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Atomic Layer Deposition (ALD): An Enablerfor Nanoscience and Nanotechnology

Roy G. Gordon

Harvard UniversityCambridge, MA

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Definitions of Chemical Vapor Deposition (CVD)and Atomic Layer Deposition (ALD)

Structures and materials made by ALD

Properties needed for CVD and ALD precursors:Volatility, Stability, Reactivity

How to design those properties into precursors:metal amidinates

High-k insulators: La2O3, LaAlO3

Outline

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Chemical Vapor Deposition (CVD)One or more gases or vapors react to form a solid product

Reaction started byheatmixing 2 vaporsplasma

Solid product can be afilmparticlenanowirenanotube

precursorvapors

byproductvapors

substrate

film

Heater

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Benefits of ALD:• Atomic level of control over film composition

⇒nanolaminates and multi-component materials• Uniform thickness over large areas and inside narrow holes• Very smooth surfaces (for amorphous films)• High density and few defects or pinholes• Low deposition temperatures (for very reactive precursors)• Pure films (for suitably reactive precursors)

Sequential, self-limiting surface reactions make alternating layers:

Atomic Layer Deposition (ALD)

Heated area = deposition zone

Precursor 1

Precursor 2

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Typical ALD Reaction for Oxides

OH OH OH

ML2

ML2

ML2MLO

MLO

MLO

HL HLHL

MLO

MLO

MLO

H2OH2O

H2O

MOHO

MOHO

MOHO

HLHL

HL

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Definitions of Chemical Vapor Deposition (CVD)and Atomic Layer Deposition (ALD)

Structures and materials made by ALD

Properties needed for CVD and ALD precursors:Volatility, Stability, Reactivity

How to design those properties into precursors:metal amidinates

High-k insulators: La2O3, LaAlO3

Outline

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Lining and Filling Holes by ALD

4 cycles 12 cycles

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Nanopores by ALD

Ion Milling

Ion Milling + ALD

May be used for rapid sequencing of DNA

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Coatings on the Outside of Particles

ALD AlN coating

ZnS particles

Used in electroluminescent back-lights for displays in cell-phones and many other devices.

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Photonic Crystals by ALD

f

500nm

1) Form crystals of silica spheres.2) ALD of Ta2O5 between the spheres3) Convert to Ta3N5 in NH34) Dissolve the silica spheres in HF

The resulting photonic crystals may be able to control light, the way semiconductors control electron transport.

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Nano-Dots by ALD

ALD rutheniumon aluminum oxide

nucleation density is ~ 20 x higher than previous metal nanocrystals

40 nm scale bar; 10 nm in insertmay be applied to flash memories

diameters ~1 to 2 nm, ~ 5 to 10 atoms across

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Nanobeads by ALD

after 500 cycles of Al2O3 after 500 cycles of iron

Growth on Single-walled Carbon Nanotubes

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Alumina Nanotubes on Carbon Nanotubes

100 nm diameter21 nm diameter7 nm diameter

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Nano-Coaxial Cable or TransistorConducting tungsten nitride (WN) concentrically around insulating aluminum oxide (Al2O3) concentrically around a conducting carbon nanotube.

Carbon Al2O3 WNAl2O3WN

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Elements included in ALD Materials

H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn Fr Ra Ac Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr

Green = Element included in at least 1 ALD Material

Red = Element not included in any ALD Material

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Oxides Made by ALD

H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn Fr Ra Ac Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr

Green = ALD process known for an Oxide of the Element

Red = no process known for ALD of any Oxide of the Element

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Pure Elements Made by ALD

H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn Fr Ra Ac Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr

Green = ALD processes known for 16 Pure Elements

Red = no process known for ALD of the Element

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Nitrides Made by ALD

H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn Fr Ra Ac Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr

Green = ALD processes known for a Nitride of the Element

Red = no process known for ALD of a Nitride of the Element

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Sulfides Made by ALD

H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn Fr Ra Ac Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr

Green = ALD processes known for a Sulfide of the Element

Red = no process known for ALD of a Sulfide of the Element

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Carbides Made by ALD

H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn Fr Ra Ac Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr

Green = ALD processes known for a Carbide of the Element

Red = no process known for ALD of a Carbide of the Element

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Fluorides Made by ALD

H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn Fr Ra Ac Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr

Green = ALD processes known for a Fluoride of the Element

Red = no process known for ALD of a Fluoride of the Element

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Current Applications of ALDElectroluminescent displays (Al2O3, AlN, ZnS)Read/Write heads in magnetic disk storage (Al2O3)Insulators in capacitors in DRAMs (Al2O3, HfO2)Insulation and spacer layers in microelectronics (SiO2, Si3N4)Metal/insulator in transistor gates (TaN/HfO2)Planar waveguides and optical filters (SiO2, TiO2)

Likely Future Applications of ALDInsulators in microelectronic capacitors (Ta2O5, SrTiO3, LaLuO3)Diffusion barriers for copper in interconnects (WN, TaN, Mn)Adhesion and seed layers for interconnects (Co4N, Ru, Cu)Sealing pores in low-k dielectrics (SiO2)Magnetic disk storage (Al2O3, Fe, Co, Ni, Cu, Ru, Mn, Pt)Nano-ElectronicsCatalysts . . .

