Low temperature plasma-enhanced atomic layer deposition of metal oxide thin films

Article abstract of DOI:10.1149/1.3428705

Many reported atomic layer deposition (ALD) processes are carried out at elevated temperatures (>150°C), which can be problematic for temperature-sensitive substrates. Plasma-enhanced ALD routes may provide a solution, as the ALD temperature window can, in theory, be extended to lower deposition temperatures due to the reactive nature of the plasma. As such, the plasma-enhanced ALD of Al₂ O₃, TiO₂, and Ta₂ O₅ has been investigated at 25-400°C using [Al (CH₃)₃], [Ti (OPⁱr)₄], [Ti(Cp ^Me) (OⁱPr)₃], [TiCp^* (OMe)₃], and [Ta (NMe₂)₅] as precursors. An O₂ plasma was employed as the oxygen source in each case. We have demonstrated metal oxide thin-film deposition at temperatures as low as room temperature and compared the results with corresponding thermal ALD routes to the same materials. The composition of the films was determined by Rutherford backscattering spectroscopy. Analysis of the growth per cycle data and the metal atoms deposited per cycle revealed that the growth per cycle is strongly dependent on the film density at low deposition temperatures. Comparison of these data for Al₂O₃ ALD processes in particular, showed that the number of Al atoms deposited per cycle was consistently high down to room temperature for the plasma-enhanced process but dropped for the thermal process at substrate temperatures lower than 250°C.

Full text of DOI:10.1149/1.3428705

Low Temperature Plasma-Enhanced Atomic Layer Deposition of
Metal Oxide Thin Films
S. E. Potts, W. Keuning, E. Langereis, G. Dingemans, M. C. M. van de Sanden and W. M. M.
Kessels
J. Electrochem. Soc. 2010, Volume 157, Issue 7, Pages P66-P74.
doi: 10.1149/1.3428705
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P66
Journal of The Electrochemical Society, 157 ͑7͒ P66-P74 ͑2010͒
0013-4651/2010/157͑7͒/P66/9/$28.00 © The Electrochemical Society
Low Temperature Plasma-Enhanced Atomic Layer Deposition
of Metal Oxide Thin Films
,z
*
*
S. E. Potts, W. Keuning, E. Langereis, G. Dingemans,
,z
*
M. C. M. van de Sanden, and W. M. M. Kessels
Department of Applied Physics, Eindhoven University of Technology, 5600 MB Eindhoven, The
Netherlands
Many reported atomic layer deposition ͑ALD͒ processes are carried out at elevated temperatures ͑Ͼ150°C͒, which can be
problematic for temperature-sensitive substrates. Plasma-enhanced ALD routes may provide a solution, as the ALD temperature
window can, in theory, be extended to lower deposition temperatures due to the reactive nature of the plasma. As such, the
plasma-enhanced ALD of Al2O3, TiO2, and Ta2O5 has been investigated at 25–400°C using ͓Al͑CH3͒ ͔, ͓Ti͑OiPr͒ ͔,
3
4
͓Ti͑CpMe͒͑OiPr͒ ͔, ͓TiCpء͑OMe͒ ͔, and ͓Ta͑NMe2͒ ͔ as precursors. An O2 plasma was employed as the oxygen source in each
3
3
5
case. We have demonstrated metal oxide thin-film deposition at temperatures as low as room temperature and compared the results
with corresponding thermal ALD routes to the same materials. The composition of the films was determined by Rutherford
backscattering spectroscopy. Analysis of the growth per cycle data and the metal atoms deposited per cycle revealed that the
growth per cycle is strongly dependent on the film density at low deposition temperatures. Comparison of these data for Al2O3
ALD processes in particular, showed that the number of Al atoms deposited per cycle was consistently high down to room
temperature for the plasma-enhanced process but dropped for the thermal process at substrate temperatures lower than 250°C.
© 2010 The Electrochemical Society. ͓DOI: 10.1149/1.3428705͔ All rights reserved.
Manuscript submitted January 18, 2010; revised manuscript received April 16, 2010. Published May 14, 2010. This was Paper
2034 presented at the Vienna, Austria, Meeting of the Society, October 4–9, 2009.
Atomic layer deposition ͑ALD͒, a method of ultrathin-film depo-
moisture permeation barrier to prevent degradation, and aluminum
oxide deposited by ALD is a popular choice. Another application
requiring low deposition temperatures is the ALD of corrosion-
resistant barrier layers, which can be applied to metal substrates.12-14
For such substrates, overheating may cause surface oxidation, which
may weaken the protective layer or change the substrates’ mechani-
cal properties. This can be problematic as corrosion-resistant appli-
cations require dense films with no defects. Generally, substrate tem-
peratures Յ150°C are required, although these have not been
widely reported so far for ALD. However, examples are known,
which are summarized in Table I.
