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edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 22 NO. 3 34 cells. An unfocused 30-MHz CMUT probe with a cell array diameter of 2 mm was used as a receiver in a through- transmission SAT system. Through-transmission images of a 2.3-mm-thick BGA taken with the CMUT probe were clearer than those taken by a conventional piezoelectric one. This is because the CMUT probe was more sensitive than the piezoelectric one at a high frequency range. This experiment demonstrated that the broadband and highly sensitive CMUT probe is suitable as a receiving probe for through-transmission SAT. CMUT probes can also be used for the SAT reflection method to improve the resolution. In developing the signal processing technique using the pulse compression method, the transmission wave- form in consideration of wave distortion in the piezoelec- tric element was optimized. The ultrasonicmeasurement was performed using pulse waves and the pulse com- pression method for a measurement specimen that was formed by stacking three layers comprising a 60-μm-thick Si wafer and20-μm-thickpolyimide filmona 400-μm-thick Si wafer with processed holes as the artificial defects in the polyimide filmof the third layer. The defect-detection sensitivity was evaluated on the basis of the standard deviation and signal-to-noise ratio (SNR). The noise value was reduced by 66% when the pulse compression method was applied. Moreover, the SNR was increased with the pulse compression method compared with that of the pulse method. Given that defects can be detected with SNR = 2, it was found that 100-μm defects could be detected with the pulse wave method and 20-μm defects with the pulse compressionmethod. In this experiment, it was confirmed that the spatial resolutionwas improvedby applying the pulse compressionmethodby comparing the conventional method with the ultrasonic measurement, and it was confirmed that it was effective for the detection of minute defects. As shown in this article, hardware and software tech- nologies improved the spatial resolution of SAT images with high sensitivity at the high frequency region. The broadband characteristics of the developed CMUT probe proved effective for improving the performance of the pulse compression method. Future study will aim to achieve even higher sensitivity by applying the pulse compression method to the CMUT probe. ACKNOWLEDGMENT This article is based on the papers “Through- Transmission Scanning Acoustic Tomography Using Capacitive Micromachined Ultrasound Transducer” by Taiichi Takezaki, Masakazu Kawano, Shuntaro Machida, and Daisuke Ryuzaki and “High Sensitivity Ultrasonic Inspection Technique using Pulse Compression Method” by Hiroki Mitsuta and Kaoru Sakai, which were presented at the 2018 IEEE International Symposiumon the Physical and Failure Analysis of Integrated Circuits (IPFA 2018). REFERENCES 1. J.A. Khan, M. Mina, L. Udpa, and S.S. Udpa: “Analysis of Scanning Acoustic Microscopy Images of IC Chips,” Review of Progress in Quantitative Nondestructive Evaluation, 1995, Vol. 14, p. 915-922. 2. Yole Development, “Status of Advanced Substrates 2018: Embedded Dies & Interconnects, Substrate like PCB Trends, 2018, Market & Technology report . 3. J. Krautkrämer and H. Krautkrämer: Ultrasonic Testing of Materials, 1983, Springer-Verlag Berlin Heidelberg. 4. M. I. Haller and B. T. 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ABOUT THE AUTHORS Hiroki Mitsuta received bachelor’s and master’s degrees in physics from Tokyo University of Science, Japan, in 2010 and 2012, respectively. He joined Research & Development Group, Hitachi Ltd. in 2012. He is engaged in the development of nondestructive inspection equipment and signal processing technology for high sensitivity detection. Taiichi Takezaki received bachelor’s, master’s and Ph.D. degrees from HokkaidoUniversity, Japan, in 2002, 2004, and 2007, respectively, all in electrical and electronics engineering. He joined Research &Development Group, Hitachi Ltd. in 2007. His current research focuses on developing micromachined devices for ultrasound and photoacoustic imaging.