IEEE ELECTRON DEVICE LETTERS, VOL. 32, NO. 8, AUGUST 2011
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Improved Resistive Switching Uniformity in Cu/HfO2/Pt Devices by Using Current Sweeping Mode Wentai Lian, Hangbing Lv, Qi Liu, Shibing Long, Member, IEEE, Wei Wang, Member, IEEE, Yan Wang, Yingtao Li, Sen Zhang, Yuehua Dai, Junning Chen, and Ming Liu, Senior Member, IEEE
Abstract—In this letter, current sweeping programming mode is proposed as an efficient method to improve the uniformity of the switching properties of resistive memory devices. Based on the measurement results of the RESET process of filament-based Cu/HfO2 /Pt devices, current sweeping mode (CSM) can significantly reduce the distributions of Roff values, as compared with the standard voltage sweeping mode. The improvement is attributed to the elimination of the intermediate resistive states due to the positive feedback of joule heat generation by the use of current sweeping. Furthermore, the uniform distribution of the Vset values of the SET process is also obtained by current sweeping, which stems from the localization of conductive filaments formation and rupture. CSM provides an effective way to achieve uniform resistance state of memory cell. Index Terms—Conductive filament (CF), resistive random access memory (ReRAM), resistive switching, uniformity.
I. I NTRODUCTION
R
ESISTIVE random access memory (ReRAM), which is featured with reversible switching between two or more resistance states, has become a promising candidate to establish next-generation embedded/high-density nonvolatile memory due to its simple structure, excellent scalability, low power, fast speed, and good compatibility with the standard complementary metal–oxide–semiconductor process [1], [2]. One major concern preventing ReRAM, particularly conductive filament (CF)-based devices, from practical memory implementation is the uniformity issue [3]. In order to obtain ReRAM devices
Manuscript received April 25, 2011; revised May 18, 2011; accepted May 18, 2011. Date of publication June 20, 2011; date of current version July 27, 2011. This work was supported in part by the Ministry of Science and Technology of China under Grant 2010CB934200, Grant 2011CBA00602, Grant 2008AA031403, and Grant 2009AA03Z306 and in part by the National Natural Science Foundation of China under Grant 60825403 and Grant 50972160. The review of this letter was arranged by Editor A. Ortiz-Conde. W. Lian and Q. Liu are with the Laboratory of Nano-Fabrication and Novel Devices Integrated Technology, Institute of Microelectronics, Chinese Academy of Sciences, Beijing 100029, China, and also with the College of Electronics and Technology, Anhui University, Hefei 230039, China. H. Lv, S. Long, W. Wang, Y. Wang, Y. Li, S. Zhang, and M. Liu are with the Laboratory of Nano-Fabrication and Novel Devices Integrated Technology, Institute of Microelectronics, Chinese Academy of Sciences, Beijing 100029, China (e-mail:
[email protected]). Y. Dai and J. Chen are with the College of Electronics and Technology, Anhui University, Hefei 230039, China. 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.2011.2157990
with uniform switching behaviors, device structures with bilayer insulators or with ultrasmall cell sizes have been proposed [4], [5]. Here, we present an alternative solution without changing the device structure and size but by utilizing current sweeping mode (CSM) during programming. In most existing ReRAM studies, voltage sweeping mode (VSM) is commonly used to program the devices. Recently, Nauenheim et al. have adopted CSM to electroforming the TiO2 ReRAM device, showing more reliable resistance switching behavior [6]. In addition, Gao et al. and Russo et al. have used CSM to accomplish the SET process, which shows better uniformity of Ron distribution [7], [8]. From these works, CSM seems quite promising to provide new opportunities to improve device performance. However, systematic investigations are still quite lacking, e.g., using CSM to program ReRAM during the RESET process. In this letter, systematic study on the effects of CSM for both the SET and RESET processes of Cu/HfO2 /Pt devices is carried out. During the RESET process, compared with VSM, CSM can enable the device to dramatically reduce its Roff distribution. This Roff uniformity is very important to enhance the ON/OFF resistance ratio and simplify the readout circuit design. Comparison of the power-generation mechanisms between these two programming methods reveals that, different from VSM, CSM can lead to the positive feedback of joule heat generation. Once the CFs begin to dissolve, the RESET process will be accelerated in a uniform manner because of increased joule heating, reducing the distribution of Roff and Rreset . For the SET process, we also find evidence that CF formation and rupture can be well localized by using the CSM programming approach, which improves the uniformity of the Vset voltages. II. E XPERIMENTS A 20-nm-thick Ti adhesion layer and a 80-nm-thick Pt bottom electrode (BE) layer were sequentially deposited on the top of the SiO2 /Si substrate. The resistive switching layer of the 20-nm-thick HfO2 material was deposited on the Pt electrode subsequently by means of electron-beam evaporation at room temperature. According to the X-ray diffraction result (not shown here), the HfO2 layer is amorphous. Then, 70-nm Cu top electrodes (TEs) covered by a 30-nm Au layer were created with a square area of 100 × 100 μm2 by a standard liftoff process. The Au layer is to reduce the oxidation of the Cu electrode
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IEEE ELECTRON DEVICE LETTERS, VOL. 32, NO. 8, AUGUST 2011
Fig. 1. Typical I–V curves of the Cu/HfO2 /Pt device. (a) VSM. (b) CSM. The inset in (b) shows the schematic of the Cu(TE) /HfO2 /Pt(BE) memory device. The BE is always grounded, whereas the TE is biased.
