MEMS Capacitance Gap Measurement via Fringing Field Elimination
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Solution Overview
Problem
Microelectromechanical systems (MEMS) devices, such as accelerometers and gyroscopes, face inaccuracies in capacitance measurements due to minor deviations in fabrication processes, leading to appreciable differences in output measurements, even among identical devices, due to fringing fields and variations in capacitive gaps.
Innovation Solution
A MEMS test structure with capacitive plates and sense electrodes is used to measure and compare capacitance signals, removing fringing field effects by processing circuitry to determine the accurate gap between the plates, allowing for optimization of operational parameters like scaling factors and offsets.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If standard semiconductor manufacturing processes are used, then production efficiency and ease of manufacture are maintained, but fabrication deviations cause appreciable differences in measurement accuracy
Solution Approach 1:
The patent creates test structures that are copies of the actual MEMS device capacitive plates and sense electrodes, allowing measurement of fabrication deviations without requiring perfect manufacturing. By copying the geometric configuration and material properties, the test structures enable accurate characterization of gap variations and fringing field effects that occur during standard semiconductor manufacturing.
Solution Approach 2:
The patent changes the measurement approach by using multiple sense electrodes at different positions and orientations to measure capacitance. By varying the measurement parameters (electrode positions, orientations, and configurations) rather than attempting to control fabrication parameters, the system achieves high measurement precision despite manufacturing variations.
2Measurement precision
If fringing field effects are included in capacitance measurement, then complete field coverage is achieved, but measurement accuracy decreases due to edge effects
Solution Approach 1:
The patent segments the capacitance measurement into multiple components by using several sense electrodes positioned at different locations. Each electrode measures a portion of the total capacitance, allowing the system to separate the useful capacitance signal from the harmful fringing field effects. By segmenting the measurement, the patent can identify and exclude edge-related measurements while retaining center-region accuracy.
Solution Approach 2:
The patent extracts and removes the fringing field portion from the capacitance measurement by comparing readings from multiple sense electrodes. The processing circuitry identifies measurements dominated by edge effects and excludes them from the final calculation, leaving only the pure capacitance signal from the center region where fringing fields are minimal.
3Measurement precision
If multiple sense electrodes are used to account for fringing fields, then measurement accuracy improves, but device complexity increases
Solution Approach 1:
The test structures serve multiple functions: they measure capacitance, characterize fringing field effects, determine gap dimensions, and provide calibration data for the actual MEMS devices. By making the test structures multi-functional, the patent achieves high measurement accuracy without proportionally increasing complexity, as the same structures perform several measurement and characterization tasks.
Solution Approach 2:
The test structures are self-contained and can be measured using the same capacitive measurement apparatus that will be used for the actual MEMS devices. The test structures automatically provide their own calibration data and characterization information without requiring external reference standards or additional measurement equipment, reducing overall system complexity.
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 enhances the accuracy of MEMS device measurements by accounting for fringing fields and gap variations, ensuring consistent performance across different manufacturing batches and wafers, thereby improving the reliability of critical functions in applications like navigation and vehicle control.
Implementation Method 1
a first sense signal representative of a first capacitance between the at least one capacitive plate and the first sense electrode
Implementation Method 2
remove a fringing portion of the compared signals based on the first sense electrode perimeter and the second sense electrode perimeter
Data Source
AI summary
A microelectromechanical system (MEMS) test structure includes a plurality of capacitors formed from sense electrodes and capacitive plates having a predetermined geometry and size associated with a related MEMS device such as a MEMS sensor. Based on the predetermined relationships between the capacitors of the test structure, and between the test structure and the MEMS devices, an effect of fringing fields on the sensed capacitances of the MEMS devices may be eliminated, and the capacitive gap of the MEMS device may be accurately measured.