Magnetoresistive Random Access Memory Devices Research Articles

MRAM Devices to Design Ternary Addressable Physically Unclonable Functions

We introduce a novel approach to constructing ternary addressable physically unclonable functions (TAPUFs) using magnetoresistive random-access memory (MRAM) devices. TAPUFs use three states (1, 0, and X) to track unstable cells. The proposed TAPUF leverages the resistance properties of MRAM cells to produce unique digital fingerprints that can be effectively utilized in cryptographic protocols. We exploit the cell-to-cell variations in resistance values to generate reliable cryptographic keys and true random numbers, which can add protection against certain attacks. To evaluate the performance of the TAPUF, various tests were conducted, including assessments of inter-cell to intra-cell variation, inter-distance, bit error rate (BER), and temperature variation. These experiments were conducted using a low-power client device to replicate practical scenarios. The obtained results demonstrate that the proposed TAPUF exhibits exceptional scalability, energy efficiency, and reliability.

Efficient Spin-Orbit Torque Switching in a Perpendicularly Magnetized Heusler Alloy MnPtGe Single Layer.

Electrically manipulating magnetic moments by spin-orbit torque (SOT) has great potential applications in magnetic memories and logic devices. Although there have been rich SOT studies on magnetic heterostructures, low interfacial thermal stability and high switching current density still remain an issue. Here, highly textured, polycrystalline Heusler alloy MnxPtyGe (MPG) films with various thicknesses are directly deposited onto thermally oxidized silicon wafers. The perpendicular magnetization of the MPG single layer can be reversibly switched by electrical current pulses with a magnitude as low as 4.1 × 1010Am-2, as evidenced by both the electrical transport and the magnetic optical measurements. The switching is shown to arise from inversion symmetry breaking due to the vertical composition gradient of the films after sample annealing. The SOT effective fields of the samples are analyzed systematically. It is found that the SOT efficiency increases with the film thickness, suggesting a robust bulk-like behavior in the single magnetic layer. Furthermore, a memristive characteristic has been observed due to a multidomain switching property in the single-layer MPG device. Additionally, deterministic field-free switching of magnetization is observed when the electric current flows orthogonal to the direction of the in-plane compositional gradient due to the in-plane symmetry breaking. This work proves that the MPG is a good candidate to be utilized in high-density and efficient magnetoresistive random access memory devices and other spintronic applications.

Giant converse magnetoelectric effect in a multiferroic heterostructure with polycrystalline Co2FeSi

To overcome a bottleneck in spintronic applications such as those of ultralow-power magnetoresistive random-access memory devices, the electric-field control of magnetization vectors in ferromagnetic electrodes has shown much promise. Here, we show the giant converse magnetoelectric (CME) effect in a multiferroic heterostructure consisting of the ferromagnetic Heusler alloy Co2FeSi and ferroelectric-oxide Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) for electric-field control of magnetization vectors. Using an in-plane uniaxial magnetic anisotropy of polycrystalline Co2FeSi film grown on PMN-PT(011), the nonvolatile and repeatable magnetization vector switchings in remanent states are demonstrated. The CME coupling coefficient of the polycrystalline Co2FeSi/PMN-PT(011) is over 1.0 × 10−5 s/m at room temperature, comparable to those of single-crystalline Fe1-xGax/PMN-PT systems. The giant CME effect has been demonstrated by the strain-induced variation in the magnetic anisotropy energy of Co2FeSi with an L21-ordered structure. This approach can lead to a new solution to the reduction in the write power in spintronic memory architectures at room temperature.

Spin–orbit torques of an in-plane magnetized system modulated by the spin transport in the ferromagnetic Co layer

Spin–orbit torque (SOT) magnetoresistive random-access memory (MRAM) devices have been proposed for energy efficient memory and computing applications. New classes of materials such as antiferromagnets, topological insulators, and semimetals can generate spins with unconventional polarization and improve the efficiency of field-free SOT switching. In this work, we report significant changes in SOTs due to a Co thin film inserted in the Pt/Co/Mg/CoFeB heterostructures. Remarkably, the damping-like effective field has been enhanced by 7.4 times after inserting a thin Co layer with weak perpendicular magnetic anisotropy (PMA), while the field-like effective field is reduced to near zero value. Independent characterizations were performed to verify the presence of the changes in SOTs following spin modulation by the Co insertion layer. In addition, we found that the dynamic spin pumping coupling between Pt/Co with weak PMA and the in-plane CoFeB could significantly modulate the effective SOTs in the heterostructure, and this effect is dependent on the thickness of the spacer Mg through long-range spin-wave mediated coupling. Our work has experimentally demonstrated a new avenue to modulate SOTs with physically sputtered metal layers, and this finding is promising to enable flexible and efficient spin polarizations for MRAM devices.

