The study of lithographic variation in resistive random access memory

Reducing the process variation is a significant concern for resistive random access memory(RRAM).Due to its ultrahigh integration density,RRAM arrays are prone to lithographic variation during the lithography process,introducing electrical variation among different RRAM devices.In this work,an optical physical verification methodology for the RRAM array is developed,and the effects of different layout parameters on important electrical characteristics are systematically investigated.The results indicate that the RRAM devices can be categorized into three clusters according to their locations and lithography environments.The read resistance is more sensitive to the locations in the array(~30%)than SET/RESET voltage(<10%).The increase in the RRAM device length and the application of the optical proximity correction technique can help to reduce the variation to less than 10%,whereas it reduces RRAM read resistance by 4×,resulting in a higher power and area consumption.As such,we provide design guidelines to minimize the electrical variation of RRAM arrays due to the lithography process.

Advances of embedded resistive random access memory in industrial manufacturing and its potential applications

Embedded memory,which heavily relies on the manufacturing process,has been widely adopted in various industrial applications.As the field of embedded memory continues to evolve,innovative strategies are emerging to enhance performance.Among them,resistive random access memory(RRAM)has gained significant attention due to its numerousadvantages over traditional memory devices,including high speed(<1 ns),high density(4 F^(2)·n^(-1)),high scalability(~nm),and low power consumption(~pJ).This review focuses on the recent progress of embedded RRAM in industrial manufacturing and its potentialapplications.It provides a brief introduction to the concepts and advantages of RRAM,discusses the key factors that impact its industrial manufacturing,and presents the commercial progress driven by cutting-edge nanotechnology,which has been pursued by manysemiconductor giants.Additionally,it highlights the adoption of embedded RRAM in emerging applications within the realm of the Internet of Things and future intelligent computing,with a particular emphasis on its role in neuromorphic computing.Finally,the review discusses thecurrent challenges and provides insights into the prospects of embedded RRAM in the era of big data and artificial intelligence.

Thermal stability and data retention of resistive random access memory with HfOx/ZnO double layers

As an industry accepted storage scheme, hafnium oxide（HfO_x） based resistive random access memory（RRAM）should further improve its thermal stability and data retention for practical applications. We therefore fabricated RRAMs with HfO_x/ZnO double-layer as the storage medium to study their thermal stability as well as data retention. The HfO_x/ZnO double-layer is capable of reversible bipolar switching under ultralow switching current（〈 3 μA） with a Schottky emission dominant conduction for the high resistance state and a Poole–Frenkel emission governed conduction for the low resistance state. Compared with a drastically increased switching current at 120℃ for the single HfO_x layer RRAM, the HfO_x/ZnO double-layer exhibits excellent thermal stability and maintains neglectful fluctuations in switching current at high temperatures（up to 180℃）, which might be attributed to the increased Schottky barrier height to suppress current at high temperatures. Additionally, the HfO_x/ZnO double-layer exhibits 10-year data retention @85℃ that is helpful for the practical applications in RRAMs.

Resistive switching characteristic and uniformity of low-power HfO_x-based resistive random access memory with the BN insertion layer

In this letter,the Ta/HfO/BN/TiN resistive switching devices are fabricated and they exhibit low power consumption and high uniformity each.The reset current is reduced for the HfO/BN bilayer device compared with that for the Ta/HfO/TiN structure.Furthermore,the reset current decreases with increasing BN thickness.The HfOlayer is a dominating switching layer,while the low-permittivity and high-resistivity BN layer acts as a barrier of electrons injection into TiN electrode.The current conduction mechanism of low resistance state in the HfO/BN bilayer device is space-chargelimited current(SCLC),while it is Ohmic conduction in the HfOdevice.

Analysis and modeling of resistive switching mechanisms oriented to resistive random-access memory

With the progress of the semiconductor industry,the resistive random-access memory（RAM） has drawn increasing attention.The discovery of the memristor has brought much attention to this study.Research has focused on the resistive switching characteristics of different materials and the analysis of resistive switching mechanisms.We discuss the resistive switching mechanisms of different materials in this paper and analyze the differences of those mechanisms from the view point of circuitry to establish their respective circuit models.Finally,simulations are presented.We give the prospect of using different materials in resistive RAM on account of their resistive switching mechanisms,which are applied to explain their resistive switchings.

