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 5μ mthick were deposited onto aluminum foils (50μ mthick) 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.