Development Of High Response MEMS Formaldehyde Gas Sensor Based On ZnO Doped Al₂O₃

As a volatile organic compound（VOCs）gas that exists widely in the environment,formaldehyde poses a huge threat to human health.The national standard stipulates that the indoor formaldehyde release limit is about 91 ppb,so it is particularly important to develop a formaldehyde gas sensor that can detect concentrations as low as ppb.At present,most gas sensors are difficult to be applied on a large scale due to the high cost of use,complex operation,and poor selectivity.Metal-oxide-semiconductor（MOS）gas sensors based on micro-electro-mechanical systems（MEMS）technology have gained considerable development due to their high responsivity,low cost,and low power consumption.In order to detect low concentration formaldehyde gas,this thesis developed film compatible MEMS formaldehyde gas sensor by preparing film gas sensitive material and optimizing process parameters.The main research work is as follows:According to the sensitive mechanism of aluminum-doped zinc oxide（AZO）thin film,the preparation process of AZO nanocomposite gas-sensitive thin film was studied.AZO nanocomposite gas-sensitive thin films were prepared by magnetron co-sputtering,and annealed at 200°C,400°C,and 600°C for 3 hours each.The micro-morphological parameters of the obtained thin films were characterized,and the gas sensors were fabricated by MEMS technology.The test results showed that the film heat-treated at 400°C had better sensing performance for the same concentration of formaldehyde.Based on this,five AZO films with different gas flow rates of argon-oxygen ratio were prepared,and the optimal operating temperature of all films in the test was 240°C.Continuous dynamic test showed that the film prepared in pure argon atmosphere showed higher responsivity to formaldehyde than the other four films,and the response recovery time was relatively short.The analysis found that the films prepared by the optimal heat treatment temperature and argon-oxygen ratio had smaller grain size and larger surface roughness,which was conducive to the adsorption of gas molecule and thus enhanced gas response characteristics.By controlling the sputtering power of Al₂O₃,the effect of Al doping content in AZO films on the gas-sensing properties of formaldehyde was studied.The test results showed that the film with the Al content of 11.44%showed the best response to the formaldehyde concentration of 100 ppb-3 ppm,and had excellent repeatability,long-term stability,humidity resistance and selectivity.XPS analysis showed that the relative percentage of oxygen vacancies in the 11.44%Al content film was as high as 71.95%,which provided more electrons and active sites for the process of gas adsorption reaction and thus improved the response performance to formaldehyde.At the same time,the surface of AZO thin film was modified with noble metal Pd by magnetron sputtering process and its gas-sensing properties was studied.The test showed that the optimal operating temperature was reduced to 210°C,and the response value to 100 ppb formaldehyde could reach 1.816.The response recovery time to formaldehyde at various concentrations was also greatly shortened.The structure design,preparation and performance test of MEMS formaldehyde gas sensor were carried out.The temperature field distribution of the sensor was simulated and the structure was determined.According to the research results of heat treatment temperature,argon-oxygen ratio,Al doping content and surface modification of noble metal Pd,the MEMS formaldehyde sensor chip of 2 mm×2 mm×0.4 mm was prepared.The test results showed that the actual optimal operating temperature of the sensor was 217.5°C,and the chip power consumption was 23.59 m W.Continuous dynamic testing of the sensor was carried out.The response increased from 1.88 to 6.05 when the formaldehyde concentration was increased from 100 ppb to 5 ppm and the response to 50 ppb formaldehyde still reached1.44.The test results also reflected that the chip had good repeatability,long-term stability,selectivity and consistency.The MEMS formaldehyde gas sensor chip developed in this thesis had laid the solid foundation for the mass production of formaldehyde sensors compatible with gas-sensitive films.