Magnetic Recording and Magnetic Memory

Bruce D Terris

IEEE Magnetics Society Summer School Virginia Commonwealth University June 5, 2019

I. Hard Disk Drives Disk Drives 1. Data Storage Industry --- still alive and thriving 2. Short History of Magnetic Recording 3. A look inside the HDD 4. Head and Media Technology 5. Energy-Assisted Recording Technology for the Future • Heat-Assisted Magnetic Recording (HAMR) • Microwave-Assisted Magnetic Recording (MAMR)

II. Magnetic Random Access Memory 1. Comparison of Memory Technologies 2. Basics of Spin-Transfer Torque MRAM and Challenges 3. Other MRAM approaches: SOT-MRAM, VCMA

Cloud Online Lifestyle 747 16 661 4-yr CAGR: +9.0% 17 582 538 529 18 330 19 22 243 4-YR CAGR% 182

99 132 -3.8% Performance Ent s 35.2% Capacity Ent B 81

E 94 93 88 89 52 3.5% Content Solutions

Exabytes 14 71 75 90 100 18.1% Digital Video 18 21 28 144 32 22.2% Gaming 113 102 98 100 -8.8% Desktop

122 Notebook 96 95 91 78 -10.6%

CY14 CY15 CY16 FC CY17 FC CY18 FC

17% 2% 8% 13% 13% YTY Change

Source: WDC Personal CE Client PC

Need to continue increasing storage density of HDDs (cost/space/power)

Big Data Data Fast Data Aggregation Streaming Analytics

INSIGHT MOBILITY Batch Analytics Machine Learning REAL-TIME PREDICTION RESULTS

Modeling

PRESCRIPTION Artificial SMART MACHINES Intelligence Scale Performance Insatiable Growth in Data HDDs continue to play an important role in the future of data storage

Total Storage Shipped (ZB) 3000 By 2020 Fast Data 2500 ~ Big Data 70%

2000 Flash of the ~10 ZBs stored will still reside on HDDs 1500 ~90% in data centers 1000 HDD 21 500 1 ZB = 10 bytes or 1 billion 1TB drives 0 CY2016 CY2017 CY2018 CY2019 CY2020 CY2021 CY2022

Source: WDC Analysis ©2017 Western Digital Corporation or its affiliates. All rights reserved. Market Glance: • 94M HDD shipped in 1st quarter 2018 • Total exabytes shipped is increasing. • Number of drives decreasing. • Average HDD cost ~$60

Towards smaller form factors

Apple iPOD • 160 GigaBytes • Two 1.8” disks • 3600 RPM • 228 Gbits/in2 • 52 MBytes/s

©2016 Western Digital Corporation or affiliates. All rights reserved. The trend to ever smaller HDD’s has stopped though … ©2017 Western Digital Corporation or its affiliates. All rights reserved. 6/4/2019 9 1 disk, 1 or 2 heads

Electronic Card Disk Enclosure Filter - cleans air

Disk(s) - stores written information

Spindle Motor & Disk Clamp - holds and spins disks

Load/Unload Ramp Actuator Voice - parks head off media Coil Magnets during non-use

Actuator Head(s) - positions the heads - writes and reads data

Flex Cable Suspension - provides electrical connection from - holds suspended heads, provides electrical suspension to electronic card connections from head to flex cable

Electronic card Provides data interface to disk controller Control operation of disk drive (spindle, actuator, position servo) Encodes written data and decodes read back data Provide read/write signals to heads via flex cable

CPP sensor bits

Slider Media

Data tracks Read Head Air bearing Trackwidth & Write Head spacing Lubricant Disk overcoat “UP” bit Disk Magnetic media

“DOWN” bit Head/Disk interface

Bit density Slider (kilobit / inch = kbpi) Disk Suspension Arm

Areal Density Track density = kbpi x ktpi (kilotrack / inch = ktpi) Spindle = Giga-bit/in2 Track Voice-coil motor actuator Typical is 2 disks 2m x 2m MFM image & 4 heads for laptop-HDD ©2016 Western Digital Corporation or affiliates. All rights reserved. Recording15 GN and 30 GN (25.7 Basics Gb/in2) and 48 GH (21.7 → Gb/inScaling2)

