Metallic nano-crystals for flash memories

Abstract

To overcome the scaling limitation of standard polysilicon floating gate flash memory, Tiwari et al. have proposed the use of a granular semiconductor floating gate. Another improvement on this idea is to replace semiconductor nanocrystals (NCs) with metallic dots to allow more charge storage per dot and more flexibility over the barrier height with respect to the insulating dielectric.To evaluate this solution, we have studied the growth of nickel and platinum NCs on 4.5 nm SiO2 tunnel oxide using the thin film metal agglomeration (dewetting) phenomenon. The dot structure was investigated by SEM and XPS and it has been shown that the dot density and size can be controlled between 1E9–5E11 cm−2 and 10–60 nm using a conventional 200 mm processing tool. Nice isolated dots with sufficient densities (>1E11 cm−2) and surface coverage (∼30%) have been elaborated at temperatures around 450 °C. It has also been shown that lower annealing temperatures (280 °C) are insufficient to form isolated dot-like structures and that surface coverage decreases rapidly at high temperature (700–850 °C). In addition, annealing tests on Ni-NCs at high temperature (850 °C) have demonstrated that tunnelling barrier can be easily destroyed by metal diffusion and chemical reaction at SiO2/Si interface (silicide formation).To conclude, using only the annealing temperature and time, it has been possible to control dot density and size to a level which approaches that of Si-based NC memories currently being studied. However, NVM technologies based on metals must solve the problem of low thermal budget, otherwise, metal diffusion into tunnelling barriers will compromise devices performances.