Depth profile analyses with sub 100-nm depth resolution of a metal thin film by femtosecond - laser ablation - inductively coupled plasma - time-of-flight mass spectrometry

Highlights: • Homogenization unit for femtosecond-laser ablation (fs-LA), which is based on a 2 stage Fourier optical processing • High-resolution depth profiling by fs-LA – inductively coupled plasma – mass spectrometry • Achieved depth resolution: 47 nm • Adaption of the well-defined formalism described by Hofmann for the evaluation of the depth resolution (S. Hofmann, Surface and Interface Analysis, 1986, 8 (2), 87-89.) • Detailed discussion of detrimental effects leading to increasing depth resolution values for increasing depth of transition Herein we investigate the capabilities of near ultraviolet femtosecond laser ablation - inductively coupled plasma – time-of-flight mass spectrometry (NUV-fs-LA-ICP-TOFMS) for sub-100 nm depth profile analysis of a Cr/Ni metal thin film. For this purpose, the laser beam is guided through a homogenization scheme, which is based on aperture-assisted diffraction and reassembly of the beam by an optical lens. Using this set up, craters with well-defined cylindrical shapes are formed. Fluences between 0.6 and 1 J/cm² were applied, resulting in mean LA up-take rate of 27 nm/pulse. Discrepancies between LA up-take rates and depth resolutions were examined by a Gaussian depth resolution function, commonly used in SIMS or AES. In this study, depth resolutions ranging from 47 nm up to 80 nm were determined.