IEEE ELECTRON DEVICE LETTERS, VOL. 32, NO. 4, APRIL 2011
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Redox Reaction Switching Mechanism in RRAM Device With Pt/CoSiOX /TiN Structure Yong-En Syu, Ting-Chang Chang, Tsung-Ming Tsai, Ya-Chi Hung, Kuan-Chang Chang, Ming-Jinn Tsai, Ming-Jer Kao, and Simon M. Sze
Abstract—This letter investigates the resistive random access memory device characteristics and the physical mechanism of a device with a TiN/CoSiOX /Pt structure. In general, the mechanism is regarded as a redox reaction in the dielectric interface between the Ti electrode and the conductive filament. Furthermore, the switching voltage is correlated only with redox reaction potential. A designed circuit is used to accurately observe the resistance switching process with a pulse generator and an oscilloscope, which reveals that the switching process is related to both time and voltage. The constant switching energy demonstrates that the switching mechanism is the redox reaction. Index Terms—Cobalt silicon oxide (CoSiOX ), nonvolatile memory (NVM), redox reaction, resistance switching.
search, cobalt silicon oxide (CoSiOX ) is chosen for the RRAM switching layer because cobalt silicide is extremely compatible with the prevalent complementary metal–oxide–semiconductor process. Currently, the switching mechanisms in RRAM are a subject of heated controversy. The most reasonable explanation is related to the filament formation and rupture. Many designed experiments have been purposed to discuss characteristic factors of the filament such as resistance, temperature, and voltage [5, 6]. In this study, a designed circuit to investigate the relationship between voltage and time reveals that formation and rupture of filaments were correlated with the total energy of applied power.
I. I NTRODUCTION
W
ITH the growing demand for powerful mobile electronic products, nonvolatile memory (NVM) has been widely applied due to its low-power-consumption requirements. However, conventional nonvolatile floating memory is expected to reach certain technical and physical limits in the future [1], [2]. In order to overcome this problem, alternative memory technologies have been extensively investigated. Among these NVMs, resistive random access memory (RRAM) has been attracting increasing interest due to its simple structure, low power consumption, rapid operation, and high density integration [3]–[9]. In such metal–insulator–metal cells, the insulator layer (switching layer) exhibits reversible electricfield-induced resistance switching between the high resistance state (HRS) and the low resistance state (LRS). In this reManuscript received December 14, 2010; accepted December 29, 2010. Date of publication February 14, 2011; date of current version March 23, 2011. This work was supported by the National Science Council of Taiwan under Contracts NSC-99-2120-M-110-001 and NSC-97-2112-M-110-009-MY3. The review of this letter was arranged by Editor W. T. Ng. Y.-E. Syu is with the Department of Physics, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan (e-mail:
[email protected]). T.-C. Chang is with the Department of Physics, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan, and also with the Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan (e-mail:
[email protected]). T.-M. Tsai, Y.-C. Hung, and K.-C. Chang are with the Institute of Materials Science and Engineering, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan (e-mail:
[email protected];
[email protected]. tw;
[email protected]). M.-J. Tsai and M.-J. Kao are with the Nanoelectronic Technology Division, Electronics and Optoelectronics Research Laboratories, ITRI, Hsinchu 31040, Taiwan (e-mail:
[email protected];
[email protected]). S. M. Sze is with the Institute of Electronics, National Chiao Tung University, Hsinchu 300, Taiwan (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.2011.2104936
II. E XPERIMENTAL S ETUP The structure of the resistive NVM was fabricated as follows: A 30-nm-thick CoSiOX thin film was deposited on TiN/SiO2 /Si substrates by RF magnetron sputtering using a target of Co : Si = 1 : 2, in Ar/O2 = 30/10 sccm mixed-gas ambient with a working pressure of 6 mtorr. Finally, Pt with a thickness of about 200 nm was deposited by DC magnetron sputtering on the CoSiOX films to complete the individually sandwiched Pt/CoSiOX /TiN memory cells. Photolithography and lift-off technique were employed to shape the cells into a square pattern with an area of 1–64 μm2 . All of the electric characteristics were measured by an Agilent B1500 semiconductor characterization analyzer, and the pulse signal was observed by an Agilent DSO 8104A oscilloscope. III. R ESULTS AND D ISCUSSION Fig. 1 shows the bipolar resistance switching characteristics for the Pt/CoSiOX /TiN device. The “forming process” is required to activate the as-deposited cells, and the memory cell was switched from HRS to LRS. The repeatable resistance switching behavior can be observed after the forming process. By sweeping the bias to negative over the reset voltage (Vreset ), a gradual decrease of the current was observed, where the cell switches from LRS to HRS, which is called the “reset process.” Conversely, the cell returns to LRS when applying a larger positive bias, which is called the set voltage (Vset ), i.e., the “set process.” During the set process, a compliance current of 10 mA is assigned to prevent permanent breakdown. The resistance ratio of HRS to LRS is approximately 102 times at the reading voltage of 0.1 V, which remains with no degradation after continuous I–V sweep measurement of 100 cycles, as shown in the left inset of Fig. 1. The distribution of the set
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IEEE ELECTRON DEVICE LETTERS, VOL. 32, NO. 4, APRIL 2011
Fig. 1. Typical bipolar resistance switching I–V curves of the Pt/CoSiOX /TiN cells. The bottom left and right insets show the HRS/LRS and the statistics of Vset during 100 resistance switching cycles, respectively.
