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

摘要

Scalability and performance of current flash memories can be improvedsubstantially by replacing the floating poly-Si gate by a layer of Si dots.This multi-dot layer can be fabricated CMOS-compatibly in very thin gate oxideby ion beam synthesis (IBS). Here, we present both experimental and theoreticalstudies on IBS of multi-dot layers consisting of Si nanocrystals (NCs). The NCsare produced by ultra low energy Si ion implantation, which causes a high Sisupersaturation in the shallow implantation region. During post-implantationannealing, this supersaturation leads to phase separation of the excess Si fromthe SiO2. Till now, the study of this phase separation process suffered fromthe weak Z contrast between Si and SiO2 in Transmission Electron Microscopy(TEM). Here, this imaging problem is resolved by mapping Si plasmon losses witha Scanning Transmission Electron Microscopy equipped with a parallel ElectronEnergy Loss Spectroscopy system (PEELS-STEM). Additionally, kinetic latticeMonte Carlo simulations of Si phase separation have been performed and comparedwith the experimental Si plasmon maps. It has been predicted theoretically thatthe morphology of the multi-dot Si floating-gate changes with increasing ionfluence from isolated, spherical NCs to percolated spinodal Si pattern. Thesepatterns agree remarkably with PEELS-STEM images. However, the predictedfluence for spinodal patterns is lower than the experimental one. Becauseoxidants of the ambient atmosphere penetrate into the as-implanted SiO2, asubstantial fraction of the implanted Si might be lost due to oxidation.