Non-volatile memory devices beyond process-scaled planar Flash technology

Abstract

Mainstream non-volatile memory technology dominated by the planar Flash transistor with continuous floating-gate has been historically improved in density and performance primarily by means of process scaling, but is currently faced with significant hindrances to its future scaling due to fundamental constraints of electrostatics and reliability. This dissertation is based on exploring two pathways for circumventing scaling limitations of the state-of-the-art Flash memory technology. The first part of the dissertation is based on demonstrating a vertical Flash memory transistor with nanocrystal floating-gate, while the second part is based on developing fundamental understanding of the operation of Phase Change Memory. A vertical Flash transistor can allow the theoretical minimum cell area and a nanocrystal floating-gate on the sidewalls is shown to allow a thinner gate-stack further conducive to scaling while still providing good reliability. Subsequently, the application of a technique of protein-mediated assembly of preformed nanocrystals to the sidewalls of the vertical Flash transistor is also demonstrated and characterized. This technique of ordering pre-formed nanocrystals is beneficial towards achieving reproducible nanocrystal size uniformity and ordering especially in a highly scaled vertical Flash cell, rendering it more amenable to scaling and manufacturability. In both forms, the vertical Flash memory cell is shown to have good electrical characteristics and reliability for the viability of this cell design and implementation. In the remaining part of this dissertation, studies are undertaken towards developing fundamental understanding of the operational characteristics of Phase Change Memory (PCM) technology that is expected to replace floating-gate Flash technology based on its potential for scaling. First, a phenomenon of improving figures of merit of the PCM cell with operational cycles is electrically characterized. Based on the electrical characterization and published material characterization data, a physical model of an evolving "active region" of the cell is proposed to explain the improvement of the cell parameters with operational cycles. Then, basic understanding is developed on early and erratic retention failure in a statistically significant number of cells in a large array and, electrical characterization and physical modeling is used to explain the mechanism behind the early retention failure.