Nucleation and growth during tungsten atomic layer deposition on SiO2 surfaces

The nucleation and growth during tungsten (W) atomic layer deposition (ALD) on SiO2 surfaces was examined using Auger electron spectroscopy (AES) techniques. Tungsten ALD was performed on a hydroxylated silica (SiO2) surface using repeated exposures to disilane (Si2H6) and tungsten hexafluoride (WF6) in an ABAB… sequence at 573 K. The AES measurements revealed that W growth is slow during the first ∼10 AB reaction cycles using ∼105 Langmuir reactant exposures. Following this initial nucleation period, the tungsten film grew linearly at 2.5 A per AB cycle. The W and Si AES signals dramatically oscillated in magnitude versus the sequential Si2H6 and WF6 exposures in the linear growth regime. Comparison of the experimental AES signals with predicted AES signals assuming different growth mechanisms determined that tungsten grows in a layer-by-layer fashion following the Frank–van der Merwe mechanism. Quantitative modeling indicated that electron backscattering effects must be included to describe the absolute experimental AES signals during W ALD. Simulations also revealed that the mechanism of tungsten nucleation during the first ∼10 AB reactions cycles will affect the surface roughness of the tungsten films. More rapid nucleation of W ALD was achieved using a larger initial Si2H6 exposure of >1010 Langmuir, or by repeatedly probing the same location on the SiO2 surface with the AES electron beam.