MEMS Resonant Strain Sensor Integration
Despite commercial availability since the 1950's, silicon strain sensors have not experienced the same success as other microdevices, such as accelerometers, pressure sensors, and inkjet heads. Strain sensors measure mechanical deformation and could be used in many structural components, improving safety, controls, and manufacturing tolerances. This thesis examines major strain sensing techniques and highlights both advantages and disadvantages of each. MEMS resonant strain gauges are identified to have superior performance over many traditional strain gauges in terms of sensitivity, resolution, stability, and size. To use these gauges, additional issues such as harsh environment survivability, strain transfer, temperature stability, and encapsulation must be solved, as detailed in this thesis.
Concerning harsh environment survivability, this work presents a MEMS resonant strain gauge fabricated from silicon carbide, which operates at 600°C, and has been tested to 64,000 G, while still resolving 0.01 microstrain in a 10 kHz bandwidth. Specific details on how to create harsh environment testing equipment are presented. Additionally, this original work identifies a unique temperature stability method based on purposely mismatched device and substrate layers. Full analytical equations are presented, and experimental confirmation of the scheme shows that temperature stability is improved from 23 ppm/°C to 3.6 ppm/°C.
All MEMS devices are created on flat substrates, which are useful when integrating electronics, but can be difficult to use when measuring strain in structural components, especially round objects. Furthermore, no work has been presented for gauges operating at high strain. To address this issue, this thesis contains the first demonstration of a MEMS resonant strain gauge operating at 1000 microstrain on a static automobile halfshaft. Details on joining the substrate to the circular halfshaft are presented, as well as how to treat the issue of strain transfer.
To protect the device, encapsulation is designed specifically to not change the strain sensitivity of the gauge. The encapsulation utilizes directional ion beam sputtering, which is experimentally shown to deposit spatially confined, extremely thin material through the release holes. Typical depositions were nanometers in thickness (<0.5% of deposited material) and on the order of tens to hundreds of femtograms.