Abstract
Resistive random access memory (RRAM) is a leading candidate in the race towards emerging nonvolatile memory technologies. The sneak path current (SPC) problem is one of the main difficulties in crossbar memory configurations. RRAM devices with desirable properties such as a selectorless, 1R-only architecture with self-rectifying behavior are potential SPC solutions. In this work, the intrinsic nonlinear (NL) characteristics and relaxation characteristics of bilayer high-k/low-k stacked RRAMs are presented. The intrinsic nonlinearity reliability of bilayer selectorless 1R-only RRAM without additional switches has been studied for their ability to effectively suppress SPC in RRAM arrays. The relaxation properties with resistive switching identification method by utilizing the activation energy (Ea) extraction methodology is demonstrated, which provides insights and design guidance for non-uniform bilayer selectorless 1R-only RRAM array applications.
Similar content being viewed by others
Introduction
In recent years, memory technology includes static random access memory (SRAM), dynamic random access memory (DRAM), flash memory are encountering challenges due to the continued scaling down of the designs1,2,3,4. Among several types of next generation memory devices, resistive random access memory (RRAM) composed of a simple metal-insulator-metal (MIM) structure has increasingly been attracting much attention as a promising candidate for next-generation nonvolatile emerging memory according to its potentially ultra-high density production probability, faster switching speed (<10 ns), compatibility with a crossbar structure with CMOS integration, lower energy consumption, and the feasibility for neuromorphic computing architecture design5,6,7,8,9,10.
The RRAM with MIM structure is simplifying memory array design by crossbar architecture, however, the leakage through the sneak paths inevitably induced while accessing this RRAM crossbar networks. The sneak paths current (SPC) problem is one of the major issues in the development of three-dimensional (3D) crossbar memory design. The SPC problem can be described as the leakage from neighboring unselected cells (USC), which significantly results in the cross-talk and distorts the data of selected cell (SC) during reading operation. To mitigate the sneak paths currents, a diode or a selector device series with a RRAM cell to form 1D-1R or 1S-1R structure has been developed11,12,13,14,15. Several solutions on selection devices including Mott transition switches, nonlinear volatile switches, threshold switches, rectifying diode devices etc. have been presented16,17,18,19,20. Unfortunately, the additional selection devices for 1S-1R configurations considerably increase fabrication process, circuit design complexity, and additional cost per chip. Therefore, a selectorless memory composed of 1R-only design architecture with nonlinear characteristics is desirable for high-density RRAM array applications.
In our previous work, we reported the selectorless RRAM in high-k/low-k bilayer stacks, in which the intrinsic nonlinearity has been demonstrated by inserting a low-k layer (e.g. SiOx layer or graphite oxide layer) and optimized by SET compliance current limit (CCL) modulation21,22,23,24. In addition, the bilayer or multilayer nonuniform metal-oxide-stacked structures for self-rectifying behavior have been studied, e.g. TiOx/HfOx, TaOx/TiOx, Al2O3/TiOx, WO3/WOx etc.25,26,27,28,29,30,31,32,33,34. However, the mechanism of the nonlinearity-CCL responses, and reliability characteristics are not yet been investigated. This work not only studied the reliability of relaxation characteristics under temperature variation, but also proposed a switching identification method which provides the potential guidance for future design of 3D sneak-path-constrained selectorless crossbar RRAM configurations.
Fabrication Process
The starting substrates were heavily-doped N + Si wafers. Titanium nitride (TiN) of 200 nm was deposited as the bottom electrode (BE). Then, 9 nm of SiOx followed by 4 nm of HfOx were deposited as resistive switching dielectric layers for realizing the bilayer selectorless structures by radio frequency (RF) sputtering method10,11,12,13. After switching layers deposition, 165 nm platinum was then deposited as top electrode (TE), as followed by lift-off method for RRAM devices. The SiOx (9 nm) single layer devices, HfOx (11 nm) single layer devices, HfOx (7 nm)/graphite (5 nm), SiOx (7 nm)/graphite (5 nm) are used as references. Graphite is deposited by the RF sputtering method as followed by the oxide layers and top electrode. For simplifying the device notifications, the abbreviation of HfOx as “H”, SiOx as “S”, and graphite as “G” are used following by the thickness of thin films. An Agilent B1500 and Lakeshore probe station were used for electrical characterization of the RRAM devices.
