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随着近年来云计算、物联网、大数据、人工智能、区块链等各种新兴技术的涌现，传统存储技术面临严峻的挑战，其中之一便是计算机体系架构中的“存储墙”问题[1-4]。传统易失性静态随机存储器（static random access memory，SRAM）和动态随机存储器（dynamic random access memory，DRAM）性能的提升不再显著，SRAM昂贵的芯片面积开销与DRAM刷新操作导致的高功耗，越来越无法满足新型应用的需求。传统的非易失存储器以FLASH应用最为广泛，但却存在着读写速度慢、擦写次数有限的问题[5-6]，无法用作系统缓存或主存。
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另外，随着近年来航天技术的飞速发展，宇航电子系统不仅面临上述问题，还面临着空间辐照环境对其保存的数据和信息的干扰与破坏等问题，这进一步提高了对存储器可靠性的要求。然而，SRAM、DRAM和FLASH均易遭受空间辐照效应的影响[7-12]。这些存储器在不进行抗辐照加固或无系统纠检错措施的情况下不仅难以胜任航天任务，甚至还会严重影响航天器的在轨安全。而对芯片和系统采取辐照加固措施，往往会造成芯片面积增加、功耗增加、速度降低[13-14]等问题。如果存储器的存储单元本身无辐射抗性或者抗性很差，则其加固更为困难。
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解决以上问题的一种有效办法是采用磁性随机存储搭建抗辐照存储系统，同时辅以其他加固措施。磁性随机存储器（magnetic random access memory，MRAM）具备非易失性、非常快的读写速度、出色的耐擦写特性、优良的数据保持能力、低功耗和存储单元天然抗辐照等优点[15-17]。其天然抗辐照能力来源于存储数据的物理机理——它通过电子的自旋方向存储数据而非电荷，前者更不容易受到辐照影响。目前，MRAM商用芯片的存储密度已高达1 Gbit/芯片[18-19]，通过多芯片封装总容量已达8 Gbit[20]。
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本文首先介绍了MRAM存储单元——磁隧道结（magnetic tunnel junction，MTJ）的基本结构及原理，简要概括了MRAM的产业化发展历程、技术变革及应用情况，然后对MRAM及MTJ的各种辐照效应进行了探讨，随后总结了近年来MRAM抗辐照加固设计方面的最新成果，最后对抗辐照MRAM的发展前景进行了展望。
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1 MRAM技术及其产业化发展历程
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1.1 磁隧道结的基本结构及原理
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MRAM利用电子的“自旋”属性存储信息，核心单元为MTJ。其基本结构如图1所示，主要包含参考层（reference layer，RL）、隧穿势垒层和自由层（free layer，FL）。RL的磁化方向是固定的，FL的磁化方向可随外磁场方向变化而变化，当FL和RL磁化方向一致时，MTJ表现为低电阻状态（RP），当FL和RL磁化方向相反时，MTJ表现为高电阻状态（RAP），两种状态可分别代表逻辑“0”和“1”（或相反）。该现象被称为隧穿磁阻效应（tunnel magnetoresistance，TMR），TMR的大小可用式（1）计算：
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TMR值越大，“0”和“1”之间的读取窗口越大，芯片设计难度越小。
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图1 磁隧道结基本结构
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Fig.1 Basic structure of magnetic tunneling junction
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早期的MTJ隧穿势垒层采用无定形态的AlOx，TMR值可达18%[21]。后来，人们发现采用单晶MgO，可使TMR值达到200%左右[22-23]，甚至达到604%[24]（理论预测可达1 000%[25-26]），所以采用单晶MgO作为隧穿势垒层成为主流。
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1.2 MRAM的产业化发展历程、技术变革及应用
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自2006年飞思卡尔推出第一款商用4 Mbit独立式MRAM，大量的研究机构投身其中，并取得了丰硕的成果（见表1）。
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如图2所示，MRAM核心单元MTJ历经了三次主要的技术变革。第一代MRAM采用磁场牵引（Toggle）写入技术[27]，MTJ的势垒层采用AlOx（Toggle-MRAM的后续产品也逐渐用MgO取代了AlOx），其RL和FL的磁化方向都平行于薄膜平面，即面内磁各向异性（in plane magnetic anisotropy，IMA）。FL采用合成反铁磁（synthetic antiferromagnetic，SAF）结构，易磁化轴方向与字线、位线各呈45°。写入操作时对字线和位线按照特定时序施加写入电流从而形成45°连续变化的三组感应磁场（如图3所示），利用磁场改变FL磁化方向，完成数据写入。基于这种技术，Everspin陆续推出了容量128 Kbit~32 Mbit的商用MRAM产品。这些商用产品的MTJ具有抗辐照特性，但其外围互补金属氧化物半导体（complementary metal oxide semiconductor，CMOS）逻辑并未进行辐照加固，仍然难以满足空间应用的要求。因此，一方面，研究机构对Everspin商用MRAM进行了辐射效应评估，辅以一些系统加固措施，以满足在一定条件范围内的应用；另一方面，Aeroflex和Honeywell从Everspin获得授权，采用其MTJ技术，并在此基础上对CMOS逻辑部分进行了抗辐照加固，随后采用陶瓷封装，并通过一系列严格的筛选及质量检验措施使其真正成为满足空间应用的宇航级抗辐照产品。
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图2 MTJ技术类型
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Fig.2 MTJ technology types
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图3 Toggle-MRAM写时序[27]
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Fig.3 Writing sequence in Toggle-MRAM[27]
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然而，Toggle磁场写入存在功耗高（mA级）、存储密度低和设计复杂等缺点，这限制了Toggle-MRAM的应用前景。第二代MRAM凭借自旋转移矩[51-52]（spin transfer torque，STT）[51-52]及垂直磁各向异性[53-54]（perpendicular magnetic anisotropy，PMA）的应用很好地解决了上述问题。首先，相较于Toggle-MRAM通过电流产生磁场而间接改变FL磁矩的写入方式，STT-MRAM是依靠电流流过MTJ直接翻转FL磁矩，从而实现数据的写入，因此又被称为纯电学写入方式。STT-MRAM写入电流的方向决定FL的磁化翻转方向，写入电流密度在106~107 A/cm2之间，且写入电流的大小随工艺尺寸的缩小而减小，写入功耗大大降低，同时免去了复杂的设计。其次，PMA的应用解决了面内磁各向异性MTJ（iMTJ）尺寸不能进一步微缩的问题，大大提高了存储密度，同时获得了更低的临界翻转电流（Ic）。因此，在产业化发展过程中，iMTJ很快被垂直磁各向异性MTJ（pMTJ）所取代，现在所说的第二代MRAM主要是指具有PMA的STT-MRAM。
