Research On Love Wave Gas Sensor Based On Functional Materials

Surface Acoustic Wave（SAW）sensors are electronic devices combined surface acoustic wave technology with sensors.Due to the superior properties,such as low cost,small size,low power consumption,ease of operation and so on,SAW sensors have been widely used in the fields of defence and military,environmental monitoring,automotive industry,wireless communications,and industrial production.In this thesis,the characteristics of Love wave sensors based on functional materials（nanorods or microridges）,as well as the properties of SAW devices based on IDTs（Interdigital Transducers）with bi-layer layout electrodes,are analyzed by the three-dimensional finite element method（3D-FEM）.The results provide a theoretical basis for improving the sensitivity of SAW sensors and the operating frequency of SAW devices with the novel structures.And the contents mainly include the following two parts:（1）The characteristics of Love wave sensors based on polyisobutylene（PIB）layers/SiO₂/ST-90°X quartz structure are studied by 3D-FEM using COMSOL software,where the PIB sensitive layer is available in three forms:thin films,nanorods and microridges.Firstly,according to the admittance and displacement results of SAW devices,it can be concluded that Love waves are excited successfully in the structures of PIB sensitive layers/SiO₂/ST-90°X quartz.Secondly,the SH vibrations of the substrate force the nanorods to vibrate in a particular mode depending on the height of the nanorods.At a critical height of nanorods,the coupled resonance between the substrate and the nanorods leading to a sharp shift in frequency,that is the transition of acoustic waves.In our case,as h_pn（the height of PIB nanorods）equals 520 nm,the mode 0 and mode 1 of Love waves coexist,and the operating frequencies are 505.8 MHz and 587.6 MHz,respectively.And the mode 1 and mode 2 of Love waves coexist when h_pnis 1260 nm.The similar phenomena are observed for the Love wave devices with PIB microridges.In the presence of coupled resonance at h_pn=520 nm,the surface stress of device reaches above 3000 MPa,which can be used to improve the sensitivity of SAW sensors.The properties of Love wave sensors based on PIB nanorods and microridges are studied by sensing six volatile organic compounds（VOCs）,the results show that the sensitivities are 2.4 times and 2.8 times higher than the sensor based on PIB films without coupling resonance,respectively.Finally,it is well known that the viscoelasticity of polymer materials is also an important factor affecting the performance of SAW devices.Therefore,the effect of viscoelastic PIB on the sensitivity of SAW sensor is further investigated,and the results indicate that the sensitivity of SAW sensors with viscoelastic PIB films,PIB nanorods and PIB microridges is attenuated by approximately 40%,70%and 65%,respectively.Although the sensitivity of Love wave sensors based on PIB nanorods and microridges is more affected by viscoelasticity,their performances are still superior to that of viscoelastic PIB thin film structure.The results mentioned above provide a new method and theoretical basis for improving the sensitivity of love wave sensors based on functional materials（nanorods or microridges,etc.）.（2）In order to meet the high-frequency development,as well as reduce the fabrication difficulty of SAW devices,we design a SAW model with bi-layer layout electrodes of IDTs,that is IDTs/（110）ZnO/IDTs/Si.The 3D-FEM is used to investigate the performance of the novel SAW devices,including operating frequency f,phase velocity v_p,electromechanical coupling coefficient K²,mass sensitivity S_mand reflection coefficient r.Firstly,the influence of bi-layer layout electrodes on the properties of SAWs are studied,and the results show that,the device will introduce the electric field with vertical component and excite strong bulk acoustic wave when the electrodes are thick.Therefore,the height of IDTs should be set small value so as to reduce the interference of the bulk acoustic wave to the Love wave.The characteristics of SAW devices are studied,in which the electrodes of IDTs are made of different materials（Al,Cu and Ag）and different width of electrodes（metallization rate of IDTs）.It is found that the phase velocity,electromechanical coupling coefficient and reflection coefficient are varied with the above parameters,and all the changes are caused by the difference of mass loading.However,the maxima of K²are obtained as h_z/λ=0.313(h_z/λis the normalized thickness of（110）ZnO for all devices.Finally,the variations of SAW period（λ）are realized by adjusting the center distance p between the upper and lower electrodes,and acoustic characteristics of love wave devices,withλ=8μm,6μm and 4μm,are analyzed.The results show that the resonant frequency f and the mass sensitivity S_mof the devices change greatly with the variation of periodic.For the device withλ=4μm,as h_z/λ=0.125,f=1.04 GHz and S_m=184.70 m²/kg,which are approximately twice that of the device withλ=8μm.Therefore,under the same lithography conditions（defined as 2μm）,the period of SAW devices with electrode bi-layer layout can be reduced to half of that of single-layer layout,then the operating frequency are double.This work provides a new way for the development of high-frequency SAW devices,as well as a theoretical basis for the expansion of its application fields.