Study Of LM-SAW Biosensors Based On The Nano-ZnO Film

The surface acoustic wave (SAW) technologies in signal processing, micro-quantity substance sensing, acousto-optic modulation and information processing have exhibited an extremely attractive prospect. The SAW technology has been matured and a variety of functions of the SAW devices have been developed. Now SAW technology has been widely applied to many scientific fields, such as radio, television, radar, communications, aerospace, oil exploration, environmental monitoring, biochemistry and so on. The surface acoustic wave devices have been used broadly depends on its many advantages, since the acoustic devices are much smaller than the electromagnetic wave devices, surface acoustic wave signal is also much easier to be handled and the mature planar semiconductor technology makes the SAW devices have good stability and repeatability. The SAW biosensors show the better aspects than other type biosensors, for example, the SAW biosensors have higher mass loading sensitivity than electricity, heat or QCM biosensors; the size is smaller, the weight is lighter, the structure of device and the preparation skill are easier to be achieved. Furthermore, the SAW biosensors have great significance for testing toxic, harmful gases and micro-mass loading.The content of this paper is divided into three parts, the simulation and optimization of ZnO/SiO₂/Si layered structure Love mode SAW biosensor for mass loading sensing, the simulation and optimization of ZnO/SiO₂/Si layered structure Love mode SAW biosensor for liquid viscosity sensing and the effects of initial tress in each Layer on the performance of SAW devices. ZnO itself is a very good sensor material, and it could possess the piezoelectric, ferroelectric, ferromagnetic, semiconductive and other properties by doping methods. Nano-ZnO possesses high adsorption which can make more biological molecules fixed on its surface. ZnO could provide an active surface for biological molecules. The property of its hydrophile, which leads to the perfect connection between the liquid the sensor surface, is necessary for the detection of the liquid characteristics. Moreover, we choose Love mode surface acoustic wave, which is a guided SH SAW and spread mainly in the film deposited on the substrate. The acousitc energy is concentrated in the guided layer, so surface disturbance could cause great responses. The sensitivity of Love wave device is higher than the conventional SAW biosensors. In addition, Love wave devices are suitable for sensing in liquids since the SH polarization cannot excite compressional waves in the adjacent medium. In this paper, a ZnO/SiO₂/Si structure Love wave biosensor is considered. The buffer layer SiO₂ is added between the guiding layer and substrate. This device structure possesses the advantages of both traditional SAW sensors and ZnO nanostructures.First, the simulation and optimization of ZnO/SiO₂/Si layered structure Love mode SAW biosensor for mass loading sensing are discussed. The main part of a SAW sensor is a SAW device. The sensitive material is coated on the path of SAW. The phase velocity of SAW will be changed once the detected objects are adsorbed on the sensitive material. The detection will be realized by measuring the changes of frequency of devices. The relations between the buffer layer and the device performance have not been discussed yet, as far as I know. The coupled electromechanical field equations in every layer are used to describe the displacement function and electric potential function, and numerical solutions are calculated out in terms of boundary conditions. And then the relations among phase velocity, electromechanical coupling coefficient, mass loading sensitivity, temperature coefficient of delay (TCD) and the structure of devices are presented. Among these results, the relations between device performances and the buffer layer are paid more attentions. Many results are first presented in this paper. Form the numerical results it is found that the thickness of the piezoelectric guiding layer has an evident effect on the electromechanical coupling coefficient, while the thickness of the SiO₂ layer has a tiny effect on it. It is also found that the mass loading sensitivity can be further improved by adding the SiO₂ layer; furthermore, the maximal sensitivity of the biosensors can be obtained by adjusting the thicknesses of the two layers. A zero TCD device also can be obtained after integrating with a proper thickness of SiO₂ thin film. Accordingly, the buffer layer is very important for the optimization of SAW devices. We try to obtain a device which possesses the mass loading sensitivity and coupling coefficient as high as possible and has zero TCD.Next, the simulation and optimization of ZnO/IDTs/SiO₂/Si layered structure Love mode SAW biosensor for liquid viscosity sensing are discussed. Liquid sensors have important applications in environmental science, industrial chemistry, medical analysis and materials characterization and so on. The mechanism of SAW liquid sensors is the similar to that of SAW mass loading sensors. The interaction between the acoustic wave and media could change the acoustic fields. It will convert the liquid characteristics, such as viscosity, density, electrical and dielectric properties into electrical signals, and then these characteristics can be identified by processing these electrical signals. The coupled electromechanical field equations in every layer and Navier-Stokes equations in liquid are used to describe the displacement function and electric potential function, and numerical solutions are calculated out in terms of boundary conditions. And then the relations among phase velocity, electromechanical coupling coefficient, viscosity sensitivity, temperature coefficient of delay and the structure of devices are presented. From the numerical results, it is found that the electromechanical coupling coefficient and viscosity sensitivity can be further improved by adding the SiO₂ thin film in this specific structure, and a zero TCD device also can be obtained after integrating with a proper thickness of SiO₂ thin film. The response of acoustic shear wave sensors to Newtonian liquid loading, i.e., changes in frequency and attenuation, is proportional to the square root of the liquid's density-viscosity product generally, neglecting changes of liquid density. But the velocity and attenuation of surface waves propagating in a fluid-loaded medium are affected by the viscosity and density of the adjacent viscous fluid in a coupled manner and both of them change simultaneously, so it is a challenging task to distinguish viscosity from density and the above approximation is of great limitation. In this paper, we try to improve the former viscosity sensitivity by taking the coupled effect of liquid density and viscosity into account. Numerical results show that the effect of coupling of viscosity and density can not be neglected. This modification could make the measure of viscosity more accurately.Finally, the effects of initial tress in each layer on the performance of SAW devices are discussed. Residual stress will be produced in the manufacturing surface wave device process, since the guided layer properties do not match the substrate material. The piezoelectric materials are often in a prestressed state to avoid brittle fracture. Therefore, it is meaningful to analyze the effect of initial stress on the Love wave propagation. It is also important to enhance the stability of the device. This paper focuses on the effects of initial stress on the device performances in ZnO/SiO₂/Si double guided layer structure devices, and the effects of initial stress in buffer layer on the device performances are gives for the first time. Moreover, we find that the effect of initial stresses on the properties of biosensors depends on the distribution of acoustic flow power in the two layers.The simulation and optimization of ZnO/SiO₂/Si layered structure Love mode SAW biosensor for mass loading sensing and liquid viscosity sensing are presented. These results are meaningful for the manufactures and applications of the ZnO/SiO₂/Si structure Love wave biosensors.