Investigation of Liquid Sensor Using a Dual-Mode Thin Film Bulk Acoustic Resonator (FBAR) Combined with Au/Cr Layers

This paper reports a dual-mode film bulk acoustic resonator (FBAR) device combined with Au/Cr layer to construct a liquid sensor. In order to obtain a liquid sensor with high sensitivity, the dual-mode FBAR with piezoelectric layer of ZnO thin film has been investigated. The dual-mode characteristics of FBAR appeared by off-axis deposition of ZnO thin film with the RF magnetron sputtering. The top and bottom electrodes consisted of a titanium (Ti) and a platinum (Pt) layers were deposited by dual-gun DC sputtering system. Owing to the chemical stability, the Pt layer is not only a perfect bottom electrode but also an etching stop layer. The gold layer (Au) is adopted for the formation of sensory area. In order to improve the adhesion between the Au layer and the Pt layer, a chromium (Cr) layer was deposited onto the backside cavity of the FBAR device before the Au layer was deposited by a DC sputter. The Au/Cr layer is used as the binding layer with analytes of DI water because of its hydrophile. The frequency response is measured using an HP8720 network analyzer and a CASCADE probe station. The resonant frequencies of longitude and shear modes in air appear at 2262.5 MHz and 1012.5MHz, whereas the resonant frequencies of longitude and shear modes in DI water are 2062.5 MHz and 955MHz, respectively.