一种基于阻变单元特性的阻变存储器修复方案

针对新型阻变存储器(RRAM)工艺良率不高的问题,提出了一种新型的修复解决方案,该方案基于阻变单元的特殊性能,即初始状态为高阻,经过单元初始化操作过程后转变为低阻。利用这样特性的阻变单元作为错误检测位、冗余单元作为修复位,提出了三种不同的组织结构来实现修复操作。三种结构由于主存储器、检验位存储器及冗余存储器的组织方式不同,达到了不同的冗余存储器利用率。最后,通过数学分析可以证明,该方案在利用了较少冗余存储器的条件下,可以将阻变存储器的错误率普遍降低10～30倍,实现了较好的修复效果。     

还原时间对AlOx/WOx RRAM开关速度的调制作用

基于128 kbit AlOx/WOx双层结构阻变存储器(RRAM)芯片,提出并验证了还原时间对RRAM开关速度的调制作用,同时设计了一种固定电压幅值逐步增大脉宽的算法用于RRAM阵列中速度的测试.还原处理的时间越长,AlOx层的厚度越薄,同时氧空位的含量增多,可加快导电细丝的形成、断裂和重新连接,进而提升芯片的开关速度.测试结果表明,还原时间由10 min增加至30 min,在4V和4.5V操作电压下,FORMING速度分布的均值分别由200 ns减小至120 ns和由100 ns减小至60 ns;在4V和4.5V操作电压下,RESET速度分布的均值分别由160 ns减小至120 ns和由120 ns减小至100 ns;SET速度分布的均值在4V电压下可由120 ns减小至80 ns.此外,还原时间的增长可以改善速度分布的一致性,减小速度的波动.     

Evidence for compliance controlled oxygen vacancy and metal filament based resistive switching mechanisms in RRAM

We present electrical evidence on asymmetric metal-insulator-semiconductor (MIS) based test structures in support of the presence of two different independent switching mechanisms in a resistive random access memory (RRAM) device. The valid mechanism for switching depends on the compliance capping (I_gl) for forming/SET transition. Our results convincingly show that low compliance based switching only involves reversible oxygen ion drift to and from oxygen gettering gate electrodes, while high compliance switching involves formation and rupture of conductive metallic nanofilaments, as verified further by our physical analysis investigations. We have observed this unique dual mode switching mechanism only in NiSi-based gate electrodes, which have a moderate oxygen solubility as well as relatively low melting point.     

Bipolar switching characteristics of low-power Geo resistive memory

We reported an ultra low-power resistive random access memory (RRAM) combining a low-cost Ni electrode and covalent-bond GeO_x dielectric. This cost-effective Ni/GeO_x/TaN RRAM device has very small set power of 2 μW, ultra-low reset power of 130 pW, greater than 1 order of magnitude resistance window, and stable retention at 85 °C. The current flow at low-resistance state is governed by Poole-Frenkel conduction with electrons hopping via defect traps, which is quite different from the filament conduction in metal-oxide RRAM.     

Direct Observation of Conversion Between Threshold Switching and Memory Switching Induced by Conductive Filament Morphology

Volatile threshold switching (TS) and non‐volatile memory switching (MS) are two typical resistive switching (RS) phenomena in oxides, which could form the basis for memory, analog circuits, and neuromorphic applications. Interestingly, TS and MS can be coexistent and converted in a single device under the suitable external excitation. However, the origin of the transition from TS to MS is still unclear due to the lack of direct experimental evidence. Here, conversion between TS and MS induced by conductive filament (CF) morphology in Ag/SiO₂/Pt device is directly observed using scanning electron microscopy and high‐resolution transmission electron microscopy. The MS mechanism is related to the formation and dissolution of CF consisting of continuous Ag nanocrystals. The TS originates from discontinuous CF with isolated Ag nanocrystals. The results of current–voltage fitting and Kelvin probe force microscopy further indicate that the TS mechanism is related to the modulation of the tunneling barrier between Ag nanocrystals in CF. This work provides clearly experimental evidence to deepen understanding of the mechanism for RS in oxide‐electrolyte‐based resistive switching memory, contributing to better control of the two RS behaviors to establish high‐performance emerging devices.     

