Dual-Fast Scanning Acoustic Microscopy (DFSAM): a high-speed dual-side ultrasound imaging system for thick sample inspection

Scanning Acoustic Microscopy (SAM) is a widely used nondestructive evaluation technique for imaging internal structures and detecting subsurface defects. In high-frequency SAM, ultrasonic attenuation limits the propagation distance of acoustic waves, reducing echo amplitude and defect detectability in thick or multilayer materials. This limitation becomes more pronounced when highly attenuating intermediate layers are present, preventing single-sided inspection from providing complete structural information. To address this acoustic challenge, this study proposes a Dual-Fast Scanning Acoustic Microscopy (DFSAM) system that performs simultaneous ultrasonic inspection from both the top and bottom surfaces of a specimen. By providing two complementary acoustic propagation paths, the proposed approach reduces the maximum ultrasonic propagation distance to any internal feature, improving echo detectability throughout the sample thickness. To implement this bidirectional inspection under high-speed operation, the scanning module employs two symmetrically arranged slider-crank mechanisms driven by a common crankshaft. This symmetrical configuration effectively counterbalances inertial forces and minimizes vibration that could degrade acoustic signal stability and image quality. Experimental vibration measurements and finite-element dynamic simulations confirm a significant reduction in vibration amplitude. Compared with existing Fast-SAM systems, the proposed DFSAM reduces the scanning time by nearly 50 % while maintaining high-resolution imaging performance. The acoustic measurement accuracy of the proposed system was validated using a standardized NAVSHIPS 1018 steel reference block, with depth and defect-size measurement errors below 0.2 %. The system was further evaluated through inspection of a power semiconductor chip, demonstrating reliable dual-sided visualization of internal structures under practical inspection conditions. These results demonstrate that the proposed DFSAM provides an efficient and reliable solution for nondestructive testing of thick and multilayer structures.