含鍺摻雜氮化矽薄膜在非揮發性記憶體應用之研究

Abstract

對最廣為使用的非揮發性記憶體 ─ 快閃記憶體而言，通常會遇到兩個瓶頸：一是在元件尺寸繼續微縮下之瓶頸，由於尺寸微縮後穿隧氧化層(或閘極氧化層)之厚度亦隨之下降，如此雖可得到較快的讀寫速度，但電荷保存時間亦隨之下降，故須在兩者之間取得平衡點；二是在多次讀寫後在穿隧氧化層品質容易劣化而產生漏電路徑，一旦有一條產生，所有儲存在浮動閘極(floating gate)的電荷都會經由此漏電路徑而全部流失掉。為了克服上述兩個瓶頸，主要有兩種改良的的方法被提出，一是SONOS非揮發性記憶體，另一種是奈米晶體(量子點)非揮發性記憶體。 在本文中，一個將前述兩種非揮發性記憶體結合之新記憶體首次被提出。利用矽鍺氮(SiGeN)三元薄膜來取代SONOS非揮發性記憶體中的氮化矽(Si3N4)薄膜，並藉由一系列的熱處理後，使鍺量子點析出，並被氮化矽所包圍，如此便完成了將兩種記憶體結合之新記憶體。由於鍺量子點及包圍鍺量子點之氮化矽皆可儲存電荷，故新記憶體的記憶視窗比單純只有鍺量子點或氮化矽薄膜來得更為大。在其它條件不變下，擁有大記憶視窗的元件比小記憶視窗的元件更容易達到十年的電荷保存時間。 此外，乾式氧化及水氣處理步驟在阻擋氧化層形成時所扮演的角色也有進一步之探討。隨著乾式氧化的時間越長，更多的鍺原子會被析出並且聚集成核，因此，在拉曼分析中鍺的訊號也就越強。由於水分子比氧分子小，故水分子比氧分子更容易得鑽入阻擋氧化層(blocking oxide)中並修補阻擋氧化層中的懸鍵(dangling bands),所以，在乾式氧化後多加一個水氣處理的步驟則會改善阻擋氧化層的品質與增強其強度。實驗中發現，30分鐘短時間乾式氧化後再加上水氣處理的元件，其抗漏電能力和60分鐘長時間乾式氧化的元件一樣好，故水氣處理步驟的引入不僅可改善阻擋氧化層的品質，同時，更可大幅的縮減製程所需之時間。

For nonvolatile semiconductor memories (NVSM), there are two limitations encountered at the present time. (1) The limited potential for continued scaling of the device structure: this scaling limitation stems from the extreme requirements on the tunnel oxide layer. To balance between program/erase speed and retention time, there is a trade-off between speed and reliability for the optimal tunnel oxide thickness. (2) The quality and strength of tunnel oxide (or tunnel dielectric) after plenty of program/erase cycles, once a leaky path has been created in tunnel oxide, all charges stored in the floating gate will be lost. Therefore, two approaches, the silicon-oxide-nitride-oxide-silicon (SONOS) and the nanocrystal nonvolatile memory devices, have investigated to overcome this oxide quality limit of the conventional floating gate NVSM. A combination of SONOS and nanocrystal NVSM is first proposed in this study. A SiGeN film is introduced to replace the nitride film in SONOS structure. After several different thermal processes, Ge in the SiGeN film will be segregated to form Ge nanodots embedded in the SiNx/SiON film. Because there are two charge-storage node sources, the nodes in Ge nanodots and in SiNx dielectric film, comparing to SONOS and Ge nanocystal NVMs, a larger memory window can be obtained. When a memory device has a larger memory window, it is easier to meet the requirement of 10-year retention. And, we hope this approach can improve the two limitations mentioned above. Besides, the roles of dry oxidation and steam treatment during blocking oxide formation are also considered. With the extension of dry oxidation time, the Ge signal in Raman spectrum increases gradually because there are more Ge atoms to be segregated from the SiGeN film and more time for Ge atoms to nucleate. A 3-mintue steam treatment is performed after 30 minutes dry oxidation. Owing to its smaller size and lower activation energy than O2 molecules, H2O molecules are more permeable through the blocking oxide and can passivate dangling bonds in the blocking oxide. The purpose of steam treatment is to strengthen the blocking oxide and improve its quality. The I-V characteristic of the sample after 30 minutes dry oxidation plus 3 minutes steam treatment is comparable with that after 60 minutes long term dry oxidation. The introduction of steam treatment can not only improve the blocking oxide quality but also reduce the thermal process duration from 60 minutes to 33 minutes.