Plasma-Assisted Atomic Layer Deposition: Basics, Opportunities, and Challenges
H. B. Profijt, S. E. Potts, M. C. M. van de Sanden, and W. M. M. Kessels
Citation: Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 29, 050801 (2011); doi:
10.1116/1.3609974
View online: http://dx.doi.org/10.1116/1.3609974
View Table of Contents: http://avs.scitation.org/toc/jva/29/5
Published by the American Vacuum Society
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REVIEW ARTICLE
Plasma-Assisted Atomic Layer Deposition: Basics, Opportunities,
and Challenges
H. B. Profijt, S. E. Potts, M. C. M. van de Sanden, and W. M. M. Kessels
a)
Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven,
The Netherlands
(Received 22 February 2011; accepted 19 June 2011; published 18 August 2011)
Plasma-assisted atomic layer deposition (ALD) is an energy-enhanced method for the synthesis of
ultra-thin films with A
˚
-level resolution in which a plasma is employed during one step of the cyclic
deposition process. The use of plasma species as reactants allows for more freedom in processing
conditions and for a wider range of material properties compared with the conventional thermally-
driven ALD method. Due to the continuous miniaturization in the microelectronics industry and
the increasing relevance of ultra-thin films in many other applications, the deposition method has
rapidly gained popularity in recent years, as is apparent from the increased number of articles
published on the topic and plasma-assisted ALD reactors installed. To address the main differences
between plasma-assisted ALD and thermal ALD, some basic aspects related to processing plasmas
are presented in this review article. The plasma species and their role in the surface chemistry are
addressed and different equipment configurations, including radical-enhanced ALD, direct plasma
ALD, and remote plasma ALD, are described. The benefits and challenges provided by the use of a
plasma step are presented and it is shown that the use of a plasma leads to a wider choice in
material properties, substrate temperature, choice of precursors, and processing conditions, but that
the processing can also be compromised by reduced film conformality and plasma damage.
Finally, several reported emerging applications of plasma-assisted ALD are reviewed. It is
expected that the merits offered by plasma-assisted ALD will further increase the interest of
equipment manufacturers for developing industrial-scale deposition configurations such that the
method will find its use in several manufacturing applications.
V
C
2011 American Vacuum Society.
[DOI: 10.1116/1.3609974]
I. INTRODUCTION
Atomic layer deposition (ALD) is a vapor-phase deposi-
tion technique in which ultra-thin films are typically synthe-
sized sub-monolayer by sub-monolayer by repeating two
subsequently executed half-cycles.
1–10
See Fig. 1 for a sche-
matic illustration of an ALD cycle. ALD offers atomic layer
precision of the growth, because the reaction of the species
dosed during the two half-cycles is self-limiting. As a conse-
quence, when sufficient precursor and reactant species are
dosed, the ALD film growth is not flux-dependent, as is the
case with deposition techniques such as chemical vapor dep-
osition (CVD) and physical vapor deposition (PVD). The
growth rate with respect to ALD is expressed as the growth
per cycle (GPC), which is typically in the range of 0.05–0.1
nm per cycle. In order to ensure that only ALD surface reac-
tions take place and not CVD-like reactions, which can
appear when precursor and reactant are present in the reactor
at the same time, a purge step is executed after each half-
cycle to remove the residual precursor or reactant species.
The total duration of a cycle is the sum of the precursor dos-
ing time, the precursor purge time, the reactant dose time
and the reactant purge time. Consequently, the duration of
one cycle cannot only be shortened by optimizing the dosing
times, but also by optimizing the purge times. During ALD,
the reactant is typically a gas, such as O
2
, or a vapor, such as
H
2
O, and the surface reactions are thermally-driven by
slightly elevated substrate temperatures (typically 150–350
C). Therefore, the method is also referred to as thermal
ALD. Besides the atomic control over the film thickness, the
self-limiting half-cycles in ALD facilitate uniform deposi-
tion over large substrates and conformal deposition in struc-
tures of high aspect ratio, as long as the dosing and purge
times are sufficiently long.
