Patterned Media Technology: Islands of Opportunity

Thomas R. Albrecht

Hitachi Global Storage TechnologiesSan Jose Research Center

2

Technology Roadmap: In Flux

2012

0.01

0.1

1

10

100

1000

10000

1980 1990 2000 2010

Are

al D

ensi

ty (G

b/sq

.in.)

100% / yr

30% / yr

60% / yrStandard FFMR headPRML channel Thin film disk

GMR head

DemosPerpendicular

Products

BPTAR?thermal instability regime

TAR?

BPMDTM?

3

Bit Density, Grain Size, and Thermal Stability

50 nm50 nm

-100 -50 0 50 100 150 200 250 300

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

ener

gy (e

V)

magnetization angle, (deg)θ

E-B

E+B

stored magnetic energy ∝ anisotropy x volumethermal energy ∝ temperature Tk

VK

B

u *}

• to increase density, need to scale grains smaller• smaller grains are thermally unstable (data erases itself!)

SOLUTIONS:• work with larger grains: bit patterned media (BPM)• work with higher anisotropy: thermally assisted recording (TAR)

CONVENTIONALCONVENTIONALMEDIAMEDIA

4

Bit Patterned Media (BPM): One Bit per Magnetic Island

PATTERNED MEDIA:• single pre-patterned large grain per bit

CONVENTIONAL MEDIA:• many small random grains per bit

5

Making BPM: The Semiconductor Industry Won’t Solve Our Problem

193 nm193 nm immersion with

water

DRAM ½ Pitch (nm)

2007 2010 2013 2016 2019

65

45

32

22

193 nm immersion with water193 nm immersion with other fluidsEUV, ML2

EUV193 nm immersion with other fluids & lenses193 nm with innovative immersion with waterImprint, ML2

EUVInnovative 193 nm immersion Imprint, ML2, Innovative Technology

16 Innovative Technology Innovative EUV, Imprint, ML2,

Research Required DevelopmentUnderway

QualificationProduction

ContinuousImprovement

FLASH

DRAM

The semiconductor industrywill not provide a lithography solution in time for patterned media

PATTERNED MEDIA

2005 ITRS Roadmap

6

Bit Patterned Media: Fabrication Overview

• Master pattern generation by rotary-stage e-beam lithography

• time consuming; features are ~20 nm diameter

• Low-cost pattern replication via nanoimprintlithography on each disk surface

• millions of replications

• Pattern transfer to substrate by reactive ion etching

• creates 1012 pillars on a substrate

• Blanket deposition of magnetic layer• material on tops of pillars forms isolated magnetic islands

e-Beam Lithography Master Mold Etching

UV-Cure Nanoimprinting

Disk Substrate Etching

Mag Layer Deposition

7

Master Pattern Generation Strategies

400 1600140012001000800600 20001800

rotary stage e-beam

e-beam + density multiplier

e-beam + self-assembly

Pattern density (Gbit/sq. inch)

J. Cheng et al. - MIT

TRACKSMASK SUPPORT LOCATION

SHADOW MASK

SUPPORT SPACER

shadow mask density multiplier

block copolymer self-assemblye-beam lithography

E. Dobisz, HGST

300 Gbit/in2

8

High Volume Pattern Replication via Nanoimprinting

• E-beam master will be expensive (many days to write one master pattern)

• Two generation nanoimprinting process envisioned for low-cost replication

• Very inexpensive: much cheaper than bit patterning used in solid-state storage

1 e-beam master10,000 replicananoimprint molds

100,000,000 imprinteddisk substrates

9

Nanoimprinting: Key Issues

Disk

Master

Replica mold• Fidelity of nano-scale features• Adhesion to substrate• Release from mold• Residual layer control• Rapid resist flow / elimination of air bubble trapping• Fast curing (oxygen inhibition, etc.)• Mold replication and mold lifetime• Resist etch resistance for pattern transfer• Scale-up to large areas + double-sided imprinting

substrate

nanoimprint resistresidual layerthickness

Successful imprint of ~50 nm period pillar features

• Patterned media may be one of the first large volume uses of nanoimprinting technology

• Relatively easy case: one “mask” and no overlay/alignment (unlike integrated circuits!)

