具有直接穿隧氧化層氮化矽記憶體資料保存特性之探討

Abstract

氮化矽記憶體(SONOS memory) 將是未來非揮發性記憶體元件的主流，它相較於快閃記憶體(flash)，SONOS有著較簡單的結構、簡單的製程，而且可以比傳統浮閘結構快閃記憶體有更佳的微縮能力(scalability)。對於SONOS來說，耐久性(endurance)及保存性(retention)是最主要的二項可靠性問題，其中由於元件的縮小，電荷的保存性(retention)則成了一個很主要的問題。對於薄氧化層SONOS來說，影響資料保存的主要漏電流有兩種成分，一種是熱放射(thermionic emission)；另一種是直接穿隧(direct tunneling) 。 在本論文中，我們將探討各種不同上氧化層(blocking oxide)及穿隧氧化層(tunnel oxide) SONOS結構的資料保存特性。經由實驗，可以了解直接穿隧的漏電流有一部份是經由上氧化層，而非一般所假設全部經由薄的穿隧氧化層。首先，我們由實驗推測出寫入及抹除時，正負電荷均存在於二氧化矽(SiO2)及矽化氮(Si3N4)的介面，這個結果幫助我們量化注入電荷及電荷流失的量。結果顯示，我們發現到較薄上氧化層SONOS元件所流失的電荷較多，尤其是當上氧化層厚度降到40A以下，而這也證明了電荷從上氧化層端所流失。由實驗結果更得知，薄的上氧化層有較佳的耐久性，較小的操作電壓等優點，但是有電荷流失，操作區間(operation window)較小等缺點。最後，我們提出一種區分兩種漏電流機制的方法，結果發現到直間穿遂(DT)在短時間主導電荷流失，但是長時間(>10000s)情況下，電荷流失成分以熱放射(thermionic emission)為多。尤其是在元件不斷的縮小化的時候，這些結果可以幫助我們更了解電荷流失的情形及其主導的物理機制。

SONOS (Silicon Oxide Nitride Oxide Silicon) will become the main stream of nonvolatile memory products because of its simplicity in structure and scalable by comparing with conventional floating gate cells. For the scaling of SONOS memory, the endurance and retention are the two major reliability issues. Also, data retention has been most crucial for the scaling of the cell. For a thin gate oxide SONOS cell in the direct tunneling regime, two leakage current components, i.e., thermionic and direct tunneling (DT), in relating to the data loss, are the two dominant mechanisms. In this thesis, data retention for various top (blocking) and bottom oxide (tunnel oxide) SONOS cells has been investigated. The direct tunneling through either tunnel or blocking oxide can also be identified experimentally. First of all, it was found that injected charge locates close to the tunnel oxide/nitride interface either during program or erase. This result is then used to identify the charge loss in retention measurement. Results show that the cell with thinner blocking oxide has more charge loss under various baking temperatures, especially when the blocking oxide is thinner than 40A. Thick blocking oxide cell has larger operation window and less charge loss, but needs larger gate voltage during program and erase and has poorer endurance. Finally, A leakage current separation technique has been developed to distinguish the two leakage components via thermionic and direct tunneling. The direct tunneling dominates the short-term leakage while the long-term leakage is dominated by thermionic emission. This will help us to understand the dominant leakage during the scaling of SONOS cells.