Reliability Of Resistive Random Access Memory Research Articles

Highly Reliable 3D Channel Memory and Its Application in a Neuromorphic Sensory System for Hand Gesture Recognition.

Brain-inspired neuromorphic computing systems, based on a crossbar array of two-terminal multilevel resistive random-access memory (RRAM), have attracted attention as promising technologies for processing large amounts of unstructured data. However, the low reliability and inferior conductance tunability of RRAM, caused by uncontrollable metal filament formation in the uneven switching medium, result in lower accuracy compared to the software neural network (SW-NN). In this work, we present a highly reliable CoOx-based multilevel RRAM with an optimized crystal size and density in the switching medium, providing a three-dimensional (3D) grain boundary (GB) network. This design enhances the reliability of the RRAM by improving the cycle-to-cycle endurance and device-to-device stability of the I-V characteristics with minimal variation. Furthermore, the designed 3D GB-channel RRAM (3D GB-RRAM) exhibits excellent conductance tunability, demonstrating high symmetricity (624), low nonlinearity (βLTP/βLTD ∼ 0.20/0.39), and a large dynamic range (Gmax/Gmin ∼ 31.1). The cyclic stability of long-term potentiation and depression also exceeds 100 cycles (105 voltage pulses), and the relative standard deviation of Gmax/Gmin is only 2.9%. Leveraging these superior reliability and performance attributes, we propose a neuromorphic sensory system for finger motion tracking and hand gesture recognition as a potential elemental technology for the metaverse. This system consists of a stretchable double-layered photoacoustic strain sensor and a crossbar array neural network. We perform training and recognition tasks on ultrasonic patterns associated with finger motion and hand gestures, attaining a recognition accuracy of 97.9% and 97.4%, comparable to that of SW-NN (99.8% and 98.7%).

Area-Selective Atomic Layer Deposition for Resistive Random-Access Memory Devices.

Resistive random-access memory (RRAM) is a promising technology for data storage and neuromorphic computing; however, cycle-to-cycle and device-to-device variability limits its widespread adoption and high-volume manufacturability. Improving the structural accuracy of RRAM devices during fabrication can reduce these variabilities by minimizing the filamentary randomness within a device. Here, we studied area-selective atomic layer deposition (AS-ALD) of the HfO2 dielectric for the fabrication of RRAM devices with higher reliability and accuracy. Without requiring photolithography, first we demonstrated ALD of HfO2 patterns uniformly and selectively on Pt bottom electrodes for RRAM but not on the underlying SiO2/Si substrate. RRAM devices fabricated using AS-ALD showed significantly narrower operating voltage range (2.6 × improvement) and resistance states than control devices without AS-ALD, improving the overall reliability of RRAM. Irrespective of device size (1 × 1, 2 × 2, and 5 × 5 μm2), we observed similar improvement, which is an inherent outcome of the AS-ALD technique. Our demonstration of AS-ALD for improved RRAM devices could further encourage the adoption of such techniques for other data storage technologies, including phase-change, magnetic, and ferroelectric RAM.

UV-laser annealing for improved resistive switching performance and reliability of flexible resistive random-access memory

Flexible resistive random-access memory (RRAM) has attracted significant attention owing to the strong demand for various flexible electronic memory devices. However, fabricating reliable and flexible RRAM is a significant challenge because of the inherently high thermal sensitivity of plastic substrates. In this study, a high-performance flexible RRAM device was fabricated without deforming a plastic substrate by utilizing a precisely controlled UV laser annealing process. The application of laser annealing in an Al/ZnO/Al flexible RRAM changes the concentration of O Frenkel defect pairs in the ZnO layer and produce a ZnO/Al mixed interface layer with high quality oxygen reservoirs. The higher concentration of O Frenkel defect pairs in the ZnO layer induces electroforming-free process, and the improved characteristics such as crystallinity, morphology, and stoichiometry of the interface area result in stable resistive switching and performance enhancement in the flexible RRAM. The laser-annealed flexible RRAM exhibits a high on/off ratio (~1.07 × 10 4 ), high cycling endurance (up to 2.5 ×10 3 cycles), and low power consumption (4.88 µW in SET state and 1.21 µW in RESET state). Importantly, the performance was maintained at a bending radius of up to 5 mm. • Laser annealing process induces a high concentration of O Frenkel defect pairs in the ZnO and forms the ZnO/Al mixed interface layer. • Laser annealing induces the EF-free process and improves the performance and reliability of flexible RRAM. • Laser annealing process does not deform flexible substrate.