during testing and to prevent the probe tip from scratching the device surfaces. The schematic of the Cu/HfO2 /Pt structure is shown in the inset in Fig. 1(b). The electrical characteristics of the fabricated memory devices were measured by a Keithley 4200-SCS semiconductor characterization system. During measurement, the TEs were biased by VSM and CSM, whereas the BEs were always kept as ground. In CSM, the sweep rate is about 3 μA/s, and in VSM, the sweep rate is about 300 mV/s.
Fig. 2. (a) Repetitive switching cycles of the Cu/HfO2 /Pt device under 120 successive SET/RESET programming. More than 104 times sensing margin is observed under CSM, as compared with 102 under VSM. (b) Statistical distributions of the LRS and the HRS under two sweeping methods. (Blue) Resistances of current sweeping. (Red) Resistances of voltage sweeping.
III. R ESULTS AND D ISCUSSION Fig. 1(a) presents the typical current–voltage (I–V ) curve by direct-current voltage sweeping. In our previous study [9], the resistive switching behavior of such Cu/HfO2 /Pt devices can be explained by the filament conduction mechanism. Both bipolar and unipolar resistive switching can be observed in the Cu/HfO2 /Pt device. However, unipolar switching is not very stable as bipolar switching because the switching voltages of Vset and Vreset in unipolar mode are sometimes overlapped. Thus, we choose bipolar mode to operate the device, as shown in Fig. 1. The sudden increase and decrease in measured current during the SET and RESET processes result from the formation and rupture of the CFs [1], respectively. The resistance of the fresh cell is about 107 Ω, and no forming operation is needed. When using VSM to set the device, the compliance current is required to protect the device from permanent breakdown. For CSM, this compliance current is not necessary. Instead, the voltage should be limited during the RESET process. This is due to the high-resistance state (HRS) being much higher than the low-resistance state (LRS) by several orders of magnitude, and the RESET transition from the LRS to the HRS will greatly increase the voltage across the device. If there is no compliance voltage setting during CSM, the device will break down. Fig. 2(a) displays the endurance cycles with repetitive SET / RESET programming. These resistances were read by using a small voltage of 0.1 V. A high ON/OFF ratio of 104 can be observed in the device under CSM. As for VSM, the ON/OFF ratio is found to be significantly reduced to 102 because of the wide dispersion of the HRS. Fig. 2(b) shows the statistical distributions of Ron and Roff for VSM and CSM. The Ron values have similar distributions for these two methods. As for the Roff values, the device under CSM exhibits a much more uniform distribution. The existence of the intermediate states is the main reason for the wide dispersion of Roff resistance. Keeping the device either on one of the intermediate states [10] or on the final state can improve the uniformity greatly.
Fig. 3. (a) Statistical distributions of Vset and Vreset in the 120 SET / RESET switching cycles. (b) Power generation during the RESET process under two sweeping methods by multiplying V with I.
