Depth positioning of silicon nanoparticles created by Si ULE implants in ultrathin SiO2

Silicon nanocrystals buried in a thin oxide can be used as charge storage elements and be integrated in standard CMOS technology to fabricate new non-volatile memory devices. In this geometry, the control of the distances between the nanocrystals layer and the two electrodes, the channel and the gate, of the MOS determines the final characteristics of the device (write-erase and retention times). In this work, we report on a systematic study of the effect of varying the beam energy (0.65 - 2 keV) and the dose (10¹⁵-10¹⁶ cm^-2) on the positioning of 2D-arrays of nanocrystals within 10 nm thick oxide after annealing at 950 and 1050°C. For this, different Transmission Electron Microscopy (TEM) methods have been used including High Resolution Electron Microscopy (HREM) for imaging isolated nanocrystals and Fresnel contrast imaging of populations of nanocrystals. Our results show that the injection distance"" can be precisely tuned in the 5-8 nm range by varying the beam energy. Moreover, very large swelling of the SiO₂ layer has been observed when increasing the implanted dose which could be the result of a partial oxidation of the Si ncs layer and/or of the SiO₂/Si interface.