Despite the low temperature requirement, it is a generalization
that the lower the substrate temperature, the higher the concentration
of impurities. Examples of possible impurities include hydroxyl
groups and carbon in Al2O3 processes6,15-17 or chlorine in the
TiCl4/H2O process,18,19 the latter being difficult to remove at low
temperatures.20 However, the choice of metal precursor is important,
as the lowest deposition temperatures have been employed using
alkyl-based precursors, resulting in very low impurity concentra-
tions in the films. This is especially evident in Al2O3,4,15,16,21-23
PtOx,24,25 and ZnO,7,8 as the alkyl ligands are highly reactive and
labile. It is also the oxidants that play an important role, particularly
in terms of reactivity. Films of fair to good quality are possible at
lower temperatures where more reactive precursors, such as ozone
or an oxygen plasma, are employed. The high reactivity of the
plasma is attributed to the fact that it contains reactive ionic and
radical oxygen species, which makes it an appealing candidate for
obtaining high purity films at temperatures as low as room
temperature.26 For this reason, we have been investigating the po-
tential of plasma-enhanced ALD as a route to Al2O3, TiO2, and
Ta2O5 thin films at temperatures down to 25°C. This paper de-
scribes the work carried out, using a variety of metal precursors, and
compare the results with thermal ALD processes from the literature.
sition by alternate dosing of gaseous precursors, gives dense, pure,
and highly conformal films with excellent step coverage. Each ALD
process is considered to have a temperature window, which is gov-
erned by the reactivity and stability of the precursors and surface
functional groups.1 The ALD temperature window typically spans
200–400°C for the majority of processes, although many studies on
metal oxides have been investigated over the range 100–600°C.2 It
is often assumed that, within the temperature window, a sub-
monolayer of material is deposited during each cycle and that the
growth per cycle remains constant with varying substrate tempera-
ture ͑Fig. 1a͒. However, in several cases it is possible to see a small
reduction in the growth per cycle with increasing temperature due to
a small loss of reactive surface groups ͑Fig. 1b͒.3 Outside the tem-
perature window, the growth per cycle can change dramatically. At
lower temperatures, excess condensation of the precursors can result
in a rapid increase in growth ͑A͒, for example, when the substrate
temperature is lower than the bubbler temperature required to obtain
an adequate vapor pressure. Also at lower temperatures, there may
be insufficient thermal energy for a chemical reaction to occur, re-
sulting in slower growth ͑B͒. At higher temperatures, decomposition
of the precursors ͑chemical vapor deposition͒ can occur, leading to
an increase in growth, which is not self-limiting ͑C͒, or, if the film
material is volatile it may evaporate, thereby reducing the growth
͑D͒. However, as previously mentioned, a change in growth per
cycle does not always accurately indicate whether or not true ALD
is occurring, exemplified by the gradual loss of surface groups with
increasing substrate temperature in Fig. 1b.
Generally, the ALD window ͑spanning 150–400°C͒ for most
metal oxide processes has proved sufficient for many applications
requiring ultrathin films. However, for emerging applications em-
ploying temperature-sensitive substrates, even temperatures as low
as 200°C can be problematic, for example, in polymers or small
organic molecules, whose structure can be affected at such tempera-
tures. Applications that require such low temperatures include or-
ganic light emitting diodes for display and lighting applications,4-8
in ZnO-based thin film transistors for ͑flexible͒ displays,9,10 and in
double patterning lithography where SiO2 is deposited on a
temperature-sensitive photoresist.11 Such devices may require a
Experimental
The experiments were carried out on three remote plasma ALD
reactors. The home-built ALD-I reactor and the commercially built
Oxford Instruments FlexAL reactor are described in detail
elsewhere.22 The Oxford Instruments OpAL apparatus, an open-load
system, was employed for the deposition of Al2O3 and operates in a
similar manner to the FlexAL, although without a turbo pump. The
base pressure was ϳ1 mTorr for the OpAL and ϳ10−3 mTorr for
the ALD-I and FlexAL, and typical operating pressures were 100–
*
Electrochemical Society Active Member.
z E-mail: s.e.potts@tue.nl; w.m.m.kessels@tue.nl
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Journal of The Electrochemical Society, 157 ͑7͒ P66-P74 ͑2010͒
P67
and ͓Ta͑NMe2͒5͔ ͑STREM Chemicals Ltd.͒, all having a purity of
Ͼ99.9%, were employed as precursorsa and were contained in stain-
less steel bubblers. Oxygen ͑purity Ͼ99.999%͒ was used to form
the plasma, and argon ͑purity Ͼ99.999%͒ was employed as the
purge gas. For the processes employing ͓Al͑CH3͒3͔,
i
͓Ti͑CpMe͒͑O Pr͒ ͔, and ͓TiCpء͑OMe͒ ͔, the oxygen flow was con-
3
3
tinuous throughout the entire process, but where ͓Ti͑OiPr͒ ͔ and
4
͓Ta͑NMe2͒5͔ were used, the flow was restricted to the plasma step.
Water ͑VWR, GBR Rectapur grade, Ͼ99.999%͒ used in thermal
ALD experiments was also held in a stainless steel bubbler at either
ambient temperature ͑FlexAL͒ or 19°C ͑OpAL͒ to reduce the vapor
pressure and prevent overdosing. The substrates were n-type Si͑100͒
wafers, which were 100 mm in diameter for the FlexAL and OpAL
reactors and ϳ40 ϫ 40 mm for ALD-I. These wafers were covered
with a thin native oxide ͑SiO2͒ layer and did not undergo any addi-
tional cleaning steps. The ALD conditions for each process are sum-
marized in Table II. Film growth during the ALD process was moni-
tored by in situ spectroscopic ellipsometry ͑SE͒, carried out using a
J. A. Woollam Inc. M2000 rotating compensator ellipsometer.21,28
The atomic composition of the films was determined by Rutherford
backscattering spectroscopy ͑RBS͒, except for the hydrogen concen-
tration, which was obtained by elastic recoil detection ͑ERD͒ analy-
sis. The detection limits for the RBS and ERD measurements are not
the same for each set of measurements and were quoted where ap-
propriate. The mass density of the films was calculated using a com-
bination of the RBS and SE data.
