This article is one of three papers to be presented exclusively on the web as part of the June 2000 JOM-e the electronic supplement to JOM.
	The following article appears as part of JOM-e, 52 (6) (2000), http://www.tms.org/pubs/journals/JOM/0006/Slaughter/Slaughter-0006.html JOM is a publication of The Minerals, Metals & Materials Society

Materials for Magnetic Memory: Overview

Magnetic Tunnel Junction Materials for Electronic Applications

TABLE OF CONTENTS
INTRODUCTION MTJ MATERIALS FOR MRAM MTJ DEPOSITION AND PROPERTIES ANNEALING EFFECTS CHARACTERIZATION AND ANALYSIS PROPERTIES OF PATTERNED BITS STATUS AND CHALLENGES ACKNOWLEDGEMENT References

Recent advances in magnetic tunnel junction material are driving the development of magnetoresistive random access memory with attributes that are competitive with semiconductor memory. The large magnetoresistance signal of this material enables fast memory-read operations. In addition, the memory is nonvolatile (the information remains stored when the power is turned off) because the information is stored in the magnetic state of the bit. The large signal also makes magnetic tunnel junction material an attractive candidate for magnetic-media read heads and other types of sensor applications.

INTRODUCTION

Since 1971, anisotropic magnetoresistive (AMR) Ni-Fe alloy thin films have been explored for use in magnetic field sensing.¹ These types of ferromagnetic thin films change resistance depending on the relative direction between film magnetization and in-plane current direction. Although the total signal change or magnetoresistance (MR) ratio, expressed as the change in resistance divided by the minimum resistance, is typically two percent for Ni-19Fe alloy, its field sensitivity is much larger than that obtained through coil winding. Unlike inductive magnetic field sensors, the AMR sensor is speed independent. Devices using this type of material include read heads in high-density hard disk drives, magnetic field sensors for a variety of applications, and magnetoresistive random access memory (MRAM).

In 1988, a new type of magnetoresistive material, termed giant magnetoresistance (GMR) material, was discovered.^2,3 The material is made of at least two magnetic layers separated by a conducting interlayer. Its resistance depends on the relative orientation between the neighboring magnetic layers. It is a maximum when the directions are antiparallel and a minimum when they are parallel. The MR ratio is 6-15% for a simple structure with two magnetic layers and up to 80% for an antiferromagnetically coupled multilayer. The strong antiferromagnetic (AF) exchange coupling seen in some multilayers forces adjacent layers to be antiparallel and is usually avoided in applications. Without the AF exchange coupling, typically MR is 6-20% for weakly coupled GMR films with two to three magnetic layers. Due to its improved signal compared to AMR material, GMR films result in enhanced device performance in most applications. GMR films have already been incorporated into commercial read heads, and development for other device applications, such as sensors and MRAM, is underway. The GMR films are most often used with current flowing in the film plane. The resistance of devices using current perpendicular to the film plane is very low, thus limiting their application potential for current-generation microelectronic lithography dimensions.

In the early 1990s, high MR was discovered for magnetic tunnel junction (MTJ) material.^4,5 MTJ material is made of at least two magnetic layers separated by an insulating tunnel barrier. The current flows perpendicular to the film plane, and the best results have been achieved with aluminum-oxide tunnel barriers. Since the initial experimental discovery of MTJ material with promising MR, the technique of producing these materials, as well as key properties, has been dramatically improved.^6,7 Tunneling MR values are now widely reported are in the 20-50% range, much higher than typical GMR films. The tunneling resistance depends exponentially on the tunnel-barrier thickness and is measured by the resistance-area (RA) product. Early work reported quite high values of RA, often in the GW-mm² range, but recent work has shown good MR down to the 10 W-mm² range. The current RA value of MTJ material is ideal for MRAM and sensor applications, but is still high for magnetic-recording read head applications.

