Thermal buckling of silicon capacitive pressure sensor

The thermal behaviour of an absolute or differential capacitive pressure sensor built from a micromachined silicon plate bonded to a Pyrex 7740 substrate has been studied. Thermal drift due to the mismatch of the thermal coefficients of expansion between the silicon, Pyrex substrate and package has been analysed numerically with the finite-element method. Two types of boundary conditions at the Pyrex substrate-package interface (representing different die-mounting configurations) are studied: fixed and free base of the Pyrex substrate. The sensor with a fixed base presents a high temperature sensitivity beyond the buckling temperature, which is small for some dimensions (105 degrees C for a circular capacitive pressure sensor, having 1000 mu m diphragm radius, 12 mu m diaphragm thickness and 200 mu m Pyrex thickness). The influence of different dimensions of the sensor has been simulated to increase the buckling temperature greatly. Buckling of the sensor with a free base occurs at negative temperatures and hence this sensor presents a feeble thermal drift at positive temperatures.