Fig. 3. (a) Equivalent circuit diagram of the designed circuit. (b) Experimental result of a triangular wave operated in the setting process.
Fig. 2. Cell area dependence of the resistance values in HRS and LRS. The inset plot shows the schematic diagrams of the driving mechanism of bipolar switching through the electrochemical reaction to form/disrupt the conducting filaments for LRS/HRS during the set/reset process.
voltage is shown in the inset by a bar chart, with the set voltage distributed in the range from 0.7 to 1.4 V, with over 80% concentrated in the region from 1 to 1.2 V. Previous reports have classified the bipolar behavior mechanism into “interface-type” and “filament-type” conductive paths [7], [8]. The Roff /Ron ratio is independent of the pattern size, shown in Fig. 2. Therefore, the formation/disruption model of localized conducting filaments is preferred as the driving mechanism of resistive switching for CoSiOX RRAM. During the forming process triggered by a sufficient electric field, oxygen ions (O2− ) are created, accompanying oxygen vacancies (V2+ O ); then, these oxygen ions (vacancies) drift toward the anode (cathode), as shown in the inset of Fig. 2. At the anode, the oxygen ions transform into oxygen and/or are absorbed by the anode. Simultaneously, at the cathode, an electroreduction process for the transition metal cations (V2+ O ) occurs, changing the oxide into a metallically conducting phase. The conducting region consists of metallic defects extending toward the anode to form the local conductive paths.
In order to accurately observe the resistance switching process, the circuit is designed by a pulse generator and an oscilloscope, as shown in the equivalent circuit in Fig. 3(a). The voltage (VRRAM ) and current (IRRAM ) on the RRAM device were obtained from the oscilloscope signal, such that VRRAM = VCH1 − VCH2 and IRRAM = VRRAM /RRRAM . To simulate the resistance switching I–V curve in DC sweep mode, the pulse generator imposed a triangular wave on the RRAM device (at HRS). The reset process was operated in DC mode to confirm the resistance state at the same HRS before the pulse setting process, with the experimental result shown in Fig. 3(b). The black and red lines show the CH1 and CH2 signals, respectively. The initial CH2 signal approaches zero because the RRAM device is in HRS. At 1.28 V, both signals vary sharply, indicating that the RRAM device switches from HRS to LRS (set process). In this study, the critical set voltage, which is the minimal voltage for a filament path formation, is defined as 1.17 V by an average set voltage at I–V sweep measurement of 100 cycles. The critical set voltage is considered irrelative to time, so it is an approximate set voltage at DC mode because the measure time of DC mode is very long. Furthermore, the delta set time (Δtset ) and delta set voltage (ΔVset ) can be defined from the transient phenomena. In general, the switching voltage is considered constant, and the operating voltage exceeds the critical value. In addition, previous research has determined that the switching voltage increases with decreasing sweeping time [9]. This study imposes a triangular wave under five leading time conditions: I (500 ns), II (1 μs), III (2 μs), IV (5 μs), and V (10 μs) with a fixed top voltage of 2 V and a trailing time of 100 ns, thus producing different sweeping voltage times. The reset process was operated in DC mode to confirm the resistance state at
SYU et al.: REDOX REACTION SWITCHING MECHANISM IN RRAM DEVICE WITH PT/COSIOX /TIN STRUCTURE
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CoSiO-based RRAM device with the Pt/CoSiOX /TiN structure. The conduction path can be assumed to be filament type due to the fact that the resistance value is unconnected with cell size. In this research, a designed circuit utilizing a pulse generator and an oscilloscope is used to observe the resistance switching process. The experimental result shows that the setting voltage is related to setting time, with ΔVset and (Δtset )−0.5 inversely proportional. These results can be used to correctly allocate voltage and time to control RRAM. In addition, the switching mechanism is proved to be the redox reaction through determining that switching energy remains constant. ACKNOWLEDGMENT Fig. 4. Delta set voltage depends on the delta set time with different pulse leading times in the setting process.