The first ALD research was conducted in the 1960s and
1970s in the former USSR and Finland, and the deposition
method was patented in 1977 by Suntola.
11
For a more
extensive review on the history of ALD, the reader is
referred to Puurunen et al.
7
In the mid-1990s, the semicon-
ductor industry became interested in ALD because a deposi-
tion method with atomic control over the film thickness and
the ability to deposit films conformally on nonplanar sub-
strates was needed. Since then, the semiconductor industry
has been the key driver of the field of ALD.
12
In 2007, Intel
introduced its first 45 nm microprocessor containing Hf-
a)
Author to whom correspondence should be addressed; electronic mail:
w.m.m.kessels@tue.nl
050801-1 J. Vac. Sci. Technol. A 29(5), Sep/Oct 2011 0734-2101/2011/29(5)/050801/26/$30.00
V
C
2011 American Vacuum Society 050801-1
based gate dielectric layers fabricated by ALD. It is expected
that, starting from the 22 nm technology node, ALD will be
used in several key process steps.
13,14
Plasma-assisted ALD is an energy-enhanced ALD
method that is rapidly gaining in popularity.
15
In plasma-
assisted ALD, also referred to as plasma enhanced ALD
(PEALD), plasma ALD and, in some cases, radical-
enhanced ALD, the surface is exposed to the species gener-
ated by a plasma during the reactant step. This process is
also illustrated in Fig. 1. Typical plasmas used during
plasma-assisted ALD are those generated in O
2
,N
2
and H
2
reactant gases or combinations thereof. Such plasmas can
replace ligand-exchange reactions typical of H
2
OorNH
3
,
and they can be employed to deposit metal oxides, metal
nitrides and metal films. Moreover, plasmas generated in
gases or vapors such as NH
3
and H
2
O have been reported,
for which there can also be a combination of plasma and
thermal ALD surface reactions taking place at the same
time.
Plasma-assisted ALD offers several merits for the deposi-
tion of ultra-thin films over thermal ALD and other vapor-
phase deposition techniques. The high reactivity of the
plasma species on the deposition surface during the plasma-
assisted ALD process allows for more freedom in processing
conditions and for a wider range of material properties.
These ideas will be addressed in detail later in this review ar-
ticle and are the primary reason why the interest in plasma-
assisted ALD has increased rapidly in recent years. This in-
terest has also been catalyzed by the many new applications
of ALD that are emerging in and outside the semiconductor
industry. Several non-semiconductor applications have set
new requirements for the ALD parameter space, which can-
not always be satisfied easily by a pure thermally-driven
ALD process.
The increasing popularity of plasma-assisted ALD is
manifested by the increasing number of recent publications
about the topic (see Fi g. 2), and the large set of thin film
materials that have been synthesized by the method (see
Table I). Such is the interest and demand in the field that the
number of ALD equipment manufacturers providing dedi-
cated plasma-assisted ALD tools has increased significantly
in the recent years. Currently (status May 2011), companies
such as ASM (Emerald (2005) and Stellar (2006)),
16
Oxford
Instruments (FlexAL (2006) and OpAL (2008)),
17
Beneq
(TFS 200 (2009)),
18
Cambridge NanoTech (Fiji (2009)),
19
Applied Materials (Applied Endura iLB (2010)),
20
Tokyo
Electron Limited (TELINDY PLUS IRad SA (2011)),
21
and
Picosun (SUNALE (2011))
22
provide tools for plasma-
assisted ALD.
The first case of plasma-assisted ALD was reported in
1991, when De Keijser and Van Opdorp of the Philips
Research Laboratories in Eindhoven, the Netherlands, pub-
lished a paper on atomic layer epitaxy (ALE) of GaAs using
H radicals.
