Micromechanical resonant cantilever sensors actuated by fringing electrostatic fields

We report on the architecture and operational principle of a resonant cantilever-type displacement sensor. The device is actuated electrostatically by a side electrode that is coplanar with the cantilever and by a gap-closing electrode positioned underneath the beam. The unique electrode geometry combined with the appropriate actuating voltages allows positioning of the cantilever in close proximity to the bistability threshold, where the frequency sensitivity to the electrode displacement is enhanced. Using a reduced order model backed by numerical simulations, the dependencies of the device frequency on the beam's deflections and the actuation voltages were mapped. We show wide-range tunability that spans a range between softening and hardening behavior. We demonstrate displacement sensing using fabricated single crystal silicon ≈2000 μm long, ≈5 μm thick cantilevers. When compared to a resonant cantilever sensor actuated solely by a gap-closing electrode, measurements from our fringing field actuated devices show a four times higher sensitivity of ≈98 Hz μm-1. The suggested approach may find applications in a broad range of micro and potentially nano-scale applications including resonant inertial, force, mass and bio-sensors.