氮化矽記憶體資料保存行為之數值分析模擬

Abstract

本篇論文主要探討氮化矽記憶元件內儲存電荷之保存性與穿隧氧化層厚度之關聯。利用特殊的數值方法並以多層儲存陷井物理模型(multiple trapping model)分析，來求解S.R.H.方程式而更精準地提出一種記憶體資料保存行為的表示法。在求解的方程式中，我們用直接穿隧(Direct-tunneling)和透過缺陷助益穿隧(trap-assisted-tunneling)來描述電荷流失的現象，並依據一種對偶的行為–儲存在缺陷電子經Frenkel-Poole激發到傳導帶而傳導帶的自由電子可能被缺陷再捕捉回去–來說明電荷在氮化矽層的行為。由上述可知，我們的研究將會說明這種自由電子再被捕捉(recapture)回儲存能態和穿隧現象來決定自由電子的濃度，並隨著穿隧氧化層厚度增加電荷再被陷阱捕獲行為更加明顯。利用這種再捕捉的行為可以阻擋電荷流失並增加資料保存的時間。而更深入的探討，在高溫的情況會增加電荷逃逸，還有穿隧氧化層stress效應都將會在這篇論文裡面討論。

The bottom oxide thickness induced the charge loss blocking effect for a SONOS type flash memory is investigated. Utilizing a numerical analysis based on a multiple trapping model for solving the Shockley-Read-Hall (SRH) rate equations, the more accurate expressions for retention behavior are developed. In these equations, the classical tunneling and the stress-induced oxide trap-assisted tunneling mainly accounts for the charge loss and the trapped charge via Frenkel-Poole excitation to conduction band coupled with the conduction free carriers recapture by the traps is used to describe the charge transition within silicon-nitride (SiN) film. The free carrier concentration is governed by two competing processes, tunneling out to Si and recaptured by SiN traps. Our study shows that in an unstressed cell the recapture process starts to dominate as the bottom oxide thickness increases. This will block the charge loss and then improve the retention time. In addition, the high temperature enhanced charge escape and the bottom oxide stress effect accelerated data loss have been considered in this thesis.