Resistive switching properties for fluorine doped titania fabricated using atomic layer deposition

This study demonstrates a new resistive switching material, F-doped TiO2 (F:TiO2), fabricated by atomic layer deposition (ALD) with an in-house fluorine source for resistive random access memory (RRAM) devices. Controlling oxygen vacancies is required since RRAM uses resistive switching (RS) characteristics by redistributing oxygen ions in oxide, and poor oxygen defect control has been shown to significantly reduce RRAM reliability. Therefore, this study designed an F based RRAM device using fluorine anions rather than oxygen defect for the main agent of RS behavior. We developed the F:TiO2 RRAM material using a novel in situ doping method in ALD and investigated its RS behaviors. The Pt/F:TiO2/Pt device exhibited forming-less bipolar RS and self-rectifying behavior by fluorine anion migration, effectively reducing the sneak current in crossbar array architecture RRAM. The doped fluorine passivated and reduced oxygen related defects in TiO2, confirmed by x-ray photoelectron spectroscopy analysis. Adopting the F-based RS material by ALD provides a viable candidate for high reliability RRAM.

On the stochastic nature of conductive points formation and their effects on reliability of MoS2 RRAM: Experimental characterization and Monte Carlo simulation

Two-dimensional (2D) monolayer molybdenum disulfide (MoS2) has been used as the non-volatile resistance switching (NVRS) layer in resistive random-access memory (RRAM) devices. In this work, an electron-beam irradiation treatment on monolayer MoS2 film to modify the defect properties has been applied to improve the reliability of the NVRS devices. Compared to unirradiated devices, the reliability of the devices with a moderate radiation has been shown to improve by up to 1.5× for yield and 11× for average DC cycling endurance. To better understand the mechanism behind, Monte Carlo simulations have been performed based on our previously proposed conductive-point model with the metal ion substitution into sulfur vacancy. The simulation yield and cycle numbers show similar trends with the experimental data. Our results provide additional insights into defect engineering to precisely control the switching properties of 2D-based RRAM devices for a wide range of applications.

A Study of the Relationship Between Endurance and Retention Reliability for a HfOₓ-Based Resistive Switching Memory

This study determines the relationship between retention and endurance reliability for a HfOx-based resistive random access memory (ReRAM). A TiN (15 nm) / HfOx (6 nm) / Ti (10 nm) / TiN (40 nm) stacked structure is fabricated and tested to verify its basic characteristics and reliability. The high resistance states (HRS) retention behavior is characterized and is found to degrade over 100x on the endured bits because there is a sequential high temperature procedure. The degradation is reduced slightly to a ~30x drop for the endured devices with one single refresh cycle. During the endurance and retention test procedures, the HRS resistance decreases because neutral oxygen vacancy filaments grow and this cannot be reversed. The I-V characteristics for endured devices are also determined. The results show that isothermal treatment causes a gradual SET and RESET process with multiple feasible states. The thermally induced filament degradation model (isolated filament vs. continuous filament) is verified by the relationship between retention and endurance reliability. Design guidance is recommended for an improvement in ReRAM reliability.

(Invited) Material and Device Design for Practical Application of Highly-Reliable and High-Density ReRAM

