Research On Piezoelectric Surface Acoustic Wave Inertial Device Based On Bonding Substrate

The surface acoustic wave(SAW)gyroscope is an inertial device that uses the SAW gyroscopic effect to measure angular rate.As a single-layer planar sensor,the SAW gyroscope has the advantages of small volume,light weight,a simple preparation process,and excellent shock and vibration resistance.This device’s applications include smartphones,smart wearable devices,automobiles,drones,and other civilian fields.In recent decades,foreign and domestic researchers have conducted theoretical and experimental research on the feasibility of SAW gyroscopes.However,this device has yet to be used for actual measurements.The main issues are the device’s low sensitivity due to the weak SAW gyroscopic effect,and the substrate material’s high frequency temperature coefficient(TCF),which results in the device’s poor temperature stability.The primary goal of this device’s research is to improve the temperature stability and sensitivity of the SAW gyroscope.A high-performance traveling wave SAW gyroscope was developed in this study.To improve the temperature stability and sensitivity of the SAW gyroscope,an innovative piezo on silicon(POS)substrate combining piezoelectric crystal film and silicon(Si)substrate was used as the substrate material.This study focuses on POS substrate preparation technology,Rayleigh wave propagation characteristics and the SAW gyroscopic effect on POS substrates,and the structural design,preparation,and experimental testing of SAW gyroscopes based on POS substrates.The following are the main research findings and highlights:(1)Smart-cut technology was used to investigate the critical technology of POS substrate preparation.The piezoelectric materials Z-X LiNbO3(LN)and 128° Y-X LN were chosen to investigate the ion implantation and annealing processes of POS substrate preparation.The depth of He ions and damage distribution in the implanted LN crystal were simulated.the effects of annealing temperature on blistering time,exfoliation mechanism of the implanted LN wafer,and surface morphology of the exfoliated LN film were studied in situ.The activation energy of the implanted LN wafer was calculated.The results of the study showed that implanted LN with different cut types had different activation energies in the annealing process under the same injection conditions.The higher the injected ion dose resulted in a lower activation energy,a lower required annealing temperature,and a faster foaming rate for the same cut type of LN crystal.When the implanted ions reached a certain dose,two stripping mechanisms of blistering and exfoliation occurred in the implanted LN crystal as the annealing temperature increased.The exfoliation mechanism produces a smoother surface roughness of the LN film than the blistering mechanism.As a result,by adjusting the annealing temperature,the surface morphology of LN film on POS substrate can be improved,reducing the subsequent polishing process time,and optimizing the preparation process.Benzo-cyclo-butene(BCB)bonding was used to successfully prepare the Z-X LN-POS substrate and the 128° Y-X LN POS substrate,as well as exfoliated and transferred LN films with good single crystal properties.(2)The surface effective permittivity method and the finite element method were used to investigate the SAW propagation characteristics of POS substrates.The Rayleigh wave characteristics of the 128° Y-X LN-SiO2 POS substrate and the 128° Y-X LN-BCB POS substrate with different structural parameters,such as the phase velocity(vp)and electromechanical coupling coefficient(K2),were investigated.Based on the Rayleigh wave’s vibration mode and acoustic field distribution,the influence of the thickness of the SiO2 layer(or the BCB layer)on the propagation characteristics of the Rayleigh wave was discussed.It was demonstrated for the 128° Y-X LN-BCB POS substrate that the BCB layer not only acts as a bonding layer but also couples more SAW vibration energy into the LN film,greatly increasing the K2 of the POS substrate.The SiO2 layer improves the K2 of the 128° Y-X LN-SiO2 POS substrate in a limited range,but it can greatly improve the TCF of the POS substrate.The 128° Y-X LN-SiO2 POS substrate(hereinafter referred to as POS substrate)performs well as the substrate material of the travelling wave SAW gyroscope because it can achieve higher K2 and lower TCF.(3)The traveling wave SAW gyroscope with differential structure’s SAW gyroscopic effect was investigated.In this study,the POS substrate was initially proposed as the SAW gyroscope substrate material.The surface effective permittivity method was used to supplement the calculation process of the SAW gyroscopic effect in the piezoelectric multilayer structure,which was based on transfer matrix theory.For the first time,the effect of POS substrate structure parameters on the SAW gyroscopic effect was investigated.The gyro gain factor η in the differential structure is defined to discuss the gyroscopic effect strength of the POS substrate under different structural parameters.Taking 128° Y-X LN as a reference,the POS-1 substrate for preparing SAW gyroscopes was optimized,with a ηy of 0.27,K2 of 4.14%,and TCF of-22.64 ppm/℃.Based on the POS-1 substrate,the SAW gyroscopic effect in uniform interdigital transducer(IDT),split IDT,EWC/SPUDT and metal dot arrays were investigated,respectively.Results demonstrated that the IDT with a low reflection coefficient can obtain a larger gyro gain factor.Increasing the thickness and area of the metal dots can effectively enhance the SAW gyroscopic effect.(4)The POS-1 substrate was used to investigate the optimal design of a traveling wave SAW gyroscope.Finite element simulation was used to investigate the relationship between COM parameters and the results of steady-state and eigenfrequency studies.Using the P-matrix cascade technology in conjunction with the method,the COM parameters of various IDTs on the POS-1 substrate were extracted,and the model of the SAW delay line was established.The low insertion loss SAW delay line was designed by optimizing structural parameters such as acoustic aperture and IDT logarithms of input IDTs.The withdrawal EWC/SPUDT structure was used to optimize the design of the SAW delay line with low insertion loss and single-mode operation as the frequency control element to obtain a SAW oscillator with high frequency stability.Integrating the research results resulted in the design parameters of the traveling SAW gyroscopes based on the POS-1 substrate.(5)Experiments were performed on the POS-1 substrate,128° Y-X LN crystal,three types of IDTs,and metal dot arrays of the SAW gyroscopic effect on the POS-1 substrate.A traveling wave SAW gyroscope was successfully developed on the POS-1 substrate.The withdrawal EWC/SPUDT structures were reversed to form a differential structure in two sets of SAW delay lines.The copper dot arrays with length 3λ/8 were arranged in the propagation path of one group of a SAW delay line.In the experimental test,the sensitivity of this device can reach 5.71 Hz/(deg/s),which is superior to the SAW gyroscope with the same structure on the LN crystal.Meanwhile,the POS-1 substrate has a TCF of-25.62 ppm/℃,which is only one-third that of the LN crystal.The results show that the POS-1 substrate,which is a high-performance substrate material for SAW gyros,can effectively improve temperature stability and enhance sensitivity.