Through-Transmission Scanning Acoustic Tomography Using Capacitive Micromachined Ultrasound Transducer

We present the first through-transmission images in scanning acoustic tomography (SAT) using a capacitive micromachined ultrasound transducer (CMUT) as a receiving probe. A CMUT cell with a narrow gap of 30 nm was used to obtain high receive sensitivity. A 30-MHz receiving CMUT probe with a -3-dB fractional bandwidth of 109 % was used in the through-transmission technique of SAT. In the imaging of artificial linear voids formed in a Si wafer using a 50-MHz piezoelectric transmitting probe, the lateral resolution was approximately 65 μm, In the reception of the transmitted pulse through a 2-mm thick acryl plate, the signal-to-noise ratio of the CMUT probe was 1.7 times higher than that of a 25-MHz piezoelectric one. In the imaging of a 2.3-mm thick ball grid array (BGA) package using the 50-MHz piezoelectric transmitting probe, the CMUT probe was more sensitive than the piezoelectric probe by 10 dB in the frequency range of 20 to 30 MHz. The resolution of the CMUT probe was higher than that of the piezoelectric one.