111
The hydrogen radicals were generated in a
remote microwave-induced plasma and transported to the
deposition surface through a quartz tube (see Fig. 3). The
atomic hydrog en was used to drive the surface reactions after
GaMe
3
and AsH
3
pulsing at substrate temperatures below
500
C, wh ich is close to the onset temperature for the ther-
mal decomposition of GaMe
3
. Subsequently, the method
remained unexplored until the end of the 1990s, when the
semiconductor industry became interested in ALD as men-
tioned earlier. Sherman filed a patent on the method in
1996,
298
after which Rossnagel and co-workers reported on
plasma-assisted ALD of Ta and Ti metal films in 2000.
206
In
the latter case, the anticipated application of the technique
was the deposition of Cu diffusion barriers in advanced
FIG. 2. (Color online) Number of publications per year on the subject of
plasma-assisted ALD, between 1991 and 2011 (status May 31, 2011). The
search was run in published abstracts using Web of Science
V
R
(Ref. 23). The
search terms included “plasma-assisted ALD,” “plasma-enhanced ALD,”
“radical enhanced ALD,” “remote plasma ALD,” “direct plasma ALD,” and
“plasma ALD.” The first report of a plasma-assisted ALD process by De
Keijser and Van Opdorp (Philips Research Laboratories, Eindhoven), pub-
lished in 1991, is also included.
FIG. 1. (Color online) Schematic representation of thermal ALD and
plasma-assisted ALD. During the co-reactant step of the cycle (the 2
nd
half-
cycle), the surface is exposed to a reactant gas or vapor such as NH
3
or H
2
O,
or to species generated by a plasma.
050801-2 Profijt et al.: Plasma-assisted ALD 050801-2
J. Vac. Sci. Technol. A, Vol. 29, No. 5, Sep/Oct 2011
TABLE I. Overview of the materials deposited by plasma-assisted ALD. The material, the precursor, the plasma gas (only the reactant gas, not the carrier gas),
the reactor type (“re” is radical-enhanced, “d” is direct-plasma ALD, “r” is remote plasma ALD, and “—” is not specified) and the references are given for proc-
esses reported up to May 31, 2011. The search was run in published abstracts using Web of Science
V
R
(Ref. 23). acac ¼ acetylacetonate, amd ¼ N,N
0
-diisopropyla-
cetamidinate, cod ¼ 1,4-cyclooctadiene, Cp ¼ g
5
-cyclopentadienyl, Cp* ¼ g
5
-pentamethylcyclopentadienyl, Cp
Et
¼ g
5
-ethylcyclopentadienyl, Cp
iPr
¼ g
5
-
isopropylcyclopentadienyl, Cp
Me
¼ g
5
-methylcyclopentadienyl, dmamb ¼ 1-dimethylamino-2-methyl-2-butanolate, dme ¼ dimethoxyethane, Et ¼ ethyl,
fod ¼ 6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octanedionate, hfac ¼ 1,1,1,5,5,5-hexafluoroacetylacetonate,
i
Pr ¼ isopropyl, Me ¼ methyl, mp ¼ 3-methyl-3-
pentoxyl,
n
Bu ¼ butyl, Ph ¼ phenyl,
t
Bu ¼ tertiary butyl, thd ¼ 2,2,6,6-tetramethyl-3,5-heptanedionate,
t
Pn ¼ tertiarypentyl, vtmos ¼ vinyltrimethoxylsilane.
Material Precursor Plasma Reactor Refs.