Applications of ALD

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Definitions of Chemical Vapor Deposition (CVD)and Atomic Layer Deposition (ALD)

Structures and materials made by ALD

Properties needed for CVD and ALD precursors:Volatility, Stability, Reactivity

How to design those properties into precursors:metal amidinates

High-k insulators: La2O3, LaAlO3

Outline

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Criteria for Both CVD & ALD Precursors

•Sufficient volatility (> 0.1 Torr at T < 200 oC)

•No thermal decomposition during vaporization

•Liquid at vaporization temperature

•Preferably liquid at room temperatureor soluble in an inert solvent

•Precursors and byproducts don’t etch films

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Criteria for ALD Precursors•Self-limited reactivity with substrate

•Self-limited reactivity with the surface made by reaction of the film with the other precursor

•Thermal decomposition not allowed

•Reactivity with substrate

•Reactivity with surface of growing film

•Thermal decomposition allowed or even needed

Criteria for CVD Precursors

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Some precursors work only in CVD, but not ALD:Ni(CO)4, W(CO)6, many alkoxides

Some precursors work in both CVD and ALD:many beta-diketonates and amidinates

Usefulness of Precursors for CVD & ALD

CVDALDMost ALD precursors

also work in CVD

Some CVD precursors also work in ALD

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Definitions of Chemical Vapor Deposition (CVD)and Atomic Layer Deposition (ALD)

Structures and materials made by ALD

Properties needed for CVD and ALD precursors:Volatility, Stability, Reactivity

How to design those properties into precursors:metal amidinates

High-k insulators: La2O3, LaAlO3

Outline

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Metal(II) AmidinatesR1 and R3 = alkyl groups

Acetamidinates: R2 = CH3

N

M

N

R1

R2

R3

N

N

R2

R1

R3

monomer dimer

N

M

N

R2

N N

M

R3 R1 R3

R1N N

R1R3

N

N

R1

R3

R2

R2

R2

Formamidinates: R2 = H

The choices of Rn affect the volatility, reactivity and stability.

M-N bonds are generally reactive to H2O, NH3, H2, etc.The chelate structure adds to the thermal stability.

Propionamidinates: R2 = CH2CH3

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1351181001038378727473736569ionic radius

ddmtert-pentyl2

Bi

m

Zn

m

Ge

m

dn-propyl2

BaSrCaMnFeMgCrCoNiR1, R3

ppdddmEt-tert-Bu

ppddddmmmisopropyl2

dddmmmccctert-butyl2

Increasing “size” of metal atom

Incr

easi

ng li

gand

bul

k

d = reactive, volatile dimer

Structures of Metal Bis-Acetamidinates

p = non-volatile polymer

m = more reactive, less stable, volatile monomer

c = crowded, less reactive, more stable, volatile monomer

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Metal(III) Amidinates

R1 and R3 = alkyl groups

Formamidinates: R2 = H

Acetamidinates: R2 = CH3

M

N N

N N

NN

C C

CR3R1

R3 R1

R1 R3

R2

R2R2

monomer

Structures of dimers are unknown, probably bridged

Propionamidinates: R2 = CH2CH3

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ddmMe2

pmmmmmEt2

La

p

d

m

c

Ce

c

Pr

m

Nd

c

Eu

c

Gd

m

Y

d

m

c

V

c

Co

c

mmmmcn-Pr2

LuSbScRuTiFeCrGaAlR1,R3

mEt-tBu

cccccccciso-Pr2

cnntert-Bu2

Increasing size of metal atom

Incr

easi

ng li

gand

bul

k

m = more reactive monomerd = low-volatility dimer

Structures of Metal(III) Tris-Amidinates

p = non-volatile polymer

c = crowded, less reactive monomern = non-existent

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Models for Lanthanum and Scandium Amidinates

La precursor reacts quickly with surface OH

Sc precursor reacts slowly with surface OH

La ion is large, so 3 amidinate ligands are not crowded

Sc ion is small, so 3 amidinate ligands are crowded

La Sc

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Zr(amd)4 and Hf(amd)4

Metal(IV) Tetra-Amidinates

Require small Rn groups such as H and CH3

CH3

N

N CH3

H

H3C N

N

CH3H

CH3N

N

CH3H

CH3

N

NH3C

H

Hf

Zr amidinate is much more stable than Zr amide, Zr(NEtMe)4

Thermal decomposition at 200 oC

Greater stability of amidinates is due to chelate structure (2 metal-nitrogen bonds instead of one)

