Research On Rayleigh Wave Surface Acoustic Wave Devices Based On Flexible Substrates

Surface Acoustic Wave（SAW）devices,with their high performance,miniaturization,and wireless and passive transmission capabilities,have been widely applied in fields such as mobile communication,defense industry,and biosensing.Traditional SAW devices are mostly fabricated based on rigid substrates,which means they cannot be bent or folded,thus failing to meet the demand for flexible SAW devices today.Currently,flexible SAW devices,with their lightweight,portability,and flexibility features,are demonstrating great application potential in emerging areas such as flexible wearable devices,electronic skin,and flexible displays.Meanwhile,real-time communication has raised higher requirements for flexible SAW devices,such as high operating frequency,high electromechanical coupling coefficient,and high temperature stability.The main challenge faced by flexible SAW devices now is the enhancement of their performance.This thesis employs the finite element method to analyze the performance of Rayleigh wave SAW devices based on flexible substrates,and combines it with the interdigital transducers（IDTs）electrode layered layout structure and two novel structures with different widths of piezoelectric materials,providing a theoretical foundation for improving the performance and high-frequency requirements of flexible SAW devices.The main contents and achievements of this research are as following:（1）To meet the demand for high-frequency SAW devices in modern technological development,a device model with an IDTs electrode layered layout is constructed.Firstly,the IDTs/AlN/IDTs/R-sapphire structure is constructed based on a rigid substrate,and the finite element method is used to analyze its acoustic performance,including conductance,phase velocity,and electromechanical coupling coefficient.The results show that the IDTs/AlN/IDTs/R-sapphire structure can excite Rayleigh waves,increasing the operating frequency by nearly twice compared to the traditional IDTs/AlN/R-sapphire structure devices.The electromechanical coupling coefficient is also improved compared to the traditional structure.In addition,the performances of Rayleigh wave device can be further improved and modulated by optimizing the electrode parameters of IDTs.Then,the IDTs electrode layered layout structure is combined with the flexible substrate Al to construct the IDTs/AlN/IDTs/Al structure.The results show that IDTs/AlN/IDTs/Al can excite Rayleigh waves,and when the ratio of the thickness of the upper aluminum electrode to the lower aluminum electrode of IDTs（Δh）is 1.2,and the electrode width w_e is 3μm,the performance is optimal,with a resonance frequency of 460 MHz and the maximum K_max² is 1.25%.The electromechanical coupling coefficient has tripled compared to the traditional IDTs/AlN/Al single-layer layout structure.At the same time,when the period is reduced,the electromechanical coupling coefficient K² and operating frequency of the device will be increased,which is consistent with the results of the double-layer layout structure based on the rigid substrate.This provides a new approach for developing high-frequency,high-performance SAW devices and offers a new way to enhance the performance of flexible SAW devices.（2）Using COMSOL simulation software,an IDTs/ZnO/Al structure based on a flexible substrate was constructed,with a focus on studying the Rayleigh wave propagation characteristics and temperature properties of the device.Firstly,the influence of the ZnO thin film and flexible substrate Al thickness on device performance was discussed.The results showed that the thickness of the flexible substrate Al has a significant impact on the TCF value of the device.When the substrate Al thickness h _Al=40μm,the IDTs/ZnO/Al structure exhibits both Rayleigh wave and Sezawa wave dual-mode.In the Sezawa wave mode,the TCF is-81.67 ppm/K,which is four times that of h _Al=160μm.Subsequently,IDTs/ZnO/ZnO/Al and IDTs/AlN/ZnO/Al structures were constructed based on the flexible substrate Al combined with a two-layer structure of different width piezoelectric materials,and the influence of the parameters of the two-layer piezoelectric materials on SAW device performance was analyzed.The results showed that both structures would enhance the device’s electromechanical coupling coefficient.By analyzing the temperature sensing characteristics of the IDTs/ZnO/ZnO/Al structure,it can be concluded that the temperature sensitivity of the device can be improved by optimizing the thickness of the first piezoelectric layer to meet the application requirements of flexible temperature sensors.Furthermore,the IDTs/ZnO/ZnO/Al structure exhibits temperature compensation effects,which can enhance the temperature stability of SAW devices.This provides a new approach to improve the temperature sensitivity of flexible temperature sensors,and offers a theoretical basis for enhancing the performance of flexible SAW devices.