The heterostructure ZnO/Al/SiO2/Si fabrication for piezoelectric SAW and BAW transducers and sensors


Zinc oxide (ZnO) thin films were deposited by pulsed laser deposition technique for making ZnO/Al/SiO2/Si Bulk and Surface Acoustic Wave (BAW and SAW) transducers. The aim is to improve the piezoelectric properties of the ZnO films, which are strongly linked to their texture and microstructure. ZnO films with a wurtzite structure were deposited on the Al electrode layer, at different substrate temperatures, within the 100-500°C range. The characterization by X-ray diffraction showed that the deposited Al electrode was (111) oriented and that the ZnO film has a c-axis (002) preferential orientation. The scanning electron microscopy of the ZnO films on Al/SiO2/Si substrate showed an evidence of compact grains with a honeycomb-like structure on surface and a columnar structure on cross-section appeared after a sequential transition of amorphous to non-oriented to crystalline phase. The results indicated that the optimum substrate temperature is of 300°C and the electrical input impedance measurement on the Al/ZnO/Al BAW structure showed a piezoelectric response, where the fundamental resonance mode was located at 160 MHz, with an electromechanical coupling coefficient Keff equal to 3.2%. The best piezoelectric response is obtained at the fourth (4th) thickness mode vibration, located at the frequency of 636.3 MHz and having Keff of 9.10%.