Design and Evaluation of a MEMS Offset Capacitive Comb Strain Sensor

An offset, gap-closing capacitive MEMS strain sensor for detecting strain of steel has been designed, fabricated, and tested both in isolation and bonded to steel. This extensometer approach benefits from low sensitivity to thermal noise typical of capacitive sensors. The design utilizes an offset parallel-plate array design so that we can achieve differential sensing in a single device layer and only three electrically isolated regions, significantly reducing fabrication complexity of typical differential capacitive sensing. The penalty for this design compared to a fully differential gap-closing capacitive extensometer is evaluated and an optimal offset ratio is found. Some fabricated sensors include an on-chip displacement actuator to simulate strain, allowing characterization of the sensor separate from bonding or packaging induced strain loss. Using a measurement circuit not optimized for this sensor, the 400 μm design exhibits a large deadband response (107 με for a drive voltage amplitude of 684 mV). The deadband value decreases with increased drive voltage amplitude and increased sensor area. Above the deadband region, the sensor has a resolution of 2.7 με. In order to confirm numerical estimates of strain loss due to the bond and die substrate, a four-point bend test fixture has also been designed, fabricated, and characterized. This test can apply strain with an accuracy of 7 με. The sensor is bonded to a steel specimen using metal foil strain gage epoxy and again evaluated. The system-level performance exhibits a deadband response below 1500 με. Above the deadband response, the sensor exhibits aresolution of 230με, confirming numerical estimations of strain loss through the bond and substrate.