High-density magnetoresistive random access memory operating at ultralow voltage atroom temperature

A novel process for highly manufacturable MRAM

Magnetoresistive random access memory (MRAM) is one of several candidates for low-power and high-density system-on-chip (SoC) memory. In this paper, we propose a novel magnetic tunneling junction (MTJ) shape, called the “sandglass” (or “hourglass”), that has both an excellent asteroid curve for preventing write disturbance and a minimum memory of 8F2, where F is the feature size. A unique magnetic switching mechanism that works under decreasing magnetic switching field was compared with that of conventional MTJs simulated using the Landau–Lifshitz–Gilbert (LLG) simulator. We fabricated both a sandglass MTJ of 260×420 nm2 and a conventional elliptic MTJ of 200×400 nm2. The excellent asteroid curve of the sandglass MTJ is confirmed which exhibits both a larger write operation margin and a 50% lower switching field than those of the conventional elliptic MTJ. The sandglass MTJ cell is a promising candidate for realizing high-density MRAM.

Progress in developing operative high-density magnetoresistive random access memory (MRAM) devices relies critically on tailoring the magnetic switching occurring in arrays of small patterned pseudo spin valve (PSV) elements. Co/Cu/NiFe PSV films, produced by sputtering in the presence of a magnetic field, have an in-plane anisotropy and switching fields of typically 10 Oe for the soft NiFe and 40 Oe for the hard Co. These films were patterned into arrays of elliptical and circular elements with dimensions of 80 nm to 10 /spl mu/m. The layered structure in these large-area arrays of compositionally modulated PSV elements is preserved through the patterning processes. Hysteresis measurements of the dot arrays show that the switching field of the hard layer increases significantly with decreasing element size, reaching 600 Oe for the smallest elements. Additionally, in patterned elements the soft layer switches prior to field reversal due to magnetostatic coupling between the layers, leading to antiparallel alignment at remanence.

As the pattern size of magnetic tunnel junctions (MTJs) becomes smaller, the vortex of magnetization and fluctuations of switching fields caused by the shape MTJ cells will cause serious writing problems. However, a MTJ structure with perpendicular anisotropy (pMTJ) has shown low saturation magnetization and shape independence during the writing process. In this study, we considered Co∕Pd multilayers that allow better tailor-design of magnetization and anisotropy. A series of calculations based on the Landau–Lifschitz–Gilbert equation were carried out on pMTJs to investigate the effect of size variations and material properties on their transfer behaviors. In a same submicrometer cell size, pMTJ shows better properties for high-density magnetoresistive random access memory (MRAM) against synthetic antiferromagnet MTJs. As the number of bilayer Co∕Pd,n, becomes larger, the coercivity and squareness were enhanced because of shape anisotropy and large effective Ku. Even with a small field, 10 Oe, along the hard axis, the drastic decrease, nearly about 80% of switching field, occurs. As a micromagnetic result, a pMTJ with a cell size of 200 nm and n=3 shows an abrupt increase in coercivity and better squareness.

20世纪80年代末期自旋电子学研究的兴起开启了现代磁存储技术发展的新纪元.1988年, 巨磁电阻 (giant magnetoresistance, GMR) 效 应 被 发 现 [2,3] .以 GMR磁电阻元件作为磁存储单元, 形成了早期GMR 效应随机存取存储器的研发.1995年, 隧道磁电阻 (tunneling magnetoresistance, TMR)效应 [4,5] 的发现掀 起了磁性隧道结(MTJ)器件的研发热潮.

차세대 메모리 소자 중 MRAM(Magnetic Random Access Memory)은 자기저항효과를 이용한 비휘발성 메모리로 기존 메모리를 대체할 것으로 주목을 받고 있다. MRAM은 자기터널 접합 소자를 이용해 구동할 수 있는데, 현재 고집적, 저 전력 소모 등의 장점을 극대화하기 위해 수직자화 특성을 갖는 자기터널 접합 개발과 스핀전달토크를 이용해 구동하는 STT-MRAM(Spin Transfer Torque-MRAM) 개발이 활발히 이루어지고 있다. 따라서 미국, 일본 등 MRAM 강국에서 고집적, 스위칭 전류 감소, 열적 안정성 등의 문제를 해결하기 위한 기술 특허 출원이 증가하고 있으며, 국내의 MRAM 연구기관에서의 특허 출원도 꾸준히 이루어지고 있다. 본고에서는 기존 국내외 특허 출원 및 등록 경향을 분석하고 향후 MRAM 개발방향을 제시하였다. Among the next generation memory, MRAM (Magnetic Random Access Memory) is worthy of notice for substituting the preexisting memory thanks to its non-volatile property and other advantages. Recently perpendicular MRAM and spin transfer torque MRAM techniques are under active investigation to realize a high density and low power consumption. As a result, there are increasing of patents applications for high density, low current density for magnetization switching and high thermal stability. In this paper, we analyze the trend of patent applications and registrations about MRAM and propose a direction of future investigation.

