By: Henry Medina
Nanoelectronics Laboratory
National Tsing Hua University


Riikka L. Puurunen. “Surface chemistry of
atomic layer deposition: A case study for the
trimethylaluminum/water process”. JOURNAL
OF APPLIED PHYSICS 97, 121301 (2005)
Cambridge Nanotech website for ALD
◦ http://www.cambridgenanotech.com/ald/
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3.
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3.
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2.
First Part
What is ALD process
Applications
Basic Characteristics of ALD
Second Part
Chemical adsorption Mechanism
Growth per cycle (GPC) Effects with T on GPC
Number of cycles vs. GPC
Third Part
1.
Requirements for Precursors
Classes
Patterning of ALD growing layers (Techniques)
1.
1.
First Part
What is ALD process

ALD is a CVD technique suitable for inorganic
material layer as oxides (high-k dielectrics),
nitrides and some metals.
◦ The success of ALD is to divide CVD process in half


•
•
Perfect for deposition of very thin layers of the
size of a monolayer* coating complex shapes
with high quality (good step coverage).
Definition: “Film deposition technique based on
sequential use of self terminating* gas-solid
reactions” **
*Definition of Monolayer and Self terminating reaction will be
explained later
** Definition taken from the main reference
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First Part
What is ALD process
Applications






Semi & Nanoelectronics
Optical
Humidity Barriers
MEMS
Nanostructures
Chemical

Semi & Nanoelectronics
◦ Gate Dielectrics
Intel has recently announced that their 45 nm generation
processors will include a high-k HfO2 gate dielectric made
by ALD. There are various reasons that ALD has become
the method of choice
 Unlike ALD, conventional evaporated films suffer from
porosity and sputtering creates defects in the sensitive silicon
surface layer.
 In addition, ALD guarantees extremely uniform and
reproducible thickness, low stress, growth on amorphous
structure and low defect density.
 Besides the industrial silicon platforms, ALD has proven
essential to create gate dielectrics on device substrates
without native oxides

Semi & Nanoelectronics
◦ Gate Dielectrics and Gate Electrodes
Gate Electrode (TiN by ALD)
High K dielectric (HfO2 by ALD)
TEM micrograph of 45nm Intel high-k
and metal gate pMOS transistor. (Source:
IEDM2007 10.2)
•
•
•
•
Prevents Cu diffusion into silicon
Refractory nature
Amorphous
Acts as an adhesion promotor for Cu and Co
Cambridge NanoTech co-authored publication
ALD Tungsten nitride (WN)
Top
33
nm
32
nm
Bottom
ALD good for AR coatings: large area precision thickness control and batch coating.
=> Graded index coatings posssible by varying the number of Al2O3/TiO2 low n/high n
layers inside a nanolaminate stack
Zaitsu et al. Applied Phyics Letters, 80, 2442, 2002
ALD-ZnO transparent conductors advantages:
 No costly indium as in ITO
 Good optical transmission
 Low resistivity (1 mOhmcm)
 Large area uniformity
 Very smooth films in contrast to ITO
Thin film transistors:
ALD of ZnO active matrix thin film
transistors possible as well.
Water vapor transmission rate of 25 nm ALD Al2O3 better than 1 mm polymer encapsulation!
WVTR <10−5 g/m2 day demonstrated
Applied Physics Letters, 89, 031915 2006
3D DRAM needs conformal coating
of high-k dielectric and metal electrode
High aspect ratio ALD
of Ta2O5 in vias of
170 nm dia, 7 microns
deep
 C=kA/d: Al2O3, ZrO2, Ta2O5
 DRAM crown
 DRAM trench
Top electrode
100 nm
Dielectric
poly Si
SiO2
100 nm
Samsung uses ALD for DRAM manufacture!
Hausmann et al.
Thin Solid films 2003.
Client: Prof. C.M. Marcus,
Harvard University.
(a) Schematic of finger gated devices. Mo gates (150
nm wide 10 nm thick) were defined lithographically on a Si/
SiO2 substrate and subsequently coated with 25 nm of HfO2
grown by low-temperature ALD. Nanotubes were grown across
these local gates by CVD and contacted with Ti/Au electrodes.
Not to scale.
(b) Atomic force micrograph of nanotubes grown across Mo
finger gates and contacted (far left and far right) by Ti/Au leads.
Note that one finger gate passes directly underneath the
nanotube-metal contact. Arrows indicate the location of the
nanotube. Finger gates are labeled as in the text.
Local gating of carbon nanotubes, Biercuk, Nano Letters 2003
Client: Nobel laureate Prof. Tsui,
Princeton University
Client: Prof. Ohno,
Tohoku University, Japan.
Client: C.M. Marcus, Harvard University.
Cambridge NanoTech co-authored publication,
Applied Physics Letters 2003.
Nickel nanotubes grown in porous alumina, then alumina etched away
Client: K. Nielsch, Max Planck Germany
K. Nielsch, Max Planck Institute, 2006
Cambridge NanoTech Inc.
Confidential
Client: K. Nielsch, Max Planck Germany
TiO2-Al2O3-TiO2 coaxial nanotubes grown with ALD inside porous alumina.
K. Nielsch, Max Planck Institute, 2006
Client: K. Nielsch, Max Planck Germany
Nature Materials 2007 Published online:
2 July 2006; doi:10.1038/nmat1673
Deposition of Al2O3 inside and around tubular shaped tobacco mozaic
virus length 300 nm, OD 18 nm, ID 4 nm. Grown < 80C
Client: K. Nielsch, Max Planck Germany
Cambridge NanoTech Inc.
Confidential
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First Part
What is ALD process
Applications
Basic Characteristics of ALD

