Nanocrystalline ZnO film layer on silicon and its application to surface acoustic wave-based streaming

A surface acoustic wave (SAW) device consisting of 1-6 microm-thick ZnO thin films deposited on Si wafer was designed, fabricated, and characterized in this study. Photolithographic protocols for interdigitated transducers (IDTs) and surface modification using fluoroalkylsilane are employed with the aim of droplet-based microfluidic actuations in bio-microsystems. A ZnO thin film was grown on a 4' silicon wafer with c-axis orientation, an average roughness of 11.6 nm, and a small grain size of 20 nm. It was found that the resonant frequencies (Rayleigh and Sezawa modes) of SAW devices move to a lower frequency range as the thickness of the ZnO thin films increases. Through the silane surface modification, a hydrophobic surface with a contact angle of 114 degrees was obtained. Finally, liquid streaming by acoustic wave was demonstrated by observing the actuation of SiO2 microparticles in a microfluidic drop.