– 79 – Piezoelectric ZnO Films Deposited by RF Magnetron Sputtering for Microfluidic Applications

Surface acoustic waves (SAW) on piezoelectric substrates find widespread application for high frequency signal pro-cessing applications. Recently, SAW induced streaming for actuation and agitation of smallest amounts of fluids has attracted considerable attention in the field of microflu-idics. This streaming causes stirring in the fluid and in-duces mixing, even for very low Reynolds numbers [1,2]. Apart from single crystal substrates like LiNbO3, low cost, high quality alternatives are required for on-chip bio-sensor applications. As an alternative, zinc oxide layers deposited by rf magnetron sputtering hold the promise of cheaper production and easier integration into the assembly. For a SAW driven setup it is particularly important to optimize the polycrystalline piezoelectric film with respect to its composition, orientation and morphological unifor-mity. Stoichiometric ZnO is a perfect semiconductor, oxy-gen deficient ZnO is a transparent conductor. Even small-est amounts of impurities (e.g. Al) could lead to a con-ductivity and a short circuit of the interdigital transducers (IDT) which generate the SAW. Therefore, it is necessary to monitor the film composition to find suitable deposition parameters. Here, ZnO layers were deposited by rf mag-netron sputtering from a ceramic ZnO target. Fig. 1 shows the composition of ZnO films grown at different temper-atures, a sputtering power of 250 Watt and a pressure of 1.2 Pa. The inset shows the composition of ZnO layers on silicon deposited at room temperature in 1.2 Pa argon atmosphere with different sputtering power examined by ERD analysis