Chapter 3 Results and discussion
3.5 XRD analysis
Fig. 3-28 shows the crystal structure of the HfOx film on the Ni/SiO2/Si substrate, it was investigated by x-ray diffractometer (XRD). No peak was observed around 30。
~ 34。. Only the Ni peaks at (111), (200), (220) was observed, which indicates that the film is amorphous state. The crystal structure was not changed after plasma treatment or metal deposition. This result may obtain the higher initial resistance and HRS.
20 30 40 50 60 70 80
In te n s it y ( a .u .)
2 theta (degree)
Without treatment CF4:O2 = 10:1 CF4:O2 = 2:8
Fig. 3-28 Crystal structure of the HfOx film by XRD analysis.
(220) (200)
(111)
Ni peaks
53
3.6 AES analysis
Fig. 3-29, Fig. 3-30 and Fig. 3-31 shows the depth profile of the region begin the top electron to bottom electron on the SiO2/Si substrate by Auger Electron Spectroscopy (AES). The sample without treatment indicates that no single of F atom observed at region of HfOx layer in the Fig. 3-29. Compared to the sample without treatment, Figs. 3-30 and 3-31 show similar results which numerous F atoms exist in the HfOx layer near the region of top electron.
0 500 1000 1500 2000
0.0 5.0x105 1.0x106 1.5x106 2.0x106 2.5x106 3.0x106 3.5x106
Without treatment
C o u n ts
Time(s)
C O Ni Hf F
Fig. 3-29 AES analysis of the sample without treatment.
54
0 500 1000 1500 2000 2500
0.0 5.0x105 1.0x106 1.5x106 2.0x106 2.5x106 3.0x106 3.5x106
Treatment of CF4:O2 = 10:1
C o u n ts
Time(s)
C O Ni Hf F
Fig. 3-30 AES analysis of the sample with treatment of CF4:O2=10:1.
0 500 1000 1500 2000 2500
0.0 5.0x105 1.0x106 1.5x106 2.0x106 2.5x106 3.0x106 3.5x106
Treatment of CF4:O2 = 2:8
C o u n ts
Time(s)
C O Ni Hf F
Fig. 3-31 AES analysis of the sample with treatment of CF4:O2=2:8.
55
3.7 XPS analysis
Fig. 3-32-Fig. 3-38 shows the chemical properties of HfOx stacks by X-ray Photoemission Spectroscopy (XPS) spectra respectively. According to previously reference [45-46], indicating that the F atoms were accumulated and formed Hf-F bond at the HfOx film by plasma treatment in Fig. 3-35.
Fig. 3-37 and Fig. 3-38 show F 1s peak which the F atoms exist in HfOx film.
And the peak located at ~685 eV corresponds to the F bonds in the bulk or surface of HfOx, indicating that F atoms are incorporated into HfOx by CF4/O2 plasma treatment.
Fig. 3-32 Hf 4f ESCA spectra of CF4 plasma treated 5 min sample [45]
56
Fig. 3-33 Hf 4f ESCA spectra of CF4 plasma treated sample [46].
57
10 15 20 25 30
Without treatment
In te n s it y ( a .u .)
Binding Energy (eV)
Hf 4f
Fig. 3-34 XPS spectra of Hf 4f of HfOx/Ni substrate.
10 15 20 25 30
CF4:O2 = 10:1
In te n s it y ( a .u .)
Binding Energy (eV)
Hf 4f
Fig. 3-35 XPS spectra of Hf 4f of HfOx/Ni substrate of the sample with treatment of CF4:O2=10:1.
58
15 20 25 30 35 40
Hf 4f
CF4:O2 = 2:8
In te n s it y ( a .u .)
Binding Energy (eV)
Fig. 3-36 XPS spectra of Hf 4f of HfOx/Ni substrate of the sample with treatment of CF4:O2=2:8.
675 680 685 690 695 700
CF4:O2 = 10:1
In te n s it y ( a .u .)
Binding Energy (eV)
F 1s
Fig. 3-37 XPS spectra of F 1s of HfOx/Ni substrate of the sample with treatment of CF4:O2=10:1.
59
685 690 695 700 705
F 1s CF4:O2 = 2:8
In te n s it y ( a .u .)
Binding Energy (eV)
Fig. 3-38 XPS spectra of F 1s of HfOx/Ni substrate of the sample with treatment of CF4:O2=2:8.
