IEEE ELECTRON DEVICE LETTERS, VOL. 29, NO. 4, APRIL 2008
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Resistive Memory Switching of CuxO Films for a Nonvolatile Memory Application H. B. Lv, M. Yin, X. F. Fu, Y. L. Song, L. Tang, P. Zhou, C. H. Zhao, T. A. Tang, B. A. Chen, and Y. Y. Lin
Abstract—Polycrystalline Cux O films produced by plasma oxidation are investigated for nonvolatile memory applications. Reversible bistable resistive switching from a high-resistance state to a low-resistance state, and vice versa, is observed in an integrated Al/Cux O/Cu structure under voltage sweeping. More than 3000 repetitive cycles are observed in 180-µm memory devices with an on/off ratio of ten times. Data testing shows that the devices meet the ten-year retention requirement for the storage of programmed logic signals. Index Terms—Cux O, on/off ratio, plasma oxidation, resistive switching, retention test. Fig. 1. (a) XPS spectrum of Cu 2p3/2 for a 10-min O-plasma-treated sample. The insert is the XRD pattern of the oxidized film. (b) Cross-sectional SEM image of a Cux O film.
I. I NTRODUCTION
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ESISTIVE random access memories (RRAMs) have attracted a great deal of attention in recent years due to their low power, low cost, and high-speed performance. It is found that a variety of materials can be utilized in RRAM applications, including binary metal oxides [1], [2], doped complex metal oxides (perovskites) [2], [3], chalcogenides [4], and even organic semiconductors [5]. Among them, binary metal oxides are much more preferred because their simple composition and low-temperature treatment are more compatible with silicon process. To explain the resistive switching characteristics, such models have been developed as conductive filament formation and rupture [6], [7] and a space-charge-limited current with trapping/detrapping process [2]. However, the intrinsic fundamental physics of the resistive switching is still unclear [8], [9]. In this letter, Cux O was studied for nonvolatile memory applications because Cu is relatively inexpensive and widely used in modern semiconductor process and also because the integration of Cux O memory cells is fully compatible with the technology of the present standard Cu interconnection. II. E XPERIMENTS A 500-nm electrochemical plated Cu was deposited on a Cu [seed layer (120 nm)]/Ta 10 nm/TaN 15 nm/SiO2 400 nm/Si substrate with a standard Novellus ECP system without further thermal treatment. Subsequently, film oxidation was performed Manuscript received August 20, 2007; revised December 05, 2007. This work was supported by the National Natural Science Foundation of China under Grant 60206005, Grant 60376017, and Grant 60676007. The review of this letter was arranged by Editor S. Chung. The authors are with the ASIC and System State Key Laboratory, Department of Microelectronics, Fudan University, Shanghai 200433, China (e-mail:
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LED.2008.917109
on a Samco RIE-10NR system for 10 min. Before oxidation, the chamber of the reactive ion etching system was cleaned with O plasma for 60 min. Then, we maintain the RF power and working pressure for O plasma oxidation at 300 W and 6 Pa with a flowing rate of pure O2 at 10 sccm. The predeposited Cu was used as the bottom electrode, whereas Al top electrodes (400 nm) were created by electron beam evaporation and liftoff process in different square sizes ranging from 20 to 900 µm. The morphologies of as-deposited films were examined on a scanning electron microscopy (SEM), whereas the crystal structure and chemical bonds were analyzed by X-ray diffraction (XRD) patterns and X-ray photoelectron spectroscope (XPS) spectra, respectively. The electrical characterizations of the Al/Cux O/Cu structure were performed on a Keithley 4200-SCS semiconductor parameter analyzer and a Keithley 3402 pulse generator. III. R ESULTS AND D ISCUSSION The composition analysis of Cux O films after the 10-min plasma oxidation was conducted by XPS. As shown in Fig. 1(a), the main peak at 932.23 eV is Cu 2p3/2 , which is a typical characteristic of Cu2 O. Nevertheless, beside the peak, there is a shoulder at 933.83 eV, which indicates the existence of CuO. The inset in Fig. 1(a) shows the XRD pattern of the film, in which only Cu2 O (111) reflection is found, indicating Cu2 O as the main composition in the film. This is consistent with the XPS results. Fig. 1(b) shows the cross-sectional SEM image. An interface between the film and the substrate is identified, revealing that the Cux O thickness is about 150 nm. Fig. 2(a) shows the typical current–voltage (I−V ) loop of a 180-µm Al/Cux O/Cu structure, where the bistable resistive switching characteristic is observed. The device can be switched from a high-resistance state (HRS) to a lowresistance state (LRS), i.e., the SET process, by sweeping the
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IEEE ELECTRON DEVICE LETTERS, VOL. 29, NO. 4, APRIL 2008
Fig. 2. (a) Typical I−V characteristics of an Al/Cux O/Cu structure at a 180-µm size with a 150-nm-thick Cux O film under voltage sweeping. The arrows represent the directions of voltage sweeping. (b) Cell size dependences of the HRS and LRS resistances. (c) Temperature dependences of the HRS and LRS.