Results and Discussion
The plasma-enhanced ALD of Al2O3, TiO2, and Ta2O5 has been
investigated at substrate temperatures down to 25°C. Every process
was routinely monitored using in situ SE to determine the film thick-
ness. The slope of the resulting thickness vs ALD cycles plot gave
the growth per cycle for the process ͑an example for the depositions
at 100°C is shown in Fig. 2͒. All ALD processes showed linear
growth at every deposition temperature reported in this paper ͑with
Figure 1. ͑Color online͒ A schematic representation of ͑a͒ an ideal ALD
temperature window, where it is assumed that a sub-monolayer has been
deposited per cycle and ͑b͒ a more commonly observed ALD window ͑es-
pecially for metal oxides͒, showing a slight reduction in growth per cycle
with increasing temperature.
the exception of the TiO2 depositions from ͓Ti͑OiPr͒ ͔ at substrate
4
temperatures Ͼ250°C͒, with no significant nucleation delay.
1000 mTorr ͑OpAL͒ and 7.5–100 mTorr ͑ALD-I and FlexAL͒.
Al2O3 was deposited using OpAL and FlexAL, TiO2 using FlexAL
and ALD-I, and Ta2O5 using FlexAL. The Ta2O5 results on FlexAL
are compared with our previous work using ALD-I.27 The com-
Aluminum oxide
.— Al2O3 has been a material of high interest
with respect to ALD ͑see the review by Puurunen͒.3 However, it is
relatively recently that low substrate temperatures have been inves-
tigated. The growths per cycle of two of the most-reported thermal
processes are compared with the plasma-enhanced route in Fig. 3.
We deposited Al2O3 using both the plasma-enhanced and thermal
processes on an Oxford Instruments FlexAL reactor21 and OpAL
reactor. The growth of the plasma-enhanced method showed a
gradual decrease from 0.17 nm/cycle at 25°C to 0.11 nm/cycle at
300°C. The decrease was essentially linear, with the exception of
the runs at 25 and 50°C, which were slightly higher. The corre-
sponding thermal process showed a maximum value of 0.10 nm/
cycle at 200–250°C. Our results for the thermal process differ from
those of Groner et al.,6 where the growth was higher at
ϳ0.13 nm/cycle. These two thermal processes were carried out in
different reactors and were clearly optimized at different tempera-
tures ͑where the peak of the data is͒. A possible reason for such a
difference in growth per cycle is the water dose. Increasing the
water dose ͑i.e., the mass of water per unit time͒ leads to an increase
in growth per cycle without affecting the film composition,29 as
more surface hydroxyl groups are created with higher doses. So
despite the difference in water dose time ͑20 ms for our work, 2 s for
Groner et al.͒, it is likely that our water flow rate was lower, afford-
ing a lower growth per cycle, because the water on the OpAL reac-
tor was actively cooled to 19°C to avoid overdosing. The reduction
pounds ͓Al͑CH3͒3͔ ͑Sigma Aldrich͒, ͓Ti͑OiPr͒ ͔ ͑Sigma Aldrich͒,
4
novel Cp-based ALD precursors ͓Ti͑CpMe͒͑OiPr͒ ͔ ͑donated by
3
SAFC Hitech, Ltd.͒ and ͓TiCpء͑OMe͒ ͔ ͑donated by Air Liquide͒,
3
Table I. Examples of low temperature ALD of metal oxides de-
posited below 150°C.
Lowest
reported
Metal
temperature
Material
precursor
Oxidant
͑°C͒
Reference
Al2O3
Al2O3
Al2O3
TiO2
TiO2
TiO2
TiO2
Ta2O5
Ta2O5
Ta2O5
PtOx
͓Al͑CH3͒ ͔
H2O
O3
O2 plasma
H2O
H2O2
H2O
H2O2
H2O
33
25
25
100
100
150
77
6
30
3
͓Al͑CH3͒ ͔
3
͓Al͑CH3͒ ͔
4, 15, 16, and 21-23
3
TiCl4
TiCl4
18 and 19
34
͓Ti͑O iPr͒ ͔
35 and 36
4
͓Ti͑O iPr͒ ͔
37
42
4
TaCl5
80
͓Ta͑NMe2͒ ͔
H2O
150
100
120
100
60
45 and 46
5
͓Ta͑NMe2͒ ͔ O2 plasma
27
24
25
7
5
͓Pt͑acac͒ ͔
O3
2
͓Pt͑CpMe͒Me3͔ O2 plasma
a acac is acetylacetonate, CH͑C=OCH3͒2; CpMe is monomethylcyclopentadienyl,
␩5-C5H4͑CH3͒; Cpء is pentamethylcyclopentadienyl, ␩5-C5͑CH3͒5; Me is methyl,
PtO2
ZnO
ZnO
͓Zn͑CH2CH3͒ ͔
͓Zn͑CH2CH3͒ ͔
H2O
H2O2
2
i
25
8
CH3; and Pr is isopropyl, CH͑CH3͒2.
