Micromechanical voltage reference using the pull-in of a beam

The pull-in voltage of a single-side anchored freestanding beam, under lateral deflection, has been investigated for application as a dc voltage reference. Two sets of electrodes, alongside the tip, are used for parallel-plate type of electrostatic actuation of the 200 m long beam in the plane of the wafer. Another set of buried electrodes is aligned with the plate electrode at the free-standing tip and is used as a differential capacitor for the simultaneous detection of the displacement, with the purpose to determine the stability border and thus the pull-in voltage. The single-end clamping ensures that the pull-in voltage is insensitive to technology-induced stresses. A two-dimensional (2–D) energybased analytical model for the static pull-in is compared with measurements. Bifurcation diagrams are computed numerically, based on a local continuation method. Devices have been designed and fabricated in an epi-poly process. Measurements are in agreement with modeling and confirm a pull-in voltage in the 9.1–9.5 V range. Reproducibility is limited by hysteresis and charging of the dielectric layer in between the electrodes. The device can be operated in feedback or as a seesaw, by using the two sets of electrodes.