Force balanced micromachined pressure sensors

Abstract

This paper discusses the fabrication and testing of two lowvoltage force-balanced pressure sensors. In these devices the force acting on a diaphragm under external pressure and its corresponding deflection are counterbalanced with the force and deflection generated in a parallel plate electrostatic actuator mechanically coupled to the diaphragm. The low balancing voltage is attained using a force multiplication scheme. The first device is implemented using an open gap actuator constructed on top of a vacuum sealed sense diaphragm. The open gap design is exposed to the measurement environment hence susceptible to contamination. The second device eliminates this problem by enclosing the actuator in an hermetically-sealed cavity. Both devices have a full scale range of 1 atmosphere. The open actuator device has a nominal capacitance of 0.34 pF and a full scale range of 70 fF. The sealed actuator device a nominal capacitance of 2.12 pF and a full scale range of 100 fF. The pressure is balanced by voltages in the range of 12-25 V demonstrating the feasibility of force balanced pressure sensor compatible with low-voltage electronics.

<title>Integration issues for pressure sensors</title>

“…For a simple parallel plate capacitive pressure sensor shown in Figure 1., the change in capacitance is given by CCi-Co (1) where C1 is the capacitance caused by the application of an external pressure P. C0 is the zero pressure capacitance and AC is the change in capacitance caused by the external pressure. Some of these issues can be appreciated when we consider the underlying principle of the capacitive pressure sensor .…”