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edfas.org ELECTRONIC DEVICE FAILURE ANALYSIS | VOLUME 22 NO. 3 28 IMPROVED SIGNAL DETECTION SENSITIVITY FOR HIGH RESOLUTION IMAGING IN SCANNING ACOUSTIC TOMOGRAPHY Hiroki Mitsuta, 1 Taiichi Takezaki, 1 Kaoru Sakai, 1 Kenta Sumikawa, 2 Masakatsu Murai, 2 and Kotaro Kikukawa 2 1 Research & Development Group, Hitachi Ltd. and 2 Hitachi Power Solutions Co. Ltd. hiroki.mitsuta.wv@hitachi.com EDFAAO (2020) 3:28-35 1537-0755/$19.00 ©ASM International ® INTRODUCTION For semiconductors or electronic devices used in consumer products, nondestructive inspection (NDI) of internal structures is required to improve yieldand reliabil- ity. Scanning acoustic tomography (SAT) has been widely used as a nondestructive inspectionmethod for detecting defects such as voids and internal delamination. [1] Today’s ever lighter, thinner, and smaller consumer products are resulting in semiconductors and other electronic devices with increasingly complex andscaled-downstructures and package formats. [2] For this reason, improving the spatial resolution in SAT is required to detect smaller defects at each bonding interface. The principle of generating ultrasonic images using an SAT apparatus is shown in Fig. 1. In conventional SAT, a pulse wave is irradiated to a specimen by an ultrasonic probe. As shown in Fig. 1, when an ultrasonic signal is transmitted into a specimen containing a layer of medium 1 and a layer of medium 2, reflection and transmission occur at the interface between the layers. The reflected signal intensity R is given by Eq 1. (Eq 1) Here, Z 1 and Z 2 are the acoustic imped- ances of medium 1 and 2, respectively, and I is the incident wave intensity. Acoustic impedance is highest for solids, lower for liquids, and even lower for gases. The acoustic impedance of gases is more than three orders of magnitude lower than that of solids. Due to this large difference in acoustic impedance, delamination and void areas reflect nearly 100% of the incident wave, providing a large contrast between bonded and defect areas in an ultrasonic image and making the detection of defects possible. The SAT apparatus acquires an inspection image by rapidly scan- ning the ultrasonic signal while displaying the reflected signal intensity or transmitted signal intensity at each point with grayscale shading. The method of measure- ment using reflected signals is called the “reflection method,” and the method using through-transmission signals is called the “transmission method.” In the reflec- tion method, the brightness values at the defect areas become larger than at the bonded areas. In the transmis- sion method, the ultrasonic signal is barely transmitted through the defect areas, therefore the brightness values are lower than those at the bonded areas. SPATIAL RESOLUTION IN SAT IMAGES An ultrasonic probe is a device that sends and receives ultrasonic signals by using a piezoelectric element to transform electrical signals intomechanical vibrations or Fig. 1 Principle of defect detection using scanning acoustic tomography.