2008 Summer School on Spin Transfer Torque

Nano scale device Nano-scale device fabricationfabrication

2-July-2008

Byoung-Chul MinByoung Chul Min

Center for Spintronics Research Korea Institute of Science and Technology

Introduction u

Moore’s Law in Action

Source: Intel

Intel Lithography Roadmap(High-volume manufacturing)(High volume manufacturing)

Source: Intel

Top-Down & Bottom-Up

Si devices shrink to Virus size

Transistor for 90-nm process

Influenza Virus

Source: Intel

Gate Oxides as thin as Atoms

Source: Intel

Spin-transfer torque

Current-induced magnetization switching

• Spin-polarized current can induce magnetization switchingby spin-transfer torque in nano-scale (< 200 nm) magneticdevices

Switching by magnetic-field Switching by spin-polarized current

devices.

urre

nt

urre

nt

witc

hing

Cu

witc

hing

CuSize of the bit

Sw

Size of the bitSw

Size of the bit Size of the bit

Size of STT devices

Yuasa et al., Nature materials 3, 868 (2004)

Ozatay et al., Nature materials 7, 567 (2008)

Spin-Momentum-Transfer (SMT) MRAM

S. A. Wolf, IBM J. RES. & DEV. 50, 101 (2006)

For non-IC applications

•High cost of semiconductor processing tools

•Limited process flexibility- No rapid prototyping possibleNo rapid prototyping possible- Not applicable on fragile substrates

(mechanical chemical layers)

•Needs for novel methods

Source: Intel

Bridging the Gap

Contents

1. Materials :- Thin Film Technology

2 Lith h2. Lithography:- Optical / Interference Lithography- E-beam/Ion-beam Lithography- E-beam/Ion-beam Lithography- Scanning Probe Techniques- Soft Lithographyg p y

3. Patterning Transfer:W t/D t hi- Wet/Dry etching

- Lift-off Technology

Planar structures

Realizing small lateral structures by:

1. (Photo) lithography2. Direct (local) processes

Collection processes and technologies:

LithographyEtching (wet; dry and reactive)Oxidation, Diffusion; Ion implantationDepositionLaser structuringg

Multilevel metallization

Thin film technology

MTJ stack

MTJ Structure TMR vs. magnetic field

Capping layer Ru (50Å) 250

CoFeB (3.0 nm)/ MgO (1.5nm)/ CoFeB (3.0nm)

Ta (50Å)

MgO (15Å)

Free layerTunnel barrier

CoFeB (30Å) 200TMR =204%RA = 43 kΩμm2

( Å)

Synthetic Pinned layer

CoFeB (40Å)

CoFe (20Å)Ru (8Å)

100

150

TMR

(%)

IrMn (140Å)

NiFe (60Å)

Buffer layer Ta (50Å)( Å) 0

50T

Si / SiO2Wafer

Ru (300Å)Ta (50Å) -800 -600 -400 -200 0 200 400 600 800

0

H (Oe)

Thin film technology

A thin film is normally made on a substrate by:PVD CVD d th t h l i PVD, CVD and other technologies.

Vacuum evaporation system

Substrate with condensing atoms

V mVacuum

Atom transport

Evaporation source

E-beam evaporation sourcep

Sputtering

•Bombardment by high energy atomic particles (ions)Bombardment by high energy atomic particles (ions)

•Ejection of atoms of the target by a momentum transfer

D i i h b•Deposition onto the substrate

Sputtering System

Basic MBE system:Structural control during thin film growthStructural control during thin film growth

Steps for making thin films1.emission of particles from source ( h hi h l ) ( heat, high voltage . . .) 2. transport of particles to substrate (free vs directed) (free vs. directed) 3. condensation of particles on substrate (nucleation and growth)( g )

Simple model:

From ad atom via nucleation to continuous filmcontinuous film

Growth process of evaporated Au on Carbon-substrate with constant deposition rateCarbon substrate with constant deposition rate

TEM photographs fromTh i s st s f thThe various stages of growth

Deposition modesp

Layer-by-layer 3D-Island model S-K model(van der Merwe) (Volmer-Weber) (Stranski-Krastanov)(van der Merwe) (Volmer Weber) (Stranski Krastanov)

