Finite Element Simulation Study Of New Structural Surface Acoustic Wave Device With High Frequency And Large Electromechanical Coupling Coefficient

Surface acoustic wave devices are widely used for filters,sensors,microfluidics and radar fields because of their light weight,small size,and high reliability.With the increasingly rapid development of electronic communication technology,it is also urgent to improve the frequency and electromechanical coupling coefficient of SAW devices.The higher wave speed usually means that the device has a higher frequency,and the larger electromechanical coupling coefficient represents a higher conversion efficiency.However,the maximum electromechanical coupling coefficient is usually achieved with a lower frequency of device.At present,it has become difficult for researchers to further increase the electromechanical coupling coefficient of SAW devices to satisfy the demand for 5G era without reducing the frequency.In this work,we constructed a"tree shape"SAW device.The structure could achieve higher frequency to meet the needs of the 5G era,while improving the electromechanical coupling coefficient of device.The device is composed of electrodes,the two-layer piezoelectric materials（The first layer of piezoelectric material is the same width as the electrode,and the second layer of piezoelectric material is the same width as the substrate）and substrate,which is different from the conventional structure.In the"tree shape"structure,AlN and diamond are selected to obtain the highest frequency.The propagation characteristics of the structure are systematically analyzed and optimized by using two-dimensional finite element method.The results show that the AlN/AlN/diamond structure can produce two different acoustic waves as an electrical signal is applied to the interdigital electrode.One acoustic wave corresponds to the low-frequency Rayleigh SAW,and the other acoustic wave corresponds to the high-frequency Rayleigh SAW.The high-frequency Rayleigh SAW based on AlN/AlN/diamond structure obtained a high frequency of 3.7283 GHz and an electromechanical coupling coefficient of 4.64%.The phase velocity of this structure is larger than that of conventional structure,and the electromechanical coupling coefficient is about four times higher than that of conventional structure.To further investigate the effect of this structure on the frequency and electromechanical coupling coefficient,piezoelectric materials of ZnO and LiNbO₃ were also used.From SAW characteristics produced by the two-by-two combination of the three piezoelectric materials,it was found that the electromechanical coupling coefficient based on the ZnO/ZnO/diamond structure was greater,up to 8.26%,about twice that of the conventional structure,at a frequency of 2 GHz.Meanwhile,a frequency transition from 1.75 GHz to 3.62 GHz could also be achieved by varying the film thickness of ZnO.As we all know,the temperature will affect the performance of SAW device.To further investigate the variation of the properties produced by the two typical structures as a function of temperature,the first-order temperature coefficient of the material is introduced in the model.It was found that the"tree shape"structure could still ensure temperature sensitivity to frequency with the increase in frequency.However,the electromechanical coupling coefficient tends to increase slightly with increasing temperature,which is very different from the conventional structure.Finally,the temperature coupling coefficient of the structure was calculated and was greater than that of conventional structure.At the same time,the temperature coupling coefficient of ZnO-based SAW device is higher than that of AlN device,indicating that the ZnO-based SAW device is not stable over temperature.This means that ZnO-based SAW device could be more suitable for temperature testing applications.