Atomic Layer Deposition (ALD): An Enabler for Nanoscience and ...

Atomic Layer Deposition (ALD): An Enabler 

for Nanoscience and Nanotechnology 

Harvard University 

Roy G. Gordon 


Cambridge, MA

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: La 2 O 3 , LaAlO 3 


Outline

Chemical Vapor Deposition (CVD) 

One or more gases or vapors react to form a solid product 

precursor 

vapors 

substrate 

Solid product can be a 

film 

particle 

nanowire 

nanotube 


Heater 

film 

byproduct 

vapors 

Reaction started by 

heat 

mixing 2 vapors 

plasma

Atomic Layer Deposition (ALD) 

Sequential, self-limiting surface reactions make alternating layers: 

Precursor 1 

Precursor 2 

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) 


Heated area = 

deposition zone

ML 2 

H 2O 

Typical ALD Reaction for Oxides 

ML 2 

ML 2 

OH OH OH 

ML 

O 

H 2O 

ML 

O 


H 2O 

ML 

O 

ML 

O 

MOH 

O 

HL 

ML 

O 

HL 

MOH 

O 

HL 

ML 

O 

HL 

MOH 

O 

HL 

HL










Outline

Lining and Filling Holes by ALD 


4 cycles 12 cycles

Ion Milling 

Ion Milling + ALD 


Nanopores by ALD 

May be used for rapid sequencing of DNA

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. 

Harvard University


Photonic Crystals by ALD 

1) Form crystals of silica spheres. 

2) ALD of Ta 2 O 5 between the spheres 

3) Convert to Ta 3 N 5 in NH 3 

4) Dissolve the silica spheres in HF 

f 

500nm 

The resulting photonic crystals may be able to control light, 

the way semiconductors control electron transport.

ALD ruthenium 

on aluminum oxide 

diameters ~1 to 2 nm, 

~ 5 to 10 atoms across 

nucleation density is 

~ 20 x higher than previous 

metal nanocrystals 

may be applied to 

flash memories 


Nano-Dots by ALD 

40 nm scale bar; 10 nm in insert

after 500 cycles of Al 2 O 3 


Nanobeads by ALD 

Growth on Single-walled Carbon Nanotubes 

after 500 cycles of iron

Alumina Nanotubes on Carbon Nanotubes 

7 nm diameter 


21 nm diameter 

100 nm diameter

Nano-Coaxial Cable or Transistor 

Conducting tungsten nitride (WN) concentrically around 

insulating aluminum oxide (Al 2 O 3 ) concentrically around 

a conducting carbon nanotube. 

WN 


Al 2 O 3 

Carbon Al 2 O 3 WN

Elements included in ALD Materials 

Green = Element included in at least 1 ALD Material 

Red = Element not included in any ALD Material 

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 

Harvard University


Oxides Made by ALD 

Green = ALD process known for an Oxide of the Element 

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

H He 






Fr Ra Ac 


Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr

Pure Elements Made by ALD 

Green = ALD processes known for 16 Pure Elements 

Red = no process known for ALD of the Element 

H He 






Fr Ra Ac 


Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr 

Harvard University


Nitrides Made by ALD 

Green = ALD processes known for a Nitride of the Element 

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

H He 






Fr Ra Ac 




Sulfides Made by ALD 

Green = ALD processes known for a Sulfide of the Element 

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

H He 






Fr Ra Ac 




Carbides Made by ALD 

Green = ALD processes known for a Carbide of the Element 

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

H He 






Fr Ra Ac 




Fluorides Made by ALD 

Green = ALD processes known for a Fluoride of the Element 

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

H He 






Fr Ra Ac 



Current Applications of ALD 

Electroluminescent displays (Al 2 O 3 , AlN, ZnS) 

Read/Write heads in magnetic disk storage (Al 2 O 3 ) 

Insulators in capacitors in DRAMs (Al 2 O 3 , HfO 2 ) 

Insulation and spacer layers in microelectronics (SiO 2 , Si 3 N 4 ) 

Metal/insulator in transistor gates (TaN/HfO 2) 

Planar waveguides and optical filters (SiO 2 , TiO 2 ) 

Likely Future Applications of ALD 

Insulators in microelectronic capacitors (Ta 2 O 5 , SrTiO 3 , LaLuO 3 ) 

Diffusion barriers for copper in interconnects (WN, TaN, Mn) 

Adhesion and seed layers for interconnects (Co 4 N, Ru, Cu) 

Sealing pores in low-k dielectrics (SiO 2 ) 

Magnetic disk storage (Al 2 O 3 , Fe, Co, Ni, Cu, Ru, Mn, Pt) 

Nano-Electronics 

Catalysts . . . 