Bit length (along track) AFC MEDIA 3 2 (FIRST INDUSTRY PRODUCT) 15 GBytes/DISK Bit width Reader Writer 2 DISK DRIVE CAPACITY (crosstrack) GLASS MEDIA SUBSTRATE

30 Gbytes RAMP LOAD/UNLOAD 25.7 Gbits/In2 Disk motion Return Pole

ADVANCED GMR HEADS

Shields Main Pole

CPP sensor Multiturn Write Coil Current-Perpendicular-to-Plane 4 Head- Hard Recording Layer Media Exchange break layer Spacing 1 Soft Magnetic underlayer Written Disk data bits Write flux Disk motion

Most straightforward method to increase storage bit density is to shrink everything → scale down (head dimensions, media thickness, media grain size, head-media spacing, etc…) Must maintain signal-to-noise ratio while scaling → New technologies to boost signal and/or reduce noise (Ex: Longituginal recording → perpendicular; Anisotopic magnetorestive sensor → giant magnetoresistive sensor)

“Slider”

Read/Write Elements ~1000 process steps Air Bearing Surface

R. Wood, ISPS 2016

Disk = SF Bay Area (95 km dia)

1bit = 1 finger (1.5 × 8cm) Fly height 5mm Head = Boeing 787

Fly speed @ O.D. = 7200 rpm × 300 km 8 I.D. = 0.12 c (c=3x10 m/s)

Fly height = 5mm 7200 rpm O.D. HDD × 1 million = “a Boeing 787 flying at 12% of the speed of 300 km circumference light (at disk’s O.D.) 5mm above a 95 km wide disk and seeking, reading and writing bits of information the size of a human’s finger”

• 2013: First helium-sealed drive from HGST, 6TB Ultrastar He6 had 5 disks • Breakthrough in sealing the helium in the drive • Current generation: 14TB, Ultrastar He14 has 8 disks • Advantage of He: less drag → less power & less noise. Thinner disks, pack in more disks → higher capacity drives

Western Digital's HGST 8TB helium-filled drives were used, whereas capacity and cost limitations ruled out SSDs.

They wrote time-slice data using a round-robin algorithm across the 32 hard drives. These drives are mounted in groups of eight in four removable modules.

The low ambient air density at the telescope sites necessitated sealed, helium-filled hard drives, both for the system disk for recorders and also in all the data recording modules. Ordinary air-filled drives crashed when tried out in 2015; the air was too thin to provide the cushion needed to support the heads.

“UP” bit

“DOWN” bit

Bit density (kilobit / inch = kbpi)

Read/write heads: 1 wafer (150 or 200 mm) has ~100,000 heads Wafer spends 4 months in the fab

Media: 20-24 sputter chambers Sputter output ~ 800-1500 disks/hour → new disk every 3.5 sec

1st Sense Sensor MR Year Density Structure Current Technology Effect (Gb/in2) Geometry

0.01 Thin-film 1979 N/A N/A Gb/in2 Inductive

Insulator Shield

NiFe Free 0.1 Lead Layer Lead Anisotropic 1991 MR Sensor Hard Bias Hard Bias CIP Gb/in2 MR Spacer NiFeX SAL Shield

Insulator Shield AP Pinned Co Layer 2 Lead Lead Hard Bias Hard Bias 1997 2 Spin Valve Giant MR CIP Gb/in Cu Spacer NiFe Free Layer Shield

Shield

CoFe/NiFe Free Layer 100 Spacer Spacer Tunneling 2006 Tunnel Valve Hard Bias Hard Bias CPP Gb/in2 Insulator Insulator MR MgO Tunnel Barrier AP Pinned CoFeB Layer Shield

? >1Tb/in2 ? ? ? ?