<sec><p indent="0mm">Magnetic random-access memory (MRAM) has emerged as a leading candidate for next-generation storage technology, offering high speed, high density, non-volatility, low power consumption, and radiation resistance. MRAM has evolved through three generations: field-driven toggle MRAM, spin-transfer torque (STT)-MRAM, and the experimental spin-orbit torque (SOT)-MRAM. The current commercial MRAM products primarily rely on STT-MRAM, which is based on Ta/CoFeB/MgO multilayers with the interfacial perpendicular magnetic anisotropy (PMA). However, STT-MRAM performance deteriorates at sub<sc>-10 nm</sc> size due to thermal instability. In contrast, the tetragonal Mn-based binary alloys exhibit stable bulk PMA and enhanced properties for sub<sc>-10 nm</sc> tunnel junction devices. Previous research has demonstrated the potential of these alloys for high-density MRAM, with magnetization switching achieved via SOT without the need for external magnetic fields. Among Mn-based binary alloys, <italic>L</italic>1₀-MnAl stands out due to its strong PMA, high spin polarization, and low magnetic damping, making it a promising material for next-generation MRAM. On the other side, the <italic>B</italic>2-CoGa alloy serves as both an ideal buffer layer and a spin current source for the epitaxial growth of PMA <italic>L</italic>1₀-MnAl thin films on GaAs substrates. Our study demonstrates that doping <italic>B</italic>2-CoGa with Pt significantly enhances the SOT efficiency in (CoGa)_{1−<italic>x</italic>}Pt<italic>_x</italic>/<italic>L</italic>1₀-MnAl bilayers while preserving the excellent PMA properties of <italic>L</italic>1₀-MnAl. </sec><sec> Experimental results show that Pt doping up to <italic>x</italic> = 0.1 keeps the PMA characteristics while simultaneously reducing the critical current density required for magnetization switching from <sc>4.63×10⁷ A/cm²</sc> to <sc>2.59×10⁷ A/cm².</sc> SOT efficiency increases from 0.042 to 0.080. This enhancement is attributed to the increased resistivity of the (CoGa)_{1−<italic>x</italic>}Pt<italic>_x</italic> layer, which does not compromise its effective spin Hall conductivity. The optimized bilayer, with <italic>x</italic> = 0.075, supports magnetic tunnel junctions with lateral size as small as <sc>8.09 nm,</sc> reducing power consumption by 37%. The SOT effect utilizes spin currents generated by phenomena such as the spin Hall effect to switch local magnetization in adjacent ferromagnetic layers. The damping-like SOT efficiency (<italic>ξ</italic>_DL), a crucial parameter for reducing device power consumption, is influenced by both spin Hall conductivity and resistivity of the material. The introduction of Pt impurities into (CoGa)_{1−<italic>x</italic>}Pt<italic>_x</italic> bilayers optimizes this balance, significantly enhancing <italic>ξ</italic>_DL. Compared to traditional heavy metals used as buffer layers, the (CoGa)_{1−<italic>x</italic>}Pt<italic>_x</italic> layer provides better lattice matching with Mn-based binary alloys, preserving their crystalline quality and PMA. This work systematically investigates the structural, magnetic, and SOT-driven magnetization switching properties of epitaxially grown (CoGa)_{1−<italic>x</italic>}Pt<italic>_x</italic>/<italic>L</italic>1₀-MnAl bilayers, focusing on the dependence of magnetization switching on Pt concentration. Deterministic SOT-induced switching is achieved for <italic>x</italic> ≤ 0.1, with optimal PMA and SOT performance observed at <italic>x</italic> = 0.075. These results demonstrate the feasibility of incorporating Pt-doped CoGa layers into MRAM designs to enhance performance and scalability. This study provides valuable insights into improving SOT efficiency in Mn-based MRAM structures by incorporating Pt-doped CoGa buffer layers. These findings contribute to the development of high-density, high-performance MRAM devices with reduced power consumption. </sec>