During the RESET process, CSM can effectively reduce the intermediate states and keep the device on the final state. Based on the filament model, the intermediate states may correspond to the incompletely ruptured conductive paths during the RESET process [11], which will require a much lower voltage to recover in the successive SET operation. As shown in Fig. 3(a), the lowest Vset under voltage sweeping is as small as 0.2 V compared with 1 V for current sweeping. Fig. 3(a) shows the statistical distribution of Vset /Vreset during the 120 SET/RESET switching cycles by VSM and CSM, respectively. Under VSM, Vset is largely fluctuated as Vset ∼ (1.07 ± 0.63 V). However, this fluctuation is considerably reduced in CSM as Vset ∼ (1.35 ± 0.31 V). The mean value increases because the HRS is uniformly distributed in a higher range under CSM. It is also noted that the RESET voltages are already quite uniform under the two sweeping methods. Thus, CSM programming can significantly improve the SET/RESET uniformity of the device, as compared with that of VSM. The mechanism to explain the reduction of the intermediate state due to CSM is also investigated. According to the mechanism of CF-based ReRAM, the RESET process is closely related to joule heating [12]–[14]. Fig. 3(b) shows the power generation of the RESET process of these two programming manners by simply multiplying the voltage with the current. As shown in Fig. 3(b), the power generation of VSM is decreased with the increase in resistance. The complete dissolution of the filaments may just depend on the further increase in sweeping voltage. Hence, VSM is helpful to realize in multilevels by controlling different RESET voltages [15] but is not preferred for the uniformity of the HRS. As for CSM, power generation is enhanced with the increase in resistance, just like positive feedback. The current increases in synchronism with the voltage.
LIAN et al.: RESISTIVE SWITCHING UNIFORMITY IN CU/HFO2 /PT DEVICES BY CSM
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IV. C ONCLUSION
Fig. 4. (a) Schematic of the parallel connection between the three cells. (b) Probability of the LRS occurring in the same device for the three parallel cells under the 120 SET / RESET cycles by the two sweeping methods. The devices were first set by applying a bias at the connected TE, and then, their resistances were individually checked.
Once the filaments start to dissolve, the rupture process will be accelerated. Hence, CSM can help achieve a more uniform HRS distribution and may be able to realize high-speed operation. As to the SET process, we also demonstrate that using CSM can localize the filament path more effectively than using VSM. First, we make an 8 × 8 crossbar array. All device areas are shaped as 30 × 30 μm2 . Then, we choose three parallel TEs from a common BE line (see Fig. 4(a) for the schematic). The common BE is kept as ground, whereas the three separated TEs are simultaneously biased. The devices were first set by an electrical stimulation applied on the TEs, which are connected together by three probes, and then, their resistances were individually checked. It is observed that, in CSM, only one of the three cells changes to ON -state, whereas the other two remain in OFF-state. While in VSM, nearly all the cells will turn from OFF -state to ON -state. The experiment result is quite obvious that the probability of the LRS occurring in the same device is much higher for CSM compared with VSM [as shown in Fig. 4(b)]. The possible reason for this result may lie in the dynamic course of transition from the HRS to the LRS. Once the filament starts to grow, the resistance of the device will be correspondingly reduced. For VSM, the current will start to increase at this trigger point. There exists a short period of current increase, which starts from the trigger point and ends at the preset value of the compliance current. In this short dynamic course, the voltage applied on these three devices is exactly equal to the sweeping voltage and does not decrease with the resistance. We believe the filaments in the different devices are more likely to be successively formed in this case. After achieving the compliance current, most of the sweeping voltage will be diminished. We observed such kind of transition short course by using ultraslow SET voltage sweeping in our previous work [11]. However, for CSM, this dynamic course is different. Once the CF starts to grow, the voltage on the devices will be decreased, and it will restrain the growth of other filaments. The test structure shown in Fig. 4(a) can be used to explain the improvement of device uniformity in CSM. This 1 × 3 cross-point array shows the probabilities of whether the CF in each cycle remains at the same position as the previous cycle or has shifted to a different position. According to the switching mechanism discussed earlier, ReRAM will be more stable and reliable for practical applications if the regions where the formation and rupture of the filaments occur can be confined. Therefore, CSM is an effective method to improve device uniformity.
