SiCN MEMS Microphone Nested Membrane Design
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Solution Overview
Problem
Existing MEMS microphone fabrication processes are susceptible to particle and moisture contamination, and require complex wet chemical etching and stiction-free drying processes, which are elaborate and not entirely effective.
Innovation Solution
A thin film-based MEMS microphone design using silicon carbonitride (SiCN) membranes that are enclosed with polysilicon electrodes, allowing for gas phase etching and encapsulation in a vacuum to prevent particle and moisture contamination, reducing the need for wet chemical etching and simplifying the drying process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If gas phase etching is used to release the membrane, then particle contamination is reduced, but the front side of the membrane becomes opened and susceptible to particle and moisture contamination
Solution Approach 1:
The patent implements a dual-membrane nested structure where an outer membrane encapsulates the inner membrane and electrode assembly. The outer membrane acts as a protective shell that seals the opened front side, preventing particle and moisture contamination while allowing the inner membrane to function. This nested configuration resolves the contradiction by providing both the benefits of gas phase etching and protection against contamination.
Solution Approach 2:
The patent uses thin film membranes (outer and inner membranes) to create a sealed encapsulation structure. These flexible thin films enclose the electrode assembly, preventing contamination while maintaining the mechanical functionality needed for acoustic sensing. The thin film approach allows the structure to remain compact while providing effective protection.
2Ease of manufacture
If wet chemical etching is used to release the membranes, then the membranes can be released, but complex stiction-free drying processes are required
Solution Approach 1:
The patent replaces wet chemical etching with gas phase etching to release the membranes. This substitution eliminates the need for complex stiction-free drying processes because gas phase etching leaves no liquid residue that would cause stiction. The mechanical drying step is replaced by a vapor-based process that inherently avoids the drying complexity problem.
Solution Approach 2:
The patent changes the etching parameter from liquid-based (wet chemical) to gas-based (vapor phase). This parameter change fundamentally alters the release mechanism, allowing membrane release without introducing liquid that would require complex drying. The gas phase process maintains ease of manufacture while eliminating drying process complexity.
3Object-affected harmful factors
If a Pancake design with two closed membranes is used, then particle immunity is improved, but wet chemical etching and complex drying processes are required
Solution Approach 1:
The patent replaces wet chemical etching with gas phase etching in the Pancake design configuration. This substitution maintains the particle immunity benefits of the dual closed-membrane structure while eliminating the need for complex stiction-free drying processes. The gas phase release method simplifies the fabrication process by removing the drying step complexity.
Solution Approach 2:
The patent employs a composite structure combining outer and inner membranes with electrode assemblies enclosed between them. This composite configuration provides particle immunity while the use of gas phase etching with sacrificial oxide layers simplifies the fabrication process by avoiding wet chemical methods and associated drying complexities.
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 SiCN membrane design provides a particle-immune and moisture-insensitive MEMS microphone with reduced complexity in fabrication, lower electrical parasitic impedance, and the ability to decouple electrical functions from mechanical components, enabling simpler and more efficient production with fewer mask planes.
Implementation Method 1
allowing for gas phase etching and encapsulation in a vacuum
Implementation Method 2
encapsulation in a vacuum to prevent particle and moisture contamination
Implementation Method 3
The displacement of the membrane due to sound pressure leads to capacity changes within the inner electrodes, which in turn can be read out from the ASIC
Data Source
AI summary
A MEMS microphone includes a substrate, a lower membrane supported on the substrate, an upper membrane suspended above the lower membrane, a first electrode supported on the lower membrane, and a second electrode supported on the upper membrane. The lower membrane and the upper membrane enclose a cavity in which the first electrode and the second electrode are located. The lower membrane and the upper membrane are each formed of silicon carbonitride (SiCN). The first electrode and the second electrode are each formed of polysilicon.