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