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Spin-torque Magnetoresistive Random Access Memory (ST-MRAM) is the subject of intense investigation since it extends MRAM technology to densities beyond those achieved with the earlier field-switched MRAM technology. This paper reviews recent developments in ST-MRAM MTJ device technology, including exciting progress in scaling MTJs down to dimensions approaching 20 nm. Fabrication issues relevant to development of ST-MRAM, including its integration with CMOS back-end-of-line (BEOL) processing, are also briefly discussed.

Magnetic random access memory (MRAM) technology, based on the use of magnetic tunnel junctions (MTJs), holds the promise of improving on the capabilities of existing charge-based memories by offering the combination of nonvolatility, speed, and density in a single technology. In this paper we review rapid-turnaround methods which have been developed or applied in new ways to characterize MRAM chips at various stages during processing, with particular emphasis on the MTJs. The methods include current-in-plane tunneling (CIPT), Kerr magnetometry, vibrating sample magnetometry (VSM), and conducting atomic force microscopy (CAFM). Use of the methods has enabled rapid learning with respect to the materials used for the MTJs, as well as tuning of the MTJ geometry in terms of size and shape and of the patterning methods employed. Examples of the use of each of the methods are presented along with interpretation of the data via critical operating parameters.

A two terminal spin-injection device is fabricated using a nanostencil process with a Co–Cu–Co stack. The stack can be deposited both by sputtering and by electron-beam evaporation. A better edge definition is observed in evaporation-deposited films under cross section transmission electron microscopy. Both methods succeeded in producing junctions with sub-100 nm lateral dimensions and show spin-injection-induced magnetic switching.

Exchange biased magnetic tunnel junction (MTJ) structures are shown to have useful properties for forming magnetic memory storage elements in a novel cross-point architecture. MTJ elements have been developed which exhibit very large magnetoresistive (MR) values exceeding 40% at room temperature, with specific resistance values ranging down to as little as ∼60 Ω(μm)2, and with MR values enhanced by moderate thermal treatments. Large MR values are observed in magnetic elements with areas as small as 0.17 (μm)2. The magnetic field dependent current–voltage characteristics of an MTJ element integrated with a silicon diode are analyzed to extract the MR properties of the MTJ element itself.

The switching current of Spin Torque Magnetic Random Access Memory (MRAM) can be reduced significantly by using perpendicularly magnetized materials. The Ta|CoFeB|MgO system provides both high tunneling magnetoresistance and perpendicular anisotropy. Using this materials system we have demonstrated basic write functionality in fully integrated Spin Torque MRAM arrays. Here, we further demonstrate device scaling down to 20 nm diameter, opening up the possibility of ultra-dense Spin Torque MRAM.

A method is developed for the fabrication of sub-100 nm current-perpendicular spin-valve junctions with low contact resistance. The approach is to use a batch-fabricated trilayer template with the junction features defined by a metal stencil layer and an undercut in the insulator. The spin-valve thin film stack is deposited afterwards into the stencil, with the insulator undercut providing the necessary magnetic isolation. Using this approach, reproducible spin-current-induced magnetic switching is demonstrated for junctions down to 50 nm×100 nm in size.

Recently developed magnetic tunnel junctions with full perpendicular magnetization that are spin-torque switchable allow for quantitative comparison of spin-torque switching statistics with a macrospin model. For typical devices above 50 nm in lateral size, the comparison suggests the presence of subvolume magnetic excitations which often dominate the switching process and which degrade the spin-torque switching efficiency. A simple model of subvolume spin-torque-driven magnetic switching is presented to account for the experimental observations. The origin of the subvolume thermal excitation is traced to a competition between the macrospin fluctuation within a simple uniaxial anisotropy potential and that of thermal magnon excitation. The subvolume excitation problem highlights the importance of improving the magnetic exchange stiffness of the junction free layer, and the reduction of junction lateral sizes below 50 nm where an improved spin-torque efficiency is seen as the switching dynamics cross over to a more macrospin-like process.

PMA spin-torque switchable junctions have been demonstrated with lower switching current and faster switching speed compared to IMA devices. They are promising for further technology exploration in solid-state memory applications.

We examine the sweep-rate dependence of magnetic switching field, Hs, in submicron magnetic tunnel junctions where shape anisotropy is dominant. Experimental data support the use of a single-domain thermal activation model for description of activated magnetic reversal in junctions 0.2 by 0.5 μm or less in size. A scaling law is obtained for the thermal activation energy which varies as the cube of junction size.