The edges of each recorded bit

follow the grain boundaries » Targetsize bit → edge noise

More noise

» Targetsize bit 500 Gb/in2: bit size ~ 15 nm x 65 nm Less noise B R →For smaller bits (higher density), SNR  W need smaller grain sizes to avoid D D increasing noise B=bit length →Scale media microstructure together with rest of recording system = 8.5 nm +/- 2.5 nm D=grain diameter Rw= reader width ©2016 Western Digital Corporation or affiliates. All rights reserved. Data Density, Grain Size, and Thermal Stability

stored magnetic energy  anisotropy x volume KuV * thermal energy  temperature and time} k BT

Hd - +  MS - + z CONVENTIONAL - +  - + H - + - + W MEDIA - + - + - + - + - + B

2.5

2.0 1.5 E + B - 1.0 EB 0.5

0.0

energy (eV) energy -0.5 5050 nmnm -100 -50 0 50 100 150 200 250 300

magnetization angle,  (deg)

• to increase density, need to scale SOLUTIONS: grains smaller • improved writability (perpendicular media) • smaller grains are thermally • work with larger grains: patterned media unstable (data erases itself!) • work with higher anisotropy: energy assisted recording

◼ Essential to continued areal density growth • Higher head fields, higher coercivity, thicker media, greater thermal stability • First product introductions in 2005 and 2006

◼ All HDD are perpendicular.

◼ Media is part of write head.

◼ Enabled new media technology, such as exchange spring

Grain size has been 2005/2006 constant since PMR was introduced, but density still increased 5-fold from 150-750 Gb/in2 Progress via reduction ~ thermal stability in magnetic cluster size limit for CoCrPt based media and distribution (lateral exchange) by using a multi-layered media structure CPCB Cap CPCB Cap ECL-1 Grain boundary, w~1nm G3 Oxide Single Single Mid-Hk G2 Oxide Mid-Hk Oxide G1 High Ku Oxide Oxide Onset Layers Ru Ru Ru Seed Layer Seed Layer Seed Layer(s) SUL-2 SUL-2 SUL-2 Ru Ru Ru D SUL-1 SUL-1 SUL-1 ©2016 Western Digital Corporation or affiliates. All rights reserved. Why Grain Size has not decreased

When using smaller grain Ru to reduce CoCrPt grain pitch, ➔ thinner grains boundaries ➔ more lateral exchange between grains ➔ larger magnetic cluster size (measured by MFM, scattering, loops,…) ➔ negates benefit of smaller grains, and also leads to larger exchange distribution

Thin boundary GP ≈ 7 nm 24 medium boundary GP ≈ 8 nm 22 GP ≈ 9 nm Thick boundary 20 Normalized Cluster 18 Size = Cluster Size / GP 16

Magnetic cluster size (nm) 14

10 7.0 8.0 9.0 10.0

Grain pitch (nm)

Solution to actually reducing grain size from here on: CPCB Cap ECL-1

➔reduce grain core, fixed boundary thickness G3 Oxide ➔ maintains cluster size, but reduces thermal stability ▪ ECL-1 ➔ need to increase Ku G2 Oxide

➔ too hard to write G1 High Ku Oxide ➔ reduce vertical exchange to allow Onset Layers more incoherent rotation, also reduced lateral exchange in cap Ru ➔ But, reduced lateral exchange also increases distributions ! Weaker vertical interlayer coupling

ECL-1

G3 Oxide ▪ ECL-1

G2 Oxide

G1 High Ku Oxide

Onset Layers

~ 150 Gb/in2 (2006) ~ 700 Gb/in2 (2012)

MCL = Magnetic Cap Layer, GRL = Granular Recording Layer

Measured grain boundary width

0 1.9 nm

50A TaN/[60A CuN/10A Ta]10/50A TaN

▪ Voronoi growth will have ~22% size distribution • Random grain seeds, isotropic 2D growth • Exchange exponential in boundary thickness Variation in grain boundary width ▪ How can we better define grain nucleation sites?

1. Shallow topographic template ~3 nm US7776388B2 (2007) patterned layer US8048546B2 (2009) - Other in following years substrate

2. Underlayer nucleation and growth

3. Co-sputtering of mag alloy and segregant

En Yang et al., Nano Lett. 15, 4726 (2016)

Innovation: Pattern crystalline Pt pillar. Grow Ru on top of it

Short range Long range

LogNormal R2 = 99.0% SigmaD/D = 13.4%

sD/D= 8.03%

EFTEM, red=Fe

B R SNR  W D D B=bit length D=grain diameter Hhead ~ Hk,media ~ Ku Rw= reader width

The problem: To increase SNR, need small grains. Small grains are thermally unstable. To avoid thermal instability, increase grain anisotropy Ku. Increasing Ku increasing media Hc and makes the medium more difficult to write.

