Conductive Bridge Random Access Memory Cells Research Articles

Multiscale Modeling of Metal-Oxide-Metal Conductive Bridging Random-Access Memory Cells: From Ab Initio to Finite-Element Calculations

Continuum modeling is a popular, computationally efficient technique that can shed light on the resistance-switching properties of conductive bridging random-access memory (CBRAM) cells. Traditional models typically rely on many fitting parameters, but this approach uses material parameters extracted either from either ab initio or machine-learned empirical calculations. As proof of concept, the authors apply the computational framework to an SiO${}_{2}$-based CBRAM cell, and reveal the relevance of Joule heating in nanoscale devices. With the proposed multiscale methodology it is possible to explore the potential of not-yet-fabricated memory cells, and to optimize their design reliably.

CBRAM technology: transition from a memory cell to a programmable and non-volatile impedance for new radiofrequency applications

Electrical resistance control programming of conductive bridging random access memory (CBRAM) radio frequency (RF) switches could benefit the development of electronically controlled non-volatile RF attenuators and other reconfigurable devices. The object of this study is to adapt a conventional CBRAM based memory cell to be used as an RF switch, and to demonstrate the feasibility of programming non-volatile RF CBRAM switches to achieve specific target resistances within a range of continuous values. The memory-RF technologic transition implies a drastic increase of the geometry in order to handle a much higher power, a decrease of the transition capacitance in order to operate at much higher frequencies, and a decrease of the LRS to a few ohms, which is critical for RF applications. These studies are initially performed on an in-house made RF CBRAM cell array at DC frequency, and then extended successfully to a co-planar waveguide (CPW) based shunt mode RF switch with an integrated CBRAM cell. Reliability of the proposed technique is validated through detailed analysis of factors like repeatability of the process, time stability of programmed states, and statistics of time taken to converge to a desired resistance value for an arbitrary RF CBRAM switch.

Lower Power, Better Uniformity, and Stability CBRAM Enabled by Graphene Nanohole Interface Engineering

Conductive-bridge random access memory (CBRAM) shows great potential as next-generation nonvolatile memory. However, the relatively high-power consumption, cycle-to-cycle and device-to-device variations are still drawbacks for industrial applications. A method of Monte Carlo simulation of CBRAM combined with a model to calculate the ${I}$ – ${V}$ curves has been developed, which investigate the formation and dissipation of filaments inside CBRAM cells. Through simulations, we conclude that the multi-filament structure is responsible for high-power consumption and the changing of regrowth filament location is responsible for the variations. We have enabled lower power, better uniformity, and stability CBRAM using graphene nanohole interface layer. Compared to CBRAM without graphene, CBRAM with inserted graphene with 0.66 nm hole shows better uniformity (60 times improvement), stability (5 times improvement), and lower power dissipation (10 times reduction). Moreover, our simulation results are matched with the experimental results of 45-nm graphene nanohole CBRAM. The reasons for better characters have been explained by the confining of the filament location with a single filament. This article shows great potential of integrating CBRAM with graphene nanohole structure.

Controllable growth of conductive filaments in sandwiched CBRAM cells using self-assembled Cu/W multilayer thin films as the electrodes

Self-assembled Cu/W multilayer thin films are fabricated and used as the top electrodes (TEs) in conductive-bridge random access memory (CBRAM) devices. The modulation period (λ) of the Cu/W multilayer structure can be well controlled by changing the substrate rotation speed during magnetron sputtering. For the λ value of 3.2 nm, the Al2O3 based CBRAM devices exhibits significantly improved resistive switching (RS) performances with effectively narrowed switching parameters and substantially increased OFF/ON ratio up to 104 at 0.10 V. Good reliability after 600 cycles is demonstrated and the retention time of 10 years at 85 °C is estimated through Arrhenius plot. In essential, the growth of conductive filaments (CFs) in Al2O3 layers is well controlled owing to the W-rich layer as the diffusion barrier. As a result, both the number and the size of CFs in Al2O3 layer are reduced if the λ value is increased. Especially, volatile RS behavior is evidenced due to the extremely slow diffusion of Cu ions across the TE/Al2O3 interface and thus the formation of tiny CFs if the λ value is up to 7.0 nm. A physical model is proposed to describe the controllable formation of CFs by using Cu/W multilayer TEs.

