The morphologies of the oxide surface and of the Si–SiO2 interface that form on special Si(111) substrates have been studied by atomic force microscopy (AFM). The substrates are totally free of atomic steps or have very low step density. Step-free regions are formed on patterned Si(111) by thermal processing. AFM scans of the same areas prior to oxidation, after oxidation, and after chemical removal of the oxide allow the relative roughnesses to be compared. The step structure of the Si(111) substrate is translated to the oxide surface even for SiO2 layers in the 10 nm range. The lack of significant displacement of the atomic steps at the Si–SiO2 interface indicates that the oxide grows by a layer-by-layer mechanism.

Surface and interface morphology may play an important role in the electrical performance of metal-oxide-semiconductor (MOS) devices with small characteristic dimensions. In previous work we showed how steps on the silicon surface influence the Si-SiO2 interface morphology and the outer oxide surface morphology following thermal oxidation [1]. The Si-SiO2 interface morphology is largely determined by the starting silicon substrate step distribution and atomic steps at the Si surface cause an inherent variation in oxide thickness after thermal oxidation. In the present study we report how roughness caused by increased interfacial step density may affect the electronic tunneling characteristics of an MOS device structure. To determine the extent to which the step morphology plays a role in the tunneling behavior of such devices, similar arrays of capacitors were fabricated on both Si surfaces with reduced step density and surfaces which had not undergone any special surface step removal treatment. The leakage currents due to tunneling for the two types of capacitors were measured and compared. Atomic steps cause an effective decrease in oxide thickness in those capacitors without reduced step density and this leads to increased leakage current.

Atomic force microscopy has been used to study the morphology of the oxide surface and the Si-SiO2 interface after oxidation of Si(111) surfaces that are either totally free of atomic steps or have well characterized low step density. The step-free areas were formed by thermally processing a patterned Si surface in which flat areas are enclosed by a square array of ridges; flow of the atomic steps into the surrounding ridge barriers produces a regular array of step-free areas each of which can be up to ∼50µm×50µm. Arrays of widely spaced steps (e.g. 5µm) can also be produced in the step-free areas. AFM scans of the same areas were taken prior to (dry) oxidation, after oxidation, and after chemical removal of the oxide. It was found that at an oxide thickness in the 5-13nm range, the initial step structure of the underlying Si substrates is translated through the oxide to the surface after oxidation with the oxide surface being somewhat rougher than the initial substrate. Furthermore, the initial step morphology of the substrate remains at the Si-SiO2 interface after etching away the oxide by HF. The interface roughness is less than that of the oxide surface. The results suggest that the initial oxidation of silicon proceeds in a ‘layer by layer’ manner and not through a preferential step-flow oxide growth mode.