High-resolution MEMS inertial sensor combining large-displacement buckling behaviour with integrated capacitive readout

Commercially available gravimeters and seismometers can be used for measuring Earth's acceleration at resolution levels in the order of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>ng</mml:mi> <mml:mo>∕</mml:mo> <mml:msqrt><mml:mi>Hz</mml:mi></mml:msqrt> </mml:mrow> </mml:math> (where g represents earth's gravity) but they are typically high-cost and bulky. In this work the design of a bulk micromachined MEMS device exploiting non-linear buckling behaviour is described, aiming for <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>ng</mml:mi> <mml:mo>∕</mml:mo> <mml:msqrt><mml:mi>Hz</mml:mi></mml:msqrt> </mml:mrow> </mml:math> resolution by maximising mechanical and capacitive sensitivity. High mechanical sensitivity is obtained through low structural stiffness. Near-zero stiffness is achieved through geometric design and large deformation into a region where the mechanism is statically balanced or neutrally stable. Moreover, the device has an integrated capacitive comb transducer and makes use of a high-resolution impedance readout ASIC. The sensitivity from displacement to a change in capacitance was maximised within the design and process boundaries given, by making use of a trench isolation technique and exploiting the large-displacement behaviour of the device. The measurement results demonstrate that the resonance frequency can be tuned from 8.7 Hz-18.7 Hz, depending on the process parameters and the tilt of the device. In this system, which combines an integrated capacitive transducer with a sensitivity of 2.55 aF/nm and an impedance readout chip, the theoretically achievable system resolution equals 17.02 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>ng</mml:mi> <mml:mo>∕</mml:mo> <mml:msqrt><mml:mi>Hz</mml:mi></mml:msqrt> </mml:mrow> </mml:math> . The small size of the device and the use of integrated readout electronics allow for a wide range of practical applications for data collection aimed at the internet of things.