Writer Writer Spin Torque Oscillator (STO) Laser

Media Media

• Microwave fields emitted by a Spin • Heat from laser allows writing Torque Oscillator (STO) allows writing higher coercivity media higher coercivity media MAMR HAMR AD Zhu1: 4 Tb/in2 Victora2: 4.7 Tb/in2 Media Grain=4.5nm, Hk=40kOe, KuV/kT=60 Grain=5.5nm, Hk=75kOe, KuV/kT>100 Head STO=45nmx12nm, 35GHz ideal in-plane rot. NFT=36nm lollipop, narrow reader 1: J.-G. Zhu, “SNR and Areal Density Gain in MAMR With Segmented Media,” IEEE Trans. Magn., vol. 50, no. 3, 3200809-1 – 3200809-9. DOI: 10.1109/TMAG.2013.2285215 2: Z. Liu,Y. Jiao, and R. H. Victora. “Composite media for high density heat assisted magnetic recording,” Appl. Phys. Lett. 108, 232402 (2016); https://doi.org/10.1063/1.4953231 Spin Torque Oscillator (STO)

• Leveraging the progress of spintronic technology, STO is the most critical development area in MAMR application

External Mag. Field

Reference Inter- Field Generation Layer layer Layer (FGL) Writer Electric Spins precession Reflection

Passage Torque Balance Spin transfer Magnetic Spin-torque oscillator (STO)

Electric Current

©2016 Western Digital Corporation or affiliates. All rights reserved. MAMR Working Principles • STO generates an AC magnetic field. • AC field assists media switching via media FMR (Ferromagnetic resonance) • Media structure tuned for optimum frequency response • MAMR technology enables writing of high Hk media. STO

MP SPL FGL TS Hac

e- Writer M

Write field AC field

Media grain Write field Media precession

©2016 Western Digital Corporation or affiliates. All rights reserved. Microwave Assisted Recording (MAMR) J. Zhu, Carnegie Mellon Univ. STO: LLG + spin transfer 휕퐦 휕퐦 = -훾 퐦 × 퐇 + 훼 퐦 × − 훽(퐦 × 퐦 × 퐩 ) 휕푡 eff 휕푡

PMR

(GHz)

ac f Microwave assisted Region

Media: Hk = 30 kOe; α =0.01 o Applied field Ha: 0.2 ns rising time, 2 away vertical direction Ha/Hk

AC field Hac: Hac/Hk = 0.1

▪ HAMR is a scalable technology. Opportunity for 1.0 to 4.0 Tbpsi ▪ FePt media allows grain size scaling to support much higher density than PMR

✓ Effective field gradient several times higher than PMR ➔ Small grains and high SNR ▪ Near Field Heating by Laser spot with Write head and making data track < 50nm

Ultra Hard FePt Media Laser + Magnetic Head

dH dH dT eff = k  dx dT dx

▪ New magnetic material alloy : FePt. C-Axis ▪ High temperature growth needed to form proper crystallographic phase – requires Disordered FePt Ordered FePt new high-temperature substrate. ▪ Multiple FePt layers and segregants required for microstructure optimization. heating Difficult to make as thick as for PMR. PMR Media HAMR Media ▪ Minimize the variation of Curie Temperature of each grain. CoPtCr ▪ Thermal design of heat sink➔ maximize FePt thermal gradient and minimize power.

dH dH dT eff = k  dx dT dx

PMR-media HAMR-media 20 nm FePt-L10 CoCrPt + segregant media (high temp.) (RT, columnar) MgO seed (diff. barrier) Ru (hp) (set media grain grain isolation) seed grain heat sink (set texture) Ru (lp) (set grain size) heat sink seed (set orientation) and adhesion

