Magnetoresistive Random Access Memory

TLDR: A review of the developments in MRAM technology over the past 20 years is presented with a particular focus on spin-transfer torque MRAM (STT-MRAM) which is currently receiving the greatest attention.

Abstract: In this paper, a review of the developments in MRAM technology over the past 20 years is presented. The various MRAM generations are described with a particular focus on spin-transfer torque MRAM (STT-MRAM) which is currently receiving the greatest attention. The working principles of these various MRAM generations, the status of their developments, and demonstrations of working circuits, including already commercialized MRAM products, are discussed.

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Figure 4, the chip is fabricated using in-plane MTJ devices in a 1T1-MTJ cell, integrated with 90nm CMOS technology and packaged in a JEDEC standard DDR3 BGA. This chip is compatible with available DDR3 memory controllers and became commercially available from Everspin Technologies, Inc. in 2015.

Figure 23. a) Number of write pulses to breakdown f an MTJ as a function of the pulse amplitude. Each

Figure 19. Comparison of various properties versus cell size between improved Process A versus

Figure 20. Improved Process A resulted in narrower MTJ cell resistance distribution (from [101]).

Figure 30: Illustration of the multilevel storage principle in TA-MRAM with soft reference layer. a) sketch of the

Figure 6. (a) A single layer of ferromagnetic materi l responds to an external field (H) by aligning with it, while the

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Magnetoresistive Random Access Memory
Dmytro Apalkov, Bernard Dieny, J. M Slaughter
To cite this version:
Dmytro Apalkov, Bernard Dieny, J. M Slaughter. Magnetoresistive Random Access Memory. Pro-
ceedings of the IEEE, Institute of Electrical and Electronics Engineers, 2016, 104, pp.1796 - 1830.
�10.1109/JPROC.2016.2590142�. �hal-01834195�

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Magnetoresistive Random Access Memory
Dmytro Apalkov
1
, Bernard Dieny
2
, J. M. Slaughter
3
1. Samsung Electronics, Semiconductor R&D Center, San Jose, California, USA.
2. SPINTEC, Grenoble Alpes Univ, CEA, CNRS, CEA/Grenoble, INAC, Grenoble, FRANCE
3. Everspin Technologies, Inc., Chandler, Arizona, USA
Abstract
A review of the developments in MRAM technology over the past 20 years is presented. The various MRAM
generations are described with a particular focus on Spin-Transfer-Torque MRAM (STT-MRAM) which is
currently receiving the greatest attention. The working principles of these various MRAM generations, the
status of their developments, and demonstrations of working circuits, including already commercialized
MRAM products, are discussed.
Keywords: MRAM, spintronics, spin electronics, magnetic tunnel junctions, tunnel magnetoresistance, spin
transfer torque, STT-MRAM, toggle, thermally assisted MRAM

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OUTLINE
1. Introduction to MRAM technology
1.1 Magnetic tunnel junction devices for MRAM
1.2 Overview of MRAM technology generations
2. Major advancements that enabled MRAM and stimulated commercial development
2.1 Tunneling Magnetoresistance
2.2 Synthetic antiferromagnet (SAF) structures and their application to MRAM
2.3 Savtchenko Switching
2.4 Spin-Torque Switching
2.5 Interfacial perpendicular magnetic anisotropy
3. Demonstrations of working MRAM circuits
4. Requirements for reliable read in an array
5. Field-written toggle MRAM
5.1 Operating principles of toggle MRAM
5.2 Performance of toggle MRAM
5.3 Applications of toggle MRAM
6. Spin-Transfer-Torque-MRAM (STT-MRAM)
6.1. Introduction
6.2. Fundamentals of STT-MRAM
6.2.1. STT-MRAM trilemma
6.2.1.1. Information recording: STT writing
6.2.1.2. Information storing
6.2.1.3. Simultaneous achievement of writability, readability and retention
6.2.2. Breaking STT-MRAM trilemma
6.2.2.1. Material improvements: damping and STT efficiency