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STT-MRAM以其较快的读写速度、较高的存储密度及出色的数据保持特性吸引了几乎所有半导体厂商（TSMC、Global Foundries、三星、Intel、UMC、海力士、索尼、IBM等）的关注与研发。其主要原因为eFLASH已止步于28 nm工艺节点，而STT-MRAM是目前最有希望的嵌入式存储器候选者[55]。此外，人们也在研究将其用于最后一级缓存，以取代数据易失的SRAM。
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相比于Toggle-MRAM，STT-MRAM缺点主要体现在擦写次数和数据保持两个指标互相矛盾，无法兼得。其原因与STT电流写入模式有关：提升MTJ的数据保持能力需提升其热稳定性，而热稳定性的提升导致STT临界翻转电流（电压）的上升，而势垒层介质的寿命（与时间相关）又与写入电流（电压）呈负相关，势垒层寿命的下降导致擦写次数降低。因此，对于pSTT-MRAM产品，只能突出优化其中一个方面。而第一代Toggle-MRAM一方面由于其为磁场写入，势垒层寿命不会受到写入电流的影响而降低，因此其擦写次数理论上是无限次；另一方面，可以通过优化其MTJ器件结构、材料和参数来提升其数据保持能力，而不用担心影响到擦写次数。经过优化，其数据保持能力可达20年（@125℃）。另外，STT-MRAM的写入速度也存在瓶颈，这一瓶颈来源于写入初期微弱的自旋转移矩导致的“初始延迟”，虽然可以通过增加写入电流来解决这一问题，但也增加了势垒层被击穿的风险。表2为几代MRAM性能的综合比较。
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为解决STT-MRAM的问题，科学家进行了大量研究，其中比较有效的方案是Miron和Liu所在的研究小组[60-65]提出的自旋轨道矩（spin orbit torque，SOT）。采用SOT效应翻转FL磁矩的MRAM被称为第三代MRAM。SOT-MTJ为三端器件，利用流经底部重金属的电流产生SOT以驱动FL完成磁化翻转。该方案将写入电流与读出电流路径分开，有效避免了写入电流对势垒层的损伤，提升了写入速度，理论上擦写次数与Toggle-MRAM一样（无限次），解决了STT擦写次数和数据保持时间的矛盾，有效提升了器件可靠性。但SOT-MTJ相比于STT-MTJ面积较大，集成度低，未成为各大机构研发的主流，市场上也暂无相关的产品。即便如此，SOT-MRAM由于其出色的写入速度和高可靠性在高速应用场合（L1~L3级缓存）和高可靠应用场合（航空航天）仍有非常大的吸引力。为了解决SOT-MRAM低集成度的问题，人们提出了很多解决措施，有望成为第四代MRAM可能的方向。例如由STT与SOT、压控磁各向异性（voltage control magnetic anisotropy，VCMA）与SOT的协同自旋矩效应构成的NAND型高密度串联结构[66-72]，以及通过将存储层由铁磁层（ferromagnet，FM）转变为SAF，利用SAF的交换偏置场的磁存储技术[58-59]（exchange bias-MRAM，EB-MRAM），有望极大提升MRAM的存储密度。
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在应用端，第一代Toggle-MRAM不仅没有被市场逐渐淘汰，反而在高可靠应用场合（例如航空航天、汽车电子等领域）扮演着重要角色。2009年瑞典公司AAC（Angstrom Aerospace Corporation）与日本东北大学合作开发的TAMU（Tohoku-AAC MEMS unit）搭载一颗小卫星SpriteSat进入了680 km的极地轨道，其中便使用了Everspin公司商用4 Mbit MRAM来取代FLASH和SRAM[73]。2011年10月，由密歇根大学研制的立方体卫星M-Cubed也使用了Everspin公司商用16 Mbit MRAM。在此后的10年里，Toggle-MRAM已大量应用于航空航天领域。此外，pSTT-MRAM已逐步应用于企业级固态硬盘的缓存，用于防止数据掉电丢失。但因其技术成熟度和可靠性等原因，尚未有应用于航天器的报道。即便如此，pSTT-MRAM在空间应用方面仍具有很大的潜力。
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MRAM的空间应用需要考虑空间辐射环境。空间环境中存在高能离子、质子、中子、电子、α粒子和γ射线等辐射源[74-75]，这些辐射源与电子元器件中半导体材料发生碰撞、电离等相互作用，可造成元器件参数漂移、功能退化甚至彻底失效，严重降低电子元器件可靠性[76-77]。MRAM作为一种新型存储器，研究其辐射效应及加固技术对提升MRAM的应用潜力意义重大。
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2 MRAM中MTJ单元的辐照效应及加固技术
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目前，对MTJ的辐照效应和机理的研究仍处于探索阶段，进行相关研究时一些概念仍然沿用辐照对MOS器件的一些术语。比如“电离总剂量效应”（total ionizing dose，TID）和“单粒子效应”（single event effect，SEE）等，而实际上这些术语对于MTJ的辐照效应并不完全恰当。例如“电离总剂量效应”主要是指由于辐照引发了MOS器件中SiO2/Si界面处“电荷的积累”，其物理效应主要跟“电荷积累”有关。而在MTJ的所谓“电离总剂量效应”的研究中并未发现“电荷积累”相关的效应。因此在MTJ的辐照效应和物理机理完全清晰之前，本节按照辐照源对MTJ辐照效应进行了分类。
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2.1 MTJ的γ辐照效应
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带能量的光子（γ及X射线等）同固体物质相互作用时，电离损伤是主要损伤。根据光子能量不同，可以与材料发生不同的电磁相互作用——光电效应（光子能量小于100 keV）、康普顿效应（光子能量在100 keV到几MeV之间）和电子-空穴对为主的效应（更高的能量），这些相互作用均带来电离损伤。除此之外，光子还可以引发位移损伤效应，但概率很低[78-79]。放射性同位素60Co γ射线源能提供均匀的沉积剂量及稳定γ光子流，非常适用于电子元器件的电离总剂量辐照试验。因此在对MTJ进行辐照研究时，人们也尝试使用了60Co γ射线源。本小节中以下的研究若无特殊说明，均是以60Co γ为辐射源。
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2.1.1 AlOx-MTJ
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AlOx-MTJ为第一代Toggle-MRAM所采用。目前尚未见到单独对AlOx-MTJ进行的总剂量效应试验，所有的试验结果均来源于MRAM芯片整体的辐照结果。根据文献[80]可知，Honeywell的Toggle-MRAM产品整体抗TID能力达到1 Mrad（Si）（注：1 rad=10-2 Gy），这意味着其内部AlOx-MTJ的抗TID能力至少也达到了1 Mrad（Si）。而Honeywell产品中MTJ采用Everspin的工艺，因此可推测，Everspin产品抗总剂量能力有限的原因来自未进行抗辐照加固的外围CMOS逻辑，而非MTJ。
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然而也有例外， Cui等[81]在对Everspin公司的1 Mbit MRAM和4 Mbit MRAM进行的辐照对比试验中发现，1 Mbit MRAM在TID为30 krad、60 krad、90 krad（Si）时均出现了随机性bit错误。相反，4 Mbit MRAM在120 krad（Si）的剂量下，在电参数失效之前未出现bit错误。分析表明两种MRAM采用了不同的MTJ结构，其中1 Mbit MRAM的MTJ结构从下到上分别是Ta、MnPt、CoFeNi、AlO、（Co/Ni）n、MnNi，4 Mbit MRAM的MTJ结构从下到上分别是Ta、MnPt、CoFe、Ru、CoFe、AlO、CoFe、NiFe。结合1 Mbit MRAM辐照后bit错误表现特征及两种不同的MTJ结构特点，其将1 Mbit MRAM出现bit错误的原因归结为其MTJ更容易受到辐照的影响。首先，两种MTJ的FL是不同的，数据存储机制不同。辐照后MTJ的TEM和EDX光谱结果显示，1 Mbit MRAM的MTJ中Mn元素向FL的扩散程度比4 Mbit MRAM的更严重。它将破坏FL薄膜的晶格结构和界面结构[82-83]，降低磁晶各向异性和界面各向异性。其次，γ射线中有少量圆极化光子，当γ射线通过薄膜时，会发生磁康普顿散射[84-85]（magnetic Compton scattering，MCS）。1 Mbit MRAM的FL的（Co/Ni）n薄膜的畴壁磁化更容易通过康普顿效应从一个方向转移到相反的方向。再次，在此过程中，γ光子激发的高能自由电子可能与晶格原子发生碰撞，导致晶格位移损伤，从而降低FL的各向异性。另外，γ射线辐照会引起晶格加热，降低FL的有效磁各向异性势垒，从而提高热辅助自旋翻转的概率。这些机制将有助于辐照过程中FL的磁矩翻转，从而改变MTJ的磁电阻，导致读取错误。由于圆偏振光子的数量非常少，读位错误的数量也很少。磁矩翻转仅改变数据存储状态，不影响器件的电磁特性和功能。因此，当再次向器件写入数据时，位错误消失。
