Microstructural control of ZnO films deposited by RF magnetron sputtering for ultrasound transducers

The aim of this study was to develop a deposition process using RF magnetron sputtering for the production of zinc oxide (ZnO) thin films on glass substrates. These ZnO films were to be used as the active piezoelectric element in very high frequency ultrasound transducers (> 300 MHz). In order to achieve piezoelectric activity the films had to be oriented with the c-axis of the ZnO grains perpendicular to the substrate surface. At the same time, a moderately high, at least 1 m=hr (17 nm=min) deposition rate was required for the production of practical devices. Prior to a full investigation into the sputtering parameters, an initial evaluation of the HHV Auto500 RF magnetron sputter coater system was performed. Using the original chamber configuration it was not possible to deposit ZnO at the required deposition rates. A modification of the growth chamber to allow a reduced target-substrate distance was successful in producing ZnO films at the required deposition rates. A systematic study into the deposition parameters and their effect on the ZnO film quality and deposition rates was then performed and it was found that strong c-axis oriented films could be deposited only when depositing at rates below 15 nm=min at a low substrate temperature (< 50 C). Depositions above this rate resulted in the growth of polycrystalline films. A two-step deposition process was designed to preserve c-axis orientation at high deposition rates up to 28 nm=min. The ZnO films were found to be highly strained due to inherent stress from the sputtering process. The deposition pressure was identified as the most critical deposition parameter for stress control. It was found that deposition above a critical pressure of 1:2 x10-² mbar was essential to prevent mechanical failure of the films. Post growth annealing was investigated and determined to be a viable technique to relax stress and improve the crystalline quality of the films. Finally a four-step deposition process was proposed to facilitate the growth of c-axis oriented ZnO films at relatively high deposition rates whilst minimising film stress