Design And Simulation Of SiC Piezoresistive Type Pressure Transducer With High Sensitivity

The working principle of the piezoresistive type pressure transducer is based on the piezoresistive effect. At present, the diffusion silicon pressure transducer can only be worked in high temperature environment through the cooling system due to the silicon semiconductor properties. Silicon carbide(SiC), a typical representative of the third generation wide band gap semiconductor material, can be widely applied in hightemperature, high-frequency, high power and harsh environment due to its wide band gap, high thermal conductivity, high breakdown field strength and excellent mechanical properties. It is expected that SiC pressure transducer can be operated in high temperature and harsh environment without cooling system. However, there are many issues, such as small output and low sensitivity, to be resolved due to the small piezoresistive coefficient of SiC material. In this master thesis, the SiC pressure transducer with high sensitivity is designed through theoretical analysis and finite element simulation. The performance characteristics of the transducer are simulated, and the factors affecting the output signal are investigated.The theoretical basis for chip design is the theory of elastic diaphragm with small deflection, and the design requirements follow the linearity, sensitivity, and safety principles. The relationship between the full-scale and diaphragm size of the chip is investigated according to the characteristics of the signal output. The distribution of the deflection and stress in the diaphragm is analyzed through finite element simulation. The optimal size of the diaphragm, with radius of 800 ?m and thickness of 40 ?m, is designed when the full-scale of the transducer is 1.5 MPa. The effect of the resistor arrangement on the output signal is studied based on the piezoresistive effect principle. It is found that the resistance value of the resistor is decreased at the edge of the diaphragm, while the value is increased at the center after applying the pressure on the diaphragm. The output signal and sensitivity are 88.57 mV, 11.81 ?V/V/kPa, respectively, for a 1.5 MPa full-scale transducer, which are higher than that of the same type pressure transducer reported in the literature.The influence of the size and position of the resistor, the size of the diaphragm and the ambient temperature on the output signal of the transducer is studied based on the simulation. Although the thickness of the resistor has little effect on the output signal, the position of the resistor has a strong influence. The highest output signal is obtained when the position of the resistor at the edge of the diaphragm is 800 ?m away from the center of the diaphragm. The influence of the diaphragm size on the output signal is dramatic. The sensitivity is proportional to the second order with respect to the ratio of radius to the thickness of the diaphragm(a/t). The influence of the temperature on the output signal is also great. The higher the temperature, the smaller of the output signal. Therefore, the performance of the SiC transducer is decreased when working in high temperature.The thermal stress distribution of the SiC transducer, after AlN packaging, is simulated, due to the similar physical properties of AlN and SiC. The simulation results show that the thermal stress between AlN and Si C is less than the mechanical stress induced from the applied pressure to the transducer when the temperature is 500?C. The effect of the thermal stress on the output signal is very small, and the stress does not lead to the rupture between the chip and package materials. The structure dynamics analysis, including modal, harmonic response and transient response analysis, is carried out. The simulation results show that the transducer has a big inherent frequency, 0.346 MHz, and a short response time, 1.75 ms.