Insertion Trade-off Effects on the Spin-Transfer Torque Memory Explored by In Situ X-ray

Magnetoresistive random-access memory (MRAM) relies on magnetic tunnel junctions (MTJs) comprising heterostructures of CoFeB/MgO/CoFeB. Nonetheless, the dielectric breakdown of MgO limits the lifespan of MRAM devices. In the current study, we terminated MgO with a Mg surface facing the CoFeB free layer to reduce the mismatch in band alignment across the barrier. The Mg-modified interface was shown to enhance the breakdown voltage while reducing the switching current and asymmetric switching behavior, with a moderate sacrifice of perpendicular magnetic anisotropy, tunneling magnetoresistance, and resistance–area product. This performance trade-off was observed in all MTJs, regardless of cell size (180, 130, and 80 nm; each size has been tested with at least 20 cells). Visualization of the proposed junction via in situ X-ray absorption spectroscopy proved effective in elucidating the reconstruction of the interface within the context of energy barrier height, asymmetric switching behavior, and the voltage-driven movement of oxygen ions. A spin-dependent band diagram was constructed to correlate the tunneling/switching properties of the MTJ with such a trade-off scenario. The atomistic simulation of the magnetic properties revealed that Mg termination lowered the switching energy barrier with an effective domain wall motion within the CoFeB free layer.

Magnetoresistive Circuits and Systems: Embedded Non-Volatile Memory to Crossbar Arrays

This overview article describes Magnetoresistive Random Access Memory (MRAM) from a circuits and systems perspective. We discuss various tradeoffs and design challenges of MRAM in three broad application areas: 1) embedded non-volatile memory (eNVMs), 2) crossbar-based analog in-memory computing, and 3) stochastic computing. Certain MRAM characteristics, such as high retention, high endurance and fast read and write operations, make them ideal for replacing the standard CMOS memories for last-level cache applications with future scaling. However, various tradeoffs in power, performance and area pose conflicting requirements on MRAM design. We explore these challenges and various circuit techniques that have been developed to mitigate them. Further, we present various requirements of memristive crossbar arrays for accelerating matrix-vector-multiplication (MVM) operations in light of MRAM devices, and highlight various challenges, design considerations, and applicability of MRAM as crossbar arrays. Finally, we will elaborate on how inherent stochasticity of MRAM devices can be leveraged for implementing energy-efficient true random number generators (TRNGs) and stochastic units for performing certain tasks, such as developing fast solvers for combinatorial optimization, and stochastic neural networks.

Effects of synthetic antiferromagnetic coupling on back-hopping of spin-transfer torque devices

A synthetic antiferromagnetic (SAF) layer is a key component in spin-transfer torque magneto-resistive random-access memory devices. This study reveals that slight fluctuations in SAF coupling at the margin of the reference layer and hard layer (i.e., concurrent reversal) can lead to write errors in the form of back-hopping (BH). It appears that variable BH behavior can be attributed to competition between antiparallel (AP) → parallel (P) and P → AP transitions associated with SAF coupling. Our conclusions are supported by careful analysis of switching phase diagrams and measurements of self-heating and voltage-controlled magnetic anisotropy. We also observed that one form of coupling provided higher perpendicular magnetic anisotropic energy and thermal stability, which is likely due to the Dzyaloshinskii–Moriya interaction (DMI) effect. Thus, minimizing variations in DMI by optimizing SAF coupling is crucial for minimizing write error rates.

