Thin Solid Films 370 (2000) 163±172
www.elsevier.com/locate/tsf
Characterization of titanium dioxide atomic layer growth from
titanium ethoxide and water
Jaan Aarik a,*, Aleks Aidla a, VaÈino Sammelselg b, Teet Uustare a, Mikko Ritala c, Markku LeskelaÈ c
È likooli Street, 50090 Tartu, Estonia
Institute of Materials Science, University of Tartu, 18 U
b
Institute of Physics, University of Tartu, 142 Riia Street, 51014 Tartu, Estonia
c
Department of Chemistry, University of Helsinki, P.O. Box 55 (A. I. Virtasen aukio 1) FIN 00014 University of Helsinki, Finland
a
Received 9 November 1999; received in revised form 10 February 2000; accepted 23 February 2000
Abstract
Atomic layer growth of titanium dioxide from titanium ethoxide and water was studied. Real-time quartz crystal microbalance measurements revealed that adsorption of titanium ethoxide is a self-limited process at substrate temperatures 100±2508C. A relatively small amount
of precursor ligands was released during titanium ethoxide adsorption while most of them was exchanged during the following water pulse.
At temperatures 100±1508C, incomplete reaction between surface intermediates and water hindered the ®lm growth. Nevertheless, the
deposition rate reached 0.06 nm per cycle at optimized precursor doses. At substrate temperatures above 2508C, the thermal decomposition of
titanium ethoxide markedly in¯uenced the growth process. The growth rate increased with the reactor temperature and titanium ethoxide
pulse time but it insigni®cantly depended on the titanium ethoxide pressure. Therefore reproducible deposition of thin ®lms with uniform
thickness was still possible at substrate temperatures up to 3508C. The ®lms grown at 100±1508C were amorphous while those grown at
1808C and higher substrate temperature, contained polycrystalline anatase. The refractive index of polycrystalline ®lms reached 2.5 at the
wavelength 580 nm. q 2000 Elsevier Science S.A. All rights reserved.
Keywords: Atomic layer deposition; Titanium dioxide; Growth mechanism; Surface roughness
1. Introduction
Titanium dioxide (TiO2) thin ®lms and dielectric structures containing TiO2 as a component have a number of
perspective applications in microelectronics and sensor
technology. In many applications, the thickness of these
®lms and structures as well as the thickness uniformity
should be very precisely controlled. Atomic layer deposition
(ALD), also known as atomic layer epitaxy [1] and molecular layering [2], is a method which allows this kind of
control with a submonolayer accuracy and enables one to
get uniform thickness even on pro®led substrates. This is
due to the cyclic self-controlled character of the ALD
process that results in a constant thickness increase in
each deposition cycle. As the thickness increase per cycle
is usually less than one monolayer of deposited substance
and, in the ideal case, it does not depend on modest variations of process parameters, the thickness control is convenient and accurate.
Several titanium compounds have been applied as precur* Corresponding author. Tel.: 1 372-7-375-877; fax: 1 372-7-375-540.
sors in atomic layer deposition of TiO2 thin ®lms [2±13].
Titanium chloride (TiCl4) is the precursor most frequently
used together with water [2±9] or hydrogen peroxide [10]
for ALD growth at substrate temperatures (Ts) ranging from
27 to 6008C. It has been shown that this precursor enables
one to grow high-quality thin ®lms. For instance refractive
index as high as 2.6 [3,8] and optical losses below 100 cm 21
[5] have been obtained for the ®lms grown by ALD from
TiCl4 and water. Moreover, using TiCl4 as the titanium
precursor, thin ®lms with different crystal structure and
physical properties can be obtained [3±6,8]. Unfortunately,
TiCl4 is a highly corrosive material. It has to be handled with
care and special measures should be applied for protection
of vacuum and gas lines. Also, signi®cant chlorine contamination has been observed in the ®lms grown at substrate
temperatures close to 1008C [5]. In addition, HCl released in
the growth process may, because of its high reactivity, readsorb and cause appearance of thickness pro®les in the gas
¯ow direction [14]. Therefore alternative titanium sources
for TiO2 atomic layer deposition are of practical interest.
Titanium isopropoxide (Ti(OCH(CH3)2)4) [11,12] and
titanium ethoxide (Ti(OCH2CH3)4) [13] combined with
0040-6090/00/$ - see front matter q 2000 Elsevier Science S.A. All rights reserved.
