Research On Novel MEMS And NEMS Volatile Organic Vapor Sensors

Volatile organic compounds (VOC) can cause seriousoutcome to both environment and health of human beings. Exposure to VOC vapors for a long time could result in some diseases such as conjunctiva inflammation, eyesight disturbance, nerve disease and even death. In addition, combining with oxygen or air, some organic vapor such as ethanol, methanol and gasoline can create explosive gas mixture when exposing to heat producer or fire. These gases are widely existed in in many workplaces like petroleum, petrochemical and metallurgy etc, which severely endanger lives and safety of human beings. Therefore, accurate and real-time detection of VOC concentration have a very important significance to protect human health and safety. In this thesis, a novel MEMS and NEMS piezoresistive vapor sensor based on the swelling effect of polymer film to detect organic vapor are presented. The main research contents are as follows:1. The novel MEMS piezoresistive vapor sensor based on the swelling effect of polymer film is studied. The structure of novel sensor is clarified. Then we discuss the sensitive mechanism of the MEMS vapor sensor and the swelling property of the polymer layer due to the absorption of vapor molecules under low vapor pressure. In addition, we also analyze the influence of the structural parameters and the supply voltage on the output performance of vapor senor.2. As a novel and sensitive vapor sensor, silicion rubber was used to detect benzene vapors and epoxy acrylate was used to detect benzene vapors and using epoxy acrylate as sensitive film to detect ethanol and methanol vapors. This proposed vapor sensor has simply structure, small size, no heating elements, low power consumption and CMOS compatibility etc. The output characteristics of the sensor for various vapors are investigated, such as, linearity, sensitivity, selectivity, reproducibility and response time etc. We also investigate the influence of ambient temperature and humidity and the influence of the parameters of the polymer layer on the output performance of the vapor sensor, which will be useful to impove the output performance of the vapor senor. The experimental results show that this MEMS vapor sensors have good linearity, sensitivity, selectivity and reproducibility. 3. To improve the sensitivity and reduce the power consumption of vapor sensor the MEMS vapor sensor, the novel NEMS vapor sensor based on giant piezoresistive effect of silicon nanowriers (SiNWs) is fabricated using CMOS-IC process. The SiNWs are embedded into NEMS diaphgram as piezoresistor and the polydimethylsiloxane (PDMS) are coated on NEMS diaphragm as film. We investigate the characteristics of this novel NEMS vapor for chloroform vapor. Compared with MEMS piezoresistive vapor sensor, the NEMS vapor sensor has smaller size, higher sensitivity, lower supply voltage and ultra-low power consumption (less than1μW).4. To further optimize the performance of the NEMS vapor sensor, the location parameters of the SiNWs are optimized using finite element analysis (FEA). The doping concentration of SiNWs and the design parameters of multi-layered NEMS diaphragm structure are optimized by experiment. Meanwhile, a simulation method for novel piezoresistive NEMS vapor sensor using thermal expansion effect of polymer to simulate swelling effect of polymer absorbing vapor in FEA is presented. The simulation output of the NEMS vapor sensor is obtained. Then, the design parameters of polymer of the NEMS vapor are optimized using this simulation method.