Clusters of Au observed by AFM

Au islands/clusters/nucleiAu islands/clusters/nuclei

Growth Modes and Surface Energies

0cos0 θγγγ fis −−= 0cos0 θγγγ fisfγ Wetting does not occur, if

sγ0θisf γγγ −>

iγ

fdhE γ2≥Wetting ifisfadhE γγγ −+=

E γ2<

fadhE γ2≥Wetting, if

No wetting if fadhE γ2

Growth Modes and Surface Energies

fadhE γ2≥Wetting, if

fadhE γ2

Growth Modes and Surface Energies

fadhE γ2≥Wetting, if

fadhE γ2

MTJ stack

MTJ Structure TMR vs. magnetic field

Capping layer Ru (50Å) 250

CoFeB (3.0 nm)/ MgO (1.5nm)/ CoFeB (3.0nm)

Ta (50Å)

MgO (15Å)

Free layerTunnel barrier

CoFeB (30Å) 200TMR =204%RA = 43 kΩμm2

( Å)

Synthetic Pinned layer

CoFeB (40Å)

CoFe (20Å)Ru (8Å)

100

150

TMR

(%)

IrMn (140Å)

NiFe (60Å)

Buffer layer Ta (50Å)( Å) 0

50T

Si / SiO2Wafer

Ru (300Å)Ta (50Å) -800 -600 -400 -200 0 200 400 600 800

0

H (Oe)

Indication of the growth of evaporated films as function of Tsubstf f subst

Tsubstr.

Indication of the growth of evaporated films as function of Tsubstf f subst

Tsubstr.

Multilayer structure

S p l tti /Si l t lli Poly-crystallineSuper lattice/Single crystalline Poly-crystalline

Interfaces

Multilayer structure

S p l tti /Si l t lliSuper lattice/Single crystalline

Interfaces

Yuasa et al., Nature materials 3, 868 (2004)

Multilayer structure

S p l tti /Si l t lli Poly-crystallineSuper lattice/Single crystalline Poly-crystalline

Interfaces

MTJ stack

MTJ Structure Textured layers

Capping layer Ru (50Å)

Ta (50Å)

MgO (15Å)

Free layerTunnel barrier

CoFeB (30Å)

( Å)

Synthetic Pinned layer

CoFeB (40Å)

CoFe (20Å)Ru (8Å)

IrMn (140Å)

NiFe (60Å)

Buffer layer Ta (50Å)( Å)

Si / SiO2Wafer

Ru (300Å)Ta (50Å)

Yuasa, J.Phys.D 40,R337 (2007)

Contents

1. Materials :- Thin Film Technology

2 Lith h2. Lithography:- Optical Lithography- E-beam/Ion-beam Lithography- E-beam/Ion-beam Lithography- Scanning Probe Techniques- Soft Lithographyg p y

3. Patterning Transfer:W t/D t hi- Wet/Dry etching

- Lift-off Technology

Fabrication of nano-scale MTJsphotoresist

B i Lith hi Basic Lithographic

Process Steps

Process flow of lithography

Radiation sourceIllumination control system

Illumination process:The resist is changed

Development:Selectively etched Illumination control system

Resist coated sampleThe resist is changed by the radiation

Selectively etched resist

Photoresist Coatingg

Exposure

Exposurep

Projection tool

reticleProjection lens system

wafer

θ

illuminator

LightSource

N A i θLens pupil, defines NA

N.A. = n sin θ

λNA

kR λ1= NASource: IMEC

Photoresist

O OHCHN

SO R

CH2n

CH3

N2

SO2R

Sensitizer (DNQ) + Novolac Resin

Novolac Resin+ converted PAC

ExposedResist

on R

ate

Novolac ResinDissolution

Dis

solu

tio

Novolac Resin+ DNQ PAC

Resistformulation

SelectivityExposure

Unexposed resist

Photoactive compoundp

ON

CO2HO

NCO

2H

Source: Micro chemicals

SO R

N2

SO R

hν

H 2O

SO R

N2

SO R

h

H 2O

SO2R SO 2R

Diazonaphthoquinonesensitizer

Indenecarboxylic acid

SO2R SO 2R

Diazonaphthoquinonesensitizer

Indenecarboxylic acid

Hydrophobic Hydrophilic

Development of resistSource: Micro chemicals

Photoresist

Source: Micro chemicals

Photoresist Contrast

PR

Oxide

High contrastHigh contrast

PR

Oxide

Low contrast

Negative resist

E-beam/Ion-beam Lithography

E-beam lithography

P i t l f th E ~30 keV

• Precise control of the energy and dose

• Imaging of electrons to form a small point < 1 nm

• No need for a physical mask

• Electron scattering in Electron scattering in solids R > 10 nm

• Slow exposure speed• Vacuum system• Vacuum system• High cost

PMMA (polmetthylmethacrylate)

