MEMS Stress Sensor Using Shared Capacitive Electrodes
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Solution Overview
Problem
Micromechanical acceleration sensors face challenges in compact design and effective compensation of mechanical stress effects, leading to sensitivity and offset errors, particularly in vertically integrated inertial sensors with limited installation space.
Innovation Solution
Incorporating additional capacitive electrodes on the CMOS ASIC's uppermost metal layer, positioned opposite to fixed top electrodes on the MEMS wafer, to measure and compensate mechanical stress-induced changes in capacitance, allowing for stress-related error correction without additional sensor core area requirements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If additional stress sensor electrodes are integrated into the micromechanical structure, then measurement precision for stress compensation is improved, but device complexity increases
Solution Approach 1:
The capacitive electrodes serve dual functions: they are part of the standard z-acceleration sensor structure for measuring acceleration in the z-direction, and simultaneously function as stress sensor electrodes for measuring mechanical stress. This multi-functionality allows stress compensation without adding separate dedicated stress sensing elements, thereby improving measurement precision while avoiding increased device complexity
Solution Approach 2:
The patent merges the acceleration sensing function and stress sensing function into a single integrated structure. The same capacitive electrodes that define the z-acceleration measurement also serve as stress indicators, combining two sensing functions into one unified system rather than requiring separate independent sensors
2Volume of moving object
If the sensor structure is miniaturized for compact design, then volume of the sensor is reduced, but manufacturing precision requirements increase
Solution Approach 1:
The patent utilizes the vertical dimension (z-direction) for sensor placement rather than expanding in the lateral plane. The capacitive electrodes are positioned on opposite sides of the micromechanical structure along the vertical axis, enabling compact lateral footprint while maintaining adequate spacing for capacitance measurement. This dimensional approach allows miniaturization without proportionally increasing manufacturing precision requirements
3Volume of moving object
If vertically integrated wafer stacks are used for miniaturization, then volume is reduced, but stress-related sensitivity drift increases
Solution Approach 1:
The patent implements a feedback mechanism where the stress sensor electrodes continuously monitor mechanical stress on the wafer stack, and this stress information is used to compensate for sensitivity drift in the acceleration measurement. The system measures the stress-induced capacitance changes and uses this feedback to correct the acceleration signal, maintaining reliability despite the compact vertical integration
Solution Approach 2:
The patent employs a composite structure combining multiple functional layers in the vertical stack: MEMS functional layers, capacitive electrode layers, and stress sensing layers. This composite wafer stack integrates different material systems and functional elements, where the capacitive structures serve both acceleration sensing and stress monitoring purposes, enabling the compact design while compensating for stress effects through the integrated measurement capability
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
Enables precise measurement and compensation of mechanical stress effects, reducing sensitivity and offset errors in z-acceleration sensors, while maintaining a compact design and high measurement sensitivity without additional area usage.
Implementation Method 1
a capacitive electrode (21), which produces a measuring capacitance with an adjacent micromechanical structural element (31) on the MEMS substrate for measuring a distance (51) between the capacitive electrode (21) and the micromechanical structural element (31)
Data Source
AI summary
A micromechanical sensor. The micromechanical sensor includes a MEMS substrate, on which a micromechanical structure including at least one sensor electrode is disposed in a cavity, and including a cap substrate, which is disposed over the micromechanical structure and closes the cavity. A capacitive electrode, which produces a measuring capacitance with an adjacent micromechanical structural element on the MEMS substrate for measuring a distance between the capacitive electrode and the micromechanical structural element, is disposed on an inner side of the cap substrate. A method for the signal correction of a sensor signal of such a micromechanical sensor is also described.