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 film quality and piezoelectric activity, films were grown at substrate temperatures 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 crystals. Under optimum deposition conditions, transparent homogenous films of high piezoelectric activity are formed from the initial stage of growth. A simple single layer model is used to predict the in situ ellipsometry measurement data for films deposited under optimum conditions, while a two layer model (duplex model) is needed to model the ellipsometry measurement data for rough films formed under conditions away from the optimum. Films grown at the optimum substrate temperature of 200 degrees C were smooth and of reproducibly good quality, having high piezoelectric activity, Although the refractive index remained constant during growth, the rate of growth was not quite linear, and was found to increase slowly with time according to a weakly quadratic law. X-ray diffraction and pulse echo techniques were used to establish that films deposited close to 200 degrees C have good oriented structure and high piezoelectric activity. We have thus shown that a strong correlation exists between the optical properties measurable with a simple in situ ellipsometer, and the appearance of highly oriented films of zinc oxide having excellent piezoelectric activity.