Depth profiling of multi-layer samples using femtosecond laser ablation

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Abstract
An all solid-state femtosecond laser (λ0 ∼775 nm, pulse duration ∼170 fs, maximum pulse energy ∼0.5 mJ) with a Gaussian beam profile was used for depth profiling of Cu–Ag and TiN–TiAlN multi-layers on silicon and iron substrates. Laser-induced breakdown spectroscopy (LIBS) in argon was used for characterisation of the Cu–Ag samples, while laser ablation in a vacuum with time-of-flight mass spectrometry (TOF-MS) was applied for the characterisation of the TiN–TiAlN samples. The thickness of the individual Cu and Ag layers was 600 nm. Each individual TiN and TiAlN layer was 280 nm thick. The LIBS experiment was performed in the pressure range 10–1000 mbar. Variation of the pulse fluence from 0.8 to 1.5 J cm−2 caused a change of the ablation rate from 15 to 30 nm per pulse. The first layers of Cu and Ag could be satisfactorily resolved by LIBS. In femtosecond laser ablation TOF-MS a lower fluence (about 0.3 J cm−2) than in LIBS could be applied. The TiN–TiAlN multi-structures were well resolved. The Gaussian-type beam of the femtosecond laser limited the contrast of the detected depth profiles in both schemes. The complementary sensing techniques enable study of technical and physical limitations in the use of femtosecond laser ablation.

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