60
3.8 Driving mechanisms
Among the many proposed resistance switching models, the filament mechanism can explain the resistance switching behavior in this study. According to the filament model, the conducting elements, such as oxygen vacancies or metal ions congregating to form stronger and more localized conducting filaments, which lead to a transition from HRS to LRS. After reset process, the conducting filaments can be ruptured by a Joule heating effect, and the device was transited from LRS to HRS. In this work of different resistance characteristics, the reason may F atoms incorporation form Hf-F bonds and reduce the defects of interface by CF4/O2 plasma treatment. Compared to the sample without treatment, the Hf-F bonds may limit some distribution of conducting filament, and the defects of interface were reduced with O/F ions incorporation by plasma treatment. It may lead conducting filament easier to disruption, because the conducting filament was limited at similar region. The vicinity the banding energy of Hf-F band is larger than Hf-O band, so both reset voltage and conducting filament can be more stability with resistive switching process. Figs. 3-39, 3-40 and 3-41 show the schematic diagrams of driving mechanism of limited conductive filament between the sample with treatment and without treatment.
When the device was completed, it shows an HRS of initial state before forming process. After the positive forming process, conducting filament paths form as a soft breakdown in the dielectric material by oxygen migration toward top electron. Fig.
3-39 shows the sample without treatment. Compared to other samples, the filament may form by distribution of random path. Fig. 3-40 shows the sample with treatment of CF4:O2=10:1. The distribution of filament path may limit by Hf-F bond, so the conducting filament formed at similar region and exhibits more stability process characteristics. Fig. 3-41 shows the sample with treatment of CF4:O2=2:8. Due to the
61
better quality of surface, the forming process needs to operation several times for switching to the LRS state. It may cause the thin film to hard breakdown or generation the oxygen vacancy of large number, so the operation cycles become tail away. Fig.
3-42 shows one possible, the Ni atoms may diffuse into the thin film, forming the Ni metal filament then dominant conduction mechanism of resistive switching. Although the switching characteristics look improved, but the amount of Ni atom may diffuse too much during the repeat forming process, causing incompleteness of rupture of Ni filament, so the operation cycles were degraded.
Fig. 3-39 Schematic diagrams of driving mechanism of the sample without treatment.
62
Fig. 3-40 Schematic diagrams of limited conductive filament driving mechanism of the sample with treatment of CF4:O2=10:1.
Fig. 3-41 Schematic diagrams of conductive filament driving mechanism of the sample with treatment of CF4:O2=2:8.
63
Fig. 3-42 Schematic diagrams of metal conductive filament driving mechanism of the sample with treatment of CF4:O2=2:8.
64
Chapter 4 Conclusions
4.1 Conclusions
In this thesis, we discuss to influence of resistive switching characteristic of resistive random access memory for different proportion of plasma treatment using HfOx as resistive switching dielectric.
The sample without treatment shows about 20 operation cycles, high leakage current, unstable distribution of resistance, more than three orders for On/Off ratio and unstable reset process.
Compared to the sample without treatment, treatment of CF4:O2=10:1 shows more operation cycles, lower leakage current, more stable distribution of LRS resistance, more than three orders for On/Off ratio and stability for reset process.
Among those samples, the sample with treatment of CF4:O2=2:8 exhibits few operation cycles, lower leakage current, more stable distribution of LRS resistance, about two orders for On/Off ratio and stability for reset process.
For the resistive switching mechanisms, the filament model with formation and rupture of the conducting filaments can explained the resistive switching characteristics of HfOx RRAM devices.
After treatment of CF4:O2=10:1 process, the resistive switching characteristics were changed, it has been demonstrated that O/F incorporation can reduce defects of HfOx film. F atoms may limit some conducting filaments region, causing the distribution of conducting filament become more stability. Hence the reset process and distribution of LRS become more stability.
In addition, the sample with treatment of CF4:O2=2:8 shows few operation cycles, it may form the HfOx surface of better quality. This surface may easily form hard breakdown with repeat operation forming process, so operation cycles become fewer.
65
Moreover repeat set process may cause effect that Ni atom diffuse into dielectric and form metal conducting filament. So resistive switching characteristics and the On/Off ratio become different to other samples, which were summarized in table 4-1.