Atomically Thin CBRAM Enabled by 2-D Materials: Scaling Behaviors and Performance Limits

Reducing the energy and power dissipation of conductive bridge random access memory (CBRAM) cells is of critical importance for their applications in future Internet of Things (IoT) device and neuromorphic computing platforms. Atomically thin CBRAMs enabled by 2-D materials are studied theoretically by using 3-D kinetic Monte Carlo simulations together with experimental characterization. The results indicate the performance potential of attoJoule energy dissipation for intrinsic filament formation and a filament size of a single atomistic chain in such a CBRAM cell. The atomically thin CBRAM cells also show qualitatively different features from conventional CBRAM cells, including complete rupture of the filament in the reset stage and comparable forming and set voltages. The scaling and variability of the CBRAM cells down to sub-nanometer size of the switching layer as realized in the experiment are systematically studied, which indicates performance improvement and increased relative variability as the switching layer scales down. The results establish the ultimate limits of the size and energy scaling for CBRAM cells and illustrate the unique application of 2-D materials in ultralow power memory devices.

Microcanonical RT-TDDFT simulations of realistically extended devices.

In this paper, real-time time-dependent density functional theory (RT-TDDFT) calculations of realistically sized nanodevices are presented. These microcanonical simulations rely on a closed boundary approach based on recent advances in the software package CP2K. The obtained results are compared to those derived from the open-boundary Non-equilibrium Green's Function (NEGF) formalism. A good agreement between the "current vs. voltage" characteristics produced by both methods is demonstrated for three representative device structures, a carbon nanotube field-effect transistor, a GeSe selector for crossbar arrays, and a conductive bridging random-access memory cell. Different approaches to extract the electrostatic contribution from the RT-TDDFT Hamiltonian and to incorporate the result into the NEGF calculations are presented.

Mechanism of Forming-Free Cu2o Solid-Electrolyte Based Conductive-Bridge Random Access Memory

Resistive random access memory (ReRAM) is one of the most promising candidates for the next-generation memory due to its simple structure, high switching speed and low power consumption [1]. Despite of such advantages, its non-volatile characteristic strongly depends on memory cell size. To overcome the weakness, conductive bridging random access memory (CBRAM) cells have been researched as an alternative to oxygen-vacancy type ReRAM cells since CBRAM cells showed independence of non-volatile memory characteristics on the memory cell size [2]. However, CBRAM cells usually require a forming process to produce metal filaments in the solid-electrolyte before memory operation. It had a negative effect on the device since it reflects a high-power consumption. Thus, in this work, Cu2O-based CBRAM cells with forming-free characteristic were presented. In addition, we mainly focused on understanding the dependency of forming-free process on annealing temperature. To confirm the presence of forming process, the resistive switching device was fabricated on a Pt-bottom electrode (BE) with a 250nm via-hole pattern, and annealed at the temperature of as-sputtered, 150, 250 and 300 ˚C. As shown in Fig. 1(a) and 1(b), as-sputtered and 150 ˚C post annealing CBRAM cells demonstrated forming-free characteristic. However, reset process was not effectively work. For the case of 300 ˚C post annealing CBRAM cell, it required forming process as shown in Figure. 1(d). CBRAM cell with 250 ˚C post annealing demonstrated both bi-stable I-V characteristic and forming free characteristic. To understand the mechanism of the forming-free process at 250 ˚C post annealing, EDS line scanning/mapping were carried out on the CBRAM cells annealed at 150, 250, 300 and 400 ˚C repectively (Fig.2 (a)-(d)). According to the EDS analysis, the diffusion amount of Ag in the Cu2O layer decreases as the annealing temperature increases. This tendency can be attributed to the decrease in the number of metal vacancies sites. In the saturated case, as shown in Fig. 2(a) and 2(b), the reset process did not effectively work due to the excessive density of VCu 2- leaving not enough room for breaking the filament. In the optimal case, as shown in Fig. 2(c), forming-free characteristic and both set/reset process worked successfully due to the optimal gap between the concentrated area of Ag and the bottom electrode. In the deficient case, as shown in Fig. 2(d), the forming process was required due to the low density of VCu 2- in solid-electrolyte layer and the high gap between the concentrated area of Ag and bottom electrode. In our study, we report the annealing temperature (250 ˚C is the optimal condition in this study) is the significant key to achieve the forming-free and stable non-volatile characteristics via controlling the concentration of negatively charged Cu vacancies, where Ag ions can diffuse through. [1] Daniele Ielmini, Semicond. Sci. Technol. 31 (2016) 063002 [2] Ilia Valov, J. Phys. D: Appl. Phys. 46 (2013) 074005 This material is based upon work supported by the Ministry of Trade, Industry & Energy(MOTIE, Korea) under Industrial Technology Innovation Program (10068055). Figure 1

