August_EDFA_Digital

edfas.org 31 ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 22 NO. 3 Fig. 3 CMUT receiving probe for through-transmission SAT: (a) overview, (b) CMUT cell array, and (c) CMUT cells; and cross- sectional electron micrographs of CMUT cell: (d) overview, (e) gap center, and (f) gap edge. Fig. 4 Through-transmission images of 2.3-mm-thick BGA package taken with (a) CMUT and (b) piezoelectric probes, (c) close-up of contact area in (a), and (d) frequency spectra of pulses transmitted through BGA package. in Figs. 4a and b. The 0.6-mm-diameter solder ballswere fully resolved in the image taken with the CMUT receiving probe as shown in Fig. 4c. The frequency spectra of the received pulses are shown in Fig. 4d. The CMUT receiving probe was more sen- sitive than the piezoelectric one by 10 dB in the frequency range of 20 to 30 MHz. The higher sensitivity at high frequencies caused the CMUT probe to have a higher lateral resolution than the piezoelectric one. From this study, we found that high- resolution imaging was possible by using a CMUT receiving probe instead of a piezo- electric one in through-transmission SAT. This is due to the features of the broadband characteristic and high receiver sensitiv- ity of the CMUT. To further improve the resolution, the frequency of the ultrasonic signal used should be higher. In the high- frequency range, however, received signals generated by a CMUT are severely attenu- ated by parasitic components of the wire between the CMUT and the IV converter. To (a) (b) (c) (d) (e) (f) (a) (b) (c) (d)