Conductive Bridge Random Access Memory Research Articles

Fabrication of a Cu‐Cone‐Shaped Cation Source Inserted Conductive Bridge Random Access Memory and Its Improved Switching Reliability

AbstractConductive bridge random access memory (CBRAM) has been regarded as a promising candidate for the next‐generation nonvolatile memory technology. Even with the great performance of CBRAM, the global generation and overinjection of cations after much repetitive switching cannot be prevented. The overinjection of cations into an electrolyte layer causes high‐resistance‐state resistance (RHRS) degradation, on/off ratio reduction, and eventual switching failure. It also degrades the switching uniformity. In this work, a Cu‐cone‐structure‐embedded TiN/TiO2/Cu cone/TiN device is fabricated to alleviate the problems of Cu‐based CBRAM, mentioned above. The fabrication method of the device, which is useful for laboratory scale experiment, is developed, and its superior switching performance and reliability compared with the conventional planar device. The insertion of the Cu cone structure allows the placement of only a limited amount of cation source in each cell, and the embedded conical structure also concentrates the applied electric field, which enables filament growth control. Furthermore, the concentrated field localizes the resistive switching on the tip area of the cone structure, which makes the effective switching area about tens of nanometers even for the much larger area of the entire electrode (several µm2).

Failure Thresholds in CBRAM Due to Total Ionizing Dose and Displacement Damage Effects

With the growing interest to explore Jupiter’s moons, technologies with +10 Mrad(Si) tolerance are now needed, to survive the Jovian environment. Conductive-bridging random access memory (CBRAM) is a nonvolatile memory that has shown a high tolerance to total ionizing dose (TID). However, it is not well understood how CBRAM behaves in an energetic ion environment where displacement damage (DD) effects may also be an issue. In this paper, the response of CBRAM to 100-keV Li, 1-MeV Ta, and 200-keV Si ion irradiations is examined. Ion bombardment was performed with increasing fluence steps until the CBRAM devices failed to hold their programed state. The TID and DD dose (DDD) at the fluence of failure were calculated and compared against tested ion species. Results indicate that failures are more highly correlated with TID than DDD. DC cycling tests were performed during 100-keV Li irradiations and evidence was found that the mobile Ag ion supply diminished with increasing fluence. The cycling results, in addition to prior 14-MeV neutron work, suggest that DD may play a role in the eventual failure of a CBRAM device in a combined radiation environment.

Key material parameters driving CBRAM device performances.

This study is focused on Conductive Bridging Random Access Memory (CBRAM) devices based on chalcogenide electrolyte and Cu-supply materials, and aims at identifying the key material parameters controlling memory properties. The CBRAM devices investigated are integrated on CMOS select transistors, and are constituted by either Ge-Se or Ge-Te electrolyte layers of various compositions combined with a Cu2GeTe3 active chalcogenide electrode. By means of extensive physical and electrical characterization, we show for a given electrolyte system that slower write is obtained for a denser electrolyte layer, which is directly correlated with a lower atomic percentage of the chalcogen element in the layer. We also evidence that the use of Ge-Se electrolyte results in larger write energy (voltage and time), however with improved state retention properties than for Ge-Te electrolyte materials. We associate these results with the stronger chemical bonding of Cu with Se, resulting both in a stabilized Cu filament and a slower Cu cation motion. More robust processing thermal stability is also observed for Ge-Se compared to Ge-Te compounds, allowing more flexibility in the integration flow design.

Excellent data retention characteristic of Te-based conductive-bridge RAM using semiconducting Te filament for storage class memory