Steps:
◦ Self-terminating reaction of the first reactant
(Reactant A)
◦ Purge or evacuation to remove non-reacted
reactant and by products
◦ Self-terminating reaction of the second reactant
(Reactant B)
◦ Purge
This is considered as one reaction cycle
2005 © All rights reserved
Cambridge NanoTech Inc.
Tri-methyl
aluminum
Al(CH3)3(g)
Methyl group
(CH3)
Al
C
H
H
H
Hydroxyl (OH)
from surface
adsorbed H2O
H
O
Substrate surface (e.g. Si)
In air H2O vapor is adsorbed on most surfaces, forming a hydroxyl group.
With silicon this forms: Si-O-H (s)
After placing the substrate in the reactor, Trimethyl Aluminum (TMA)
is pulsed into the reaction chamber.
Cambridge NanoTech Inc.
Confidential
2005 © All rights reserved
Cambridge NanoTech Inc.
Methane reaction
product CH4
H
Reaction of
TMA with OH
H
H
C
H
H
H
H
C
C
H
H
Al
O
Substrate surface (e.g. Si)
Trimethyl Aluminum (TMA) reacts with the adsorbed hydroxyl groups,
producing methane as the reaction product
Al(CH3)3 (g) + : Si-O-H
(s)
:Si-O-Al(CH3)2
(s)
+ CH4
Cambridge NanoTech Inc.
Confidential
2005 © All rights reserved
Cambridge NanoTech Inc.
Methane reaction
product CH4
Excess TMA
H
H
H
C
C
H
Al
O
Substrate surface (e.g. Si)
Trimethyl Aluminum (TMA) reacts with the adsorbed hydroxyl groups,
until the surface is passivated. TMA does not react with itself, terminating the
reaction to one layer. This causes the perfect uniformity of ALD.
The excess TMA is pumped away with the methane reaction product.
Cambridge NanoTech Inc.
Confidential
2005 © All rights reserved
Cambridge NanoTech Inc.
H 2O
O
H
H
H
H
H
C
H
C
Al
O
After the TMA and methane reaction product is pumped away,
water vapor (H2O) is pulsed into the reaction chamber.
Cambridge NanoTech Inc.
Confidential
2005 © All rights reserved
Cambridge NanoTech Inc.
Methane reaction product
New hydroxyl group
Methane reaction
product
Oxygen bridges
H
O
Al
Al
O
Al
O
H2O reacts with the dangling methyl groups on the new surface forming aluminumoxygen (Al-O) bridges and hydroxyl surface groups, waiting for a new TMA pulse.
Again metane is the reaction product.
2 H2O (g) + :Si-O-Al(CH3)2
(s)
:Si-O-Al(OH)2
(s)
+ 2 CH4
Cambridge NanoTech Inc.
Confidential
2005 © All rights reserved
Cambridge NanoTech Inc.
H
O
Al
O
Al
O
Al
O
The reaction product methane is pumped away. Excess H2O vapor does not react with
the hydroxyl surface groups, again causing perfect passivation to one atomic layer.
Cambridge NanoTech Inc.
Confidential
2005 © All rights reserved
Cambridge NanoTech Inc.
H
H
O
O
Al
O
H
O
Al
O
O
Al
O
Al
O
Al
O
O
O
O
O
Al
O
O
Al
Al
Al
O
O
O
One TMA and one H2O vapor pulse form one cycle. Here three cycles are shown, with
approximately 1 Angstrom per cycle. Each cycle including pulsing and pumping takes e.g. 3 sec.
Two reaction steps in each cycle:
Al(CH3)3 (g) + :Al-O-H
(s)
2 H2O (g) + :O-Al(CH3)2
(s)
:Al-O-Al(CH3)2
(s)
+ CH4
:Al-O-Al(OH)2
(s)
+ 2 CH4
Cambridge NanoTech Inc.
Confidential

