Real-Time Interface Prediction During Laser Processing of Thin Film Layers by High-Resolution Femtosecond Laser-Induced Breakdown Spectroscopy

Abstract

A novel method to determine when the laser ablation crater reaches the interface between layers in real time during the high-resolution laser processing of multilayer thin films is reported. Femtosecond laser-induced breakdown spectroscopy (LIBS; wavelength = 343nm, pulse duration = 550fs) was adopted to predict the interface location during laser ablation with lateral (~ 3μm) and depth (~ 250nm) resolutions typically required for thin-film products in the industry. Rather than identifying the intersection of the intensity profiles of the upper and lower layers, the laser shot at which the LIBS signal intensity of the lower-layer material exceeded the noise level was monitored in the proposed method. A procedure for estimating the noise level excluding non-noise peaks from the measured LIBS spectrum was introduced. It was shown that the proposed method can accurately predict when the ablation crater reaches the interface between layers by using a single LIBS spectrum even when the LIBS signal intensity fluctuates highly owing to the low pulse energy to achieve the desired spatial resolutions. Averaging of the LIBS spectra was not necessary to reduce noise, as is generally the case with noisy LIBS data. The accuracy of the proposed method was verified experimentally by examining the cross-section of an ablation crater produced with a focused ion beam. This revealed that the center of the ablation crater reached the interface, and a shallow layer near the top of the lower film was ablated at the laser shot number predicted by the proposed method. © The Author(s), under exclusive licence to Korean Society for Precision Engineering 2025.