Nanoscale Resistive Random Access Memory Research Articles

Digital and analog resistive switching in Lu-doped piezoelectric [formula omitted] film

Recently, in the field of artificial intelligence, neuromorphic devices have received significant attention for building artificial neural networks. Nano-scale resistive random access memory (RRAM) is one of the emerging electrical components for neuromorphic computing. In the present study, the synaptic characteristics are investigated in the piezoelectric BiFe1−xLuxO3 (BFLO) and Bi1−xLuxFeO3 (BLFO) (x = 0.05) based RRAM devices in a vertical configuration. Both devices show bipolar resistive switching behavior with good endurance characteristics. However, analog switching with potentiation and depression response is observed in Ag/BFLO/FTO device. The change in resistance and leakage current flow across the device is explained by the space charge limiting current conduction mechanism with regard to thermally generated and injected charge carriers. Both films possess strong electro-mechanical response with the ferroelectric and ferro-elastic domain walls oriented along 710, 1090, and 1800 directions. The electro-mechanical response and resistive switching properties make the materials viable for sensing, energy harvesting, and non-volatile memory technology. Moreover, the BFLO-based synaptic device has potential application in neuromorphic computation.

Direct measurement of nanoscale filamentary hot spots in resistive memory devices.

Resistive random access memory (RRAM) is an important candidate for both digital, high-density data storage and for analog, neuromorphic computing. RRAM operation relies on the formation and rupture of nanoscale conductive filaments that carry enormous current densities and whose behavior lies at the heart of this technology. Here, we directly measure the temperature of these filaments in realistic RRAM with nanoscale resolution using scanning thermal microscopy. We use both conventional metal and ultrathin graphene electrodes, which enable the most thermally intimate measurement to date. Filaments can reach 1300°C during steady-state operation, but electrode temperatures seldom exceed 350°C because of thermal interface resistance. These results reveal the importance of thermal engineering for nanoscale RRAM toward ultradense data storage or neuromorphic operation.

Performance Enhancement of a Time-Delay PUF Design by Utilizing Integrated Nanoscale ReRAM Devices

Currently the semiconductor industry is in search of a Physically-Unclonable-Function (PUF) implementation, which combines high reliability and uniqueness with low area and power consumption. The characteristics of emerging nanoscale Resistive Random Access Memory (ReRAM) devices fulfill most of these properties, as they exhibit inherent variability with low area consumption. Of particular interest is that the resistive states of ReRAM devices show a strong dependence on the distribution of grain boundaries within the device, which leads to variability in total device resistance. In this work we transform the classic CMOS time-delay PUF (TD-PUF) utilizing integrated nanoscale ReRAM devices to achieve better performance metrics including uniqueness and reliabilitiiy. The enhanced design exploits the property of high resistance variability of ReRAMs for the design of a ReRAM based delay stage that exhibits excellent uniqueness. Accurate simulation and characterization of the proposed PUF was achieved by extracting resistance values, temperature dependence and usage stress of ReRAM devices fabricated in-house and their application in the proposed TD-PUF are discussed. A 24 stage time-delay PUF utilizing 48 ReRAM devices was simulated and results show excellent reliability with respect to environmental parameters. A temperature range of 0 to 125°C was simulated and an optimum reliability was observed at 0.79 V. A supply voltage noise of ±30 mV had no impact on the uniqueness and reliability. The proposed design was compared against two pure CMOS implementations of a TD-PUF. The comparison was performed with respect to the aforementioned metrics and under the same environmental conditions, showing up to 5 times increase in performance.

Recombinant azurin-CdSe/ZnS hybrid structures for nanoscale resistive random access memory device

In the present study, we developed a biohybrid material composed of recombinant azurin and CdSe-ZnS quantum dot to perform as a resistive random access memory (ReRAM) device. Site specific amino acid sequences were introduced in azurin to bind with the surface of CdSe-ZnS nanoparticle allowing the formation of a hybrid and voltage-driven switching enabled to develop a resistive random access memory (ReRAM) device. The analytical measurements confirmed that the azurin and CdSe-ZnS nanoparticles were well conjugated and formed into a single hybrid. Further, reversible, bistable switching along with repeatable writing-reading-erasing processes on individual azurin/CdSe-ZnS hybrid at nanoscale was achieved on the hybrid device. The device was programmed tested for 50 cycles with an ON/OFF ratio and measured to be of three orders of magnitude. The developed device shown good stability and repeatability and operates at low voltages thus makes it promising candidate for future memory device applications.

