Surface acoustic wave (SAW) devices with 64 μm wavelength were fabricated on a zinc oxide (ZnO) film deposited on top of an ultra-smooth nanocrystalline diamond (UNCD) layer. The smooth surface of the UNCD film allowed the growth of the ZnO film with excellent c-axis orientation and low surface roughness, suitable for SAW fabrication, and could restrain the wave from significantly dissipating into the substrate. The frequency response of the fabricated devices was characterized and a Rayleigh mode was observed at ∼65.4 MHz. This mode was utilised to demonstrate that the ZnO/UNCD SAW device can be successfully used for microfluidic applications. Streaming, pumping, and jetting using microdroplets of 0.5 and 20 μl were achieved and characterized under different powers applied to the SAW device, focusing more on the jetting behaviors induced by the ZnO SAW.

To date, most surface acoustic wave (SAW) devices have been made from bulk piezoelectric materials, such as quartz, lithium niobate or lithium tantalite. These bulk materials are brittle, less easily integrated with electronics for control and signal processing, and difficult to realize multiple wave modes or apply complex electrode designs. Using thin film SAWs makes it convenient to integrate microelectronics and multiple sensing or microfluidics techniques into a lab-on-a-chip with low cost and multi-functions on various substrates (silicon, glass or polymer). In the work, aluminum nitride (AlN)-based SAW devices were fabricated and characterized for discrete microfluidic (or droplet based) applications. AlN films with a highly c-axis texture were deposited on silicon substrates using a magnetron sputtering system. The fabricated AlN/ Si SAW devices had a Rayleigh wave mode at a frequency of 80.3 MHz (with an electromechanical coupling coefficient k 2 of 0.24 % and phase velocity v p of 5,139 m/s) and a higher-frequency-guided wave mode at 157.3 MHz (with a k 2 value of 0.22 % and v p of 10,067 m/s). Both modes present a large out of band rejection of *15 dB and were successfully applied for microfluidic manipulation of liquid droplets, including internal streaming, pumping and jetting/ nebulization, and their performance differences for microfluidic functions were discussed. A detailed investigation of the influences of droplet size (ranging from 3 to 15 lL) and RF input power (0.25-68 W) on microdroplet behavior has been conducted. Results showed that pumping and jetting velocities were increased with an increase of RF power or a decrease in droplet size.