• all RT deposition process (no inter-diffusion) • high temperature media deposition (L10-order) • high pressure Ru grain isolating layer • MgO seed also acts as diffusion barrier • 1:1 grain relation between seed and media • continuous grains in MgO seed • step by step structure built up of grain • no 1:1 grain relation between seed and media orientation, grain size and grain isolation • epitaxy sets grain orientation, easy axis

In addition to traditional PMR media parameters, new media parameters become important for HAMR, such as optical and thermal layer design, thermal gradients, Tc, sigma Tc, …

50 Counts LogNormFit

40 =8.4 nm = 8.4 nm w=0.18 30 w = 0.18

20 Counts

0 0 2 4 6 8 10 12 14 16 18 20 Grain Diameter (nm)

- small grains - larger grains - smaller grains - tight size distribution - tight size distribution - tight size distribution - spherical grains - more columnar grains - even more columnar grains - low signal - increased signal - further increased signal - rough media - smoother media - smoother media

Laser Induced Protrusion Oxidation of Carbon Overcoats

NFT protrusion & HAMR Head-Disk Arrhenius Equation Interface Head: 100 - 300oC Activation S. A. Arrhenius , Z. Phys. Chem. 4, 226, 1889. Energy

Reactant Thermal Actuator Bulge Heat

C(s) + O2(g) → CO2(g) Media: Products 400-600oC

▪ New heat sources (laser diode, scattered light, NFT heating) cause protrusion over various time-scales and Apex rounding length-scales and new challenges for maintaining low head due to heat disk spacing and high recording performance. ▪ High temperatures can cause oxidation (combustion) of thin carbon overcoats on head and disk over the lifetime of a product. ▪ Intense optical/thermal fields can lead to carbonaceous build up on NFT. Contact can also lead to back heating.

©2017 Western Digital Corporation or its affiliates. All rights reserved. Conclusions on HDD • HDDs will be around for a long time to come • The modern HDD is packed with high-tech engineering of many disciplines • Major discoveries at the basic research level enabled the rapid growth of storage capacity • Energy-assisted recording – MAMR or HAMR – is a promising future technology for recording densities beyond 1Tb/in2. Engineering breakthroughs are needed to make it a reality.

HDD Areal Density 1.E+01

1.E+00

1.E-01

1.E-02

1.E-03

1.E-04

1.E-05

1.E-06 Arealdensity Tbits/in2 1.E-07

1.E-08

Yuasa et al, MRS Bulletin May 2018

• Access device required for each memory element • Eliminate sneak paths • Reduce power and unintended selection • Apply V/2 on word and bit lines – only V on selected cell • Diodes or non-linear I-V devices • Unipolar or dipolar depending on memory cell type

• STT-MRAM is the only candidate to replace SRAM in cache memory and DRAM in both embedded and stand alone applications. • STT-MRAM also provides high performance alternative for embedded flash (NOR) in applications where non-volatility is required. • PCM and RRAM are better suited for storage applications where higher STT-MRAM is the only emerging memory technology that combines endurance, density is required and speed and energy efficiency of SRAM and DRAM with non-volatility of Flash. lower endurance and Figure from: slower speed can be A. D. Kent and D. C. Worledge, Nature Nanotech. 10, 187 (2015). tolerated. ©2017 Western Digital Corporation or its affiliates. All rights reserved. Energy-speed performance of emerging memories Based on reported experimental results

STT-MRAM speed has advantage in terms of speed and energy efficiency over other emerging memory technologies.

H.-S. P. Wong, C. Ahn, J. Cao, H.-Y. Chen, S. B. Eryilmaz, S. W. Fong, J. A. Incorvia, Z. Jiang, H. Li, C. Neumann, K. Okabe, S. Qin, J. Sohn, Y. Wu, S. Yu, X. Zheng, “Stanford Memory Trends,” https://nano.stanford.edu/stanford- memory-trends, accessed May 16, 2018.

More than 20 years ago: Field-MRAM

1st research program: IBM/Motorola (1995) 1st product: Freescale/Everspin (2006)

Issue in scaling – field disturb of half-select bits No selector !