Oxide nanolayer improving RRAM operational performance

The microstructure of Co-O, the interface between Co-O and electrodes, and the resistance switching in Co-O have been investigated. The Co-O film on the Pt electrode has quite dense crystallized grains and smooth surface, making it appropriate for the ultrahigh density non-volatile memory. About 5 nm thick interfacial oxide nanolayer including Ta₂O₅ was confirmed at the Co-O/Ta interface, which was spontaneously formed owing to the low oxygen potential of Ta. The low current resistance switching and operational stability in Pt/Co-O/Ta are attributed to the interfacial layer acting as the embedded load resistance component in the forming process. The engineering for the nanoregion at the interface is crucial in order to improve the performance of the resistance switching random access memory.     

Filament diffusion model for simulating reset and retention processes in RRAM

Resistive switching memory (RRAM) devices are attracting an increasing interest as a possible future technology for ultra-scaled, high-density nonvolatile/dynamic memory. Although the RRAM concept is promising from the integration and scaling viewpoints, the switching mechanism and its controllability are still under debate. This paper addresses the modeling of reset and retention processes in unipolar resistive-switching memory devices. Reset transition and data loss are described in terms of the dissolution of a conductive filament, which is modeled by thermally-activated diffusion of defects/dopants. Carrier transport, Joule heating and diffusion of oxygen ions/vacancies during the electrical pulse and/or the annealing are modeled within a 3D numerical solver. The model can account for the observed dependence of reset voltage on the width of the applied triangular pulse and on the initial resistance for NiO-based RRAM devices. Retention simulations as a function of annealing temperature also agree with available data. The model provides a first example of device simulation tool for the design and the exploration of scaling and performance of RRAM cells.     

Control of filament size and reduction of reset current below 10 μA in NiO resistance switching memories

Resistive-switching memory (RRAM) is receiving a growing deal of research interest as a possible solution for high-density, 3D nonvolatile memory technology. One of the main obstacle toward size reduction of the memory cell and its scaling is the typically large current I_reset needed for the reset operation. In fact, a large I_reset negatively impacts the scaling possibilities of the select diode in a cross-bar array structure. Reducing I_reset is therefore mandatory for the development of high-density RRAM arrays. This work addresses the reduction of I_reset in NiO-based RRAM by control of the filament size in 1 transistor-1 resistor (1T1R) cell devices. I_reset is demonstrated to be scalable and controllable below 10 μA. The significance of these results for the future scaling of diode-selected cross-bar arrays is finally discussed.     

Resistive Random Access Memory Cells with a Bilayer TiO2/SiOX Insulating Stack for Simultaneous Filamentary and Distributed Resistive Switching

In order to fulfill the information storage needs of modern societies, the performance of electronic nonvolatile memories (NVMs) should be continuously improved. In the past few years, resistive random access memories (RRAM) have raised as one of the most promising technologies for future information storage due to their excellent performance and easy fabrication. In this work, a novel strategy is presented to further extend the performance of RRAMs. By using only cheap and industry friendly materials (Ti, TiO₂, SiO_X, and n⁺⁺Si), memory cells are developed that show both filamentary and distributed resistive switching simultaneously (i.e., in the same I–V curve). The devices exhibit unprecedented hysteretic I–V characteristics, high current on/off ratios up to ≈5 orders of magnitude, ultra low currents in high resistive state and low resistive state (100 pA and 125 nA at –0.1 V, respectively), sharp switching transitions, good cycle‐to‐cycle endurance (>1000 cycles), and low device‐to‐device variability. We are not aware of any other resistive switching memory exhibiting such characteristics, which may open the door for the development of advanced NVMs combining the advantages of filamentary and distributed resistive switching mechanisms.     

