Nanocluster Formation by Phase Separation in Ultra-Thin Ion Implanted Gate Oxides

Non-volatile memories concepts based on semiconductor nanocrystals (NC) embedded in the gate oxide of MOS transistors with have attracted much in-terest. In order to synthesize the NC’s, ion implantation followed by annealing is the most compatible method with the current CMOS technology. In this contribution, fundamental studies of NC formation by phase separation in very-low energy ion implanted ultra-thin gate oxides will be presented. Ki-netic 3D lattice Monte-Carlo (MC) simulations show that for low ion fluences (low concentrations) the NC formation proceeds via nucleation, growth and Ostwald ripening. However, for high fluences spinodal decomposition results in a laterally connected network of nanostructures resembling a conventional floating gate memory. In general, the Si/SiO2 interface being in close neighbourhood to the NC’s has substantial influence on the NC evolution. Specifically, it leads to a NC-free tunnel oxide at the interface. Although the analysis of Si NC’s in SiO2 is a challenge for current methods, TEM investiga-tions of Si+ implanted gate oxides on (001) Si are in qualitative agreement with our predictions based on MC simulations. However, quantitatively, the fluence necessary for Si NC formation exceeds the predicted one by up to an order of magnitude. Recent studies have shown that this discrepancy origi-nates from oxidation of a large fraction of implanted Si by moisture absorbed in the damaged SiO2 surface layer.