In this work, we explore the electrical properties and data retention of Te-based conductive-bridge random-access memory (CBRAM) of ZrxTe1−x/Al2O3/Pt cells. The virgin resistance and forming voltage are decreased with increasing Zr composition (0.5 ≦ x ≦ 0.7) and decreasing electrolyte thickness. The resistance of the conductive filament (CF) formed in the Te-CBRAM shows semiconducting behavior that is decreased with increasing temperature, whereas a Cu-based CBRAM shows metallic behavior. Furthermore, the conductance change of Te-based CBRAM, during the filament dissolution step, occurs with lower conductance units than those of Cu/Ag-based CBRAM. The most differentiable characteristics of both devices are the data retention. Te-based CBRAM shows better data stability at high temperature (150 °C) than Cu-based device. Accelerated tests (250, 270, and 300 °C) were performed to understand the data retention of the Te-CBRAM, yielding excellent retention characteristics (10 years at 177 °C) despite its relatively low activation energy (Ea, 1.07 eV) than Cu/Ag- based devices. We believe that the excellent retention properties of Te-based devices are more influenced by the wide effective CF size than by Ea.

Perspective: Uniform switching of artificial synapses for large-scale neuromorphic arrays

Resistive random-access memories are promising analog synaptic devices for efficient bio-inspired neuromorphic computing arrays. Here we first describe working principles for phase-change random-access memory, oxide random-access memory, and conductive-bridging random-access memory for artificial synapses. These devices could allow for dense and efficient storage of analog synapse connections between CMOS neuron circuits. We also discuss challenges and opportunities for analog synaptic devices toward the goal of realizing passive neuromorphic computing arrays. Finally, we focus on reducing spatial and temporal variations, which is critical to experimentally realize powerful and efficient neuromorphic computing systems.

Hybrid‐RRAM toward Next Generation of Nonvolatile Memory: Coupling of Oxygen Vacancies and Metal Ions

AbstractHere, the impact of copper and oxygen vacancy balance in filament composition as a key factor for oxide‐based conductive bridge random access memories (hybrid resistive random access memories (HRRAMs)) performances is investigated. To this aim, several RRAM technologies are studied using various resistive layers and top electrodes. Material analyses allow to highlight the hybrid aspect of HRRAM conductive filament. Density functional theory simulations are used to extract microscopic features and highlight differences from a material point of view. Integrated RRAM technology performances such as window margin, endurance, and retention are then measured to analyze copper and oxygen vacancy influence on device characteristics. A new RRAM classification correlating filament composition and memory performances is proposed.

Analysis of tail bits generation of multilevel storage in resistive switching memory**Project supported by the Ministry of Science and Technology of China (Grant Nos. 2016YFA0203800, 2016YFA0201803, and 2018YFB0407502), the National Natural Science Foundation of China (Grant Nos. 61522408, 61334007, and 61521064), Beijing Municipal Science & Technology Commission Program, China (Grant No. Z161100000216153), and Huawei Data Center Technology Laboratory.

The tail bits of intermediate resistance states (IRSs) achieved in the SET process (IRSS) and the RESET process (IRSR) of conductive-bridge random-access memory were investigated. Two types of tail bits were observed, depending on the filament morphology after the SET/RESET operation. (i) Tail bits resulting from lateral diffusion of Cu ions introduced an abrupt increase of device resistance from IRS to ultrahigh-resistance state, which mainly happened in IRSS. (ii) Tail bits induced by the vertical diffusion of Cu ions showed a gradual shift of resistance toward lower value. Statistical results show that more than 95% of tail bits are generated in IRSS. To achieve a reliable IRS for multilevel cell (MLC) operation, it is desirable to program the IRS in RESET operation. The mechanism of tail bit generation that is disclosed here provides a clear guideline for the data retention optimization of MLC resistive random-access memory cells.

Theoretical investigation of the Ag filament morphology in conductive bridge random access memories

Conductive bridge random access memories (CBRAMs) usually involve active Ag or Cu metals, where the formation of metal filaments accounts for the low resistance state. For the application of neuromorphic computation, it is highly desirable to develop artificial neurons and synapses, which utilize the complicated volatile or nonvolatile resistive switching phenomena, respectively. This can be achieved by controlling the morphology and stability of the filaments, which requires a deep understanding of the filament formation and disruption mechanisms. Using ab initio calculations, we explored the physical mechanism behind various Ag filament morphologies and growth modes, using GeSe, ZrO2, SiO2, and a-Si as the examples. The roles of Ag and Ag+ stability inside the dielectric, the migration barrier of Ag+, and the Ag+ solvation effect have been investigated in detail. A comprehensive model has been proposed, which in particular could explain the diverse Ag filament morphology experimentally observed in sputtered SiO2 and PECVD SiO2. Our theoretical approach can serve as a pre-screening method in designing new solid-state electrolyte materials of CBRAM, aiming at new functionalities in neuromorphic computation or in-memory logic computing.