Resistive switching characteristics of Ni/HfO_2/Pt ReRAM

This study investigated the resistive switching characteristics of the Ni/HfCVPt structure for nonvolatile memory application.The Ni/HfO_2/Pt device showed bipolar resistive switching(RS) without a forming process, and the formation and rupture of conducting filaments are responsible for the resistive switching phenomenon.In addition,the device showed some excellent memory performances,including a large on/off ratio( 3×10~5),very good data retention( 10~3 s @ 200℃) and uniformity of switching parameters.Considering these results,the Ni/HfO_2/Pt device has the potential for nonvolatile memory applications.     

Flexible and transparent ReRAM with GZO memory layer and GZO-electrodes on large PEN sheet

Fabrication of flexible transparent resistive random access memory (FT-ReRAM) which consists of Ga doped ZnO (GZO) film not only as a memory layer but also as electrodes on the large Poly Ethylene Naphthalate sheet was attained by introducing RF plasma assist DC magnetron sputtering method. The averaged transmittance in the visible region (400-800 nm) was 66%. The memory effect was studied by using conducting atomic force microscope. It was suggested that the increase of Joule heating and oxygen vacancy density enhances memory effect, which is consistent with the redox model which has been proposed as the switching mechanism for conventional ReRAM. Stable and repeatable bi-polar resistive switching by application of the low voltage less than 2 V and low current less than 100 μA was confirmed in the FT-GZO-ReRAM. Reset switching, which is a switching from the low to the high resistance states, in GZO-ReRAM was confirmed to be smooth and continuous, which will enable a multilevel application. It was suggested that the smooth and continuous reset was brought about by Ga-doping.     

Highly sustainable mechanical/electrical resistance switching behaviors via one-dimensional Ag/TiO2 core-shell resistive switchable materials in flexible composite

Composite-based resistance switching random access memory (ReRAM) has great potential for application in flexible and wearable electronics. However, its large operating parameters and low reliability still have some limitations in realizing practical applications, which is derived from its high dependence on the orientation and dispersion of the filler in the composite layer. Here, we proposed a novel composite system that does not depend heavily on the orientation or dispersion of the fillers within the composite film of the ReRAM device. The AgNW/TiO₂ core-shell nanowires inducing superb resistance switching behavior were fabricated. The composite resistance switching (RS) film was prepared by mixing the one-dimensional core-shell particles and poly (vinyl alcohol) (PVA) dielectric matrix. The composite RS film exhibited remarkable resistance switching behavior with extremely low/uniform operating voltage (V_set ~ 0.13 ± 0.013 V, and V_reset ~ −0.10 ± 0.012 V), and the reliable switching behavior was maintained for up to ~200,000 mechanical deformation cycles under 3 mm of bending radius. To evaluate the resistance switching mechanism of the composite-type ReRAM, the structural analysis and device modeling were performed.     

基于双端口随机存储器IDT7133的多处理器系统中数据共享的研究

介绍了双端口随机存储器（以下简称“DPRAM”）芯片IDT7133的外部管脚功能及其芯片特点。针对多处理器系统中的数据存储与共享问题,着重阐述了基于普通单口RAM和基于DPRAM芯片IDT7133两种不同的多处理器系统应用方案。通过对两个方案的比较,认识到基于DPRAM芯片IDT7133的应用方案在数据共享及数据传输中的显著优势。最后还谈到了IDT7133芯片内部的数据存储方式,为该方案在现实中的合理运用提供了保证。     

ZnO growth on Si with low-temperature CdO and ZnO buffer layers by molecular-beam epitaxy

Low-temperature (LT) buffer-layer techniques were employed to improve the crystalline quality of ZnO films grown by molecular-beam epitaxy (MBE). Photoluminescence (PL) spectra show that CdO, as a hetero-buffer layer with a rock-salt structure, does not improve the quality of ZnO film grown on top. However, by using ZnO as a homo-buffer layer, the crystalline quality can be greatly enhanced, as indicated by PL, atomic force microscopy (AFM), x-ray diffraction (XRD), and Raman scattering. Moreover, the buffer layer grown at 450°C is found to be the best template to further improve the quality of top ZnO film. The mechanisms behind this result are the strong interactions between point defects and threading dislocations in the ZnO buffer layer.     

