Molecular based flash cell for low power flash application: Optimization and variability evaluation

Abstract

The field of molecular electronics continues to spur interest in the quest for miniaturization and reduction of operational power of electron devices. Most of the systems described in the literature are based on organic molecules, such as benzene, ferrocene and fullerenes. However, the use of inorganic molecules known as polyoxometalates (POMs) (see Fig.l and Fig.2) could offer several important advantages over the conventional and organic based devices. Our present work shows that POMs are more compatible with existing CMOS processes than organic molecules and they can replace the polysilicon floating gate in contemporary flash cell devices [2]. The interest in POMs for flash cell applications stems from the fact that POMs are highly redox active molecules and that they can also be doped with electronically active heteroatoms [3]. They can undergo multiple reversible reductions/oxidations, which makes them attractive candidates for multi-bit storage in flash memory cells. The molecular charge storage is localised, thus minimising cross-cell capacitive coupling, which arises from charge redistribution on the sides of a poly-Si floating gate (FG) and is one of the most critical issues with flash memories. Although this benefit is presently realised in floating gates by charge-trapping dielectric or by a metallic nano-cluster array, both technologies exhibit large variability. Charge-trap memories suffer variation in trap-density and trap energy and the size and density of nano-clusters is difficult to control. This precludes their ultimate miniaturization. In fact, the concept of using molecules as storage centers has already been demonstrated for organic redox-active molecules [1]. Here, using full 3D simulations, we evaluate correlation between the device performance (in terms of threshold voltage VT) and statistical variability, arising from the random dopant fluctuations (RDF) and POM fluctuations (POMF).