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.

130 nm 7T SOI SRAM总剂量与单粒子协和效应研究

为进一步阐明SOI器件中总剂量效应(TID)与单粒子效应(SEE)间的协和效应,本文基于SOI工艺特征尺寸为130 nm的国产7T结构SRAM进行了相关研究。通过对4组SOI SRAM开展了不同TID辐照后的SEE实验,得到器件单粒子翻转(SEU)截面随TID的变化规律。SOI SRAM的SEU截面在TID辐照后呈现明显的降低,最大在750 krad(Si)剂量辐照后下降80.5%。器件的饱和截面呈现随剂量增加而下降的趋势,最大下降19.5%,研究中未发现SEU阈值的明显变化。分析认为,延迟晶体管N5的等效关态电阻因为TID辐照而增加,该现象会造成N5的延迟作用增强,是该款器件SEU截面下降的主要原因。采用这种7T结构的SOI SRAM的抗SEE性能会随其在轨累积剂量的增加而逐渐增强,这为今后电子器件的抗辐射加固提供了启示。

Universal memory based on phase-change materials:From phase-change random access memory to optoelectronic hybrid storage

The era of information explosion is coming and information need to be continuously stored and randomly accessed over long-term periods,which constitute an insurmountable challenge for existing data centers.At present,computing devices use the von Neumann architecture with separate computing and memory units,which exposes the shortcomings of“memory bottleneck”.Nonvolatile memristor can realize data storage and in-memory computing at the same time and promises to overcome this bottleneck.Phase-change random access memory(PCRAM)is called one of the best solutions for next generation non-volatile memory.Due to its high speed,good data retention,high density,low power consumption,PCRAM has the broad commercial prospects in the in-memory computing application.In this review,the research progress of phase-change materials and device structures for PCRAM,as well as the most critical performances for a universal memory,such as speed,capacity,and power consumption,are reviewed.By comparing the advantages and disadvantages of phase-change optical disk and PCRAM,a new concept of optoelectronic hybrid storage based on phase-change material is proposed.Furthermore,its feasibility to replace existing memory technologies as a universal memory is also discussed as well.

Recovery of single event upset in advanced complementary metal-oxide semiconductor static random access memory cells

Using computer-aided design three-dimensional (3D) simulation technology,the recovery mechanism of single event upset and the effects of spacing and hit angle on the recovery are studied.It is found that the multi-node charge collection plays a key role in recovery and shielding the charge sharing by adding guard rings.It cannot exhibit the recovery effect.It is also indicated that the upset linear energy transfer (LET) threshold is kept constant while the recovery LET threshold increases as the spacing increases.Additionally,the effect of incident angle on recovery is analysed and it is shown that a larger angle can bring about a stronger charge sharing effect,thus strengthening the recovery ability.

Total ionizing radiation-induced read bit-errors in toggle magnetoresistive random-access memory devices

The 1-Mb and 4-Mb commercial toggle magnetoresistive random-access memories(MRAMs) with 0.13 μm and 0.18-μm complementary metal–oxide–semiconductor(CMOS) process respectively and different magnetic tunneling junctions(MTJs) are irradiated with a Cobalt-60 gamma source. The electrical functions of devices during the irradiation and the room temperature annealing behavior are measured. Electrical failures are observed until the dose accumulates to 120-krad(Si) in 4-Mb MRAM while the 1-Mb MRAM keeps normal. Thus, the 0.13-μm process circuit exhibits better radiation tolerance than the 0.18-μm process circuit. However, a small quantity of read bit-errors randomly occurs only in 1-Mb MRAM during the irradiation while their electrical function is normal. It indicates that the store states of MTJ may be influenced by gamma radiation, although the electrical transport and magnetic properties are inherently immune to the radiation. We propose that the magnetic Compton scattering in the interaction of gamma ray with magnetic free layer may be the origin of the read bit-errors. Our results are useful for MRAM toward space application.