bias voltage up to 1.8 V with the current compliance of 5.0 mA, regardless of the voltage polarity. Similarly, the transition from the LRS to the HRS, i.e., the RESET process, can be achieved by a second voltage sweeping. The bistable resistive characteristic is symmetric in the voltage polarity, as shown in Fig. 2(a), which indicates that the predominant switching mechanism is a bulk effect rather than an interface effect. The resistivity of the initial Cux O film in our study is on the order of 106 Ω · cm, much lower than that of the electrodeposited cuprous oxide [10], which is 109 −1012 Ω · cm. It seems that Cux O grown with high-energy O plasma at room temperature introduces more defects than those grown by electrodeposition. The large amount of defects, including oxygen vacancies and crystal boundary, leads to a dramatic drop in the resistivity of the Cux O film. Fig. 2(b) is the cell area dependence of the HRS and LRS resistances. The data for each specific size were examined in four different cells at a read voltage of 0.1 V. The HRS resistance increases as the cell area decreases, whereas the LRS resistance is almost independent of the cell area, indicating the formation of local conductive filaments during the SET process. In contrast, the RESET process follows the rupture of those local conductive paths. Fig. 2(c) illustrates the temperature dependence of the resistances of the HRS and LRS. To minimize the influence of the extra electrical field, the resistance was measured at as low as 0.1 V. It is found that values of both the HRS and LRS resistances have a negative temperature coefficient. Fig. 3(a) shows the resistance variation with the pulse amplitude and the pulsewidths fixed at 1 µs and 1 ms for SET and RESET, respectively. When the rising voltage reaches a critical value, the resistance switching occurs. Fig. 3(b) shows the HRS and LRS as functions of pulsewidth with the SET and RESET voltages at +3 and −0.6 V, respectively. When the pulsewidth is long enough, the resistance switching occurs. The typical operation pulsewidth ranges from 10−8 to 10−6 s for SET and from 10−6 to 10−3 s for RESET. This great fluctuation of the SET and RESET times of the memory cells in our study results in difficulty in programming the device as a practical high-speed nonvolatile memory. The intrinsic physics of the fluctuation is still under investigation. Fig. 4 shows the dependence of the resistance on the repetitive switching cycle under bipolar voltage pulses in a 180-µm cell. The device was
Fig. 3. (a) Pulse amplitude dependence of the resistance for an Al/Cux O/Cu structure at a 180-µm size with widths at 1 µs and 1 ms for SET and RESET, respectively. (b) HRS and LRS as functions of pulsewidth for an Al/Cux O/Cu structure at a 180-µm size with the SET and RESET voltages at 3 and −0.6 V, respectively.
Fig. 4. Cycle dependence of the resistance for an Al/Cux O/Cu structure at a 180-µm size in series with a 5000-Ω resistor under the current compliance of 1 mA.
programmed by pulses of 4 V/10 µs for SET and −1 V/1 ms for RESET. The SET compliance current was approximately kept at 1 mA with a 5000-Ω series resistor in the circuits. More than 3000 program/erase cycles are observed with a ten times HRS/LRS ratio, demonstrating that the device is quite promising for a nonvolatile memory application. For the purpose of a practical memory application, an intrinsic data retention test was carried out. The standard procedure for this assessment relies on the evaluation of the time-dependent resistance change under different annealing
LV et al.: MEMORY SWITCHING OF Cux O FILMS FOR A NONVOLATILE MEMORY APPLICATION
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Fig. 5. (a) Time-dependent HRS resistance at 130 ◦ C, 140 ◦ C, and 150 ◦ C for an Al/Cux O/Cu structure at a 180-µm size. (b) Arrhenius plot and extrapolation for the retention time.
