PLD elaboration of piezoelectric ZnO thin film for 540 MHz Al/ZnO/Pt bulk acoustic wave resonator

The ZnO thin films in Al/ZnO/Pt/Ti/SiO₂/Si Bulk Acoustic Wave (BAW) resonators are realized by a reactive Pulsed Laser Deposition (PLD) technique. It is of most interest to improve their piezoelectric characteristics. These later are intimately linked to the microstructure, the texture and the growth conditions. In this work, the piezoelectric characteristics of ZnO thin films have been investigated. Wurtzite ZnO thin films were prepared on Pt(111) at different substrate temperatures (100-500 °C). The 200 nm of Pt on 20 nm Ti were prepared by Electron Beam Deposition (EBD) technique at room temperature on 2 μm SiO₂ substrate. This SiO₂ film has been realized by Silicon wet thermal oxidation step at 1100 °C. The top electrode of the BAW was made by thermal evaporation of Aluminum; the piezoelectric film was confined in a circle of 1.8 mm radius. X-Ray Diffraction (XRD) characterization showed that the deposited Pt (bottom electrode) has (111) preferential orientation. It also showed that the ZnO films were c-axis (002) oriented. The Scanning Electron Microscopy (SEM) of the realized ZnO thin films showed evidence of compact grains with honeycomb-like structure on surface and evidence of columnar structure on cross-section. The measurements indicate that all substrate temperatures are suitable to obtain a good quality of ZnO, but T_s=300 °C is the optimum. In order to evaluate the piezoelectric properties of the BAW, measurements of the electrical input impedance and admittance have been performed. They showed evidence of piezoelectric response. The resonance frequency was obtained at 524.5 MHz and the anti-resonance frequency at 540.9 MHz. The electromechanical coupling coefficient K_eff² was evaluated to be 7.26 %, which is an indicator of a good piezoelectric response. From this measured values, the thickness d and the wave velocity V_p in the ZnO slab were de- uced to be respectively 6.15 μm and 6451.35 m/s and the piezoelectric constant e₃₃ was found to be equal to 1.21 C/m².