Review of Semiconductor Flash Memory Devices for Material and Process Issues.

Vertically integrated NAND (V-NAND) flash memory is the main data storage in modern handheld electronic devices, widening its share even in the data centers where installation and operation costs are critical. While the conventional scaling rule has been applied down to the design rule of ≈15 nm (year 2013), the current method of increasing device density is stacking up layers. Currently, 176-layer-stacked V-NAND flash memory is available on the market. Nonetheless, increasing the layers invokes several challenges, such as film stress management and deep contact hole etching. Also, there should be an upper bound for the attainable stacking layers (400-500) due to the total allowable chip thickness, which will be reached within 6-7 years. This review summarizes the current status and critical challenges of charge-trap-based flash memory devices, with a focus on the material (floating-gate vs charge-trap-layer), array-level circuit architecture (NOR vs NAND), physical integration structure (2D vs 3D), and cell-level programming technique (single vs multiple levels). Current efforts to improve fabrication processes and device performances using new materials are also introduced. The review suggests directions for future storage devices based on the ionic mechanism, which may overcome the inherent problems of flash memory devices.

Electrically-generated memristor based on inkjet printed silver nanoparticles.

A method to electrically induce memristor performance from inkjet-printed silver (Ag) nanoparticles is presented, which is effective on a specifically designed hourglass-shaped Ag metal device. Joule heating-induced oxidation in the bottleneck region, when applying a high current to the device, results in a metal-electrolyte-metal structure produced from just a single metal ink for the memristor operation. This electrically induced memristor shows a nonuniform dispersion of the Ag nanoparticles within the oxide electrolyte layer, depending on the bias polarity adopted during the initial metal rupture process. A versatile and useful range of controllable memristor behaviors, from volatile threshold switching to nonvolatile unipolar as well as bipolar resistive switching, are observed based on the reversible rejuvenation and rupture of the Ag nanofilaments according to the Ag cation migration within the oxide electrolyte. The interplay between the electric field induced redox reaction and thermal diffusion of the Ag nanoparticles constitutes the primary reason for the different switching behaviors, further supported by thermo-field simulation results. The bipolar switching memristor demonstrates reliable endurance even under harsh DC switching conditions with low power consumption compared with its unipolar switching operation. The observed range of controllable switching behavior can be exploited for future low power flexible memory, as a selector in crossbar memory architecture, synaptic learning, and others.

Nociceptive Memristor.

The biomimetic characteristics of the memristor as an electronic synapse and neuron have inspired the advent of new information technology in the neuromorphic computing. The application of the memristors can be extended to the artificial nerves on condition of the presence of electronic receptors which can transfer the external stimuli to the internal nerve system. In this work, nociceptor behaviors are demonstrated from the Pt/HfO2 /TiN memristor for the electronic receptors. The device shows four specific nociceptive behaviors; threshold, relaxation, allodynia, and hyperalgesia, according to the strength, duration, and repetition rate of the external stimuli. Such nociceptive behaviors are attributed to the electron trapping/detrapping to/from the traps in the HfO2 layer, where the depth of trap energy level is ≈0.7 eV. Also, the built-in potential by the work function mismatch between the Pt and TiN electrodes induces time-dependent relaxation of trapped electrons, providing the appropriate relaxation behavior. The relaxation time can take from several milliseconds to tens of seconds, which corresponds to the time span of the decay of biosignal. The material-wise evaluation of the electronic nociceptor in comparison with other material, which did not show the desired functionality, Pt/Ti/HfO2 /TiN, reveals the importance of careful material design and fabrication.

The current limit and self-rectification functionalities in the TiO2/HfO2 resistive switching material system.

To replace the present NAND flash memory, resistance switching random access memory (ReRAM), which has both memory and selection functionalities with a simple metal-insulator-metal structure should be implemented. To accomplish this goal, ReRAM must be self-rectifying, low-power-consuming, and highly uniform, and it must have reliable states. In this work, the Pt/TiO2/HfO2-x/TiN resistive switching memory structure showed self-rectifying resistive switching behavior with unprecedented unique I-V curves. This is named "self-current saturation," which can give an extremely uniform variation of the low resistance state. The plausible reasons for the whole switching behavior, including the unique I-V curves, in this material system are presented herein. The diffusion of Ti along the grain boundaries of HfO2 down to the bottom electrode TiN and the defect formation within the HfO2 layer near the TiO2/HfO2 interface made the resistance switching device have the characteristics of both the unidirectional diode and electronic bipolar switching devices.