Impact of Etch Process on Hafnium Dioxide Based Nanoscale RRAM Devices

Resistive Random Access Memory (RRAM) is a new class of electronic device whose resistance level can be modulated by feeding a current through the device. The resistance level (memory state) of RRAM devices is non-volatile, making them a viable replacement for flash memory. In addition, this characteristic can be utilized for various circuit level designs including neuro-inspired hardware. For instance, RRAM devices can emulate the relative strength of neural synapses by encoding variable resistance states. In this contribution we integrated a RRAM devices with CMOS fabricated on a 300mm wafer platform. The IBM 65nm 10LPe process technology was used as a base for this customized integration effort, with modifications to accommodate the RRAM devices between metal 1 (M1) and metal 2 (M2). By changing the M1 interconnect to tungsten it is possible to utilize front-end-of-the-line (FEOL) tools for the hafnium dioxide atomic layer deposition. Above the custom tungsten M1 layer, a dual tungsten/copper via structure was developed to accommodate RRAM device fabrication. The hafnium dioxide RRAM switching layer was capped with a titanium oxygen getter layer and an inert titanium nitride electrode. The capping layers were etched with reactive ion etch (RIE) whereas two approaches were tested for the hafnium dioxide removal: 1) a wet etch with diluted hydrofluoric acid (dHF) and 2) a BCl3 based reactive ion etch (RIE) process. The test platform for the vertically-integrated RRAM devices included: 1) 12x12 crossbar arrays with 100x100 nm2 size devices 2) single RRAM (1R) devices with sizes ranging from 100x100 nm2 to 10x10 µm2 and 3) 1 transistor 1 RRAM (1T1R) structures with 100x100 nm2 size devices. The impact on the RRAM devices for both etch processes was investigated with respect to the physical structure, the electrical characteristics and the yield. The electrical characteristics include simple I-V DC sweeps, pulse-based endurance measurements, retention measurements as well as pulse dependent behavior showing an incremental reset. The functional principle these RRAM devices is consistent with a valence change mechanism (VCM), which is filament-based, and therefore area independent. RRAM devices fabricated using the dHF wet etch with layer thicknesses of 5.8 nm of hafnium dioxide and 6 nm of titanium exhibited endurance values exceeding 108 cycles with an average on/off resistance ratio of 5 and set/reset voltages below 1/-1.5 V. The dHF etch negatively impacted the physical device structure of the RRAM devices, which was reflected by the resulting electrical performance. By moving to a dry etching (RIE) process, an improvement in yield, reliability, endurance and retention is expected. In particular, yield is expected to increase due to reduced undercut of the titanium oxygen getter layer, which is characteristic of the dHF etch. In summary, this work shows the importance of etch performance and selectivity for optimization of RRAM device performance.

Impact of Etch Process on Hafnium Dioxide Based Nanoscale RRAM Devices

Resistive Random Access Memory (RRAM) is a novel form of non-volatile memory that is expected to play a major role in future computing and memory solutions. We have developed memristive RRAM devices and integrated logic devices on a 300mm wafer platform with the IBM 65nm 10LPe process technology. The RRAM device consists of an inert tungsten bottom electrode, HfO2 based active switching layer, a Ti oxygen exchange layer and a TiN top electrode. The TiN/Ti capping layers were etched with reactive ion etch (RIE), whereas two approaches were tested for the HfO2 removal: a wet etch with diluted hydrofluoric acid, and a BCl3/O2 based RIE process. The impact on the RRAM devices for both etch processes was investigated with respect to the physical structure, the electrical characteristics, and yield. The electrical characteristics include simple I-V DC sweeps and pulse-based endurance measurements.

