(A) comparative study on curing effects by joule heat for stoichiometry split of charge trap nitride in FinFET SONOS based flash memory

Abstract

Improvement of threshold voltage shift and retention characteristics due to increase in traps in the tunnel oxide film and interface after repeated write and erase operations is a research item for a long time in 3D NAND flash memory, which has become a major storage device due to the recent increase in the Internet of Things, cloud systems, and mobile applications. The 3D NAND flash memory is formed by vertically stacking cells, and due to the characteristics of the structure, a pad can be formed by connecting wires to both sides of a gate electrode serving as a word line. In this study, focusing on the structural advantages of the 3D NAND flash memory, it was confirmed that the deterioration of the threshold voltage and the retention characteristics were improved due to Joule heating through the gate and the gate electrode were thermally recovered. In addition, the effect of improving cell characteristics through joule heat recovery, which is the main film material for storing electric charges, for each silicon nitride film with N-rich or Si-rich was compared. As a result, it is confirmed that the N-rich silicon nitride film device on the tunnel oxide film further is improved the cell threshold voltage degradation and retention characteristics when the device was recovered by Joule heat. This is because N-rich silicon nitride has a high trap density (N$_T$) in both the initial state and the state after repeated write and erase operations, but it is recovered after Joule heat. It is confirmed this by the power spectral density and trapping density through low-frequency noise measurements (LFN). However, it is confirmed that N-rich silicon nitride had better initial retention properties than Si-rich silicon nitride. Because of these characteristics, it may be better to use N-rich silicon nitride for personal mobile application products where initial information retention characteristics may be important. On the other hand, it may be better to use Si-rich silicon nitride for server-oriented products of companies where cell characteristics deterioration due to repeated writing and erasing is more important. Therefore, it is expected that it will be possible to select how to use the silicon nitride material in connection with the Joule heat recovery according to the characteristics of each product.