MEMS Encapsulation Using Polysilicon and Sacrificial Layers
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Solution Overview
Problem
Conventional methods for fabricating microelectromechanical systems (MEMS) face challenges in integrating high-performance integrated circuits with MEMS on a common substrate due to damage or destruction of mechanical structures, poor encapsulation materials causing tensile stress and inadequate vacuum environments, and limitations in using non-silicon based materials.
Innovation Solution
A method involving the deposition of sacrificial layers and encapsulation layers, such as polycrystalline silicon or gallium arsenide, to form a chamber around the mechanical structure, allowing for the integration of MEMS with high-performance integrated circuits while minimizing stress and enabling the use of non-silicon based materials.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional oxide encapsulation techniques are used to protect the mechanical structure, then the mechanical structure is sealed in a chamber, but the oxide exhibits high tensile stress at corners or steps and provides poor coverage where the underlying surface exhibits significant spatial transitions
Solution Approach 1:
The patent changes the material parameter from conventional oxide to polysilicon, which has different mechanical properties including lower tensile stress and better conformality, thereby resolving the contradiction between encapsulation integrity and coverage uniformity
Solution Approach 2:
The patent uses a composite encapsulation structure with multiple layers including polysilicon and other materials, where each layer contributes different properties to achieve both stress reduction and improved coverage
2Reliability
If conventional oxide encapsulation is used to encapsulate the mechanical structure, then the structure is protected, but the deposited film impacts the integrity of the mechanical structures and performance of the MEMS
Solution Approach 1:
The patent introduces a sacrificial layer that is temporarily present during fabrication and then completely removed, serving its protective function only during the encapsulation process and eliminating any potential contamination issues in the final device
Solution Approach 2:
The sacrificial layer is extracted or removed after serving its purpose, leaving no harmful residue that could contaminate the mechanical structure or affect MEMS performance
3Adaptability or versatility
If additional processing is performed to integrate high performance integrated circuitry with MEMS on the same substrate, then integration is achieved, but the mechanical structures are damaged or destroyed
Solution Approach 1:
The patent performs preliminary encapsulation of the mechanical structures before subsequent integrated circuit processing steps, creating a protective barrier that allows aggressive processing chemistry to be used without damaging the sensitive mechanical elements
4Reliability
If conventional oxide encapsulation is used, then the mechanical structure is sealed, but an insufficient vacuum is provided where a vacuum is desired as the operating environment
Solution Approach 1:
The patent changes the encapsulation material from conventional oxide to polysilicon, which provides superior vacuum barriers due to its lower outgassing rate and better sealing properties, thereby achieving both reliable chamber sealing and high-quality vacuum environment
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 enhances the integrity and performance of MEMS by reducing stress, improving encapsulation, and facilitating the integration of diverse materials, thereby overcoming the limitations of conventional techniques.
Implementation Method 1
depositing a sacrificial layer over at least a portion of the mechanical structure
Implementation Method 2
depositing a first encapsulation layer over the sacrificial layer
Implementation Method 3
depositing a first encapsulation layer over the sacrificial layer
Implementation Method 4
depositing a second encapsulation layer over or in the vent to seal the chamber
Implementation Method 5
at least a portion of the sacrificial layer is removed to form the chamber
Data Source
AI summary
There are many inventions described and illustrated herein. In one aspect, the present invention is directed to a MEMS device, and technique of fabricating or manufacturing a MEMS device, having mechanical structures encapsulated in a chamber prior to final packaging. An embodiment further includes location of a piezoelectric material as part of a semiconductor sensing structure. The semiconductor sensing structure, in conjunction with the piezoelectric material, can be used as a sensing device to provide an output signal associated with a sensed event.