The basic concept of MRAM uses magnetization direction as information storage and the resultant resistance difference for information readout. The development of MRAM began approximately ten years ago in response to the need for a durable, radiation-hard, nonvolatile RAM.⁸ The potential of the technology has improved dramatically with each advance in magnetic materials. The first material used was AMR Ni-Fe-Co/Ta-N/Ni-Fe-Co sandwich films. The MR ratio was limited to 2%, and the actual MR used in memory states was only about 0.5%. The low signal was responsible for the relatively slow read-access time of around 250 ns. The critical dimension of the cell was larger than 1 mm, which lagged behind semiconductor memories by many generations. Such memory technology is very attractive for niche markets such as space applications, but it is not competitive with general semiconductor memories in speed, density, and cost.

The discovery of the GMR effect was a boost to MRAM technology. Not only is the signal strength larger, but the characteristics of the physical phenomenon itself are well suited for MRAM, which uses magnetic-moment direction as information storage and the resultant MR difference for sensing. A submicrometer critical dimension of the MRAM cell is essential for its competitiveness in the general memory market.

A number of different memory-storage methods using different types of GMR films have been explored for application in high-density memory.^9,10 Since the sheet resistance of the GMR film is small compared to that of a complementary metal-oxide semiconductor (CMOS) transistor, a number of GMR MRAM memory cells must be connected in series with a CMOS transistor, so that total resistance from the memory cells is much larger than that from the transistor. Although good from a design point of view, this scheme effectively decreases the usable signal, making it difficult to design a high-speed memory.

MTJ material is quickly finding applications in MRAM and magnetic-field sensing.^11,12 Major advantages of MTJ material include a larger signal, from 20-50% depending on the polarization of the magnetic electrodes used, and its tunable RA, depending on barrier thickness and degree of oxidation. It is possible to make MTJ memory cells with one cell in series with a minimum-size silicon CMOS transistor for isolation. This kind of high-density architecture is suitable for fast-speed, low-power memory applications.¹²

MTJ MATERIALS FOR MRAM

 

Figure 1. A schematic of an MTJ memory cell. Arrows indicate possible directions of magnetic movement.

The MTJ material stack includes two magnetic layers separated by a thin dielectric barrier and a mechanism to pin the polarization of one of the magnetic layers in a fixed direction (Figure 1). The polarization direction of the free magnetic layer is used for information storage. The resistance of the memory bit is either low or high, depending on the relative polarization (parallel or antiparallel) of the free layer with respect to the pinned layer. An applied field can switch the free layer between the two states. In an MRAM array, orthogonal lines pass under and over the bit, carrying current that produces the switching field. The bit is designed so that it will not switch when current is applied to just one line, but will always switch when current is flowing through both lines that cross at the selected bit.

The tunneling MR can be understood in terms of a two-band model in which the d-band is split into spin-up and spin-down bands with different density of states at the Fermi energy. When the magnetization of the layers is parallel, the majority-band electrons tunnel across to the majority band of the opposing electrode and the minority to the minority band. When they are antiparallel, the majority/minority band electrons are forced to tunnel into the minority/majority band of the opposing electrode. The reduced number of states available for tunneling between the ferromagnetic layers when the layers are antiparallel results in an increased tunneling resistance, as compared to parallel.

The reality is somewhat more complicated than this simple picture; in fact, the barrier material also plays a role in both the magnitude and sign of this phenomenon.¹³ However, this simple two-band model is sufficient for understanding the MR of the AlOx-based junctions considered here. The tunneling MR ratio is defined, in direct analogy with other types of MR, as the difference in resistance between the two states divided by the resistance in the low state. Because the conduction is perpendicular to the layers, the device resistance scales as the inverse of its in-plane area, and the material is characterized by its RA product.

The two most critical layers in the MTJ stack are the AlOx tunnel barrier and the free layer. The tunnel barrier is very thin, £ 20 Å, and the tunneling resistance is exponentially dependent on its thickness. In addition to being free of pinholes and very smooth, it must be extremely uniform over the wafer, since small variations in the AlOx thickness result in large variations in the resistance. MR uniformity and the absolute resistance of the cell are critical, since the absolute value of the MTJ resistance is compared with a reference cell during read mode. In patterned bits, the thickness of the free layer is directly related to the field, or current, required for switching the bit. A thin free layer less than 50 Å is desirable because it results in low switching fields for low-power write operations. For MRAM to succeed in large-scale manufacturing, processes and deposition tools must be developed that provide an unprecedented level of uniformity and reproducibility.