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The surface must be in a controlled state, e.g.
heated
Parameters to be adjusted:
◦ Reactants (precursors)
◦ Substrate
◦ Temperature
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1.
First Part
What is ALD process
Applications
Basic Characteristics of ALD
Second Part
Chemical adsorption Mechanism
Both have been seen in ALD applications

Self terminating Reaction
Both have been seen in ALD applications but Chemisorption by ligand
exchange is preferred because is associated with (a)

For ALD process ligand exchange is preferred
◦ For ligand exchange saturation in the process is
due to 2 factors:
 (a) Steric Hidrance
 (b) Number of Reactive
Surface Sites
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1.
2.
First Part
What is ALD process
Applications
Basic Characteristics of ALD
Second Part
Chemical adsorption Mechanism
Growth per cycle (GPC) Effects with T on GPC


Unit used in ALD system to describe different
process.
Ideal would be growth one monolayer per
cycle but this is not true due to steric
hindrance

3 Models to describe the effect of steric
hindrance on GPC



Model I: Just few processes can be assumed
as model I ( GPC is around 25% of a
monolayer)
Model II: Often used to model many ALD
processes. (Gives growths of GPC under 20%
of a monolayer)
Model III: Is applied only for some ideal ALD
processes as AlMe3/H2O. Most optimistic
(GPC around 30% of a monolayer)

Basically depends on the surface and the
reactant, an there are 4 possibilities


Most of the papers said AlMe3/H2O for
Al2O3 over SiO2 is rather insensitive to T,
there are documentation from 80oC to 300oC
so it’s assumed as model (b)
This is not completely true because T affect
the adsorption time on surface so the cycle
time per precursor should be adjusted to wait
for the self terminating reaction*.
*On FAQ of ALD Savannah is possible to find some
information related of how to fix this problem
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1.
2.
3.
1.
2.
3.
First Part
What is ALD process
Applications
Basic Characteristics of ALD
Second Part
Chemical adsorption Mechanism
Growth per cycle (GPC) Effects with T on GPC
Number of cycles vs. GPC

Due to the substrate there are 4 types:
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1.
2.
3.
1.
First Part
What is ALD process
Applications
Basic Characteristics of ALD
Second Part
Chemical adsorption Mechanism
Growth per cycle (GPC) Effects with T on GPC
Number of cycles vs. GPC
Third Part
1.
Requirements for Precursors
Classes

Ligand precursor
◦ To prepare the surface for next layer, and define
the kind of material to growth i.e. H2O for oxides,
N2 or NH3 for nitrides, etc.

Main Precursor (metallic precursor)
◦ Highly reactive (usually this means volatile
precursors), thermally stable, and full-fill the
requirement for self terminating reaction
◦ For Oxides

H2O this is the material preferred because of its physical properties




Easily decompose at low temperature by ligand exchange on H2 (Gas) and O attached to the
substrate
O2
O3
ROH (Alcohols with organic Chains)
◦ Nitrides


