Experimental validation of the SDTrimSP-3D code

Dedicated ion-beam experiments were performed to validate the dynamic 3-D version of the SDTrimSP code, SDTrimSP-3D. SDTrimSP is a Monte-Carlo code based on the binary collision approximation which can simulate the transport of energetic particles in matter. SDTrimSP-3D builds upon the pre-existing expansion of the original code to accommodate 2-D targets, SDTrimSP-2D. In this work, well-defined, 3-D targets whose surface morphology changed during ion bombardment were studied. These targets consisted of pillars of approximately 200 nm in height, 100 nm in diameter and spaced 200 nm, distributed in an orthogonal formation on a mirror-polished Si substrate. The pillars were made out of Si in the case of three samples and Ta in the case of one sample. Scanning Electron Microscopy (SEM) imaging of Focused-Ion-Beam (FIB) prepared cross sections prior to exposure were employed to construct a 3-D model of the target morphology for the corresponding SDTrimSP-3D simulations. The samples were exposed to a 5 keV Ar + beam to various fluence steps in SIESTA, a high-current ion source setup designed for well-defined sputtering experiments. The sample morphology was characterized at various positions before and after each fluence step via SEM imaging of FIB-prepared cross sections. Three exposure geometries were investigated: ion bombardment under incidence normal to the target surface, bombardment under 45°incidence collinear to the rows of columns, and bombardment under 45°incidence with the columnar structures rotated 15°relative to the ion beam. Cross-section images of the samples were compared with cross sections modelled by SDTrimSP-3D, providing excellent agreement with the experimental data at all fluence steps. In the case of the Ta sample, SDTrimSP-3D was able to correctly model the geometry of both the Ta columns and the Si substrate, thereby validating its use for fully 3-D targets of variable composition exposed under geometrically complex scenarios. With the aid of this new simulation tool, the evolution of arbitrary synthetic or measured surface morphologies and compositions and their impact on physical processes such as sputtering can now be calculated as a function of the impinging particle fluence.