Results and Discussion
The schematic of 3D crossbar 1S-1R array memory configuration is shown in Fig. 1(a). Figure 1(b) shows the transmission electron microscopy (TEM) image of bilayer HfOx/SiOx stacked device. The TEM sample is prepared by focused ion beam milling method with the scanning electron microscope (SEM). To initiate the resistive switching, a single voltage sweep electroforming process with a current limit was applied to induce a soft breakdown. After electroforming, the device manifests an improved conductance as the conductive filament (CF) connects the TE and BE, thus resulting in a low-resistance state (LRS) of the RRAM. The reset process can then be applied to rupture the CF, resulting in a high-resistance state (HRS). Then, the soft-breakdown process was performed by single sweeping the voltage until current abruptly increased to a compliance current limit (CCL) of 1 mA, as shown in Fig. 1(a). Voltage was applied to the bottom electrode (TiN) with the top electrode (Pt) connected to ground. By switching set and reset operation, the CF can be repeatedly connected/ruptured, and allowing reversible transition cycles between HRS and LRS. The SET process i.e. switching from HRS to LRS took place in positive polarity, while the RESET occurred in negative polarity.
Figure 1(c) shows bipolar resistive switching I-V characteristics during DC voltage sweeps for single HfOx layer (H11) and bilayer selectorless RRAMs (H4S9 and H7G5). The I-V characteristics of H11 (black dash) and H4S9 (red) have been shown in our previous work22. In this experiment, the total thickness of bilayer and single layer devices are designed to be ~11 to 13 nm, in order to reduce the influence of bias stress i.e. overset during the electroformation process. The extra bias stress though forming process with potentially different shape of filaments is being avoided in here. After the electroforming process, the resistive switching performance was stabilized by 30 DC voltage sweep cycles. The SET process for oxide-based RRAMs was performed by applying to 3 V forward/reverse double sweep with 1 mA CCL to program to LRS. The RESET process was done by sweeping up to −2.1 V, where current decreased as the voltage was swept from around RESET voltage, and the devices were then programmed into HRS. The nonlinear nature in selectorless RRAM is shown to mitigate the SPC because the LRS of selected cell can be read at a “high-voltage” region (i.e. −0.8 V), while the sharp conductance drops at “low-voltage region” (i.e. −0.4 V or −0.28 V) effectively suppresses SPC through unselected cells in reading schemes (e.g. V/2 or V/3 read schemes)35,36. In other words, the sneak paths current can be constrained by leverage the nonlinearity of the self-rectifying current-voltage characteristics.
The nonlinearity is defined as the current at the read voltage (Vread) divided by the current at the one third read voltage (1/3 Vread) with V/3 scheme (half of read voltage (1/2 Vread) with V/2 scheme). The on-state of the selected cell (SC) is read at a “high-voltage” (i.e. Vread) region, while the sharp conductance decreases at “low-voltage region” (i.e. 1/3 Vread or 1/2 Vread) effectively suppresses the sneak current through the unselected cells (USC). The nonlinearity of H7G5 (NL~120) stacked device exhibits ~24 times of improvement over that of H11 device (NL~5) with SET CCL of 1 mA, which suggests a significant increased current at Vread by inserting the high-k layer i.e. HfOx in bilayer devices, with reduced current at 1/3 Vread (~10−5 A) for both structures. The high-k layer i.e. HfOx in H7G5 is shown to enhance the higher currents than those of H4S9 are around −0.8 V, where the same high-k layer and same SET CCL of 1 mA (i.e. switching gap in low-k layer) are utilized.
