Time Resolved Studies of Focused Ion Beam Induced Tungsten Deposition

ABSTRACTIn this study we investigate the nucleation and growth mechanisms of tungsten films processed by focused ion beam (FIB) induced chemical vapor deposition. For our investigation we use a 50 keV Ga+ ion beam focused on the substrate target and tungsten hexacarbonyl (W(CO)6) as precursor gas. Mediated by the substrate the energy of the impinging ions leads to the decomposition of the tungsten hexacarbonyl molecules adsorbed on the substrate into volatile parts and nonvolatile residues forming a metal deposit. Time resolved FIB secondary electron microscope imaging in combination with atomic force microscopy reveal first the formation of isolated nuclei and further their coalescence finally resulting in the formation of a contiguous metal layer. Despite the local impacts of the ion beam within the irradiated area of the substrate the localization of the nucleation spots is neither correlated to the spot centers nor to the scan path of the ion beam. After formation, the nanoscale tungsten nuclei preserve their positions and typical shapes during further deposition. Only after merging the nuclei to a contiguous tungsten layer, a further regime of growth sets on which is characterized by deposition of tungsten on a tungsten surface. In this regime the deposition process is determined by the total ion dose and the average current density the samples are subjected to. In this regime, deposition yields up to 3.5 atoms per incident gallium ion are achieved. The contiguous layer quality is determined by Auger electron analysis. The measured growth data were interpreted by adopting the analytic Ruedenauer Steiger approach mainly incorporating ion current density, precursor gas transformation rate, and ion induced sputtering. As a result, the critical ion current density, where ion sputtering exceeds deposition, was identified by the model. Because the model shows excellent agreement with the measurement it should be suitable for further survey concerning focused ion beam process development.