The problems in the thinning of Oxide/Nitride/Oxide (ONO) films for memory capacitors of DRAM are reviewed. There are two key points in forming the ONO films of excellent leakage current characteristics, that is, the improvement of the surface before nitride deposition and of the nitride deposition parameter. These points were investigated by using XPS and RBS. The native oxide layer grows up at the surface of poly-crystalline silicon film playing a roll as an electrode for the memory capacitor. This layer is changed into oxynitride by NH₃ annealing, and silicon-rich transition layer formed in the early stage of nitride deposition disappears as a quantity of nitrogen involved in the native oxide film saturates. The resistance of nitride film in thermal oxidation is improved in a nitrogen-rich film. The composition of nitride film depends not only on the flow rate of NH₃ and SiH₂Cl₂ but also on a deposition temperature. Nitride films deposited at the temperatures near 750°C showed good performance.