The introduction of commercial foil-strain gauges in the 1950s revolutionized the study of structural and mechanical engineering, as well as material science. These sensors provide an unprecedented window into the fundamental behavior of materials and solids. Coupled with the advent of inexpensive computing power, strain gauges allow engineers to monitor the behavior of structures in the field easily. Refined over the years, the basic foil gauge now meets the demands of a vast array of applications and environmental conditions in both research and production.
Unfortunately, conventional foil strain gauges are limited in their capability to measure small strain fields. They suffer from diminishing sensitivity, signal, and fatigue life as they get smaller. The current generation of miniaturized foil gauges boasts theoretical gauge lengths of 200μm (e.g., Tokyo Sokki Kenkyujo QFLG-02), but has low signal and sensitivity (at maximum 1μm resolution) and lifetimes (1x106 strain cycles under ideal conditions). Further, foil gauges function best under uniform strain field. To insure good signal, a uniform strain field over the whole of their polyimide-backing layer is preferable, thus increasing the effective gauge length to the size of the gauge’s polyimide substrate – 3.5 mm for the QLFG-02.
A similar technology – semiconductor strain gauges – shows promise for measuring small strain fields. Commercial gauges of this type are available with gauge lengths of approximately 700 μm and sensitivity approximately 75 times greater then foil gauges (Micro Flexitronics - L2-350 series). However, they suffer from large non-linear temperature behavior, and also require uniform strain fields similar to their foil counter parts.
In addition, both types of strain gauge require expensive, bulky sense equipment to achieve good strain sensitivity as well as exotic surface preparation and bonding to insure that the strain in the substrate is conveyed to the sensor. These issues restrict both of these sensors primarily to laboratory applications. Micro electro-mechanical systems (MEMS) offer a promises as a means for overcoming these limits. As early as the 1980s, the piezoelectric characteristics of silicon and silicon oxides led researchers to develop MEMS-based strain gauges . Since then, researchers have investigated various approaches and fabrication technologies including optical , capacitive -, and resonant  methods. Resonating sensors are of particular interest due to their simple design and potentially high sensitivity. Most recently, work at the University of California at Berkeley -, and the University of Southampton  demonstrates that a dual-ended tuning fork (DETF) design offers promise as a micro strain sensor. In addition, advances in bonding techniques suggest that installation and hermetic sealing of MEMS strain sensors can be achieved on a chip scale without exotic surface preparation. Finally, the fact that MEMS structures are typically fabricated using IC processing techniques, makes integration of the sensor and its control electronics apossibility, allowing for in situ installation of these sensors on preexisting devices.
May 31, 2004
Lippmann, J. M. (2004). Design and Fabrication of MEMS Resonant Strain Sensor in SOI: Research Report. United States: University of California, Berkeley.