2
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P68
Journal of The Electrochemical Society, 157 ͑7͒ P66-P74 ͑2010͒
Table II. The ALD process conditions for the metal oxides reported in this work. The purges for the ALD processes where
†Ti„CpMe…„OiPr…3‡ and †TiCpء„OMe…3‡ were used only incorporated an argon flow for part of the duration. In these cases, the pumping and
argon (Ar) times are given in the order they occurred during the purge. In all other cases, a continuous Ar flow was employed throughout the
cycle. “Oxidant” refers to an O2 plasma for plasma-enhanced ALD and water for thermal ALD. The conditions for the Ta2O5 process on the
ALD-I reactor are reported elsewhere.27
Variable
Al2O3
Al2O3
TiO2
TiO2
TiO2
Ta2O5
ALD
Reactor
Precursor
Plasma-enhanced
OpAL and FlexAL OpAL and FlexAL FlexAL
Thermal
Plasma-enhanced
Plasma-enhanced
ALD-I
Plasma-enhanced
ALD-I
Plasma-enhanced
FlexAL
͓Al͑CH3͒ ͔
͓Al͑CH3͒ ͔
͓Ti͑OiPr͒ ͔
͓Ti͑CpMe͒͑OiPr͒ ͔
͓TiCpء͑OMe͒ ͔
͓Ta͑NMe2͒ ͔
3
3
4
3
3
5
Tsubstrate
Tprecursor
25–400°C
ϳ25°C
25–400°C
ϳ25°C
25–400°C
45°C
100–400°C
70°C
50–300°C
110°C
25–250°C
65°C
Precursor dose
0.02 s
0.05 s
4 s
5 s
4 s pumping,
4 s Ar
4 s
4 s pumping, 2 s Ar,
2 s pumping
100 W
5 s
Precursor purge
Plasma power
Oxidant time
3 s
400 W
2 s
10 s
—
0.02 s
4 s
400 W
12 s
5 s
200 W
5 s
100 W
5 s
5 s
2 s pumping, 0.5 s Ar,
2 s pumping
7.5 mTorr
Oxidant purge
O2 flow
0.5 s
60 sccm
10 s
—
1.5 s
60 sccm
4 s pumping
7.5 mTorr
15 s
50 sccm
100 sccm ͑precursor step͒,
150 sccm ͑purges͒,
50 sccm ͑plasma͒
Ar flow
20 sccm
200 sccm
30 sccm
15 mTorr
15 mTorr
in growth per cycle for the thermal ALD process with decreasing
temperature below the maximum value appears to be a result of
insufficient thermal energy for the reaction between water and the
methyl surface groups to occur as rapidly, resulting in fewer reactive
hydroxyl surface groups with which ͓Al͑CH3͒3͔ can react. Unlike
water, the oxygen plasma is sufficiently reactive with the surface
methyl groups down to room temperature, resulting in an almost
linear decrease in growth per cycle from 25–300°C. At higher depo-
sition temperatures for the thermal processes with water, the growth
per cycle profile follows that of the plasma-enhanced process. The
decrease in growth per cycle, as illustrated in Fig. 1b, is due to the
gradual reduction of reactive surface groups, such as hydroxyls,
with increasing deposition temperature.3,12,15,16
thermal process ͑10 s for this work and 5–180 s for Groner et al.6͒,
which allows for a significantly reduced overall cycle time for the
plasma-enhanced process. The thermal ALD process employing
ozone as the oxidant reported by Kim et al.30 gave the lowest growth
per cycle above 100°C ͑ϳ0.09 nm/cycle͒. Ozone is more reactive
than water toward the surface methyl groups, forming hydroxyl,
methoxy, formate, and carbonate surface groups in the process.31,32
Therefore, Kim et al. suggested that the dramatic increase in growth
per cycle observed at deposition temperatures below 100°C was
attributed to the insufficient removal of the aforementioned oxygen-
based surface groups in the steps subsequent to the oxidation be-
cause there was insufficient thermal energy with which to remove
them.30 Also in this case, the ozone purge was kept constant at 3 s at
33–100°C, where a longer purge at lower temperatures may have
helped to reduce the impurities. In comparison, our plasma-
Another important consideration is the oxidant purge time ͑Fig.
4͒. The post-plasma purge times required ͑0.5 s͒ were significantly
lower than those required to purge water from the system in the
Figure 2. ͑Color online͒ The increase in film thickness with the number of
ALD cycles as measured by in situ SE ͑deposition temperature = 100°C͒.
Figure 3. ͑Color online͒ A comparison of growths per cycle for the ALD of
Al2O3 from ͓Al͑CH3͒ ͔. Shown here are the thermal process with water, as
3
3
5
reported by Groner et al.,6 the thermal process with ozone, as reported by
Kim et al.,30 and our plasma-enhanced ͑FlexAL͒ and thermal processes
͑FlexAL and OpAL combined͒.21 All lines serve as a guide to the eye.
The increase was linear for each precursor: ͑a͒ ͓Al͑CH3͒ ͔, ͑b͒ ͓Ta͑NMe2͒ ͔
͑ALD-I reactor͒,27 ͑c͒ ͓Ta͑NMe2͒ ͔ ͑FlexAL reactor͒, ͑d͒ ͓Ti͑OiPr͒ ͔, ͑e͒
5
4
͓Ti͑CpMe͒͑OiPr͒ ͔, and ͑f͒ ͓TiCpء͑OMe͒ ͔.
