Mechanistic Aspects of Alkylchlorosilane Coupling Reactions

Toluene solutions of monochlorosilane and trichlorosilane coupling agents are shown to react differently with hydrated and dry silicon surfaces. For typical hydrated surfaces produced by piranha (H2O2 + H2SO4) etching, the saturation coverages of octadecyldimethylchlorosilane (ODMS) (one reactive Si−O bond) are approximately a third that of octadecyltrichlorosilane (ODTS) (three reactive Si−O bonds). In contrast, for samples vacuum baked at 400 °C, the saturation coverages of ODTS and ODMS are comparable. These data are consistent with coupling reactions limited to the surface hydroxyl groups. The ODTS saturation coverage on vacuum-fired silicon samples can be increased by soaking in water followed by a second ODTS exposure. From X-ray photoelectron spectra (XPS) and secondary ion mass spectra (SIMS) obtained as a function of solution exposure time, saturation coverage is shown to take at least 2 h and to be limited by the concentration of surface hydroxyl groups; a direct correlation is seen in SIMS between the exposure dependent gain in the surface alkyl signal and the loss of the SiOH signal. The coefficient of friction of silicon (piranha etched) is reduced by an order of magnitude by saturation coverages of ODTS, but continued rubbing for over 50 cycles at a normal loading of 245 mN results in decay of the friction coefficient to the value appropriate for uncoated silicon. These results as well as others from the literature are discussed in this paper in terms of a mechanism for the reaction of alkylchlorosilanes with silicon.