Numerical study of geometry variations in a balanced MEMS-loudspeaker

Nanoscopic electrostatic drive (NED) actuators point to a MEMS sound transducer for in-ear applications. Moving actuators placed each within a cavity produce the sound. The alternating actuation of the two NED layers along the clamped-free beam is realised by a bias voltage with opposite polarity at the outer electrodes and the signal voltage at the inner electrode. An upper limit for the drive voltage is the pull-in voltage. A short circuit due to a contact between the electrodes is the result, as the electrostatic forces exceed the elastic forces. We analyse the AC pull-in voltage at various bias levels with a 2D stationary finite element method (FEM) by an arc-length solver in consideration of trench variations due to fabrication tolerances. A lower bound for the pull-in voltage combinations estimates device failure. In addition, an upper bound is necessary to rate the variance of the actuator’s deflection. FEM simulations are compared to general conclusions by a lumped parameter model (LPM). The deflection’s sensitivity towards fabrication tolerances is lower at higher signal-to-bias as well as lower bias levels. For operation points with the same deflection a higher safety margin towards the pull-in is realizable at higher signal-to-bias-levels.