MEMS Outgassing Shield for Hermetic Cavity Isolation
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Solution Overview
Problem
Hermetically sealed MEMS devices face pressure issues due to outgassing from backend films used in integrated circuits, which can lead to failed performance in devices like accelerometers and gyroscopes, as the released gases increase sealed pressure beyond desired levels, making it difficult to maintain a low-pressure environment within the device.
Innovation Solution
A capped micromachined device design that includes a MEMS structure with a cap featuring an outgassing shield, isolation walls, and an interconnection chamber, where the shield layer acts as a complete outgas barrier between the cap and the MEMS chamber, and the isolation walls create separate hermetically sealed cavities for the MEMS structure and interconnections, preventing gas from entering the MEMS chamber.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If hermetic sealing is used to encapsulate the MEMS device, then the device achieves a sealed cavity environment, but outgassing from backend films causes pressure buildup that degrades device performance
Solution Approach 1:
The device is divided into two separate hermetic cavities: a first hermetic cavity containing the MEMS structure and a second hermetic cavity containing the integrated circuit. This segmentation isolates the MEMS structure from outgassing sources in the integrated circuit cavity, allowing each cavity to maintain its own pressure environment independently.
Solution Approach 2:
Electrically conductive interconnections extend through the hermetic seal from the substrate to the integrated circuit, serving as intermediaries that provide electrical coupling between the two hermetic cavities without compromising the hermetic seal integrity. This allows electrical functionality while maintaining physical separation of the cavities.
2Ease of operation
If electrical connections are made through the hermetic seal, then electrical coupling is achieved, but the seal integrity may be compromised
Solution Approach 1:
The device separates electrical interconnection functions from hermetic sealing functions by extending conductive interconnections through the seal rather than integrating them into the seal structure itself. This segmentation allows the seal to maintain integrity while providing electrical pathways.
Solution Approach 2:
The hermetic seal acts as an intermediary that physically separates the two cavities while allowing electrical interconnections to pass through. The seal material and structure are designed to accommodate these interconnections without compromising its barrier function.
3Device complexity
If a single hermetic cavity is used to simplify structure, then device complexity is reduced, but outgassing affects the entire cavity including the MEMS structure
Solution Approach 1:
The single cavity is segmented into two separate hermetic cavities by the hermetic seal, creating distinct enclosed spaces. The first cavity houses the MEMS structure while the second cavity houses the integrated circuit, preventing cross-contamination of gases between the two regions.
Solution Approach 2:
The integrated circuit is extracted from the MEMS cavity and placed in a separate hermetic cavity. This extraction removes the outgassing source (backend films of the integrated circuit) from proximity to the MEMS structure, eliminating the harmful effect while maintaining electrical connectivity through the seal.
Data Source
AI summary
A capped micromachined device has a movable micromachined structure in a first hermetic chamber and one or more interconnections in a second hermetic chamber that is hermetically isolated from the first hermetic chamber, and a barrier layer on its cap where the cap faces the first hermetic chamber, such that the first hermetic chamber is isolated from outgassing from the cap.