氟應用於二氧化鉿儲存層非揮發性記憶體之研究

Abstract

為了節省晶圓成本，增加產率，降低操作電壓，所製作的元件尺寸不斷微縮，其中非揮發性記憶體(NVM)也是如此，高密度記憶單元、低功率消耗、快速讀寫操作、以及良好的可靠(Reliability)，又是在微縮之下的目標與方向。其中可靠度的改善一直是許多論文所研究的議題，以資料保存時間(Retention)和耐操度(Endurance)的改良為主要方向。 本篇論文中，閘極採用乾氧成長二氧化矽(dry SiO2)作為穿隧氧化層(tunnel oxide)，其中以二氧化鉿取代傳統的SONOS結構的氮化矽（Si3N4）作為儲存層，並在沉積完二氧化鉿後，進行四氟化碳電漿處理(CF4 plasma treatment)技術將氟擴散進入二氧化鉿，接著沉積 由四乙氧基矽烷(TEOS)組成的二氧化矽作為阻擋氧化層(blocking oxide)。雖然二氧化鉿寫入時比氮化矽擁有較大的記憶窗口(window) ，但二氧化鉿在資料保存時間上的表現是遜色於氮化矽的。為了改善資料保存時間，我們進行四氟化碳電漿處理，將氟的擴散進入二氧化鉿，氟與鉿鍵結形成Hf-F，因此二氧化鉿儲存層中低能階的陷阱被修補，而高能階陷阱仍留下，減少記憶體在寫入後儲存電荷逃逸漏電之現象，進而得到較佳的資料保存時間。 我們將會電容和SOHOS記憶體元件作基本電性量測，藉此獲取資料保存時間趨勢。在材料分析方面，也使用X光光電子能譜術(XPS)，分析將氟擴散的電漿處理前後，是否形成鉿與氟鍵結情形；使用二次離子質譜儀(SIMS)分析顯示氟所分佈的位置。 根據實驗結果，確實驗證四氟化碳電漿處理，確實地改善了以二氧化鉿作為儲存層的非揮發性記憶體，在資料保存時間上的不足，此外，在製程中加上四氟化碳電漿處理，合用於傳統SONOS記憶體製程，並且屬於較低溫的製程，不會增加過多二氧化鉿形成結晶的負擔，因此我們認為這樣的製程技術對未來的記憶體元件是可以期待與考慮使用的。

In order to save the cost, increase the throughput and decrease operation voltage, many kinds of the devices are continually scaling. Undoubtedly, the nonvolatile memory (NVM) device is one of the scaling devices. Moreover, Current requirements of nonvolatile memory are the high density cells, low-power consumption, high-speed operation and good reliability for the scaling down devices. There are many studies that discuss how to improve the reliability including retention and endurance. In this study, silicon oxide (SiO2) is grown for tunnel oxide layer. The trapping layer of traditional SONOS structure is silicon nitride (Si3N4) which is replaced with hafnium oxide (HfO2). After HfO2 is deposited, we proceed to CF4 plasma treatment. Then, SiO2, composed of tetraethoxy silane (TEOS), is deposited as blocking oxide. While the memory window of HfO2 is larger than Si3N4, the retention performance -ivof HfO2 is worse than Si3N4. In order to improve the retention performance, we use CF4 plasma treatment to diffuse fluorine into HfO2 after HfO2 is deposited. The fluorine is incorporated into the HfO2 trapping layer and then formed of Hf-F bonding with hafnium. Because of CF4 plasma treatment, the shallow traps would be recovered but the deep traps would still be left. The carriers’ de-trapping effect is decreased after program operation. Hence, the retention performance would be better. We probe into the electrical characteristics of the capacitors and the SOHOS memory devices. From fundamental electric characteristic data, we could know retention information. In materials analysis, X-ray photoelectron spectroscopy (XPS) is used to analyze the devices which were carried out CF4 plasma treatment and confirm if there is the Hf-F bonding. Besides, the secondary ion mass spectroscopy (SIMS) analyses show depth profiles of fluorine in the devices. According to the experiment results, the retention performance of NVM, carried out CF4 plasma treatment, is actually improved. Moreover, CF4 plasma treatment is compatible with the conventional SONOS memory process. It is also the low thermal budget process which would not make the HfO2 film more crystallized. Therefore, we all consider the process technology is potential and expectable gradually for the memory device in the future.