Table 4-1 Operation results of three samples.
66
4.2 Future Work
We have succeeded in developing the HfOx based RRAM device with surface plasma treatment, which could improve resistive switching properties. In the future, we can decrease the thickness of film which may reduce the forming voltage of device, improving for the increase of forming voltage. The HfOx can stack another one layer (such as TiOx) to form bi-layer which may achieve the self-compliance current, avoiding the requirement of compliance current. Further, the other material analysis can performed, such as secondary ion mass spectrometer (SIMS) may clear understand the condition of diffusion of Ni atoms. The parameter of temperature of treatment can rise to increase of reaction of bonding which may improve stability of device.
We are hoping that future research will focus on these views to realize the commercial applications of RRAM which we hope will ultimately result in better uniformity and reliability of its manufacture.
67
References
[1] K. Kim, J. H. Choi, and H. s. Jeong, “The future prospect of nonvolatile memory,”
in Proc. on VLSI Tech. Dig., 2005, pp. 88-94.
[2] Daniele Ielmini, “Reliability issues and modeling of Flash and post-Flash memory,” Microelectronic Engineering 86, 2009, pp. 1870-1875.
[3] C. kang, J. Choi, J. Sim, C. Lee, Y. Shin, J. Park, J. Sel, S. Jeon, Y. Park, and K.
Kim, “Effects of lateral charge spreading on the reliability of TANOS (TaN/AlO/SiN/Oxide/Si) NAND flash memory,” IEEE, 45th Annual International Reliability Physics Symposium, 2007, pp. 167-170.
[4] H. T. Lue, S. Y. Wamg, E. K. Lai, K. Y. Hsieh, R. Liu, and C. Y. Lu, “A High-Speed BE-SONOS NAND Flash Utilizing the Field-Enhancement Effect of FinFET,” in IEDM Tech. Dig., 2007, pp. 913-916.
[5] T. Nakamura, Y. Fujimori, N. Izumi, and A. kamisawa, “Application of Sr2Nb2O7
Family Ferroelectric Films for Ferroelectric Memory Field Effect Transistor,” Jpn.
J. Appl. Phys., vol. 37, no. 3B, 1998, pp. 1325-1327.
[6] R. Moazzami, “Ferroelectric thin film technology for semiconductor memory,”
Semicond. Sci. Technol., vol. 10, 1995, pp. 375-390.
[7] T. hayashi, Y. Igarashi, D. Inomata, T. Ichimori, T. Mitsuhashi, K. Ashikaga, T.
Ito, M. Yoshimaru, M. Nagata, S. Mitarai, H. Godaiin, T. Nagahama, C. Isobe, H.
moriya, M. Shoji, Y. Ito, H. Kuroda, and M. Sasaki, “A novel stack capacitor cell for high density FeRAM compatible with CMOS logic,” in IEDM Tech. Dig., 2002, pp. 543-546.
[8] J. M. Slaughter, R. W. Dave, M. Durlam, G. Kerszykowski, K. Smith, K. Nagel, B.
Feil, J. Calder, M. DeHerrera, B. Garni, and S. Tehrani, “High speed toggle MRAM with MgO-based tunnel junctions,” in IEDM Tech. Dig., 2005, pp.
68 873-876.
[9] R. Leuschner, U. K. Klostermann, H. Park, F. Dahmani, R. Dittrich, C. Grigis, K.
Hernan, S. Mege, C. Park, M. C. Clech, G. Y. Lee, S. Bournat, L. Altimime, and G. Mueller, “Thermal Select MRAM with a 2-bit Cell Capability for beyond 65 nm Technology Node,” in IEDM Tech. Dig., 2006, pp. 1-4.
[10] C. C. Hung, Y. S. Chen, D. Y. Wang, Y. J. Lee, W. C. Chen, Y. H. Wang, C. T.
Yen, S. Y. Yang, K. H. Shen, C. P. Chang, C. S. Lin, K. L. Su, H. C. Cheng, Y. J.
Wang, D. L. Tang, M. J. Tsai, and M. J. Kao, “Adjacent-Reference and Self-Reference Sensing Scheme with Novel Orthogonal Wiggle MRAM Cell,” in IEDM Tech. Dig., 2006 pp. 1-4.