PII: S 0040-609 0(00)00911-1
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J. Aarik et al. / Thin Solid Films 370 (2000) 163±172
water have also been used as precursors for atomic layer
deposition of TiO2. The ALD growth has been performed in
the substrate temperature ranges of 50±350 and 200±4008C
for titanium isopropoxide [11,12] and titanium ethoxide
[13], respectively. Although thin ®lms of uniform thickness
have been obtained from these precursors, the growth rate
signi®cantly depended on the substrate temperature as well
as on the precursor doses [12,13]. A disadvantage of titanium ethoxide has been a rather low growth rate (0.03±0.04
nm/cycle) at substrate temperatures below 3008C. For
instance, at substrate temperatures of 100±1508C, TiO2
growth rates exceeding 0.07 nm per cycle [5] have been
obtained using titanium chloride and water as precursors.
Different growth rate could be due to the larger size of the
titanium ethoxide molecule compared with that of titanium
chloride [13]. However, some other reasons, such as incomplete exchange reactions during precursor pulses and low
density of activated adsorption sites, can also be proposed to
explain slow thin ®lm growth. Therefore kinetic studies are
inevitable to understand the reaction mechanisms and
processes limiting the deposition rate.
In the present study we investigated the kinetics of atomic
layer deposition of titanium dioxide thin ®lms from titanium
ethoxide and water by using the quartz crystal microbalance
(QCM) method [5,6,15]. Besides real-time measurement of
the mass increase per complete ALD cycle, the method
enabled us to record the deposition kinetics within a single
ALD cycle. In this way we obtained valuable information
providing a possibility to draw some conclusions about
surface reactions and optimize the conditions for thin ®lm
growth.
2. Experimental
A hot-wall low-pressure (250 Pa) ALD reactor equipped
with a quartz crystal microbalance (QCM) [15] was used for
growing the thin ®lms as well as for the real-time measurements. Titanium dioxide thin ®lms were deposited from
alternating titanium ethoxide and water vapor pulses led
into the reaction zone with N2 carrier gas of 99.999% purity.
In order to remove the gaseous reaction products and to
avoid overlapping of precursor pulses, the reactor was
purged with pure carrier gas after each precursor pulse.
The vapor pressure of titanium ethoxide was determined
by the temperature of the titanium ethoxide cell varied
from 48±878C during the QCM measurements and kept at
788C while growing the thin ®lms for ex situ studies. Water
vapor was generated in a container kept at 208C whereas the
vapor ¯ow into the reactor was controlled with a calibrated
needle valve.
In the real-time QCM experiments, the thin ®lm was
grown directly on the mass sensor. However, before starting
the real measurements a 2±3 nm thick TiO2 buffer-layer was
always grown on the sensor. This allowed us to obtain data
that described the reactions on the surface of TiO2 and did
not depend on the characteristics of the bare mass sensor
surface. In order to reduce the experimental error and avoid
misleading effects of transient processes that usually
appeared during the ®rst ALD cycles applied after changing
deposition parameters, the mass sensor signal was recorded
for 3±5 subsequent cycles. Then the data corresponding to
the ®rst cycle were neglected and the average response
describing the mass changes during one ALD cycle was
calculated from the rest of data.
The thin ®lms for post-growth characterization were
deposited on fused silica and (100)-oriented silicon
substrates. During the thin ®lm growth the substrate surface
was parallel to the gas ¯ow direction. The substrate
temperature ranged from 100 to 3508C.
The composition of the ®lms was determined with the
electron probe microanalysis (EPMA) [16] and Auger electron spectroscopy (AES) methods. X-ray diffraction (XRD)
and re¯ection high energy electron diffraction (RHEED)
methods were used to study the phase composition and
crystallinity. The optical thickness, refractive index, and
extinction coef®cient were determined from transmission
spectra using the method proposed by Swanepoel [17].
Atomic force microscopy (AFM) was used to investigate
the surface morphology.
3. Results and discussion
3.1. Deposition kinetics
In order to characterize adsorption of precursors, the time
dependence of the thin ®lm mass was ®rst studied at relatively long cycle times. The curves recorded at the substrate
temperatures of 215 and 3508C during a single ALD cycle
are depicted in Fig. 1a. In both cases the titanium ethoxide
pulse, ®rst purge time, H2O pulse and second purge time
were 20, 10, 10 and 10 s, respectively. As can be seen in Fig.
