MEMS Sensor Z-Axis Parasitic Capacitance Reduction
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Solution Overview
Problem
Commercial MEMS accelerometers measuring z-axis movement face reliability issues due to parasitic capacitance between electrodes and the substrate, affecting the accuracy of z-axis motion detection.
Innovation Solution
A MEMS sensor design featuring a movable mass with a radially outward reference structure electrically coupled to it, along with z-plane MEMS and reference electrodes, minimizes parasitic capacitance by using a cap to encapsulate the structures and applying potentials to measure changes in capacitance accurately.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If an electrode is positioned under the inertial mass to sense z-axis motion capacitively, then z-axis movement measurement is enabled, but parasitic capacitance between the electrode and substrate degrades measurement reliability
Solution Approach 1:
The patent extracts the reference structure from the substrate plane and positions it radially outward from the MEMS structure. This spatial separation removes the source of parasitic capacitance (the reference structure) from the problematic area beneath the inertial mass, eliminating the harmful electrical interaction with the substrate while preserving the capacitive sensing function for z-axis motion detection.
Solution Approach 2:
The patent transitions the reference structure from a planar configuration (on the substrate plane) to a three-dimensional configuration (radially outward from the MEMS structure). This dimensional change allows the reference structure to be electrically coupled to the inertial mass while physically positioned outside the z-plane sensing area, thereby reducing parasitic capacitance with the substrate.
2Reliability
If a reference structure is positioned radially outward from the MEMS structure, then parasitic capacitance is reduced, but device complexity increases
Solution Approach 1:
The patent merges the reference structure with the inertial mass through electrical coupling, creating a unified sensing element. This combination allows the reference structure to function as part of the capacitive sensing system rather than as a separate component, reducing the number of independent elements while achieving the goal of reduced parasitic capacitance.
Solution Approach 2:
The reference structure positioned radially outward serves multiple functions: it provides a reference potential for capacitive sensing, reduces parasitic capacitance with the substrate, and maintains electrical coupling with the inertial mass. This multi-functionality reduces the need for additional separate components, thereby managing device complexity while improving reliability.
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 configuration enhances the accuracy of z-axis movement measurement by reducing noise from parasitic capacitance and maintaining reliability, while allowing for effective sensing across all three axes with a single inertial mass.
Implementation Method 1
The movable structure and the fixed structures form a capacitor having a capacitance that changes when the movable structure moves relative to the fixed structures in response to applied forces or acceleration
Implementation Method 2
parasitic capacitance between the electrode and the substrate may undesirably impact the reliability of this measurement
Data Source
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AI summary
A MEMS sensor includes a substrate and a MEMS structure coupled to the substrate. The MEMS structure has a mass movable with respect to the substrate. The MEMS sensor also includes a reference structure electrically coupled to the mass of the MEMS sensor. The reference structure is used to provide a reference to offset any environmental changes that may affect the MEMS sensor in order to increase the accuracy of its measurement.