Fabrication Of Single Crystal LiNbO₃ Thin Films And Surface Acoustic Wave Devices

The surface acoustic wave sensor has the advantages of small size,high sensitivity and can be measured wirelessly and passively.It is very suitable for working in complex,closed and harsh environment which is inconvenient for wires.However,the sensor based on rigid piezoelectric substrate cannot be bent,so it is difficult to be applied to heterogenic surfaces such as curved surfaces.Flexible thin film surface acoustic wave sensor is suitable to be applied to rotating parts and curved surfaces because of its flexibility,low weight and low centrifugal force.Based on this background,a flexible surface acoustic wave sensor with the structure of LiNbO₃ film/BCB bonding layer/flexible metal substrate is proposed and designed in the thesis,and its preparation research is carried out.The main content of the thesis is as follows:1.Single crystal LiNbO₃ thin films were prepared on Ta substrate by crystal ion slicing method with BCB adhesive layer.SRIM software was used to simulate the process of ion implantation,and it was found that He⁺or vacancy were approximately Gaussian distribution along with the distance to the implantation surface.It is found that the average number of collisions is more than 100 for per He⁺,and the total number of O vacancies is the highest,which is the most important defect type.During the process of bonding and exfoliation of LiNbO₃ thin film,the phenomenon of cracks in the films was analyzed by three-dimensional finite element simulation of thermal stress.Finally,by reducing the annealing temperature and metal substrate thickness to reduce the thermal stress,a complete single crystal LiNbO₃ thin film was obtained.The full width of half maximums of LiNbO₃ thin film is about 0.3°,the thickness is 740 nm,and the RMS roughness is18.1 nm.2.The propagation characteristics of acoustic wave in“LiNbO₃/BCB/Ta”structure were simulated by COMSOL software.The effects of LiNbO₃ thin film and BCB thickness on particle vibration mode,surface acoustic wave velocity v and electromechanical coupling coefficient K² were also studied.When the thickness of BCB is less than 0.1λ,BCB can limit the acoustic wave energy within the LiNbO₃ thin film.While the thickness of BCB is more than 0.2λ,BCB can absorb the acoustic wave energy,forming the bulk acoustic wave mode.v is significantly reduced regardless of BCB thickness.3.A surface acoustic wave device with target structure was prepared by exfoliation method,and a device with“LiNbO₃ thin film/BCB/Ni substrate”structure was prepared for comparison.It is found that Rayleigh wave resonance was not detected in the surface acoustic wave device with target structure,while the resonance peak of“LiNbO₃ thin film/BCB/Ni-based substrate”has an intensity of about 3d B at 53MHz.Combined with the simulation,we made sure that the resonance here was caused by Rayleigh wave.By comparing the measurement results of the two devices,we believe that the fairly thick BCB bonding layer compared with operation wavelength and the high surface roughness of Ta substrate may be the major reasons that the target device had no resonance peak.The corresponding improvement measures are put forward according to our analysis.