Evolution of tungsten film deposition induced by focused ion beam

Abstract

Direct write metallization is an important approach for circuit modification and prototyping. We investigate the evolution of the chemical vapor deposition of tungsten induced by a 50 keV focused Ga+ ion beam. Time resolved imaging in combination with atomic force microscopy reveals that chemical vapor deposition of tungsten by focused ion beam proceeds via two clearly distinguishable regimes of layer growth. Deposition starts with the nucleation of nanoscale tungsten deposits scattered over the substrate surface. Despite local impacts of the ion beam within the irradiated area of the substrate the localization of the nucleation spots is not correlated to the scan path of the ion beam. The nanoscale tungsten particles preserve their position and typical shape during further deposition. Only after merging of the particles into a contiguous tungsten layer, does the second regime of growth characterized by deposition of tungsten on a tungsten surface set in. In this regime the deposition process is determined by the total ion dose and the average current density the sample was subjected to. Deposition yields up to 3.5 atoms per incident gallium ion are achieved. The layer quality is determined by Auger electron analysis, which shows fractions of Ga, C, Si and O in the W layer. Depth profiling by secondary ion mass spectroscopy showed the depth profiles of these constituents and confirmed the existence of a 50–100 nm thick transition zone between the tungsten layer and the substrate. Electrical resistivity of metal layers of 250 μ Ω cm and current densities up to 3.5×106 A/cm2 are measured by means of van der Pauw test structures. In order to give a concise description of the experimental findings the data were interpreted utilizing an analytic model that mainly incorporates the precursor gas coverage, precursor gas transformation cross section and ion induced sputtering. The critical ion current density, where ion sputtering exceeds the deposition, was identified by the model. Because the model shows excellent agreement with the measurement it should be suitable for a further survey of focused ion beam process development.