X-Ray Scattering from Silicon and Thin Films on Silicon

X-ray scattering techniques were used to study silicon wafers coated with hydrocarbon films that were either chemically bound or wetting the surface. Specular reflectivity (SR) studies of alkylsiloxane monolayers, formed by self-assembly of the relevant trichlorosilane precursor, revealed that the alkane chains are nearly all trans and oriented perpendicular to the surface. Grazing in-plane diffraction (GID) measurements of the in-plane structure showed that the monolayers possess only short range liquid-like order with correlation lengths of approximately 45A. SR indicated that incomplete alkylsiloxane monolayers are relatively uniform and smooth with a density similar to the complete monolayers but a reduced film thickness. In particular, there is no evidence for islands of molecules on the surface. GID studies found considerably more in -plane disorder than the complete monolayers and evidence of microscopic inhomogeneity. Off-specular reflection studies of the wetting of uncoated silicon wafers by cyclohexane demonstrated that for thin adsorbed films (less than about 50A) the liquid surface roughness is correlated with the substrate surface roughness, which is evident from interference in off-specular diffuse scattering near the specular condition. This interference is absent in the scattering from thicker films, indicating a decoupling of the substrate and liquid surface roughnesses. The results are explained in terms of a competition between the liquid surface tension and non-retarded van der Waals forces. The thickness of the adsorbed film was controlled by differentially heating the substrate above the temperature of the bulk liquid. For incompletely wet surfaces such as the methyl terminated alkylsiloxane monolayers, only very thin (~1A) adsorbed films form since the terminal methyl group inhibits complete wetting. The wetting of silicon surfaces coated with partial alkylsiloxane monolayers is a function of the alkylsiloxane coverage; low coverage surfaces are completely wet, while high coverage surfaces are incompletely wet, with the monolayer "swelling" due to the incorporation of cyclohexane. Diffraction from the Si(100)/SiO_2 interface is described. GID scattering indicates that the interface region has 2 x 1 periodicity with the most likely interface configuration a modified dimer structure.