temperatures. At a specific annealing temperature, the time for the resistance falling below a reference value is defined as the retention failure time. In our test, each of the data was checked three times on three identical cells. Fig. 5(a) shows the timedependent HRS value of 180-µm Al/Cux O/Cu cells at three different temperatures. The time for the resistance falling below 500 Ω is defined as the retention failure time. At 150 ◦ C (the green line), a two-step change of resistance was observed, as occasionally happened to some samples, probably due to the formation of tiny filaments at the early stage of the resistive switching. With the creation of more conductive filaments, the cell resistance abruptly decreases, indicating the completion of the resistive switching. Fig. 5(b) shows the retention extrapolation with the Arrhenius plot, following the work of Pirovano et al. [11]. As shown by the data, the ten-year retention time can be reached at 98.6 ◦ C, which is good enough for a practical nonvolatile memory application. Despite the various models developed for explaining the resistive switching mechanism, the underlying physics of this issue is still disputable. From our experimental results, the conductive filament formation and rupture models are more suitable for interpreting the switching phenomenon. The linear I−V curve and cell area independence of the LRS predict the local conductive filament formation during the SET process. It is reasonable to deduce that many local conductive Cux+ζ O regions exist in the Cux O films, resulting from the large amount of oxygen vacancies and crystal boundary. The SET process could be originated from these conductive local regions linked with each other, as well as the formed conductive filaments. However, the reoxidation of the Cux+ζ O phase in some parts of the filaments makes the RESET process possible. Finally, the activation energy Ea for the ionic reoxidation, as a result of the involvement of the active O movement, is calculated through the solid-line fitting in Fig. 5(b). IV. C ONCLUSION The reproducible bistable resistive switching is observed in polycrystalline Cux O films grown with the plasma oxidation technique. By XRD and XPS measurements, the film is mainly composed of Cu2 O coupled with a small composition of CuO.
More than 3000 repetitive switching cycles of an Al/Cux O/Cu structure are demonstrated with a ten times on/off resistance ratio. Moreover, the ten-year retention time at 98.6 ◦ C is extrapolated by the Arrhenius plot. Local conductive filament formation and rupture model are considered essential to the resistive switching. Considering the excellent performance, the Cux O-based RRAM shows high potential for a nonvolatile memory application. R EFERENCES [1] C. Rohde, B. J. Choi, D. S. Jeong, S. Choi, J. S. Zhao, and C. S. Hwang, “Identification of a determining parameter for resistive switching of TiO2 thin films,” Appl. Phys. Lett., vol. 86, no. 26, p. 262 907, Jun. 2005. [2] A. Beck, J. G. Bednorz, C. Gerber, C. Rossel, and D. Widmer, “Reproducible switching effect in thin oxide films for memory applications,” Appl. Phys. Lett., vol. 77, no. 1, pp. 139–141, Jul. 2000. [3] S. Srivastava, N. K. Pandey, P. Padhan, and R. C. Budhani, “Current switching effects induced by electric and magnetic fields in Sr-substituted Pr0.7 Ca0.3 MnO3 films,” Phys. Rev. B, Condens. Matter, vol. 62, no. 21, pp. 13 868–13 871, Dec. 2000. [4] D. Adler, M. S. Shur, M. Silver, and S. R. Ovshinsky, “Threshold switching in chalcogenide-glass thin films,” J. Appl. Phys., vol. 51, no. 6, pp. 3289–3309, Jun. 1980. [5] T. Oyamada, H. Tanaka, K. Matsushige, H. Sasabe, and C. Adachi, “Switching effect in Cu:TCNQ charge transfer-complex thin films by vacuum codeposition,” Appl. Phys. Lett., vol. 83, no. 6, pp. 1252–1254, Aug. 2003. [6] E. L. Cook, “Model for the resistive–conductive transition in reversible resistance-switching solids,” J. Appl. Phys., vol. 41, no. 2, pp. 551–554, Feb. 1970. [7] D. C. Kim, S. Seo, S. E. Ahn, D. S. Suh, M. J. Lee, B.-H. Park, I. K. Yoo, I. G. Baek, H. J. Kim, E. K. Yim, J. E. Lee, S. O. Park, H. S. Kim, U.-I. Chung, J. T. Moon, and B. I. Ryu, “Electrical observations of filamentary conductions for the resistive memory switching in NiO films,” Appl. Phys. Lett., vol. 88, no. 20, pp. 202 102.1–202 102.3, May 2004. [8] A. Sawa, T. Fujii, M. Kawasaki, and Y. Tokura, “Hysteretic current–voltage characteristics and resistance switching at a rectifying Ti/Pr0.7 Ca0.3 MnO3 interface,” Appl. Phys. Lett., vol. 85, no. 18, pp. 4073–4075, Nov. 2004. [9] S. Seo, M. J. Lee, D. H. Seo, E. J. Jeong, D. S. Sur, Y. S. Joung, and I. K. Yoo, “Reproducible resistance switching in polycrystalline NiO films,” Appl. Phys. Lett., vol. 85, no. 23, pp. 5655–5657, Dec. 2004. [10] A. E. Rakhshani, “The role of space-charge-limited-current conduction in evaluation of the electrical properties of thin Cu2 O films,” J. Appl. Phys., vol. 69, no. 4, pp. 2365–2369, Feb. 1991. [11] A. Pirovano, A. Redaelli, F. Pellizzer, F. Ottogalli, M. Tosi, D. Ielmini, A. L. Lacaita, and R. Bez, “Reliability study of phase-change nonvolatile memories,” IEEE Trans. Device Mater. Rel., vol. 4, no. 3, pp. 422–427, Sep. 2004.
