Piezoelectric films for 100-MHz ultrasonic transducers

Piezoelectric aluminum nitride (AlN) and zinc oxide (ZnO) thin films were d eposited 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 (ceram ic) materials and components. A Krimholtz, Leedom, Matthaei (KLM) model was used to aid in the design of the ultrasonic transducer. Design criteria in cluded piezoelectric material properties and thickness, electrode material and thickness, substrate dielectric properties, active area diameter and ba ndwidth and sensitivity requirements. The preliminary designs consisted of Ni/AlN/Ni and Ni/ZnO/Ni on BK-7 glass and fused silica substrates. AIN was chosen because of its a high longitudinal wave velocity of 10 700 m/s and h igh dielectric strength of 20 MV/cm. Strongly oriented (002) AlN and ZnO co atings, up to 50 mum thick, were deposited onto transducer substrates. Tran sducers with strongly oriented (002) crystal orientation displayed the high est longitudinal/shear (L/S) wave ratio of 19.6 dB. Crystalline structure w as found to depend primarily on substrate temperature, substrate placement, and substrate material. Substrate temperatures ranged between 100 and 400 degreesC. Strongly (002) oriented AlN films were deposited at substrate tem peratures as low as 100 degreesC. The L/S ratio decreased with the onset of the (103) and other crystal orientations, which resulted from off-normal a datom flux incidence during deposition. Near normal incidence flux angles w ere needed to obtain the (002) crystal orientation. Substoichiometric ZnO f ilms were obtained at substrate temperatures below 300 degreesC. The transd ucers operated at frequencies between 50 and 100 MHz, with performance very close to that of the design. (C) 2000 Elsevier Science B.V. All rights res erved.