Silicon oxide-nitride-oxide multilayers (ONO stacks) attract considerable interest for the charge storage structures in non-volatile memory devices [1 -2]. Ultra-thin ONO stacks are commonly prepared by thermal growth of a SiO2 layer (bottom oxide) on silicon, followed by low-pressure chemical vapor deposition (LPCVD) of Si3N4. Subsequently, the top oxide is either grown by the nitride re-oxidation or deposited by LPCVD. The typical thickness of individual layers in the ONO stacks ranges from 5 nm to 15 nm. The critical structural and compositional parameters that affect electrical performance of ONO based devices include the physical density of the amorphous oxiddnitride layers and depth distributions of oxygen, nitrogen, and hydrogen atoms. In this study, we applied (i) spatially-resolved electron-energy loss spectroscopy (EELS) in a transmission electron microscope and (ii) secondary ion mass spectroscopy (SIMS) to analyze elemental distributions in the differently processed ONO stacks deposited on Si. Additionally, densities of individual layers in the ONO stacks were measured using X-ray reflectometry (XRR). EELS spectrum-imaging in a dedicated scanning transmission electron microscope (STEM) revealed radiation-induced nitrogen segregation to the Si/SiO2 interfaces; the extent of nitrogen segregation increased visibly with increasing the radiation dose. The EELS metrology was optimized to obtain artifact-free data. Artifactfree measurements revealed lack of detectable nitrogen segregation to the Si/SiO2 interfaces regardless of processing conditions used. SIMS analysis demonstrated that higher thermal budget used to process the top oxide layer yields lower hydrogen content at the Si/SiO2 interface and broader nitrogen distribution across the top SiO2/Si3N4 interface. No nitrogen segregation to the Si/SiO2 interfaces was observed consistent with the EELS measurements. XRR measurements revealed a clear dependence of the density of the top oxide layer on the processing conditions. EXAFSEXELFS measurements on the O-K edge were used to probe radial-distribution function in the individual oxide layers of differently processed stacks; however, no significant difference for the first WO coordination shells was observed. The results of combined chemical and structural analyses were correlated with electrical performance of ONO-based flash-memories.