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2.1.2 MgO-MTJ
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新一代MRAM均采用了MgO作为MTJ势垒层，为了评估这种MTJ的性能及对空间辐照环境的适应性，科学家进行了大量研究。Amara-Dababi等[86]认为γ辐射可以在MgO中产生电子-空穴对，当MgO内捕获的电荷密度足够高时，会导致MgO击穿。因此研究γ辐射对MgO-MTJ的影响是有必要的。
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Ren等[87]对MgO-iMTJ进行的辐照效应研究结果表明，使用γ射线以9.78 rad/min的剂量率对MTJ进行辐照，累积剂量达10 Mrad（Si）后，MTJ的RP、RAP和矫顽力（Hc）均未发生统计学意义上的变化。同年， Hughes等[88]试验结果也有着相同的结论，总剂量达到10 Mrad（Si）后，MTJ膜堆的磁性、Hc、有效磁化强度、自由层阻尼系数、电阻面积矢量积（resistance area product，RA）均未发生统计学意义上的变化。两人研究的MTJ器件均为IMA，尺寸太大，集成度低，应用潜力有限。而PMA有效减小了MTJ的尺寸，提高了存储密度。但由于PMA的主要来源是界面各向异性，电离辐射可能引起的界面缺陷在pMTJ中比在iMTJ中更受关注。
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Zink等[89]对pMTJ的γ辐照研究结果表明，辐照剂量累计达到1 Mrad（Si）后，MTJ的电阻和TMR变化不大。另外，TID可以引起pMTJ关键性能参数（热稳定因子、临界翻转电压Vc和写入能量）的变化。但这些影响在尺寸较大的MTJ中更为显著，当MTJ尺寸较小时影响很小，因此有利于MRAM集成度的提升。Montoya等[90]研究了极端剂量（1.47 Mrad（SiO2））的γ辐射对纳米级pMTJ的影响，试验结果表明辐照后pMTJ的TMR、临界翻转磁场和Ic均无统计学意义上的变化。此外，在γ辐照下，热激活的pMTJ翻转率几乎没有变化，表明γ辐射并不影响pMTJ翻转的过程。
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除了以上针对单界面pMTJ的研究外， Wang等[91]研究了CoFeB/MgO双界面pMTJ[92]在不同剂量γ辐照下的辐射特性。其制备的双界面CoFeB/MgO多重膜的结构如图4（a）所示。MTJ基于热SiO2基底，底部为CuN、Ta种子层，从下至上分别为[Co/Pt]6、Co、Ru、Co、[Co/Pt]3、W、CoFeB、MgO、CoFeB、W、CoFeB、MgO、Ta、Ru。试验结果表明在总剂量分别为5 Mrad、10 Mrad、20 Mrad（Si）时，Hc相比辐照前分别增加了5.5%、16.1%和19.7%，而饱和磁化强度Ms则没有变化。但是，如果照射剂量足够大（247 Mrad或475 Mrad（Si）），磁性就会被破坏。Hc的增加是多种因素共同作用的结果，包括MCS、两层（Co/Pt）n多层膜之间的Ru层被破坏、双界面结构中FL的有效厚度受辐照影响而增加，以及γ辐照引起的电离能沉积对畴壁运动的阻止作用。在247 Mrad（Si）或475 Mrad（Si）辐照下，磁性几乎消失，可能原因为热应力和内应力[93-94]。从图4所示的显微镜图像可以看出，多层膜存在由热应力和内应力引起的微米级裂纹。薄膜热应力是由多层薄膜结构与衬底之间热膨胀系数的差异引起的。此外，γ辐照引起的电离能沉积或温度分布不均匀也会产生热应力进而影响磁性能[95-96]。因此，辐射引起的热应力和内应力可能会破坏MTJ的磁性。
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图4 双界面pMTJ多重膜结构和样品的表面形貌
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Fig.4 Structure of the double-interface pMTJ multifilms and surface appearance of samples
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综上，AlOx-MTJ抗γ辐照的能力与MTJ的结构、材料及参数有关。MgO-iMTJ抗γ辐照能力至少可达10 Mrad（Si）以上。小尺寸MgO-pMTJ抗γ辐照能力至少可达1 Mrad（Si）以上。γ光子对MTJ的效应或损伤主要包含：MCS、位移损伤和热应力。良好γ辐照抗性的获得需要对MTJ的结构、材料及参数进行特殊设计，以削弱这些效应或损伤。
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2.2 MTJ的重离子辐照效应
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重离子是指原子序数Z≥2的离子。在空间中，这些重核离子虽然通量较低，但能量很高（为数十MeV到数百GeV），通过材料时产生强烈电离且很难屏蔽，这是SEE发生的主要原因。对于大多数重离子而言，大部分能量转移是产生电离效应，只有少部分带来位移损伤。
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MTJ由多层薄膜堆叠而成，各层薄膜厚度普遍很小（Å/nm量级），当总注量达到一定程度时，重离子轰击很可能会对多层纳米薄膜造成位移损伤，导致器件电学和磁学特性的改变。
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2.2.1 AlOx-MTJ
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Al2O3作为MTJ的势垒层对MTJ的特性起着重要作用。虽然Al2O3是良好的抗辐射绝缘体[97]，并且常用于强辐射环境，但当其为薄膜形式时对快速重离子辐照的反应可能并不理想。在快速重离子照射其他材料（例如Ge[98-99]）时，已经观察到了块体形式和薄膜形式不同的抗辐照行为。因此研究辐照对AlOx-MTJ的影响时，必须重新评估重离子辐照对Al2O3的影响。
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2003年， Schmalhorst等[100]研究了Mn83Ir17/Co70Fe30/AlOx/Ni80Fe20结构的MTJ在Al膜被等离子体氧化后立即被低能Ar离子束辐照后器件的输运特性。经过150 eV离子辐照后，器件电阻增加了40倍，TMR值和介电稳定性有所降低。该作者认为导致该现象的原因是离子辐照引起势垒层中Al和O的原子配比发生了局部变化，从而导致势垒层存在局部缺陷。同年，Conraux等[101]对AlOx介质的MTJ进行了C、O、Ni离子效应研究，结果表明MTJ受单粒子影响很小但不可逆，因此，AlOx-MTJ并非对单粒子辐照完全免疫。研究表明随着注量率的增加，TMR值逐渐降低，该作者将这一现象的原因归结为AlOx势垒层中Al和O化学计量的变化——势垒中的O消耗，而非互相扩散作用。2005年， Sacher等[102]研究了Co/Al2O3/Co结构的MTJ在Ar和He离子辐照下的输运特性。对于这两种离子，MTJ的RA随辐照离子能量的增加而增大，而TMR值减小。该研究表明辐照离子的能量和种类会影响辐照离子在MTJ中的分布，影响局域电子态的数目以及界面处缺陷的形成和自旋极化状态。同年，Banerjee等[103]研究了Si和Ag离子辐照对Co/Al2O3/Ni80Fe20结构的MTJ特性的影响。如图5所示，采用能量为70 MeV、总注量为5×1011 ions/cm2的Si离子辐照后，TMR值由初始的20.3%降低到了18.2%，Hc发生显著变化，并伴随着RAP和RP的下降；采用能量为200 MeV、总注量为1×1011 ions/cm2的Ag离子辐照后，MTJ被完全破坏，TMR效应完全丧失，且MTJ电阻大幅下降。为了解释所观测到的变化，该文献作者结合库仑爆炸模型[104]和热脉冲模型[105]进行了分析，认为快速重离子在Al2O3中诱导的缺陷可以近似认为是多条并行的导电路径，这些导电路径降低了整体结电阻。当离子通过这些层时，可能会在FM/势垒层界面上出现小程度的原子重新分布。另外，原子位移、原子聚集在单个膜层以及原子穿过界面将产生强自旋依赖散射和自旋独立散射，这可能与TMR的降低有关。