MTJ DEPOSITION AND PROPERTIES

The layers of the MTJ stack are formed by sputter-deposition techniques with deposition rates in the Ångstrom-per-second range. Two such techniques have been applied successfully--physical vapor deposition, specifically planar magnetron sputtering, and ion-beam deposition. The tools and techniques used for the metal-layer deposition are the same as those used for GMR films.^14,15

The best methods for producing the insulating tunnel barrier are not yet clear; various techniques are currently under study throughout the world. The best results to date are for AlOx tunnel-barrier layers made by depositing a metallic aluminum layer, between 5 Å and 15 Å thick, and then oxidizing it by one of several methods. We have studied several types of plasma oxidation¹⁶ as well as oxidation in air and ion-beam oxidation. Additional techniques studied by other groups include oxidation by glow-discharge plasma,⁵ atomic-oxygen exposure, and ultraviolet-stimulated O₂ exposure.¹⁷

The necessity of controlling the magnetic properties of the magnetic layers introduces special requirements on the deposition process. For example, most ferromagnetic materials have an inherent magnetic anisotropy that is related to ordering on an atomic scale.¹⁸ The direction of this anisotropy can be set during the deposition of the layer by applying a magnetic field across the wafer. The resulting uniaxial anisotropy is observed as magnetic easy and hard directions in the magnetization of the layer. Since the anisotropy axis affects the switching behavior of the material, the deposition system must be capable of projecting a uniform magnetic field across the wafer, typically in the 20-100 Oe range, during deposition. Other magnetic properties, such as coercivity and magnetorestriction, also are dependent on the deposition process and must be controlled by the choice of magnetic alloy and deposition conditions. Because the switching field of a patterned bit depends directly on the thickness of the free layer, the requirements for thickness uniformity and repeatability are strict. A total combined variation of less than 2% will be needed. These tolerances are currently met by R&D deposition tools, but are not standard for semiconductor production tools.

MTJ material cannot be tested in blanket form. Since the current must pass perpendicular to the layers, it must be patterned so that the top and bottom electrodes can be separately contacted. In addition, one must be careful about the resistance of the electrodes, even in a four-point probe measurement, since current-distribution effects can produce erroneous results when the junction resistance is low.¹⁹

Figure 2. An MR curve for an MTJ bit. The resistance is low/high when the polarization of the magnetic layers is parallel/antiparallel.

For the material studies presented here, standard contact lithography techniques were used to form 300 mm ´ 100 mm rectangular junctions. Testing was done using a four-point electrical prober. One set of the current and voltage probes was placed on pads in contact with one MTJ electrode, and the other set of probes was contacted to the other electrode. An external field was applied, and field versus resistance of MTJ was measured. The MR was then extracted from these measurements; a typical MR loop is shown in Figure 2. Bottom and top electrodes of 400 Å thick aluminum were used to provide low-resistance contacts to the junctions. For the study of small bits, a multimask process with deep ultraviolet lithography was used.

For a submicrometer-patterned MTJ device to have a resistance that is suitable for MRAM, the tunnel-barrier thickness must be on the order of 20 Å or less. In addition to being pinhole free and very smooth, the AlOx tunneling barrier must be extremely uniform over a wafer. Since the resistance of the junction is exponentially dependent on thickness, small variations in the AlOx thickness result in large variations in the resistance.¹⁶ The uniformity and absolute values of the resistance, in addition to the MR values of the cells, are important parameters for the read operation, since in preferred architecture the cell signal, which depends on cell resistance and MR, is compared with a nearby reference cell during read operation.

