Readout of MEMS capacitive sensors beyond the condition of pull-in instability

The ongoing transition between micro and nano electromechanical systems (MEMS and NEMS) based on capacitive sensing requires a solution to the problem of pull-in instability and, on the whole, to the relevance of the electrostatic readout forces with respect to other forces to be sensed (inertial, magnetic,…). The paper analyzes the case of a differential accelerometer which is dimensioned to be subject to instability (i.e. with a negative equivalent stiffness), when combined with a constant voltage readout of ±1.8 V. A charge controlled readout principle, based on an electronic feedback loop, is presented. Its switching capacitor VLSI implementation, built in a CMOS 0.35 μm technology is used to demonstrate, through experimental results, that this approach is effective in the minimization of the electrostatic readout forces.