Nanoscale CuO solid-electrolyte-based conductive-bridging, random-access memory cell with a TiN liner

The Conductive-bridge random-access memory (CBRAM) cell is a promising candidate for a terabit-level non-volatile memory due to its remarkable advantages. We present for the first time TiN as a diffusion barrier in CBRAM cells for enhancing their reliability. CuO solid-electrolyte-based CBRAM cells implemented with a 0.1-nm TiN liner demonstrated better non-volatile memory characteristics such as ~ 106 AC write/erase endurance cycles with 100-μs AC pulse width and a long retention time of ~ 7.4-years at 85 °C. In addition, the analysis of Ag diffusion in the CBRAM cell suggests that the morphology of the Ag filaments in the electrolyte can be effectively controlled by tuning the thickness of the TiN liner. These promising results pave the way for faster commercialization of terabit-level non-volatile memories.

Multi-Level CuO-Based Conductive-Bridging-Random-Access-Memory Cell Embedded with Au Ncs

Recently, memory industry has demanded terabit-level memory cell of high density to have bit cost-competition. However, current memory technology such as DRAM and NAND flash memory has a physical limit due to their charge-storage system. Conductive bridging random access memory (CBRAM) is considering as one of the promising candidate for next generation non-volatile memory (NVM) because it has many advantages such as low power consumption, and large on/off ratio. In particular, CBRAM has a possibility of terra-bit-level nonvolatile memory as a post flash memory due to their 4F2simple structure of a top reactive electrode, electrolyte, and bottom inert electrode, and they showed a bipolar switching memory characteristic.[1] We have developed a nanoscale CBRAM cell with a structure of TiN/CuO/Ag/TiN. It showed good nonvolatile memory performance such as as a ~1.23 X 102 memory margin (Ion/Ioff), ~3 X 106 AC set/reset endurance cycles by with 100-μs AC pulse width by sustaining a 1.31 X 102 memory margin, ~6.63-years retention time at 85 °C by sustaining a 3.63 X 102 memory margin, 100 ns program speed, and multi-level (four level) cell operation.[2] However, for achieving commercial-level non-volatile memory cell, the CBRAM cell with Ag or Cu electrode has some problem such as poor stability and reliability due to undesired diffusion of Ag and Cu ions in solid-electrolyte. Thus, the undesired diffusion of Ag and Cu ions leads to the formation of multiple and thick filaments resulting in degradation of write/erase endurance cycles. To solve this issue, we applied Cu-Te alloy electrode for CuO solid-electrolyte-based CBRAM cell. Also, to enhance the nonvolatile memory characteristics of the CBRAM cell, Au nano-crystals (NCs) were intentionally inserted between the CuO solid electrolyte and TiN bottom electrode, where the Au NCs were created by depositing Au thin-film via thermal evaporation. Thus, the Au NCs inserted in CBRAM cell made intensive fields be formed on Au NCs and then localized Cu-ion bridging filaments could be formed on Au NCs deposited in solid electrolyte earlier than on TiN bottom electrode. Since it was well-known that the CBRAM was operated by metal-filaments formed in solid electrolyte, well-controlled metal-filaments could be a key factor of nonvolatile memory characteristics. Therefore, inserting the Au NCs in the CBRAM cell could enhance the nonvolatile memory characteristics and reduce the forming voltage of CBRAM cell due to controlling the random position and orientation of Cu-ion bridging filaments. The CBRAM cell embedded with proper Au NCs exhibited the enhanced nonvolatile memory characteristics such as AC set/reset endurance cycles of 1.0 X 107, read endurance cycles of 1.0 X 1010, and retention time at 85 oC of ~16 years. Moreover, the multi-level cell operation of the CBRAM cell embedded with proper Au NCs also was obtained with improved AC set/reset endurance cycles of 1.0 X 105, and retention time of 1.0 X 105sec. * This work was financially supported by the Industrial Strategic Technology Development Program (10039191, The Next Generation MLC PRAM, 3D ReRAM, Device, Materials and Micro Fabrication Technology Development) funded by the Ministry of Trade, Industry and Energy (MOTIE), Republic of Korea and the Brain Korea 21 Plus, Republic of Korea. Figure 1

