Sensitivity Enhancement in Vacuum Packaged Resonant MEMS Strain Sensors with On-Chip Strain Amplification Mechanism

A new strain sensor design based on vacuum packaged Double-Ended Tuning Fork silicon resonators with a strain amplification mechanism is presented. The sensors are fabricated with a Silicon On Insulator MEMS process including a thin-film vacuum encapsulation technology that permits to obtain high-Q resonators operating at a vacuum level around 160 mTorr. Within the encapsulation, a strain amplification structure consisting in a long beam connected to the DETF resonator is integrated in order to increase the sensitivity of the device to the applied strain. Thanks to such amplification, an unprecedented sensitivity of 200 Hzvarepsilon in bending tests on steel is demonstrated, operating the sensors in closed loop with an external readout circuit.