Ion irradiation through SiO2/Si interfaces: Non-conventional fabrication of Si nanocrystals for memory applications

Abstract Si nanocrystals for multi-dot floating-gate memories have been produced by non-conventional ion beam synthesis. The stack (50 nm poly-Si)/(15 nm SiO 2 )/(Si substrate) of MOSFETs has been exposed to 50–100 keV Si + irradiation, the Si + ions comes to rest below the gate oxide within the (0 0 1) Si substrate. Thus, Si excess in the gate oxide of the MOS structure is formed mainly by ion beam mixing of the upper poly-Si/SiO 2 interface and the lower SiO 2 /(0 0 1) Si interface. Subsequently, the metastable SiO x of the two ion beam mixed interfaces has been transformed into stable Si and SiO 2 by rapid thermal annealing (RTA). Adjacent to the recovering interface, narrow SiO 2 zones become denuded of excess Si, whereas the more distant tails of excess Si form nanocrystals in the gate oxide. nMOSFET characteristics in terms of write/erase voltage, programming time, endurance and retention have been evaluated.