Applications of ALD










Outline

Criteria for Both CVD & ALD Precursors 

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

•No thermal decomposition during vaporization 

•Liquid at vaporization temperature 

•Preferably liquid at room temperature 

or soluble in an inert solvent 

•Precursors and byproducts don’t etch films 

Harvard University

Criteria for CVD Precursors 

•Reactivity with substrate 

•Reactivity with surface of growing film 

•Thermal decomposition allowed or even needed 


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

Usefulness of Precursors for CVD & ALD 

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 

Most ALD precursors 

also work in CVD 

Some CVD precursors 

also work in ALD 


CVD 

ALD










Outline

Acetamidinates: R2 Formamidinates: R 

= CH3 2 = H 

R 2 

R 1 

N 

N 

R 3 

M 


Metal(II) Amidinates 

Propionamidinates: R 2 = CH 2 CH 3 

R 1 

N 

N 

R 3 

R 2 

R 1 and R 3 = alkyl groups 

monomer dimer 

R 2 

R 3 R1 R 3 

R 1 

N 

N 

R 3 

N N 

M 

M 

N N 

M-N bonds are generally reactive to H 2 O, NH 3 , H 2 , etc. 

The chelate structure adds to the thermal stability. 

The choices of R n affect the volatility, reactivity and stability. 

R 2 

R 2 

N 

N 

R 1 

R 1 

R 3 

R 2

Increasing ligand bulk 

Structures of Metal Bis-Acetamidinates 

tert-pentyl 2 

tert-butyl 2 

isopropyl 2 

Et-tert-Bu 

n-propyl 2 

R 1 , R 3 

ionic radius 


c 

m 

m 

Ni 

69 

c 

m 

d 

d 

Co 

65 

c 

m 

d 

Cr 

73 

m 

Ge 

73 

m 

Zn 

74 

m 

d 

Mg 

72 

m 

d 

Fe 

78 

Increasing “size” of metal atom 

Mn 

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

d 

d 

83 

m 

Bi 

103 

m 

d 

Ca 

100 

m = more reactive, less stable, volatile monomer 

m 

d = reactive, volatile dimer 

d 

d 

d 

p 

p 

Sr 

118 

d 

d 

p 

p 

Ba 

135 

p = non-volatile polymer

R 2 

R 1 

N N 

R3 R1 N 

R 2 

C 

M 

C N N C 

R 1 R 3 


Metal(III) Amidinates 

N 

monomer 

R 3 

R 2 

Formamidinates: R 2 = H 

Acetamidinates: R 2 = CH 3 

Propionamidinates: R 2 = CH 2 CH 3 

R 1 and R 3 = alkyl groups 

Structures of dimers are unknown, probably bridged

Increasing ligand bulk 

Structures of Metal(III) Tris-Amidinates 

tert- 

Bu 2 

iso- 

Pr 2 

Et- 

t Bu 

n-Pr 2 

Et 2 

Me 2 

R 1 ,R 3 

c 

Al 

c 

Ga 

n 

c 

c 

m 

m 

Cr 


c 

Co 

c 

V 

c 

Fe 

n 

c 

m 

m 

d 

Ti 

m 

Ru 

m 

m 

d 

Sc 

m 

Sb 

Increasing size of metal atom 

n = non-existent 

c = crowded, less reactive monomer 

c 

c 

c 

c 

m 

m 

m 

Lu 

c 

m 

d 

Y 

m 

Gd 

c 

Eu 

c 

Nd 

m = more reactive monomer 

d = low-volatility dimer 

m 

Pr 

c 

Ce 

c 

m 

d 

p 

p 

La 

p = non-volatile polymer

Models for Lanthanum and Scandium Amidinates 

La ion is large, so 3 amidinate 

ligands are not crowded 

La 

La precursor reacts 

quickly with surface OH 


Sc ion is small, so 3 amidinate 

ligands are crowded 

Sc 

Sc precursor reacts 

slowly with surface OH

Zr(amd) 4 and Hf(amd) 4 


Metal(IV) Tetra-Amidinates 

Require small R n groups 

such as H and CH 3 

H 3C 

H 

H 3C N 

H 

N 

CH 3 

N 

N 

Hf 

CH 3 

CH 3 

N 

N 

CH 3 

H 

N CH 3 

N 

H 

CH 3 

Thermal decomposition at 200 o C 

Zr amidinate is much more stable 

than Zr amide, Zr(NEtMe) 4 

Greater stability of amidinates is 

due to chelate structure (2 metalnitrogen 

bonds instead of one)










