Investigation of ballistic current in scaled Floating-gate NAND FLASH and a solution

Abstract

Cell-to-cell interference constraints dictate that the Floating-gate (FG) of NAND FLASH cells be scaled down to very small thicknesses of a few nm. For the first time, we investigate and quantify the ballistic transport that occurs across ultra-thin poly-Si FGs during programming and experimentally determine its mean free path. This ballistic current has an adverse impact on the reliability of the Inter-poly dielectric (IPD) and potentially limits scaling. We have also demonstrated a solution to this problem in the form of an ultra-thin metal FG and have shown three orders of magnitude lesser ballistic current than poly-Si of same thickness. Further, we have demonstrated functional metal FG down to 3 nm.