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图5 电阻随外磁场的变化关系（+5 mV偏置，室温）[103]
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Fig.5 Plot of resistance versus applied magnetic field at RT at a bias of +5 mV[103]
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通过以上研究可以看出，重离子辐照可能会对AlOx-MTJ多层膜造成位移损伤，在膜层或界面处产生缺陷，也可能会影响MTJ中局域电子态，进而影响自旋极化状态，最终导致MTJ的TMR值、RA等关键参数发生变化。
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2.2.2 MgO-MTJ
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与AlOx-MTJ相比，MgO-MTJ的薄膜结构和材料特性发生了极大的改变。由于AlOx为非晶材料，对电子的散射作用严重。穿过势垒层的电子的自旋极化率被AlOx消耗，导致AlOx-MTJ的TMR值普遍较低。而在CoFeB非晶层上可以生长出＜001＞晶向的MgO薄膜，且单晶化程度更高。MTJ膜堆沉积完成后，通过热退火工艺使得临近MgO势垒层的CoFeB薄膜以＜001＞晶向MgO为模板结晶，从而使自旋极化电子更高效穿透势垒层，自旋极化率几乎保持不变，因此单晶MgO比多晶AlOx-MTJ的TMR值更高[22-26]。
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有研究表明，在经历10~100个15 MeV Si离子（施加偏压）[106]辐照后，MgO-pMTJ的RAP、RP和TMR值基本保持不变，表现出良好的抗单粒子特性。同时，MTJ的热稳定性因子没有显著变化，这意味着数据保持特性并未发生变化。
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然而，也有一些实验表明离子辐照会对MTJ薄膜结构产生一定的损伤，导致器件电学或者磁学特性发生一定程度的退化。Singh等[107]用能量为120 MeV、总注量为1×1011~1×1013 ions/cm2的Ag离子垂直照射Fe/MgO/Fe/Co多层膜，观察到垂直磁滞回线的形状发生了显著变化。辐照后MgO层中Fe、Co和Au原子有所增加，界面处Fe的价态发生了变化，重离子通过材料时会诱导磁矩沿其方向排列，这三方面的因素改变了辐照后膜堆的Hc。Xiao等[108]的研究表明，pMTJ经历低注量（1011 ions/cm2）的Ta离子辐照后，TMR、Vc和临界翻转磁场基本不受影响，电学特性基本无变化；但随着注量的增加，pMTJ受损伤程度增加，其TMR、Vc和临界翻转磁场逐渐退化，甚至在总注量达到1013 ions/cm2后完全失去磁阻效应。这种退化主要来源于CoFeB/MgO界面的结构损伤：①辐照离子破坏了电子隧穿势垒的对称性，降低了自旋极化；②辐照离子降低了结构的PMA。Kobayashi等[109]研究了各种类型的高能离子（250 MeV Ar、322 MeV Kr、400 MeV Fe、454 MeV Xe和490 MeV O）对10 nm尺寸下pMTJ的影响。研究发现490 MeV O离子可以在MTJ中产生可恢复的位翻转，即软错误，而其他离子却无此现象。其还试验了电压对翻转可能产生的影响，但没有检测到。例如在400 MeV Fe辐照中，在+0.2 V（接近翻转临界电压）条件下也不会出现翻转；而相反，即使减小偏置电压到-0.01 V，O离子轰击仍然可以导致翻转。他们认为软错误发生的原因可能是重离子轰击引起局部温度升高，从而引发了铁磁层的磁化翻转。Park等[110]研究了Cr离子束对iMTJ和pMTJ的影响。研究发现，器件性能的退化与Cr离子辐照引起的位移损伤密切相关，Cr离子辐照位置主要集中在距离表面的几纳米处。因此，iMTJ的损伤位置主要出现在铁磁层顶层，从而仅减小了其磁化强度。但是pMTJ的损伤位置会扩展到FM/势垒层界面，从而降低TMR。该研究提供了一个提高抗单粒子能力的方法，即通过引入保护层来阻止离子穿过MTJ。Coi等[111]研究了995 MeV Xe离子对pMTJ的影响，实验结果表明pMTJ对SEE的敏感性比较低。重离子辐照对MgO的结晶度有类似退火的作用，使得TMR值轻微的增大。辐照离子造成的晶体缺陷可以作为成核中心，形成更多的成核位点，导致Hc降低。界面缺陷的增加和重离子对SAF的损伤会降低PMA，另外，重离子辐照也可能通过热激发或热诱导效应影响MTJ。Wang等[112]研究了Ta离子和Kr离子对双界面CoFeB/MgO MTJ的辐照效应。在总注量为1011 ions/cm2和能量为1 907 MeV的Ta离子辐照后，双界面产生了一定的损伤，导致Hc存在一定程度的降低而且无法恢复；而总注量为5×1010 ions/cm2和能量为2 060 MeV的Kr离子辐照破坏了MTJ的体特性，导致Ms下降。然而，双界面MTJ的电性能几乎不受Kr离子辐照的影响。
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2021年，Alamdar等[113]研究了SOT-pMTJ的辐照效应。研究结果表明，其对于总注量1012 ions/cm2的Ta离子具有较好的抗辐照能力，但在更高注量（1012~1014 ions/cm2）的辐照环境下，磁性膜堆的Hc和PMA变小，主要源于底部CoFeB/MgO界面处层间原子混合的增强作用。越接近底部重金属层和衬底，受损伤的程度越大。
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上述研究采用不同能量、不同注量的各种离子对各种结构的MTJ进行了辐照实验和分析，根据实验结果和提出的理论机制可以看到，无论是AlOx还是MgO基的MTJ，对单粒子并不是完全免疫的。重离子的轰击会对膜堆结构产生一定的位移损伤，其电学特性或者磁学性能会发生一定程度的退化，在非常大的总注量或者能量下甚至会失去磁阻效应。某些重离子的轰击也能产生软错误，但机理尚未清晰。为了加强MTJ的抗重离子辐照能力，可以从膜堆结构或者材料上进行优化。
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然而，离子辐照并非只带来坏处，若能掌握其与MTJ中各种材料的相互作用规律，可以尝试利用离子辐照对材料进行可控修饰和改性，从而实现所需的材料特性。快速重离子辐照对材料造成的变化往往是不可逆的，如结构变化、界面变化或通过形成缺陷导致的相的构成变化等[114-115]。高能离子穿过固体时失去能量，要么与原子中电子产生非弹性碰撞，要么与原子核产生弹性散射。前者往往造成与材料特性的变化，后者主要造成原子的位移。这使人们可以研究MTJ界面和势垒变化对磁性能、自旋输运和隧穿行为的影响，以便于更好地制作特殊应用的MTJ。实际上，利用离子辐照对磁薄膜的磁特性进行改性已经得到一些研究[116-126]。例如：Co/Pt体系的磁界面各向异性因离子辐照而受到抑制[125]；FePt（Pd）合金的磁界面各向异性因化学有序而增加[126]；在类似的系统中，将抗蚀剂掩模与He或Ga聚焦离子束[127-128]结合使用，也观察到了纳米尺度上的磁性能变化。这些可能在超高密度存储的垂直磁介质中得到潜在应用。
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2.3 MTJ的质子辐照效应
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质子是银河宇宙射线和太阳宇宙射线的主要组成部分，是空间辐射最重要的辐射来源。银河宇宙射线中质子能量为几MeV到几百GeV，太阳宇宙射线中质子能量为数百MeV。质子入射材料通过卢瑟福散射，将能量传递给材料，引起原子激发、电离或位移。质子在材料中的能量损失具有以下特点：①对于低等和中等能量的质子（100 MeV以下），其能量损失的主要方式是使作用物质中的原子和分子激发或电离，其他方式的能量损失可以忽略不计；②随质子能量的升高，核反应在总能量损失中变得逐渐显著，这时弹性核散射的能量损失很小，可以忽略；③由于粒子减速而造成的轫致辐射能量损失可忽略不计。因此，分析质子对MTJ的影响主要从原子的激发、电离、位移三个方面着手。
