Investigation for Narrow Cell Threshold Voltage Distribution in NOR Flash Device

Abstract

A NOR memory density has been increased 2-fold every 2 years satisfying Moor's law in 90 nm generation. This density increase results from the scaling down of cell size to one-half that in previous generation. However, since this cell scaling down is faced with several critical device problems, further scaling down seems markedly difficult. As one of the scaling barriers, the distribution of cell threshold voltage is specifically critical. In general, as the area of a cell transistor decreases, the effect of process variation on cell threshold voltage variation becomes larger. In this paper, we indicate several effects of process variation on cell threshold voltage variation and present a new solution. Even if cell transistor area is scaled down, tunnel oxide thickness should be maintained constant reducing a bird-beak oxide. Furthermore, the space length of metal contact to gate should be to eliminate cell threshold variation due to a coupling effect. To solve this critical problem, we perform bird-beak oxidation by radical oxidation and a contact process using an enlarged metal contact scheme.