Assmann, W.

Krenner, Hubert J.

Watje, K.

Wixforth, Achim

English

OPUS Augsburg

– 79 –Piezoelectric ZnO Films Deposited by RF Magnetron Sputtering forMicrofluidic ApplicationsK. Wätje a, H.J. Krenner a, A. Wixforth a, and W. Assmanna Lehrstuhl für Experimentalphysik I, Universität AugsburgSurface acoustic waves (SAW) on piezoelectric substratesfind widespread application for high frequency signal pro-cessing applications. Recently, SAW induced streaming foractuation and agitation of smallest amounts of fluids hasattracted 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-sensorapplications. As an alternative, zinc oxide layers depositedby rf magnetron sputtering hold the promise of cheaperproduction and easier integration into the assembly.For a SAW driven setup it is particularly important tooptimize the polycrystalline piezoelectric film with respectto 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 necessaryto monitor the film composition to find suitable depositionparameters. Here, ZnO layers were deposited by rf mag-netron sputtering from a ceramic ZnO target. Fig. 1 showsthe composition of ZnO films grown at different temper-atures, a sputtering power of 250 Watt and a pressure of1.2 Pa. The inset shows the composition of ZnO layerson silicon deposited at room temperature in 1.2 Pa argonatmosphere with different sputtering power examined byERD analysis.100 200 300 400 500 600 7000,60,81,01,21,41,6100 200 3000,60,81,01,2 Ar Ar:O2 (75:25)O/Zn  Temperature [°C]sputtering Power [watt]room temperature O/ZnFig. 1: Composition of ZnO layers, measured with ERDA for filmsdeposited at different temperatures (P=const.=250 Watt) and differ-ent sputtering powers (T=const.=RT).At room temperature the composition seems to be rela-tively unaffected by the rf power. Additional oxygen (upto 30%) in the sputtering chamber shows no impact on thefilm composition. Additional XPS measurements of theAuger peak, LMM2 and valence band edge of Zn foundno indication of metallically bonded Zn atoms within theXPS detection limit. Therefore, the films are chemicallyidentical to pure ZnO. As outlined in Fig. 1, sputteringat higher substrate temperatures leads to a decrease ofthe oxygen-zinc ratio. This might be due to the higherdesorption probability of the oxygen in the layer surface.Adding oxygen to the sputtering gas prevents a decrease ofthe oxygen-zinc ratio at higher temperatures to maintainthe correct stoichiometry. To excite SAW metallic IDTare defined on the top of the ZnO layer by a lift off pro-cess. Together with the sound velocity vSAW of the layerand the IDT layout, the resonance frequency f is givenby f = vSAW · λ−1. The wavelength λ equates two timesthe finger spacing of the IDT. For the mixing experimentin Fig. 2, stoichiometric ZnO layers on glass substrateswere deposited at 250 Watt in pure Argon at RT. Filmswere structured with tapered IDT and a finger pair dis-tance from 23.0-26.0 µm. Water droplets with a volume of1 µl were pipetted onto the IDT. To visualise the inducedstreaming, a small amount of polystyrol beads was addedto the droplet. As the diameter of the beads is 4.6 µm, thediffusion constant is very low (1 · 10−13m/s), and can beneglected and no stirring in the droplet would take placewithout SAW induced streaming . Fig. 2a and b show twoimages taken at different agitation frequencies. This mix-ing experiment was performed directly on the IDT withan amplitude power of 16 dBm. As the SAW- velocity isconstant, a higher frequency requires lower finger- pair dis-tances. Therefore the SAW-path migrates from the rightside (Fig. 2a) to the left side of the IDT, where the dis-tance of the finger-pairs is narrower (Fig. 2b) and thestream lines follow accordingly as indicated by the arrows.98MHz 110MHzFig. 2: Mixing experiment on a ZnO film with a tapered IDT atdifferent frequencies.This experiments clearly demonstrates that the fabri-cated ZnO films are a suitable substrate for SAW drivenmicrofluidic devices.References[1] A. Wixforth, Superlattice and Microstructures 33 (2004) 389[2] T. Frommelt et al., Phys. Rev. Lett. 100 (2007) 34502

Piezoelectric ZnO films deposited by RF magnetron sputtering for microfluidic applications

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Piezoelectric ZnO films deposited by RF magnetron sputtering for microfluidic applications

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