Effects of Film Thickness and Ar/O2 Ratio on Resistive Switching Characteristics of HfOx-Based Resistive-Switching Random Access Memories

Cu/HfOx/n^＋Si devices are fabricated to investigate the influence of technological parameters including film thickness and Ar/02 ratio on the resistive switching （RS） characteristics of HfOx films, in terms of switch ratio, endurance properties, retention time and multilevel storage. It is revealed that the RS characteristics show strong dependence on technological parameters mainly by altering the defects （oxygen vacancies） in the film. The sample with thickness of 2Onto and Ar/O2 ratio of 12：3 exhibits the best RS behavior with the potential of multilevel storage. The conduction mechanism of all the films is interpreted based on the filamentary model.

Graphene resistive random memory - the promising memory device in next generation

Graphene-based resistive random access memory （GRRAM） has grasped researchers＇ attention due to its merits com- pared with ordinary RRAM. In this paper, we briefly review different types of GRRAMs. These GRRAMs can be divided into two categories： graphene RRAM and graphene oxide （GO）/reduced graphene oxide （rGO） RRAM. Using graphene as the electrode, GRRAM can own many good characteristics, such as low power consumption, higher density, transparency, SET voltage modulation, high uniformity, and so on. Graphene flakes sandwiched between two dielectric layers can lower the SET voltage and achieve multilevel switching. Moreover, the GRRAM with rGO and GO as the dielectric or electrode can be simply fabricated. Flexible and high performance RRAM and GO film can be modified by adding other materials layer or making a composite with polymer, nanoparticle, and 2D materials to further improve the performance. Above all, GRRAM shows huge potential to become the next generation memory.

Effect of Pulse and dc Formation on the Performance of One-Transistor and One-Resistor Resistance Random Access Memory Devices

We investigate the effect of the formation process under pulse and dc modes on the performance of one transistor and one resistor （1 T1R） resistance random access memory （RRAM） device. All the devices are operated under the same test conditions, except for the initial formation process with different modes. Based on the statistical results, the high resistance state （FIRS） under the dc forming mode shows a lower value with better distribution compared with that under the pulse mode. One of the possible reasons for such a phenomenon originates from different properties of conductive filament （CF） formed in the resistive switching layer under two different modes. For the dc forming mode, the formed filament is thought to be continuous, which is hard to be ruptured, resulting in a lower HRS. However, in the case of pulse forming, the filament is discontinuous where the transport mechanism is governed by hopping. The low resistance state （LRS） can be easily changed by removing a few trapping states from the conducting path. Hence, a higher FIRS is thus observed. However, the HRS resistance is highly dependent on the length of the gap opened. A slight variation of the gap length will cause wide dispersion of resistance.

Carbon-based memristors for resistive random access memory and neuromorphic applications

As a typical representative of nanomaterials,carbon nanomaterials have attracted widespread attention in the construction of electronic devices owing to their unique physical and chemical properties,multi-dimensionality,multi-hybridization methods,and excellent electronic properties.Especially in the recent years,memristors based on carbon nanomaterials have flourished in the field of building non-volatile memory devices and neuromorphic applications.In the current work,the preparation methods and structural characteristics of carbon nanomaterials of different dimensions were systematically reviewed.Afterwards,in depth discussion on the structural characteristics and working mechanism of memristors based on carbon nanomaterials of different dimensions was conducted.Finally,the potential applications of carbon-based memristors in logic operations,neural network construction,artificial vision systems,artificial tactile systems,and multimodal perception systems were also introduced.It is believed that this paper will provide guidance for the future development of high-quality information storage,high-performance neuromorphic applications,and highsensitivity bionic sensing based on carbon-based memristors.

