Ozone-Based Atomic Layer Deposition of Alumina from TMA:
Growth, Morphology, and Reaction Mechanism
S. D. Elliott*
Tyndall National Institute, Lee Maltings, Cork, Ireland
G. Scarel, C. Wiemer, and M. Fanciulli
CNR-INFM MDM National Laboratory, Via C. OliVetti 2, 20041 Agrate Brianza, Italy
G. Pavia
STMicroelectronics, Via C. OliVetti 2, 20041 Agrate Brianza, Italy
ReceiVed April 18, 2006. ReVised Manuscript ReceiVed May 25, 2006
We examine the effect of growth temperature in the 150-300 °C range on the structural and
morphological properties of Al
2
O
3
films deposited using atomic layer deposition, contrasting the effect
of H
2
O and O
3
as oxygen sources. Trimethylaluminum (TMA) is the metal source. A mechanism for the
O
3
reaction is investigated using ab initio calculations and provides an explanation for the observed
temperature dependence. The simulations show that hydroxyl groups are produced at the surface by the
oxidation of adsorbed methyl groups by O
3
. This is confirmed by the measured rates; both H
2
O and O
3
processes show molar growth rates per cycle that decrease with increasing reactor temperature, consistent
with a decrease in hydroxyl coverage. At no temperature does the O
3
process deposit more Al
2
O
3
per
cycle than the H
2
O process. Morphological characterization shows that O
3
as the oxygen source yields
lower-quality films than H
2
O; the films are less dense and rougher, especially at low growth temperatures.
The existence of voids correlates with the low film electronic density. This may indicate the low mobility
of surface hydroxyl at low temperatures, an effect that is washed out by repeated exchange with the
vapor phase in the H
2
O case.
1. Introduction
The atomic layer deposition (ALD) of alumina (Al
2
O
3
)
using ozone (O
3
) is studied over a wide temperature range
and compared with the well-known H
2
O process, focusing
on the growth rate, film density, roughness, and reaction
mechanism.
ALD is a type of chemical vapor deposition (CVD) in
which precursors in gas form are admitted separately into
the reactor in alternate pulses. Each precursor chemisorbs
individually onto the substrate, rather than reacting in the
gas phase.
1
Gas-phase reactions are avoided by purging with
inert gas between each pulse. Under ideal conditions,
substrate-precursor reactions are self-limiting and the surface
is saturated with precursor fragments at the end of each pulse.
Ligands in the fragments are eliminated by reaction with the
other precursor during the next pulse. Because growth
reactions occur only at the surface, deposition is slow, but
with the advantage of atomic-level control of film thickness,
as well as unparalleled conformality and uniformity. ALD
is therefore used for the deposition of nanometer-thin films
in high-aspect-ratio structures, such as alumina films in
dynamic random access memory (DRAM) trenches,
2
read-
write heads,
3
or in the interpoly dielectric stack of flash
memories.
4
The use of ozone (O
3
) as oxygen precursor in metal-oxide
ALD has stemmed from its higher activity for ligand
elimination relative to H
2
O. It is therefore needed for
particularly stable metal (M) precursors, such as -diketo-
nates. For instance, Y
2
O
3
has been deposited from yttrium
-diketonate and O
3
,
5
and ZrO
2
has been deposited from
various zirconium cyclopentadienyls and O
3
6
or H
2
O.
7
It is
interesting that both of these examples show temperature
windows over which the growth rate is approximately
constant. Another reason for a preference for O
3
is that H
2
O
adsorbs on reactor walls and is difficult to purge.
Along with these processing advantages, higher-quality
film is sometimes produced by O
3
ALD when compared with
H
2
O. Much current research is directed toward the use of
ALD of high permittivity films (e.g., ZrO
2
,
7
HfO
2
,
8,9,10
and
Al
2
O
3
)
11,12,13
in the microelectronics industry. For the deposi-
* To whom correspondence should be addressed. E-mail: simon.elliott@
tyndall.ie. Tel: 353-21-4904392. Fax: 353-21-4270271.
(1) Suntola, T. Mater. Sci. Rep. 1989, 4, 261-312.
(2) Ha, D.; Shin, D.; Koh, D. H.; Lee, J.; Lee, S.; Ahn, Y. S.; Jeong, H.;
Chung, T.; Kim, K. IEEE Trans. Electron DeVices 2000, 47, 1499.
(3) Paranjpe, A.; Gopinath, S.; Omstead, T.; Bubber, R. J. Electrochem.
Soc. 2001, 148, G465-G471.
(4) Lee, T. P.; Jang, C.; Haselden, B.; Dong, M.; Park, S.; Bartholomew,
L.; Chatham, H.; Senzaki, Y. J. Vac. Sci. Technol., B. 2004, 22, 2295-
2298.
(5) Putkonen, M.; Sajavaara, T.; Johansson, L.-S.; Niinisto ¨ , L. Chem. Vap.
Deposition 2001, 7, 44-50.
(6) Putkonen, M.; Niinisto ¨, L. J. Mater. Chem. 2001, 11, 3141-3147.
(7) Niinisto ¨ , J.; Putkonen, M.; Niinisto ¨ , L.; Kukli, K.; Ritala, M.; Leskela ¨,
M. J. Appl. Phys. 2004, 95, 84-91.
(8) Lee, S.-I.; Owyang, J. S.; Senzaki, Y.; Helms, A.; Kapkin, K. Solid
State Technol. 2003, 46, 45-46.
3764 Chem. Mater. 2006, 18, 3764-3773
10.1021/cm0608903 CCC: $33.50 © 2006 American Chemical Society
Published on Web 07/13/2006