We have undertaken the characterization of the low-temperature direct electron cyclotron resonance (ECR) N2O plasma process on the technically important Si(100) surface by the combined use of the newly developed ultrahigh vacuum (UHV)-based contactless capacitance-voltage (C-V) and x-ray photoelectron spectroscopy (XPS) techniques. An UHV-based contactless C-V measurement system allowed for the first time a direct in,situ assessment of electronic properties of "free＇＇ Si surfaces and the surfaces covered with an ultrathin insulator layer. The UHV contactless C-V analysis indicated that a oxynitride/Si interface with a minimum interface state density of 1 X 10(11) eV(-1) cm(-2) could be realized by the ECR N2O oxynitridation process at 400 degrees C. The angle-resolved XPS study showed that the formation of a phase-separated Si3N4/SiO2 structure is important to realize such a interface at low temperatures. The electronic and the chemical properties of the interfaces were found to be strongly correlated.