The early failure yields are as 62.5%, 37.5%, and 7.7% for H7G5, S7G5, H4S9, respectively, which depicts the H4S9 as having better DC cycling endurance than graphite-based selectorless RRAMs37. The number of word lines (N) assessment with nonlinearity by utilizing the V/3 read schemes are showed in Fig. 2(a), for single layer and bilayer selectorless RRAMs (median of 30 devices). The reading voltages are −0.8 V for fully read on selected cell, and −0.28 V for unselected cells. The bilayer devices i.e. H4S9 (or H7G5) have the nonlinearity of ~14 times (or ~18 times) higher than single layer device i.e. H11. After the calculation of array size by taking into account 10% read margin, the number of word line (i.e. the maximum array size) are 80 for H4S9 and 120 for H7G5, respectively. Although the H4S9 has slightly lower nonlinearity than H7G5, the early failure yield is also lower in H4S9 than in H7G5. In other words, there is a tradeoff between the reliability of memory window with the nonlinearity37.
In addition, the nonlinearity readouts under various temperature conditions with SET compliance currents limit (CCL) modulation is showed in the Fig. 2(b). The results show the nonlinearity properties are not affected by the ambient temperature under vacuum (~2.5 mtorr). The SET CCL modulation is applied under room temperature of 300 K, and 20 cycles for switching stabilization of each CCL condition are applied on devices. After DC cycles, the ambient temperature decreases from 300 K to 150 K, and the nonlinearity is characterized by V/2 scheme after the target temperature is reached for 5 minutes. The temperature elevation (i.e. 340 K, orange curve) is also applied and the nonlinearity shows slightly decrements comparing to the cooling process, which is thought to be suggested that the thermal effect on filamentary structures decreases the bandgap and increase the effective dielectric constant resulting in nonlinearity decrement38.
The relaxation characteristics of conductive filament with varied SET CCL of 0.1 mA and 2 mA for H11, S9, H4S9 (median of 10 tested devices for each structure) are compared and showed in Fig. 3(a). The normalized current (%) is defined as the current of time (It) divided by initial current (I0) multiply by 100%. The current drift is the difference between two current percentages, i.e. I0-It/I0. Here, the current drift of 5% is chosen as a criterion to extract the activation energy. In other word, the time value utilized for Ea extraction is as It have 5% of current drift (i.e. normalized current is 95%). The retention testing is applied every 60 seconds, and read voltage is 0.1 V. The results showed the current drift is larger as the SET CCL is lower in all the device structures, e.g. current drift ~5% with CCL of 0.1 mA while <1% with CCL of 2 mA on S9 after 4000 seconds. This is thought to be suggested that the conductive filamentary structures are thicker with higher SET CCL (e.g. 2 mA), which results in better retention and less current drift39. On the other hand, the H11 showed the greater relaxation behavior (current drift ~2.5%) in the comparison of H4S9 (current drift ~3.8%) and S9 (current drift ~5%).
Figure 3(b) shows the current drift as a function of time on S9 devices under various temperate conditions. With increasing temperature, the larger the filamentary structures relaxation occurs, i.e. ~11% under 393 K, ~8% under 358 K, ~5% under 333 K, ~4% under 298 K after 1 hour. Based on the observation of different relaxation behaviors with temperature on various devices, the methodology of switching identification is proposed (Fig. 3(c)). The Arrhenius equation and extracted activation energy (Ea) are utilized, where the t is relaxation time under 5% current drift, T is the ambient temperature (in kelvin) during retention testing, k is the gas constant of 8.314 × 10−3 kJ mol−1K−1. By comparing the extracted activation energy value as an indicator, the information of filamentary structure composition and resistive switching can possibly be identified, which will be discussed in next session.
