雙層介電質薄膜之電阻式記憶體之研究

Abstract

在記憶體領域中，電阻式隨機存取記憶體具備有非揮發性、高切換速度、低操作電壓和結構簡單等優點而佔有一席之地。由於製程簡易，近來電阻式記憶體吸引大量目光去取代非揮發性記憶體主流快閃記憶體成為主要記憶體元件。然而，電阻式記憶體仍有不少的缺陷待解決，目前主要缺點為: 電性均勻度不佳、無確定統一的機制等問題。本篇論文中，透過雙層介電質薄膜之電阻式記憶體，利用薄膜不同堆疊的順序來提升元件與元件之間的電性均勻度以及增加資料儲存密度。 首先，本論文研究氮化鈦 TiN / 有或無鈦 Ti (10 nm) / 氧化鉿 HfOx （6 nm） / 氮化鈦 TiN 以及 氮化鈦 TiN / 有或無鈦 Ti (10 nm) / 氧化鋁 AlOx （1 或 2 nm）/ 氧化鉿 HfOx （6 nm） / 氮化鈦 TiN 之電阻式記憶體。結果顯示出此兩種結構有沉積鈦的元件，它們的操作電壓都會明顯下降。因為鈦對於氧高親和力，所以在介電層裡的氧空缺數量會增加，從而降低初始阻值。由於氧空缺的數量增加有利於金屬絲的形成，元件的操作次數也會因此而增加。如果只針對電性均勻度時，無論元件有沒有鈦，1奈米厚的氧化鋁沉積在6奈米厚的氧化鉿表現最出色。因為透過兩層不同材料特性的介電薄膜，使得金屬絲有效的進行局部形成與斷裂，從而提升元件與元件之間的電性均勻度。可是2奈米厚的氧化鋁沉積在6奈米厚的氧化鉿表現卻不是最出色，由於氧離子的分析結果發現2奈米厚的氧化鋁不容易和氮化鈦 / 鈦反應，導致裡面的氧空缺的量太少，從而影響整體的均勻性以及電性，但1奈米厚的氧化鋁不僅可以和氮化鈦 / 鈦反應形成氧空缺，而且藉由雙層介電層的材料特性差異來提升電性的均勻度。 其次，本論文研究氮化鈦 TiN / 鈦 Ti (10 nm) / 氧化鉿 HfOx（6 nm）/ 氧化鋁 AlOx （1或2 nm） / 氮化鈦 TiN 之電阻式記憶體。結果顯示出透過改變set的停止電壓(stop voltages)與固定reset停止電壓，兩種結構的元件都有多重儲存狀態之特性(multilevel characteristics)。當氮化鈦 TiN / 鈦 Ti (10 nm) / 氧化鉿 HfOx（6 nm）/ 氧化鋁 AlOx （2 nm） / 氮化鈦 TiN的set停止電壓等於4V時，在高電壓下，低阻態(Low Resistance States, LRS)的曲線會低於高阻態(High Resistance States, HRS)的曲線；在小電壓下，LRS的曲線會高於HRS的曲線。因為在高電壓下氧化鉿裡已經形成金屬絲，而當電流流過氧化鋁(AlOx)時，電子會被裡面的缺陷捕捉，導致電流變小。當外加電壓持續變小的時候，此時在金屬絲在氧化鉿裡已經完全形成，相較於在HRS時金屬絲尚未完全形成，因此電流穿遂的距離變短，其電流就變高。當set停止電壓等於5V或以上，I-V曲線會越趨近一般的電阻式記憶體的特性。因為大電場會導致氧化鋁裡產生大量氧空缺，所以電流傳導主要來源從Fowler-Nordheim (FN)穿遂變成氧空缺傳道。 當氮化鈦 TiN / 鈦 Ti (10 nm) / 氧化鉿 HfOx（6 nm）/ 氧化鋁 AlOx （1 nm） / 氮化鈦的set停止電壓等於3V時也有與2奈米厚的氧化鋁類似現象。當set的停止電壓持續變大，在氧化鋁的氧空缺數量增加，所以電流傳導主要來源就會變成氧空缺傳道。 當電壓等於reset停止電壓時，這時候電場的方向相反，所以被捕捉的電子因此而脫離，使得元件回到最剛開始的狀態。 除此之外，氮化鈦 TiN / 鈦 Ti (10 nm) / 氧化鉿 HfOx（6 nm）/ 氧化鋁 AlOx （2 nm） / 氮化鈦 TiN元件在正偏壓以及負偏壓下都有阻值逐漸變化的現象，而此現象可以應用到類神經的應用。 總結，本論文以氮化鈦 TiN/ 鈦 Ti / 氧化鋁 AlOx / 氧化鉿 HfOx / 氮化鈦 TiN與氮化鈦 TiN/ 鈦 Ti / 氧化鉿 HfOx/ 氧化鋁 AlOx / 氮化鈦 TiN的電阻式記憶體元件分別具有更佳的電性均勻度、多重儲存狀態以及可以應用於類神經之特性，其結果具有未來的非揮發性記憶體應用之潛力。