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Oldham等[129]对Freescale的MR2A16A进行研究，器件经受能量为89 MeV和189 MeV、注量为2×1010~1×1011 ions/cm2的质子辐照。结果表明，当器件处于静态或读/写状态时，辐照均未造成内部存储信息错误，处于读/写状态时也未造成读/写过程错误。这说明AlOx-MTJ和外围CMOS逻辑均未受到显著影响。Zhang等[130]对Everspin的16 Mbit Toggle-MRAM进行了质子辐照试验，质子能量为3 MeV。结果表明，当质子注量低于1×1011 ions/cm2时，3颗MRAM都能正常工作，电参数未观察到明显漂移；当质子注量达到2.5×1011 ions/cm2时，观察到读取错误，电特性参数超出正常规格。退火一定时间后，MRAM性能有了很大的恢复。该作者认为芯片电特性参数超标的原因是质子作用于MOSFET栅极，诱导了显著的捕获电荷，最终导致了TID效应。
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Hughes等[88]单独对MgO-iMTJ进行了质子辐照研究，结果表明，在总注量为1×1011 ions/cm2、5×1011 ions/cm2、1×1012 ions/cm2和能量为2 MeV、220 MeV的质子辐照下MTJ膜堆的磁性、Hc、有效磁化强度、自由层阻尼系数、RA和TMR值均未发生变化。Park等[110]也对MgO-iMTJ进行了质子辐照研究，结果表明，在经历能量为50 keV、100 keV、200 keV、20 MeV和注量为1×1012 ions/cm2、1×1013 ions/cm2、1×1014 ions/cm2的质子束照射后，其磁滞回线、FL和RL的磁矩、RL的Hc、TMR值均未发生统计学意义上的变化，表明质子束引起的位移损伤可忽略不计。
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综上，质子辐照对AlOx-MTJ和MgO-iMTJ单元影响并不明显，MTJ参数均未发现统计学意义上的变化。
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2.4 MTJ的中子辐照效应
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中子是空间粒子辐射的一个重要因素。大气中的反照中子、高能粒子与航天器物质作用产生的二次中子，是航天器内部中子的两个主要来源。由于中子不带电，必须靠近原子核10-15 m时才会与原子核发生相互作用，因此中子在材料中的穿透能力很强。中子与原子核碰撞时将能量传递给原子核，产生带电的次级粒子引起电离作用。中子与物质的相互作用分为散射和吸收两大类。散射包括弹性散射和非弹性散射，吸收包括辐射俘获、放出带电粒子和核裂变等。在中等和高等能量下，中子的强子相互作用与质子的情况类似。然而，在低能量情况下非常不同。低能质子在布拉格峰（Bragg peak）处迅速失去能量，并且在接近原子核时会受到强的库伦势垒作用；而低能中子在热能的情况下（动能Ekin≤20 MeV）依然可以发生复杂的强子相互作用。
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Hirose等[131]认为，理论上中子可以通过以下三个过程与MTJ相互作用：①单步过程：中子可能直接撞击MTJ并对其造成影响。②两步过程：中子击中MTJ周围的材料，诱导核反应，产生二次离子，二次离子可能撞击MTJ。③三步过程：中子击中晶体管周围的材料，产生二次离子，继而产生许多电子和空穴；产生的噪声载流子聚集在漏端，产生电流，从而影响MTJ。基于上述的第二个过程，通过理论预测高能中子可能导致pMTJ的软错误，但预期风险小于1×10-6 FIT/pMTJ，而热中子（能量远小于1 MeV）几乎没有风险。Ren等[87]对MgO-iMTJ进行的中子效应研究结果表明，MTJ对超热中子（中子总注量2.9×1015 ions/cm2，注量率5×1010 ions/（cm2·s-1），能量0.1 eV~10 MeV）不敏感，试验前后TMR和Hc均未发生变化。而该注量可导致SiO2和高k半导体材料的不可逆位移损伤[132]。Narita等[133]研究了快中子辐照对MgO-pMTJ的影响，结果表明未施加偏置电压的MTJ经过能量为1 MeV和注量为3.79×1012 ions/cm2的中子辐照后，电阻-磁场曲线、RP和RAP、TMR值均未发生变化。Montoya等[90]研究了总注量为2.8×1015 ions/cm2的中子辐照对MgO-pMTJ的影响，试验结果表明辐照后MTJ的TMR值、临界翻转磁场和临界翻转电流均无统计学意义上的变化。
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综上，从试验结果看，中子对MgO-iMTJ和MgO-pMTJ的参数并未产生统计学意义上的影响。
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2.5 MTJ器件加固技术
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研究表明，小尺寸MgO-pMTJ的抗γ辐照能力至少可达1 Mrad（Si）以上，已远远满足航天工程应用要求（100 krad（Si））。质子和中子对MTJ器件参数并未产生明显的改变。
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AlOx-MTJ抗总剂量的能力可能与MTJ的结构、材料及参数有关。因此，良好TID抗性的获得需要对MTJ的结构、材料及参数进行特殊设计。FL采用（Co/Ni）n多层薄膜可能会获得比CoFe薄膜更好的抗TID能力。
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多项实验表明重离子辐照会对磁性薄膜产生一定的损伤，造成器件性能的退化。目前尚未有MTJ器件单粒子加固方法研究的相关报道，通过各类失效模型的分析和研究，本文提出以下可能的加固方法：①MTJ周围沉积一层特殊介质作为保护层，阻止高能离子对MTJ的侧壁甚至薄膜内部进行破坏；②MTJ顶层沉积特殊材料作为保护层，有效降低高能离子的穿透性；③优化磁性薄膜结构，在合适的位置引入阻挡层，既有效阻止高能离子的迁移，又不影响MTJ器件参数。MTJ器件的加固技术研究目前还处于初始阶段，需要更多的研究机构去发掘和探索有效且实际的加固方法。
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3 MRAM中CMOS逻辑的辐照效应和抗辐照技术
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文献[80]详细统计并分析了Everspin（Freescale）、Aeroflex和Honeywell三家公司的不同型号MRAM产品的TID和SEE试验结果。试验结果表明影响产品抗TID和SEE能力的短板来源于未加固的CMOS逻辑，而非MTJ单元。经采用外延片、SOI工艺等加固手段后，MRAM产品抗辐照能力显著提升[17，31，33-35，129，134-147]。
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因此空间应用的MRAM除了MTJ单元需要对辐照具有较强的抗性以外，其外围的CMOS逻辑同样需要具备较强的抗辐照能力。针对CMOS逻辑的电离总剂量效应和单粒子闩锁（single event latchup，SEL）加固方法已有很多研究成果，也同样适用于MRAM的加固。本节针对MRAM电路的逻辑特点，分析CMOS逻辑的辐照效应和易发生单粒子翻转的敏感节点和行为，并简要概括近年来抗单粒子设计加固方法的研究情况。
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3.1 CMOS逻辑的辐照效应
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CMOS逻辑的辐照效应主要包括总剂量效应和单粒子效应。
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总剂量效应的主要辐射来源有γ射线、电子、X射线等。对CMOS逻辑的主要影响为：随着总剂量的累积，MOSFET阈值电压漂移和漏电逐渐增大，CMOS逻辑性能降低，严重的直接丧失功能。引起的效应是半永久性或永久性的，适当退火（加电场或加热）可部分恢复，完全恢复相当困难[78]。TID对CMOS器件的影响随着工艺尺寸的缩小而降低，当工艺节点降到65 nm及以下时，其对CMOS的影响已经很小，几乎可以忽略[148]。
