INVESTIGATION OF SURFACE SPUTTERING AND IONIZATION PROCESSES UNDER NON-REACTIVE LIGHT ION IRRADIATION: TOWARDS 4D SIMS IMAGING

The progressive trend to miniaturize samples presents a challenge to materials characterization techniques in terms of both lateral resolution and chemical sensitivity. The latest generation of focused ion beam (FIB) platforms has allowed to advance in a variety of different fields, including nanotechnology, geology, soil, and life sciences. State-of-the-art ultra-high resolution electron microscopy (EM) devices coupled with secondary ion mass spectrometry (SIMS) systems have enabled to perform in-situ morphological and chemical imaging of micro- and even nanosized objects to better understand materials by studying their properties correlatively.
However, SIMS images are prone to artefacts induced by the sample topography as the sputtering yield changes with respect to the primary ion beam incidence angle. Knowing the exact sample topography is crucial to understand SIMS images. Moreover, using non-reactive primary ions (Ne+) produced in a gas field ion source (GFIS) allows to image in SIMS with an excellent lateral resolution of < 20 nm, but it comes with a lower ionization probability compared to reactive sources (e.g., Cs+) and due to small probe sizes only a limited number of atoms are sputtered, resulting in low signal statistics.
This thesis focused first on taking advantage of high-resolution in-situ EM-SIMS platforms for applications in specific research fields and to go beyond traditional correlative 2D imaging workflows by developing adapted methodologies for 3D surface reconstruction correlated with SIMS (3D + 1). Applying this method to soil microaggregates and sediments allowed not only to enhance their visualization but also to acquire a deeper understanding of materials’ intrinsic transformation processes, in particular the organic carbon sequestration in soil biogeochemistry.
To gain knowledge of the influence of the topography on surface sputtering, using model samples the change of the sputtering yield under light ion bombardment (He+, Ne+) for different ranges of incidence angles of the primary ion beam was studied experimentally. This data was compared to Monte Carlo simulation results and fitted with existing sputtering model functions. We showed thus that these models developed and studied for heavier ions (Ar+, Cs+) are also applicable to light ions (He+, Ne+). Additionally, an algorithm used to correct SIMS images with respect to topographical artefacts resulting from local changes of the sputtering yield was presented.
Finally, the contribution of oxygen on positive SI yields was studied for non-reactive primary ions (25 keV Ne+) under high primary ion current densities (up to 10^20 ions/(cm2 ∙ s)). It was shown that in order to maximize and maintain a high ionization probability oxygen needs to be provided continuously to the surface. Secondary ion signal enhancement of up to three orders of magnitude were achieved for silicon, opening the doors for SIMS imaging at both highest spatial resolution and high sensitivity.