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.

TAOS based Cu/TiW/IGZO/Ga2O3/Pt bilayer CBRAM for low-power display technology

We demonstrate the characteristics of a conductive-bridging random access memory (CBRAM) with Cu/TiW/InGaZnO/Ga2O3/Pt stack structure. The addition of a thin metal-oxide layer (4.5 nm-thick Ga2O3) in the bottom of the CBRAM device significantly increases the off-state resistance (ROFF) and the memory window. The IGZO bi-layer CBRAM shows the excellent memory performances, such as low operation current (down to 50 μA), high on/off resistance ratio (>103), high switching endurance (up to 103 cycles) and the capability of multi-level tuning. Meanwhile, high thermal stability was also achieved. Three decades of resistance window is constantly maintained beyond 104 s at 85 °C. The resistive switching stability and electrical uniformity of bi-layer IGZO/Ga2O3 CBRAM device are obviously enhanced as compared with the one only with a single layer of IGZO film. These results have given a great potential for the transparent amorphous oxide semiconductor (TAOS)-based material utilizing in CBRAM stacks and integrating into the display circuits for future memory-in-pixel applications.

Enhanced electrical behavior from the galvanic effect in Ag-Cu alloy electrode conductive bridging resistive switching memory

In this study, an Ag-Cu alloy was chosen as the electrode in conductive bridging random access memory (CBRAM), with results indicating a significant decrease in forming voltage. In addition, resistive switching characteristics as well as a retention test indicated better stability and a resistive switching window of at least an order. The switching time of the Ag-Cu alloy CBRAM is shorter than that of both Ag and Cu electrode CBRAMs under fast current-voltage (fast I-V). The experimental result indicated that the mechanism was dominated by the galvanic effect. Active atoms (Ag) captured electrons of inactive atoms (Cu) and generated metallic ions (Cu ions) in the alloy electrode. Cu ions drifted into the insulator and generated a conductive path when applying voltage bias. The use of this alloy as an electrode in CBRAM can significantly decrease forming voltage and enhance CBRAM characteristics.

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

Ar-Plasma-Modulated Optical Reset in the SiO2/Cu Conductive-Bridge Resistive Memory Stack