Ag Ag(O
2
C
t
Bu)(PEt
3
)H
2
re 24
Ag(O
2
C
t
Bu)(P
n
Bu
3
)H
2
re 24
Al AlH
3
(NEtMe
2
)H
2
d 25,26
Al
2
O
3
AlH
3
(MeNC
4
H
4
)O
2
d 27
AlMe
2
(O
i
Pr) O
2
— 28
AlMe
3
O
2
d, r, re, — 28–67
CO
2
— 68
N
2
/O
2
d, — 32,69–72
N
2
O d 73
AlN AlCl
3
NH
3
/H
2
d 74,75
AlMe
3
NH
3
d, r 42,76,77
H
2
/N
2
r 78
AlO
x
N
y
AlMe
3
O
2
/N
2
d 30,38,79
AlSi
x
O
y
AlMe
3
and Si(OEt)
4
O
2
/N
2
d 80
O
2
— 81
AlTi
x
O
y
AlMe
3
and Ti(O
i
Pr)
4
O
2
d 82–85
N
2
O d 85
Co Co(amd)
2
NH
3
d 86
Co(Cp)(amd) NH
3
d 87
Co
2
(CO)
8
H
2
r 88
H
2
/N
2
r 89
CoCp
2
NH
3
r 90,91
CoCp(CO)
2
H
2
r 92,93
H
2
/N
2
r 89
NH
3
r 90
Co
3
O
4
CoCp
2
O
2
r 94
CoSi
2
CoCp
2
NH
3
and SiH
4
d 95,96
Cu Cu(acac)
2
H
2
d, re 97–100
Cu(hfac)(vtmos) H
2
— 101
Cu(thd)
2
H
2
r 102
Cu(g
2
-OC(Et)(Me)CH
2
NMe
2
)H
2
— 103
Er
2
O
3
Er(thd)
3
O
2
re 104
Ga
2
O
3
[Ga(Me)
2
NH
2
]
3
O
2
d 105–110
GaAs GaMe
3
and AsH
3
H
2
re 111
GaTi
x
O
y
[Ga(Me)
2
NH
2
]
3
and Ti(NMe
2
)
4
O
2
d 105,107,108,112
GeSb
x
Te
y
Ge(NMe
2
)
4
and Sb(NMe
2
)
4
and Te
i
Pr
2
H
2
d 113
Ge
i
Bu
4
and Sb
i
Pr
3
and Te
i
Pr
2
H
2
d 114
HfN Hf(NMe
2
)
4
H
2
d 115–117
H
2
/N
2
d 117
N
2
d, r 117,118
HfO
2
Hf(NEt
2
)
4
O
2
d, r, — 119–133
N
2
O d, — 126,134
Hf(NEtMe)
4
O
2
d, r, re 33,34,135–139
Hf(NMe
2
)
4
O
2
d, r, — 116,140–142
O
2
/N
2
r 143
Hf(OH)
3
NH
2
O
2
— 144
Hf(mp)
4
O
2
r 127
Hf(O
t
Bu)
4
O
2
re 145
HfAl
x
O
y
Al(Me)
3
and Hf(NEtMe)
4
O
2
d 33,34
HfO
x
N
y
Hf(NMe
2
)
4
O
2
/N
2
r 140,143,146
Hf(NEt
2
)
4
O
2
/N
2
r 147
HfSi
x
O
y
Hf(NEtMe)
4
and Si(NMe
2
)
3
HO
2
r 147
Hf(O
t
Bu)
4
and Si(OEt)
4
O
2
re 148
050801-3 Profijt et al.: Plasma-assisted ALD 050801-3
JVST A - Vacuum, Surfaces, and Films
TABLE I. Continued.
Material Precursor Plasma Reactor Refs.