Ferromagnetic resonance has been used to study the magnetic properties and magnetization dynamics of polycrystalline Fe$_{1-x}$V$_{x}$ alloy films with $0\leq x < 0.7$. Films were produced by co-sputtering from separate Fe and V targets, leading to a composition gradient across a Si substrate. FMR studies were conducted at room temperature with a broadband coplanar waveguide at frequencies up to 50 GHz using the flip-chip method. The effective demagnetization field $4 \pi M_{\mathrm{eff}}$ and the Gilbert damping parameter $\alpha$ have been determined as a function of V concentration. The results are compared to those of epitaxial FeV films.

The discovery of perpendicular magnetic anisotropy (PMA) in Ta|CoFeB|MgO and the subsequent development of perpendicularly magnetized tunnel junctions at IBM is reviewed. The fast-turn-around method used for screening materials for interface PMA by measuring the moment/area and anisotropy field of in-plane materials as a function of CoFeB thickness is presented, including the data as a function of seed-layer material which led to the discovery of PMA in Ta|CoFeB|MgO. Magnetic and electrical data are reported for the first PMA magnetic tunnel junction we made using this material. By inserting a thin Fe layer at the Ta|CoFeB interface, a substantial increase in the PMA energy density was obtained. Pure Fe layers (which required the use of a TaMg seed) greatly improved the thermal stability, allowing annealing up to 400 °C.

To understand the near-interface magnetism in manganites, uniform, ultra-thin films of La_{0.67}Sr_{0.33}MnO_3 were grown epitaxially on single crystal (001) LaAlO_3 and (110) NdGaO_3 substrates. The temperature and magnetic field dependent film resistance is used to probe the film's structural and magnetic properties. A surface and/or interface related dead-layer is inferred from the thickness dependent resistance and magnetoresistance. The total thickness of the dead layer is estimated to be $\sim 30 \AA$ for films on NdGaO_3 and $\sim 50 \AA$ for films on LaAlO_3., Comment: 11 pages, 4 figures

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We have demonstrated purely electrical manipulation of the magnetic anisotropy of a Co0.6Fe0.2B0.2 film by applying only 8 V across the CoFeB/oxide stack. A clear transition from in-plane to perpendicular anisotropy was observed. The quantitative relationship between interface anisotropy energy and the applied electric-field was determined from the linear voltage dependence of the saturation field. By comparing the dielectric stacks of MgO/Al2O3 and MgO/HfO2/Al2O3, enhanced voltage control was also demonstrated, due to the higher dielectric constant of the HfO2. These results suggest the feasibility of purely electrical control of magnetization with small voltage bias for spintronics applications.

CoFeB-based magnetic tunnel junctions with perpendicular magnetic anisotropy are used as a model system for studies of size dependence in spin-torque–induced magnetic switching. For integrated solid-state memory applications, it is important to understand the magnetic and electrical characteristics of these magnetic tunnel junctions as they scale with tunnel junction size. Size-dependent magnetic anisotropy energy, switching voltage, apparent damping, and anisotropy field are systematically compared for devices with different materials and fabrication treatments. Results reveal the presence of sub-volume thermal fluctuation and reversal, with a characteristic length-scale of the order of approximately 40 nm, depending on the strength of the perpendicular magnetic anisotropy and exchange stiffness. To have the best spin-torque switching efficiency and best stability against thermal activation, it is desirable to optimize the perpendicular anisotropy strength with the junction size for intended use. It also is important to ensure strong exchange-stiffness across the magnetic thin film. These combine to give an exchange length that is comparable or larger than the lateral device size for efficient spin-torque switching.

Spin-transfer torque magnetic random access memory (STT-MRAM) is one of the most promising emerging non-volatile memory technologies. MRAM has so far been demonstrated with a unique combination of density, speed, and non-volatility in a single chip, however, without the capability to replace any single mainstream memory. In this paper, we demonstrate the basic physics of spin torque switching in 20 nm diameter magnetic tunnel junctions with perpendicular magnetic anisotropy materials. This deep scaling capability clearly indicates the STT MRAM device itself may be suitable for integration at much higher densities than previously proven.