1a, the titanium ethoxide pulse causes a signi®cant increase
in the deposit mass. However, the mass starts to increase
after a delay only. This kind of behavior indicates that a
well-de®ned adsorption wave is formed in the reactor. As
the adsorption wave propagates with a ®nite rate from the
reactor inlet towards the mass sensor, the response does not
appear immediately after switching on the reactant pulse.
The delay time depends on the adsorption capacity of the
reactor walls, on the concentration of precursor molecules in
the gas phase and on the ¯ow rate of the carrier gas [18].
The adsorption of titanium ethoxide rather well saturates
at the substrate temperature of 2158C (Fig. 1a). Thus, a
surface intermediate layer which does not adsorb additional
titanium ethoxide is formed at this temperature. At 3508C,
however, the behavior of the mass sensor signal is somewhat different. After a steep increase observed in the beginning of the titanium ethoxide pulse, the ®lm mass continues
to rise with a nearly constant rate. As the rate of the unsaturated deposition increased with the substrate temperature,
J. Aarik et al. / Thin Solid Films 370 (2000) 163±172
Fig. 1. Mass sensor signal as a function of time recorded (a) at substrate
temperatures of 350 and 2158C during a single ALD cycle and (b) at
substrate temperatures of 300 and 1508C during series of titanium ethoxide
pulses (the upper curves) and a series of ALD cycles (the lowest curve).
the most probable reason of that is decomposition of titanium ethoxide. Indeed, as the O/Ti ratio is as high as four in
a Ti(OCH2CH3)4 molecule, titanium dioxide can grow in the
decomposition process without supplying any other oxygen
165
precursor. The unsaturated adsorption could also be caused
by hydrolysis of surface intermediate species or gaseous
titanium ethoxide by water vapor that may reside in very
small amounts in the carrier gas. However, as will be
discussed in Section 3.2, the latter process unlikely affects
the growth in our experiments.
Fig. 1a demonstrates that a small but clearly observable
mass decrease occurs during the purge time used after the
titanium ethoxide pulse. Possible reasons for this effect are
desorption of surface intermediate species formed during
adsorption of titanium ethoxide and/or desorption of decomposition products of these surface intermediates. Even faster
decrease in the mass sensor signal appears when the water
pulse is switched on. Consequently a signi®cant amount of
reaction products desorbs from the surface in this reaction
step. Thus the surface intermediate species formed during
the titanium ethoxide adsorption contain ligands that are
removed and/or decomposed by water pulse. As can be
seen in Fig. 1a, during the water pulse the mass stabilizes
on the level, which is higher than that before starting the
ALD cycle. After switching off the H2O pulse the mass
sensor shows no remarkable changes. This fact con®rms
that the reaction products able to desorb from the surface
are completely removed, no signi®cant amount of excess
water adsorbs on the ®lm surface during the H2O pulse,
and the solid reaction product causing the mass increment
Dm0 is stable.
The behavior of the ®lm mass recorded during a series of
ALD cycles is depicted in Fig. 1b (the lowest curve). For
comparison, the effect of successive titanium ethoxide
pulses is also shown. One can see in the ®gure that the
adsorption capability of substrate surface rapidly decreases
if the titanium ethoxide pulses do not alternate with the H2O
pulses. At 1508C, for instance, no adsorption can be
recorded during the second titanium ethoxide pulse, already.
At the same time the surface exposed to H2O readily adsorbs
titanium ethoxide. No qualitative changes in the adsorption
of titanium ethoxide occurred even if the purge time used
after a H2O pulse was prolonged up to several hours. Therefore the steep mass increase that appears after switching on
the titanium ethoxide pulse, is a result of adsorption rather
than gas phase reactions which, in principle, may contribute
to the growth when the titanium ethoxide and water pulses
overlap with each other.
Obviously the effect of overlapping is sensitive to the
length of the purge times. It was established that the increase
in Dm0 was below 5 and 15% when the purge times following the titanium ethoxide and water pulses, respectively,
were reduced from 10 to 2 s. By contrast, Dm0 increased
by 40±60% when the purge was decreased from 2 to 0.1 s
whereas the most signi®cant increase in Dm0 appeared at the
purge times below 0.5 s. Therefore overlapping of the
precursor pulses obviously contributes to the ®lm growth
at short purge times but its effect is insigni®cant at the
purge times of about 2 s and longer.
