Film thickness dependence of silicon reduced LPCVD deposited tungsten on native oxide thickness

The thickness of tungsten films obtained by the silicon reduction of WF/sub 6/ depends strongly on the thickness of the native oxide. The thinner this oxide, the thinner a continuous tungsten film is formed, owing to a more homogeneous rate of reaction between WF/sub 6/ and silicon across the wafer surface. Tungsten depositions have been performed at 4000C on p-type (100) single crystal wafers with ''native'' oxides ranging from approximately 3 to 60A. At the lowest initial oxide thickness, films of approximately 100-200A are obtained, reaching 400 to 500A for a 12 to 15A native oxide. Above 40A, the tungsten film thickness approaches zero, thus marking the onset of selectivity. Auger elemental depth profiles of these samples are discussed and correlated with ellipsometric measurements of the native prior to tungsten deposition