Physical sketch:

Symbolic description F BL

Free (storage) layer MTJ Tunnel barrier (MgO) Reference (pinned) layer

Figure from: D. Apalkov et al., Proceedings of IEEE 104, 1796 (2016) WL

SL ©2017 Western Digital Corporation or its affiliates. All rights reserved. Physics of the writing operation Writing operation of STT-MRAM is based on spin transfer torque switching of the magnetization

toy model Landau-Lifshitz-Gilbert-Slonczsewski 훕 훕 푑퐦 H α = −γ휇 퐦 × 퐇 − 훼γ휇 퐦 × 퐦 × 퐇 푑푡 0 0 ℏ 퐽 푆푇푇 +γ휇0휂 퐦 × 퐦 × 퐦RL 훕α 2푒 푀푠푡 ′ ℏ 퐽 푆푇푇 +γ휇0휂 퐦 × 퐦RL 훕H 퐦RL 퐦 2푒 푀푠푡

퐇 • 훕푆푇푇oposses 훕 and if large 훕푆푇푇 α α 훕α 훕H H enough it causes switching of Tunneling electrons from one electrode interact with the direction of 퐦. second electrode and are either reflected back or • 훕푆푇푇 affects precessional transmitted with new spin orientation. By 훕푆푇푇 H 퐦 α frequency, but does not cause conservation of angular momentum, the second switching of 퐦. electrode feels a torque acting upon it. If incoming electric current is high enough, torque can be large enough to switch m. 퐦RL

Perpendicular bits in-plane bits single uniaxial anisotropy shape + easy plane anisotropy

top view top view 퐦 퐦 X STT needs to overcome side view side view demagnetization field H

J. Z. Sun, Phys. Rev. B 62, 570 (2000)

2푒훼 2푒훼 퐻 퐽 = 휇 푀 푡퐻 퐽 = 휇 푀 푡 퐻 + ⊥ 푐0 ℏ휂 0 푠 퐾 푐0 ℏ휂 0 푠 퐾 2

Lower 퐽푐0 for perpendicular STT-MRAM => faster, more power efficient, denser, better endurance than in-plane MRAM

Read: Write with high current Tunnel Magnetoresistance Read with low current

Write: Spin Transfer Torque

Non-volatility of STT-MRAM is due to uniaxial magnetic anisotropy of the free (storage) layer Hk

• For thermally activated magnetization reversal of a single domain particle with uniaxial and easy plane anisotropy only, switching probability 푃푆푊 over time t is described by the Neel-Brown relaxation time formula with relaxation time and energy barrier E : 1 b 퐸 = 휇 푀 퐻 푉 푏 2 0 푠 푘 푡 퐸푏 푃푆푊 푡 = 1 − exp − 휏 = 휏0exp 휏 푘퐵푇

휏0 - inverse attempt frequency ~ 1 ns W. F. Brown, Phys. Rev. 130, 1677 (1963).

퐸푏 ∆ = - thermal stability factor 푘퐵푇

-7 o Most applications require PSW < 10 at elevated temperatures (> 40 C or more) -> ∆ > 60 Smaller bits need higher magnetic anisotropy in order to maintain the required thermal stability Impossible to maintain high enough thermal stability with just shape anisotropy below ~60 nm size

High density applications -> perpendicular STT-MRAM

MgO iPMA 푒푓푓 휇0푀푠퐻 푉 Free layer (CoFeB based) ∆ = 푘 Boron sink nanolayer (Ta) 2푘퐵푇 MgO iPMA (due to hybridization of Fe-O orbitals) 푆 푒푓푓 2퐾푈 퐻퐾 = − 푁푧푧푀푠 Reference layer (CoFeB based) 휇0푀푠푡퐹퐿 Ru 푆 퐾푈 - interfacial magnetic anisotropy Hard layer (Co/Pt multilayer) energy density 푁푧푧- demagnetization factor

Interfacial PMA competes with demagnetization energy Effective perpendicular magnetic anisotropy is difference between interfacial anisotropy field and demagnetization field 푒푓푓 Possible to obtain high enough 퐻퐾 to have ∆ > 70 for small device size

cap MgO cap Free layer (CoFeB based) Boron sink: Ta,W,Mo,Hf

MgO barrier

Reference layer: CoFeB based Ru, Ir, Rh

Hard layer: Co/Pt, Co/Ni, Co/Pd multilayer

Yuasa et al, MRS Bulletin May 2018

• Demonstration of stand-alone 4 Gb STT-MRAM main memory (to replace DRAM) by SK Hynix/Toshiba.

