Annealing Effect on Structural, Functional, and Device Properties of Flexible ZnO Acoustic Wave Sensors Based on Commercially Available Al Foil

This paper reports post-annealing of zinc oxide (ZnO) films on flexible foil substrates in order to improve the functional and acoustic wave sensing performance. ZnO films of 5
$\mu \text{m}$
thick were deposited onto aluminum foils (50
$\mu \text{m}$
thick) using magnetron sputtering and then annealed in air at different temperatures between 300 °C and 500 °C. Effects of post-annealing on structural, optical, and device properties of the ZnO films and ZnO/Al foil acoustic wave devices were investigated. A temperature of 350 °C was identified as the optimized annealing temperature, which resulted in good light transmission, improved crystallinity, reduced film stress/defects, and increased amplitude of reflection signals of both Lamb and Rayleigh waves. The annealed ZnO/Al acoustic wave devices demonstrated a large temperature coefficient of frequency and a good linearity, revealing the potential for precise temperature sensing.