Research On Acoustic Devices Based On Piezoelectric Composite Substrates

Surface acoustic wave(SAW)is a kind of mechanical wave propagating on a solid surface,which has the characteristics of high integration,fast signal transmission speed,high sensitivity,and low power consumption,so it has received extensive attention and research in scientific research and engineering applications.However,there are some problems in SAW devices prepared based on piezoelectric single crystal substrate materials,such as the low operating frequency of traditional SAW devices due to the characteristics of the material itself;Secondly,due to the high loss caused by the internal energy scattering of the material and the structural limitations,such devices often have a large beam width and size,which limit their application in high-density integrated chips.In this paper,the SAW characteristics of LN-on-SiC and other composite substrates are systematically studied and optimized in view of the problems faced by traditional SAW devices prepared based on piezoelectric single crystal substrate materials,such as low frequency,high loss,large device size and beam width.The specific research content and achievements are as follows:(1)Based on the design idea of preparing high-quality and large-bandwidth SAW devices on piezoelectric thin film-on-high acoustic impedance composite substrates,the complex multiphysics coupling system is mathematically approximated by finite element analysis method,and the theoretical model of SAW excitation propagation and reception is established,and the spectral characteristics and field distribution maps of SAW devices prepared on characteristic composite substrates are obtained.Based on the analysis of the displacement field distribution,it is first confirmed that the substrate can excite the Rayleigh wave and the Love wave through the interdigital electrode(IDT)defined by simple lithography under different tangential and propagation directions,and the results show that the LN piezoelectric film can obtain the largest electromechanical coupling coefficient of the Love wave in the XY tangential downward.At Z30°X cut downward,only Rayleigh waves are present.At the same time,the change of excitation mode will obviously affect the electromechanical coupling coefficient of the device,and changes in IDT electrode material,thickness,and duty factor are mainly due to the application of mass loading effects to the actual device.Based on the simulation results,the IDT structure in a new SAW device is designed,and the working frequency and bandwidth of the device are optimized by adjusting the tangential direction and size of the piezoelectric film,and better acoustic wave propagation characteristics can be obtained.(2)In terms of preparation process,the IDT of SAW devices is mainly prepared by lithography technology and deposition technology,which can achieve high-precision pattern definition and fine structure preparation,to ensure the coupling efficiency between electrode and piezoelectric film.In addition,reactive ion etching enables precise graphical design of microstructures,including phonon crystals and ridged waveguides,which enables structural control with submicron accuracy to ensure consistent device performance and simulation results.Finally,through experimental testing and characterization analysis,the performance and ultralow temperature characteristics of the designed SAW devices are verified.(3)Relying on the deep understanding of the design of high-quality SAW devices by regulating sound waves,the sound waves are effectively regulated by adjusting the energy bands of phonon crystals with the help of energy band engineering,to realize miniaturized and high-quality SAW devices.Based on the energy band and transmission spectrum characteristic curve on the high local LN-on-SiC composite substrate,the band structure of micropillars and microporous phonon crystals on XY cut LN-on-SiC composite substrates was studied,and through the mode shape analysis of phonon crystals at the edge of the Brillouin zone,it was found that there were two basic acoustic modes in the structure,namely Rayleigh mode and Love mode,and the bandgap height of phonon crystals could be effectively adjusted by changing the geometric parameters of micropillars and micropores.Moreover,the bandgap change of micro-pillar and micro-well phonon crystal does not show a linear relationship with the change of its geometric parameters.The transmission spectrum analysis of these two phonon crystals shows that the transmission spectrum is highly consistent with the bandgap of the corresponding phonon crystal.A SAW resonator based on a micro-wells phonon crystal designed with a frequency of about 5 GHz based on XY cut LN-on-SiC composite substrate was demonstrated.The period of the phonon crystal is 0.5 μm,the depth and diameter are 0.25μm and 0.28 μm,respectively,and the number of layers of the phonon crystal is set to 20,and it is found that the quality factor(Q-factor)of the new SAW resonator based on the large bandgaps based phonon crystal as the mirror can exceed 1 X 104 compared with the traditional SAW resonator based on the reflective electrode as the mirror.(4)Based on the LN-on-SiC composite substrate,a SAW ridge waveguide that can effectively limit the acoustic wave on the propagation surface is proposed and designed.Through numerical simulation experiments,it is found that IDT on ridge waveguides can excite Love waves and Rayleigh waves,and the electromechanical coupling coefficient of the former is much greater than that of the latter.By studying the acoustic transmission characteristics of straight waveguides,curved waveguides,and split waveguides,it is found that these three types of waveguides have excellent characteristics,such as operating frequency from 2.25 GHz to 5.10 GHz when the straight waveguide width is 0.5 μm.The effective operating bandwidth reaches 77.55%.When the waveguide and the coupling waveguide are close to each other,the two will be coupled,such as when the width of the waveguide and the coupling waveguide are 0.5 μm,the gap is 0.1 μm,and the operating frequency of the device is 4 GHz;The critical coupling length is 128 μm.A SAW ring resonator built on an XY cut LN-on-SiC composite substrate with a radius of 10μm was shown,and the SAW echo wall patterns in this resonator could be excited by placing IDTs on the waveguide,and its Q-factor reached 6.9×104,showing extremely high performance.Through numerical simulation experiments,the material system was extended to AlN-on-SiC composite substrates,and basic SAW components such as straight waveguides,bended waveguides,split beam waveguides,and ring resonators were designed and validated on this substrate.This study provides an important theoretical and experimental basis for further optimizing and applying ridge waveguides in composite substrates.