Process Simulations on Size and Location Control of Si Nanocrystals at Ion Beam Synthesis in Thin Gate Oxides

Silicon nanocrystal (NC) based nonvolatile memories are currently an active subject of study. In order to synthesize NCs in the gate oxide, ion implantation followed by annealing is the most compatible method with the current CMOS technology. Although, the process of phase separation during annealing and the influence of the very close Si/SiO₂ interface is not completely understood.


In this contribution, binary collision simulations of highfluence Si implantation are combined with kinetic 3D lattice Monte Carlo simulations of NC formation by phase separation during annealing. For low concentrations of implanted Si, NCs form via nucleation, growth and Ostwald ripening, whereas for high concentrations Si separates from SiO₂ by spinodal decomposition. In both regimes, the close Si/SiO₂ interface has substantial influence on the NC formation. Specifically, it leads to a self-adjusted NC-free tunneling oxide at the interface, which has the just right thickness (2 .. 4 nm) to act as barrier for NC charging by direct electron tunneling. However, the evolution of NCs during annealing differs in the two regimes. It is shown that a constant tunneling distance and a constant mean NC diameter can be achieved in the nucleation regime at high NC densities ( > 1x10¹² cm^-2). This is not the case for spinodal decomposition. Thus, it is predicted that the technological demands on the NC synthesis for nonvolatile memories are fulfilled best in the nucleation regime.