Nanoscale resistive switching memory device composed of NiO nanodot and graphene nanoribbon nanogap electrodes

We report a nanoscale resistive random access memory (RRAM) device consisting of a NiO nanodot and graphene nanoribbon (GNR) nanogap electrodes. The GNR nanogap was established by an electroburning process induced by applying voltage ramp stress between the two ends of the GNR. A NiO nanodot was then deposited between the nanogapped GNR electrodes by an elaborately controllable dip pen lithography method using a nickel carbonate [Ni2(CO3)(OH)2] solution. The nanoscale GNR/NiO/GNR RRAM device exhibited reliable unipolar resistive switching characteristics with low SET/RESET voltages and currents, which might be the result of its miniaturized size and well-defined Ohmic contacts between the NiO nanodot and GNR electrodes.

Origin of the OFF state variability in ReRAM cells

This work exploits the switching dynamics of nanoscale resistive random access memory (ReRAM) cells with particular emphasis on the origin of the observed variability when cells are consecutively cycled/programmed at distinct memory states. It is demonstrated that this variance is a common feature of all ReRAM elements and is ascribed to the formation and rupture of conductive filaments that expand across the active core, independently of the material employed as the active switching core, the causal physical switching mechanism, the switching mode (bipolar/unipolar) or even the unit cells' dimensions. Our hypothesis is supported through both experimental and theoretical studies on TiO2 and In2O3 : SnO2 (ITO) based ReRAM cells programmed at three distinct resistive states. Our prototypes employed TiO2 or ITO active cores over 5 × 5 µm2 and 100 × 100 µm2 cell areas, with all tested devices demonstrating both unipolar and bipolar switching modalities. In the case of TiO2-based cells, the underlying switching mechanism is based on the non-uniform displacement of ionic species that foster the formation of conductive filaments. On the other hand, the resistive switching observed in the ITO-based devices is considered to be due to a phase change mechanism. The selected experimental parameters allowed us to demonstrate that the observed programming variance is a common feature of all ReRAM devices, proving that its origin is dependent upon randomly oriented local disorders within the active core that have a substantial impact on the overall state variance, particularly for high-resistive states.

Nanoscale Plasmonic Memristor with Optical Readout Functionality

We experimentally demonstrate for the first time a nanoscale resistive random access memory (RRAM) electronic device integrated with a plasmonic waveguide providing the functionality of optical readout. The device fabrication is based on silicon on insulator CMOS compatible approach of local oxidation of silicon, which enables the realization of RRAM and low optical loss channel photonic waveguide at the same fabrication step. This plasmonic device operates at telecom wavelength of 1.55 μm and can be used to optically read the logic state of a memory by measuring two distinct levels of optical transmission. The experimental characterization of the device shows optical bistable behavior between these levels of transmission in addition to well-defined hysteresis. We attribute the changes in the optical transmission to the creation of a nanoscale absorbing and scattering metallic filament in the amorphous silicon layer, where the plasmonic mode resides.

Nanoscale RRAM-based synaptic electronics: toward a neuromorphic computing device

Efforts to develop scalable learning algorithms for implementation of networks of spiking neurons in silicon have been hindered by the considerable footprints of learning circuits, which grow as the number of synapses increases. Recent developments in nanotechnologies provide an extremely compact device with low-power consumption. In particular, nanoscale resistive switching devices (resistive random-access memory (RRAM)) are regarded as a promising solution for implementation of biological synapses due to their nanoscale dimensions, capacity to store multiple bits and the low energy required to operate distinct states. In this paper, we report the fabrication, modeling and implementation of nanoscale RRAM with multi-level storage capability for an electronic synapse device. In addition, we first experimentally demonstrate the learning capabilities and predictable performance by a neuromorphic circuit composed of a nanoscale 1 kbit RRAM cross-point array of synapses and complementary metal–oxide–semiconductor neuron circuits. These developments open up possibilities for the development of ubiquitous ultra-dense, ultra-low-power cognitive computers.

Nanoscale resistive random access memory consisting of a NiO nanodot and Au nanowires formed by dip-pen nanolithography

We demonstrate the nanoscale resistive random access memory (RRAM) element consisting of an approximately 30nm diameter NiO nanodot and two bridging Au nanowires, formed by a dip-pen nanolithography technique using nickel carbonate (Ni2(CO3)(OH)2) and [AuCl4]− complex solutions, respectively. The Au/NiO/Au nanowire resistive switch exhibits typical unipolar switching characteristics with high performance at low Set and Reset voltages.