Integrated ultrasonic piezoelectric transducers for medical applications

Recent development trends in piezoelectric devices are towards smaller size, higher resonant frequencies and a low driven voltage. For high-frequency transducers in medical imaging applications, thin (i.e.<50 mum) piezoceramic elements are necessary. These are usually produced by lapping and machining, however the elements tend to chip and break, and this is a major problem with joining the element to the backing. This problem might be avoided by an integrated device with a thick piezoelectric layer on a suitable substrate that may also serve as a backing. To provide this function, Pb(Zr,Ti)O-3 (PZT) ceramics were chosen for the substrate, theoretical density of PZT is close to 8000 kg/m(3). PZT thick films with a thickness around 40 mum were succesfully processed on gold-coated alumina and gold-coated PZT substrates. To lower the firing temperature of the PZT layers to 800-900degreesC the composition was modified with lead germanate (PGO) that forms a low-temperature liquid phase. Films with 80-90% of theoretical density and adequate dielectric properties were prepared by sintering PZT thick films at 800degreesC. Film thickness and porosity were estimated from SEM images, chemical composition was checked by EDS. The thick films on PZT substrate appear to be a good solution for transducer fabrication due to their relatively high attenuation (which limits the required backing thickness) and high acoustical impedance, very close to the thick-film acoustical impedance, which increases the transducer bandwidth. Moreover, the high value of thickness coupling factor allows a high transducer sensitivity to be obtained.