T. Wu, M. Best - HGST

10

Magnetic Island Formation Approaches

B. Terris et al.

Prepatterned substrate

Magneticfilm

Isolated islands

Trench material

Unpatterned substrate

Magneticfilm

Isolated islands

Method 1: Prepatterned Substrate• clean, fast etching (RIE)

• Si, SiO2, Si3N4

• etch products volatile• “trench material” is present

• possible noise source• large topography (~40 nm)

• needs planarization

Method 2: Patterned Magnetic Film• dirty, slow etching (IBE)

• Co, Pd, Ni, etc.• redep of nonvolatile products

• island edge damage (ion bombardment)• no trench material• less topography

TEM cross sectionJ. Risner - HGST

11

Trench Noise

Cusp WAS

A. Moser, HGST

• 200 nm islands on 400 nm period (low density makes problem easy to see)

• DC erase (40 mA)

• Write one track (1250 fc/mm) (40 mA) – trench noise badly distorts readback

• DC erase with low write current (5 mA) – DC erases trench; islands unaffected

• Trench noise signal can be very large!

12

R/W Testing of BPM Disks (e-Beam Direct Write)

Spin Stand R/W Testing• well isolated islands at various densities• spin stand writing• no write synchronization or servo yet• both spin stand and MFM readback

100 Gbit/in2 (MFM readback)

100 Gbit/in2 (Spinstand readback)

Offs

et(u

m)

190 191 192 193 194 195 196 197 198

-0.2

0

0.2

50nm period, with sync mark

X. Wu, D. Kercher,

J. Risner, M. Best - HGST

13

Mag Layer Challenge: Switching Field Distribution

-12000 -10000 -8000 -6000 -4000 -2000

0.0

0.2

0.4

0.6

0.8

1.0

[Co(3.5)Pd(8.5)] x 8reference layer

4 16 65 180 300 500

Mic

ro-K

err s

igna

l [ar

b. u

nits

]

H [kOe]

( Gbit/in2)Pd (8.5Å)Co (3.5Å)

Co-Pd Multilayer Media: [Co(3.5)Pd(8.5)]x8 Pd(20)

Magnetic Force Microscopy 180 Gb/in2: Broad switching field distribution

Switching field distribution is broader than desired

Higher density (smaller islands) shows increased distribution width

Places challenging demands on write head field gradient

1000 Oe5000 Oe6000 Oe7000 Oe8000 Oe9000 Oe

M. Best - HGST

O. Hellwig - HGST

14

Scaling Behavior of Switching Field Distribution

• Assume: • Gaussian dist. of nucleation fields• Nucleation volume ~25 x ~25 x t nm*

50 nm islands 500 nm islandHn = 1000 Oeσ = 100 Oe

• Island reversal occurs when the lowest nucleation site switches

Nucleation site* Rok Dittrich et al. J. Appl. Phys. 97 (2005) 10J705

Reference: T.Thomson, G. Hu, B.D. Terris, Phys. Rev. Lett. 96(25) (2006) 257204(4).