Nonvolatile Memory-Operation Mechanism for Ag-Doped PEO-Based Cbram-Cells

Conductive bridging random access memory (CBRAM) is considering as one of the promising candidate for next generation non-volatile memory (NVM) because it has many advantages such as low power consumption, and large on/off ratio. In particular, CBRAM has a possibility of terra-bit-level nonvolatile memory as a post flash memory due to their 4F2simple structure of a top reactive electrode, polymer electrolyte, and bottom inert electrode, and they showed a bipolar switching memory characteristic.[1] CBRAM is affected by material and state of reactive electrode because CBRAM can operate via forming or breaking metal filament in polymer electrolyte by migrating metal cation. However, only a few materials can be commercially used for reactive electrode of the CBRAM cells. To overcome such limitation, we studied the CBRAM cell fabricated with Ag doped polymer electrolyte. Metal filament was formed by doping Ag ions at Ag-doped polymer electrolyte, instead of migrating metal cation from reactive electrode. [2] Filament forming mechanism for Ag-doped PEO-based CBRAM cells was investigated by fabricating planar type CBRAM cells, which has a structure of Ag-doped PEO (Poly Ethylene Oxide) between Pt electrodes. As shown in Fig. 1, CBRAM cell was fabricated on SiO2 grown on a Si wafer. Arrays of Pt electrode (width and length, 1,000 and 10 um) were patterned on the 100 nm-thick Pt electrode. Then, Ag-doped PEO was spin-coated on the array of Pt electrode. Note that, the Ag-doped PEO is made by mixing the AgNO3 and PEO with a co-solvent of acetonitrile and ethanol at 30 oC for 3 hrs. AgNO3 is separated into Ag+ and NO3 - ions in solvent. Ag+ion locate negative defect site. Ag+ ions move cathode and are reduced to Ag atoms when the positive bias was applied to anode and the negative bias was applied to cathode. The reduced Ag atoms at the cathode lead to a growth of the Ag filament, which finally reaches the anode.[3] The shape of Ag filament is changed by the compliance current. We investigated relationship between their nonvolatile memory characteristics and the shape of Ag filament. Fig. 2 shows various CBRAM characteristics depending on compliance current. It was observed that reset voltage increased from -0.1 to -0.6 V when compliance current increase from 10-6 to 10-3A. But set voltage and high resistance state (HRS) were not changed. Fig. 3 shows SEM images for Ag filament depending on compliance current. It indicates that Ag filament is reduced when compliance current decreased from 10-3 to 10-6A. The reset voltage of CBRAM cells increased with compliance current. It implied that higher compliance current led to forming the thicker filament. This result was corresponds with SEM image. We present NVM operation mechanism of Ag-doped PEO-based CBRAM by revealing of relationship between Ag filament and memory characteristics. * This work was financially supported by the Industrial Strategic Technology Development Program (10039191, The Next Generation MLC PRAM, 3D ReRAM, Device, Materials and Micro Fabrication Technology Development) funded by the Ministry of Trade, Industry and Energy (MOTIE), Republic of Korea and the Brain Korea 21 Plus, Republic of Korea.. Reference [1] M. Tada, K. Okamoto, T. Sakamoto, M. Miyamura, N. Banno, and H. Hada: IEEE Trans. Electron Devices 58 (2011) 4398. [2] Han-Vit Jeoung, Hyun-Min Seung, Kyoung-Cheol Kwon, Jong-Sun Lee, Myung-Jin Song, Ki-Hyun Kwon, Dong-Won Kim, and Jea-Gun Park: The 14thNon-Volatile Memory Technology Symposium P1-022 [3] Yu Chao Yang, Feng Pan, Qi Liu, Ming Liu, and Fei Zeng: Nano Lett., Vol. 9, No. 4, 2009 Figure 1