Ir Ir(Cp
Et
)(COD) NH
3
d, — 149,150
La
2
O
3
La(Cp
Et
)
3
O
3
re 151,152
La(Cp
iPr
)
3
O
2
r, re 53,139,153,154
LaHf
x
O
y
La(Cp
iPr
)
3
and Hf(NEtMe)
4
O
2
re 139,155
NbN Nb(N
t
Bu)(NEtMe)
3
H
2
,H
2
/N
2
,NH
3
r 156,157
Ni Ni(dmamb)
2
NH
3
,H
2
d 158
“Bis-Ni(II)” H
2
— 159
Ni(Cp
Et
)
2
H
2
— 160
NiSi
2
Ni(dmamb)
2
NH
3
/SiH
4
d 95
Pd Pd(hfac)
2
H
2
r 161,162
Pd(hfac)
2
H
2
/N
2
r 163
Pt Pt(Cp
Me
)Me
3
O
2
r 164
PtO
2
Pt(Cp
Me
)Me
3
O
2
r 164
Ru Ru(Cp
Et
)
2
NH
3
d, r ,— 165–174
H
2
/N
2
d,— 175–178
RuCp(CO)
2
Et O
2
r 179
Ru(Cp)
2
NH
3
— 173
Ru(Cp
Et
)(NC
4
H
4
)NH
3
d, — 180,181
Ru(1-
i
Pr-4-MeC
6
H
4
)(1,3-C
6
H
8
)NH
3
d 182,183
SiO
2
SiH
4
N
2
O — 184
SiH
2
(NEt
2
)
2
O
2
d 185
SiH
3
NH
2
O
2
d 186
Si(NMe
2
)
4
and Si(NMe
2
)
3
Cl (mix) O
2
/N
2
d 187
Si(OEt)
4
O
2
r 188
[SiMe
2
O-]
4
O
2
— 189
SiN
x
SiH(N
i
PrH)
3
NH
3
d 182
SnO
2
Sn(O
2
CMe)
2
(
n
Bu)
2
O
2
— 190–194
SrO Sr(C
5
H
2
i
Pr
3
)
2
(dme) O
2
r 195
Sr(C
11
H
19
O
2
)
2
O
2
— 196–198
Sr(thd)
2
O
2
d 199
SrTaO
6
Sr[Ta(OEt)
5
(OCH
2
CH
2
NMe
2
)]
2
O
2
d 200
Sr[Ta(OEt)
5
(OCH
2
CH
2
OMe)]
2
O
2
d 201,202
SrTiO
3
Sr(thd)
2
and Ti(O
i
Pr)
4
O
2
d 196–198
Ti(Cp*)(OMe)
3
and Sr(C
5
H
2
i
Pr
3
)
2
(dme) O
2
r 195
SrBi
x
Ta
y
Sr[Ta(OEt)
5
(OCH
2
CH
2
OMe)]
2
and BiPh
3
(mix)
O
2
d 202,203
Ta TaCl
5
H
2
r 204–206
TaO
x
Ta(NMe
2
)
5
O
2
r 56,207–209
O
2
/N
2
r 209
Ta(OEt)
5
O
2
re, — 210,211
TaC
x
N
y
Ta(N
t
Bu)(NEt
2
)
3
H
2
d 212,213
NH
3
d 214
CH
4
/H
2
d 215
Ta(N
t
Pn)(NMe
2
)
3
H
2
d 216
Ta(NMe
2
)
5
H
2
r 217
TaN
x
TaCl
5
H
2
/N
2
r, — 218–220
TaF
5
H
2
/N
2
d 221
Ta(NMe
2
)
5
H
2
r, — 207,222–224
N
2
r 225
H
2
/N
2
r 223,226
NH
3
r, — 174,223,225
Ta(N
t
Bu)(NEt
2
)
3
H
2
d, r 227–229
NH
3
— 181
Ta(N
t
Bu)(NEtMe)
3
H
2
— 230
Ta(N
i
Pr)(NEtMe)
3
H
2
/N
2
r 231
Ta(N
t
Pn)(NMe
2
)
3
H
2
— 175
Ti TiCl
4
H
2
r 206,232
TiN
x
TiCl
4
H
2
/N
2
d, r 31,51,136,222,233–238
H
2,
D
2
re 239
050801-4 Profijt et al.: Plasma-assisted ALD 050801-4
J. Vac. Sci. Technol. A, Vol. 29, No. 5, Sep/Oct 2011