15

Dipolar Contribution to Switching Field Distribution

• At higher density, dipolar fields (4πMs) become significant source of SFD• Can reduce by reducing Ms, but this leads to reduced readback signal

Hellwig et al, APL 90, 162516 (2007)

16

Write Synchronization

GMR read elementinductive writeelement

CONTINUOUS MEDIA

PATTERNED MEDIA

CONTINUOUS MEDIA:

Magnetic transitions can be written at arbitrary positions

Timing controlled by fixed clock

PATTERNED MEDIA:

Writing must be synchronized to passage of individual islands under head

Misaligned writing produces errors

System must sense phase and frequency of islands and synchronize write current switching

50 nm50 nm

17

The “Uncertainty Zone” – Modeling Performance of BPM

writepole

width of uncertainty zone

island pitch

write sync margin

writing zone

uncertainty zone

hardest island writablesoftest island (re)writable

writepole

media motion

just finished writing this island

-40 -20 0 20 40 60 800

0.2

0.4

0.6

0.8

1

1.2

1.4

Mea

nFie

ldef

f [T]

DX [nm]

DY=0 nmVolume averaged effective field Heff

+3.5σ SFdistribution• 1σ = 8%• 1E-4 BER

hardest island with interaction

uncertainty zone

softest island with interaction

anisotropycenter

RULE: No head switch with center of any island within uncertainty zone

THEREFORE: Island spacing must be greater than width of uncertainty zone

CONSEQUENCE: Wide switching field distribution and finite head field gradient limit densityM. Schabes - HGST

18

Areal Density [Tb/in2] Design Charts for BPM

σprint=3 nm

σprint=2 nm

switching field distribution of the islands

gradient of the effective write field profile

Are

alD

ensi

ty [T

b/in

2 ]

areal density contours [Tb/in2]pooled fabrication tolerance

• centroid jitter• shape jitter

pooled synchronization tolerance• write synchronization jitter• NRO, etc.

BERw=10-6

K1=2.7x105 J/m3

thermal stabilitycontours [K1V/kBT] at 300 K: need >40

disqualified region of parameter space due toreadback jitter or thermal stability

2 Tb/in2 design exampleσHsw=1400 Oegrad(Heff)=390 Oe/nmEb=110 kBT

realistic regime M. Schabes – HGSTTo be presented at PMRC 2007

19

BAR Effects – Tradeoffs with Fabrication and Sync Tolerances

BAR = 1: 4 Tb/in2

M. S

chab

es-H

GST

tighter tolerances

BAR = 2: 3 Tb/in2

BAR = 4: 1.6 Tb/in2 BAR = 4: 4 Tb/in2

20

Low BAR Write Field Penalty

• Write field scales poorly if BAR decreased compared to conventional (BAR) recording

• Solid angle modeling shows that reduced pole tip area caused by lowered BAR results in strongly reduced field

• Modeled geometry assumes a fixed pole width and thickness ratio of 2 and constant values for media and head to media dimensions

• Reduced write field attacks the very advantage that BPM is intended to achieve (thermal stability with good writability)

BAR reduced by 5: Area decrease by 86%

Same BARArea decrease by 29%

BAR reduced by 5: Field drop 58%

T. Olson - HGST

Same BARField drop 11%

Increase Areal density by 40 % Increase Areal density by 40 %

21

How Far Can BPM Go with Exchange Spring?

Use Co3Pt islands and (graded) exchange springs

M. Schabes - HGST

Limited by thermal stability at ~ 30 Tb/in2

Control over lateral coupling:Exchange spring more easily applied to BPM than conventional PMR

22

Pre-Patterned Servo

data tracks servo “sector” track ID Gray code

“quad burst”tracking pattern

track direction (circumferential)

Very precise servo features created along with data track islands

Eliminates need for separate servowriting operation Z. Bandic - HGST

23

Bit Patterned Media: Summary

• Thermal stability drives the need for BPM• Media fabrication looks feasible

• Master pattern generation by high resolution e-beam lithography• Pattern replication by UV-cure nanoimprint lithography• Etching of substrate or mag layer

• Achieving suitable magnetic properties is challenging• Island switching field distribution (limits recording density due to finite head

field gradient)• BPM is Extendible

• Thermal stability may be possible well beyond 10 Tb/in2 with exchange spring

• Integration of patterned media into a hard disk drive requires:• Write synchronization• Pre-patterned servo (reduces cost)

A. Moser - HGST