面向SOT-MRAM的磁屏蔽仿真设计与优化

磁随机存储器由于其高读写速度、接近无限次的读写次数和低功耗等优异特性受到越来越多的关注。磁随机存储器采用磁场方向来操作存储位,外部磁场很容易对磁结产生干扰导致读写错误的发生。因此采用磁屏蔽封装结构来减小外部磁场对存储器的影响具有重要意义。本文针对第四代自旋轨道矩磁随机存储器的磁屏蔽优化设计进行研究,根据自旋轨道矩磁随机存储器的面内磁场敏感特性,采用平行式磁屏蔽封装形式,建立了磁屏蔽有限元模型,分析了磁屏蔽结构因素(间距,面积和厚度)和磁饱和现象对磁屏蔽效果的影响,为磁存储器磁屏蔽设计提供了新思路。

Perpendicular magnetic tunnel junction and its application in magnetic random access memory

Recent progresses in magnetic tunnel junctions with perpendicular magnetic anisotropy(PMA) are reviewed and summarized. At first, the concept and source of perpendicular magnetic anisotropy(PMA) are introduced. Next, a historical overview of PMA materials as magnetic electrodes, such as the RE–TM alloys TbFeCo and GdFeCo, novel tetragonal manganese alloys Mn–Ga, L10-ordered(Co, Fe)/Pt alloy, multilayer film [Co, Fe, CoFe/Pt, Pd, Ni, Au]N, and ultra-thin magnetic metal/oxidized barrier is offered. The other part of the article focuses on the optimization and fabrication of CoFeB/MgO/CoFeB p-MTJs, which is thought to have high potential to meet the main demands for non-volatile magnetic random access memory.

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_x/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_x/BN bilayer device compared with that for the Ta/HfO_x/TiN structure.Furthermore,the reset current decreases with increasing BN thickness.The HfO_x layer 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_x/BN bilayer device is space-chargelimited current(SCLC),while it is Ohmic conduction in the HfO_x device.

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.

Ionizing radiation effect on single event upset sensitivity of ferroelectric random access memory

The impact of ionizing radiation effect on single event upset(SEU) sensitivity of ferroelectric random access memory(FRAM) is studied in this work. The test specimens were firstly subjected to ^(60)Co γ-ray and then the SEU evaluation was conducted using ^(209)Bi ions. As a result of TID-induced fatigue-like and imprint-like phenomena of the ferroelectric material, the SEU cross sections of the post-irradiated devices shift substantially. Different trends of SEU cross section with elevated dose were also found, depending on whether the same or complementary test pattern was employed during the TID exposure and the SEU measurement.

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.

Synergistic effects of total ionizing dose on single event upset sensitivity in static random access memory under proton irradiation

Synergistic effects of the total ionizing dose(TID) on the single event upset(SEU) sensitivity in static random access memories(SRAMs) were studied by using protons. The total dose was cumulated with high flux protons during the TID exposure, and the SEU cross section was tested with low flux protons at several cumulated dose steps. Because of the radiation-induced off-state leakage current increase of the CMOS transistors, the noise margin became asymmetric and the memory imprint effect was observed.

基于全裕度SA的高读可靠性STT-MRAM两位量化器设计

受隧道磁阻比(Tunnel Magneto Resistance,TMR)下降、工艺偏差和热波动等因素影响,先进工艺节点(亚25 nm)下的自旋转移力矩磁随机存储器(Spin Transfer Torque Magnetoresistive Random Access Memory,STT-MRAM)的读取裕度降低,读写正确率下降。纠错码(Error Correction Codes,ECC)技术方案能有效提升STT-MRAM读取可靠性。但是,常见的一位硬判决量化器无法发挥多位量化软判决ECC编解码算法的优势,而基于模数转换器的软判决量化器在实现上又要以牺牲面积和功耗为代价。提出的面向STT-MRAM的两位量化器采用全裕度灵敏放大器(Sensitive Amplifier,SA)和电阻量化判决门限,显著降低了量化器的电路复杂度和读写错误率。结果表明,在TMR和低读取裕度条件下,基于提出的两位量化器和极化码(Polar)编码的ECC算法能实现优于基于一位量化器的里德-索洛蒙博斯-乔赫里-霍克文黑姆码(Bose,Chaudhuri&Hocquenghem,BCH)编解码方案的输出帧错误率(Frame Error Rate,FER)。此外,本文提出在STT-MRAM级联信道模型中考虑实际SA引入的读判据错误,用于准确评估读取裕度对读正确率造成的影响。结果表明,基于全裕度SA的两位电阻量化方案能够显著提升判决器的有效读取裕度,减小SA引入的读判据错误,在TMR≤90%时,实现比采用一位硬判决量化器的BCH码平均低47%的输出FER。

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 compared 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(r GO) 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 r GO 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.

