Detailed investigations of polymer/metal multilayer matching layer and backing absorber structures for wideband ultrasonic transducers

Detailed investigations of multilayer front and back matching layers and a novel backing absorber have been conducted, the detailed theory for which was presented in a previous paper. To design useful structures using the simple proposed equations, the material parameters of the constituent layers must be identified. Therefore, polyimide (for the matching layer) and adhesive-backed copper tape (for the absorber) were characterized by bonding them to polyvinylidene fluoride-trifluoroethylene P(VDF-TrFE) copolymer ultrasonic transducers and then applying a parameter-fitting algorithm to the resulting impedance data. A double matching layer was designed using an 11-μm PVDF (inner) and 23-μm copper (outer) multilayer construction in the first matching section followed by a 75μm polyimide layer as a typical quarter-wave- length material in the second (outermost) matching section. This structure was bonded to 330-μm PZT with air backing and the reflection waveform from a short pulse was captured. The FFT frequency response showed a 3.1-MHz bandwidth centered at 6.4 MHz, which agreed with the Mason's model analysis. The use of multiple layers of copper tape as a back- ing absorber was also investigated. At 3 MHz, the measured impedance was 4 MRayl, attenuation was 220 dB/cm, and velocity was 890 m/s, which agreed with the design theory. The 4-MRayl copper-tape structure was bonded to a back matching structure made from one layer of polyimide and one layer of brass (multilayer matching), and the effectiveness of the backing absorber made of 10 layers of copper tape on a 3-MHz transducer was confirmed.