Femtosecond laser ablation

Abstract

Femtosecond laser ablation was investigated as a solid sampling method for elemental chemical analysis. In comparison to the sampling with longer laser pulses, two aspects could be improved by using ultrashort pulses: elimination of the elemental fractionation from the ablation crater, which is necessary for an accurate quantitative analysis, and better control of the material removal (especially for metals), which increases the spatial resolution of microanalysis. Basic aspects of ultrashort laser ablation were studied using laser induced fluorescence (LIF), laser induced breakdown spectroscopy (LIBS) and laser ablation time of flight mass spectrometry (LA-TOF-MS). The results of the LIF study of plasma expansion fit to the spherical expansion model. The spectroscopic studies of plasma emission and determination of the excitation temperature served to find suitable conditions for quantitative elemental analysis by LIBS. The highly energetic ions (10 keV) that antecede the plume expansion were observed under high vacuum conditions in the TOF spectrometer during ablation of metals and semiconductors. Measurements with additional acceleration indicate high ionisation states.The elimination of the elemental fractionation was demonstrated in LIBS and fs laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) experiments on metals, alloys and glass samples. The in-depth profiling of thin layered structures was proposed as the most promissing application of the femtosecond laser ablation in chemical analytics. Rapid analysis of multilayered samples with depth resolution better than 100 nm and lateral resolution of 10-50 µm is feasible. A homogeneous laser beam intensity profile and a high pulse-to-pulse energy stability are the prerequisites for a successful in-depth profiling.