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Definitions of Chemical Vapor Deposition (CVD)and Atomic Layer Deposition (ALD)

Structures and materials made by ALD

Properties needed for CVD and ALD precursors:Volatility, Stability, Reactivity

How to design those properties into precursors:metal amidinates

High-k insulators: La2O3, LaAlO3

Outline

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Thermogravimetric Analysis of Lanthanum Amidinates

NHC N

NCHNNHC

N La

N C N

NCNNC

N La

CH3

CH3H3C

N C N

NCNNC

N La

CH3

CH3H3C

H3C CH3

CH3

CH3CH3

H3C

=> Vaporization temperature increases with molecular mass

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Vapor Pressures of Lanthanum Precursors

=> La(iPr2-fmd)3 is most volatile La compound known, 60 mTorr at 100 oC

La(iPrCp)3

NHC N

NCHNNHC

N La

0.1 Torr

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ALD of La2O3

=> 0.16 nm per cycle

=> negligible delay in nucleation on SiH

NHC N

NCHNNHC

N La

tris(N,N’-diisopropyl-formamidinato)lanthanum(iPr2-fmd)3La

Precursors: H2O and

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Growth per La Cycle for ALD LaAlO3

Bubbler temperature 90 to 120 oCSubstrate temperature 300 oC

Growth even at bubbler temperature <100 oC

=> ALD saturation at 0.08 nm per La cycle

NHC N

NCHNNHC

N La

tris(N,N’-diisopropyl-formamidinato)lanthanum(iPr2-fmd)3La

Precursors: Me3Al, H2O and

120 oC110 oC

100 oC90 oC

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Composition of ALD LaxAl1-xO3/2

Growth conditions: Bubbler temperature 120 oCSubstrate temperature 300 oC

=> Composition control by changing ratio of precursor doses

=> 2 x as many Al atoms as La atoms per dose

NHC N

NCHNNHC

N La

tris(N,N’-diisopropyl-formamidinato)lanthanum(iPr2-fmd)3La

Precursors: Me3Al, H2O and

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Precursor Reactivity with SiH Surface by IR

=> Completely uniform surface coverage by La amidinate

Hf alkylamide only reacts with half of the Si-H bonds on the surface even after many cycles

La amidinate reacts with nearly all the Si-H bonds in only 3 cycles

Details of the infrared data were given at AVS Conference ALD 2007 byJ. Kwon, M. Dai, E. Langereis, Y. Chabal, K.-H. Kim and R. G. Gordon.

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TEMs of ALD LaAlO3 and GdScO3

=> Sharp interfaces with silicon without interlayers

=> Uniform nucleation and thickness

LaAlO3

Si2 nm

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Leakage Current through ALD La2O3Vapor source: a solution of the La precursor (mp 194 oC) vaporized with an MKS MDD liquid delivery system

Low leakage current similar to films made from a bubbler.

=> negligible carbon contamination from solvent

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Comparison of leakage

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.010-8

10-6

10-4

10-2

100

102

SiO2

ALD LaAlO3

ALD GdScO3

ALD LaScO3

ALD HfO2 from IMEC (Ref.1) MOCVD HfO2 from IMEC (Ref.2) ALD HfO2 from IBM (Ref.3) Sputter HfO2 (Ref.4)

Leak

age

(A/c

m2 )

EOT (nm, |Vg-Vfb|=1V)

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SummaryALD requires volatile precursors with self-limited reactivity

and high thermal stability

Precursors with these properties are known for most elements

ALD is a proven process with many current applications

Many more uses for ALD are expected in the future

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AcknowledgementsMetals: Booyong Lim, Antti Rahtu, Jin-Seong Park, Venkateswara PallemCu, Co: Zhengwen Li, Séan Barry, Don Keun Lee, Harish Bhandari, Hoon KimRuthenium: Huazhi Li, Titta Aaltonen, Jun NiMetal Nitrides: Jill Becker, Seigi Suh, Esther Kim, KyoungKyoung--ha ha KimMetal oxides: Dennis Hausmann, Philippe de Rouffignac, Jin-Seong Park,KyoungKyoung--ha ha Kim, Leo Rodriguez, Mike Coulter, Jean Sébastien Lehn, ShengXu, Hongtao Wang, Yiqun LiuTEM: Damon Farmer; Hongtao Wang, SEMATECH, Applied MaterialsDRAM trenches supplied by Infineon (Qimonda)La, Co, Cu and Ru precursors supplied by Rohm and Haas Electronic MaterialsSiO2 and W precursors supplied by Sigma-Aldrich Company

SEMs and Electrical Analysis by Daniel Josell, NISTSupported by the US National Science Foundation and Intel Corporation