A nuclear microprobe analysis of laser treated aluminium implanted with antimony

Abstract

Laser processing of materials, being a rapid melt quenching process, quite often produces a surface which is far from being ideally smooth for ion beam analysis. We have used a 50 μm × 50 μm microbeam of 3.05 MeV He + (obtained from the Harwell nuclear microprobe) to overcome such problems during the RBS analysis of laser treated aluminium implanted with Sb (30 keV, 1.7 × 10 17 Sb + ions/cm 2). Laser treatment was done in air with single pulses (7 ns fwhm, peak energy density 2–5.7 J/cm 2) obtained from a Nd: glass laser operating in a TEM 00 mode. Scanning electron microscopy on the laser treated area shows a very smooth central region surrounded by an annular ring with coarseness gradually increasing radially outwards till the melt threshold is reached. A radial RBS scan with the microbeam reveals distinctly different Sb depth profiles for coarse and smooth regions. The coarse regions extending up to an absorbed energy density of 0.2 J/cm 2, show flat Sb concentration profiles up to depths ∼3.5 μm (uper limit determined by RBS with 4.8 MeV He + using the BARC Van de Graaf accelerator). Such anomalous depth profiles are shown to be incompatible with liquid phase diffusion and are interpreted as a consequence of convective effects. The smooth regions at higher absorbed power do not show such convective effects. The Sb profiles become broader with considerable Sb depletion near the surface. A detailed liquid phase diffusion analysis incorporating melt front motion and segregation effects has been performed. Effective distribution coefficient = 1 (as compared to the equilibrium value of ∼7), diffusion coefficient ∼5 × 10 3 cm 2/s and evaporation loss ⪅15% are parameters obtained. The use of a nuclear microprobe is shown to be indispensable for analysis of laser treated materials exhibiting a wide range of surface coarseness.