Discrete microfluidics based on aluminum nitride surface acoustic wave devices

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 k2 of 0.24 % and phase velocity vp of 5,139 m/s) and a higher-frequency-guided wave mode at 157.3 MHz (with a k2 value of 0.22 % and vp 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 μL) 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