阻变存储器及其集成技术研究进展

在各种新型非挥发性存储器中,阻变存储器(RRAM)具有成为下一代存储器的潜力.介绍了RRAM器件的基本结构,分类总结了常用的材料以及制备工艺,对RRAM阵列的集成方案进行了比较,并讨论了目前存在的问题;最后,对RRAM的研究趋势进行了展望.     

新型阻变存储器内的电压解决方案

相对于现在流行的FLASH型存储器,新型阻变存储器(resistive-RAM,RRAM)有很多优势,比如较高的存储密度和较快的读写速度。而针对RRAM的读写操作特性,提出了一种适用于新型阻变存储器的提供操作电压的电路。该方案解决了新型存储器需要外部提供高于电源电压的操作电压的问题,使得阻变存储器能应用于嵌入式设备。同时,对工艺波动和温度波动进行补偿,从而降低了阻变存储器的读写操作在较差的工艺和温度环境下的失败概率,具有很强的实际应用意义。该设计采用0.13μm标准CMOS 6层金属工艺在中芯国际(SMIC)流片实现,测试结果表明,采用此电路的RRAM能正确地进行数据编程和擦除等操作,测试结果达到设计要求。     

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。     

一种基于HfO2材料的RRAM波动性模型

基于HfO₂材料制作了一种具有良好非易失性的阻变存储器(RRAM)。根据ECM导电细丝机制,建立了动态的Verilog-A模型,该模型包含了RRAM的波动特性。对模型进行直流电压扫描验证,与实际器件的电流-电压特性曲线进行拟合。验证结果表明,该模型具备RRAM器件优良的电学特性,波动特性的加入对电路的前期设计具有指导意义。对模型进行脉冲仿真,仿真结果表明高、低阻态之间的鉴别窗口大于100倍,转变时间小于30 ns。     

CuO纳米线阻变存储器的组装及电阻开关特性

用热氧化法在空气中加热铜片制备了CuO纳米线(CuO NWs),通过FESEM对纳米线表面进行了观察,并用液体转移法组装成功了一种简单的阻变存储器件。通过I-V测试系统观察到了Cu/CuO NWs/Cu器件表现出了明显的双极型和单极型。最后通过对比高阻态(HRS)和低阻态(LRS)的表面形貌,解释了Cu/CuO NWs/Cu器件的阻变机制。     

阻变存储器外围电路关键技术研究进展

阻变存储器(RRAM)是一种前景非常好的未来通用存储技术,也是当前国际学术界和工业界研究的热点。主要介绍了存储器外围电路的电路设计,并介绍了阻性存储器外围电路,包括验证电路、写电路、参考模块方案和形式、限流等关键技术的原理,重点讨论了提升复位操作速度,改善高阻值离散性,参考方案的设计和参考单元的组成,用限流实现低功耗操作的方法及其发展趋势。     

两类阻变机理及性能改善方法的研究

在新型非易失性存储领域,结构简单、高速低耗的阻变存储器具有巨大优势和很强的竞争力.简要介绍了阻变存储器的结构及其两个电阻转变行为.总结了两类阻变机理,探讨了阻变存储器性能优化的方法,以及优化方法在阻变性能与器件的可靠性和稳定性之间如何取得平衡统一的问题,并展望了其前景.     

阻变存储阵列的自动化测试系统

阻变存储器(RRAM)是一种新型的不挥发存储技术,研究阻变存储器阵列规模的存储性能以及可靠性问题是推进RRAM实用化的关键.目前通用的基于微控探针台的半导体参数分析的常规测量系统无法完成对阵列的自动化测试.利用半导体参数分析仪(4200-SCS)、开关矩阵以及相关外围电路搭建了一套针对阻变存储阵列的自动测试系统,实现了1MbitRRAM芯片的初始阻态分布的读取、初始化测试、存储单元的自动化编程/擦除操作.测试结果表明,该测试系统可以实现阻变存储阵列的自动化测试,为进一步工艺参数和编程算法的优化设计奠定基础.     

超薄氧化铝层的电场调制效应

研究了基于高k介质材料的阻变存储器的写入/擦除 (SET/RESET) 特性和物理机制.研究发现基于NbAlO材料的阻变存储器SET/RESET电压具有较大波动性, 通过结构优化, 在Al2O3/NbAlO/Al2O3纳米薄片堆垛结构器件中获得高度稳定性的可重复的阻变特性.基于电场调制效应, 提出了一种统一的电阻开关模型去模拟阻变存储器的SET/RESET行为, 并探讨了单层阻变薄膜的阻变存储器中由导电单元形成和湮灭的巨大随机性引起的阻变特性分布.当在NbAlO基阻变存储器中嵌入超薄Al2O3膜后, 阻变存储器的SET/RESET电压稳定性将显著提升, 其原因在于采用堆垛结构的阻变器件中各介质层中的电场重新分布并精确可控, 因此导电细丝的导通/断裂通过电场调制作用稳定均匀地在发生在具有高电场的薄缓冲层介质层中.     

