(鎳、鈦與鎢)氧化物之電性及應用於電阻式隨機記憶體研究

Abstract

近年來，由於傳統浮動閘極結構之非揮發性記憶體(Flash memory)在小於二十奈米節點技術將面臨其物理極限，一些新式非揮發性記憶體如磁阻式記憶體(MRAM)、相變化記憶體(PCRAM)、以及電阻式記憶體(RRAM)之研究在下世代資訊儲存應用也變得越趨重要。另一方面，由於某些電阻式記憶體材料具備CMOS製程完全相容、高存取速度、高可靠度、以及高容量等優越特性在下世代記憶體元件中具有高商業價值潛力，故本篇論文也聚焦於RRAM之相關研究。 雖然有很多金屬氧化物材料可應用於RRAM元件，然而與CMOS製程完全相容的材料包含氧化鎳、氧化鈦以及氧化鎢三種材料而且在RRAM特性上也有不錯的表現。在CMOS製程中鎳化矽(NiSi)用於中段製程，其作用為降低金氧半導體(MOS)接面上的阻值，氮化鈦(TiN)則用於中段以及後段製程(BEOL)，其目的為增加金屬附著性以及當作擴散阻障層，而金屬鎢(W)則廣用於中段接觸(Contact)製程，為CMOS元件與後段電路連接的主要金屬材料，綜觀以上特點，以氧化鎳、氧化鈦或是氧化鎢作為電阻式記憶體元件材料可與現今半導體CMOS製程完全相容。而在本篇論文中將深入探討這三種材料之RRAM特性包括電性表現、電阻翻轉機制以及相關之應用。 在實驗中我們發現氧化鎳材料屬於多晶結構而且具有雙極式 (Bipolar) RRAM特性，其電性顯示出薄膜特性而且電性表現與氧化程度有著直接的關係，氧化程度越高的樣品其電阻變化也相對明顯，而電子的傳導遵循著蕭基發射(Schottky emission)傳導機制，藉由此一機制可以得知蕭基能障的變化與元件電阻的變化息息相關，此外更可以得知氧化程度越低的樣品其介電常數越高，這也進一步解釋了低氧化程度的樣品在電阻翻轉過程中需要更大的外加電壓才足夠抵銷樣品內部所產生的反向電場。 類似於氧化鎳材料系統，氧化鈦材料同樣擁有雙極式RRAM特性，其電性表現顯示出明顯的界面特性，薄膜厚度對其電性表現影響不大，其電子的傳導機制與氧化鎳材料相同都遵循著蕭基發射傳導機制，而且蕭基能障的變化也與元件電阻的變化有著直接的關係，而在氧化鈦/氧化矽複合材料的實驗當中進一步證實氧化鈦RRAM的界面特性，而且此一複合材料的資訊保存能力也比單一氧化鈦薄膜有著明顯的改善。 而在氧化鎢薄膜的實驗當中，我們發現這種材料有著多方面應用的潛力，舉例來說，它不但可以用於單次儲存(OTP)而且可以用於多次儲存(MTP)記憶體裝置，而在多次儲存記憶體裝置中又可以使用雙極式以及單極式(Unipolar)操作來控制其電阻阻態，而且耐久性皆大於一千次，由於這種材料也具有相當大的電阻阻距，因此非常適合用於多層單元(MLC)儲存記憶體的應用，而且在製程微縮的實驗中得知氧化鎢電阻式記憶體有著製程微縮的可行性，這種材料也展現出高可靠度特性，例如在攝氏二百五十度的高溫環境之下表現出超過一千小時的熱穩定性。 而在電子傳導機制中氧化鎢電阻式記憶體之低電阻狀態表現出接近於導體的minimum-metallic-conductivity (MMC) 傳導機制，而高電阻狀態則遵循著variable-range-hopping (VRH) 傳導機制，而且根據計算可以得到高電阻狀態之躍遷距離大約為十五□。 綜觀以上三種材料的電性表現，氧化鎳以及氧化鈦材料由於界面之電荷效應的影響在電子特性上並沒有比較傑出的表現，然而由於塊材特性的氧化鎢材料擁有包括單次儲存(OTP)、多次儲存(MTP)、多層單元(MLC)儲存記憶體應用、雙極式操作以及單極式操作等優越特性，在RRAM領域有著較高的商業應用潛力。