Reliable Five-Nanosecond Writing of Spin-Transfer Torque Magnetic Random-Access Memory

We report reliable 5 ns switching of spin-transfer torque magnetoresistive random-access memory devices of nominal size 43 nm and a resistance area product of 11 Ω·µm2. We measured 256 devices with a 100% write-error-rate (WER) yield at a WER floor of $10^{-6}$ and a steep WER slope as a function of voltage. A single device had a WER less than $10^{-10}$ for 5 ns write pulses. We show promising 3 ns switching performance, with a 94% WER yield at a $10^{-6}$ WER floor, for 64 devices of nominal size 50 nm.

Networked Power-Gated MRAMs for Memory-Based Computing

Emerging nonvolatile memory technologies open new perspectives for original computing architectures. In this paper, we propose a new type of flexible and energy-efficient architecture that relies on power-gated distributed magnetoresistive random access memory (MRAM). The proposed architecture uses a network-on-chip (NoC) to interconnect MRAM-based clusters, processing elements, and managers. The NoC distributes application-specific commands to MRAM devices by means of packets. Configurable network interfaces allow to transform MRAM devices into smart units able to respond to incoming commands. In this context, three types of MRAM designs are proposed with different power-gating policies and granularities. A relevant database search engine case study is considered to illustrate the benefits of this proposed architecture. It is implemented with a sparse-neural-network approach and simulated in SystemC with different scenarios including hundreds of database queries. Hardware designs and accurate power estimations have been conducted. The obtained results demonstrate important power reduction with database hit rates of about 94%. Targeting 65-nm technology, energy savings reach 87% when compared with an static random access memory-based implementation. Moreover, a new asymmetric read/write MRAM type provides from 39% to 50% energy reduction with respect to the other fixed-granularity models. This results in a low-power, highly scalable, and configurable implementation of memory-based computing.

Highly efficient voltage control of spin and enhanced interfacial perpendicular magnetic anisotropy in iridium-doped Fe/MgO magnetic tunnel junctions

Voltage control of spin enables both a zero standby power and ultralow active power consumption in spintronic devices, such as magnetoresistive random-access memory devices. A practical approach to achieve voltage control is the electrical modulation of the spin–orbit interaction at the interface between 3d-transition-ferromagnetic-metal and dielectric layers in a magnetic tunnel junction (MTJ). However, we need to initiate a new guideline for materials design to improve both the voltage-controlled magnetic anisotropy (VCMA) and perpendicular magnetic anisotropy (PMA). Here we report that atomic-scale doping of iridium in an ultrathin Fe layer is highly effective to improving these properties in Fe/MgO-based MTJs. A large interfacial PMA energy, Ki,0, of up to 3.7 mJ m−2 was obtained, which was 1.8 times greater than that of the pure Fe/MgO interface. Moreover, iridium doping yielded a huge VCMA coefficient (up to 320 fJ Vm−1) as well as high-speed response. First-principles calculations revealed that Ir atoms dispersed within the Fe layer play a considerable role in enhancing Ki,0 and the VCMA coefficient. These results demonstrate the efficacy of heavy-metal doping in ferromagnetic layers as an advanced approach to develop high-density voltage-driven spintronic devices.

Temperature dependence of the interfacial magnetic anisotropy in W/CoFeB/MgO

The interfacial perpendicular magnetic anisotropy in W/CoFeB (1.2 ∼ 3 nm)/MgO thin film structures is strongly dependent on temperature, and is significantly reduced at high temperature. The interfacial magnetic anisotropy is generally proportional to the third power of magnetization, but an additional factor due to thermal expansion is required to explain the temperature dependence of the magnetic anisotropy of ultrathin CoFeB films. The reduction of the magnetic anisotropy is more prominent for the thinner films; as the temperature increases from 300 K to 400 K, the anisotropy is reduced ∼50% for the 1.2-nm-thick CoFeB, whereas the anisotropy is reduced ∼30% for the 1.7-nm-thick CoFeB. Such a substantial reduction of magnetic anisotropy at high temperature is problematic for data retention when incorporating W/CoFeB/MgO thin film structures into magneto-resistive random access memory devices. Alternative magnetic materials and structures are required to maintain large magnetic anisotropy at elevated temperatures.

