高速操作及高資料保存特性SONOS型式快閃記憶體之研究

Abstract

傳統的浮動閘極(floating gate) 快閃式記憶體，受限於資料保存(data retention)問 題，穿隧氧化層(tunnel oxide)無法繼續微縮，SONOS 則具有可微縮性(scalability)、可以 有較佳的資料保存、低電壓操作、適合箝入式(embedded)設計等優點。SONOS 結構快 閃式記憶體的研究，在國內研究則相當缺乏，本計劃著眼於此，規劃三年時程的研究計 劃，建立一套完整的SONOS Flash cell 架構，嘗試利用Nitride 介電層及金屬化合物電荷 儲存層(charge storage)結構的SONOS-type 快閃式記憶體，以達到同時可以高速操作及較 佳資料保存(dada retention)特性等優點，建立相關製程模組，並探討其漏電流機制及可 靠性的分析方法。 本計劃預期完成的工作有： 第一年計劃工作的執行，我們將更改傳統 SONOS 中ONO layer 的設計，將ONO layer 改為Nitride 介電層/金屬化合物/Nitride(NMN)結構，以期達到可以高速操作、具高可靠 性、較佳data retention 的SONOS 快閃式記憶體元件設計。主要工作包含：建立high-k nitride 做為tunnel oxide、成長高可靠性低漏電流tunnel oxide 技術、開發metal-storage node 材料，建立相關製程模組，以及各種不同tunnel nitride 結構、storage node 的各項 性能(performance)及可靠性的研究。 第二年主要是延續第一年的元件製作及各種操作模式，完成具體的可靠性評估，包含： Cell Endurance、Data retention 等的可靠性量測，元件在P/E cycling 以後，tunnel oxide 所造成氧化層傷害的可靠性量測方法研究及各項leakage current 模型建立，依第一年完 成的不同的tunnel nitride、blocking nitride 厚度條件sample，研究出scaling 的設計準則。 最後一年則是利用 metal-NC 技術取代前一年的電荷儲存層於SONOS cell 的製作，期能 達到更好的data retention 的目標及操作速度、operation window、可靠性等的評估，建立 一套利用metal-NC 技術於SONOS cell 製程與元件的設計準則。 最後成果預期可以有世界級水準的論文發表，研究成果更可做為產業界下一世代快閃記 憶體的重要參考。

The floating gate cell is limited by the data retention characteristics as a result of the non-scalability of tunnel oxide, while SONOS cell has gained more advantages such as scalability, better data retention, low voltage operation, and exhibits much better compatibility with logic circuits for embedded applications. At the present time, we are in short of the design manpower as well as the technology on the SONOS as it still remains at the early stage. Based on this understanding, we proposed a 3-year plan to achieve a goal for establishing a complete study of the SONOS cell technology. First, we take both advantages of the defect-free nitride as the tunneling dielectric and use metal composite layer as the charge storage in the design of SONOS cell. It can be expected to achieve both the high programming speed and the long retention characteristics. Then, the evaluation of these cells will be supported by using the sophisticated reliability characterization technique developed by us, as well as establishing the most important leakage current models related to the cell reliabilities. This projects include three phases. In the first year, instead of using ONO layer, the nitride/metal/nitride(NMN) layer will be developed for the charge storage. It includes a tunnel nitride and a metal(or metal composite) layer sandwiched between the nitride. Basic measurements of the performance and reliability will then be performed. In the second year, the fabricated cells in the first year will be verified and with intensive study on the cell endurance, charge loss etc. reliability evaluations. The method to characterize the program/erase induced oxide damage and the study of all the leakage components will be will be established. Finally, a scaling rule of the ONO film will be developed. In the final year, metal nanocrystal(NC) structure will be used to fabricate the NC memory, which has been targeted for achieving much better retention, while its programming speed, operation window, reliability need to be examined.. Based on a split of samples, the scaling as well as reliability design window will then be developed. Finally, these results are expected to reach a world class with academic excellence and provide a guideline for next generation flash memory technology applications.