[11] R. Neale, D. Nelson, Gordon Moore, “Nonvolatile and reprogrammable, the read-mostly memory is here,” Electronics, 1970, pp. 56-60.
[12] S. Lai and T. Lowrey, “OUM-A 180 nm nonvolatile memory cell element technology for stand alone and embedded applications,” in IEDM Tech. Dig., 2001, pp. 803-806.
[13] S. Lai, “Current Status of the Phase Change Memory and Its Future,” in IEDM Tech. Dig., 2003, pp. 255-258.
[14] A. Redaelli, A. Pirovano, F. Pellizzer, A. L. Lacaita, D. Ielmini, and R. Bez,
“Electronic switching effect and phase-change transition in chalcogenide materials,” IEEE Electron Device Lett., vol. 25, no. 10, 2004, pp. 684-686.
[15] D. S. Suh, K. H. P. Kim, J. S. Noh, W. C. Shin, Y. S. Kang, C. Kim, Y. Khang, and I. K. Yoo, “Critical Quenching Speed Determining Phase of Ge2Sb2Te5 in Phase Change Memory,” in IEDM Tech. Dig., 2006, pp. 1-4.
[16] Takashi Nakamura, Yoshikazu Fujimori, Naoki Izumi and Akira Kamisawa,
“Fabrication Technology of Ferroelectric Memories,” Jpn. J. Appl. Phys., vol. 37, pp. 1325-1327, 1998.
69
[17] R. Moazzami, “Ferroelectric thin film technology for semiconductor memory,”
Semicond. Sci. Technol., vol. 10, no. 375, 1995.
[18] Rainer Waser, Regina Dittmann, Georgi Staikov, and Kristof Szot,
“Redox-Based Resistive Switching Memories –Nanoionic Mechanisms, Prospects, and Challenges,” Adv. Mater., vol. 21, pp. 2632-2663, 2009.
[19] H. Y. Lee, P. S. Chen, C. C. Wang, S. Maikap, P. J. Tzeng, C. H. Lin, L. S. Lee, and M. J. Tsai, “Low-Power Switching of Nonvolatile Resistive Memory Using Hafnium Oxide,” Jpn. J. Appl. Phys., vol. 46, no. 4B, pp. 2175-2179, 2007.
[20] D. S. Jeong, H. Schroeder, U. Breuer, and R. Waser, “Characteristic electroforming behavior in Pt/TiO2/Pt resistive switching cells depending on atmosphere,” J. Appl. Phys., vol. 104, no. 12, pp. 123-716, 2008.
[21] K. Tsunoda, K. Kinoshita, H. Noshiro, Y. Yamazaki, T. Iizuka, Y. Ito, A.
Takahashi, A. Okano, Y. Sato, T. Fukano, M. Aoki, and Y. Sugiyama, “Low Power and High Speed Switching of Ti-doped NiO ReRAM under the Unipolar Voltage Source of less than 3 V,” in IEDM Tech. Dig., 2007, pp. 767-770.
[22] M. J. Lee, Y. Park, B. S. Kang, S. E. Ahn, C. Lee, K. Kim, W. Xianyu, G.
Stefanovich, J. H. Lee, S. J. Chung, Y. H. Kim, C. S. Lee, J. B. Park, I. G. Baek, and I. K. Yoo, “2-stack 1D-1R Cross-point Structure with Oxide Diodes as Switch Elements for High Density Resistance RAM Applications,” in IEDM Tech. Dig., 2007, pp. 771-774.
[23] S. Muraoka, K. Osano, Y. Kanzawa, S. Mitani, S. Fujii, K. Katayama, Y. Katoh, Z. Wei, T. Mikawa, K. Arita, Y. Kawashima, R. Azuma, K. Kawai, K.
Shimakawa, A. Odagawa, and T. Takagi, “Fast switching and long retention Fe-O ReRAM and its switching mechanism,” in IEDM Tech. Dig., 2007, pp. 779-782.
[24] K. Aratani, K. Ohba, T. Mizuguchi, S. Yasuda, T. Shiimoto, T. Tsushima,
70
T.Sone, K. Endo, A. Kouchiyama, S. Sasaki, A. Maesaka, N. Yamada, and H.
Narisawa, “A Novel Resistance Memory with High Scalability and Nanosecond Switching,” in IEDM Tech. Dig., 2007, pp. 783-786.
