Humidity Sensors Based On MEMST Thin Film Piezoelectric Resonators And Graphene Oxide

As one of the common sensors,humidity sensors are widely used in meteorology,industrial process control,agricultural planting and food storage.With the development of the Internet of Things,emerging applications such as smart homes and health care require humidity sensor to be smart,integrated,miniaturized,and cost-effective,in addition to the high sensing performance of the humidity sensors.At present,the most effective way to achieve sensor miniaturization and low cost is to use the MEMS technology to prepare MEMS humidity sensors.Compared with other MEMS humidity sensors,piezoelectric resonant humidity sensors have advantages of high sensitivity,low power consumption,strong anti-interference ability,large test range and quasi-digital output signals,which are more suitable for application in the Internet of Things.At present,MEMS piezoelectric resonant humidity sensors still have many problems to be solved,such as poor comprehensive humidity sensing performance,poor repeatability of the coating process of humidity sensing materials,lock of theoretical and experimental verification of humidity response mechanism,and sensors need further miniaturization and integration.To counter the issues above,this thesis uses CMOS-compatible AIN thin film as the piezoelectric material,and the AIN film is combined with the surface highly doped Si to form an AIN/Si layered structure to realize the preparation of piezoelectric resonator with good chemical and thermal stability.At the same time,the graphene oxide film with the properties of good hydrophilic property,fast water absorption and desorption speed is chosen as the humidity sensing material.The researche contents of the this thesis are focuse on testing of comprehensive humidity sensing performance of the graphene oxide(GO)film based SAW resonant humidity sensors,studying of the GO film coating process,analysing of the specific humidity response mechanism of the GO film based SAW resonant humidity sensor,and exploring a smaller size piezoelectric resonator as the humidity sensing platform.Firstly,the overall performance of the AIN piezoelectric resonator combined with GO film in the humidity detection process is initially explored by investigating GO film(formed by drop casting)based SAW humidity sensor,the test results show that this humidity sensor has better sensing performance than othe SAW humidity sensor in terms of sensitivity(including absolute sensitivity and relative sensitivity)and thermal stability,and has a large humidity test range(5%RH-95%RH),low humidity hysteresis(less than 3%RH),good stability and repeatability,fast response and recovery,and good selectivity to humidity.Then,the surface tension of the GO solution is used to realize the preparation of uniform and thickness controllable GO films.After transferring these uniform GO films to the surface of the SAW resonators,the humidity test repeatability between the SAW resonant humidity sensors are effectively improved.Through comparison of experiments,it is found that increasing the thickness and oxidation degree of GO film can improve the test sensitivity of the SAW resonant humidity sensors.On this basis,large-sized uniform GO films are prepared by a modified vacuum filtration method,so as to solve the problem that the GO film prepare by the surface tension method cannot be applied to the mass processing of the humidity sensor,and further improve the test repeatability between the humidity sensors.Meanwhile,by comparing the humidity sensing performanc of different SAW resonators(different resonant frequencies)after coating the same thickness of GO films,it can be conclude that increasing the resonant frequency of the SAW resonator can also improve the sensitivity of the SAW resonant humidity sensors.According to the theoretical analysis,the conductivity,viscoelasticity and mass change of GO film are there possible reasons for the change of resonant frequency of SAW resonant humidity sensor.On this basis,the complex impedance spectrum of the IDE device coated with GO film is analyzed by AC complex impedance analysis,to study the effects of humidity change and test signal frequency on the electrical properties of GO film,and using the electron-ion hybrid conduction theory to analyze the causes of changes in the electrical properties of GO films.Subsequently,the equivalent circuit models corresponding to the Mason models of QCM device before and after coating GO film are used to establish the relationship between the shear modulus,mass of the GO film and the electrical parameters of GO film coated QCM device.After that,the changes of conductivity,viscoelasticity(shear modulus)and mass of the GO film are converted into the resonant frequency change of the corrrsponding SAW resonant humidity sensor,and it is conclude that the resonant frequency changes of the SAW resonator caused by the mass changes of GO film is the main humidity sensing mechanism of the GO film based SAW resonant humidity sensor.Finally,it is verified by experiments that the non-linear humidity response of the SAW resonant humidity sensors(corresponding to the nonlinear increase of the mass of GO film),which is due to the fact that the water molecules infiltrating into the interior of the GO film and increasing the layer spacing of the GO film when humidity increases,thereby increasing the water absorption capacity of the GO film.Using a piezoelectric microcantilever resonator instead of a SAW resonator as a humidity sensing platform to solve the problem that the size of the SAW resonator is relative large,and it is difficult to further miniaturize and array.Test results show that the microcantilever resonant humidity sensor has a higher relative sensitivity than SAW resonant humidity sensor when its size is one order of magnitude smaller than SAW resonant humidity sensor.Whereafter,based on the analysis of the humidity sensing mechanism of microcantilever resonant humidity sensor,the relative sensitivity and absolute sensivity of the sensor are further improved by reducing the equivalent mass of the resonator and using higher-order modes of the resonator.At last,the microcantilever resonant humidity sensor is integrated into the breathing mask,and combined with the closed-loop self-oscillation detection circuit for human respiratory monitoring.Simulation experiment results show that this respiratory monitoring system can effectively monitor the respiratory frequency and patterns of human body.