S.-W. Chung et al., IEDM16 -659 (2016)

• Demonstration of 8Mb 1T-1MTJ STT-MRAM embedded in 28nm CMOS logic (to replace SRAM) by Samsung.

Y. J. Song et al., IEDM16 - 663 (2016)

Significant development of STT-MRAM for both stand alone and embedded memory platforms.

June 2017 According to reports, Taiwan Semiconductor Manufacturing Company (TSMC) is aiming to start producing embedded MRAM chips in 2018 using a 22 nm process. This will be initial "risk production" to gauge market reception

Sept 2017 --- GLOBALFOUNDRIES Announces Availability of Embedded MRAM on Leading 22FDX® FD-SOI Platform

March 2019 SAN FRANCISCO — Samsung announced commercial production of its first embedded MRAM (eMRAM) product based on its 28-nm FD-SOI process.

Scaling cell size < 20 nm for high density memories

휇0푀푠퐻푘푉 As volume of the free layer decreases Hk has to increase in order to maintain the required ∆ = thermal stability -> increase in iPMA 2푘퐵푇

2푒훼 Need switching current reduction for lowering power, endurance, higher speed, and 퐽 = 휇 푀 푡퐻 푐0 ℏ휂 0 푠 푘 shrinking the transistor footprint -> reduced damping  is crucial and/or Mst

Fabrication of high density memory arrays -> research into milling and etching methods for MTJs in tight pitch arrays Maintaining tight distribution of relevant resistances, voltages and Δs at small dimensions -> materials and process uniformity Increasing TMR may be required to compensate for wider R distributions Alternative memory cell designs may be required for sub-10 nm scaling, as well as ultrafast applications that require high endurance and power efficiency, such as cache memories

Dmytro Apalkov, J. Slaughter

• Improve STT efficiency – Reduce damping and Mst – Double MTJ – increase torque – Add spin polarization layers • High Density Patterning – Ion beam etching at pitch • Small devices – Shape anisotropy • Crosspoint MRAM • SOT and VCMA

• Basic SOT-MRAM cell: - consists of spin-orbit torque layer (typically a heavy metal like Pt, Ta) in contact with MTJ - to write current is passed through SOT layer, while to read it is passed through MTJ - cell is 3-terminal and requires2 transistors per cell to controllably write and read

• SOT-MRAM potential advantages: - higher endurance (as write voltage is never applied across tunnel barrier) - lower write-energy (as write current is applied through low resistance metal instead of high- resistance MTJ) - higher writing speed (as higher overdrive current can be applied due to the above) - lower write error rate (for the same reason as the above)

Figure from: Y. Kim et al., IEEE Trans. Electron Devices 62, 561 (2015)

S. Fukami et al, Nature Nanotech 11, 621 (2016)

Type Z Type Y Type X PMA In-plane In-plane Highest density Lower density Lower density fastest Precessional, slower fastest Highest current Lowest current Middle current Bias field (x-y) No field Bias field (z) ©2017 Western Digital Corporation or its affiliates. All rights reserved. SOT Switching (IMEC)

Li et al, MRS Bulletin 2018 Amiri, IEEE Trans Magn 51, 2015

• The VCMA is used to select the bit to write

• This can work in a chain; in a cross-point configuration, however, sneak paths would make it impossible to read the bit.

• In theory, this idea gets bit density close to that of a 1T-1R memory, since the large “write” transistor is shared by many bits, and each bit has a small “read” transistor that requires very little current.

• Read can be done in a polarity that helps increase the energy barrier, reducing read disturb

• STT-MRAM is the only emerging memory technology that combines endurance, speed and energy efficiency of SRAM and DRAM with non-volatility of Flash • Working chips have been demonstrated • Several companies are offering/developing MRAM for embedded and stand alone memory applications • Further materials, process and device physics developments are needed for pushing the memory cell size below 20 nm, to enable > 10 Gb memory capacities. • New technology applications are becoming feasible with new physics