Resistive switching characteristics of Ti/ZrO_2/Pt RRAM device

In this paper, the bipolar resistive switching characteristic is reported in Ti/ZrO2/Pt resistive switching memory de- vices. The dominant mechanism of resistive switching is the formation and rupture of the conductive filament composed of oxygen vacancies. The conduction mechanisms for low and high resistance states are dominated by the ohmic conduc- tion and the trap-controlled space charge limited current （SCLC） mechanism, respectively. The effect of a set compliance current on the switching parameters is also studied： the low resistance and reset current are linearly dependent on the set compliance current in the log-log scale coordinate; and the set and reset voltage increase slightly with the increase of the set compliance current. A series circuit model is proposed to explain the effect of the set compliance current on the resistive switching behaviors.

Improved resistive switching stability of Pt/ZnO/CoO_x /ZnO/Pt structure for nonvolatile memory devices

For Pt(Ag)/ZnO single-layer/Pt structure,random 10 formation and rupture of conductive filaments composed by oxygen vacancies or metallic ions often cause dispersion problems of resistive switching(RS)parameters,which is disadvantageous to devices application.In this study,ZnO/CoOx/ZnO(ZCZ)tri-layers were utilized as the switching layers to investigate their RS properties as compared with ZnO-based single-layer devices.It is interestingly noted that Pt/ZCZ/Pt devices show quite stable bipolar RS behaviors with little resistance value fluctuations compared to Ag/ZCZ/Pt devices and Pt(Ag)/ZnO/Pt devices,which minimize the dispersion of the resistances of RS.This highly stable RS effect of Pt/ZCZ/Pt structure would be promising for high density memory devices.

Electric-field control of perpendicular magnetic anisotropy by resistive switching via electrochemical metallization

Electric-field control of perpendicular magnetic anisotropy(PMA) is a feasible way to manipulate perpendicular magnetization,which is of great importance for realizing energy-efficient spintronics.Here,we propose a novel approach to accomplish this task at room temperature by resistive switching(RS) via electrochemical metallization(ECM) in a device with the stack of Si/SiO_(2)/Ta/Pt/Ag/Mn-doped ZnO(MZO)/Pt/Co/Pt/ITO.By applying certain voltages,the device could be set at high-resistance-state(HRS) and low-resistance-state(LRS),accompanied with a larger and a smaller coercivity(H_(C)),respectively,which demonstrates a nonvolatile E-field control of PMA.Based on our previous studies and the present control experiments,the electric modulation of PMA can be briefly explained as follows.At LRS,the Ag conductive filaments form and pass through the entire MZO layer and finally reach the Pt/Co/Pt sandwich,leading to weakening of PMA and reduction of H_(C).In contrast,at HRS,most of the Ag filaments dissolve and leave away from the Pt/Co/Pt sandwich,causing partial recovery of PMA and an increase of H_(C).This work provides a new clue to designing low-power spintronic devices based on PMA films.

Investigation of resistive switching behaviours in WO_3-based RRAM devices

In this paper, a WO3-based resistive random access memory device composed of a thin film of WO3 sandwiched between a copper top and a platinum bottom electrodes is fabricated by electron beam evaporation at room temperature. The reproducible resistive switching, low power consumption, multilevel storage possibility, and good data retention characteristics demonstrate that the Cu/WO3/Pt memory device is very promising for future nonvolatile memory applications. The formation and rupture of localised conductive filaments is suggested to be responsible for the observed resistive switching behaviours.

基于28 nm RRAM的可重构真随机数发生器

基于阻变存储器(RRAM)电流饥饿型环形振荡器的真随机数发生器(TRNG)方案,提出交替读写的操作模式,优化熵源重构机制.针对现有RRAM在多操作周期下跨导线性化问题,提出熵可配置电阻窗口钳位电路.通过减小电阻窗口获得RRAM非线性的最大化,所提电路能够有效避免RRAM出现过度置位、过度复位的现象,保证熵源稳定性.基于UMC 28 nm HKMG工艺对TRNG进行流片.输出数据统计性测试结果通过了NIST SP800-22所有测试集的真随机数标准测试.检测结果表明,在高斯分布的95%置信区间,所有统计数据的自相关函数值均落在-0.003~0.003,输出序列具有良好的随机性.

Analysis of tail bits generation of multilevel storage in resistive switching memory

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.