Outline


Thermogravimetric Analysis of 

Lanthanum Amidinates 

N 

N 

H 

C N 

N 

HC 

La 

H 3C 

N 

CH 

N 

N C N 

N 

C 

La 

N 

CH 3 

N 

C 

N 

CH 3 

CH 3 

N C N 

N N 

C 

N 

C 

N 

La 

H3C CH3 H3C H3C => Vaporization temperature increases with molecular mass 

CH 3 

CH 3 

CH 3 

CH 3

Vapor Pressures of Lanthanum Precursors 

=> La(iPr 2 -fmd) 3 is most 

volatile La compound known, 

60 mTorr at 100 o C 


N 

H 

C N 

N 

HC 

N 

La 

N 

N 

CH 

La(iPrCp) 3 

0.1 

Torr

Precursors: 

H 2 O and 


ALD of La 2 O 3 

tris(N,N’-diisopropylformamidinato)lanthanum 

( i Pr 2 -fmd) 3 La 

N 

H 

C N 

N 

HC 

N 

La 

N 

N 

CH 

=> 0.16 nm per cycle 

=> negligible delay 

in nucleation on SiH

Growth per La Cycle for ALD LaAlO 3 

Precursors: 

Me 3 Al, H 2 O and 

90 o C 




110 o C 

100 o C 

120 o C 

Bubbler temperature 90 to 120 o C 

Substrate temperature 300 o C 

=> ALD saturation at 

0.08 nm per La cycle 

Growth even at bubbler 

temperature

Precursors: 

Me 3 Al, H 2 O and 


Composition of ALD La xAl 1-xO 3/2 



Growth conditions: 

Bubbler temperature 120 o C 

Substrate temperature 300 o C 

=> Composition control 

by changing ratio of 

precursor doses 

=> 2 x as many Al atoms 

as La atoms per dose 

N 

H 

C N 

N 

HC 

N 

La 

N 

N 

CH

Precursor Reactivity with SiH Surface by IR 

La amidinate reacts 

with nearly all the 

Si-H bonds in only 

3 cycles 

Hf alkylamide only 

reacts with half of 

the Si-H bonds on 

the surface even 

after many cycles 

=> Completely uniform surface coverage by La amidinate 

Details of the infrared data were given at AVS Conference ALD 2007 by 

J. Kwon, M. Dai, E. Langereis, Y. Chabal, K.-H. Kim and R. G. Gordon. 

Harvard University

2 nm 

TEMs of ALD LaAlO 3 and GdScO 3 

=> Sharp interfaces with silicon without interlayers 

=> Uniform nucleation and thickness 


LaAlO 3 

Si

Leakage Current through ALD La 2 O 3 

Vapor source: a solution of the La precursor (mp 194 o C) 

vaporized with an MKS MDD liquid delivery system 

Low leakage current similar to films made from a bubbler. 

=> negligible carbon contamination from solvent 

Harvard University

Leakage (A/cm 2 ) 

10 2 

10 0 

10 -2 

10 -4 

10 -6 

10 -8 


Comparison of leakage 

SiO 2 

ALD LaAlO 3 

ALD GdScO 3 

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 

EOT (nm, |V g -V fb |=1V) 

ALD LaScO 3 

ALD HfO 2 from IMEC (Ref.1) 

MOCVD HfO 2 from IMEC (Ref.2) 

ALD HfO 2 from IBM (Ref.3) 

Sputter HfO 2 (Ref.4)


Summary 

ALD 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


Acknowledgements 

Metals: Booyong Lim, Antti Rahtu, Jin-Seong Park, Venkateswara Pallem 

Cu, Co: Zhengwen Li, Séan Barry, Don Keun Lee, Harish Bhandari, Hoon Kim 

Ruthenium: Huazhi Li, Titta Aaltonen, Jun Ni 

Metal Nitrides: Jill Becker, Seigi Suh, Esther Kim, Kyoung-ha Kyoung ha Kim 

Metal oxides: Dennis Hausmann, Philippe de Rouffignac, Jin-Seong Park, 

Kyoung-ha Kyoung ha Kim, Leo Rodriguez, Mike Coulter, Jean Sébastien Lehn, Sheng 

Xu, Hongtao Wang, Yiqun Liu 

TEM: Damon Farmer; Hongtao Wang, SEMATECH, Applied Materials 

DRAM trenches supplied by Infineon (Qimonda) 

La, Co, Cu and Ru precursors supplied by Rohm and Haas Electronic Materials 

SiO 2 and W precursors supplied by Sigma-Aldrich Company 

SEMs and Electrical Analysis by Daniel Josell, NIST 

Supported by the US National Science Foundation and Intel Corporation

Atomic Layer Deposition (ALD): An Enabler for Nanoscience and ...

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