Zinc Oxide Piezoelectric Thin Films for Bulk Acoustic Wave Resonators

・ Abstract – Characteristics of Zinc oxide (ZnO) piezoelectric thin films have been investigated for thin film bulk acoustic wave (BAW) resonators with relationship to bottom electrodes. A Metal/ZnO/Metal/SiO2 membrane structure, which has secondary harmonics, was adopted for the BAW resonators. The ZnO thin films, showed clear c-axis orientation, were made by rf sputtering on bottom electrodes. Epitaxially grown ZnO thin films on Au and Pt electrodes on bottom electrodes is observed by high-resolution cross section transmission electron spectroscopy. However, ZnO thin film on Al shows three stages growth: c-axis oriented ZnO/none-oriented ZnO/amorphous ZnO/amorphous Al2O3 structure. ZnO thin film on Ni has disordered orientation because of surface roughness of Ni. We made two types of BAW resonators: with Au/Ti bottom electrodes for 200MHz BAW resonator and Pt/Al for 1.9GHz. Both BAW resonators with Au/Ti/ZnO/Au/Ti/SiO2 and Al/ZnO/Pt/Al have good resonant characteristics. Among Au (111), Ti (001) and ZnO (001) planes the lattice misfit is very small, and also among Pt (111), Al (111) and ZnO (001) the misfit is the same as the misfit among Au (111), Ti (001) and ZnO (001). That is one of the main reasons why the BAW resonators made in these experiments have superior characteristics. From these results electrical characteristics of BAW resonators strongly depend on crystal orientation of ZnO thin films, and they obey both crystallinity and surface morphology of bottom electrodes. Characteristics of BAW resonators using ZnO thin films were affected by conditions of the bottom electrodes.