3
3
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Journal of The Electrochemical Society, 157 ͑7͒ P66-P74 ͑2010͒
P69
Figure 4. ͑Color online͒ A comparison of the oxidant purges required for
various Al2O3 ALD processes: the plasma-enhanced and thermal processes
on OpAL, the thermal process with water ͑note the break in the scale͒, as
reported by Groner et al.,6 and the thermal process with ozone, as reported by
Kim et al.30,48
enhanced route gave a more consistent growth per cycle profile
down to 25°C and provided the highest growths per cycle over
50–300°C with the shortest cycle times.
In terms of film composition, the films deposited by both thermal
and plasma-enhanced ALD in this work were over-stoichiometric at
deposition temperatures below 200°C ͑Fig. 5a͒. The similar reduc-
tion in hydrogen content ͑Fig. 5b͒ with increasing substrate tempera-
ture suggests that this is due to the incorporation of hydroxyl groups
in the film.16 In all cases, the carbon content was below the RBS
detection limit ͑1 atom %͒; although thermal effusion measurements
have suggested that a small quantity of carbon was present in the
films deposited by this process.17 The mass density ͑Fig. 5c͒ was
ϳ3.0 g cm−3 at deposition temperatures of 200°C and above for
the plasma-enhanced process, which is close to the bulk value of
Al2O3 ͑3.1 g cm−3͒.33 A gradual decrease was observed below
200°C to a minimum of 2.6 g cm−3 at 25°C. This decrease in mass
density is likely to be the result of excess –OH in the film, which
accounts for the higher than expected growths per cycle obtained
below 100°C. The thermal water process showed no significant dif-
ference in terms of impurity content, and the mass density of the
films over the temperature range was still within error, although
slightly lower than that for the plasma-enhanced process. The ther-
mal process with ozone gave over-stoichiometric films at all tem-
peratures up to 300°C with up to 5 atom % carbon contamination.30
The growth per cycle profiles are apparently all different, which,
in part is caused by the varying mass densities. However, determi-
nation of the number of ͑metal͒ atoms deposited per cycle, obtained
by dividing the atom density from RBS by the total number of ALD
cycles, can show if this is the case. For example, if the number of
atoms deposited per cycle does not vary significantly over a tem-
perature range, any variation in growth per cycle is most likely due
to the variation in mass density; however, if there is a difference in
atoms deposited, other factors, such as chemical vapor deposition
͑CVD͒ or surface group loss, may be occurring. Figure 6 shows the
growth per cycle data for the plasma-enhanced and thermal pro-
cesses on both the FlexAL and OpAL reactors and the number of Al
atoms deposited per cm2 in a cycle. In terms of films deposited by
plasma-enhanced ALD, there is no significant difference between
the growths per cycle from either reactor; but for the thermal pro-
cess, more atoms appear to have been deposited by the OpAL reac-
tor. The OpAL reactor is not equipped with a turbo pump or load
lock, so it may not be able to purge the water away as effectively as
in the FlexAL reactor and the chamber vacuum has to be disrupted
for sample loading. However, a greater difference in mass density
Figure 5. ͑Color online͒ The film characteristics of the Al2O3 films obtained
on the OpAL reactor. ͑a͒ The ͓O͔/͓Al͔ ratio of the films ͑the line serves as a
guide to the eye͒, ͑b͒ the hydrogen content of the films, and ͑c͒ the mass
density of the films.
would be expected for such a difference in Al atoms deposited per
cycle where the growth per cycle remains the same. There is a clear
gradient for Al atoms deposited per cycle with the plasma-enhanced
process, showing the effect of dehydroxylation ͑loss of surface
groups͒ with increasing substrate temperature. A similar gradient
was observed for the growth per cycle, although this is enhanced at
25 and 50°C as a result of the decreased film density and increased
OH concentration ͑as shown in Fig. 5͒. For the thermal processes,
the effect of lower temperatures is clear; with the number of Al
atoms deposited being half of that of the plasma-enhanced process at
100°C, suggesting that the rate of reaction is lower at these tempera-
tures. The temperature at which the thermal process was optimized
͑i.e., the saturation of each ALD step was determined, which was
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P70
Journal of The Electrochemical Society, 157 ͑7͒ P66-P74 ͑2010͒
Figure 7. ͑Color online͒ A comparison of growths per cycle for the ALD of
TiO2. Shown here are thermal processes: TiCl4 and water, as reported by
Aarik et al.18,19 ͓Ti͑OiPr͒ ͔ and water, as reported by Ritala et al.35,36 and
4
͓TiCpء͑OMe͒ ͔ and ozone, as reported by Katamreddy et al.40 These are
3
compared to the plasma-enhanced processes using ͓Ti͑OiPr͒ ͔,
4
͓Ti͑CpMe͒͑O Pr͒ ͔ and ͓TiCpء͑OMe͒ ͔.38 The higher growths per cycle for
i
3
3
͓Ti͑OiPr͒ ͔ resulting from anatase formation are shown as hollow squares.
4
Lines serve as a guide to the eye.
using ͓Ti͑CpMe͒͑OiPr͒ ͔ were slightly higher than those of the other
3
two Ti precursors, as more Ti atoms were deposited per cycle for
this precursor ͑Fig. 8a͒. Again, a gradual decrease in growth per
cycle was observed with increasing substrate temperature for all of
the Ti precursors, which can be attributed to increasing mass density.
