Nanometric SiC overlayer synthesis has been performed via an ultrahigh vacuum-compatible microwave electron cyclotron resonance plasma source. The H-2 plasma streaming onto a Si(001) substrate, whose temperature T(s) could be varied from room temperature to 850-degrees-C, activates and dissociates CH4 molecules. The films are characterized in situ by angle-resolved photoemission techniques. Without the H-2 plasma, no surface reaction of CH4 is observed with the Si irrespective of T(s) up to 850-degrees-C and exposures up to 10(6) L. H-2 plasma excitation leads to the rapid formation of a thin SiC overlayer in the whole T(s) range. For temperatures below a threshold of about 700-degrees-C where thermal interdiffusion between Si and C is negligible, the SiC overlayer thickness rapidly saturates in the nanometric range and the SiC formed is not structured. This thickness is essentially determined by ion penetration in the substrate which can be increased by negative biasing. Above this T(s), SiC growth increases rapidly and the film becomes textured near 800-degrees-C, as the growth of beta-SiC(001) aligned with Si(001) can be observed. The SiC topmost-layer structure is critically dependent on the plasma conditions with respect to the thermal processing at the film growth interruption. When the plasma is switched off before heating, the surface is essentially Si rich and oxidizable. In the opposite case, the H-2 plasma etches the Si-terminated overlayer and passivates the surface.