Capabilities of Femtosecond Laser Ablation Inductively Coupled Plasma Mass Spectrometry for Depth Profiling of Thin Metal Coatings

Abstract

The capabilities of ultraviolet femtosecond laser ablation inductively coupled plasma mass spectrometry (UV-fs-LA-ICPMS) for depth profile analysis of thin metal coatings were evaluated. A standard sample consisting of a single Cr thin layer of 500 nm +/- 5% on a Ni substrate was used. A fast washout was obtained by a high-efficiency aerosol dispersion ablation cell (V approximately 1 cm3), which allowed single-shot analysis with increased depth resolution. Laser ablation was performed in helium at atmospheric pressure conditions. A laser repetition rate of 1 Hz and low laser fluence (<0.5 J/cm2) were used. Very low ablation rates (<10 nm/pulse) were determined by atomic force microscopy (AFM). Information about the crater geometry and morphology was investigated using scanning electron microscopy and AFM. The depth resolution, calculated via the maximum slope of the tangent in the layer interface region, was smaller than 300 nm. Our data indicate that UV-fs-LA-ICPMS represents a powerful combination of high lateral and depth resolution for the analysis of thin metal coatings. Moreover, an overall ion yield, defined as the ratio of detected ions and ablated atoms, of approximately 5 x 10-5 was estimated for the chromium layer under the operating conditions chosen. The absolute amount of ablated material per laser pulse was approximately 1 pg, which corresponds to a detection limit of 180 microg/g.

Critical revision of GD-MS, LA-ICP-MS and SIMS as inorganic mass spectrometric techniques for direct solid analysis

“…Whereas TOF mass spectrometers allow the simultaneous sampling and fast sequential detection of ions (20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30), the acquisition speed of quadrupole and sector field mass spectrometers are limited by the number of isotopes of interest or the required magnet jumps to reach all the isotopes. Developments in sector field (fast field generator) improved the acquisition speed for transient signals, with similar capabilities as known for quadrupole instruments.…”

“…LA-ICP-MS allows trace and ultra trace element analysis at high lateral and depth resolution (few mm and hundreds of nm, respectively). 25,26 Moreover, the sampling process is not limited to the physical and chemical properties of the materials of interest and, therefore, a wide variety of samples such as conducting, non-conducting, hard, soft, and coated solids can be directly analyzed. [27][28][29][30] All the samples just need to fulfil one general requirement; they must be kept in a stable form (e.g.…”

“…Therefore, fs-laser sampling is able to provide chemical information with high spatial resolution and sensitivity. 26 However, a pure comparison of thermal relaxation times does not take into account the correlation of heat diffusion and evaporation rate of molten material, which was recently found to be strongly fluencedependent.…”

Inorganic mass spectrometry as a tool for characterisation at the nanoscale

“…Despite the Gaussian laser beam profile employed in such experiments, the first layers of such samples could be well resolved in both cases. In addition, Pisonero et al [30] evaluated the capabilities of fs-LA-ICP-quadrupole MS (QMS) for depth profile analysis of metallic coatings (500-nm Cr layer on a Ni substrate) using a special low-volume ablation cell. The use of low laser fluences (approximately 0.4 J cm −2 ) together with a low laser repetition rate (1 Hz) provided very low LA rates (less than 6 nm per pulse), which corresponded to only approximately 60 atomic layers per shot.…”

“…1 Crater profiles across the central part of the crater at increasing number of femtosecond laser shots (15, 30, 45, 75, 90, 105, 130 and 150). (From [30]; reproduced with permission of the American Chemical Society) 34]. The idea behind NF-LA consists in the formation of radiation intensity singularities by means of very small conductive objects (e.g.…”