Conducting electrons passing through a nanostructured ferromagnetic metal excite a torque on the magnetization by the exchange interaction between their spins and the magnetization. In 1996, it was theoretically proposed that this torque, called spin-transfer torque, or simply spin torque, can switch the magnetization direction in the ferromagnet when the current density reaches a sufficiently large but experimentally available value [1]. The experimental observation of the spin torque switching was first reported in Co/Cu metallic multilayer system in 2000 [2]. These results significantly advanced the research development of magnetic random access memory (MRAM), which is one of the promising candidates for future non-volatile memory. A preferable feature of the spin torque switching for practical application is that the current density for the switching decreases with decreasing the memory size, leading the low-power consumption in the writing. Recently, it was experimentally shown that the switching current density can be reduced to on the order of 106 A/cm2 by keeping a high thermal stability larger than 60 in FeB/MgO-based magnetic tunnel junctions [3]. The research on MRAM has required not only the technical development but also advances in fundamental physics. An important problem for realization of MRAM is the accurate evaluation of the thermal stability. The thermal stability in MRAM is defined as the uniaxial anisotropy energy of the ferromagnet divided by the temperature. A high thermal stability is necessary to keep retention time of MRAM longer than ten years. Experimentally, the thermal stability has been evaluated by measuring the dependence of the switching probability on the current magnitude. Here, the current magnitude is sufficiently small, and thus, the thermal fluctuation plays a key role to switch the magnetization, making the switching probabilistic. The value of the thermal stability was obtained by fitting the switching probability with the Arrhenius formula [4], which has been a conventional and traditional method to evaluate thermal stability in several physical, chemical, and biological systems. The spin torque switching current at zero-temperature, which determines the power of the writing in MRAM, was also determined from the switching probability. However, recently, we pointed out problems in the analysis. The Arrhenius law assumes that the force acting on a Brownian particle is conservative, i.e., the force is a gradient of a potential. On the other hand, the spin torque is a non-conservative force. Therefore, it is unclear whether the evaluation of the thermal stability in MRAM with the Arrhenius law provides the accurate evaluation. Also, nonlinearity of the magnetization dynamics has been neglected in the previous work by focusing on only a small fluctuation near the equilibrium. In my study, we show recent our progress on theoretical study of magnetization dynamics in MRAM in the presence of both spin torque and thermal fluctuation [5]. We noticed that, in the thermally activated region, the magnetization precesses almost on the constant energy curve many times during the switching. This fact allows us to average the equation of motion, called the Landau-Lifshitz-Gilbert (LLG) equation, on the constant energy curve. Deriving the Fokker-Planck equation from the LLG equation, we develop the general theory of the probabilistic switching by the spin torque. We also quantitatively calculate the spin torque switching rate by using both analytical and numerical methods, which enables us to take into account the nonlinearity of the magnetization dynamics. We found that, the switching probability can be written in the form of Arrhenius formula. However, its scaling with respect to the current is unconventional because of the non-conservative nature of the spin torque. This results indicate that both the thermal stability and the switching current at zero temperature had been underestimated in the previous work. The former is preferable for MRAM study because it guarantees a high thermal stability in the currently succeeded structures. On the other hand, the latter will be a problem from the view point of the low-power consumption. In this talk, we will show the introduction, the points of the problems, and the details of our theory. We will also discuss about future direction of MRAM study.

Key attributes of MRAM (magnetoresistive random access memory) technology are known as non-volatility with high speed and density, radiation hardness, and unlimited endurance. A lot of results have been announced for commercial market. It is anticipated that MRAM would play an important role in future memory market through its unique, functional advantages. For high density MRAM as a standalone memory, several technological issues related with MRAM core cells should be preferentially solved, we demonstrated 1 Kbit MRAM array fabricated by combination of hybrid technology of standard deep sub-micron CMOS and MTJ process. The main issues in the array, which can be fundamental limitation of MRAM technology, are considered. The topic covers basic issues of deep sub-micron MRAM core cell and consider the work related to the MRAM issues, such as cell stability and switching process.

Spin-transfer torque (STT) based magnetoresistive random-access memory (MRAM) is of great current interest for embedded applications, due to its high speed, high endurance, and long retention time. However, state-of-the-art STT MRAM requires a large switching current that leads to reliability issues, high write energy, and limits the density scaling. In this talk, we will show that a conventional current-controlled MTJ can be changed to a voltage-controlled device if we replace the thin oxide barrier with a resonant tunneling barrier. Such a simple modification will combine two established quantum phenomena: interlayer exchange coupling and resonant tunneling within the scaled magnetic structure to enable a resonant-exchange controlled (REC) magnetization switching [1]. REC-driven magnetization switching can substantially lower the write energy, decouple the energy and speed of operations, eliminate the reliability issues, and enable denser MRAM technologies. A voltage-controlled MTJ has a high baseline resistance, which may cause compatibility issues with standard CMOS. We will discuss a magnetic gain cell structure that consists of a REC MTJ and two transistors for a large separation in the output current for the two memory states while retaining the low power advantages of a voltage-controlled write mechanism [2]. We analyze the REC MTJ using quantum-transport simulations on a CoFeB/MgO/Ru/MgO/CoFeB structure and combine the results with an empirical SPICE model to analyze the magnetic gain cell using 45 nm CMOS models in HSPICE. Our simulation results suggest approximately 103 times change in the read current in the gain cell between the two magnetic states. The expected energy and delay are 29.5 fJ/bit and 1.6 ns respectively for a write operation and 7.6 ~ 39 fJ/bit and 0.6 ~ 1 ns respectively for a read operation, thus promising for low energy, fast, and high-density REC MRAM technology. 

