Analysis characterisation and optimisation of temperature coefficient parameters in capacitive pressure sensors

The literal expression of the temperature coefficient is derived in the case of plane sensing cells. It states that the thermal behaviour of the capacitance depends on the cell dimensions and the temperature coefficients of the fixed plate and the internal cavity. Measurements carried out in a set of sensors, constituted of aluminium fixed plates and thin silicon membranes bonded to Pyrex substrates, show that the amplitude of the capacitance thermal coefficient is very sensitive to the bonded length and the fixed plate thickness. Analyses of experimental results indicate that, very often, thermo-mechanical deformations of the internal cavity determine the amplitude of the capacitance temperature coefficient. Nevertheless, they also demonstrate the feasibility of sensing cells in which the capacitance temperature coefficient is very small and practically equal to the temperature coefficient of the fixed plate. In these cells, the fabrication of the fixed plate from ultra thin films and/or metals featuring small thermal expansion coefficients should give capacitance temperature coefficient lower than 44 ppm/°C. Finally it is shown that analyses of isomorphous sensors allow to separate the contributions of the fixed plate and the cell structure and in addition to withdraw the effects of packaging and parasitic capacitors.