High-Resolution and Real-Time Interface Detection of Thin Multilayer Films Using Femtosecond Laser-Induced Breakdown Spectroscopy

Abstract

Laser-Induced Breakdown Spectroscopy (LIBS) is an advanced spectroscopic technique utilized for elemental analysis of materials. The technique involves directing a high-intensity laser pulse onto a material’s surface, inducing rapid energy absorption and generating a microplasma. As this plasma cools, it emits characteristic spectral emissions, which can be analyzed to determine the elemental composition of the material. LIBS is widely employed in various fields, including industrial quality control, environmental monitoring, and biomedical diagnostics, due to its rapid, non-contact nature, minimal sample preparation, and real-time analytical capabilities. However, conventional LIBS is inherently limited by spatial resolution and signal stability, particularly when applied to precision laser processing of thin film layers. Femtosecond LIBS (fs-LIBS) has emerged as a high-resolution alternative, significantly reducing thermal damage and enhancing spectral accuracy. This study explores the application of high-resolution fs-LIBS for real-time interface detection during the laser processing of thin film layers. In prior studies on interface detection using laser-induced plasma spectroscopy, there have been no reports of successful plasma signal acquisition or interface identification under laser processing conditions with a lateral resolution below 10 μm. This limitation is primarily attributed to the intrinsic difficulty of generating stable and analyzable plasma emissions when the ablation volume is extremely small. As the lateral resolution increases, the resulting plasma becomes highly confined and unstable, significantly hindering the reliability of emission-based detection methods. In the present study, stable plasma signals were successfully acquired using laser-induced craters with a lateral resolution of approximately 3 μm and a depth resolution of around 400 nm. This result demonstrates that the proposed method is highly applicable to the precision processing of multilayer thin films, particularly in applications where feature sizes are on the order of a few micrometers, such as in semiconductor and display panel manufacturing. Accurate detection of layer transitions within multi-layered structures is critical for advanced manufacturing processes, including semiconductor fabrication and optical coatings. Traditional interface detection techniques often lack the necessary spatial and temporal resolution for precise material removal, leading to inefficiencies and defects. In contrast, fs-LIBS facilitates real-time monitoring of laser-material interactions, enabling precise control over depth profiling and interface identification. The research systematically examines the influence of key laser parameters—including pulse duration, fluence, and repetition rate—on the accuracy of interface detection. A series of experiments were conducted using femtosecond laser pulses to process thin film layers while simultaneously capturing emission spectra to determine layer composition and thickness. Furthermore, this study introduces a novel real-time interface detection method based on high-resolution fs-LIBS. Unlike previous approaches relying on the crossover point of signal intensities between upper and lower layers, our method defines the interface as the point where the emission signal of the lower layer surpasses the noise threshold. To quantitatively identify noise regions, various statistical techniques—including signal classification, interquartile range (IQR), and confidence interval analysis—were employed. These methods enabled the effective separation of true signal from background noise and allowed for the reliable determination of signal occurrence. As a result, even under low pulse energy conditions with weak emission intensity, the interface could be detected without signal averaging, thereby demonstrating strong applicability to real-time processing environments. The findings demonstrate that fs-LIBS significantly enhances the resolution and accuracy of interface detection compared to conventional laser processing methods. The ability to detect layer transitions in real time improves process control, minimizes defects, and increases manufacturing efficiency. The results further underscore the potential of fs-LIBS in high-precision applications, such as microelectronics, aerospace, and biomedical engineering, where thin film processing plays an essential role. Ultimately, this research presents a novel approach to integrating high-resolution laser processing with real-time analytical techniques. By leveraging fs-LIBS, manufacturers can achieve superior accuracy in multi-layered material processing, contributing to advancements in next-generation fabrication technologies.