Bit error rates below 10-11 are reported for a 4-kb magnetic random access memory chip, which utilizes spin transfer torque writing on magnetic tunnel junctions with perpendicular magnetic anisotropy. Tests were performed at wafer level, and error-free operation was achieved with 10 ns write pulses for all nondefective bits during a 66-h test. Yield in the 4-kb array was limited to 99% by the presence of defective cells. Test results for both a 4-kb array and individual devices are consistent and predict practically error-free operation at room temperature.

A three-terminal spin-torque-driven magnetic switch is experimentally demonstrated. The device uses nonlocal spin current and spin accumulation as the main mechanism for current-driven magnetic switching. It separates the current-induced write operation from that of a magnetic tunnel junction based read. The write current only passes through metallic structures, improving device reliability. The device structure makes efficient use of lithography capabilities, important for robust process integration.

We present room-temperature measurements of high-frequency magnetization fluctuation (mag noise) in MgO-based nanopillar magnetic tunnel junctions (MTJs) biased with a direct current (dc). In the frequency range of 1–13 GHz, double mag-noise peaks are observed for some MTJs while others only show a single mag-noise peak. The in-plane field dependence of the mag-noise peak frequency is consistent with the Kittel formula. For all MTJs measured, the bias-dependent shift in the mag-noise peak frequency has a pronounced asymmetry. In addition, we find nonmonotonic variations in peak linewidth as a function of the external in-plane magnetic field and of the dc bias current. These suggest the possible involvement of nonmacrospin modes in spin-torque-dependent thermal mag-noise generation.

Toggle magnetic random access memory (MRAM) has been proposed to solve the problems of small switching margins and half-select activated errors found in Stoner-Wohlfarth MRAM. However, it is widely acknowledged that the switching fields required for toggle MRAM are substantially larger than those needed for Stoner-Wohlfarth MRAM. Previously published reports on toggle switching use large toggle start fields around 75Oe. Here we examine, both experimentally and with a single-domain model, how both the toggle start and end fields vary with free layer intrinsic anisotropy, thickness, width, aspect ratio, and interlayer exchange coupling. By optimizing these parameters, we obtain 400nm width devices with toggle start fields below 30Oe.

We have measured the subtle changes in magnetic reversal of weakly interacting arrays of submicrometer magnetic particles as a function of particle spacing and interpreted the data within the context of a model assuming a normal distribution of switching fields plus dipole fields between particles. The measurements were performed on a Kerr magnetometer and showed the changes in switching characteristics of the particle array as a function of array spacing in the two in-plane directions. The Monte Carlo calculations included the effect of changing particle pitch in two in-plane directions and explained the characteristic change in sharpness of the switching transition alternatively as a frustration or as an enhanced sharpness due to cascading transitions, depending on whether the hard- or easy-axis dipole coupling is dominant. This method is extensible to nanoscale particles and provides an estimate of the moment of the particle, a measurement which is difficult to obtain via other experimental methods.

Conception et fonctionnement du microscope destine a l'etude des surfaces. Faible derive thermique

The performance of thin‐film devices can be limited by thermal failure. A model for thermal power dissipation of a thin‐film device on a glass substrate has provided the basis for the solution of the heat flow equation. Solutions for the maximum temperature as a function of power input and rectangular device and substrate dimensions have been obtained by digital computations. These results have been applied to the TFT and to the Mead device, and it is concluded that the Mead device will suffer thermal destruction before it can achieve a figure of merit greater than 10 Mc/sec whereas the TFT is not limited by thermal considerations until a figure of merit of several hundreds of Mc/sec has been achieved.

An investigation of the electrical properties of CdSe thin‐film triodes has demonstrated the existence of shallow trapping levels distributed from 0.02 to 0.06 eV below the edge of the conduction band. These values have been experimentally determined by a combination of optical and field effect techniques. The investigation also indicates that field excitation of electrons from such levels is a fundamental mechanism of operation in the thin‐film field‐effect triode. Close correlation has been obtained between the dependence of device current on applied gate potential and the probability of field excitation of carriers with the field strengths achieved with these values of potential.

Understanding the magnetic switching behavior in micron and submicron scale specimens is important for a number of applications. In this study, magnetic-tunnel junctions of various sizes and shapes were fabricated and their switching behavior was studied in detail. Using exchange bias to offset the magnetic response of one electrode, the response of the other (free) electrode was determined from measurements of junction resistance. Switching threshold curves were measured by sweeping magnetic fields in both easy and hard direction. Single domain like switching was observed in some of our smallest submicron junctions. The observed behavior was compared with predictions from the Stoner–Wohlfarth rotational model and from numerical calculations.