E-beam lithography

50 nm lines 80 nm lines

Organic resist PMMA ~ 7 nm

Inorganic resist ~ 1-2 nm

Source: TU Delft

Inorganic resist 1 2 nm

E-beam lithography

20 nm 30 nm

50X50 nm2HSQ(1300A)

SiO2

Use of Focused Ion Beam:Direct structuringg

AFM S s ith si s f 50 730 AFM: Squares with sizes of 50-730 nm

Contents

1. Materials :- Thin Film Technology

2 Lith h2. Lithography:- Optical / Interference Lithography- E-beam/Ion-beam Lithography- E-beam/Ion-beam Lithography- Scanning Probe Techniques- Soft Lithographyg p y

3. Patterning Transfer:W t/D t hi- Wet/Dry etching

- Lift-off Technology

Pattern Transfer

Pattern Transfer

Photoresist

Substrate

MTJ film stack

Pattern

Lift-offEtching

Wet/ Dry EtchingWet/ Dry Etching

Etching: Wet and DryWet etching Dry etching

Ion beam etching Isotropic Ion beam etching, plasma etching, reactive ion etching (RIE)

IsotropicResolution limited by film thickness

Different Dry Etching TechniquesTechniques

Sputtering Chemical Sputtering Isotropic Etching

Ion enhanced Ion enhancedenergetic inhibitor

Vertical Etch

Side-wall re-depositionO.Auciello (1981) , JVST 19 p841

e (Å

/min

)Et

ch R

ate

P G Glö (1975) JVST 12 28Beam Angle (deg.)

P.G. Glöersen (1975) , JVST 12 p28

67/5Ion Beam Etching Acceleration Grid Contamination

Beam Acc.

Acceleration Grid Contamination

Beam

500 V 0 V -350 VAr+ Ar

High-energyion

High-energyneutral

Grid GridVoltage

Ar+ Ar

eLow-energyneutral

Substrate

Ar Ar

Low-energyion

Contamination of acceleration grid materials

The contamination is proportional to (the acceleration voltage)2The contamination is proportional to (the acceleration voltage)2Optimum acceleration voltage = 15 ~20 % of the beam voltage

P R Puckett “Ion Beam etching” P.R. Puckett , Ion Beam etching in J. L. Vossen and W. Kern ed., Thin film process II, (Academic Press, San Diego, 1991)

TrenchTilt Angle

0°

10°

20°

30°MTJ

40°FMBarrierFM

R. E. Lee (1979) , JVST 16 p164

FM

Plasma Dry Etchery

Plasma Etching Stepsg p

Typical Dry Etch Chemistries

Deep Reactive Ion Etching (DRIE)

a) Resist patterningb) Et hib) Etchingc) Passivationd) Etchingd) Etching

Dry Etching Equipment

Through-wafer etched interconnectsDry Etcher

Source: STS

Lift-offLift off

E-beam litho & Lift-off

Sub-micron magnetic dots by LIL and lift-off process

1 μm

by LIL and lift-off process

Co dots evaporated through the shadow

mask

1 μm

Arrays of holes in photoresist with overhang structure

Diameter: 500 nm Thi k 100Thickness: 100 nm

27/41

Sub-micron magnetic dots Vortex state

Vortex coreVortex core

MFM Image of Co dots

A. Wachowiak (2002), Science 298, p577T. Shinjo (2000), Science 289, p930

500-nm-diameter 100-nm-thick

28/41

Contents

1. Materials :- Thin Film Technology

2 Lith h2. Lithography:- Optical Lithography- E-beam/Ion-beam Lithography- E-beam/Ion-beam Lithography- Scanning Probe Techniques- Soft Lithographyg p y

3. Patterning Transfer:W t/D t hi- Wet/Dry etching

- Lift-off Technology

Emerging Nano-patterning Methods

S EPFLSource: EPFL