The switching characteristics of the Cu/HfO2 /Pt memory device have been investigated by using two different kinds of electrical sweeping modes. More uniform device parameters of Roff and Vset were achieved by using CSM, as compared with VSM. The improvement may originate from the positive feedback of joule heat generation during the RESET process, with reduction of intermediate state formation. Moreover, CSM can also help localize the position of filament paths, preventing the formation of additional paths once the SET process happens. This letter provides an important solution to improve the switching uniformity of ReRAM. R EFERENCES [1] R. Waser and M. Aono, “Nanoionics-based resistive switching memories,” Nat. Mater., vol. 6, no. 11, pp. 833–840, Nov. 2007. [2] M. Kund, G. Beitel, C.-U. Pinnow, T. Röhr, J. Schumann, R. Symanczyk, K.-D. Ufert, and G. Müller, “Conductive bridging RAM (CBRAM): An emerging non-volatile memory technology scalable to sub 20 nm,” in IEDM Tech. Dig., 2005, pp. 754–757. [3] H. B. Lv, H. J. Wan, and T. A. Tang, “Improvement of resistive switching uniformity by introducing a thin GST interface layer,” IEEE Electron Device Lett., vol. 31, no. 9, pp. 978–980, Aug. 2010. [4] 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, “Improvement of resistive memory switching in NiO using IrO2 ,” Appl. Phys. Lett., vol. 88, no. 23, p. 232 106, Jun. 2006. [5] Y. S. Chen, H. Y. Lee, P. S. Chen, P. Y. Gu, C. W. Chen, W. P. Lin, W. H. Liu, Y. Y. Hsu, S. S. Sheu, P. C. Chiang, W. S. Chen, F. T. Chen, C. H. Lien, and M.-J. Tsai, “Highly scalable hafnium oxide memory with improvements of resistive distribution and read disturb immunity,” in IEDM Tech. Dig., 2009, pp. 1–4. [6] 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, no. 12, p. 122 902, Mar. 2010. [7] B. Gao, X. Y. Chang, B. Sun, H. W. Zhang, L. F. Liu, X. Y. Liu, R. Q. Han, T. B. Wu, and J. F. Kang, “Identification and application of current compliance failure phenomenon in RRAM device,” in Proc. VLSI-TSA, Jun. 2010, pp. 144–145. [8] U. Russo, C. Cagli, S. Spiga, E. Cianci, and D. Ielmini, “Impact of electrode materials on resistive-switching memory programming,” IEEE Electron Device Lett., vol. 30, no. 8, pp. 817–819, Aug. 2009. [9] Y. Wang, Q. Liu, S. Long, W. Wang, Q. Wang, M. Zhang, S. Zhang, Y. Li, Q. Zuo, J. Yang, and M. Liu, “Investigation of resistive switching in Cu-doped HfO2 thin film for multilevel non-volatile memory applications,” Nanotechnology, vol. 21, no. 4, p. 045 202, Jan. 2010. [10] J. Park, M. Jo, J. Lee, S. Jung, S. Kim, W. Lee, J. Shin, and H. Hwang, “Improved switching uniformity and speed in filament-type RRAM using lightning rod effect,” IEEE Electron Device Lett., vol. 32, no. 1, pp. 63– 65, Jan. 2011. [11] Q. Liu, C. M. Dou, Y. Wang, S. B. Long, W. Wang, M. Liu, M. H. Zhang, and J. N. Chen, “Formation of multiple conductive filaments in the Cu/ ZrO2 :Cu/Pt device,” Appl. Phys. Lett., vol. 95, no. 2, p. 023 501, Jul. 2009. [12] U. Russo, D. Ielmini, C. Cagli, and A. L. Lacaita, “Self-accelerated thermal dissolution model for reset programming in unipolar resistiveswitching memory (RRAM) devices,” IEEE Trans. Electron Devices, vol. 56, no. 2, pp. 193–200, Feb. 2009. [13] W. Wang, S. Fujita, and S. S. Wong, “Reset mechanism of TiOx resistance-change memory device,” IEEE Electron Device Lett., vol. 30, no. 7, pp. 733–735, Jul. 2009. [14] W. Guan, M. Liu, S. Long, Q. Liu, and W. Wang, “On the resistive switching mechanisms of Cu/ZrO2 :Cu/Pt,” Appl. Phys. Lett., vol. 93, no. 22, p. 223 506, Dec. 2008. [15] M. Terai, Y. Sakotsubo, S. Kotsuji, and H. Hada, “Resistance controllability of Ta2 O5 /TiO2 stack ReRAM for low-voltage and multilevel operation,” IEEE Electron Device Lett., vol. 31, no. 3, pp. 204–206, Mar. 2010.
