MEMS Z-Axis Accelerometer Anchoring for Thermal Drift Reduction
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Solution Overview
Problem
Microelectromechanical z-axis accelerometers experience measurement errors due to thermal drifts caused by substrate deformations, leading to offset and sensitivity drifts in acceleration detection.
Innovation Solution
A microelectromechanical structure with a modified mechanical coupling configuration where the sensing mass is anchored to the substrate at points corresponding to the fixed electrodes, allowing it to displace similarly to the electrodes during substrate deformations, thereby minimizing variations in capacitance and reducing thermal drifts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If the sensing mass is anchored at the center of gravity, then the structure is simple and symmetric, but thermal drifts cause measurement errors
Solution Approach 1:
The single central anchoring element is segmented into multiple anchoring elements distributed at specific positions corresponding to the fixed electrodes. This segmentation allows each anchoring element to independently compensate for substrate deformations in its local region, reducing thermal drifts while maintaining structural feasibility.
Solution Approach 2:
The anchoring elements act as intermediaries between the substrate and the sensing mass, positioned at strategic locations to transmit and compensate for substrate deformations. This intermediary positioning allows the sensing mass to follow substrate movements, minimizing the impact of thermal expansions on measurement accuracy.
2Measurement precision
If the sensing mass is positioned close to the substrate, then capacitance sensitivity is improved, but substrate deformations cause offset and sensitivity drifts
Solution Approach 1:
The anchoring structure implements local quality by positioning anchoring elements at specific locations corresponding to the fixed electrodes rather than using a uniform central anchoring. This localized positioning creates different mechanical coupling characteristics at different positions, allowing the sensing mass to better follow substrate deformations locally while maintaining close proximity for high capacitance sensitivity.
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
The solution effectively reduces thermal drifts and associated measurement errors, maintaining accurate acceleration detection by ensuring the sensing mass displaces in sync with the substrate deformations, thus minimizing sensitivity and offset drifts.
Implementation Method 1
the package of a microelectromechanical sensor is in fact subject to deformations as the temperature varies owing to the different coefficients of thermal expansion of the materials of which is made
Implementation Method 2
The first and second fixed electrodes 5a, 5b hence define, together with the sensing mass 3, a first detection capacitor and a second detection capacitor with plane and parallel faces, designated by C 1 , C 2
Data Source
Figure 1a~1b
Figure 2a~3
Figure 4~5b
AI summary
A MEMS detection structure (10) comprising a substrate (2) having a top surface (2a) with at least a first fixed-electrode arrangement (5a); a sensing mass (3) extending in a plane (xy) and suspended above the substrate (2) and above the first fixed-electrode arrangement (5a) at a separation distance; and connection elastic elements (8a, 8b) supporting the sensing mass (3) for being freely rotatable out of the plane (xy), modifying the separation distance as a function of a detected quantity along an axis (z) orthogonal to the plane (xy). The MEMS detection structure further comprises a coupling mass (12), suspended above the substrate (2) and connected to the sensing mass (3) via the connection elastic elements (8a, 8b) and an anchoring arrangement (14, 15), which anchors the coupling mass (12) to the substrate (2) with at least a first point of constraint (13), set at a distance from the rotation axis (A) and in a position corresponding to the first fixed-electrode arrangement (5a).