line routing. This segmentation allows each module to be optimized independently while maintaining good insulation quality through the DC guard structure, avoiding the processing complexity of full integration.
Solution Approach 2:
The DC guard structure acts as an intermediary that compensates for the longer signal line path in distributed architecture. By actively managing leakage current paths, it maintains insulation quality equivalent to integrated designs while allowing the benefits of modular construction and simplified processing.
4Area of stationary object
If flip-chip mounting is used for miniaturization, then device size is reduced, but insulation resistance decreases due to substrate dielectric properties
Solution Approach 1:
The DC guard structure is positioned between the signal line and surrounding conducting structures in the flip-chip configuration. It intercepts leakage current paths that would otherwise flow through the substrate dielectric, maintaining high insulation resistance despite the compact flip-chip layout.
Solution Approach 2:
By maintaining the DC guard structure at the same potential as the signal line, the potential difference across the substrate dielectric is minimized. This dramatically reduces leakage current through the substrate, enabling compact flip-chip mounting while maintaining high insulation resistance.
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 approach significantly reduces noise levels and allows for cost-effective production of MEMS microphones by using cheaper organic substrates while maintaining effective insulation and miniaturization, enhancing the signal quality and reducing thermal noise.
Implementation Method 1
A minimum distance between the signal line and the conducting structure is kept of at least 200 μm or, even better, at least 500 μm in order to reduce a DC leakage current flowing through the insulation resistance driven by a potential difference Ubias between a membrane potential and ground
Implementation Method 2
an electromagnetic shielding structure to reduce noise
Implementation Method 3
employing a multilayer substrate for improved insulation
Data Source
AI summary
A MEMS microphone having an improved noise performance due to reduced DC leakage current is provided. For that, a minimum distance between a signal line of the MEMS microphone and other conducting structures is maintained. Further, a DC guard structure fencing at least a section of the signal line is provided.