Investigation of Rayleigh wave and Love wave modes in [formula omitted] ZnO film based multilayer structure

Abstract

Piezoelectric ZnO film-based multilayer structures with versatile surface acoustic wave (SAW) modes are promising for use in the cutting-edge lab-on-chip-level bio-sensing applications which are desired to integrate sensing and microfluidics functions on the same miniature chip. This paper investigates a multilayer structure of SiO2/ZnO112¯0/r-sapphire with the feasibility of Rayleigh and Love wave modes along the two orthogonal directions. Theoretical calculations are performed to analyze different SAW modes and to optimize the structure parameters accordingly. ZnO is prepared by magnetron sputtering and its optimal fabrication parameters of sputtering pressure of 0.2 Pa, pure oxygen ambient gas and substrate temperature (500 °C) are determined towards the preferred 112¯0 orientation. The crystallographic characteristics and surface morphology of the fabricated ZnO film are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and atomic force microscopy (AFM). The surface vibration trajectory of the two modes both are measured using a laser vibrometer. It is verified the SAW mode parallel to the c-axis (0002) is a Rayleigh wave while that along 11¯00 is an SH-SAW/Love wave. As the SAWs encounter a PDMS cell filled with water on the film surface, the Love wave shows an insignificant acoustic damping of less than 2 dB and a reliable transmission response is maintained. In contrast, the acoustic energy of the Rayleigh wave based devices mostly radiates into liquid accompanied by an insertion loss of 13 dB and leading to obvious streaming.