Renu Whig

Bio: Renu Whig is an academic researcher from Motorola. The author has contributed to research in topics: Tunnel magnetoresistance & Layer (electronics). The author has an hindex of 11, co-authored 28 publications receiving 818 citations.

Recent developments in magnetic tunnel junction MRAM

Abstract: We summarize our progress on Magnetoresistive Random Access Memory (MRAM) based on Magnetic Tunnel Junctions (MTJ). We have demonstrated MTJ material in the 1-1000 k/spl Omega/-/spl mu/m/sup 2/ range with MR values above 40%. The switching characteristics are mainly governed by the magnetic shape anisotropy that arises from the element boundaries. The switching repeatability, as well as hard axis selectability, are shown to be dependent on both shape and aspect ratio. MTJ memory elements were successfully integrated with 0.6 /spl mu/m CMOS technology, achieving read and program address access times of 14 ns in a 256/spl times/2 MRAM.

A Fully Functional 64 Mb DDR3 ST-MRAM Built on 90 nm CMOS Technology

Abstract: A spin torque magnetoresistive random access memory (ST-MRAM) holds great promise to be a fast, high density, nonvolatile memory that can enhance the performance of a variety of applications, particularly when used as a non-volatile buffer in data storage devices and systems. Towards that end, we have developed a fully functional 64 Mb DDR3 ST-MRAM built on 90 nm CMOS technology. The memory is organized in an 8-bank configuration that can sustain 1.6 GigaTransfers/s (DDR3-1600). We have run standard memory tests, such as a March6N pattern, on the full 64 Mb at 800 MHz with 0 fails for greater than 10 5 cycles. Full functionality was also verified from 0°C to 70°C with no significant change in performance. The bits are magnetic tunnel junctions (MTJs) having an MgO tunnel barrier and a magnetic free layer made of a CoFeB-based alloy with an in-plane magnetization, but with an out-of-plane anisotropy reduced by more than 50% due to an enhanced perpendicular surface anisotropy. To enable the 64 Mb performance, we developed an MTJ stack that has low switching voltage (Vsw), high breakdown voltage (Vbd), and excellent switching reliability with tight distributions. The ST switching distribution has σ ≈ 10%, and we found excellent agreement with a single Gaussian distribution down to an error rate . For our optimized material, the Vsw/Vbd ≈ 0.3, and the separation between Vsw and Vbd is ≈ 25σ. The energy barrier to magnetization reversal (Eb) was characterized using both time-dependent coercivity and higher temperature to accelerate reversal. We found the average Eb ≈ 70kbT.

Process integration of a single chip three axis magnetic field sensor

Abstract: A semiconductor process integrates three bridge circuits, each include magnetoresistive sensors coupled as a Wheatstone bridge on a single chip to sense a magnetic field in three orthogonal directions. The process includes various deposition and etch steps forming the magnetoresistive sensors and a plurality of flux guides on one of the three bridge circuits for transferring a “Z” axis magnetic field onto sensors orientated in the XY plane.

Spin-torque magnetoresistive memory element and method of fabricating same

Abstract: A spin-torque magnetoresistive memory element has a high magnetoresistance and low current density. A free magnetic layer is positioned between first and second spin polarizers. A first tunnel barrier is positioned between the first spin polarizer and the free magnetic layer and a second tunnel barrier is positioned between the second spin polarizer and the free magnetic layer. The magnetoresistance ratio of the second tunnel barrier has a value greater than double the magnetoresistance ratio of the first tunnel barrier.

CMOS-embedded STT-MRAM arrays in 2x nm nodes for GP-MCU applications

Abstract: Perpendicular Spin-Transfer Torque (STT) MRAM is a promising technology in terms of read/write speed, low power consumption and non-volatility, but there has not been a demonstration of high density manufacturability at small geometries. In this paper we present an unprecedented demonstration of a robust STT-MRAM technology designed in a 2x nm CMOS-embedded 40 Mb array. Key features are full array functionality with low BER (bit error rate), process uniformity and reliability, 10 years data retention at 125C with extended endurance to ∼ 107 cycles. All achieved with standard BEOL process temperatures. Data retention post 260°C solder reflow temperature cycle is demonstrated.

Spintronics: Fundamentals and applications

Abstract: Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic physical principles underlying the generation of carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport in that spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors discuss in detail spin decoherence mechanisms in metals and semiconductors. Various theories of spin injection and spin-polarized transport are applied to hybrid structures relevant to spin-based devices and fundamental studies of materials properties. Experimental work is reviewed with the emphasis on projected applications, in which external electric and magnetic fields and illumination by light will be used to control spin and charge dynamics to create new functionalities not feasible or ineffective with conventional electronics.