Influence of carbon content on the copper-telluride phase formation and on the resistive switching behavior of carbon alloyed Cu-Te conductive bridge random access memory cells

In this paper, we investigate the influence of the carbon content on the Cu-Te phase formation and on the resistive switching behavior in carbon alloyed Cu0.6Te0.4 based conductive bridge random access memory (CBRAM) cells. Carbon alloying of copper-tellurium inhibits the crystallization, while attractive switching behavior is preserved when using the material as Cu-supply layer in CBRAM cells. The phase formation is first investigated in a combinatorial way. With increasing carbon content, an enlargement of the temperature window in which the material stays amorphous was observed. Moreover, if crystalline phases are formed, subsequent phase transformations are inhibited. The electrical switching behavior of memory cells with different carbon contents is then investigated by implementing them in 580 μm diameter dot TiN/Cu0.6Te0.4-C/Al2O3/Si memory cells. Reliable switching behavior is observed for carbon contents up to 40 at. %, with a resistive window of more than 2 orders of magnitude, whereas for 50 at. % carbon, a higher current in the off state and only a small resistive window are present after repeated cycling. This degradation can be ascribed to the higher thermal and lower drift contribution to the reset operation due to a lower Cu affinity towards the supply layer, leading cycle-after-cycle to an increasing amount of Cu in the switching layer, which contributes to the current. The thermal diffusion of Cu into Al2O3 under annealing also gives an indication of the Cu affinity of the source layer. Time of flight secondary ion mass spectroscopy was used to investigate this migration depth in Al2O3 before and after annealing, showing a higher Cu, Te, and C migration for high carbon contents.

A Thermally Stable and High-Performance 90-nm ${\rm Al}_{2}{\rm O}_{3}\backslash{\rm Cu}$-Based 1T1R CBRAM Cell

In this paper, we optimize the stack of a 90-nm CMOS-friendly <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm W}\backslash{\rm Al}_{2}{\rm O}_{3}\backslash{\rm Cu}$</tex></formula> conductive-bridging random access memory cell integrated in the one-transistor/one-resistor configuration. We show that the excellent Cu buffering properties of a TiW layer inserted at the <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm Al}_{2}{\rm O}_{3}\backslash{\rm Cu}$</tex></formula> interface make it possible, on one hand, to ensure cell integrity after back-end-of-line processing at 400 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$^{\circ}{\rm C}$</tex></formula> and, on the other, to obtain excellent memory performances. After optimization of the <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">${\rm Al}_{2}{\rm O}_{3}$</tex></formula> layer thickness, the cell exhibits highly controlled set and reset operations, a large memory window, fast pulse programming (10 ns) at low voltage <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$({&lt;}{\rm 3}~{\rm V})$</tex></formula> , and low-current (10 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu{\rm A}$</tex></formula> ), and multilevel operation. Finally, <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$10^{6}$</tex></formula> cycles of write endurance lifetime with up to a three-decade memory window is demonstrated, and state stability is assessed up to 125 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{\circ}{\rm C}$</tex></formula> .