Development of Peninsula-Structured Diaphragm for Micro Piezo-Resistive Pressure sensor

This thesis was devoted to the development of micro pressure sensors which can measure 0-1 bar with high linearity and high sensitivity at a resolution of 0.014 bar or 1.4%. To acquire a better design, the finite element method was employed to analyze the stress distribution of the square sensing diaphragm with four peninsula-structure bosses attached to the four central edges. Conventional piezoresistors are embedded by doping process at the positions of maximum stresses responding to the applied pressure, the present study fabricated the isolated piezoresistors by removing the excessive material of the device layer which was properly doped in a SOI wafer. This design would improve the sensitivity and linearity of the sensor, while reduce fabrication processes. Two piezoresistors in each sensor were connected with two dummy resistors on the metal lines to form a Wheatstone bridge. It can convert the unbalanced signal of the piezoresistors resulted from the deformed diaphragm where the pressure force was applied. Five models involving three sizes in square diaphragm and three sizes in piezoresistor were designed for numerical simulation coupling mechanical and electrical fields. The results of simulation indicate that the maximum stresses occur at the centers of the edges of the diaphragm. It also provides sufficient information to validate that the respondent stresses are within the allowable strength of material and significant voltage output from the Wheatstone bridge constructed by the piezoresistors for accurate measurement. The modules were fabricated on a silicon-on-insulator wafer by semiconductor manufacturing processes. Upon finishing the manufacturing, cutting, and packaging processes, detail calibrations were carried out to characterize sensitivity and linearity of pressure sensor. Two pressure sensors for each five models were tested and calibriated at the room temperature. All sensors demonstrate satisfactory performances in sensitivity and linearity in the test pressure range. The best sensor exhibits a sensitivity of 34.57 mV/V/bar and linearity in the order of 0.71 % per full scale output. The sensitivity of sensors can be amplified to 100 mV/V/bar by inserting a tuned resistor to the instrumentation amplifier for convenient applications. The developed modular micro pressure sensor can be integrated into modern transportation vehicles as well as for general measurements. Since the MENS processing is compatible with the CMOS manufacturing, the present micro pressure sensing module can be integrated with the other electric control circuits.