Overview of emerging nonvolatile memory technologies

Abstract: Nonvolatile memory technologies in Si-based electronics date back to the 1990s. Ferroelectric field-effect transistor (FeFET) was one of the most promising devices replacing the conventional Flash memory facing physical scaling limitations at those times. A variant of charge storage memory referred to as Flash memory is widely used in consumer electronic products such as cell phones and music players while NAND Flash-based solid-state disks (SSDs) are increasingly displacing hard disk drives as the primary storage device in laptops, desktops, and even data centers. The integration limit of Flash memories is approaching, and many new types of memory to replace conventional Flash memories have been proposed. Emerging memory technologies promise new memories to store more data at less cost than the expensive-to-build silicon chips used by popular consumer gadgets including digital cameras, cell phones and portable music players. They are being investigated and lead to the future as potential alternatives to existing memories in future computing systems. Emerging nonvolatile memory technologies such as magnetic random-access memory (MRAM), spin-transfer torque random-access memory (STT-RAM), ferroelectric random-access memory (FeRAM), phase-change memory (PCM), and resistive random-access memory (RRAM) combine the speed of static random-access memory (SRAM), the density of dynamic random-access memory (DRAM), and the nonvolatility of Flash memory and so become very attractive as another possibility for future memory hierarchies. Many other new classes of emerging memory technologies such as transparent and plastic, three-dimensional (3-D), and quantum dot memory technologies have also gained tremendous popularity in recent years. Subsequently, not an exaggeration to say that computer memory could soon earn the ultimate commercial validation for commercial scale-up and production the cheap plastic knockoff. Therefore, this review is devoted to the rapidly developing new class of memory technologies and scaling of scientific procedures based on an investigation of recent progress in advanced Flash memory devices.

Perpendicular magnetic anisotropy at transition metal/oxide interfaces and applications

Abstract: Spin electronics is a rapidly expanding field stimulated by a strong synergy between breakthrough basic research discoveries and industrial applications in the fields of magnetic recording, magnetic field sensors, nonvolatile memories [magnetic random access memories (MRAM) and especially spin-transfer-torque MRAM (STT-MRAM)]. In addition to the discovery of several physical phenomena (giant magnetoresistance, tunnel magnetoresistance, spin-transfer torque, spin-orbit torque, spin Hall effect, spin Seebeck effect, etc.), outstanding progress has been made on the growth and nanopatterning of magnetic multilayered films and nanostructures in which these phenomena are observed. Magnetic anisotropy is usually observed in materials that have large spin-orbit interactions. However, in 2002 perpendicular magnetic anisotropy (PMA) was discovered to exist at magnetic metal/oxide interfaces [for instance Co(Fe)/alumina]. Surprisingly, this PMA is observed in systems where spin-orbit interactions are quite weak, but its amplitude is remarkably large—comparable to that measured at Co/Pt interfaces, a reference for large interfacial anisotropy (anisotropy ∼1.4 erg/cm2 = 1.4 mJ/m2). Actually, this PMA was found to be very common at magnetic metal/oxide interfaces since it has been observed with a large variety of amorphous or crystalline oxides, including AlOx, MgO, TaOx, HfOx, etc. This PMA is thought to be the result of electronic hybridization between the oxygen and the magnetic transition metal orbit across the interface, a hypothesis supported by ab initio calculations. Interest in this phenomenon was sparked in 2010 when it was demonstrated that the PMA at magnetic transition metal/oxide interfaces could be used to build out-of-plane magnetized magnetic tunnel junctions for STT-MRAM cells. In these systems, the PMA at the CoFeB/MgO interface can be used to simultaneously obtain good memory retention, thanks to the large PMA amplitude, and a low write current, thanks to a relatively weak Gilbert damping. These two requirements for memories tend to be difficult to reconcile since they rely on the same spin-orbit coupling. PMA-based approaches have now become ubiquitous in the designs for perpendicular STT-MRAM, and major microelectronics companies are actively working on their development with the first goal of addressing embedded FLASH and static random access memory-type of applications. Scalability of STT-MRAM devices based on this interfacial PMA is expected to soon exceed the 20-nm nodes. Several very active new fields of research also rely on interfacial PMA at magnetic metal/oxide interfaces, including spin-orbit torques associated with Rashba or spin Hall effects, record high speed domain wall propagation in buffer/magnetic metal/oxide-based magnetic wires, and voltage-based control of anisotropy. This review deals with PMA at magnetic metal/oxide interfaces from its discovery, by examining the diversity of systems in which it has been observed and the physicochemical methods through which the key roles played by the electronic hybridization at the metal/oxide interface were elucidated. The physical origins of the phenomenon are also covered and how these are supported by ab initio calculations is dealt with. Finally, some examples of applications of this interfacial PMA in STT-MRAM are listed along with the various emerging research topics taking advantage of this PMA.

A spin–orbit torque switching scheme with collinear magnetic easy axis and current configuration

Abstract: Spin-orbit torque, a torque brought about by in-plane current via the spin-orbit interactions in heavy-metal/ferromagnet nanostructures, provides a new pathway to switch the magnetization direction. Although there are many recent studies, they all build on one of two structures that have the easy axis of a nanomagnet lying orthogonal to the current, that is, along the z or y axes. Here, we present a new structure with the third geometry, that is, with the easy axis collinear with the current (along the x axis). We fabricate a three-terminal device with a Ta/CoFeB/MgO-based stack and demonstrate the switching operation driven by the spin-orbit torque due to Ta with a negative spin Hall angle. Comparisons with different geometries highlight the previously unknown mechanisms of spin-orbit torque switching. Our work offers a new avenue for exploring the physics of spin-orbit torque switching and its application to spintronics devices.