The extracted Ea values as a function of SET CCL with various temperatures are shown in Fig. 4 (black curve for H11; blue curve for S9; red curve for H4S9). The temperature of 300, 335, 360 K have been used in the retention measurements and relaxation behavior characterizations. The extracted activation energy (Ea) values based on Arrhenius equation are in the range of ~0.7 to 1.8 eV for single layer HfOx, and ~0.3 to 0.4 eV for single layer SiOx (Fig. 4, left panel). The extracted Ea values for H4S9 bilayer selectorless devices with SET CCL modulation is showed in Fig. 4 (right panel). The preliminary result shows the nonlinearity characteristics of H4 and H11 with SET CCL of 1 mA are 3.08 and 3.04, respectively22,23,24. The nonlinearity is independent on the thickness of HfOx single layer devices, so the H11 as the reference sample to avoid extra voltage stress. Noted the Ea of HfOx (4 nm) is of ~1.87 eV at CCL of 1 mA, and not showing significant differences than HfOx (11 nm) (~1.67 eV). The extracted Ea value of H4S9 bilayer devices is in the median of ~0.32 eV as CCL is of 1 mA, and ~1.46 eV, ~0.7 eV, ~0.7 eV as CCL are of 0.1, 0.3, and 2 mA, which suggested that the resistive switching at SET CCL of 1 mA has Si and O ionized defects involved in the filament structures than other CCL conditions. The analysis of RESET process (i.e. filament rupture process) is the based on the “hourglass model” as well as quantum point contact (QPC)40,41 model to present the oxygen vacancies or metal ions movements during switching process. The relaxation behavior of filament utilized here for Ea extraction is also analyzed based on the hourglass model, where the thinnest part of conductive filament i.e. bottle neck is only composed several metal atoms. During the relaxation process, the conductance of CF continues to decrease until fully ruptured the CF, where the metal filament dissolution process determines the process i.e. M-M bonds continue to break which requires less bond dissociation energy than M-O formation42,43. When the last atom is dissolved, the conductive filament is finally ruptured to HRS. Besides, the relaxation of Si-CF is faster than Hf-CF (Fig. 3(a)) which corresponds to the bond energy of Si-Si (~3.2 eV) is lower than Hf-Hf (~4.02 eV), while the bond energy of Si-O (8.15~8.42 eV) is similar to which of Hf-O (8.16~8.43 eV)42,43. In other words, the Si metal filamentary structure is comparably weaker than Hf metal filamentary structures which have higher Ea and lower reaction rate for LRS relaxation. The bond energy of Si-Si bond is ~20% lower than of Hf-Hf bond, which explains the lower extracted Ea value showed in the SiOx single layer devices. According to Figs 2(b) and 4 (right panel), the H4S9 with SET CCL of 1 mA is showing the higher nonlinearity related to the Si filamentary structure than other CCL conditions, which depicts the optimized nonlinearity can be achieved by both modulating the CCL and insertion of a low dielectric constant layer.
Conclusion
In conclusion, the intrinsic nonlinearity has been demonstrated in bilayer selectorless 1R-only RRAM without additional diode/transistor selector elements, which are beneficial in suppressing SPC in the high-storage-class crossbar memory array configuration. The resistive switching identification method utilizing reliability of relaxation properties, SET CCL modulation, and activation energy extraction have been reported, where the Ea is ~0.7 to 1.8 eV for single layer HfOx, ~0.3 to 0.4 eV for single layer SiOx, respectively. The relaxation characteristics and resistive switching identification provide the insights and mechanism understanding of bilayer selectorless 1R-only RRAM for high storage class crossbar memory configuration.
References
DeBoer, S. Memory Technology: The Core to Enable Future Computing Systems. 2018 IEEE Symposium on VLSI Technology, 3–6 (2018).
Chen, Y. Reshaping Future Computing Systems with Emerging Nonvolatile Memory Technologies. IEEE Micro 39(1), 54–57 (2019).
Imani, M. & Rosing, T. S. Approximate CPU and GPU Design Using Emerging Memory Technologies. Approximate Circuits Springer, Cham., 383–398 (2019).
Jain, A. K., Lloyd, S. & Gokhale, M. Performance Assessment of Emerging Memories Through FPGA Emulation. IEEE Micro 39(1), 8–16 (2019).
Yu, S. Neuro-inspired computing with emerging nonvolatile memory. Proceedings of the IEEE, 106(2), 260–285 (2018).
Park, S. et al. Neuromorphic speech systems using advanced ReRAM-based synapse. IEEE International Electron Devices Meeting, 25-6 (2013).
Woo, J. et al. Improved synaptic behavior under identical pulses using AlO x/HfO 2 bilayer RRAM array for neuromorphic systems. IEEE Electron Device Letters 37(8), 994–997 (2016).
Jo, S. H. et al. Nanoscale memristor device as synapse in neuromorphic systems. Nano letters 10(4), 1297–1301 (2010).
Gao, L. et al. Fully parallel write/read in resistive synaptic array for accelerating on-chip learning. Nanotechnology 26(45), 455204 (2015).
