Time-of-flight medium energy backscattering (TOF-MEBS) is a derivative of conventional Rutherford backscattering (RBS) that uses medium energy ions and a time-of-flight detector resulting in improved surface sensitivity, increased depth resolution, and reduced target damage compared to conventional RBS. In this study, thin dielectric materials and their lower interfaces were analyzed with TOF-MEBS using 270 keV He+ projectiles. Dielectric materials systems studied included Al2O3, SiO2, and ZrO2 on silicon. Film thicknesses ranged from approximately 10 to 200 A. Data analyses were performed by modeling the ion interactions with material structures and performing a non-linear least squared fit to the backscattering spectra. Simulation fit results were improved up to 40% for alumina and 60% for zirconia samples by including interfacial layers on the order of a few nanometers thick in the material structure models. The significant improvement to fits resulting from the addition of extremely thin interfacial layers demonstrates the technique's applicability to ultra-thin film characterization. (C) 2004 American Vacuum Society.