MEMS Microphone Diaphragm Surface Roughness for Stiction Control
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Solution Overview
Problem
MEMS devices, such as microphones, face yield loss and reliability failures due to stiction issues caused by smooth surfaces sticking together when exposed to humidity, leading to degraded performance.
Innovation Solution
The surfaces of MEMS devices, specifically the diaphragm and backplate, are processed to have a Hurst exponent of less than or equal to 0.5 and a root mean squared (RMS) roughness greater than 1 nanometer, with organic coatings like fluorocarbons to reduce surface energy and create a hydrophobic effect, minimizing wetting and stiction by forming cavities that oppose water droplet wetting.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If the surfaces of the diaphragm and backplate are made smooth, then the manufacturing precision is improved, but the reliability deteriorates due to stiction when exposed to humidity
Solution Approach 1:
The patent changes the surface roughness parameter from smooth to intentionally roughened surfaces with specific Hurst exponents (≤0.5) and RMS roughness (>1nm). This parameter change transforms the surface properties to create hydrophobic characteristics that prevent stiction while maintaining manufacturing feasibility through standard fabrication processes like chemical etching or plasma treatment.
Solution Approach 2:
The patent applies an organic coating layer (fluorocarbon or hydrocarbon-based) over the roughened surface to create a composite structure. This composite combines the mechanical support of the substrate with the hydrophobic properties of the organic coating, achieving both structural integrity and stiction resistance through the synergistic combination of materials.
2Reliability
If the surface roughness is increased to prevent stiction, then the reliability is improved, but the manufacturing precision deteriorates
Solution Approach 1:
The patent specifies precise parameter ranges for surface roughness (Hurst exponent ≤0.5, RMS roughness >1nm) to achieve the optimal balance between stiction resistance and manufacturability. By defining these specific parameters, the invention guides the fabrication process to produce surfaces that are rough enough to prevent stiction but controlled enough to be manufactured with standard precision techniques.
Solution Approach 2:
The patent applies different surface treatment characteristics to different regions or aspects of the surface. The Hurst exponent controls the fractal dimension and self-affine properties of the surface, while RMS roughness controls the amplitude of surface variations. This local quality approach allows optimization of specific surface characteristics for stiction resistance without compromising overall manufacturing feasibility.
3Reliability
If an organic coating is applied to reduce surface energy, then the reliability is improved, but the device complexity increases
Solution Approach 1:
The patent specifies the chemical composition parameters of the organic coating (fluorocarbon or hydrocarbon-based) to achieve optimal hydrophobicity with minimal process complexity. By selecting from these specific material classes, the invention simplifies the coating process while maintaining effective stiction resistance through the inherent low surface energy properties of these materials.
Solution Approach 2:
The organic coating serves as a sacrificial protective layer that can be easily applied and removed if needed. Using conventional coating techniques with readily available materials (fluorocarbon or hydrocarbon) provides an economical solution that adds minimal complexity while delivering significant reliability improvement through hydrophobicity.
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 effectively reduces stiction and maintains device performance by preventing water droplets from wetting the surfaces, allowing the diaphragm to return to its rest position without being impeded by droplets, thus enhancing the reliability and yield of MEMS devices.
Implementation Method 1
an organic coating is applied to further lower surface energy, mimicking the 'lotus flower effect' to prevent droplets from wetting and adhering
Implementation Method 2
creating a hydrophobic surface that prevents droplets from wetting and adhering, allowing the diaphragm to move freely and respond to acoustic signals without being impeded by surface tension forces
Data Source
AI summary
A MEMS microphone has a backplate and a movable diaphragm that together form a variable capacitance. The backplate has a backplate surface and, in a corresponding manner, the diaphragm has a diaphragm surface that faces the backplate surface. At least one of the backplate surface and the diaphragm surface has at least a portion with a Hurst exponent that is less than or equal to about 0.5.