Analysis of the resistive switching behaviors of vanadium oxide thin film

We demonstrate the polarization of resistive switching for a Cu/VOx/Cu memory cell.The switching behaviors of Cu/VOx/Cu cell are tested by using a semiconductor device analyzer（Agilent B1500A）,and the relative micro-analysis of I-V characteristics of VOx/Cu is characterized by using a conductive atomic force microscope（CAFM）.The I-V test results indicate that both the forming and the reversible resistive switching between low resistance state（LRS） and high resistance state（HRS） can be observed under either positive or negative sweep.The CAFM images for LRS and HRS directly exhibit evidence for the formation and rupture of filaments based on positive or negative voltage.The Cu/VOx/Cu sandwiched structure exhibits reversible resistive switching behavior and shows potential applications in the next generation of nonvolatile memory.

Remarkable Resistance Change in Plasma Oxidized TiOx/TiNx Film for Memory Application

We report the experimental phenomenon of large resistance change in plasma oxidized TiOx/TiNx film fabricated on W bottom-electrode-contact （W-BEC） array. The W-BEC in diameter 26Ohm is fabricated by a 0.18μm CMOS technology, and the TiOx/TiNx cell array is formed by rf magnetron sputtering and reactive ion etching. In current-voltage （I- V） measurement for current-sweeping mode, large snap-back of voltage is observed, which indicates that the sample changes from high-resistance state （HRS） to low-resistance state （LRS）. In the I-V measurement for voltage-sweeping mode, large current collapse is observed, which indicates that the sample changes from LRS to HRS. The current difference between HRS and LRS is about two orders. The threshold current and voltage for the resistance change is about 5.0- 10^-5 A and 2.5 V, respectively. The pulse voltage can also change the resistance and the pulse time is as shorter as 30 ns for the resistance change. These properties of TiOx/TiNx film are comparable to that of conventional phase-change material, which makes it possible for RRAM application.

Bipolar resistive switching in Cr-doped TiO_x thin films

The electric-pulse-induced resistive switching effect is studied for Tio.s5Cro.15Ox （TCO） films grown on Ir-Si substrates by pulsed laser deposition. Such a TCO device exhibits bipolar switching behaviour with an electric-pulse- induced resistance ratio as large as about 1000% and threshold voltages smaller than 2 V. The resistive switching characteristics may be understood by resistance changes of a Schottky junction composed of a metal and an n-type semiconductor, and its nonvolatility is attributed to the movement of oxygen vacancies near the interface.

Bipolar resistance switching in the fully transparent BaSnO_3-based memory device

The fully transparent indium-tin-oxide/BaSnO3/F-doped SnO2 devices that show a stable bipolar resistance switching effect are successfully fabricated. In addition to the transmittance being above 87% for visible light, an initial forming process is unnecessary for the production of transparent memory. Fittings to the current-voltage curves reveal the interfacial conduction in the devices. The first-principles calculation indicates that the oxygen vacancies in cubic BaSnO3 will form the defective energy level below the bottom of conduction band. The field-induced resistance change can be explained based on the change of the interracial Schottky barrier, due to the migration of oxygen vacancies in the vicinity of the interface. This work presents a candidate material BaSnO3 for the application of resistive random access memory to transparent electronics.

Temperature dependences of ferroelectricity and resistive switching behavior of epitaxial BiFeO_3 thin films

We investigate the resistive switching and ferroelectric polarization properties of high-quality epitaxial BiFeO3 thin films in various temperature ranges. The room temperature current-voltage（I-V） curve exhibits a well-established polarization-modulated memristor behavior. At low temperatures（〈 253 K）, the I-V curve shows an open circuit voltage（OCV）, which possibly originates from the dielectric relaxation effects, accompanied with a current hump due to the polarization reversal displacement current. While at relative higher temperatures（〉 253 K）, the I-V behaviors are governed by both space-charge-limited conduction（SCLC） and Ohmic behavior. The polarization reversal is able to trigger the conduction switching from Ohmic to SCLC behavior, leading to the observed ferroelectric resistive switching. At a temperature of〉 298 K, there occurs a new resistive switching hysteresis at high bias voltages, which may be related to defect-mediated effects.