Accurate TEM measurements of the injection distances in nanocrystal based memories

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. The control of the distances between the nanocrystals layer and the two electrodes of the MOS determines the final characteristics of the device (write-erase and retention times). This 2D arrays of ncs can be elaborated by ion implantation of Si at low energy. In this paper, we compare two transmission electron microscopy methods that can be used to extract such distances in 10 nm-thick SiO₂ layers implanted with Si at low energy. We demonstrate by using image simulations that conventional electron microscopy under out-of-Bragg and strongly underfocussed conditions is the fastest and most efficient technique to be used for routine measurements at subnanometer resolution. Our results show that the injection distance in such devices can be precisely tuned from 5 to 8 nm by adjusting the Si implantation energy from 0.65 keV to 2 keV.