The loss of surface groups is not likely to be a significant factor for
the Cp-based precursors, as the Ti atoms deposited per cycle were
constant over the temperature range. At 25°C, the growth per cycle
Figure 6. ͑Color online͒ ͑a͒ The growths per cycle for plasma-enhanced and
thermal ALD of Al2O3 on both the OpAL and FlexAL reactors. ͑b͒ The Al
atoms deposited per cm2 in a cycle. All lines serve as a guide to the eye.
obtained using ͓Ti͑OiPr͒ ͔ was higher than the expected trend,
4
200°C in these cases͒ apparently gave the highest growth per cycle
and the highest number of Al atoms deposited. At higher tempera-
tures, the growth per cycle and atoms per cycle decrease in line with
the plasma-enhanced process as a result of the loss of surface
groups.
which is a result of the excess OH groups in the film, as for the
Al2O3 films ͑Fig. 8b-d͒. All precursors gave significantly higher
growths per cycle with the plasma-enhanced ALD process than for
those obtained for the thermal routes using ͓Ti͑OiPr͒ ͔ and water, as
4
reported by Ritala et al.35 and Rahtu and Ritala,36 and
͓TiCpء͑OMe͒ ͔ and ozone, as reported by Katamreddy et al.40 This
In summary, the plasma-enhanced ALD of Al2O3 generally gives
a higher growth per cycle at low temperatures than the correspond-
ing thermal processes using water or ozone, although the tempera-
ture at which the thermal process is optimized and the water flow
rate play an important role. The film compositions of the plasma-
enhanced and thermal processes were comparable, with the concen-
tration of –OH increasing with decreasing substrate temperature.
Carbon was not detected in the films ͑RBS detection limit
= 1 atom %͒. Analysis of the Al atoms deposited per cycle showed
that the temperature window could easily be extended down to room
temperature with fixed cycle settings for the plasma-enhanced ALD
of Al2O3. For the thermal process, temperatures lower than the op-
timization temperature could be considered outside the temperature
window; however, if saturation curves were determined for each
deposition temperature and the water flow rate were increased, this
could be overcome.
3
shows the benefits of the high reactivity of the plasma; although it is
somewhat surprising that ozone, itself a highly reactive gas, could
not allow for a higher growth per cycle. The growths per cycle of
the TiCl4 and water process were extended down to 100°C by Aarik
et al.18,19 The data were more comparable to the plasma-enhanced
process at substrate temperatures around 150°C because TiCl4 is
more reactive toward surface groups than ͓Ti͑OiPr͒ ͔ ͑see Fig. 7͒.
4
The lower data points for the TiCl4/water process at 150 and 175°C
were reported to be the result of etching of the film by HCl formed
during the ALD process at these temperatures.19 The increase in
growth per cycle at higher temperatures for the TiCl4/H2O process
was attributed to the crystallization of the films, and below 150°C
the increase was due to the condensation of water on the substrate,
leading to a dramatic increase in surface hydroxyl groups. The pres-
ence of such additional surface groups at low temperatures was less
prominent for the plasma-enhanced ALD using ͓Ti͑OiPr͒ ͔,
Titanium dioxide
4
.— TiO2 is
a
well-studied ALD mat-
͓Ti͑CpMe͒͑OiPr͒ ͔, and ͓TiCpء͑OMe͒ ͔.
erial,18,19,34-37 and a selection of the low temperature ALD studies is
summarized in Fig. 7. For the plasma-enhanced ALD of TiO2, we
employed ͓Ti͑OiPr͒ ͔, ͓Ti͑CpMe͒͑OiPr͒ ͔, and ͓TiCpء͑OMe͒ ͔ as
3
3
The number of Ti atoms deposited per cycle is shown in Fig. 8a.
For the two Cp-based precursors, the Ti deposited was generally
consistent across the temperature ranges studied, implying that these
precursors are less affected by surface dehydroxylation than
͓Al͑CH3͒3͔ in the Al2O3 process, where there is a distinct negative
gradient in the data ͑Fig. 5b͒. The data also complement the growth
4
3
3
precursors.38 The heteroleptic mono Cp-based alkoxides are not suf-
ficiently reactive with water during ALD,39 but they are reactive
with ozone40 and an oxygen plasma. The growths per cycle for our
plasma-enhanced processes were 0.05–0.06 nm/cycle, over the
deposition temperature range 100–250°C. The growths per cycle
per cycle values in Fig. 7, whereby ͓Ti͑CpMe͒͑OiPr͒ ͔ deposited
3
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Journal of The Electrochemical Society, 157 ͑7͒ P66-P74 ͑2010͒
P71
more Ti atoms per cycle and afforded a higher growth per cycle,
when compared with ͓TiCpء͑OMe͒ ͔. In addition to the experimen-
3
tal inaccuracy of RBS, the ͓Ti͑OiPr͒ ͔ is not so straightforward, as
4
there is no real, clear-cut linear trend. The rise in Ti atoms deposited
at 300°C can be attributed to the decomposition ͑CVD͒ of the pre-
cursor, as a significant amount of carbon ͑ϳ17 atom %͒ was
present in films deposited at that temperature. For depositions using
͓Ti͑OiPr͒ ͔ at temperatures Ն250°C, crystallization was observed
4
in the anatase form, resulting in higher growths per cycle.28 At 250
and 300°C, the growth began in the amorphous phase at a lower
growth per cycle, after which anatase TiO2 was deposited at a much
higher rate. The transitions were observed between 400–700
cycles at 250°C and above 200 cycles at 300°C. Where
i
͓Ti͑CpMe͒͑O Pr͒ ͔ was the precursor, anatase TiO2 was also ob-
3
served at substrate temperatures Ն250°C, but this did not seem to
have such a dramatic effect on the growth per cycle, suggesting that
the CpMe group inhibits the anatase formation to some extent.
