Research On Capacitive Pressure Sensors Based On Multi-Layered Strcture And Corresponding CMOS Compatible Process

Micromachined pressure sensors are one of the important research areas in MEMS, which have found wide applications in areas such as automotive systems, industrial control, environmental monitoring and biomedical diagnostics. The capacitive pressure sensor translates a pressure change into a capacitance variation, which tends to provide higher sensitivity, lower temperature coefficients, more robust structure and lower power consumption compared to piezoresistive devices. As the capacitance value is inversely proportional to the distance between the two electrodes, conventional capacitive devices show nonlinear characteristics in response to the pressure, causing the sensor trimming more difficult. In addition, the cavities between two electrodes for the capacitive pressure sensors and the electrodes feedthrough out of the cavity complicate the fabrication process, resulting in the increased cost and reduced reliability of the devices.In this thesis, a novel pressure sensor employing a sandwich structure with dielectric layers between two electrodes, which can be fabricated with 3-mask process, is proposed for the first time in order to overcome the processing difficulties mentioned above. The structure is utilized for the advantages of stress compensation. In addition to the simplicity of fabrication, the solid-state capacitor has also more robust structure, intrinsic larger initial capacitance value beneficial for following interfaced circuit processing. Improved sensitivity, linearity, high reliability and lower cost can be achieved with such a structure. Mechanical thermal model has been proposed for the analysis of the sensor structure, which is evaluated by finite element method. The load-deflection analysis, thermal warpage and temperature effects were performed by finite element method. An improved model is proposed for the first time in this paper based on the electrostriction enhancement effect of elastic dielectric materials by inserting an additional item into the relationship, which exhibits the relative dielectric constant change due to the strain gradient along the thickness of the dielectric layer. A good interpretation of the experiment results is given by the model. The related parameter is extracted from the experiment results. The sensors have been implemented by SOG process. Sensors with P⁺⁺/SiO₂/Si3N4/Au structure were fabricated by a 3-mask process. The side lengths of the sensors are 800μm,1000μm,1200μm,1500μm, respectively. The sensitivity of the sensors varies from 0.008 pF/hPa to 0.02 pF/hPa. The nonlinearity is about 1.2% and the maximum hysteresis is about 3.3%. TCO at 1010 hPa is about 192.3ppm. The residual pressure in the cavity is about 80 hPa. The sensor with P⁺⁺/SiO₂/Au structure was also implemented by a 4-mask process. The side length of the membrane is 1400μm. The sensitivity is 0.012pF/hPa. The nonlinearity is about 3.2% and the hysteresis is about 2.4%, respectively. TCO is about 282.4ppm. The residual pressure is about 82 hPa. A preliminary stability measurement has been performed for two weeks. No measurable drift was found using the YD 2810B Precision LCR meter with a resolution of 0.01 pF.An interface capacitor-voltage converter circuitry based on switched-capacitor circuit (SC) is proposed. The sensitivity of the converter is 2mV/5fF by HSPICE simulation and verification. The setting time of the circuitry is 0.5μs with a setting accuracy of 0.01%. The equivalent input thermal noise is about 7.137μV. The bandwidth is 300 kHz and the power dissipation of the main part of the circuit is 14.42 mW. According to the test result, the linear voltage output is -4V-3V.The integration of transducers and on-chip interface circuitry has become a new focus in MEMS development in order to achieve high functionality and performance, low cost and realization of the system-on-chip (SOC) concept. A fully CMOS compatible capacitive pressure sensor is proposed based onthe multi-layered capacitive pressure sensor structure. CMOS pre-processing and post-processing were utilized to implement such a microsystem. No additional steps were added to CMOS technology, and post-processing steps are separated without effecting standard IC process flow, making the sequence and completeness of the standard process not disrupted. Key processing steps in post-processing were...