Utilization of Semiconductors Piezoresistive Properties in Mechanical Strain Measurements under Varying Temperature Conditions for Structural Health Monitoring Applications

Abstract: Strain gauges have been powerful tools in experimental stress analysis. This importance is expected to continue, even though other means of strain measurement are continually introduced to the market. The strain gauge is a simple device that can be easily installed to measure mechanical strain. By far, the one-dimensional single-filament has been the most common strain gauge. Conventional strain gauges are typically made from thin-foil metal; thus the resistivity change under stress is insignificant. On the other hand, semiconductor material demonstrated considerable resistivity change as a result of the applied mechanical strain or stress. In this work, the piezoresistive characteristics of silicon crystal are investigated to realize a robust Micro Electro Mechanical (MEMS) strain sensor. This sensor has been developed to withstand harsh environmental conditions, such as those in Structural Health Monitoring (SHM) applications. Silicon strain gauges have demonstrated higher gauge factor, sensitivity, and accuracy compared to conventional thin-foil strain gauges. Unfortunately, silicon strain gauges suffer from large temperature effect, which influences their performance dramatically. This temperature effect puts various challenges on the development and application of semiconductor sensors. On top of these challenges are the temperature compensation of the output signal, packaging, and fabrication. In addition, transferring strain through different structural layers causes substantial loss in the sensed strain values. Piezoresistivity theory is presented and applied to develop a new MEMS strain sensor. Taking into account all geometric and material characteristics, various tools and techniques, such as indicial equations, Finite Element Modeling (FEM) (ANSYS10.0®), and experimental evaluation, were employed to go through the development cycle of the piezoresistive strain sensor. In addition, alignment errors during microfabrication have been investigated. The proper microfabrication parameters and piezoresistors configuration were selected by investigating the silicon crystal material properties based on the crystallographic directions. A microfabrication process flow has been developed exploring a group of fabrication processes available at the University of Alberta micromachining and nanofabrication facility (NanoFab). In order to minimize the loss in the transferred strain, geometric features were created in the silicon substrate. These geometric features have resulted in stress discontinuity in their vicinity, which introduced the concept of geometrical gauge factor. The geometrical gauge factor is a new concept that can ‘virtually’ improve the performance of any piezoresistive sensor by utilizing the silicon carrier to increase the differential stress around the piezoresistive sensing elements. The main limitation to use this concept is whether or not material properties will accommodate the resulted stress concentration without failing the sensor. The developed sensor was evaluated, tested, and characterized at the University of Alberta and Syncrude Canada Edmonton Development Research Center. Uniaxial tension was utilized to calibrate a number of chip designs. The temperature coefficient of resistance (TCR) was also evaluated. Preliminary packaging procedure was proposed and applied. Comparing the performance characteristics of the developed MEMS strain sensor to a 350Ω thin-foil strain gauge showed that the piezoresistive MEMS strain sensor had better performance characteristics: sensitivity and resolution, with room for significant improvements. In addition, the MEMS strain sensor can be successfully applied under varying temperature conditions. The solution of the above challenges and the small size of MEMS sensors have resulted in a novel MEMS-based strain sensor with low power consumption, compared to conventional strain gauges. This low power consumption promotes this sensor in wireless SHM systems as the sensing unit, which can extend such technology to wider range of applications. The MEMS strain sensor has potential to provide a valuable tool to improve the current SHM systems as well as allowing higher number of sensors to be economically deployed. As a result, the reliability of both the SHM system and equipment will be enhanced, which will reflect positively on the economic performance of the equipment.