MEMS Proof Mass Rough Surface Stiction Reduction
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Solution Overview
Problem
Conventional MEMS devices, particularly Z-axis accelerometer devices, face stiction issues due to the smooth surfaces of single crystal silicon proof masses and polysilicon electrodes, which complicates the design, operation, and manufacturability of integrated circuit MEMS sensors.
Innovation Solution
The introduction of selectively etched rough surfaces on the proof mass and electrode elements, using non-homogeneous etch processes and polymer deposition, reduces stiction by increasing surface roughness and creating an air sensing gap, while a high temperature eutectic seal ring bonding structure forms a vacuum cavity to protect the inertial transducer element.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If smooth surfaces are used for the proof mass and electrode, then manufacturing precision is improved, but stiction increases causing reliability degradation
Solution Approach 1:
The patent applies local quality by creating rough surfaces only at the sensing gap interfaces between the proof mass and electrode, while maintaining smooth surfaces elsewhere for manufacturing precision. This localized surface modification reduces stiction at critical contact points without compromising overall device fabrication quality.
Solution Approach 2:
The patent changes the surface roughness parameter at the sensing gap surfaces to reduce stiction. By controlling the roughness through etching processes, the patent optimizes the balance between manufacturing precision and stiction resistance, transforming a harmful effect (stiction) into a manageable parameter.
2Reliability
If anti-stiction coatings are applied to reduce stiction, then reliability is improved, but the eutectic bonding process for gyrometer sensors is interfered with
Solution Approach 1:
Instead of applying anti-stiction coatings globally, the patent uses local quality by selectively roughening only the sensing gap surfaces where stiction occurs. This localized approach avoids interfering with the eutectic bonding process while still providing stiction reduction at the critical interfaces.
Solution Approach 2:
The patent replaces the chemical approach of anti-stiction coatings with a mechanical surface modification approach (etching to create rough surfaces). This substitution eliminates the need for additional coating materials that would interfere with bonding processes, while achieving the same stiction reduction goal through physical surface topology changes.
3Reliability
If rough surfaces are created by etching to reduce stiction, then reliability is improved, but device complexity increases
Solution Approach 1:
The patent merges the surface roughening step with existing fabrication processes by performing the etching as part of the structural release etch process. This combination integrates the stiction reduction step into the normal manufacturing flow without adding separate dedicated process steps, thereby minimizing the increase in device complexity.
Solution Approach 2:
The patent performs preliminary action by preparing the rough surfaces during the structural release etch process before final device assembly. This timing allows the surface modification to be completed as part of the standard fabrication sequence, avoiding the need for additional post-processing steps and reducing overall process complexity.
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 between the proof mass and electrode, enhancing the operational reliability and manufacturability of MEMS sensors by creating a controlled environment for the inertial transducer elements, thereby improving the performance and integration of MEMS devices.
Implementation Method 1
at least a first sensing gap surface of the first monocrystalline semiconductor proof mass element is a first rough surface that has been selectively etched to reduce stiction
Implementation Method 2
a high temperature eutectic seal ring bonding structure to form a vacuum in a first sensor cavity which houses the first inertial transducer element
Data Source
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AI summary
A method and apparatus are described for fabricating a high aspect ratio MEMS sensor device having an inertial transducer element formed in a multi-layer semiconductor structure, where the first inertial transducer element comprises a first monocrystalline semiconductor proof mass element and a second conductive electrode element separated from one another by an air sensing gap, and where at least a first sensing gap surface of the first monocrystalline semiconductor proof mass element is a first rough surface that has been selectively etched to reduce stiction between the first monocrystalline semiconductor proof mass element and the second conductive electrode element.