In situ monitoring of sputtered zinc oxide films for piezoelectric transducers

We report the application of in situ ellipsometry to monitor the growth by RF reactive sputtering of zinc oxide thin film piezoelectric transducers on silicon substrates. To establish the effect of substrate temperature on fi lm quality and piezoelectric activity, films were grown at substrate temper atures of 100-300 degrees C. Films of refractive index as high as 1.940 are achieved compared with refractive index of 1.99 for bulk zinc oxide crysta ls. Under optimum deposition conditions, transparent homogenous films of hi gh piezoelectric activity are formed from the initial stage of growth. A si mple single layer model is used to predict the in situ ellipsometry measure ment data for films deposited under optimum conditions, while a two layer m odel (duplex model) is needed to model the ellipsometry measurement data fo r rough films formed under conditions away from the optimum. Films grown at the optimum substrate temperature of 200 degrees C were smooth and of repr oducibly good quality, having high piezoelectric activity, Although the ref ractive index remained constant during growth, the rate of growth was not q uite linear, and was found to increase slowly with time according to a weak ly quadratic law. X-ray diffraction and pulse echo techniques were used to establish that films deposited close to 200 degrees C have good oriented st ructure and high piezoelectric activity. We have thus shown that a strong c orrelation exists between the optical properties measurable with a simple i n situ ellipsometer, and the appearance of highly oriented films of zinc ox ide having excellent piezoelectric activity. (C) 1999 Elsevier Science S.A. All rights reserved.