氧化鍺覆蓋層在二氧化鈦與鋯鈦酸鉛基底之電阻式隨機存取記憶體之研究

Abstract

隨著行動數位生活的到來，在智慧型手機、數位相機和行動裝置對於非揮發性記憶體的需求急遽上升。非揮發性記憶體在技術上已經發展到可以兼具高速，高密度和低功耗的功能。但是隨著持續不斷的微縮，快閃記憶體將會面臨到的挑戰是薄穿穿隧化層所帶來的持久度下降。在長時間的操作下，厚度在10奈米以下的穿隧氧化層會使電子直接穿隧或藉由缺陷穿隧而造成在浮柵的電荷損失。因此，數種非揮發性記憶體如鐵電隨機存取記憶體、磁性隨機存取記憶體和電阻式隨機存取記憶體被廣泛的研究。在電阻式隨機存取記憶體方面有許多具有發展潛力的候選人，包含屬於多元鈣鈦礦種類的PCMO、PZT、SZO、STO和二元金屬氧化物的氧化鎳、二氧化鈦、二氧化鉿、氧化鋁和氮氧化鉿。要達到可以應用的記憶體必須符合低功率損耗、與目前的互補型金屬氧化物半導體的相容性、高速操作、能持續微縮和簡單的金屬-絕緣層-金屬的三層結構。而電阻性操作機制是歸因於導電細絲的形成與斷裂，並且與誘捕/去誘捕、再氧化/還原和氧離子的Mott轉換的遷移有關係。另外，隨機細絲的形成與破裂會導致操作電壓與記憶體在連續操作下的狀態產生波動，並且可能會導致嚴重的控制和讀取的問題。 首先，我們製作了Ni/GeOx/PbZr0.5Ti0.5O3/TaN電阻式操作記憶體，在單極操作模式下，雙層的Ni/GeOx/PZT/TaN電阻式隨機存取記憶體擁有著與缺乏共價鍵介電質GeOx的Ni/PZT/TaN RRAM相比超過100倍的阻值視窗、85度的高溫持久度和高達2000次的直流操作次數。 接下來為了改善分佈特性，我們利用氧化鍺(GeOx)與二氧化鈦(TiO2)來形成Ni/GeOx/TiOx/TaN電阻式操作記憶體來得到更好的元件到元件之間 (device-to-device)的分佈特性與持久性。此電阻式記憶體表現出30 毫瓦的操作功率、10萬次操作特性與高溫85度的持久度。 為了比較不同的共價鍵絕緣層，我們比較了室溫下製作氧化鋁(AlOx)和氧化鍺(GeOx)在二氧化鈦上的差異。雖然氧化鋁/二氧化鈦與氧化鍺/二氧化鈦在操作電流與開/關特性上有著諸多的相似，但是氧化鋁/二氧化鈦電阻式記憶體與氧化鍺/二氧化鈦相比之下表現出較差的持久度與操作一致性。在持久度方面，氧化鋁/二氧化鈦電阻式記憶體由於較高的高阻態電流和與氧化鍺/二氧化鈦相比較低的0.4電子伏特的活化能，因此只能達到60度的持久度。 最後，為了避免氧化鍺受到大氣中水氣的影響而退化，我們製作了鍺摻雜二氧化矽的電阻式記憶體來達到單層的簡單結構、一萬次的60毫秒高速操作和高溫85度的良好持久度。

With the arrival of Digital Mobile Life, the demands for nonvolatile memory (NVM) have significantly increased, such as mobile phones, digital cameras and portable devices. NVM technology has been developed to obtain high speed, high density and low power consumption. But, when the technology node continuously scaled down, the flash memory faces the challenge of retention drop caused by thin tunneling oxide. During long-term operation, the thickness of tunneling oxide below 10 nm will cause the charge lost in floating gate due to direct tunneling current or defects formed in the tunneling oxide. Therefore, several types of NVMs such as ferroelectric random access memory (FeRAM), magnetic random access memory (MRAM), and resistive random access memory (RRAM) are being investigated. There are many candidate materials for RRAM application including perovskite materials such as Pr0.7Ca0.3MnO3, PbZr0.5Ti0.5O3, SrZrO3, SrTiO3, and binary metal oxides such as NiO, TiO2, HfO2, Al2O3, and HfON. Such a memory application should have the merits of low power consumption, compatibility of the current complementary metal oxide semiconductor (CMOS) process, high-speed operation, high scalability, and simple metal-insulator-metal (MIM) tri-layer structure. The resistive switching mechanisms are attributed to the formation and rupture of the conducting filaments, which are related to trapping/detrapping, reoxidization/reduction, and Mott transition performed by O2− migration. Moreover, the stochastic formation and rupture of the filaments cause the fluctuation of the operation voltages and the memory states during continuous switching cycles, which may lead to severe control and readout problems. First of all, we have fabricated the Ni/GeOx/PbZr0.5Ti0.5O3/TaN resistive switching memory. Under unipolar-mode operation, the bilayers Ni/GeOx/PZT/TaN RRAM shows a large resistance window of >102, 85℃ retention, and a good DC cycling of 2000 cycles, which are significantly better than those shown by the single-layer Ni/PZT/TaN RRAM without the covalent-bond-dielectric GeOx . Next, to further improvement the distribution, we use GeOx with TiO2 layer to form Ni/GeOx/TiOx/TaN resistive random access memory for better device-to-device distribution and retention. This RRAM device shows low 30μW switching power (9μA at 3V; −1μA at −3V), 105 cycling endurance and good retention at 85 oC. To compare the different covalent bond dielectric with GeO, we fabricated AlOx/TiOx and GeOx/TiOx resistive random access memory at room temperature. The AlOx/TiOx and GeOx/TiOx RRAM exhibit similar set/reset powers and switching window, but the AlOx/TiOx RRAM shows much poor data retention and poor switching uniformity as compared to the GeOx/TiOx RRAM. In retention test, AlOx/TiOx RRAM presents poor 60 oC data retention due to high currents for high resistance state (HRS) with low activation energy (Ea) of only 0.4 eV, which is much lower than 0.52 eV of GeOx/TiOx RRAM. Finally, to avoid the degradation of the GeO layer caused by hydraulic in atmosphere, we fabricated Ge-SiO resistive random access memory to achieve simple structure of single layer, high speed operation of 60s for 104 cycling endurance and good retention at 85 oC.