| The thesis studies on the performances of the surface acoustic wave humidity sensors based on the (1120)ZnO/R-sapphir bi-layered structure, as well as related FEM simulations. The full text is divided into five chapters, in addition to chapter Ⅰ and chapter Ⅴ, which give Introduction and Conclusion respectively, the contents mainly include three parts:(1) Preparations and humidity response tests of Love wave humidity sensors; (2) Analysis the acoustic and electric characteristics of surface acoustic wave propagating in the (1120)ZnO/R-sapphir bi-layered structures by FEM in COMSOL 5.0; (3) Analysis the humidity responses of the surface acoustic wave sensors based on the (1120)ZnO/R-sapphir bi-layered structure by FEM in COMSOL 5.0.1. Preparations and humidity response tests of Love wave humidity sensorsZnO is a multifunctional piezoelectric material possessing unique electrical, optical, acoustic, and mechanical properties. In early 1970s, due to its moderate coupling coefficients and ease of deposition. It has been applied in wide fields including bulk acoustic wave (BAW) and surface acoustic wave (SAW) resonators, filters and sensors, and micro- and nano-electromechanical systems (MEMS and NEMS). Researchers have developed a lot of study on the surface acoustic wave devices based on the ZnO film.Humidity sensors have achieved increasing applications in industrial processing agricultural technique, and environmental control. Different sensing techniques, including impedance, capacity, optics, field effect transistor (FET), quartz crystal microbalance (QCM), and surface acoustic wave (SAW), etc., have been explored for humidity detections. Among various sensing technologies, the SAW sensors are very stable devices with high sensitivity, and widely used in many fields, such as gas detection, humidity and bio-chemical sensing and so on. There are some researches that have been carried out about SAW humidity sensors. Most of the work are focused on Raleigh wave devices for humidity sensing. While Love wave is another kind of SAW, which propagates in bi-layered structures. Different from Raleigh wave devices, Love wave devices almost radiate no energy into the liquid above the devices due to they do not contain the displacement perpendicular to the surfaces. Love wave sensors have widely applied to liquid and biological sensing.In chapter Ⅱ, (1120)ZnO thin film is epitaxially grown on R-sapphire substrates by RF magnetron sputtering. The Love wave sensors are fabricated by lithographing interdigital transducers on the surfaces of ZnO thin film/R-sapphire structures. The humidity responses of the love wave sensors are tested by the humidity test system (including the test of frequency shifts and insertion loss variations). Based on BET model, the microscopic processes of the water vapor adsorption for surface acoustic wave sensors are analyzed. Then the frequency shifts of the Love wave sensors for humidity responses can be fitted by BET model.When the humidity is less than 70%, the calculated results can be fitted with the experimental data accurately.2. Analysis the characteristics of surface acoustic waves propagating in the (1120)ZnO/R-sapphir bi-layered structuresThe surface acoustic wave devices with piezoelectric thin-film multilayer structures have been widely studied. The use of materials with different properties of multilayer compositions can develop the advantages of each material. It is not only possible to improve the performances of surface acoustic wave devices; to implement and integrate with other circuit elements; to reduce the sizes and costs of the devices, but also to explore new devices and expand the range of applications. For examples, the piezoelectric film with high electromechanical coupling coefficient can be overlaid on a non-piezoelectric material with a higher acoustic speed in order to improve the acoustic speed of the device, or covered on low piezoelectric materials to improve the electromechanical coupling coefficient, and/or, a film with positive temperature coefficient is covered on a material with negative temperature coefficient to provide a structure with zero temperature coefficient; in addition, the piezoelectric materials may also be covered by a sensitive film, for sensing temperature, humidity, pressure, gas flow rate, etc., to produce a sensitive sensor.To analysis the characteristic of the surface acoustic wave with the multilayer piezoelectric film, the general methods including transfer matrix based on the partial wave theory, stiffness matrix method are used. With the finite element (FE) software develops more and more maturely and computer equipment increases computing capability continuously, the calculations of the surface acoustic wave propagations in multilayer structures have been studied in many researches.In the chapter Ⅲ, the characteristics of surface acoustic wave propagating in the (1120)ZnO/R-sapphire bi-layered structure have been studied including wave mode, sound speed, electromechanical coefficient and admittance curve. Based on the calculating results, the electromechanical coefficient of Rayleigh wave that propagates along the [0001] orientation in the (1120)ZnO/R-sapphire bi-layered structure has maximum value when hf/λ= 0.3; (hf is the film thickness, λ is the wavelength) while the Love wave that propagates along the [1100] orientation in the (1120)ZnO/R-sapphire bi-layered structure has a maximum value when hf/λ= 0.13. In general, the electromechanical coefficient of Love wave is larger than the Rayleigh wave.3. Analysis the humidity response of the surface acoustic wave sensorsThe surface acoustic wave technology is widely used in different acoustic devices, the method to design the surface acoustic wave devices is improved continuously, more and more complex structures and performances have been improved. In general, the methods to analyze surface acoustic wave devices can be divided into two categories:one is phenomenological model which refers to similar concept or model to study conveniently and practically, in which the general methods include Impulse Response Model, P-Matrix model and the coupling of modes (COM) and so on. Another is accuracy model which is from the basic wave equations and the boundary conditions to meet the eigenvalues and get accurate and reliable results, in which the general methods include Green Function Model, FEM/BEM method (finite element method and boundary element method) and so on. The FEM/BEM method is a precise theoretical tool for simulation of SAW device.Many theoretical studies on SAW gas sensors have been reported. The basic theoretical model for SAW gas sensors is mainly Wohljent’s method. Later, Martin expanded the method and his model is also applicable for the case of an acoustically thick film. The main problem they focused on is the changes in SAW device performance caused by a known layer on the device surface. Other methods such as the transfer matrix method are often used to calculate the frequency shift caused by the mass load. The transfer matrix method is much more complicated although it has a more accurate solution. The finite element analysis method to study SAW sensor is more accuracy and integrally compared to the methods described above. Based on superior performance of finite element method and sophisticated software, the simulations of surface acoustic wave sensors by finite element method have developed many related researches.In the chapter IV, the FEA (Finite Element Analysis) model for SAW humidity sensor based on (1120)ZnO/R-sapphire bi-layered structure is built in COMSOL5.0. The humidity can be characterized with the thickness of the water layer adsorbed on the surface of the SAW humidity sensor in the FEA model. The insertion losses of the two modes of SAW humidity sensors in a range of frequency domain are calculated. At the same time, the sensitivities of the SAW humidity sensors are calculated with different thickness of ZnO film. The calculated results indicate that Electromechanical coupling coefficient of the film may have a great influence on the sensitivity of the sensor. Theoretical derivation displays that two mechanisms, including mechanical and electrical affection of water molecules layer adsorbed on the surface of the SAW humidity sensor, independently affect the surface acoustic wave propagation. Based on the FEA model, the mechanical and electrical effects of water layer adsorbed on the SAW humidity sensors to frequency response and sensitivity are calculated respectively.Finally, in chapter V, the conclusions and prospects are presented. |
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