Comparison of the Atomic Layer Deposition of Tantalum Oxide Thin Films Using Ta(NtBu)(NEt2)3, Ta(NtBu)(NEt2)2Cp, and H2O.

The growth characteristics of Ta2O5 thin films by atomic layer deposition (ALD) were examined using Ta(NtBu)(NEt2)3 (TBTDET) and Ta(NtBu)(NEt2)2Cp (TBDETCp) as Ta-precursors, where tBu, Et, and Cp represent tert-butyl, ethyl, and cyclopentadienyl groups, respectively, along with water vapor as oxygen source. The grown Ta2O5 films were amorphous with very smooth surface morphology for both the Ta-precursors. The saturated ALD growth rates of Ta2O5 films were 0.77 Å cycle-1 at 250 °C and 0.67 Å cycle-1 at 300 °C using TBTDET and TBDETCp precursors, respectively. The thermal decomposition of the amido ligand (NEt2) limited the ALD process temperature below 275 °C for TBTDET precursor. However, the ALD temperature window could be extended up to 325 °C due to a strong Ta-Cp bond for the TBDETCp precursor. Because of the improved thermal stability of TBDETCp precursor, excellent nonuniformity of ∼2% in 200 mm wafer could be achieved with a step coverage of ∼90% in a deep hole structure (aspect ratio 5:1) which is promising for 3-dimensional architecture to form high density memories. Nonetheless, a rather high concentration (∼7 at. %) of carbon impurities was incorporated into the Ta2O5 film using TBDETCp, which was possibly due to readsorption of dissociated ligands as small organic molecules in the growth of Ta2O5 film by ALD. Despite the presence of high carbon concentration which might be an origin of large leakage current under electric fields, the Ta2O5 film using TBDETCp showed a promising resistive switching performance with an endurance cycle as high as ∼17 500 for resistance switching random access memory application. The optical refractive index of the deposited Ta2O5 films was 2.1-2.2 at 632.8 nm using both the Ta-precursors, and indirect optical band gap was estimated to be ∼4.1 eV for both the cases.

A crossbar resistance switching memory readout scheme with sneak current cancellation based on a two-port current-mode sensing.

This paper describes a novel readout scheme that enables the complete cancellation of sneak currents in resistive switching random-access memory (RRAM) crossbar array. The current-mode readout is employed in the proposed readout, and a few critical advantages of the current-mode readout for crossbar RRAM are elucidated in this paper. The proposed scheme is based on a floating readout scheme for low power consumption, and one more sensing port is introduced using an additional reference word line. From the additional port, information on the sneak current amount is collected, and simple current-mode arithmetic operations are implemented to cancel out the sneak current from the sensing current. In addition, a simple method of handling the overestimated-sneak-current issue is described. The proposed scheme is verified using HSPICE simulation. Moreover, an example of a current-mode sense amplifier realizing the proposed cancelling technique is presented. The proposed sense amplifier can be implemented with less hardware overhead compared to the previous works.

A study of the transition between the non-polar and bipolar resistance switching mechanisms in the TiN/TiO2/Al memory.

Thermochemical and electronic trapping/detrapping mechanism-based resistance switching in TiO2 is one of the most extensively researched topics in the field of resistance-switching random access memory (ReRAM). In this study, the subtle correlation between the formation and rupture of the Magnéli-based conducting filament (CF), which is the mechanism of non-polar thermochemical-reaction-based switching, and the electron trapping/detrapping at the defect centers, which is the mechanism of bipolar electronic switching, is examined in detail. The chemical interaction between the TiN top electrode and the TiO2 layer generates a stable and immobile electron trapping layer, which is called a "switching layer", whereas the thin region between the just-mentioned switching layer and the remaining Magnéli CF after the thermochemical reset comprises a non-switching layer. The seemingly very complicated switching behavior with respect to the bias polarity, compliance current, and detailed biasing sequence could be reasonably explained by the phenomenological model based on the combined motions of the CF, switching layer, and non-switching layer. Light-induced detrapping experiments further supplement the suggested switching model.