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单粒子效应主要来源为高能质子、重离子等。对CMOS逻辑的主要影响为：单粒子辐照MOS器件中的反向偏置P-N结时产生电子-空穴对，电子-空穴对的数量取决于线性能量转移（linear energy transfer，LET）、粒子的能量和轨迹、硅衬底（结构、掺杂）和局域电场。例如，一个LET为1 MeV·cm2/mg的粒子可以沉积约10 fC/μm轨道长度的电荷[149-150]。空穴或电子被晶体管中的电场驱动产生电流脉冲，造成相应节点的电压上升或下降，从而产生单粒子瞬态[151]（single event transient，SET）。当这种SET发生在锁存结构且辐照产生的电荷总量超过临界值，将引起锁存结构中数据发生变化，即产生单粒子翻转（single event upset，SEU）。随着工艺节点的降低，单粒子多位翻转（single event multiple upset，SEMU）正成为新兴纳米CMOS技术中高能粒子碰撞的主要效应。SEMU是指粒子撞击芯片并影响多个敏感节点，甚至在SEU加固的芯片中也会发生多位翻转的现象[152-153]。
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式中：为最大注入电流，Qinj为最大注入电荷，τ1为收集时间常数（典型值为150 ps），τ2为粒子径迹确立时间常数（典型值为50 ps）。图6为不同电荷量情况下瞬态电流脉冲形状。
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3.2 MRAM单粒子敏感模块分析
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图7和图8分别为并行接口和串行接口MRAM的典型逻辑设计架构。在并行接口MRAM设计中，主要逻辑模块包括控制信号（）缓冲模块、地址译码模块、存储阵列、数据写入缓冲及写入模块、灵敏放大器及输出驱动模块。除此之外，有的MRAM设计可能出于时序设计等原因在数据输入和输出通路中增加了数据寄存器。
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图6 不同电荷量情况下瞬态电流脉冲形状[157]
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Fig.6 Pulse shape of transient current under different charge quantities[157]
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图7 并行接口MRAM逻辑设计架构
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Fig.7 Typical logic design architecture of parallel interface MRAM
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图8 串行接口MRAM典型逻辑设计架构
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Fig.8 Typical logic design architecture of serial interface MRAM
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相比于并行接口MRAM，串行接口MRAM中存储阵列的结构未发生变化，数据写入和读出存储单元的方式也未发生变化。其主要区别是将地址输入方式改为了时钟计数方式，因此增加了地址计数器（易失）；数据的输入和输出仅靠SI和SO两个端口实现，因此需要数据寄存器（易失）实现数据串-并和并-串转换功能；另外针对电路的状态控制增加了指令寄存器（易失）和状态寄存器（非易失）。
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在并行接口和串行接口MRAM的逻辑架构中，可能发生单粒子翻转的模块或单元有：灵敏放大器（一般含交叉放大锁存结构）、寄存器和MTJ单元。其余电路模块均为组合逻辑，无锁存结构，不会发生数据翻转，但会产生SET。针对锁存结构，需预防与时钟沿同时刻的SET噪声信号被寄存器收集而转变为SEU，此时可考虑采用C单元[158]或延迟锁存[150，159]的方法解决。另外，时钟树也是SET敏感结构，时钟上的SET会引发寄存器数据更新，若输入端口数据与原存储数据不同，则引发错误，需对时钟树进行SET加固[160-162]。
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3.3 读写电路单粒子敏感点分析及其加固措施
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读写电路是MRAM核心模块，也是单粒子加固的重点模块。当读写模块受到单粒子影响时，可能引发读写故障，甚至改变MTJ磁化状态，造成非易失性数据错误。读故障通常包括锁存结构翻转故障、判读故障和干扰故障。如果单粒子轰击读出放大器中锁存结构，可能引起翻转故障；若引发读取或参考支路中电压波动，可能发生判读故障；若通过MTJ的读电流大于其临界翻转电流，就会产生读干扰，导致MTJ翻转[163-164]。对于设计良好的感应放大器电路，辐射引起的读取干扰几乎不会发生。另外，在写入过程中，当MTJ翻转受到干扰，导致翻转延迟超过施加的持续时间时，可能会导致写入失败[165]。
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3.3.1 读电路
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图9（a）为未进行辐照加固设计的典型的灵敏放大器结构——预充电式灵敏放大器[166]（precharge sensitive amplifier，PCSA）。当该结构处于读取模式时，单粒子若轰击MOS管P1~P4、N1和N2，均有可能引发敏感节点Q和QB电位上升或下降，一旦电位超过反相器的开关阈值，就会造成电平翻转，从而发生单粒子翻转效应。同时，该结构与其他基于锁存结构的灵敏放大器一样，存在自锁的风险。当重离子在预充电信号Pre上升沿前瞬间轰击敏感节点（例如N5或N6）时，有可能引起Q或QB的电位值无法预充到电源电压Vdd，锁存器可能被锁定在未知状态。另外，在预充或读取过程中，若N6（或N5）漏极遭受单粒子轰击，则会引发N6漏极到地SET电流，若此时B点电位较高且辐照引发的电子-空穴对数量足够多，SET电流将超过MTJ的写电流阈值，造成MTJ数据改写。此外，在读取过程中，N5或N6管遭受轰击还可能引发A和B两点电压紊乱，导致放大器误判，读出错误结果。
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在STT-MRAM设计中，为了防止读干扰问题（非辐照环境也存在），限制MTJ两端电压，人们加入了一对钳位NMOS（N3和N4），并设置了钳位电压Vclamp[167]（见图9（b）），问题得到了有效解决。在辐照环境下，这对钳位NMOS仍然能有效发挥抗干扰作用，当N5或N6遭受辐照时，虽然会产生电子-空穴对，但由于A和B点电压被钳位，SET电流将低于MTJ的临界翻转电流，无法引发MTJ状态翻转。
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图9 预充电式灵敏放大器
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Fig.9 Precharge sensitive amplifier
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读取电路的抗辐照加固主要围绕上述问题展开。研究人员从机理、仿真、逻辑设计优化等方面进行了探索和研究。为了分析单粒子对STT-MRAM造成的影响， Yang等[165]提出了一种综合的辐射诱导软错误的分析框架，通过跨层建模和仿真准确捕捉单粒子辐照对MTJ、读写电路和存储阵列的影响。Sakimura[163]和Wakimura[164]等针对STT-MRAM中SET电流引发MTJ翻转的概率分别进行了仿真分析。前者仿真结果表明当一个LET为14 MeV·cm2/mg的中子辐照芯片时，SET电流引发临界翻转电流为30 μA的MTJ产生状态翻转的概率大于10-12；后者仿真结果如图10所示，对于相同的临界翻转电流和LET，若粒子垂直辐照NMOS管漏极中心位置，则MTJ翻转概率大约为10-20。虽然可以通过增加MTJ临界翻转电流的方式来降低其受SET电流的影响，但会增加写入功耗和时间。Chabi等[168]针对这一问题提出了一种新型的抗辐照灵敏放大器设计，通过较小的面积和适度的性能牺牲显著提升了MTJ单元的抗辐照翻转能力。Wang等[169-171]针对SOT-MRAM读取放大器中锁存结构易受辐照而发生翻转的问题，提出了一种抗SEU以及两种同时抗SEU和双节点翻转（double-node LET upset，DNU）的灵敏放大器结构（其中一种结构如图11所示）。三种结构均采用了DICE双互锁原理[172]作为理论基础，利用特殊的逻辑结构防止电压波动扩散到其他敏感节点，利用反馈对被轰击节点进行充放电，在数百皮秒内快速恢复初始状态。
