Multi-Dot Floating-Gates in MOSFETs for Nonvolatile Memories - Their Ion Beam Synthesis and Morphology

Scalability and performance of current flash memories can be improved substantially by novel devices based on Multi-Dot Floating Gate MOSFETs. The multi-dot layer in the very thin gate oxide can be fabricated CMOS-compatibly by ion beam synthesis (IBS). Here, we present both experimental and theoretical studies on IBS of multi-dot layers consisting of Si nanocrystals (NCs). The NCs are produced by ultra low energy Si ion implantation, which causes a high Si supersaturation in the shallow implantation region. During post-implantation annealing, this su-persaturation leads to phase separation of the excess Si from the SiO₂. Till now, the study of this phase separation suffered from the weak z contrast between Si and SiO₂ phases in Transmission Electron Microscopy (TEM). Here, this imaging problem is solved by Energy Filtered Scanning Transmission Electron Microscopy (EFSTEM). Additionally, kinetic lattice Monte Carlo simula-tions of Si phase separation have been performed and compared with EFSTEM images. It has been predicted theoretically that the morphology of the multi-dot Si floating gate changes with increasing ion fluence from isolated, spherical NCs to percolated spinodal Si pattern. These pat-tern agree remarkably with EFSTEM images. However, the predicted fluence for spinodal pattern is lower than the experimental one. Because oxidants of the ambient atmosphere penetrate into the as-implanted SiO₂, a substantial fraction of the implanted Si is lost due to oxidation.