MR values above 30% for RA in the 1-1,000 kW-mm² range have been obtained by optimizing aluminum thickness and oxidation time. Figure 3 illustrates the behavior of MR and the RA product for MTJ material with Ni-Fe alloy electrodes. For aluminum thickness above 9 Å, MR peaks at 35% with an RA in the 1-10 kW-mm² range as desired for the MRAM elements. The peak indicates that either over-oxidizing or under-oxidizing the aluminum reduces MR. Over-oxidizing results in oxidation of the magnetic electrode beneath the barrier, while under-oxidizing leaves metallic aluminum at that bottom interface. For the series of samples shown in the figure, MR drops abruptly for aluminum thickness (d_Al) below 9 Å, probably due to roughness at the tunnel-barrier interfaces that leads to partial shorts or tunneling hot spots. The RA increases exponentially with plasma oxidation time in the region of the best MR. Studies of varying d_Al with constant oxidation time also exhibit exponentially increasing RA with d_Al in the region of best MR. Because of the exponential dependence on both aluminum thickness and oxidation time, producing MTJ material with good resistance uniformity over an entire wafer is challenging. However, with excellent aluminum thickness uniformity, RA uniformity of 10% 1-sigma over a 150 mm wafer can be routinely obtained.

Figure 3. The (left) MR and (right) RA product for MTJ material with Ni-Fe alloy electrodes showing the dependence on plasma oxidation time and aluminum thickness before oxidation.		Figure 4. (left) The dependence of MR on thickness of the Ni-Fe free layer for bottom-pinned MTJ material. (right) Hysterisis loops for free layers with thickness as labeled.

A thin, free magnetic layer is desirable to obtain low switching fields in patterned bits. However, there are fundamental limits on how thin it can be made. Figure 4 shows how tunneling MR depends on the free-layer thickness for MTJ material with Ni-Fe alloy electrodes. These data indicate a transition from ferromagnetic to superparamagnetic behavior for d_NiFe below 15 Å. Further analysis shows that these hysteresis curves are consistent with the growth of small Ni-Fe islands on the AlOx tunnel barrier that coalesce at a Ni-Fe coverage near 9 Å.

ANNEALING EFFECTS

Figure 5. The effect of annealing on the MR and RA of MTJ material.

The difference between as-deposited and low-temperature (T < 300°C) annealed material is dramatic. Typically, MR increases significantly and resistance decreases slightly after a low-temperature anneal, as shown in Figure 5. At temperatures above 300°C, MR degrades as resistance increases. Each data point in the figure represents the average of about ten die across a wafer. Clearly, there are changes occurring in the tunnel barrier or interfaces with the barrier that strongly impact the properties of the spin-dependent tunneling. The practical implications of this behavior are described under Status and Challenges.

To better understand the changes that occur near the interfaces in the tunnel junction during the initial low-temperature anneal, x-ray photoelectron spectroscopy experiments were performed on simplified structures. Samples consisting of typical bottom electrode layers covered by AlOx were inserted into an ultrahigh vacuum chamber for annealing and x-ray photoelectron spectroscopy analysis using a magnesium-anode x-ray source and a double-pass cylindrical-mirror electron energy analyzer. Changes occurred in the photoelectron peaks corresponding to the Fe 2p1/2 and 2p3/2 levels when one sample was annealed at 250°C and 300°C. A large shoulder on the right side of the 2p1/2 peak was observed and correlated to the presence of oxidized iron in the vicinity of the AlOx barrier. The chemical state of the iron oxide is unknown, but the peak shift was similar to FeO rather than Fe₂O₃. On annealing at 250°C, the FeOx signal was lower, and after the 300°C anneal it was lower still. These results imply that some of the iron is oxidized together with the aluminum layer, but is at least partially reduced again to metallic iron when annealed. The initial state may be due to intermixing of iron and aluminum when the aluminum is deposited. The intermixed iron is then oxidized with the aluminum, giving rise to the large FeOx peak. Since the enthalpy of formation of aluminum oxide is larger (more negative) than iron oxide, the reduction effect could be due to a competition between the iron and aluminum for the available oxygen, which favors the aluminum.^12,20

CHARACTERIZATION AND ANALYSIS

Understanding the properties of thin magnetic films is essential to engineering a reliable device. Characterizing how the magnetic layers react to deposition, seed layers, thermal anneal, operating temperature, and stress is important to ensure that these thin layers will withstand the rigors of processing, packaging, and operation. The permutations of magnetic layer and seed, deposition technique, thermal anneal, stress, etc. lead to a large number of structures to be investigated. A summary of magnetic properties and characterization is beyond the scope of this article, but this section focuses on one experiment that illustrates the type of issues and characterization techniques encountered. This experiment involves using Pt-Mn to pin the magnetization direction of Ni-Fe.