RRAM的氧空位与金属细丝机制SPM的比较

应用扫描探针显微镜(SPM)技术实现了氧化物阻变薄膜局部区域高低阻态的互相转变。通过电激励、编程和擦除等操作,控制细丝的产生和断裂,实现了阻变薄膜局域的重复编程/擦除操作。用该方法分别研究了氧空位机制与金属导电细丝机制的氧化物薄膜的阻变特性,对两种机制做了对比研究。结果表明:在阻变存储器(RRAM)中氧空位机制在导电细丝和数据密度方面要高于金属细丝机制。同时,金属细丝机制阻变薄膜部分区域因编程/擦除操作发生了永久性形貌变化,可能对阻变器件的电极产生永久性破坏,这说明氧空位机制阻变薄膜在未来的高密度存储上具有较好的应用前景。     

Preface to the Special Issue on Beyond Moore:Resistive Switching Devices for Emerging Memory and Neuromorphic Computing

Traditional charge-based memories,such as dynamic random-access memory(DRAM)and flash,are approaching their scaling limits.A variety of resistance-based memories,such as phase-change memory(PCM),magnetic random-access memory(MRAM)and resistive random-access memory(RRAM),have been long considered for emerging memory applications thanks to their non-volatility,fast speed,low power,and compact size for potentially high-density integration.     

Resistive random access memory and its applications in storage and nonvolatile logic

The resistive random access memory (RRAM) device has been widely studied due to its excellent memory characteristics and great application potential in different fields. In this paper, resistive switching materials, switching mechanism, and memory characteristics of RRAM are discussed. Recent research progress of RRAM in high-density storage and nonvolatile logic application are addressed. Technological trends are also discussed.     

Comparison of three different architectures for active resistive memories

Resistive memory (RRAM) is one of the strong emerging technologies in modern memory field. This type of memories has the potential to be the replacement of several current memory types. As any new technology, RRAM brings new challenges concerning technology and design. This work discusses some aspects concerning the design of active resistive memories and compares three possible memory architectures.     

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.     

Low-Energy Oxygen Plasma Injection of 2D Bi2Se3 Realizes Highly Controllable Resistive Random Access Memory

Resistive random access memory (RRAM) based on ultrathin 2D materials is considered to be a very feasible solution for future data storage and neuromorphic computing technologies. However, controllability and stability are the problems that need to be solved for practical applications. Here, by introducing a damage-less ion implantation technology using ultralow-energy plasma, the transport mechanisms of space charge limited current and Schottky emission are successfully realized and controlled in RRAM based on 2D Bi₂Se₃ nanosheets. The memristors exhibit stable resistive switching behavior with a high resistive switching ratio (>10⁴), excellent cycling endurances (300 cycles), and great retention performance (>10⁴ s). The reliability and controllability of Bi₂Se₃ memory endowed by oxygen plasma injection demonstrate the great potential of this ultralow-energy ion implantation technology in the application of 2D RRAM.     

On Using the Volatile Mem-Capacitive Effect of TiO<Subscript>2</Subscript> Resistive Random Access Memory to Mimic the Synaptic Forgetting Process

In this work, we report on mimicking the synaptic forgetting process using the volatile mem-capacitive effect of a resistive random access memory (RRAM). TiO₂ dielectric, which is known to show volatile memory operations due to migration of inherent oxygen vacancies, was used to achieve the volatile mem-capacitive effect. By placing the volatile RRAM candidate along with SiO₂ at the gate of a MOS capacitor, a volatile capacitance change resembling the forgetting nature of a human brain is demonstrated. Furthermore, the memory operation in the MOS capacitor does not require a current flow through the gate dielectric indicating the feasibility of obtaining low power memory operations. Thus, the mem-capacitive effect of volatile RRAM candidates can be attractive to the future neuromorphic systems for implementing the forgetting process of a human brain.     

A journey towards reliability improvement of TiO2 based Resistive Random Access Memory: A review

A Resistive Random Access Memory (RRAM), where the memory performance principally originated from ‘resistive’ change rather than ‘capacitive’ one (the case with conventional CMOS memory devices), has attracted researchers across the globe, owing to its unique features and advantages meeting the demands of future generation high-speed, ultra low power, nano dimensional memory devices. A large family of semiconducting oxides have been investigated as insulator for Resistive Random Access Memory (RRAM), amongst which TiO₂ is one of the potential candidate, principally owing to some of its remarkable advantages e.g. wide band gap, high temperature stability and high dielectric constant with flexibility to offer both unipolar and bipolar switching, which are essential for RRAM device applications. In this review article, we tried to represent the long voyage of TiO₂ based RRAM, towards the improvement of the reliability aspects of the device performance in a comprehensive manner. Starting with the key factors like oxygen vacancies, Ti interstitials and electroforming, which are responsible for resistive switching phenomenon, various material preparation techniques for RRAM development have been discussed with emphasis on relative merits and bottlenecks of the process. The factors like electrode material and geometry, device structuring, doping, compliance current, annealing effect etc., which play the pivotal role in determining the switching performance of the device, have been reviewed critically. Finally, the article concludes with the comparison of different TiO₂ based RRAM devices followed by the prediction of possible future research trends.     

Current-Sweep Operation on Nonlinear Selectorless RRAM for Multilevel Cell Applications

Bilayer selectorless resistive random-access memories (RRAM) have been demonstrated by utilizing the intrinsic nonlinear resistive switching (RS) characteristics, without additional transistor or a selector integration. The bilayer structures, i.e. high-k layer/low-k layer stacks, are highly scalable while suppressing the sneak path currents (SPC) and reading error in the crossbar RRAM array. The nonlinearity (NL) modulation is also investigated by different operating schemes, and a multilevel cell application is demonstrated with the current-sweep method. The results provide additional insights into the development and optimization of bilayer selectorless RRAMs with high nonlinearity, good memory window, and low switching energy (∼ 40 pJ/bit), which enable the high-density storage and low-power crossbar array memory applications.