Yao, P. et al. Face classification using electronic synapses. Nature communications 8, 15199 (2017).
Ji, L. et al. Integrated one diode–one resistor architecture in nanopillar SiO x resistive switching memory by nanosphere lithography. Nano letters 14(2), 813–818 (2013).
Ciprut, A. & Friedman, E. G. On the write energy of non-volatile resistive crossbar arrays with selectors. 2018 19th International Symposium on Quality Electronic Design (ISQED), 184–188 (2018).
Deng, Y. et al. RRAM crossbar array with cell selection device: A device and circuit interaction study. IEEE transactions on Electron Devices 60(2), 719–726 (2013).
Liu, C. & Hai L. A weighted sensing scheme for ReRAM-based cross-point memory array. 2014 IEEE Computer Society Annual Symposium on VLSI (2014).
Wang, Y., Rong, L., Wang, H. & Wen, G. One-step sneak-path free read scheme for resistive crossbar memory. ACM Journal on Emerging Technologies in Computing Systems (JETC) 13(2), 25 (2017).
Kim, S. et al. Performance of threshold switching in chalcogenide glass for 3D stackable selector. In 2013 Symposium on VLSI Technology, T240–T241 (2013)
Jo, S. H., Kumar, T., Narayanan, S., Lu, W. D. & Nazarian, H. 3D-stackable crossbar resistive memory based on field assisted superlinear threshold (FAST) selector. 2014 IEEE international electron devices meeting, 6–7 (2014)
Burr, G. W. et al. Large-scale (512kbit) integration of multilayer-ready access-devices based on mixed-ionic-electronic-conduction (MIEC) at 100% yield. Symposium on VLSI Technology (VLSIT), 41–42 (2012)
Virwani, K. et al. Sub-30nm scaling and high-speed operation of fully-confined access-devices for 3D crosspoint memory based on mixed-ionic-electronic-conduction (MIEC) materials. 2012 International Electron Devices Meeting, 2–7 (2012).
Son, M. et al. Excellent Selector Characteristics of Nanoscale $\hbox {VO} _ {2} $ for High-Density Bipolar ReRAM Applications. IEEE Electron Device Letters 32(11), 1579–1581 (2011).
Chen, Y. C. et al. Graphite-based selectorless RRAM: improvable intrinsic nonlinearity for array applications. Nanoscale 10(33), 15608–15614 (2018).
Chen, Y. C. et al. Built-In Nonlinear Characteristics of Low Power Operating One-Resistor Selector-Less RRAM by Stacking Engineering. ECS Transactions 80(10), 923–931 (2017).
Chen, Y. C., Chang, Y. F. & Lee, J. C. Selector-Less Graphite Memristor: Intrinsic Nonlinear Behavior with Gap Design Method for Array Applications. ECS Transactions 85(1), 11–19 (2018).
Chen, Y. C., Lin, C. C., Kim, S. & Lee, J. C. Selectorless Oxide-Based Resistive Switching Memory with Nonuniform Dielectric for Low Power Crossbar Array Applications. ECS Transactions 89(3), 45–51 (2019).
Lee, H. D. et al. IEEE Symposium on VLSI Technology (VLSIT), pp. 151–152 (2012).
Woo, J. et al. Selector-less RRAM with non-linearity of device for cross-point array applications. Microelectronic Engineering 109, 360–363 (2013).
Lee, S. et al. Engineering oxygen vacancy of tunnel barrier and switching layer for both selectivity and reliability of selector-less ReRAM. IEEE Electron Device Letters 35(10), 1022–1024 (2014).
Chakrabarti, S., Samanta, S., Maikap, S., Rahaman, S. Z. & Cheng, H. M. Temperature-Dependent Non-linear Resistive Switching Characteristics and Mechanism Using a New W/WO 3/WO x/W Structure. Nanoscale research letters 11(1), 389 (2016).
Lee, S. et al. Effect of AC pulse overshoot on nonlinearity and reliability of selectorless resistive random access memory in AC pulse operation. Solid-State Electronics 104, 70–74 (2015).
