An epitaxial 3C-SiC/Si(100) system is of special interest for future microelectronics technology, since top performance SiC-based devices are created in a few-micrometer SiC film, simultaneously avoiding high costs of SiC wafer production. The buffer layer is an important aspect of epilayer growth, damping the lattice mismatch between two materials. We have chosen the Si(100) carbonisation technique of H. Nagasawa and K. Yagi /Phys. Stat. Sol. (b) 202 (1997) 335/ for buffer layer formation, due to its highest potential for commercial application. The silicon surface was carbonised by C2H2 diluted in H2 (ratio 1:10) under the pressure of ~10-1 Pa and in the temperature range from 700oC to 1100oC. The XPS investigation shows the onset of bulk SiC growth starting from 1000oC. This is an undesired scenario, because such process is accompanied by Si outdiffusion from the substrate and leads to a strained 3C-SiC/Si(100) interface with voids and micropipes /R. Scholz et al, Appl. Phys. A 64 (1997) 115/, thus producing rough morphologies of the buffer layer and subsequently of the 3C-SiC film itself. On the contrary, we observe a mild carbonisation in the range of 800oC to 900oC, where the SiC thickness is of the order of 1 monolayer (ML) - as judged from the XPS data. This scenario corresponds to an ideal buffer layer, namely where C atoms penetrate several ML deep into the Si substrate, while a completed ML of SiC is formed on its surface. This would be an ideal template for growing an epitaxial 3C-SiC film, with a smooth transition between two different lattice constants. We detect a small amount of O impurities in our buffer layers, coming from residual atmosphere of the growth chamber and from the contact with the environment. Their influence on the main 3C-SiC film, determined by AFM investigation of the surface topography, is determined by correlating the O concentration with the quality of the films grown on the corresponding buffer layer.