Spin-transfer torque magnetic random access memory (STT-MRAM) is a novel, magnetic memory technology that leverages the base platform established by an existing 100+nm node memory product called MRAM to enable a scalable nonvolatile memory solution for advanced process nodes. STT-MRAM features fast read and write times, small cell sizes of 6F 2 and potentially even smaller, and compatibility with existing DRAM and SRAM architecture with relatively small associated cost added. STT-MRAM is essentially a magnetic multilayer resistive element cell that is fabricated as an additional metal layer on top of conventional CMOS access transistors. In this review we give an overview of the existing STT-MRAM technologies currently in research and development across the world, as well as some specific discussion of results obtained at Grandis and with our foundry partners. We will show that in-plane STT-MRAM technology, particularly the DMTJ design, is a mature technology that meets all conventional requirements for an STT-MRAM cell to be a nonvolatile solution matching DRAM and/or SRAM drive circuitry. Exciting recent developments in perpendicular STT-MRAM also indicate that this type of STT-MRAM technology may reach maturity faster than expected, allowing even smaller cell size and product introduction at smaller nodes.

For reliable operation, individual cells of an STT-MRAM memory array must meet specific requirements on their performance. In this work we review some of these requirements and discuss the fundamental physical principles of STT-MRAM operation, covering the range from device level to chip array performance, and methodology for its development.

We report our progress on material improvement, device design, wafer processing, integration with CMOS, and testing of STT-RAM memory chips at 54 nm node with cell sizes of 14 and 28 F2 (F=54 nm). A dual tunnel barrier MTJ structure was found to have lower and more symmetric median spin transfer torque writing switching currents, and much tighter parallel to antiparallel switching current distribution. In-plane MTJ devices write endurance data, read and write soft error rates data and simulation fits, and solutions to the long write error rate tail at fast write speeds are discussed.

A first-order phase transition in the magnetization of Fe films, driven by an applied magnetic field, was first reported by Hathaway and Prinz [1]. Further studies were performed on this phase transition using anisotropic magnetoresistance measurements by Riggs and Dahlberg [2]. Here we report the extension of these studies to include temperatures between 77K and 293K. Emphasis is on a determination of the fourth-order and uniaxial anisotropy constants (K1 and Ku). It is shown that the temperature dependence of the anisotropy energies in these films varies with thickness, which may be useful in sorting out the origin and magnitude of different contributions to the total effective anisotropy. The present study suggests that similar studies of (110) iron on other substrates might contribute to achieving a better understanding of in-plane anisotropies in epitaxial films.

STT-RAM (Spin-Transfer Torque Random Access Memory) is a second-generation magnetic random access memory (MRAM) technology that is fast, non-volatile, durable, and scalable to future technology nodes [1-2]. In this paper, we present the latest advances in in-plane and perpendicular STT-RAM development and outline STT-RAM's future prospects, applications and roadmap.

Spin-transfer torque random access memory (STT-RAM) is a potentially revolutionary universal memory technology that combines the capacity and cost benefits of DRAM, the fast read and write performance of SRAM, the non-volatility of Flash, and essentially unlimited endurance. In order to realize a small cell size, high speed and achieve a fully functional STT-RAM chip, the MgO-barrier magnetic tunnel junctions (MTJ) used as the core storage and readout element must meet a set of performance requirements on switching current density, voltage, magneto-resistance ratio (MR), resistance-area product (RA), thermal stability factor (¿) , switching current distribution, read resistance distribution and reliability. In this paper, we report the progress of our work on device design, material improvement, wafer processing, integration with CMOS, and testing for a demonstration STT-RAM test chip, and projections based on modeling of the future characteristics of STT-RAM.

Non-volatile STT-RAM (spin transfer torque random access memory) is a new memory technology that combines the capacity and cost benefits of DRAM, the fast read and write performance of SRAM and the non-volatility of Flash with essentially unlimited endurance. It has excellent write selectivity, excellent scalability beyond the 45 nm technology node, low power consumption, and a simpler architecture and manufacturing process than firstgeneration, field-switched MRAM. A magnetic tunnel junction (MTJ) device (Fig. 1) is used as the information storage memory element, and its magneto-resistance is used for information read-out. To make the STT-RAM technology competitive with mainstream semiconductor memories, the writing current has to be reduced so that the MTJ can be switched by a minimum sized CMOS transistor. In this paper, we discuss our approaches and results in writing current reduction; device read and write performances; robustness against read disturb switching and barrier break down; and prospects of scaling to future smaller nodes.

We present an analytical treatment of the Camley–Barnaś theory of the giant magnetoresistance (GMR) in magnetic layered structures and obtain an exact and general expression for the resistivity. We used this expression to evaluate the resistivity and GMR numerically, comparing the results with experimental observations.

We have developed a laboratory course focusing on the physics of materials. This course, taught in place of a “conventional” lecture-only solid state or condensed matter physics course, helps to prepare students for the technical work force and also serves as a solid, broad-based foundation for students bound for graduate school. In addition, the course illustrates the increasingly interdisciplinary nature of physics. Classroom activities and experiments concentrate on four materials classes: metals, ceramics, semiconductors, and polymers. Experiments include electrical conductivity of metals and semiconductors; ionic conductivity of ceramics; superconductivity in metals, alloys, and ceramics; preparation and characterizations of metallic thin films; scanning tunneling microscopy; magnetic properties of materials; impedance spectroscopy of solid electrolytes; phase diagram determination by differential thermal analysis and x-ray diffractometry; Hall effect in pure and doped semiconductors; dielectric response of polymers; and mechanical properties of polymers.

We begin by identifying the fascinating behavior of most of the thin, flexible magnets normally described as “refrigerator magnets.” Simple procedures are described that illustrate the intriguing physics of these magnets, which produce fields characterized by spatial variations that are quite abrupt compared to those of the more common ceramic magnets. A method is suggested for producing profiles of this spatial variation. The method is based on the use of magnetic field sensors that employ a novel phenomenon called giant magnetoresistance. After reviewing GMR and the sensors, we describe our field-mapping procedure and discuss the results.

We have conducted a survey of the temperature variation of the magnetoresistance in a series of FeMn exchange-biased spin-valve structures. These permalloy-based samples were prepared in an ion-beam sputtering system and feature Co layers inserted at the interfaces with the Cu spacer layer to enhance the interfacial spin-dependent scattering. Typical values for the MR are 4.0% at 295 K and 8.4% at 8 K (sample with 15 Å Co thickness). A control sample with no cobalt showed MR values of 1.3% and 4.3% for those same temperatures. Both the MR ratio and the un-normalized magnetoresistive change ΔR are plotted vs temperature. The MR ratio for the cobalt-enhanced samples exhibits nearly linear decrease with rising temperature. The sample with no cobalt exhibits a temperature variation deviating substantially from linearity, with an upward curvature. The temperature dependence for the MR in these spin valves is examined in the light of interchannel spin-mixing and intrachannel scattering.