Three-Axis MEMS Accelerometer with Varying Gap Sensing
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Solution Overview
Problem
Current micro-fabrication approaches struggle to create three-axis capacitive MEMS accelerometers with differential sensing mechanisms, particularly for vertical-axis accelerometers, which are essential for comprehensive acceleration sensing in applications like inertial navigation and motion control, due to limitations in sensitivity and complexity of fabrication processes.
Innovation Solution
A three-axis capacitive MEMS accelerometer is developed using a SOI on glass process, featuring a novel out-of-plane vertical axis accelerometer design with a movable proof mass between two fixed electrodes, enabling differential capacitive sensing and integrating with lateral axis accelerometers on the same substrate, utilizing the handle layer of the SOI wafer as a top electrode for both sensing and environmental protection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a vertical-axis accelerometer is implemented using varying overlap area method with vertical comb electrodes, then capacitive sensing is achieved, but sensitivity is low and fabrication process becomes complicated
Solution Approach 1:
The patent inverts the conventional varying overlap area method by using a varying gap distance between horizontal comb electrodes instead. The movable proof mass with horizontal comb electrodes moves vertically relative to stationary horizontal comb electrodes, creating capacitance changes through gap variation rather than overlap area variation. This inversion achieves higher sensitivity while simplifying the fabrication process.
Solution Approach 2:
The patent employs SOI (Silicon-On-Insulator) wafer technology that combines silicon structural layers with buried oxide layers. The buried oxide layer serves dual purposes as both structural support and release layer, enabling the varying gap mechanism while simplifying fabrication. This composite structure allows for precise control of electrode gaps and reduces manufacturing complexity.
2Adaptability or versatility
If a three-axis accelerometer is fabricated by assembling separate single-axis accelerometers on different substrates, then three-dimensional acceleration sensing is achieved, but packaging size increases and misalignment problems occur
Solution Approach 1:
The patent merges three separate single-axis accelerometer structures into a single integrated three-axis accelerometer on one substrate. The device combines two lateral axis sensing elements (X and Y axes) with one vertical axis sensing element (Z axis) sharing a common proof mass and substrate, achieving three-dimensional acceleration sensing while reducing packaging size and eliminating misalignment issues between separate devices.
Solution Approach 2:
The patent creates a universal accelerometer structure where a single proof mass serves multiple sensing functions. The same proof mass structure is used for both lateral axis sensing (through horizontal comb electrodes) and vertical axis sensing (through vertical comb electrodes), allowing one component to perform multiple measurement functions simultaneously.
3Ease of manufacture
If conventional micro-fabrication approaches are used for vertical-axis accelerometers, then fabrication is possible, but differential sensing mechanism cannot be implemented
Solution Approach 1:
The patent segments the capacitor structure into movable and stationary parts with distinct electrode configurations. The proof mass with horizontal comb electrodes is segmented from the stationary horizontal comb electrodes, and both are positioned between upper and lower electrodes. This segmentation enables differential capacitive sensing by creating symmetric capacitor pairs that can be differentially read out, improving measurement precision while using conventional fabrication.
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 allows for the fabrication of a compact, sensitive, and reliable three-axis accelerometer with reduced packaging size and cost, achieving hermetic sealing and improved cross-axis sensitivity, while maintaining the same working principle for all sensing elements, thus overcoming the challenges of existing technologies.
Implementation Method 1
a varying gap differential capacitive sensing three-axis accelerometer
Implementation Method 2
differential capacitive sensing is a promising approach due to the further advantages including high sensitivity, low noise, and low power dissipation characteristics
Implementation Method 3
SOI based MEMS accelerometers have been reported that are designed and fabricated successfully
Data Source
Figure 1A~1D
Figure 1E~3
Figure 4~6
AI summary
The invention relates to a three axis capacitive mems accelerometer on a single substrate. In this invention a varying gap differential capacitive sensing three-axis accelerometer using SOI on glass process is introduced. The out of plane axis accelerometer which is developed in the present invention can be used for fabrication of either a three axis accelerometer with a single proof mass or an individual single axis accelerometers on the same substrate. Additionally the out of plane axis accelerometer which is developed in the present invention, the handle layer of the SOI wafer is used as packaging layer.