Xu, X. et al. IEEE Symposium on VLSI Technology (VLSIT), pp. 1–2 (2016).
Chen, F. T., Chen, Y. S., Wu, T. Y. & Ku, T. K. Write scheme allowing reduced LRS nonlinearity requirement in a 3D-RRAM array with selector-less 1TNR architecture. IEEE Electron Device Letters 35(2), 223–225 (2014).
Lanza, M. et al. Advanced Electronic Materials 5(1), 1800143 (2019).
Chand, U., Huang, K. C., Huang, C. Y. & Tseng, T. Y. Mechanism of nonlinear switching in HfO 2-based crossbar RRAM with inserting large bandgap tunneling barrier layer. IEEE Transactions on Electron Devices 62(11), 3665–3670 (2015).
Zhou, J. et al. Very low-programming-current RRAM with self-rectifying characteristics. IEEE Electron Device Letters 37(4), 404–407 (2016).
Ciprut, A. & Eby, G. F. Energy-efficient write scheme for nonvolatile resistive crossbar arrays with selectors. IEEE Transactions on Very Large Scale Integration (VLSI) Systems 26(4), 711–719 (2018).
Zackriya, M., Kittur, H. M. & Chin, A. A novel read scheme for large size one-resistor resistive random access memory array. Scientific reports 7, 42375 (2017).
Chen, Y. C., Wu, X., Chang, Y. F. & Lee, J. C. Nonlinearity Enhancement by Positive Pulse Stress in Multilevel Cell Selectorless RRAM Applications. In 2018 76th Device Research Conference (DRC) 1–2 (2018).
Chen, Y. C. et al. Internal filament modulation in low-dielectric gap design for built-in selector-less resistive switching memory application. Journal of Physics D: Applied Physics 51(5), 055108 (2018).
Tsunoda, K. et al. Low power and high speed switching of Ti-doped NiO ReRAM under the unipolar voltage source of less than 3 V. In 2007 IEEE International Electron Devices Meeting, 767–770 (2007).
Li, Y., Long, S., Liu, Q., Lv, H. & Liu, M. Resistive Switching Performance Improvement via Modulating Nanoscale Conductive Filament, Involving the Application of Two‐Dimensional Layered Materials. Small 13(35), 1604306 (2017).
Raghavan, N. et al. Modeling the Impact of Reset Depth on Vacancy-Induced Filament Perturbations in HfO2 RRAM. IEEE Electron Device Letters 34(5), 614–616 (2013).
Luo, Y. R. & Kerr, J. A. Bond dissociation energies. CRC Handbook of Chemistry and Physics 89, 89 (2012).
Luo, Y. R. Handbook of bond dissociation energies in organic compounds. CRC press (2002).
Acknowledgements
The authors acknowledged the constructive discussions from Dr. Yao-Feng Chang in Intel Corporation (USA), and Prof. Sungjun Kim in Chungbuk University (South Korea). The authors acknowledged the technology incubation scholarship supported by Ministry of Education (MOE), Taiwan.
Author information
Authors and Affiliations
Contributions
J.L. and Y.-C.C. developed the concepts and designed the experiment. Y.-C.C., C.-Y.L. and C.-C.L. design the experimental setup and analyzed the data. S.-T.H. assisted in experimental setup for material analysis. Y.-C.C., B.F. and J.L. interpreted the results and wrote the paper.
Corresponding author
Ethics declarations
Competing Interests
The authors declare no competing interests.
Additional information
Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
Rights and permissions
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.
About this article
Cite this article
Chen, YC., Lin, CC., Hu, ST. et al. A Novel Resistive Switching Identification Method through Relaxation Characteristics for Sneak-path-constrained Selectorless RRAM application. Sci Rep 9, 12420 (2019). https://doi.org/10.1038/s41598-019-48932-5
Received:
Accepted:
Published:
DOI: https://doi.org/10.1038/s41598-019-48932-5
This article is cited by
-
Self-aligned TiOx-based 3D vertical memristor for a high-density synaptic array
Frontiers of Physics (2024)
-
An Account of Natural Material-Based Nonvolatile Memory Device
Proceedings of the National Academy of Sciences, India Section A: Physical Sciences (2023)
Comments
By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.