Abstract:
The field of instrumentation of pressure sensors with piezoresistive or capacitive
detection is constantly evolving. Currently, these sensors are the largest segment of the
MEMS market. They are found in many areas as the first element of the measurement chain.
The main goal of thesis in hand is to study and model the thermo mechanical
behavior of these sensors. We have begun our work with a bibliographic study on the state of the art of these devices, giving some concepts on the mechanical and physical properties of the material used in the realization of these sensors. Then, we have studies the deflection at the center of the membrane and the normal stress acting on its edges as a function of pressure and temperature at rest. In addition, the effect of doping and temperature on the output voltage of the sensor and the influence of the configuration of the gauges were also studied.
Given that the increase in temperature causes considerable drifts in the output
voltage, which are due not only to the manufacturing process but also to Joule heating, this prompted us to devote a whole study to the thermal drifts generated by this effect in the piezoresistive pressure sensors. We have described the influence of the supply voltage, the operating time and its geometric parameters on its output characteristics.
The obtained results confirmed that the low polarization voltage should be applied to
dimensions of the membrane and to the length of the piezoresistive element. On the other
hand, the results showed that the self-heating is substantially reduced during a short operating time of the device and the temperature assumes a steady state value beyond 100 min and becomes independent of time.
In order to evaluate the reliability of the sensor, we have coupled the obtained
expression with that of the KANDA to evaluate the piezoresistivity coefficient π44 and the
sensitivity as a function of the bias voltage and the time operation of the device. The results showed that π44 and the sensitivity to pressure are inversely proportional to the supply voltage and the operating time, they are proportional to the length of the diaphragm. Finally, in the last part of this thesis, a study is devoted to optimizing the performance of capacitive pressure sensors.