Resistive Random Access Memory (RRAM) has been widely studied as a promising next-generation memory with advantageous characteristics such as high-speed and low-power operation[1]. In particular, the SiO2/Cu conductive-bridge random access memory (CBRAM), where electric-field-driven formation/dissolution of Cu metal filaments provides resistance switching, has been drawing much attention because of its compatibility with the backend interconnect process. Recently, we have demonstrated negative photo conductivity (NPC) or light-induced resistance reset (LIR) in the SiO2/Cu stack by disrupting the electrically-formed filamentary breakdown path[2]. It is considered that the NPC is caused by recombination of light-excited oxygen ions and oxygen vacancies created in the breakdown path during electrical forming. Although the exact mechanism is still elusive, the finding suggests a potential to realize an electro-optical control of memory devices. In our pursuit of the mechanism(s) behind the abovementioned NPC phenomenon, we reveal here an impact on NPC by Ar plasma surface treatment, which is reported to increase oxygen vacancies in the oxide electrolyte[3]. The effect of light illumination on the electrical property of the Ar-plasma-treated Cu sample (SiO2 10nm/Cu) and control sample (SiO2 5nm/Si) is investigated via ultra-high vacuum conductive atomic force microscope or CAFM (Fig. 1). During forming, a negative bias voltage is supplied to the probe with a preset current compliance of 1.1 nA. The area of the probe-oxide contact is estimated to be ~26nm2[4]. From the current-voltage (I-V) curves for the Cu or control sample depicted in Fig. 2, it is evident that the switching voltage from the high resistance state (HRS) to the low resistance state (LRS) in the Cu sample is shifted from -6V to -4V by 33% after the Ar plasma treatment, while the breakdown voltage in the control sample is almost unchanged. Therefore, the electrical formation of Cu filaments is considered to be accelerated by Ar plasma treatment. Fig. 3 shows typical response to light in the post-forming Cu sample and percentage of NPC occurrence calculated from 30 tested locations in each of the Cu and control sample. The light intensity used was 1 mW/cm2. Clearly, Ar plasma treatment suppresses the NPC in both samples and especially enlarges the proportion of no-NPC cases in the Cu sample. The FT-IR spectra (Fig. 4) of the pre- and post-plasma-treated SiO2 film show the intensities of HO-H stretching (3300cm-1) and SiO-H stretching (3650cm-1) become higher after the Ar plasma treatment. This rise implies the destruction of the Si-O network near the surface and the increase of various defects like Si dangling bonds and oxygen vacancies where water in the air may be adsorbed. Based on the results, we propose a possible explanation for the mechanism of the lower switching voltage in the Cu sample and the decreased percentage of NPC occurrence with Ar plasma treatment in both samples. For the control sample, Ar plasma treatment may sputter oxygen ions and create more oxygen vacancies near the surface of SiO2 [3], which helps forming a larger conducting filament composed of oxygen vacancies. This can be confirmed by a higher current in the reverse I-V curve after Ar plasma exposure (Fig. 2 (c)(d)). Since oxygen ions are depleted, the NPC response (due to recombination of light-excited oxygen ions and vacancies) is suppressed. In the case of the Cu sample, the oxygen vacancies created in the surface of SiO2 by Ar sputtering may adsorb water molecules. The resultant hydroxyl groups (OH-) reportedly promote Cu ion (Cu+) diffusion into SiO2 [5] and reduce the switching voltage during forming (Fig. 5). When more Cu ions migrate into SiO2, part of the conducting filament made up of oxygen vacancies there is replaced by a Cu filament (Fig. 5 (e)), which is not readily disrupted due to the stronger metal-metal bond (which requires higher energy for cleavage than that of the light used[2]). Therefore, the conducting path composed of more Cu metal filaments remain after light illumination (Fig.5 (f)), leading to the suppression of the NPC response.

Effect of dysprosium and lutetium metal buffer layers on the resistive switching characteristics of Cu–Sn alloy-based conductive-bridge random access memory

The conductive-bridge random access memory (CBRAM) has become one of the most suitable candidates for non-volatile memory in next-generation information and communication technology. The resistive switching (RS) mechanism of CBRAM depends on the formation/annihilation of the conductive filament (CF) between the active metal electrode and the inert electrode. However, excessive ion injection from the active electrode into the solid electrolyte reduces the uniformity and reliability of the RS devices. To solve this problem, we investigated the RS characteristics of a CuSn alloy active electrode with different compositions of Cux–Sn1–x (0.13 < X < 0.55). The RS characteristics were further improved by inserting a dysprosium (Dy) or lutetium (Lu) buffer layer at the interface of Cux–Sn1–x/Al2O3. Electrical analysis of the optimal Cu0.4–Sn0.73/Lu-based CBRAM exhibited stable RS behavior with low operation voltage (SET: 0.7 V and RESET: −0.3 V), a high on state/off state resistive ratio (106), AC cyclic endurance (>104), and stable retention (85 °C/10 years). To achieve these performance parameters, CFs were locally formed inside the electrolyte using a modified CuSn active electrode, and the amount of Cu-ion injection was reduced by inserting the Dy or Lu buffer layer between the CuSn active electrode and the electrolyte. In particular, conductive-atomic force microscopy results at the Dy or Lu/Al2O3 interface directly showed and defined the diameter of the CF.