Directional etch of magnetic and noble metals. I. Role of surface oxidation states

An organic chemical etch process based on tailoring the surface oxidation state was found to be effective in realizing directional etch of magnetic and noble metals for their integration and application in magnetoresistive random access memory devices. Using Pt, a noble metal, as a test case, plasma treatments with sulfur- and oxygen-based chemistries were able to oxidize Pt0+ to Pt2+ and Pt4+, which can be effectively removed by selected organic chemistries. The most effective control of the surface oxidation states of Pt was achieved with an O2 plasma, which was then applied with similar effectiveness to other transition and noble metals. By quantifying the reaction rate, the oxidation of transition metals (Fe and Co) was shown to follow an inverse log rate law, while that of noble metals (Pd and Pt) follows a parabolic rate law. This work highlights the importance of the surface oxidation states of magnetic and noble metals in enabling directional etch by organic chemistry.

Ion beam assisted organic chemical vapor etch of magnetic thin films

An ion beam-assisted organic vapor etch process is demonstrated for patterning magnetic metal elements for potential applications in magnetoresistive random access memory devices. A thermodynamic analysis was performed to evaluate the feasibility of a chemical etch process, leading to the selection of acetylacetone (acac) and hexafluoroacetylacetone (hfac) chemistries. First, etching of cobalt and iron in acac and hfac solutions was studied, and it was determined that acac etches Co preferentially over Fe with a Co:Fe selectivity of ∼4, while hfac etches Fe preferentially over Co with an Fe:Co selectivity of ∼40. This motivates the use of acac and hfac to etch Co and Fe, respectively, but the etch rate was, in the gas phase, too small to be considered a viable process. An argon ion beam was employed in between organic vapor exposures and resulted in significant enhancement in the etch rates, suggesting an ion-enhanced chemical etching process is viable for the patterning of these magnetic metal elements.

BEOL compatible high tunnel magneto resistance perpendicular magnetic tunnel junctions using a sacrificial Mg layer as CoFeB free layer cap

Perpendicularly magnetized MgO-based tunnel junctions are envisaged for future generation spin-torque transfer magnetoresistive random access memory devices. Achieving a high tunnel magneto resistance and preserving it together with the perpendicular magnetic anisotropy during BEOL CMOS processing are key challenges to overcome. The industry standard technique to deposit the CoFeB/MgO/CoFeB tunnel junctions is physical vapor deposition. In this letter, we report on the use of an ultrathin Mg layer as free layer cap to protect the CoFeB free layer from sputtering induced damage during the Ta electrode deposition. When Ta is deposited directly on CoFeB, a fraction of the surface of the CoFeB is sputtered even when Ta is deposited with very low deposition rates. When depositing a thin Mg layer prior to Ta deposition, the sputtering of CoFeB is prevented. The ultra-thin Mg layer is sputtered completely after Ta deposition. Therefore, the Mg acts as a sacrificial layer that protects the CoFeB from sputter-induced damage during the Ta deposition. The Ta-capped CoFeB free layer using the sacrificial Mg interlayer has significantly better electrical and magnetic properties than the equivalent stack without protective layer. We demonstrate a tunnel magneto resistance increase up to 30% in bottom pinned magnetic tunnel junctions and tunnel magneto resistance values of 160% at resistance area product of 5 Ω.μm2. Moreover, the free layer maintains perpendicular magnetic anisotropy after 400 °C annealing.

Viable chemical approach for patterning nanoscale magnetoresistive random access memory

A reactive ion etching process with alternating Cl2 and H2 exposures has been shown to chemically etch CoFe film that is an integral component in magnetoresistive random access memory (MRAM). Starting with systematic thermodynamic calculations assessing various chemistries and reaction pathways leading to the highest possible vapor pressure of the etch products reactions, the potential chemical combinations were verified by etch rate investigation and surface chemistry analysis in plasma treated CoFe films. An ∼20% enhancement in etch rate was observed with the alternating use of Cl2 and H2 plasmas, in comparison with the use of only Cl2 plasma. This chemical combination was effective in removing metal chloride layers, thus maintaining the desired magnetic properties of the CoFe films. Scanning electron microscopy equipped with energy-dispersive x-ray spectroscopy showed visually and spectroscopically that the metal chloride layers generated by Cl2 plasma were eliminated with H2 plasma to yield a clean etch profile. This work suggests that the selected chemistries can be used to etch magnetic metal alloys with a smooth etch profile and this general strategy can be applied to design chemically based etch processes to enable the fabrication of highly integrated nanoscale MRAM devices.

