Development of high quality ultrathin dielectric films of thickness less than 100 Angstrom has become an important research subject due to the scaling down of semiconductor devices. In the present study, samples with 20-100 Angstrom SiO2 on silicon were prepared by dry oxidation, and subsequently subjected to rapid thermal nitridation (RTN) using NH3 in the temperature range of 800-1200 degrees C. X-ray photoelectron spectroscopy and surface charge spectroscopy were applied to study the nitrogen distribution in the dielectric layers, the changes in the interface state density, D-it, and the dielectric breakdown field strength due to the nitrogen incorporation. It was found that nitrogen was mainly incorporated to the dielectric at the dielectric/Si interface without any consumption of silicon in the substrate during RTN. In the study of the electrical properties, we found that RTN led to a slight decrease in D-it and an increase in the breakdown field strength.