Uniform Self-rectifying Resistive Switching Behavior via Preformed Conducting Paths in a Vertical-type Ta2O5/HfO2-x Structure with a Sub-µm(2) Cell Area.

To replace or succeed the present NAND flash memory, resistive switching random access memory (ReRAM) should be implemented in the vertical-type crossbar array configuration. The ReRAM cell must have a highly reproducible resistive switching (RS) performance and an electroforming-free, self-rectifying, low-power-consumption, multilevel-switching, and easy fabrication process with a deep sub-µm(2) cell area. In this work, a Pt/Ta2O5/HfO2-x/TiN RS memory cell fabricated in the form of a vertical-type structure was presented as a feasible contender to meet the above requirements. While the fundamental RS characteristics of this material based on the electron trapping/detrapping mechanisms have been reported elsewhere, the influence of the cell scaling size to 0.34 µm(2) on the RS performance by adopting the vertical integration scheme was carefully examined in this work. The smaller cell area provided much better switching uniformity while all the other benefits of this specific material system were preserved. Using the overstressing technique, the nature of RS through the localized conducting path was further examined, which elucidated the fundamental difference between the present material system and the general ionic-motion-related bipolar RS mechanism.

Resistance switching behavior of atomic layer deposited SrTiO3 film through possible formation of Sr2Ti6O13 or Sr1Ti11O20 phases.

Identification of microstructural evolution of nanoscale conducting phase, such as conducting filament (CF), in many resistance switching (RS) devices is a crucial factor to unambiguously understand the electrical behaviours of the RS-based electronic devices. Among the diverse RS material systems, oxide-based redox system comprises the major category of these intriguing electronic devices, where the local, along both lateral and vertical directions of thin films, changes in oxygen chemistry has been suggested to be the main RS mechanism. However, there are systems which involve distinctive crystallographic phases as CF; the Magnéli phase in TiO2 is one of the very well-known examples. The current research reports the possible presence of distinctive local conducting phase in atomic layer deposited SrTiO3 RS thin film. The conducting phase was identified through extensive transmission electron microscopy studies, which indicated that oxygen-deficient Sr2Ti6O13 or Sr1Ti11O20 phase was presumably present mainly along the grain boundaries of SrTiO3 after the unipolar set switching in Pt/TiN/SrTiO3/Pt structure. A detailed electrical characterization revealed that the samples showed typical bipolar and complementary RS after the memory cell was unipolar reset.

Electronic resistance switching in the Al/TiO(x)/Al structure for forming-free and area-scalable memory.

Electronic bipolar resistance switching (eBRS) in an Al/TiOx/Al structure, where the TiOx layer was reactively sputter-deposited, was examined in conjunction with a structural analysis using transmission electron microscopy. A thin (3-5 nm) insulating Al(Ti)Ox layer was formed at the bottom Al electrode interface, which provided the necessary asymmetric potential barrier for the eBRS to emerge, whereas the top Al electrode interface appeared to have provided the fluent carrier (electron) injection. The set and reset switching were related to the trapping and detrapping of the carriers at the trap centers, the characteristic energy of which was ∼0.86 eV, across the entire electrode area. The general features of this material system as the feasible RS memory were insufficient: endurance cycle, <∼8000, and retention time at 85 °C, 10(6) s. However, the detailed analysis of the switching behavior based on the space-charge limited current conduction mechanism, and its variation with the switching cycles, provided useful information on the general features of the eBRS, which could also be applicable to other binary (or even ternary) metal-oxide RS systems based on the electronic switching mechanism.

Pt/Ta2 O5 /HfO2- x /Ti resistive switching memory competing with multilevel NAND flash.

Pt/Ta2 O5 /HfO2- x /Ti resistive switching memory with a new circuit design is presented as a feasible candidate to succeed multilevel-cell (MLC) NAND flash memory. This device has the following characteristics: 3 bit MLC, electroforming-free, self-rectifying, much higher cell resistance than interconnection wire resistance, low voltage operation, low power consumption, long-term reliability, and only an electronic switching mechanism, without an ionic-motion-related mechanism.