Due to the fact that traditional nonvolatile memory (Flash memory) with polycrystalline floating-gate structure will face the physical limitation below 20nm technology node, some emerging non-volatile memories such as magnetic random access memory (MRAM), phase change random access memory (PCRAM), and resistive random access memory (RRAM) are widely investigated. Since the advantages of RRAM include CMOS fully compatibility, high speed operation, good reliability, and high capacity, RRAM exhibits high potential in commercial applications for the next generation. The RRAM study is thus the main theme of this thesis. Although there are several materials successfully revealing resistance bistability under a certain electrical operation, it’s a pity that their CMOS compatibility is limited. Recently, metallic oxides by nickel (Ni), titanium (Ti), and tungsten (W) are reported to present good RRAM performance. Meanwhile, in the CMOS integration, nickel silicide (NiSi) is used in the middle-end-of-line (MEOL) process for reducing the contact resistance of both N- and P-MOS. Titanium nitride (TiN) is utilized respectively for back-end-of-line (BEOL) and MEOL processes to increase the metal adhesion and diffusion barrier capability. Tungsten (W) is widely accepted in the contact plug fabrication and it is the main material for the interconnection between CMOS device and BEOL circuit. Accordingly, RRAM materials with nickel oxide (NiOx), titanium oxide (TiOx), and tungsten oxide (WOx) are also the CMOS fully compatible materials. In this thesis, we discuss the RRAM characteristics of all these three materials in details, including the electrical performance, the switching model, and the applications. In NiOx-based RRAM study, bipolar resistance switching behavior in polycrystalline thin film shows obvious thin film contribution and oxygen content effects on on/off ratio; that is, the higher oxygen content sample exhibits higher on/off ratio. Also, its conduction mechanism entirely follows the Schottky emission, and the on/off ratio is strongly barrier height dependent. According to Schottky emission simulation, NiOx-based RRAM thin film with low oxygen content presents higher dielectric constant. The result could explain why NiOx-based RRAM thin film with low oxygen content needs higher energy to countervail the inner opposite electric field in the resistive switching process. In TiOx-based RRAM study, similar to the previous NiOx-based system, TiOx-based RRAM also shows bipolar resistance switching behavior. It is clear that the interfacial characteristics between TiOx-based thin film and electrode dominates the electrical performance, while the thickness effect of TiOx thin film on resistance switching properties is relatively minor. Again, similar to NiOx-based system, the transportation follows the Schottky emission and the on/off ratio is found to be strongly barrier height dependent. In addition, the designed experiment, TiOx / SiO2 hybrid system, indicates the importance of interfacial contribution, and the data retention is further found to be improved by this hybrid system. In WOx-based RRAM study, we found this system has high potential for several applications, such as the memory device with one-time-programming (OTP) and multi-times-programming (MTP) functionality. For MTP application, resistive state of WOx-based system can be switched reversibly by either bipolar or unipolar operation with the cycle endurance of exceeding 1000. On the other hand, this RRAM system is suitable for the multi-level-cell (MLC) application, too, due to its sufficient on/off ratio higher than 1000X. The highly scalable ability of this system is also discussed in this thesis. Meanwhile, a reliable characteristics is also demonstrated: high thermal stability of over 1000 hours at 250℃. With the mathematical fitting of WOx-based RRAM system, the electron conduction of high resistance state and low resistance state respectively follow the variable-range-hopping (VRH) transportation and minimum-metallic-conductivity (MMC). According to the fitting results, the hopping distance of high resistance state is found to be around 15□. NiOx, TiOx, and WOx materials, which are all CMOS fully compatible without contamination risk, are summarized here for RRAM applications. For the overall comparison of RRAM functionality, interfacial contribution (NiOx or TiOx) cannot provide better characteristics than bulk contribution (WOx) because the electron charging affects at interface to worsen the electrical performance of such kind of RRAM system. And the system with bulk contribution in turn could provide high potential for several commercial applications on electron devices, such as OPT, MTP, MLC, and unipolar operation as well.