Pattern imprinting in deep sub-micron static random access memories induced by total dose irradiation

Pattern imprinting in deep sub-micron static random access memories(SRAMs) during total dose irradiation is investigated in detail. As the dose accumulates, the data pattern of memory cells loading during irradiation is gradually imprinted on their background data pattern. We build a relationship between the memory cell's static noise margin(SNM) and the background data, and study the influence of irradiation on the probability density function of ?SNM, which is the difference between two data sides' SNMs, to discuss the reason for pattern imprinting. Finally, we demonstrate that, for micron and deep sub-micron devices, the mechanism of pattern imprinting is the bias-dependent threshold shift of the transistor, but for a deep sub-micron device the shift results from charge trapping in the shallow trench isolation(STI) oxide rather than from the gate oxide of the micron-device.

抗辐照MRAM研究进展

新型非易失磁性随机存储器(magnetic random access memory,MRAM)具有读写速度快、数据保持时间长、功耗低等优点,引起了研究人员的广泛关注。其优异的抗辐照能力被人们深入挖掘,有望进一步应用于航天等领域。本文回顾了MRAM的产业化发展历程、技术变革及应用情况,列举了近年成熟的MRAM产品,对不同的代际MRAM的优缺点进行了剖析;对MRAM核心存储单元——磁隧道结(magnetic tunnel junction,MTJ)和外围基于互补金属氧化物半导体(complementary metal oxide semiconductor,CMOS)的读写电路的辐射效应分别进行了探讨;总结了近年来MRAM抗辐照加固设计方面的最新成果;对抗辐照MRAM在航空航天领域甚至核能领域的发展前景进行了展望。

双端口SRAM抗写干扰结构的优化设计

针对双端口静态随机存储器(SRAM)通常存在写干扰而导致数据写入困难的问题,基于经典位线电平复制技术提出了一种新型的位线电平复制结构。基于SMIC 28 nm CMOS工艺对位线电平复制结构进行设计,通过优化控制逻辑的组合电路,缩短位线电平复制操作的开启时间,提高了数据写入SRAM的速度,使设计的SRAM可在更高频率下正常工作,同时降低了动态功耗。仿真结果显示,在0.9 V工作电压下,相对于经典位线电平复制结构,采用新结构设计的SRAM的写入时间缩短了约27.4%,动态功耗降低了约48.1%,抗干扰能力得到显著提升。

关于3D堆叠MRAM热学分析方法的研究

本文针对3D堆叠磁随机存储器(Magnetic Random Access Memory,MRAM)的热学分析问题,在有限元法和热阻网络法的基础上,提出了一种局部等效法,可高精度并且快速地分析3D堆叠MRAM的热学分布.与有限元法相比,该方法使用直观方便,克服了有限元法建模与求解复杂耗时的问题;与热阻网络法相比,局部等效法具有保持较高精度的特点,解决了热阻网络法针对带夹层和硅通孔(Through Silicon Via,TSV)的复杂封装问题时存在较大误差的问题.对比结果表明,使用本文提出的方法得出的各叠层的上表面温度误差均小于0.05℃,精度与有限元法一致,并且更便捷高效.同时对应的建模结构简单,避免了热阻网络法将含铜柱的夹层和含铜柱的硅层分开考虑的不准确性.本文的研究可为未来多层3D堆叠MRAM热学特性相关的设计与分析提供指导.

基于双MCU与双端口SRAM的高速传感系统设计

随着半导体设备的广泛使用,对传感器测量系统的需求也大大增加,传统的传感器测量系统将MCU(微控制单元)与传感器组合使用,用于实时采集、处理和分析数据,但是受限于存储容量、处理速度、测量精度等无法满足高速实时处理数据的需求。基于此,设计了一种基于双端口SRAM的高速传感系统,使用双MCU结合双端口SRAM结构实现更高的数据吞吐量和更快的处理速度。测试表明,在相同主频和相似程序工作流程下,双MCU传感系统比单MCU传感器测量系统的工作时间更快,且双MCU系统完成相同工作所需的时间随着芯片的主频的降低,差距成倍增加。研究结果为采用低主频芯片实现高实时性响应系统提供了思路。