Evolution of the shape of the conducting channel in complementary resistive switching transition metal oxides.

Ultimate control of the defect distribution and local conduction path in a bipolar resistive switching (BRS) Pt/TiO2/Pt sample, which was in a unipolar reset state, is provided by means of voltage pulsing and the resulting time-transient current analysis. The limited amount of oxygen vacancies in this system allowed reversibly switching-diode-like current-voltage curves, which was also confirmed in another Magnéli-phase-containing Pt/WO3/Pt sample. Such careful control of the defect distribution allowed the achievement of a complementary resistive switching (CRS) curve even from a single switching layer. The unlimited vacancy source in the Pt/TiO2/TiO2-x/Pt sample did not allow the switching-diode type and the CRS behavior. The data retention of the on-state in the BRS was critically dependent on the shape of the rejuvenated conduction channel. The required time to lead to the rejuvenation of the conducting channel was ∼70-100 ns when the threshold voltage for the BRS set of ∼-1 V was applied.

Ionic bipolar resistive switching modes determined by the preceding unipolar resistive switching reset behavior in Pt/TiO2/Pt.

Various types of bipolar resistive switching (BRS) at the filament ruptured region by the unipolar resistive switching (URS) reset in the structure Pt/TiO2/Pt were categorized in terms of operation polarity and switching parameters. The differences in BRS behavior, even under identical current-voltage switching, are closely related to the previously performed URS reset parameter, especially the power consumed during the reset process. Various modes of BRS from the URS reset status in the structure Pt/TiO2/Pt are reported, and interpreted in terms of a distinct oxygen vacancy configuration in the ruptured region of a Magnéli filament.

Highly improved uniformity in the resistive switching parameters of TiO2 thin films by inserting Ru nanodots.

Limiting the location where electron injection occurs at the cathode interface to a narrower region is the key factor for achieving a highly improved RS performance, which can be achieved by including Ru Nanodots. The development of a memory cell structure truly at the nanoscale with such a limiting factor for the electric-field distribution can solve the non-uniformity issue of future ReRAM.

Optimization of chemical structure of Schottky-type selection diode for crossbar resistive memory.

The electrical performances of Pt/TiO(2)/Ti/Pt stacked Schottky-type diode (SD) was systematically examined, and this performance is dependent on the chemical structures of the each layer and their interfaces. The Ti layers containing a tolerable amount of oxygen showed metallic electrical conduction characteristics, which was confirmed by sheet resistance measurement with elevating the temperature, transmission line measurement (TLM), and Auger electron spectroscopy (AES) analysis. However, the chemical structure of SD stack and resulting electrical properties were crucially affected by the dissolved oxygen concentration in the Ti layers. The lower oxidation potential of the Ti layer with initially higher oxygen concentration suppressed the oxygen deficiency of the overlying TiO(2) layer induced by consumption of the oxygen from TiO(2) layer. This structure results in the lower reverse current of SDs without significant degradation of forward-state current. Conductive atomic force microscopy (CAFM) analysis showed the current conduction through the local conduction paths in the presented SDs, which guarantees a sufficient forward-current density as a selection device for highly integrated crossbar array resistive memory.

Memristive tri-stable resistive switching at ruptured conducting filaments of a Pt/TiO2/Pt cell.

A tri-stable memristive switching was demonstrated on a Pt/TiO2/Pt device and its underlying mechanism was suggested through a series of electrical measurements. Tri-stable switching could be initiated from a device in unipolar reset status. The unipolar reset status was obtained by performing an electroforming step on a pristine cell which was then followed by unipolar reset switching. It was postulated that tri-stable switching occurred at the location where the conductive filament (initially formed by the electroforming step) was ruptured by a subsequent unipolar reset process. The mechanism of the tri-stable memristive switching presented in this article was attributed to the migration of oxygen ions through the ruptured filament region and the resulting modulation of the Schottky-like interfaces. The assertion was further supported by a comparison study performed on a Pt/TiO2/TiO(2-x)/Pt cell.