Strain sensing on steel surfaces using vacuum packaged MEMS resonators

The paper presents a technology for strain sensing on steel using resonant MEMS packaged in vacuum. For this purpose, a custom sensor fabrication technology and a novel vacuum packaging technique have been developed. The MEMS sensors have been fabricated by surface micromachining of thick (15 ?m) Silicon On Insulator substrates with heavily doped handle layers (? = 0.005 ? cm). Using this process, Double-Ended Tuning Fork (DETF) parallel-plate resonators with reduced coupling gaps (less than 1 ?m) have been fabricated, using a high-performance Deep Reactive Ion Etching performed on submicrometer features realized by near-UV lithography combined with a maskless line narrowing technique. The devices have been bonded to a thin steel bar by epoxy glue, packaged in vacuum and tested by applying strain to the bar, showing good tolerances to packaging parasitics, measurement reversibility, and strain sensitivity of 10 Hz/??.