Determination of depth resolution from measured sputtering profiles of multilayer structures: Equations and approximations

UV post-ionization laser ablation ionization mass spectrometry for improved nm-depth profiling resolution on Cr/Ni reference standard

RATIONALE

Laser ablation combined with mass spectrometry forms a promising tool for chemical depth profiling of solids. At irradiations near the ablation threshold, high depth resolutions are achieved. However, at these conditions, a large fraction of ablated species is neutral and therefore invisible to the instrument. To compensate for this effect, an additional ionization step can be introduced.

METHODS

Double-pulse laser ablation is frequently used in material sciences to produce shallow craters. We apply double-pulse UV femtosecond (fs) Laser Ablation Ionization Mass Spectrometry to investigate the depth profiling performance. The first pulse energy is set to gentle ablation conditions, whereas the second pulse is applied at a delay and a pulse energy promoting the highest possible ion yield.

RESULTS

The experiments were performed on a Cr/Ni multi-layered standard. For a mean ablation rate of ~3 nm/pulse (~72 nJ/pulse), a delay of ~73 ps provided optimal results. By further increasing the energy of the second pulse (5-30% higher with respect to the first pulse) an enhancement of up to 15 times the single pulse intensity was achieved. These conditions resulted in mean depth resolutions of ~37 and ~30 nm for the Cr and Ni layers, respectively.

CONCLUSIONS

It is demonstrated on the thin-film standard that the double-pulse excitation scheme substantially enhances the chemical depth profiling resolution of LIMS with respect to the single-pulse scheme. The post-ionization allows for extraordinarily low ablation rates and for quantitative and stoichiometric analysis of nm-thick films/coatings.