The magnetic random access memory (MRAM) is considered one of the potential candidates that will replace current on-chip memories (RAM, EEPROM, and flash memory) in the future. The MRAM is fast and does not need a high supply voltage for read/write operations, and is compatible with the CMOS technology. It can also endure almost unlimited read/write cycles. These combined advantages of RAM and flash memory make it a potential choice for SOC. In this paper, we present the write disturbance fault (WDF) model for MRAM, i.e., a fault that affects the data stored in the MRAM cells due to excessive magnetic field during the write operation. We also construct the SPICE macro model for the magnetic tunneling junction (MTJ) device of the toggle MRAM to obtain circuit simulation results. We then present an MRAM fault simulator called RAMSES-M, based on which we derive the shortest test for the proposed WDF model. The test is shown to be better and more robust as compared with the conventional March C-test algorithm. We also present a March 17 N diagnosis algorithm for identifying WDF. A 1 Mb MRAM chip has been designed and fabricated using a CMOS-based 0.18-mum technology. The proposed WDF model is justified by chip measurement results, with the march test results reported. Finally, specific MRAM fault behavior and test issues are discussed.

With the advent of system-on-chip (SOC), the demand for embedded memory cores increases rapidly. The magnetic random access memory (MRAM) is considered one of the potential candidates that replace current on-chip memories (RAM, EEPROM, and flash memory) in the future. The MRAM has a high speed and does not need high supply voltage for read/write operations, so it has the advantages of RAM and flash memory, making it a potentially good choice for SOC. The testing of MRAM, however, has not been fully investigated. In this work we classify and analyze the MRAM defects and their behavior, and propose its fault models. We have built a SPICE model of MRAM cell and performed defect injection and simulation of a real MRAM circuit. The circuit has been implemented and fabricated with a novel 0.18 m technology. The simulation results regarding the correlation between the defects and conventional fault models show that most of the defects can be covered by the stuck-at fault model. The test data based on the fabricated chips show that the stuck-at faults do cover most of the defects on the chips. However, from the experiment we also have identified two new faults, i.e., the Multi-Victims fault and Kink fault.

The magnetic random access memory (MRAM) is considered one of the potential candidates that will replace current on-chip memories (RAM, EEPROM, and flash memory) in the future. The MRAM is fast and does not need a high supply voltage for read/write operations. It can also endure almost unlimited read/write cycles. These combined advantages of RAM and flash memory make it a potential choice for SOC. In this paper, we present the write disturbance fault (WDF) model for MRAM, i.e., a fault that affects the data stored in the MRAM cells due to excessive magnetic field during the write operation. The proposed WDF model is justified by chip measurement results. We also construct the SPICE macro model for the magnetic tunneling junction (MTJ) device of the toggle MRAM to obtain circuit simulation results. An MRAM chip has been designed and fabricated using a CMOS-based 0.18mum technology. We also present an MRAM fault simulator called RAMSES-M, based on which we derive the shortest test for the proposed WDF model. The test is shown to be better and more robust as compared with March C. Finally, we present a March 17N diagnosis algorithm for identifying the WDF

Magnetoresistive random access memory (MRAM) is a promising non-volatile memory technology that can be utilized as an energy and space-efficient storage and computing solution, particularly in cache functions within circuits. Although MRAM has achieved mass production, its manufacturing process still remains challenging, resulting in only a few semiconductor companies dominating its production. In this review, we delve into the materials, processes, and devices used in MRAM, focusing on both the widely adopted spin transfer torque MRAM and the next-generation spin-orbit torque MRAM. We provide an overview of their operational mechanisms and manufacturing technologies. Furthermore, we outline the major hurdles faced in MRAM manufacturing and propose potential solutions in detail. Then, the applications of MRAM in artificial intelligent hardware are introduced. Finally, we present an outlook on the future development and applications of MRAM.

The characteristics of a magnetoresistive random access memory (MRAM) and a giant magnetoresistive (GMR) transpinnor, which is an active solid-state device, have been investigated using /spl alpha/-Fe/sub 2/O/sub 3/ bottom GMR spin-valves. Patterned /spl alpha/-Fe/sub 2/O/sub 3/ bottom GMR spin-valve devices exhibited excellent device performance, which is related to the high GMR ratio, high initial device resistivity, and good magnetic properties. The magnetic and electrical device testing results of MRAM and transpinnor suggest that they are potential candidates in high-density memories. The possibility of fabricating both an operating device (transpinnor) for the MRAM and a MRAM memory element on the same substrate offers the potential of reducing the real MRAM cell size.