the same HRS before the pulse setting process. Fig. 4 shows the result of ΔVset − Δtset at different pulse leading times and reveals that the setting process is also related to time. In order to determine the physical mechanism, the result of the fitting curve conforms to the following formula: ΔVset = a(Δtset )−0.5 + b = 1.69 × 10−4 (Δtset )−0.5 + 0.01. Therefore (ΔVset )2 × Δtset = constant.
(1)
However, based on electric chemical reaction theory, the reaction can only be driven with sufficient energy. We calculate the setting energy (WRRAM ) over the critical voltage in the RRAM device from a critical position to a setting point as follows: Δt set
(ΔIRRAM × ΔVRRAM ) · dt
WRRAM =
0
= (ΔVset )2 × Δtset
1 3RRRAM
.
(2)
The setting energy of RRAM is constant in the same resistance state, which is proved by substituting (1) into (2). In this study, the switching mechanism is proved to be related to the redox reaction by confirmation of a constant switching energy. IV. C ONCLUSION In conclusion, we have investigated the bipolar resistance switching characteristics and physical mechanism of the
This work was performed at the National Science Council Core Facilities Laboratory for Nano-Science and NanoTechnology in the Kaohsiung–Pingtung area and assisted with the Nanoelectronic Technology Division, Electronics and Optoelectronics Research Laboratories, ITRI. R EFERENCES [1] S. Tiwari, F. Rana, K. Chan, H. Hanafi, W. Chan, and D. Buchanan, “Volatile and non-volatile memories in silicon with nano-crystal storage,” in IEDM Tech. Dig., 1995, pp. 521–524. [2] T. C. Chang, S. T. Yan, P. T. Liu, C. W. Chen, S. H. Lin, and S. M. Sze, “A novel approach of fabricating germanium nanocrystals for nonvolatile memory application,” Electrochem. Solid-State Lett., vol. 7, no. 1, pp. G17– G19, 2004. [3] M.-J. Lee, S. I. Kim, C. B. Lee, H. Yin, S.-E. Ahn, B. S. Kang, K. H. Kim, J. C. Park, C. J. Kim, I. Song, S. W. Kim, G. Stefanovich, J. H. Lee, S. J. Chung, Y. H. Kim, and Y. Park, “Low-temperature-grown transition metal oxide based storage materials and oxide transistors for high-density non-volatile memory,” Adv. Funct. Mater., vol. 19, no. 10, pp. 1587–1593, May 2009. [4] R. Soni, P. Meuffels, A. Petraru, M. Weides, C. Kügeler, R. Waser, and H. Kohlstedt, “Probing Cu doped Ge0.3 Se0.7 based resistance switching memory devices with random telegraph noise,” J. Appl. Phys., vol. 107, no. 2, p. 024517, Jan. 2010. [5] D. Ielmini, C. Cagli, and F. Nardi, “Resistance transition in metal oxides induced by electronic threshold switching,” Appl. Phys. Lett., vol. 94, no. 6, p. 063 511, Feb. 2009. [6] S. B. Lee, S. C. Chae, S. H. Chang, J. S. Lee, S. Seo, B. Kahng, and T. W. Noh, “Scaling behaviors of reset voltages and currents in unipolar resistance switching,” Appl. Phys. Lett., vol. 93, no. 21, p. 212 105, Nov. 2008. [7] A. Sawa, “Resistive switching in transition metal oxides,” Mater. Today, vol. 11, no. 6, pp. 28–36, Jun. 2008. [8] C. Yoshida, K. Kinoshita, T. Yamasaki, and Y. Sugiyama, “Direct observation of oxygen movement during resistance switching in NiO/Pt film,” Appl. Phys. Lett., vol. 93, no. 4, p. 042106, Jul. 2008. [9] U. Russo, D. Ielmini, C. Cagli, and A. L. Lacaita, “Self-accelerated thermal dissolution model for reset programming in unipolar resistive-switching memory (RRAM) devices,” IEEE Trans. Electron Devices, vol. 56, no. 2, pp. 193–200, Feb. 2009.