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

The main bottlenecks limiting the practical applications of current magnetoresistive random access memory (MRAM) technology are its low storage density and high writing energy consumption. Although a number of proposals have been reported for voltage-controlled memory device in recent years, none of them simultaneously satisfy the important device attributes: high storage capacity, low power consumption and room temperature operation. Here we present, using phase-field simulations, a simple and new pathway towards high-performance MRAMs that display significant improvements over existing MRAM technologies or proposed concepts. The proposed nanoscale MRAM device simultaneously exhibits ultrahigh storage capacity of up to 88 Gb inch−2, ultralow power dissipation as low as 0.16 fJ per bit and room temperature high-speed operation below 10 ns.

Three-dimensional magneto-resistive random access memory devices based on resonant spin-polarized alternating currents

Selective switching of a magneto-resistive random access memory (MRAM) multilayer stack is demonstrated using resonant spin-polarized alternating currents (AC) superimposed on spin-polarized direct currents. Finite element micromagnetic simulations show that the use of frequency triggered AC allows one to maximize the transferred spin transfer torque selectively in order to merely reverse the magnetization of a single storage layer in a stack. Using layers with different resonance frequencies, which are realized by altering the anisotropy constants, allows one to address them by tuning the AC frequency. A rapid increase of the storage density of MRAM devices is shown by using three-dimensional sandwich structures.

Inherent spin transfer torque driven switching current fluctuations in magnetic element with in-plane magnetization and comparison to perpendicular design

This paper presents a systematic micromagnetic modeling study on switching current fluctuations in both in-plane and perpendicular spin-transfer-torque (STT) magnetoresistive random access memory devices. For the magnetic tunnel junction (MTJ) with in-plane magnetization, high-order spin wave modes are excited during a STT-driven switching, which leads to an inherently broad switching current distribution. If the MTJ size is not sufficiently small, a stable vortex can be formed over a wide range of current amplitudes. In contrast, the excitation of such high-order spin waves is absent in STT switching of MTJs with perpendicular magnetic anisotropy. Consequently, the fluctuation in switching current amplitude or pulse duration is significantly smaller in comparison.

Magnetic Properties and Writing Characteristics of Magnetic Clad Lines in Magnetoresistive Random Access Memory Devices

Structural and magnetic properties of magnetic clad lines used for magnetoresistive random access memory (MRAM) devices were analyzed to optimize the clad-line fabrication process. Patterns of 520 nm lines and 280 nm spaces with a depth of 300 nm with magnetic cladding films of Ta/NiFe/Ta were fabricated to characterize the structure and magnetic properties of cladding films by transmission electron microscopy and magnetic measurements. A high susceptibility with a small hysteresis was obtained when a NiFe cladding film highly oriented to fcc (111) was formed perpendicular to the sidewall depth direction. The optimized cladding film was integrated in MRAM cells and we confirmed that the clad lines reduced the flop current of a toggle-MRAM cell by more than 50% compared with a cell having unclad lines. The flop current distribution can be reduced by applying a magnetic field of 12 kOe in the direction of word lines at room temperature. We successfully confirmed writing operation in a 4-Mbit MRAM cell with these optimized magnetic clad lines.

Improvement of Thermal Stability of Magnetoresistive Random Access Memory Device with SiN Protective Film Deposited by High-Density Plasma Chemical Vapor Deposition

Embedded magnetoresistive random access memory (MRAM) with multi-level interconnects necessitates that magnetic tunnel junction (MTJ) devices have a thermal stability of 350 °C or higher during fabrication. We have improved thermal stability of MRAM devices using SiN protective film deposited by high-density plasma chemical vapor deposition (HDP-CVD) at 200 °C. The MTJ devices with HDP-CVD SiN protective film did not degrade after post-annealing at 350 °C, which suggests that the HDP-CVD process reduced oxide metal on the etched surface of the MTJ devices and that the SiN film blocked H2O diffusion from the interlayer dielectric film during post-annealing at 350 °C. We also fabricated a 1-kbit MRAM array and experimentally demonstrated thermal stability at 350 °C.