[25] C. Nauenheim, C. Kuegeler, A. Ruediger, and R. Waser, “Investigation of the electroforming process in resistively switching TiO2 nanocrosspoint junctions,”
Appl. Phys. Lett., vol. 96, p. 122902, 2010.
[26] P. Gonon, M. Mougenot, C. Vallée, C. Jorel, V. Jousseaume, H. Grampeix, and F.
El Kamel, “Resistance switching in HfO2 metal-insulator-metal devices,” J. Appl.
Phys., vol. 107, p. 074507, 2010.
[27] Buh, Gyoung-Ho; Hwang, Inrok and Park, Bae Ho, “Time-dependent electroforming in NiO resistive switching devices,” Appl. Phys. Lett., vol. 95, p.
142101, 2009.
[28] S. M. Sze, “2nd ed. New York: John Wiley & Sons,” Physics of Semiconductor Devices, 1981, pp. 402-407.
[29] J.J. Yang, M.D. Pickett, X. Li, D.A.A. Ohlberg, D.R. Stewart and R.S. Williams,
“Memristive switching mechanism for metal/oxide/metal nanodevices,”
Nanotechnology, vol. 3, pp. 429-433, 2008.
[30] Rainer Waser, Regina Dittmann, Georgi Staikov and Kristof Szot, “Redox-Based Resistive Switching Memories –Nanoionic Mechanisms, Prospects, and Challenges,” Adv. Mater., vol. 21, pp. 2632-2663, 2009.
[31] Sheng-Yu Wang, Dai-Ying Lee, Tseung-Yuen Tseng and Chih-Yang Lin,
“Effects of Ti top electrode thickness on the resistive switching behaviors of rf-sputtered ZrO2 memory films,” Appl. Phys. Lett., vol. 95, p. 112904, 2009.
[32] Min Kyu Yang, Jae-Wan Park, Tae Kuk Ko, and Jeon-Kook Lee, “Bipolar resistive switching behavior in Ti/MnO2/Pt structure for nonvolatile memory devices,” Appl. Phys. Lett., vol. 95, p. 042105, 2009.
71
[33] Yong-Mu Kim and Jang-Sik Lee, “Reproducible resistance switching characteristics of hafnium oxide-based nonvolatile memory devices,” J. Appl.
Phys., vol. 104, p. 114115, 2008.
[34] Seunghyup Lee, Heejin Kim, Dong-Jin Yun, Shi-Woo Rhee, and Kijung Yong,
“Resistive switching characteristics of ZnO thin film grown on stainless steel for flexible nonvolatile memory devices,” Appl. Phys. Lett., vol. 95, p. 262113, 2009.
[35] Hsin-Hung Huang, Wen-Chieh Shih, and Chih-Huang Lai, “Nonpolar resistive switching in the Pt/MgO/Pt nonvolatile memory device,” Appl. Phys. Lett., vol.
96, p. 193505, 2010.
[36] Akihito Sawa, “Resistive switching in transition metal oxides,” Materialstoday, vol. 11, 2008, pp. 28-36.
[37] Kuibo Yin, Mi Li, Yiwei Liu, Congli He, Fei Zhuge, Bin Chen, Wei Lu, Xiaoqing Pan, and Run-Wei Li, “Resistance switching in polycrystalline BiFeO3
thin films,” Appl. Phys. Lett., vol. 97, p. 042101, 2010.
[38] A. Odagawa, H. Sato, I.H. Inoue, H. Akoh, M. Kawasaki and Y. Tokura,
“Colossal electroresistance of a Pr0.7Ca0.3MnO3 thin film at room temperature,”
Phys. Rev., no. B 70, p. 224403, 2004.
[39] Dong-jun Seong, M. Jo, D. Lee and H. Hwang, “HPHA Effect on Reversible Resistive Switching of Pt/Nb-Doped SrTiO3 Schottky Junction for Nonvolatile Memory Application,” Electrochemical and Solid-State Letters, vol. 10, no. 6, pp.
168-170, 2007.
[40] D. S. Shang, L. Shi, J. R. Sun, B. G. Shen, F. Zhuge, R. W. Li, and Y. G. Zhao,
“Improvement of reproducible resistance switching in polycrystalline tungsten oxide films by in situ oxygen annealing,” Appl. Phys. Lett., vol. 96, p. 072103, 2010.