基于Ni/HfO2/Pt结构的阻变存储器的高温稳定性研究

这篇文章研究了Ni/HfO₂/Pt结构的阻变存储器的阻变特性。Ni/HfO₂/Pt器件显示出了双极阻变特性并且没有forming过程,我们认为导电细丝的形成和破灭是器件阻变的本质机理。此外,器件有很好的转变性能和保持特性：转换窗口达到3?10₅, 在高温200_oC下的保持时间大于10₃s,并且具有均匀的转变参数。考虑到以上优良性能,Ni/HfO₂/Pt结构的阻变存储器很有在非挥发性存储器领域中具有很高的应用价值。     

ZnO/(Ni)薄膜的制备与发光性能的研究

采用直流磁控溅射法制备了ZnO/(Ni)薄膜.研究了氧分压及Ni掺杂对ZnO薄膜的结构、光致发光特性及薄膜中的几种本征缺陷如氧空位(VO)、锌空位(VZn)、氧位锌(OZn)、锌位氧(ZnO)、间隙氧(Oi)、间隙锌(Zni)等浓度变化的影响.实验结果表明,随着氧分压的增大,466nm处的蓝色发光峰增强,掺Ni后蓝色发光峰也增强.通过分析,推测出蓝色发光峰可能是由ZnO薄膜中的间隙锌(Zni)点缺陷引起的.     

选择性BF_2~+离子注入对提高DRAM刷新时间的研究

动态随机存储器(dynamic random access memory,DRAM)电容器在存储高电位数据"1"时,将影响邻近记忆单元区晶体管栅极电场分布,从而导致漏电流增加,降低了刷新时间.研究提出针对位元线接触区、有选择性的浅掺杂漏极离子注入BF2+方案来改善刷新时间,模拟分析了其注入离子分布及电迁移,发现在位元线接触区硅基单侧浅表层形成了富硼离子注入区,且最大电迁移深度仅为60 nm,由此减少了对其它掺杂区的影响.电性测试结果表明,BF2+离子剂量与开启电压成正比,重复实验证明,该方案有良好的可再现性;分析结果表明,增加BF2+离子注入剂量能提高开启电压对制造偏差的容差能力;栅极关键尺寸在(90±15)nm波动范围内晶圆样品的NMOS电性测试结果表明,该离子注入法能保持与原有工艺的良好匹配性.进一步的分析结果指出,若开启电压升高,则刷新时间将会减少,若开启电压为0.8 V时,该离子注入方案能使刷新时间从180ms提升到不小于300ms.改良幅度达66.7%.模拟及实验分析结果表明,该离子注入方案能应用于深微米进程的研究与生产中.     

压印技术制备超高密度Si_2Sb_2Te_5基相变存储阵列

采用低成本、高效率的压印技术实现了高密度相变存储器(PCRAM)存储阵列的制备,开发出Si2Sb2Te5(SST)新材料的4Gbit/inch2存储阵列,存储单元面积为0.04μm2;利用SEM观测压印获得的光刻胶图形阵列以及刻蚀后的SST存储阵列,其单元外形均具有高度的一致性,且单元特征尺寸的3倍标准差均小于6nm;利用AFM研究了SST存储单元的I-V特性,阈值电压为1.56V,高、低电阻态阻值变化超过两个数量级。实验结果表明了SST新材料及压印技术在PCRAM芯片中的应用价值。     

CuxSiyO阻变存储器单粒子和总剂量辐照效应

对CuxSiyO结构的阻变存储器(RRAM)进行了总剂量(TID)以及单粒子辐照(SEE)实验.总剂量实验中,1T1R样品分别接受总剂量为1,2和3 kGy (SiO2)的60Co γ射线辐射.样品中没有出现低阻在辐照后翻转的现象.单粒子辐照实验中,16 kbit RRAM阵列样品分别接受线性能量转移(LET)值最高达75 MeV的4种离子束的辐射.样品中没有出现低阻在辐照后翻转的现象.除一组无翻转外,存储单元的高阻到低阻的翻转率皆在0.06％左右.实验结果证实了RRAM中已经形成的导电通道不会受辐照影响,因此不存在低阻翻转为高阻的现象.