Research On Preparation Of Single Crystal LiNbO₃ Thin Film And Thin Film Bulk Acoustic Wave Resonator

The thin film bulk acoustic wave?BAW?filter has been applied to 4G and other communication technologies due to its advantages of small size,high frequency and integration.Along with the development of 5G communication technology,there is new performance requirements for radio frequency devices,requiring the filter to develop towards large bandwidth.The performance of BAW resonator,as the main component of the BAW filter,the electromechanical coupling coefficient?k_t²?of which directly affects the bandwidth of the filter.And the electromechanical coupling coefficient of the BAW resonator is mainly determined by the k_t² of the piezoelectric film.Therefore,the BAW resonator and the piezoelectric materials are vital,which attracts research in the academic and business circles.However,currently-applied Al N-based BAW resonator has a low k_t²?<7.5%?.That is often used as a narrow-band filter?relative bandwidth<4.3%?,which cannot meet the requirement of large bandwidth of 5G filters?relative bandwidth>8%?.Therefore,it is vital to find piezoelectric materials with high k_t² and prepare the BAW resonator with high k_t².In view of this situation,this dissertation focuses on the orientation research of single-crystalline piezoelectric materials,the preparation of single-crystalline piezoelectric films,film surface modification treatment,and the preparation research of BAW resonators.The main research contents are summarized as follows:This dissertation carried out the design of the functional layer of BAW resonator.Single-crystalline LiNbO₃?LN?film was proposed as the piezoelectric layer of the BAW resonator.Moreover,a new model simulation method was builded to calculate the k_t² and resonant mode of LN film with different orientations.Through the analysis of the influence of electrode type and shape on the resonant mode and resonant frequency,Al and irregular pentagon were served as the electrode and electrode shape of the BAW resonator,respectively.Through the simulation of Euler angle transformation and finite element method,the k_t² and resonant mode of Y-cut LN material with different rotation angles were obtained,which lays a theoretical foundation for the choice of LN orientation for BAW resonator.Based on the Crystal-Ion-Slicing?CIS?technique,both LN films with different orientations were fabricated,which verifies the applicability and compatibility of CIS technique on the preparation of Y-cut LN films with different rotation angles.The both LN films have good crystalline quality and uniform thickness,but there is a large surface roughness.Therefore,a non-contact ion polishing technology,low-energy Ar⁺irradiation,was applied in surface treatment.And the effect of irradiation parameters on surface roughness was explored.The results indicated that it not only reduces the surface roughness,also removes the surface damage layer resulted from ions implantation.Moreover,the electrical properties of the LN film after the surface treatment have been significantly improved.And results showed that the single-crystalline LN film continues the performance of the LN single crystal bulk material.In order to obtain a BAW resonator with high k_t²,43^°Y-cut LN film was proposed as the piezoelectric layer of the BAW resonator.The k_t² of LN film reaches 23%,which is more than 3 times that of the Al N material.Then,a gap-type BAW resonator based on the LN single crystal film was prepared by means of CIS technique.The k_t² of BAW resonator is about 14.7%,which is about twice the k_t² of the Al N-BAW resonator.But there is a problem of film warping above the cavity.In order to improve the performance of BAW resonator,the solidly-mounted-type resonator?SMR-BAW?was prepared and studied,adopting two differenct integrated approaches.In the forward growth technology,the Bragg reflector was grown on the surface of the substrate from the bottom up.In this structure,it was proposed that the Benzocyclobutene?BCB?layer is as an acoustic reflection layer to improve the reflection efficiency of acoustic waves.In the reverse growth technology,the Bragg reflector was grown on the surface of the implanted plate from top to bottom.The k_t² of the prepared BAW resonator reached 20%,which is more than twice the k_t²of the Al N-BAW resonator.In summary,LN single crystal film with a special orientation was proposed as the piezoelectric layer of the BAW resonator.And the applicability and compatibility of the CIS technique on the preparation of Y-cut LN films with different rotation angles was verified.Based on the technology,BAW resonator was successfully fabricated.The k_t² of the device reached 20%,which is more than twice the k_t² of the current commercial Al N-BAW resonator.