The advantages of resistive random access memory (RRAM) were non-volatile, high switching speed, low operation voltages, and simple structure. Thus, RRAM had high competitiveness in the field of memory technology. Recently, RRAM had attracted great attention and be supposed to replace the conventional non-volatile memories (NVM), such as the flash memory technology, owing to the simpler fabrication process. Nevertheless, there were still lots of drawbacks required to be solved for the RRAM devices, such as bad uniformity and uncertain switching mechanism. In this thesis, bi-layered dielectric films with different stacking orders were used to achieve the better uniformity and multilevel characteristics. In the first part, the RRAM devices with TiN / with and without Ti (10 nm) / HfOx (6 nm) / TiN and TiN / with and without Ti (10 nm) / AlOx (1 or 2 nm) / HfOx (6 nm) / TiN were fabricated. The devices with Ti interposing layer revealed lower operation voltages than the devices without Ti because there were more oxygen vacancies in the dielectric films due to the high oxygen affinity of Ti. Furthermore, the number of endurance cycles for the devices with Ti was more than the device without Ti. For the comparison of uniformity, AlOx (1 nm) + HfOx (6 nm) bi-layered dielectric films RRAM showed the best uniformity because the formation and ruptured of CFs could occur effectively due to the different material characteristics between the two dielectric films. In contrast, the reason of AlOx (2 nm) + HfOx (6 nm) bi-layered dielectric films didn’t show the best uniformity because 2 nm-thick AlOx was too thick to form the oxygen vacancies for the conduction filaments. In the second part, the RRAM devices with TiN / Ti (10 nm) / HfOx (6 nm) / AlOx (1 or 2 nm) / TiN were also fabricated. Both of them showed the multilevel characteristics by changing the set stop voltages at positive bias and kept the reset stop voltages the same at negative bias. For the TiN / Ti / HfOx (6 nm) / AlOx (2 nm) / TiN device at Vset,stop = 4V, the low resistance state (LRS)’s curve was lower than the high resistance state (HRS)’s curve at high voltage region. Then the LRS’s curve became higher than the HRS’s curve at low voltage region. It was because at high voltage region, the conductive filaments (CFs) were formed completely in HfOx led to the Fowler-Nordheim (FN) tunneling current in AlOx. The defects in AlOx would trap the electrons, so the current became smaller. In contrast, the LRS’ current became larger at the small voltage region than the initial HRS’s current since the CFs had formed in the HfOx. When the Vset,stop became larger and larger, more oxygen vacancies were formed in AlOx and so the current conduction was dominated by oxygen vacancies. It exhibited the typical RRAM characteristics when the Vset,stop = 6 V. For the AlOx with 1 nm in thickness, the TiN/ Ti (10 nm) / HfOx (6 nm) / AlOx (1 nm) / TiN also revealed the similar phenomenon to the 2nm AlOx ones. When the set stop voltages became larger and larger, the CFs were almost formed completely in AlOx. Therefore, the current conduction was dominated by oxygen vacancies. When the reset stop voltages were applied, the direction of electric field were reversed and so the trapped electrons would be detrapped. Therefore, the device returned to the initial state. Moreover, TiN / Ti (10 nm) / HfOx (6 nm) / AlOx (2 nm) / TiN devices showed the gradual resistance change and this characteristic could be applied in the neuromorphic field. In this thesis, the TiN / Ti / AlOx / HfOx / TiN and TiN / Ti / HfOx / AlOx / TiN could improve the uniformity and create multilevel / neuromorphic characteristics, accordingly. They implied that such devices had the potential applications in the future three-dimensional (3D) non-volatile (NVM) circuits.