□ A research team led by Prof. Chang Jin Ho from the Department of Electrical, Electronic, and Computer Science at DGIST (President Lee Kunwoo) has developed a screw lens-focused ultrasound transducer by etching a threaded structure onto the side of the lens to dramatically reduce internal noise, which degrades the quality of ultrasound imaging. This transducer enables clear detection of even ultra-fine defects inside semiconductor chips, and it is expected to contribute to the advancement of quality inspection technologies for high-reliability semiconductors used in electric vehicles and the aerospace industry.
□ With the recent growth of the electric vehicle and aerospace industries, the reliability of power semiconductors—which must withstand high voltages and heat—has become more critical than ever. Ultrasonic testing offers superior penetration capabilities compared to X-rays or optical inspection; thus, it is primarily used to detect microscopic cracks or empty spaces (voids) inside semiconductors without damaging the devices.
□ Semiconductor inspection equipment used in industrial sites primarily employs a method that transmits ultrasonic waves through a water column (the "waterfall" method). However, increasing the sensor size is limited due to space constraints. This has resulted in a persistent problem in which irregular "edge noise"—caused by ultrasonic waves reflecting and interfering with each other inside the small lens—significantly reduces image clarity.
□ Inspired by the wedge-shaped sound-absorbing structure found on the walls of an anechoic chamber (sound-absorbing room), which prevents sound reflection, Prof. Chang's team developed a groundbreaking solution to this problem. They densely engraved slanted, screw thread-shaped grooves onto the smooth side surface of the ultrasonic lens. As ultrasonic waves directed toward the side surface of the lens strike the inclined surfaces of the screw threads, they scatter in multiple directions, physically eliminating the source of the phenomenon where reflected waves converge to create noise.
□ Through computer simulations, the research team established optimal thread specifications (height: 500 μm, inclination angle: 30°) that offer excellent noise suppression and ease of fabrication, thereby presenting new standard guidelines for lens design. This technology can be easily and immediately applied to conventional compact industrial inspection equipment because it only requires machining the outer surface of the lens without altering the sensor's overall size or internal core components.
□ When the newly developed transducer was used to inspect automotive power module chips, microstructures and defects inside the chips (e.g., metal wires one-quarter the thickness of a human hair and microscopic bubbles, which were completely obscured by noise when using conventional sensors) were revealed with a clarity rivaling that of conventional computed tomography (CT) scans.
□ "This research is significant because it solved the problem of internal lens noise—the greatest challenge in designing ultrasonic sensors optimized for industrial inspection equipment—simply by machining the lens shape," explained Prof. Chang of the Department of Electrical, Electronic and Computer Science at DGIST. "This effectively lays the groundwork for the mass production of ultra-high-frequency ultrasonic transducers operating at 100 MHz or higher." He continued, "Through joint research with semiconductor manufacturers, we will conduct tests for various real-world process defects in the future to accelerate the development of next-generation non-destructive testing systems tailored to on-site needs."
□ This research was supported by the STEAM program of the Korean Government's Ministry of Science and ICT (MSIT) and the National Research Foundation of Korea, the InnoCORE program of the MSIT, and the AI Star Fellowship Support program of the Institute of Information & Communications Technology Planning & Evaluation (IITP). DGIST postdoctoral researcher Kang Sungwoo and integrated master's-Ph.D. student Park Hyunkyu participated as co-first authors in this research, and the paper was published in NDT & E International, a prestigious international journal considered the most authoritative in the field of non-destructive testing.
