Deposition&Properties Of BN-ZnO High Acoustic Piezoelectric Material And Design Of High Frequency SAW Device Cell

With drastic increase in communication frequency band, high frequency, high power SAW device has become critical device in high frequency communication. In order to improve the center frequency, one way is to reduce the feature size of the IDT and another way is to adopt the substrate or piezoelectric material with high acoustic property. Based on the high acoustic multi‐layer diamond film structure, the optimization, deposition process and device design of piezoelectric material has attracted much attention and become one of the most popular hotpots. In this dissertation, based on c‐BN、ZnO piezoelectric thin film, the following innovative work on design and construction of diamond multilayer structure high‐frequency SAW device has been done:(1) Cubic phase c‐BN films was successfully prepared at high content (up to96%) on Si and diamond substrate using RF magnetron sputtering and the Root‐Mean‐Square value of surface roughness is found to be less than2.3nm. Piezoelectric performance tests show that the cubic boron nitride film maintains good piezoelectric properties, and polarization reversal can be achieved. Copper (Cu) and platinum (Pt) are used as bottom electrode in piezoelectric test and the maximum effective relative piezoelectric coefficients in micro‐region were0.119and0.122, respectively. Therefore the piezoelectric property of the Pt bottom electrode sample is found to be more sensitive.(2) Planarization research was performed based on the optimization of ZnO film deposition process. Surface with roughness of1.2nm （20×20μm2） was obtained with optimization of pH value and polishing downforce, and the corresponding CMP mechanism is also explored. With planarization process, a notable enhancement, as well as significant improvement piezoelectric signal in ZnO film could be observed, which would be favorable for further increase in stability of SAW device performance.(3)Based on the analysis on piezoelectric properties of single layer BN and ZnO film,(ZnO/BN)n composite film is constructed and fabricated, and the impact of composite stacking film on the micro area piezoelectric properties were also explored. Results showed that the relative piezoelectric coefficient in stack structure is higher than that of single c‐BN film. When the composite proportion for ZnO and BN films were50nm:50nm, highest piezoelectric coefficient and best piezoelectric uniformity can be observed in the sample, which makes it suitable for high frequency SAW device fabrication.(4)With optimized high cubic phase content c‐BN film and BN‐ZnO stacking structure, SAW device units were fabricated respectively. On BN/diamond/Si multi‐layer, Al interdigital transducer with feature size of1.5μm was made with integrated circuit lithography process. The center frequency of the bare device unit could reach1.65GHZ, with insertion loss of21.244dB and phase velocity of9900m/s. On (ZnO/BN)n/Diamond/Si multi‐layers, IDT with interdigital width of2.0μm was fabricated. the center frequency, insertion loss and phase velocity for the bare device unit were1.12GHz,10.648dB and8960m/s, respectively.