Spatial Projection of Electronic Conductivity: The Example of Conducting Bridge Memory Materials

Conducting bridge random access memory materials have special promise for FLASH memory, other applications beside, and also special potential for continued miniaturization. They are electronic materials of unique flexibility. Here, we offer new models of Cu‐doped alumina, and reveal qualitative differences in the behavior of transition metal ions in chalcogenide and oxide hosts, showing that Cu clusters in an amorphous alumina host, in contrast with chalcogenides in which the metal atoms do not cluster. We further elucidate the processes of electron transport. To determine these, the Kubo–Greenwood formula is cast in a form to enable the estimate of a space‐projected conductivity. The method reveals those parts of the networks that may conduct a current (or absorb radiation at frequency ω).

Non-volatile resistive switching in CuBi-based conductive bridge random access memory device

Resistive switching devices, which are dominated by metal cation based conductive filament formation/rupture, are called programmable memory, or conductive bridge random access memory (CBRAM), and are widely expected to replace existing memory devices. In this letter, CuBi alloy was used as an active electrode to control the over-diffusion of Cu ions into the solid electrolyte of a CBRAM. In addition, resistive switching performance was improved by inserting lutetium and dysprosium metals, which acted as a buffer layer at the interface of the active electrode (CuBi) and the dielectric layer (Al2O3). When optimized, the Cu0.55Bi0.45/Lu(Dy)/Al2O3/Pt showed excellent resistive switching performance. This improvement can be explained by the high controllability of Cu in Cu-Bi electrode with the Lu (Dy) buffer layer. The formation of intermediate oxide buffer layers at the CuBi/Lu(Dy) interface was analysed using XPS. Moreover, conductive-atomic force microscope measurements helped to define the inverted cone shape of the metallic conductive filament.

Solid-State Conductive-Bridging Reconfigurable RF-Encoding Particle for Chipless RFID Applications

We report the design and first results of a reconfigurable RF-encoding particle (REP) for chipless RFID tag, based on a Nafion conductive-bridging random-access memory cell. The developed tag consists of a modified shorted dipole as REP, equipped with a copper-Nafion-aluminum metal-insulator-metal (MIM) nonvolatile ionic-bridging switch. The MIM switch is similar to an RC parallel network, which tunes the electrical length of the resonator. The chipless tags switch between two different resonant frequencies, in the set and reset states of the switch. An electrical model is also developed for the REP to explain the mechanism of switching. The experimental results are in close agreement with the simulation results using commercial electromagnetic simulators. This letter is a proof of concept of electronically reconfigurable chipless tags based on integrated switches which have the potential to be directly printed along with the tag.

Current Optimized Coset Coding for Efficient RRAM Programming

Emerging nonvolatile memory technology based on resistive switching random access memory, especially conductive-bridge random access memory (CBRAM), has become a proficient contender for the replacement of charge-based flash memory technology in certain applications. In this brief, we present the detailed characterization and analysis of programming current consumption of CBRAM chips. We found that trends in write current consumption specific to bit switching order exist, with local and global minima for 2- or 3-bit switching. We further characterized in detail, the programming current consumption for specific 2- or 3-bit switching combinations. We then propose a new encoding algorithm: current optimized coset coding (CoCC), which exploits the observed unique current consumption signatures of the technology for programming current minimization. Experimental validation of the proposed algorithm showed an overall current reduction of ~6% in case of random data-write cycling experiments.

Role of Al2O3 thin layer on improving the resistive switching properties of Ta5Si3-based conductive bridge random accesses memory device

Ta5Si3-based conductive bridge random access memory (CBRAM) devices have been investigated to improve their resistive switching characteristics for their application in future nonvolatile memory technology. Changes in the switching characteristics by the addition of a thin Al2O3 layer of different thicknesses at the bottom electrode interface of a Ta5Si3-based CBRAM devices have been studied. The double-layer device with a 1 nm Al2O3 layer has shown improved resistive switching characteristics over the single layer one with a high on/off resistance ratio of 102, high endurance of more than 104 cycles, and good retention for more than 105 s at the temperature of 130 °C. The higher thermal conductivity of Al2O3 over Ta5Si3 has been attributed to the enhanced switching properties of the double-layer devices.