Electric-pulse-induced resistance change phenomena in PrCaMnO3 were reported in 2000 [1]. From this finding, Resistive Random Access Memory (ReRAM) using this phenomena attracted both academic and industrial researchers as a promising candidate of next-generation nonvolatile memory. In 2013, we started the world’s first mass-production of ReRAM-embedded micro-controller unit. Among issues to be solved for its practical application, the most fundamental problem was that the mechanism of resistive switching (RS) remained unclear, causing the lack of guide for the design of materials and devices for ReRAMs. Therefore, we investigated the mechanism of RS and considered the appropriate materials and device structures for ReRAM from the viewpoint of device reliability. Based on the clarified RS mechanism, we successfully developed highly-reliable Ta oxide ReRAM. ReRAM cells with top electrode/Ta oxide/bottom electrode was fabricated. Ta oxide was deposited by reactive sputtering. Device structures, RS characteristics and reliability of ReRAMs were evaluated by electrical measurements and physical analyses.In order to elucidate the mechanism of RS, chemical states in ReRAM cells for initial resistance state, high-resistance state (HRS) and Low-resistance state (LRS) were directly observed by hard x-ray photoemission spectroscopy. The ratio of Ta 4d photoemission peaks for Ta sub-oxide (TaOx) to Ta2O5 increases with the decrease in the resistance of ReRAM cells (from Initial to HRS to LRS). This indicates that the mechanism of RS is redox reactions in Ta oxide layer. Based on the RS model, the guidelines for the material selection of oxide and electrode were presented. Oxide material should be bi-stable: The Gibbs free energy for the reaction between oxides with different valence states such as TaO2/Ta2O5should be small. Electrode material should not be easy to oxidize: Standard electrode potential of the electrode metal should be sufficiently higher than that of material constituting the RS oxide [2]. We next investigated the detailed structure of Ta oxide ReRAM and found that the RS locally occurred in small conductive area (Conductive Filament: CF) consisting of oxygen-deficient Ta oxide [3]. Therefore, we evaluated the retention characteristics of Ta oxide ReRAMs focusing on the CF. Experimental results indicate that the retention degradation of LRS/HRS is caused by the oxygen diffusion into/out of CF. From detailed analyses, we clarified that the control of CF characteristics including filament size and the density of oxygen vacancy was the key for the improvement of the reliability [4]. Utilizing these insights, we established the reliability model of Ta oxide ReRAM and successfully started the mass-production of ReRAM. The delivery of higher-density ReRAM is essential for further spread of ReRAM in the market. The following two the issues are inevitable for the realization of higher-density ReRAM; (1) scaling down of ReRAM cells and (2) low-current operation, which may degrade the reliability. Therefore, we addressed these challenges and successfully demonstrated the good array-level reliability with scaled-down cells and low-current operation by controlling the CF from the standpoint of fabrication process and device structure, respectively [5,6].

Analysis on the Filament Structure Evolution in Reset Transition of Cu/HfO2/Pt RRAM Device.

The resistive switching (RS) process of resistive random access memory (RRAM) is dynamically correlated with the evolution process of conductive path or conductive filament (CF) during its breakdown (rupture) and recovery (reformation). In this study, a statistical evaluation method is developed to analyze the filament structure evolution process in the reset operation of Cu/HfO2/Pt RRAM device. This method is based on a specific functional relationship between the Weibull slopes of reset parameters’ distributions and the CF resistance (Ron). The CF of the Cu/HfO2/Pt device is demonstrated to be ruptured abruptly, and the CF structure of the device has completely degraded in the reset point. Since no intermediate states are generated in the abrupt reset process, it is quite favorable for the reliable and stable one-bit operation in RRAM device. Finally, on the basis of the cell-based analytical thermal dissolution model, a Monte Carlo (MC) simulation is implemented to further verify the experimental results. This work provides inspiration for RRAM reliability and performance design to put RRAM into practical application.

Ab initio modeling of oxygen-vacancy formation in doped-HfOx RRAM: Effects of oxide phases, stoichiometry, and dopant concentrations

Doping techniques have been widely investigated to improve the performance and reliability of resistive random-access memory. In this paper, oxide phases, non-stoichiometry, and dopant concentrations are identified as three critical factors that affect the characteristics of doped HfOx. Using ab initio calculations, their effects on Al, Ti, and Si dopants are thoroughly investigated and compared with experimental studies. In particular, the inconsistent observations regarding the effects of Al dopants on forming voltages are explained by the variation of oxygen stoichiometry. Only non-stoichiometric oxide will activate the dopant effects to significantly reduce the forming voltages.

Effects of RESET Current Overshoot and Resistance State on Reliability of RRAM

Current overshoot has severe effects on the reliability of resistive random access memory (RRAM). It is well known that the current overshoot during the SET process is caused by parasitic capacitance. In this letter, we observed a different type of current overshoot during the RESET process. The RESET current overshoot was confirmed to have severe effects on the endurance of RRAM. We also demonstrated the relation between the current overshoot and the intrinsic capacitive elements of each state of RRAM. Finally, an optimized pulse shape was proposed to minimize the current overshoot and was experimentally verified to significantly improve the variability and endurance in a typical RRAM device with a W/Zr/ ${\rm HfO}_{2}$ /TiN structure.