A capacitive strain sensor module has been developed to fulfill the desired requirements such as high sensitivity, high accuracy, extended temperature range, and high signal bandwidth for advanced industrial applications.
The capacitive strain sensor employs a novel mechanical amplifier using buckled beam suspensions to mechanically amplify the applied displacement signal, thus resulting in a 20 dB increment of the differential capacitance output. The detailed principle description and the analysis of the mechanical amplifier are provided, including the full analytical modeling and the Finite-Element-Analysis modeling. The modeling accuracy has been verified by the measurement results of a fabricated test structure with the maximum discrepancy less than 5%. The fundamental noise analysis indicates that the fundamental Brownian motion noise of the mechanical structure has an input-referred noise power spectral density of 3x10-7 micro-strain/sqrtHz. Therefore, the sensor can be operated in ambient without requiring a vacuum packaging, thus substantially reducing the system packaging complexity and cost.
Understanding and predicting the transfer behavior of strain from the test substrate to the sensing element is another critical step toward achieving the high accuracy requirement. The thesis provides the detailed analysis on the strain transfer behavior and parametrically establishes the relationship between the strain transfer behavior and device material and geometrical parameters. Based on the analysis, a novel folded-spring silicon backing structure is designed to reduce the strain transmission loss, yet still providing the microfabrication batch process capability to reduce the device fabrication cost. The analysis shows the structure stiffness has been reduced close to three orders in magnitude, thus substantially improve the transmission ratio from 60% to 99.2%. Additionally, the transmission ratio is less sensitive to bonding adhesive thickness, a key advantage to achieve high measurement consistency.
The overall capacitive strain sensor module has a sensor dimension of 2mmx3mm and delivers a nominal capacitance of 0.23 pF, and a measured capacitance output sensitivity of 280 aF/micro-strain. Integrated with the developed low-noise C/V converter interface electronics, the overall system is able to detect a minimum strain of 0.033 micro-strain with the maximum range of 1000 micro-strain, indicating an equivalent dynamic range of 89 dB has been achieved. Tested from room temperature to 112°C, the integrated system shows a maximum variation of 2.9% in sensitivity.