NH3
N2
◦ To growth pure materials the ligand precursor should be selected
depending on the main precursor
Recent Cambridge NanoTech experiment:
replacement of H2O with beer
+
Cambridge NanoTech Inc.
Confidential
=
Al2O3 grown with
H2O/TMA
Al2O3 grown with beer/TMA
Both same thickness. Beer LD is similar because the water vapor is
distilled from the cylinder with beer, and thus pure H2O.This
demonstrates how the intrinsic distillation of the vapor draw process in
the system reduces the need for high purity precursors.
Cambridge NanoTech Inc.
Confidential
◦ Inorganic
 Elements
React with nonmetal compounds and hidrogen compounds
As elements don’t carry extra ligands*
Selective reactivity (just few elements)
* Free ligands are associated with impurities grown in the film
◦ Inorganic
 Halides
Variety of materials grown
Reactive
Stable in Temperature
No extra ligands
Drawbacks: gaseous byproducts containing hydrogen nonmetal
reactants e.g. HCl
Corrosive etching the film and can be
readsorb at the surface
Metal Organic
 Non direct metal carbon bounds
 Organometallic
Metal Organic
 Non direct metal carbon bounds
◦ Alkoxides
◦ ß-diketanones
◦ Aminidates
Metal Organic
 Non direct metal carbon bounds
◦ Alkoxides
 Decompose at low temperature, usually lower than
200oC
 The decomposition produces the oxide already
 If deposited on the surface the process lose the
conformality
 Alcohol as by-product, promote readsorption
 The chains have high content of carbon and hydrogen
Metal Organic
 Non direct metal carbon bounds
◦ Alkoxides
Metal Organic
 Non direct metal carbon bounds
◦ ß-diketanones
 Before Cyclopentadienyls were widely used to grow
alkaline-earth metals
 Bulky chains, causing a marked steric hindrance so
GPC very low
 As alkoxides the M-O ligand is difficult to remove or
change for Nitrogen so is not suitable for nitrides
Metal Organic
 Non direct metal carbon bounds
◦ ß-diketanones
 Before Cyclopentadienyls were widely used to grow
alkaline-earth metals
 Bulky chains, causing a marked steric hindrance so
GPC very low
 As alkoxides the M-O ligand is difficult to remove or
change for Nitrogen so is not suitable for nitrides
Metal Organic
 Non direct metal
carbon bounds
◦ ß-diketanones
Metal Organic
 Non direct metal carbon bounds
◦ Amidinates
 New… from 2003 not widely studied
 Decomposition at around 300oC
 Seems to be self terminating process but GPC larger
than a monolayer has been reported
 Maybe for decomposition or readsorption
Metal Organic
 Non direct metal carbon bounds
 Organometallic
 Alkyls
 Cyclopentadienyls
Metal Organic
 Organometallic
◦
Alkyls: M-Cn-H2n+1
The reactivity of Alkyls reactant based is considered of
the highest in ALD process e.g. AlMe3/H2O has a GPC
of 30% of a monolayer at 300 oC
◦
Alkyls: M-Cn-H2n+1
The reactivity of Alkyls reactant based is considered of
the highest in ALD process e.g. AlMe3/H2O has a GPC
of 30% of a monolayer at 300 oC
Metal Organic
 Organometallic
◦
Cyclopentadienyls : M-5 Carbon ring
For deposition of pure metals as Ru (RuCp 2/O2). These
reactants have been used since long time but gain
popularity after 2000 however there are few
information about the process.
Metal Organic
◦ Cyclopentadienyls : M-5 Carbon ring
For deposition of pure metals as Ru (RuCp 2/O2). These
reactants have been used since long time but gain
popularity after 2000 however there are few
information about the process.
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1.
2.
3.
1.
2.
3.
1.
2.
First Part
What is ALD process
Applications
Basic Characteristics of ALD
Second Part
Chemical adsorption Mechanism
Growth per cycle (GPC) Effects with T on GPC
Number of cycles vs. GPC
Third Part
1.
Requirements for Precursors
Classes
Patterning of ALD growing layers (Techniques)
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
Photoresist
PMMA
Surface modification chemicals to become
hydrophobic
Etching
On the other hand surface modification is
also used to promote adhesion (hydrophilic)

Photoresist*
◦
◦
◦
◦
Rhodium growth Rh(acac)3/O2.
Temperature too high for photoresist ??(300oC)
Additional the use of HMDS inhibit the film growth.
Film grown over Oxide
* K. J. Parka and G. N. Parsons. “Selective area atomic layer
deposition of rhodium and effective work function
characterization in capacitor structures”. Applied Physics
Letters 89, 043111 2006

PMMA*
◦ Titanium Dioxide TiCl4/H2O and Ti(OCH(CH3)2)/H2O.
◦ At 160oC
◦ For TiCl4.
 Cl React with PMMA, PMMA difficult to remove when more than
150 cycles are applied.
◦ For Ti(OCH(CH3)2).
 No reaction with PMMA, PMMA easy to remove by normal
solvents.
◦ There’s no specification about the substrate
* Ashwini Sinha, Dennis W. Hess, and Clifford L. Henderson. “Area
selective atomic layer deposition of titanium dioxide: Effect of
precursor chemistry”. J. Vac. Sci. Technol. B 24(6) Nov/Dec 2006

Surface Modification*
◦ APS
◦ HMDS
APS treatment at 150oC
APS treatment at 150oC

Surface Modification*
◦ APS
 Not good results but I never try to modify parameter
 150oC too high for APS??
◦ HMDS

Surface Modification*
◦ ODTS and Octadecene. Liquid and are baked for
thermal reaction
◦ For positive and negative patterning.
◦ Surface treated before growth
* Rong Chen, Stacey F. Bent. “Chemistry for Positive Pattern
Transfer Using Area-Selective Atomic Layer Deposition”.
Adv. Mater. 2006, 18, p.1086–1090

Surface Modification
◦ Surface treated before growth to increase adhesion
◦ For Si.
 Piranha treatment (H2SO4/H2O2 7:3)
 HF 2%
Note: Sample are immediately transfer to next process
Generate Hydride termination (Si-H).

Etching
 After coating the surface with ALD film, the excess can
be removed by wet etching or dry etxhing
 Wet etching: NaOH for Al2O3 has given good results