High attenuation structured porous silicon backing for high-frequency ultrasound transducer applications

The use of silicon wafer as backing of high-frequency ultrasonic transducer is a solution that allows electronics’ integration on the wafer. Nevertheless, the acoustic attenuation of silicon is too low to meet the requirements of this transducer constitutive element. To significantly increase this acoustic attenuation, a double step process on the rear face of the silicon wafer is proposed. First, square-base micro-pyramids were etched into the silicon and distributed in a staggered pattern. Next, porous silicon layer covering micro-pyramids was formed. These multi-structured silicon wafers deliver high acoustic attenuation (26 dB at 25 MHz). These substrates were used to manufacture high-frequency single element transducers. For this purpose, a thin film of piezoelectric copolymer poly(vinylidene fluoride-co-trifluoroethylene) with electrodes were bonded to the opposite side of the structured substrate. Electroacoustic responses measured at the focal distance on a metal plane target, allow to deduce amplitude difference between the two first echoes (from the back side of the structured silicon) at 24 dB. The center frequency is at 26 MHz and the fractional bandwidth at 86%. Finally, B-mode images of a tungsten wire (80 µ m) were successfully obtained and no additional echo was observed. These results suggest that these new compact transducers could be used for high-frequency medical imaging.