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6.2.2.2. Device design improvements
6.2.2.2a. In-plane STT-MRAM with PPMA
6.2.2.2b. Perpendicular MTJ design (p-STT-MRAM)
6.2.2.2c. MTJ structure with two MgO barriers: DMTJ
6.3. STT-MRAM: Remaining challenges
6.3.1. Patterning process
6.3.2. Switching probability: write and read error rates
6.3.3. Read disturb
6.3.4. Long-term data retention
6.3.5. MTJ endurance and breakdown
6.4. STT-MRAM Chip demonstrations
6.5. STT-MRAM: Conclusion and outlook
7. Thermally Assisted-MRAM (TA-MRAM)
7.1. Dilemma between retention and writability / benefit from thermally-assisted writing
7.2. Heating in MTJ due to tunneling current
7.3. In-plane TA-MRAM
7.3.1. Write selectivity achieved by a combination of heating and field
7.3.2. Reduced power consumption thanks to low write field and field sharing
7.4. TA-MRAM with soft reference: Magnetic logic unit (MLU)
7.5. TA-MRAM with soft reference: Multilevel storage
7.6 Thermally assisted STT-MRAM
7.7 Remaining challenges and conclusion regarding Thermally Assisted MRAM
8. Conclusion

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1. Introduction to MRAM technology
Magnetoresistive random access memory (MRAM) is a class of solid-state storage circuits that store data as
stable magnetic states of magnetoresistive devices, and read data by measuring the resistance of the devices to
determine their magnetic states. In practice, the magnetoresistive devices are integrated with CMOS circuitry to make
chips that are compatible with mass-produced semiconductor electronics. Such circuits have been designed around a
variety of magnetoresistive devices, but commercially-produced MRAM products, and the vast majority of MRAM
technologies being developed for future commercial MRAM technologies, are based on magnetic tunnel junction
(MTJ) devices. All of these circuits are resistive memories in terms of the read operation; it is the method of writing
the magnetic state that sets apart the different types of MRAM technology. Some of the heavily-studied write methods
include Stoner-Wolfarth-type field switching, Savtchenko switching (also a field-switching method), spin-torque
switching, and thermally-assisted switching (heat with field or spin torque). Two methods have so far been
commercialized: toggle MRAM, which uses Savtchenko switching[1], has been in mass production since 2006,[2]
and spin-torque switched MRAM is in the early-stages commercial production[3].
Advances in MRAM technology have been closely linked with advances in the understanding of magnetic
and magneto-transport properties of ultra-thin films, including: tunneling magnetoresistance (TMR), MgO-based MTJ
materials for giant TMR, synthetic antiferromagnet (SAF) structures, interfacial perpendicular magnetic anisotropy
(PMA), and spin-transfer torque (STT). The application of scientific discovery to commercial technology seen in this
field is striking in its breadth and speed of adoption. In this paper we review the major developments that are driving
accelerating interest and adoption of MRAM, key considerations for functionality and scaling to higher densities, and
the status of the major technology types.
1.1 Magnetic Tunnel Junction Devices for MRAM
Figure 1 shows the most basic magnetic tunnel junction structure, two ferromagnetic layers separated by a
dielectric spacer layer, the tunnel barrier. When the tunnel barrier is very thin, typically < 2nm, quantum mechanical
tunneling of electrons through the barrier makes the MTJ behave like a resistor having a resistance that depends
exponentially on the barrier thickness and is proportional to the inverse of the in-plane barrier area. The tunneling
current is spin-polarized, due to the asymmetric band structure of the ferromagnetic electrodes, giving rise to the
tunneling magnetoresistance as shown in Figure 1. The relative orientation of the magnetizations in these two layers
determines the resistance of the MTJ device. For most materials, the resistance is low when the magnetizations of the
two layers are parallel, because the majority band electrons can tunnel into the majority band on the opposite side of
the barrier. When the orientation is antiparallel, the resistance is high since the majority band electrons have to tunnel
into the minority band of the opposite electrode.