Optimization of ZnO Piezoelectric Thin Films and Bottom Electrodes in Thin Film BulkAcoustic Wave Resonator

Abstract

A thin film bulk acoustic wave resonator (TBAR) has been fabricated using a ZnO piezoelectric thin film sandwiched by top and bottom electrodes on a SiO2 diaphragm by MEMS techniques. The elctrode/ZnO/electrode/SiO2 structure can be designed to cancel a temperature coefficient of frequency constant (TCF) by optimizing the ZnO/SiO2 thickness ratio, because the TCF of ZnO is negative, and that of SiO2 is positive. The ZnO thin film on the SiO2 shows a c-axis orientation by RF sputtering. The crystallinity of the ZnO thin film is influenced by surface conditions of under layer. ZnO thin films have been deposited on bottom electrode, that is Au/NiCr, Au/Ti and Au/Cr, respectively. The Au/Ti/ZnO/Au/Ti/SiO2 structure thin film bulk acoustic resonator shows the best resonant characteristics in this experiment. The resonant characteristics depend on the crystallinity of the ZnO thin film. The resonant resistance at 205MHz using a Au/Ti bottom electrode is about 8% less than that using a Au/Cr electrode. The X-ray diffraction result shows that the crystallinity of ZnO is greatly influenced by the crystallinity of the bottom electrode. The buffer electrode between an Au electrode and substrate give an influence on both the crystallinity of the ZnO thin film and the resonant characteristics through the Au electrode.