四、 Results
4.5 Metal-oxide RRAM organization
In the results of our metal-oxide RRAM experiments, three materials, including NiOx, TiOx, and WOx, exhibit the resistive switching character. Both NiOx and TiOx
exhibit the bipolar resistive switching behavior and their conduction mechanism follows the Schottky emission.
However, the WOx shows the best RRAM performance in our experiment. Both bipolar and unipolar operation resistive switching character was found in this grade WOx material. Moreover, this material can also be used for one-time-programming (OTP), multi-time-programming (MTP), and multi-level-cell (MLC) application. It also shows the shrinkable potential and the threshold voltage reducing with the shrinking ratio. Moreover, this WOx RRAM shows good reliability performance, including better cycle endurance, good thermal stability, better stress performance, and good data retention. The conduction mechanism of high and low resistance state is variable-range-hopping (VRH) and the minimum-metal-conductivity (MMC), respectively.
104
Chapter 5 : Summary
We have demonstrated the CMOS fully compatible NiOx, TiOx, and WOx based resistance random access memories successfully in this study. All three materials exhibit clear bistable resistive switching character and their resistive characters show the relationship between barrier high and on/off ratio.
Polycrystalline NiOx shows the RRAM performance relationship between thin film and the oxygen content. Its conduction mechanism follows the Schottky emission, and the Schottky emission curve shows the relationship between dielectric constant and oxygen content. According to the calculation of this curve, samples with low oxygen content have a higher dielectric constant. This result explains that samples with low oxygen content need more applied voltage to countervail the inner opposite electric field in the resistive switching process.
Although TiOx follows the Schottky emission, the electric character exhibits the interface contribution and thickness independence relationship. TiOx / SiO2 hybrid system also indicates that the interface contribution and the performance of data retention, cycle endurance, and read disturb can be improve by this hybrid system.
However, the WOx based RRAM shows more potential than others in the non-volatile memory application such as one-time-programming (OTP), multi-times-programming (MTP), multi-level-cell (MLC), shrinkable memory cell, and the unipolar operation. The conduction mechanism of tungsten oxide follows variable-range-hopping (VRH) in high resistance state (HRS) and the electrical behavior of low resistance state (LRS) is close to minimum-metal-conductivity (MMC).
Moreover, the good performance such as high on/off ratio (>1000), good cycle endurance (>1000), high thermal stability (>2000 hrs at 250℃), good read disturb (>1000 sec at 1V), high speed operation (<10ns), small cell size (~9nm), and low power consumption (<10uA) of tungsten oxide based RRAM shows highly potential for the next generation non-volatile memory applications.
According to the results of this study, all three NiOx, TiOx and WOx are CMOS fully compatible materials without contamination risk. For the overall comparison of RRAM functionality, NiOx or TiOx cannot provide better characteristics than WOx. The WOx based RRAM could provide high potential for several commercial applications on electron devices, such as OTP, MTP, MLC, and unipolar operation as well. Therefore, we believe it is suitable for application in the next generation non-volatile memory device.
105
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Shih-Chang Tsai, Ching-Hsiung Lee, Sheng-Hui Hsieh, Chun-Fu Chen, Yen-Hao Shih, Kuang-Yeu Hsieh, Rich Liu, and Chih-Yuan Lu, “Tungsten Oxide Resistive Memory Using Rapid Thermal Oxidation of Tungsten Plugs”, Japanese J. of Appl.
Phys., vol.49, p.04DD17, (2010).
113
[90]. Seonghyun Kim, Kuyyadi P. Biju, Minseok Jo, Seungjae Jung, Jubong Park, Joonmyoung Lee, Wootae Lee, Jungho Shin, Sangsu Park, and Hyunsang Hwang,
“Effect of Scaling WOx-Based RRAMs on Their Resistive Switching Characteristics”, IEEE Electron Device Lett., vol.32, no.5, p671, (2011).
[91]. W. C. Chien, Y. C. Chen, E. K. Lai, F. M. Lee, Y. Y. Lin, Alfred T. H. Chuang, K.
P. Chang, Y. D. Tao, T. H. Chou, H. M. Lin, M. H. Lee, Y. H. Shih, K. Y. Hsieh, Chih-Yuan Lu, Appl. Phys. A, 102, 901, (2011).
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Publication List Paper :
[1]. M. D. Lee, C. K. Lo, T. Y. Peng, S. Y. Chen, Y. D. Yao, “Endurance study of switching characteristics in NiO films”, J. Magn. Magn. Mater. vol.310, p.e1030, (2007).
[2]. M. D. Lee, C. H. Ho, C. K. Lo, T. Y. Peng, Y. D. Yao, “Effect of Oxygen Concentration on Characteristics of NiO-Based Resistance Random Access Memory”, IEEE Trans. Magn., vol.43, no.2, 939, (2007).
[3]. M. D. Lee, C. H. Ho, Y. D. Yao, “CMOS Fully Compatible Embedded Non-Volatile Memory System With TiO2-SiO2 Hybrid Resistive-Switching Material”, IEEE Trans. Magn., vol.47, p.653, (2011).
[4]. M. D. Lee, C. K. Lo, T. Y. Peng, K. L. Yau, S. Y. Chen, Y. D. Yao, “Epitaxial layer of MgO (001) grown on Si (001) wafer by e-beam evaporation”, J. Magn. Magn.
Mater. vol.304, p.e44, (2006).
[5]. M. D. Lee, N. V. Nong, N. P. Thuy, Y. D. Yao, S. F. Lee, Y. Liou, Y. Y. Chen, C. R.
Wang, “Temperature dependence of magnetic properties in Ni-Mn-Ga shape memory alloys”, phys. stat. sol. (c), vol.1, p.3579, (2004).
[6]. C. H. Ho, E. K. Lai, M. D. Lee, C. L. Pan, Y. D. Yao, K. Y. Hsieh, R. Liu, C. Y. Lu,
[6]. C. H. Ho, E. K. Lai, M. D. Lee, C. L. Pan, Y. D. Yao, K. Y. Hsieh, R. Liu, C. Y. Lu,