A ferromagnetic thin film is pinned when placed in contact with an AF thin film. For an uncoupled, free, ferromagnetic film, the magnetic orientation of the film displays a hysteretic behavior pointing in the direction of the last applied saturating field. If a saturating field is applied and then taken away, the magnetic orientation of this free film will be in the direction of that field. If the direction of the applied saturating field is reversed and again taken away, the magnetic orientation of the film will be reversed. Thus, in zero applied field, either orientation is possible. A ferromagnetic film pinned by an AF layer displays similar behavior, but has an offset. In zero field, the ferrromagnetic film will align in one direction. An exchange coupling between the ferromagnetic and AF layers, at their mutual interface, causes the ferromagnetic layer to be preferentially aligned in one direction. For our memory devices, this preferential alignment or pinning is used to lock one layer in a fixed direction. Much of our work on AF pinning materials, and of others in the field, has revolved around manganese-based antiferromagnetic materials such as Pt-Mn, Ir-Mn, Rh-Mn, and Fe-Mn.

Figure 6. The annealing behavior of a thin film stack: Ta(50 Å)/Ni-Fe(30 Å)/Pt-Mn(300 Å)/Ni-Fe(60 Å)/Ta(100 Å). The main figure shows the evolution of the q-2q x-ray diffraction spectrum as the (111) oriented f.c.c. Pt-Mn transforms to f.c.t. Pt-Mn during anneal. The insets show the corresponding change in the M-H loop of the Ni-Fe when the antiferromagnetic f.c.t. Pt-Mn exchange couples to it.

Figure 7. Calculated hysteresis curves for a 0.6 ´ 1.2 mm2 bit with 0 Oe and 20 Oe hard-axis fields applied.

Pt-Mn is a particularly interesting pinning material because it remains AF at relatively high temperatures. Unlike many of the commonly used AF alloys, as-deposited Pt-Mn is not AF. Instead, this material must be post annealed, resulting in a phase transformation from face-centered cubic (f.c.c.) to a face-centered tetragonal (f.c.t.) crystal structure. The f.c.t. phase of Pt-Mn is AF and will pin an adjacent ferromagnetic film. This behavior is shown in Figure 6. This figure displays x-ray and magnetic characterization of a Ni-Fe ferromagnetic layer pinned by a Pt-Mn AF layer. In the x-ray diffraction data, the phase transformation to f.c.t. from f.c.c is clearly seen to occur between a ten minute and 30 minute anneal at 275°C. The inset magnetic hysteresis loops (magnetization vs. applied field) show how the pinning strength increases accordingly with annealing time. The shift and broadening of the Ni-Fe hysteresis loop in the annealed material is characteristic of a pinned ferromagnetic film. Once pinned, the exchange bias causes the magnetic orientation of the film to be in one direction at zero applied field. The data shown here are for only one thickness of Pt-Mn, one thickness of surrounding Ni-Fe layers, and a seed layer of tantalum. Other layer thicknesses and seed layers can produce a variety of different results.

PROPERTIES OF PATTERNED BITS

Understanding and controlling the micromagnetic behavior of MTJ elements is essential for reproducible and reliable switching characteristics.^21,22 The switching field is mainly governed by the magnetic-shape anisotropy that arises from the element boundaries. Hence, bit size, shape, and aspect ratio all play roles in controlling the micromagnetic arrangement and, therefore, the switching behavior.

Ideally, bits with a single magnetic domain would coherently rotate in response to the selecting and switching fields in an MRAM device. In real elements, the magnetic configuration is complicated by the presence of edges and is not single-domain in the ideal sense. Therefore, switching is strongly dependent on the details of the patterned shape.