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图10 MTJ翻转概率与Ic0及LET的关系[164]
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Fig.10 MTJ switching probability as a function of Ic0 and LET[164]
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3.3.2 写电路
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图12为MRAM典型写逻辑原理图，STT与SOT-MRAM可采用相同的写驱动逻辑。WEN为写使能信号（低电平有效），Data为写入数据，BL为位线，SL为源线，WL1、WWL和RWL为选择管控制信号。写“0”时，P1、N3（或N4）、N2管打开，N1和P2关闭；写“1”时，P2、N3（或N4）、N1打开，P1和N2关闭。静态或者读取时WEN为高电平，P1和N1均关闭，输出为高阻态；同时P2关闭，N2开启，输出低电平。读取状态时N3（或N5）打开，N4关闭，N2为读取电流提供对地通路。该电路中无锁存结构，因此仅需考虑SET的影响：
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图11 SOT-MRAM的读/写电路
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Fig.11 Read/write circuit for SOT-MRAM
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1）SET可能直接改写MTJ数据。若器件处于读取状态，N3（或N5）和N2开启，此时若P1管漏极被单粒子轰击则形成从P1衬底到漏极的导电通路，或P1栅极遭受SET脉冲直接导致P1管开启，两者均会引发SET电流按照写“0”的方向流动。若原数据为“1”，当SET电流大小及持续时间超过MTJ的临界翻转电流和临界翻转时间（图13为STT-MTJ翻转电流与写入脉冲的关系），则MTJ将发生翻转，SET转化为SEU。另外，若器件处于写入状态，Data信号中若含SET脉冲，则造成写电流反向，MTJ可能被写入相反数据，若恰处于写脉冲末期，则直接造成写入错误。
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2）SET可能影响写操作，造成写入时间增加甚至写入失败。MTJ的磁化翻转是一个累积场效应的结果，STT/SOT电流产生的扭矩在驱动磁化到另一个稳态的过程中起着重要的作用。这个过程的中断或干扰将延迟MTJ翻转时间（图14展示了粒子轰击对STT-MTJ翻转的影响）。若器件处于写入状态，例如写“0”，则单粒子轰击N1或P2漏极，均可造成写电流在短时间内降低甚至直接降为0；P1、N1、P2、N2、N3、N4栅极也可能遭受SET电压脉冲，造成写电流短暂时间内降低或停止；另外，WEN信号若含SET，也会导致写操作短暂停止，这些情况均会延长写入时间，若恰逢写周期的末尾，则可能导致写入失败。此外若Data信号中含SET脉冲，则写电流反向，MTJ可能被写入相反数据，若恰处于写周期末期，则直接造成写入错误；若处于其他时间段，则会增加写入时间（但不会影响写入结果）。如果粒子轰击发生在MTJ翻转的末端或之后，则原始的MTJ翻转时间不会受到影响。对于这类现象可通过增加MTJ的临界翻转电流或写入脉冲宽度来解决，但会增加写入功耗和降低写入速度。
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图13 STT-MTJ翻转电流与写入脉冲的关系
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Fig.13 Relationship between STT-MTJ switching current and write pulse
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图14 粒子轰击对STT-MTJ翻转的影响[165]
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Fig.14 Effect of particle bombardment on STT-MTJ switching[165]
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Wang等[169-171]认为STT-MTJ受SET的影响较小，因为多数情况下SET脉冲的宽度远远小于写脉冲的宽度，不足以引起STT-MTJ的翻转。然而，对于SOT-MTJ，其翻转速度很快，最快可达亚纳秒级[179-181]，更容易受到SET的影响。因此该团队针对SOT-MRAM，提出了两种抗单粒子加固的写电路结构，有效提升了抗单粒子性能。
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3.4 非易失锁存器加固技术
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MTJ技术的出现和发展使得非易失性锁存器有了更新的实现方式。人们不断开发出利用MTJ实现非易失锁存的逻辑结构[182-183]，同时也在努力赋予这些锁存结构良好的抗单粒子辐照能力。
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2012年， Lakys等[157]基于典型的PCSA提出一种新型抗辐照锁存器结构，如图15所示。该结构主要由两个PCSA及将两者紧密结合起来的组合逻辑组成。当重离子轰击敏感节点使其中一个PCSA发生SEU时，两个PCSA输出结果不同，组合逻辑检测到差异并使读取电路重新进行预充及读取操作，从而将结果纠正。该结构也能避免自锁的情况，即使重离子在“CLK”上升沿到来之前的瞬间轰击敏感节点，造成其中一个PCSA预充失败并输出结果错误，组合逻辑也会使电路进入重置状态，重新预充并读出正确结果。该结构由于具有两对相同的互补输出（Q0/QB0和Q1/QB1），其行为与传统的DICE型锁存器相同。例如，它可以与一个DICE类型的从锁存器相关联，以实现抗辐照主从触发器，并具有额外的非易失性特征。但这种结构缺点也很明显：它只能应对单个粒子对单节点带来的影响，无法应对多节点翻转（multiple node upset，MNU）；同时，该结构使得MTJ结的数量从两个增加到四个，用到了较多的MOS管，不仅需要翻倍的写入与读出功耗，还需要更大的面积。2014年，Zhang等[184]基于C单元和PCSA提出一种抗辐照锁存结构，如图16所示。此结构也具备与Lakys′s锁存结构类似的优缺点，可有效抵抗SEU，但功耗高、面积大，也无法应对MNU。每个数据节点，即每个C单元的输出，是由其他两个具有相同逻辑值的节点决定的。例如，节点N1的逻辑值依赖于节点N2和节点N4的逻辑值，若N2与N4同时发生SEU，节点N1的值将无法纠正，产生错误结果。
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图15 Lakys′s锁存结构[157]
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Fig.15 Lakys′s latch structure[157]
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Kang等[185]提出了一种可以应对双节点翻转（double node upset，DNU）的锁存结构，如图17所示。相较于图15、图16这两种结构，该结构所需MTJ数量更少，所用CMOS更少，在成本、功耗、集成度方面有着更好的表现。
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图16 Zhang′s锁存结构[184]