In terms of film composition, films deposited by plasma-
enhanced ALD from all of our Ti precursors gave stoichiometric
TiO2 at 50°C and above ͑Fig. 8b͒. The film from ͓Ti͑OiPr͒ ͔ at
4
25°C was over-stoichiometric, which, as with the ͓Al͑CH3͒3͔/water
process, is the result of the incorporation of hydroxyl groups in the
film, evident by the 8 atom % hydrogen present ͑Fig. 8c͒ and the
lower mass density of the film ͑Fig. 8d͒. At other temperatures, the
carbon and hydrogen concentrations were below their detection lim-
its for RBS and ERD ͑1 and 5 atom %, respectively͒ where
͓Ti͑OiPr͒ ͔ was the precursor. The stoichiometry of the films depos-
4
ited by ͓Ti͑CpMe͒͑OiPr͒ ͔ and ͓TiCpء͑OMe͒ ͔ were the same as
3
3
those obtained using ͓Ti͑OiPr͒ ͔ ͑and so the data overlap in Fig.
4
8b͒, and no carbon was detected at any temperature for the Cp-based
precursors. Hydrogen was detected in the films from
͓TiCpء͑OMe͒ ͔ ͑2–3 atom %͒, and similar quantities are possibly
3
present in the films from the other two precursors, as the detection
limit in those experiments was higher. As such, it is difficult to
directly compare the hydrogen content of our films with those of
Ritala et al.,35 as they reported a hydrogen content of ϳ0.3 atom %
15
͑measured using nuclear reaction analysis with a N2+ beam͒,
which is below the lowest detection limit ͑1 atom %͒ for ERD in our
experiments. The films grown from TiCl4 and water at 100°C were
also stoichiometric but contained 1.2 atom % chlorine, which can
lead to films with a high surface roughness, due to the aforemen-
tioned etching.41 In terms of mass density ͑Fig. 8d͒, the films depos-
ited by all precursors had densities over the range 3.6–4.2 g cm−3
,
which is comparable to the bulk density of anatase TiO2
͑3.9 g cm−3͒.33 The films from ͓Ti͑OiPr͒ ͔ had the most variation
4
in density owing to the larger experimental inaccuracy in the RBS
experiments. The low density at 25°C was a result of –OH incorpo-
ration as mentioned previously, whereas the higher density of
4.18 g cm−3 at 300°C is likely to be a result of the crystallization.
For the noncrystalline films below 200°C, the high density is a
feature of the plasma-enhanced ALD method.
The plasma-enhanced ALD processes TiO2 using ͓Ti͑OiPr͒ ͔,
4
͓Ti͑CpMe͒͑OiPr͒ ͔, and ͓TiCpء͑OMe͒ ͔ afford higher growths per
3
3
cycle compared with the ͓Ti͑OiPr͒ ͔ and water process. They were
4
preferable to the TiCl4/H2O process, in that the possibility of the
films containing chlorine is negligible. The films grown in this study
were consistently stoichiometric down to 50°C.
Tantalum oxide
.— Ta2O5 is another thoroughly studied ALD
material, particularly via thermal routes using halide-,42,43
alkoxide-,44 and amide-based27,45,46 precursors. However, films
grown at Ͻ150°C are not so commonly known. We recently re-
ported the low temperature plasma-enhanced ALD of Ta2O5 using
͓Ta͑NMe2͒5͔ on the ALD-I reactor,27 which gave a growth of
ϳ0.08 nm/cycle over the temperature range 100–225°C ͑Fig. 9͒,
which is comparable to the TaCl5/water process above 100°C.42 All
films were stoichiometric Ta2O5 with a ͓O͔/͓Ta͔ ratio of 2.5 and
Figure 8. ͑Color online͒ Compositional details of the TiO2 films deposited
by plasma-enhanced ALD. ͑a͒ The Ti atoms deposited per cycle, ͑b͒ the
͓O͔/͓Ti͔ ratio ͑the ratio for stoichiometric TiO2 being represented by a
dashed line͒, ͑c͒ the hydrogen content, and ͑d͒ the mass density of the films.
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P72
Journal of The Electrochemical Society, 157 ͑7͒ P66-P74 ͑2010͒
Figure 9. ͑Color online͒ A comparison of growths per cycle for the ALD of
Ta2O5 from ͓Ta͑NMe2͒ ͔. Shown here are the thermal process with water
5
and the plasma-enhanced process, as reported by Maeng et al.45,46 and our
plasma-enhanced processes on ALD-I27 and FlexAL. The TaCl5/H2O pro-
cess is also shown for comparison.42 Lines serve as a guide to the eye.
contained hydrogen ͑Fig. 10c͒. No nitrogen or carbon was detected.
