High Aspect-Ratio Multi-Axis Capacitive Accelerometer Design
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Solution Overview
Problem
Existing microelectromechanical systems (MEMS) accelerometers face challenges in achieving high sensitivity and low noise with a small footprint, as they require large proof-mass and increased device height to reduce thermal mechanical noise, which complicates the fabrication process and increases the device footprint.
Innovation Solution
The design incorporates a transducer structure with a high aspect ratio, featuring a proof-mass and support beam with capacitive transduction gaps that vary in height, allowing for increased sensitivity and reduced noise by optimizing the gap dimensions and proof-mass size, while using deep reactive ion etching and additional mass deposition to enhance sensitivity and reduce Brownian noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If device height is increased to reduce thermal mechanical noise, then sensitivity is improved, but device footprint increases
Solution Approach 1:
The patent transitions from planar (2D) capacitor plates to vertically stacked (3D) capacitor structures. By stacking capacitor plates in the vertical dimension, the sensing area is effectively increased without expanding the device footprint, thereby improving sensitivity while maintaining a compact form factor.
Solution Approach 2:
The patent implements nested capacitor structures where multiple capacitor plates are stacked within a compact vertical space. The proof mass is positioned between stacked capacitor plates, creating a nested arrangement that maximizes the sensing area within a small footprint, effectively resolving the contradiction between sensitivity and device size.
2Reliability
If proof-mass size is increased to reduce Brownian noise, then noise performance is improved, but device complexity increases
Solution Approach 1:
Instead of increasing proof-mass area in the planar dimension which would increase device footprint and complexity, the patent stacks multiple capacitor plates vertically. This allows the effective sensing mass to be increased by utilizing the vertical dimension, improving noise performance without proportionally increasing device complexity.
Solution Approach 2:
The patent employs composite structures combining proof mass material with stacked capacitor plate structures. This composite arrangement allows the system to achieve equivalent or superior noise performance to larger monolithic proof masses while maintaining simpler fabrication processes and device architecture.
3Measurement precision
If capacitive gap is reduced to increase sensitivity, then sensitivity is improved, but manufacturing precision requirements increase
Solution Approach 1:
The patent stacks capacitor plates in the vertical dimension, creating multiple capacitive gaps in series. This approach distributes the total capacitance change across multiple gaps, allowing each individual gap to be larger while maintaining high overall sensitivity, thereby reducing manufacturing precision requirements for each gap.
Solution Approach 2:
The patent combines multiple capacitor structures in series to achieve high sensitivity. By merging multiple capacitive elements, the system accumulates the capacitance change from each element, providing high overall sensitivity with larger, more manufacturable gap dimensions that are less sensitive to fabrication variations.
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 enables the creation of accelerometers with improved sensitivity and reduced noise performance, achieving sub-μg/√Hz noise floor within a smaller footprint, suitable for multi-axis applications, and allows for the integration with CMOS integrated circuits.
Implementation Method 1
The sensing electrodes may be used to measure capacitance across a gap formed between the one or more electrodes and the transducer structure. Motion of the transducer structure causes size of the gap to vary
Implementation Method 2
using deep reactive ion etching and additional mass deposition to enhance sensitivity
Implementation Method 3
using deep reactive ion etching and additional mass deposition to enhance sensitivity and reduce Brownian noise
Data Source
AI summary
The design and fabrication of a multi-axis capacitive accelerometer is presented with sub-μg resolution based on CMOS-compatible fabrication technology that can provide large proof-mass, high-aspect ratio and a large sense electrode area within a smaller footprint that previous accelerometers. In some instances, the device footprint can be reduced by placing the sense electrodes near the top or bottom of the transducer structure such that motion of the transducer causes size of the sense gap to vary in a direction that is parallel with longitudinal axis of the support beam for the transducer structure. An extra mass can also be added to the top of the transducer structure to increase sensitivity.