Surface Processes in CVD: Laser- and Low Energy Electron-Induced Decomposition of W(CO)₆ on Si(111)-(7×7)

Abstract

The decomposition of W(CO)₆ adsorbed on Si(111)-(7×7) using low energy electrons and ultraviolet photons has been investigated under ultrahigh vacuum conditions. This work is motivated by a desire to understand the mechanism for laser- and electron-assisted chemical vapor deposition (CVD) of tungsten using volatile coordination complexes and to specifically understand the role of the surface in these processes. Both electron stimulated and photo-assisted decomposition of the adsorbed W(CO)₆ are observed. No thermal decomposition of the W(CO)₆ occurs under the conditions of these experiments, based on independent temperature programmed reaction experiments, ruling out the possibility of laser- or electron-induced heating as the cause of decomposition. Furthermore, the interaction of the W(CO)₆ with the Si(111)-(7×7) surface is shown to be exceedingly weak based on the fact that the desorption energy is 9.46 ± 0.77 kcal/mol. Desorption of CO is induced during both ultraviolet photolysis and electron bombardment. Carbon monoxide is exclusively evolved during ultraviolet photolysis: no W-containing fragments are desorbed. During electron bombardment, a small amount of the W(CO)₆ is desorbed, accounting for ∼10% of the desorption. In both cases, CO-containing W fragments remain on the surface after decomposition at low surface temperature. The remaining surface fragments do not undergo further photolysis at 308 nm but do react thermally. Competing desorption and dissociation of CO are thermally induced resulting in carbide and oxide impurities in the deposited material. The fact that strongly bound W(CO)_x fragments are trapped on the surface is proposed as a limiting factor in the purity of tungsten deposits using the decomposition of W(CO)₆.