Silicon Carbide Coating for MEMS Stiction Reduction
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Solution Overview
Problem
Stiction forces in microelectromechanical systems (MEMS) devices, such as accelerometers, cause parts to adhere and become unusable due to electrostatic, Van der Waals, and residual contamination forces, with traditional methods to reduce stiction either decreasing sensitivity or increasing device size.
Innovation Solution
Forming a near-uniform silicon carbide layer on polysilicon surfaces using carbon from TEOS-based silicon oxide sacrificial films during fabrication, controlling annealing parameters to achieve a stable and effective anti-stiction coating.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional methods (increasing spring constants or increasing distance between parts) are used to reduce stiction, then stiction forces are reduced, but device sensitivity to acceleration decreases
Solution Approach 1:
A silicon carbide layer is introduced as an intermediary coating between the polysilicon surfaces that would otherwise be in direct contact. This intermediate layer reduces stiction forces through its low friction properties while allowing the original mechanical structure (springs and part distances) to remain unchanged, thereby preserving acceleration sensitivity.
Solution Approach 2:
The surface properties of the polysilicon parts are changed by forming a silicon carbide coating. This parameter change (surface material composition and friction characteristics) reduces stiction without altering the bulk mechanical properties or dimensions of the device components, thus maintaining sensitivity.
2Reliability
If traditional methods (increasing spring constants or increasing distance between parts) are used to reduce stiction, then stiction forces are reduced, but device size increases
Solution Approach 1:
The silicon carbide layer serves as a mediator that enables close proximity or contact between parts without stiction problems. This eliminates the need to increase the distance between parts or stiffen springs, allowing the device to maintain its compact original size.
Solution Approach 2:
By changing the surface friction parameters through silicon carbide coating, the device can operate with smaller clearances and softer springs while avoiding stiction, thereby reducing overall device volume without sacrificing reliability.
3Measurement precision
If surfaces are kept at close proximity to maintain sensitivity, then device sensitivity is improved, but stiction forces increase causing parts to adhere
Solution Approach 1:
The silicon carbide layer acts as a protective intermediary between opposing surfaces. It allows the surfaces to remain at close proximity for high sensitivity while the coating's low friction properties prevent adhesion and stiction, resolving the contradiction between closeness and reliability.
Solution Approach 2:
The friction and adhesion parameters of the surface are fundamentally changed by the silicon carbide coating. This enables the surfaces to be positioned close together for sensitivity while the modified surface properties prevent stiction, achieving both high sensitivity and reliability simultaneously.
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
Reduces stiction forces while maintaining device sensitivity and allowing for smaller device sizes, enabling improved performance and increased density of MEMS devices.
Implementation Method 1
Stiction is a static friction force that is a recurring problem with typical MEMS devices. When two surfaces with areas below the micrometer range come into close proximity, the surfaces may adhere together due to electrostatic and/or Van der Waals forces.
Implementation Method 2
annealing the first silicon layer and the sacrificial layer. The annealing includes heating the first silicon layer and the sacrificial layer to a temperature sufficient to form a silicon carbide layer at an interface region between the first silicon layer and the sacrificial layer
Implementation Method 3
forming a near-uniform silicon carbide layer on polysilicon surfaces using carbon from TEOS-based silicon oxide sacrificial films during fabrication
Data Source
AI summary
A mechanism is provided for reducing stiction in a MEMS device by forming a near-uniform silicon carbide layer on silicon surfaces using carbon from TEOS-based silicon oxide sacrificial films used during fabrication. By using the TEOS as a source of carbon to form an antistiction coating, all silicon surfaces can be coated, including those that are difficult to coat using standard self-assembled monolayer (SAM) processes (e.g., locations beneath the proof mass). Controlled processing parameters, such as temperature, length of time for annealing, and the like, provide for a near-uniform silicon carbide coating not provided by previous processes.