Micromagnetic simulations²³ of single-element switching behavior have been performed. Figure 7 is a plot of the calculated hysteresis curves for a 0.6 mm² ´ 1.2 mm² ellipse with 0 Oe and 20 Oe hard-axis select fields. The predicted behavior is in excellent agreement with the experimental measurements of real devices. Figure 8 shows a vector representation of the micromagnetic structure of this element during the switching transition with zero applied hard-axis field. The curling of the end domains is evident at the start of the transition. The reversal begins at the ends and sweeps through the body of the element, giving a crisp transition in the hysteresis curve.

Figure 8. Vector representation of the micromagnetic behavior during a switching transition with zero applied hard-axis field. (from top to bottom) -29 Oe easy-axis field immediately before transition; transition wall sweeps through element; and -30 Oe easy-axis field immediately after switch. Click on the image below to download a QuickTime movie (~871 kb) depicting the transition described above.

STATUS AND CHALLENGES

We have successfully integrated MTJ-based MRAM bits with CMOS in a fully fabricated 256 ´ 2 test vehicle in which the MTJ memory cells were inserted into the back end of a 0.6 mm CMOS process. The read-address access time is 14 ns, and the read-cycle time is 24 ns, consuming 800 mA of current at 3 V operation at room temperature. The program access time is 14 ns. This performance is very encouraging for 0.6 mm technology and is anticipated to improve significantly at smaller lithography dimensions. These results indicate that MRAM has the potential to be a competitive memory with the attributes of high-speed read and write, as well as nonvolatility.

One of the challenges involved in the integration of MRAM technology is temperature compatibility with the CMOS process. Several standard CMOS process steps occur at or above 400°C. As shown in Figure 5, the MR of typical MTJ material begins to degrade at temperatures above 300°C and drops sharply by 400°C. Thus, for a working memory either the MTJ material must be improved to withstand these standard process temperatures, or low-temperature processes must be developed for MRAM technology.¹² For our demonstration circuits, special low-temperature processes were used to prevent the MTJ material from being exposed to higher temperatures during MRAM processing. Improvements in the thermal endurance that would make the materials compatible with standard processes would enhance the manufacturability of the technology.

Obtaining very uniform RA over large wafers is another challenge. Techniques that have been explored include forming the aluminum-oxide tunnel barrier with air; reactive sputtering; plasma oxidation with plasma source; plasma oxidation with power introduced from the target side; and plasma oxidation with power introduced from the substrate side.¹⁶ The results show that all techniques can be made to work. Plasma oxidation is favored due to its simplicity and manufacturing compatibility. It was also discovered that different oxidation methods used in this study caused little difference in MTJ resistance uniformity. The latter is mainly determined by the aluminum metal thickness uniformity. Modeling based on Simmons' theory supports the experimental finding. This illustrates that the key to better MTJ RA uniformity is to improve the aluminum metal layer thickness uniformity.

A final challenge is producing MTJ material with very low RA. As bit sizes are reduced, MRAM may require material with lower RA. In addition, use in hard-disk read heads would require a much lower resistance for the first generation of product. Obtaining a thinner tunnel barrier without losing MR is one of the key factors to achieving low RA. The MR loop shown in Figure 2 is for an MTJ structure with only 0.7 nm of aluminum oxidized to form the tunnel barrier. Its RA is only 480 W-mm². This low RA value is appropriate for future generations of MRAM and close to the range that will be useful for read heads.

ACKNOWLEDGEMENT

This work was partially supported by the U.S. Defense Advanced Research Projects Agency.

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J.M. Slaughter, E.Y. Chen, R. Whig, B.N. Engel, J. Janesky, and S. Tehrani are with Motorola Labs, Physical Sciences Research Laboratories.

For more information, contact J.M. Slaughter, Motorola Labs, MD-EL508, 2100 East Elliot Road, Tempe, Arizona 85284 (UPDATED ADDRESS: Mail Drop ML34, Motorola Labs, 7700 South River Parkway, Tempe, Arizona 85284).