Plane-wave-like laser ultrasonic inspection of multilayer delaminations using beam-expanded excitation and...

Multilayer structures are widely used in energy, aerospace, and microelectronic packaging owing to their high structural efficiency and functional integration. However, their reliability is often compromised by planar interfacial defects such as delaminations. Reliable nondestructive detection of these defects remains challenging, particularly under all-optical conditions, because conventional laser ultrasonic inspections typically rely on point-like thermoelastic excitation. This generates highly divergent bulk waves with oblique incidence, leading to weak interfacial reflections and complicated echo interpretation. In this work, a plane-wave-like laser ultrasonic inspection strategy enabled by beam-expanded excitation and migration-based reconstruction is proposed. To overcome the reduced optical energy density inherent in beam expansion, a reflective photoacoustic transducer is introduced to significantly enhance the optical-to-acoustic conversion efficiency while maintaining thermoelastic operation. The resulting near-plane longitudinal-wave illumination ensures stable, high-contrast reflections from planar interfaces. Subsequently, to compensate for the degradation of lateral resolution caused by the spatially extended excitation source, phase-shift migration (PSM) is employed to reconstruct the ultrasonic wavefield. Experimental validation on multilayer chip-like Cu-TIM-Si specimens containing circular and square delaminations demonstrates reliable reflection-mode defect detectability across different defect geometries. After PSM reconstruction, contrast-to-noise ratio (CNR) values above 12 dB and sizing errors within 0.2 mm are achieved. By synergizing physical wavefront control with reconstruction-based signal processing, this work provides a robust, all-optical solution for the reflection-mode inspection of layered structures.