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Fig.16 Zhang′s latch structure[184]
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图17 Kang′s锁存结构[185]
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Fig.17 Kang′s latch structure[185]
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4 总结与展望
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商用MRAM未进行辐照加固设计，因此难以直接胜任航天任务。MRAM应用于航天器，在抗辐照方面需要具备两个条件：一是MTJ单元本身具备一定的天然抗辐照能力；二是外围CMOS逻辑需采取针对性的抗辐照加固措施，达到一定的抗辐照指标。
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以目前研究来看，对MTJ特性影响较大的粒子主要是重离子和γ光子，质子和中子未见统计学意义上的影响。重离子和γ光子影响MTJ的主要方式或效应主要有：①辐照粒子与MTJ材料（尤其是磁性材料）中电子发生相互作用，影响电子的自旋，进而影响各膜层的磁性；②重离子与MTJ材料原子核发生弹性相互作用，带来位移损伤，破坏原膜层结构和界面完整性，使MTJ电学性能和磁性能发生变化（这方面γ光子造成影响的概率较低，可忽略不计）；③粒子辐照MTJ后带来热效应，使MTJ内部产生热应力，从而改变MTJ膜层和界面的物理性状，带来永久损伤，也可能引起两个状态之间的势垒降低，带来软错误。这些效应的物理作用机理目前尚未清晰。如何通过优化MTJ的结构、材料和参数来进一步提升MTJ的抗辐照性能仍是一个需要研究的课题。
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CMOS逻辑加固方面：对于总剂量效应和单粒子闩锁，可采用一些通用的加固方法，这方面已有大量的研究成果可借鉴；对于单粒子翻转和单粒子瞬态加固，应重点对数据写入、读出结构以及数据/状态寄存器进行加固，这方面虽然有许多文献报道，但多基于仿真，实际抗单粒子效果仍需结合所用工艺进行试验证实。
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抗辐照MRAM在航空航天领域甚至核能领域将具有广阔的应用前景。随着Everspin、TSMC、三星、Global Foundries等国际半导体领军企业在MRAM上的研发投入不断增加，MRAM工艺技术将不断提高，集成度和可靠性将进一步提升，成本也会逐步降低。同时随着人们对MTJ辐照效应和机理以及对CMOS逻辑辐照加固技术的进一步探索，MRAM的抗辐照能力也将越来越强。抗辐照MRAM将朝着更强的抗辐照能力、更高的集成度（STT或其他技术）、更快的速度（SOT或其他技术）、更高的可靠性（SOT或其他技术）、更强的抗外界磁场干扰能力（EB-MRAM或其他技术）方向发展。从抗辐照MRAM的应用场景角度看，随着技术的不断进步，MRAM将首先取代读写速度较慢、容量较小的EEPROM（容量1 Mbit以内）；然后取代读写速度较慢且对辐照较为敏感的NOR FLASH（容量4 Mbit~1 Gbit）；进而取代数据易失且功耗较大的外部缓存DRAM（容量4 Gbit及以上）；若擦写次数达到1016，写延迟达到1 ns以内，单元面积小于SRAM，则将有望作为一级缓存取代SRAM。
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摘要
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。
Abstract
The novel non-volatile MRAM(magnetic random access memory) has the advantages of fast read and write speed, long data retention time and low power consumption, which attracts wide attention from researchers. Its excellent anti-irradiation capabilities are explored in depth, and further applications in aerospace and other fields are expected. The industrial development, technological changes and applications of MRAM were reviewed, the mature MRAM products of recent years were listed, and the advantages and disadvantages of different generations of MRAM were analyzed. The radiation effects of MTJ(magnetic tunnel junction) and read/write circuit based CMOS(complementary metal oxide semiconductor) were discussed. The recent achievements in anti-radiative hardening design for MRAM were summarized. The development prospect of anti-irradiation MRAM in aerospace field and even nuclear energy field was prospected.
图1 磁隧道结基本结构
Fig.1 Basic structure of magnetic tunneling junction
表1 MRAM产业化发展历程——主要研究机构及代表产品
Tab.1 MRAM industrialization development process—main research institutions and representative products
图2 MTJ技术类型
Fig.2 MTJ technology types
表2 不同代际MRAM产品性能比较
Tab.2 Performance comparison of MRAM products in different generations
图4 双界面pMTJ多重膜结构和样品的表面形貌
Fig.4 Structure of the double-interface pMTJ multifilms and surface appearance of samples
图5 电阻随外磁场的变化关系（+5 mV偏置，室温）[103]
Fig.5 Plot of resistance versus applied magnetic field at RT at a bias of +5 mV[103]
图6 不同电荷量情况下瞬态电流脉冲形状[157]
Fig.6 Pulse shape of transient current under different charge quantities[157]
图7 并行接口MRAM逻辑设计架构
Fig.7 Typical logic design architecture of parallel interface MRAM
图8 串行接口MRAM典型逻辑设计架构
Fig.8 Typical logic design architecture of serial interface MRAM
图9 预充电式灵敏放大器
Fig.9 Precharge sensitive amplifier
图11 SOT-MRAM的读/写电路
Fig.11 Read/write circuit for SOT-MRAM
图13 STT-MTJ翻转电流与写入脉冲的关系
Fig.13 Relationship between STT-MTJ switching current and write pulse
基本信息
中图分类号: TN4
文献标识码: A
DOI: 10.11887/j.cn.202306020
文章编号: 1001-2486(2023)06-174-22
基金信息
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稿件历史
收稿日期: 2022-03-14
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