Piezoelectric films for 100-MHz ultrasonic transducers

Abstract Piezoelectric aluminum nitride (AlN) and zinc oxide (ZnO) thin films were deposited by reactive magnetron sputtering for use in ultrasonic transducers in the 100- to 300-MHz regime. Pulse-echo ultrasonic transducers are being developed to image subsurface microstructure in metal and non-metal (ceramic) materials and components. A Krimholtz, Leedom, Matthaei (KLM) model was used to aid in the design of the ultrasonic transducer. Design criteria included piezoelectric material properties and thickness, electrode material and thickness, substrate dielectric properties, active area diameter and bandwidth and sensitivity requirements. The preliminary designs consisted of Ni/AlN/Ni and Ni/ZnO/Ni on BK-7 glass and fused silica substrates. AlN was chosen because of its a high longitudinal wave velocity of 10 700 m/s and high dielectric strength of 20 MV/cm. Strongly oriented (002) AlN and ZnO coatings, up to 50 μm thick, were deposited onto transducer substrates. Transducers with strongly oriented (002) crystal orientation displayed the highest longitudinal/shear (L/S) wave ratio of 19.6 dB. Crystalline structure was found to depend primarily on substrate temperature, substrate placement, and substrate material. Substrate temperatures ranged between 100 and 400°C. Strongly (002) oriented AlN films were deposited at substrate temperatures as low as 100°C. The L/S ratio decreased with the onset of the (103) and other crystal orientations, which resulted from off-normal adatom flux incidence during deposition. Near normal incidence flux angles were needed to obtain the (002) crystal orientation. Substoichiometric ZnO films were obtained at substrate temperatures below 300°C. The transducers operated at frequencies between 50 and 100 MHz, with performance very close to that of the design.