Ultra-Short Pulses Improving Performance and Reliability in Flash memories

Abstract

Reliability and performance of both NAND and NOR flash memories strongly depend on the physics of Fowler Nordheim (FN) tunneling, a mechanism widely used in writing operations. In fact, the large number of involved cells and the strong sensitivity to technological parameter variations cause wide threshold voltage distributions after FN tunneling. Overerase, erratic phenomena and tunnel oxide degradation (TOD) with long-term consequences on data retention are examples of FN related issues. Recently it has been shown that the use of high voltage ultra-short pulses (<1mus) separated by a recovery time (gt;10mus) (hereafter called pulsed operation, PO) can improve reliability by reducing TOD, and, hence, SILC. Performance and reliability improvement have been demonstrated on 7 nm and 3.5 nm tunnel oxides showing the wide range of applicability of the methodology. Nevertheless, so far, these important studies have only involved single cells or small arrays of a few thousands of cells on a wafer. This work shows, for the first time, experimental results based on the application of PO on large arrays of flash cells. Issues and limits as well as improvements in terms of reliability and performance are here discussed