A novel method to predict the effect of static charges on the pull-in voltage and touch-point pressure of the capacitive transducer with square diaphragm

Microelectromechanical systems (MEMS) use silicon based dielectric films such as SiO2 and Si3N4 for providing insulation. In capacitive transducers, these layers are used as insulation layers for capacitive actuators to prevent short-circuiting of electrodes by contact of electrodes and in case of touchmode capacitive pressure sensors (TMCPS), the dielectric layer provides for overload protection and provides for operation in the pressure ranges in which the diaphragm comes in contact with dielectric. The pressure at which diaphragm just touches the dielectric is referred to as touch-point pressure. In case of actuator, pull-in voltage is a critical parameter that decides the onset of instability in operation of the device. Silicon based dielectric thin films have the tendency to store or trap static charges, the presence of which alters the ideal behaviour of dielectrics. These trapped static charges introduced due fabrication processes; handling and operation affect the operation of capacitive transducers by influencing the pull-in voltage and by affecting the touch-point pressure. This paper presents a novel methodology, which predicts the pull-in voltage and touch-point pressure in presence of the static charges in a dielectric. The method proposed is less complex and less time consuming. Closed form expressions have been derived for pull-in voltage, touch-point pressure and critical displacement in presence of static charges. The results are compared with those obtained by experiment.