The mass density varied from 7.8 g cm−3 at 100°C to 8.1 g cm−3
at 225°C, which is comparable to the bulk density value of Ta2O5
͑8.24 g cm−3͒.33 Maeng et al. previously carried out a comparison
of the thermal and plasma-enhanced ALD of Ta2O5 from
͓Ta͑NMe2͒5͔ using water and an oxygen plasma, respectively.45,46
The
thermal
process
afforded
comparable
growths,
ϳ0.085 nm/cycle, to those obtained in our plasma-enhanced pro-
cess at 200–300°C ͑Fig. 9͒. The growths per cycle for the thermal
process increased at both the higher and lower substrate tempera-
tures due to precursor decomposition ͑CVD͒ and condensation of
water, respectively. The plasma-enhanced ALD of Ta2O5 using
͓Ta͑NMe2͒5͔ reported by Maeng and co-workers gave much higher
growths per cycle of ϳ0.12 nm. These films had a ͓O͔/͓Ta͔ ratio of
2.6, and no nitrogen or carbon incorporation was reported ͑the hy-
drogen content was not commented upon͒. The mass density re-
ported, ϳ7.75 g cm−3 ͑94% of the bulk value of Ta2O5 although
the precise substrate temperatures were not reported͒, was slightly
lower than our previously reported values,27 but this difference can-
not be the reason for such a significant difference in growth. It may
be a result of the use of different ALD equipment, where the design
of the reactor may affect the way in which precursors are introduced
to the substrate. With the exception of our results, plasma-enhanced
ALD using ͓Ta͑NMe2͒5͔ afforded higher growths per cycle than the
TaCl5/water process, although the ͓Ta͑NMe2͒5͔/water process gave
similar growths per cycle.
To investigate the use of different reactors, the ͓Ta͑NMe2͒5͔
plasma-enhanced ALD process was transferred to from the ALD-I to
the FlexAL reactor, extending the investigation down to 25°C. The
growths per cycle obtained bore more resemblance to the results of
Maeng et al. than those obtained using ALD-I. The increase in
growth per cycle at temperatures below 100°C was significant,
which was again a result of the low mass density of the film as the
number of Ta atoms deposited did not change dramatically over the
temperature range ͑Fig. 10a and d͒. At 250°C, there was also an
increase in growth per cycle, due to the decomposition of the
͓Ta͑NMe2͒5͔ precursor. This was consistent with the thermogravi-
metric analysis of the precursor in the literature,47 which showed the
onset of decomposition at around 200°C. The effect of the CVD is
clearly shown by the almost twofold increase in the Ta deposited at
250°C, and to a much lesser extent at 200°C.
Figure 10. ͑Color online͒ Compositional data for the Ta2O5 films deposited
on the FlexAL and ALD-I reactors. ͑a͒ Ta atoms deposited per cycle ͑lines
serve as a guide to the eye͒, ͑b͒ ͓O͔/͓Ta͔ ratio, the ratio of stoichiometric
Ta2O5 is represented by a dashed line, ͑c͒ the hydrogen concentration, and
͑d͒ the mass densities of the films.
Despite the similarity in the numbers of atoms being deposited
per cycle between the ALD-I and FlexAL reactors, the film compo-
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Journal of The Electrochemical Society, 157 ͑7͒ P66-P74 ͑2010͒
P73
sitions were different ͑Fig. 10b-d͒. The films deposited on the
ALD-I reactor had a density close to that of the bulk, were stoichi-
ometric Ta2O5, and contained very low concentrations of hydrogen.
At 50 and 100°C on the FlexAL reactor, the films were over-
stoichiometric with an excess of oxygen ͑Fig. 10b͒. The films also
contained signifiant concentrations of hydrogen ͑up to 35 atom %͒,
which are very high when compared with the Al2O3, TiO2, and other
Ta2O5 processes. However, in common with these processes, the
hydrogen concentration decreased with increasing substrate tem-
perature. It is likely that there is a significant presence of OH groups
in the film; therefore, it might just be coincidence that the film at
25°C was stoichiometric, as there was a significant presence of hy-
drogen and oxygen in that film. Carbon and nitrogen were not de-
tected at any temperature ͑RBS detection limit = 5 atom % in each
case͒. There was a positive correlation between substrate tempera-
ture and film density ͑Fig. 10d͒, but the densities were much lower
than expected for a good ALD film, less than half the bulk value
below 50°C. This does not compare with the 94% of the bulk value
reported by Maeng et al.,46 despite similar growths per cycle, prob-
ably due to the excess of hydroxyl groups. Why the film densities
were low and hydroxyl groups were present in such high quantities
and why the mass densities were so low, is still unclear. The differ-
ences observed between the two reactors could be a result of the
composition of the plasmas used, which interact with the NMe2
ligands in different ways. However, such a difference in growth per
cycle and film density has not been observed for other processes. A
possible reason is that the ion flux and ion energies in the ALD-I
system are generally higher than in the FlexAL reactor. The plasma
composition in all reactors and the effects on film growth are cur-
rently under investigation.
of atoms deposited per cycle aids in the clarification of where the
boundaries of the temperature window lie as it is not affected by
variations in film density.
Acknowledgments
The research leading to these results has received funding from
the European Community’s Seventh Framework Programme ͑FP7/
2007-2013͒ under grant agreement number CP-FP213996-1. The au-
thors thank L. R. J. G. van den Elzen, J. C. Goverde, and D. Hooge-
land for their assistance in the depositions and J. J. A. Zeebregts, M.
J. F. van de Sande, and C. A. A. van Helvoirt for their technical
assistance, support, and advice. SAFC Hitech Ltd. and Air Liquide
are also thanked for their donations of ͓Ti͑CpMe͒͑OiPr͒ ͔ and
3
͓TiCpء͑OMe͒ ͔, respectively.
3
Eindhoven University of Technology assisted in meeting the publication
costs of this article.
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4
3
͓TiCpء͑OMe͒ ͔ precursors gave higher growths per cycle than the
3
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4
3
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