In this study, the ladder-type filters based on back-etched thin film bulk acoustic resonator (TFBAR) were fabricated with several patterns to investigate the influence on their frequency responses. The highly c-axis oriented ZnO films were deposited on silicon substrates by reactive RF magnetron sputtering. The optimal two-step deposition temperature for ZnO films is 100 ¢J, which is obtained by means of SEM AFM, and XRD analysis. According to the experimental results, it leads to good resonance responses as TFBAR filters are fabricated with the patterns of large resonance area, two stages and the ratio of shunt/series resonance area is equal to two. Herein, conventional thermal annealing (CTA) was adopted to improve the frequency responses of TFBAR filters. Because CTA treatment can release stress and improve surface roughness of ZnO and Pt films, it enhances the frequency responses of TFBAR filters. The optimal CTA treatment temperature for TFBAR filters is 400 ¢J. Finally, TFBAR filters show the good performances with insertion loss of -8.138 dB, band rejection of 10.9 dB and bandwidth of 37.125 MHz.

This study describes the design and fabrication of dual-mode film bulk acoustic resonator (TFBAR) devices to construct wafer level T-ladder type filters. Reactive radio-frequency (RF) magnetron sputtering method was used to deposit c-axis- tilted ZnO piezoelectric thin films. The piezoelectric ZnO thin films were deposited by a two-step method at room temperature with off-axis. In this investigation, off-axis distance was varied to determine the optimal growth parameters of the tilted piezoelectric thin film. The SEM and XRD analysis reveal that ZnO thin films deposited at off-axis distances of 35 mm yielded a highly textured and sufficiently-tilted ZnO piezoelectric layer for dual-mode TFBAR. Additionally, the ZnO piezoelectric layer with off-axis distances of 35 mm exhibited enhanced competitive growth, and had a c-axis-tilted angle of 5¢X. To explore the relationship between the c-axis-tilted angle and the dual-mode resonance frequency responses (fL and fS) of TFBAR, two TFBAR devices were fabricated with ZnO c-axis tilted at 4.4¢X and 5¢X, respectively. The TFBAR device with 5¢X-tilted ZnO layer exists shear and longitudinal resonant modes. The center-frequency of longitudinal resonant mode is 2.2 times that of the shear resonant mode. The longitudinal mode is suitable for designing as a communication receiver (Rx) device at WCDMA band. On the other hand, the shear mode of TFBAR is suitable for EGSM-900 band. To optimize the characteristics, the filter was annealed by CTA treatment in 400 ¢J. For the frequency responses of the longitudinal wave, the insertion loss was upgraded from -5.77 dB without annealing to -4.85 dB as annealed, the band rejection was reduced from 13.57 dB to 12.65 dB, the bandwidth was broaden from 69.69 MHz to 73.12 MHz. On the other hand, for the frequency responses of the shear wave, the insertion loss was upgraded from -9.94 dB to -8.21 dB, the band rejection was reduced from 13.74 dB to 13 dB, the bandwidth was decreased from 28.13 MHz to 28.12 MHz.