The data illustrating the dependence of Dm0 on different
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J. Aarik et al. / Thin Solid Films 370 (2000) 163±172
Fig. 2. Mass changes (a) Dm0 and (b) Dm2 as functions of substrate temperature. Titanium ethoxide source temperature is 718C and H2O pressure is 6
Pa.
process parameters are presented in Fig. 2a. The ®gure
shows that the variations of Dm0 are within ^20% when
the substrate temperature rises from 100 to 2508C. Also,
the effect of titanium ethoxide pulse time and following
purge time is rather weak at these temperatures. At temperatures 200±2508C, however, a noticeable increase of Dm0
with the substrate temperature starts. Also a clearly observable in¯uence of the titanium ethoxide pulse time on Dm0
appears at these and especially at higher temperatures.
However, the in¯uence of the purge time is still smaller
than it could be expected taking into account the mass
decrease observed at 3508C during the purge time applied
after titanium ethoxide pulse (Fig. 1a). Therefore it is
reasonable to conclude that the mass decrease observed
during the purge time is caused by desorption of the titanium precursor ligands and/or decomposition products of
them, only, rather than by desorption of titanium-containing
surface intermediate species.
An interesting result is that neither the titanium ethoxide
pulse time nor the following purge time affect the mass
decrease Dm2 (Fig. 2b) even at the substrate temperatures
where the Dm0 values obviously depend on the titanium
ethoxide pulse time. It was checked at 3508C that neither
the H2O pulse time varied from 1 to 10 s in¯uenced the Dm2
value. Consequently, at this temperature, the surface abundance of titanium precursor ligands, which can be removed
during water pulse and/or preceding purge time, completely
saturates with increasing titanium ethoxide pulse time
although the amount of adsorbed titanium does not.
Fig. 2b shows that Dm2 as a function of the substrate
temperature has a maximum at about 2758C. Therefore at
this temperature, the largest amount of ligands is removed
during the H2O pulse. The main reason of the decrease in
this amount at higher temperatures is that due to thermal
decomposition, more ligands are released or decomposed
during titanium ethoxide adsorption, already. At temperatures
below 225±2508C the decrease of Dm2 can be explained by
exchange reactions which probably occur on the surface
during titanium ethoxide adsorption. It is generally known
that the oxide surfaces exposed to water vapor are terminated
with hydroxyl groups at suf®ciently low temperatures.
Provided that adsorbing titanium ethoxide reacts with surface
hydroxyl groups [13], some ligands can be removed in adsorption process. Thus, an increasing role of the exchange reaction
between surface hydroxyl groups and adsorbing titanium
ethoxide is one reason why the Dm2 value decreases at the
substrate temperatures below 2508C.
Another possible reason for smaller Dm2 values measured at
low temperatures, is that the water pulse does not remove all
precursor ligands from the ®lm surface. Indeed, Dm0 as a function of H2O pressure (Fig. 3a) and pulse time (Fig. 3b) does not
saturate at substrate temperature of 1008C, although it saturates at 215 and 3508C. Similarly, no saturation of Dm2 was
observed at T s ˆ 1008C. Instead, the Dm2/Dm0 ratio was
almost invariant. These facts show that the reactions between
surface intermediates and water vapor are not completed and
small Dm2 values measured at temperatures close to 1008C
may really be explained by incomplete removal of precursor
ligands. Low reactivity of H2O towards the surface intermediate species is not surprising. For instance, in the tantalum oxide
ALD process, the sticking coef®cient of water to the surface
treated with tantalum ethoxide is about an order of magnitude
smaller, than the sticking coef®cient of tantalum ethoxide to
the surface treated with water [19]. Of course, low reactivity of
H2O may, in principle, be compensated with higher pressure or
longer pulse times. Indeed, signi®cant increase in the growth
rate and obvious tendency towards saturation was observed at
1008C, as well, when the H2O pressure and pulse time exceeding 20 Pa and 5 s, respectively, were applied (Fig. 3). Unfortunately, longer purging is needed at higher H2O pressures in
order to evacuate the excess of H2O from the reactor. Increase
of the purge and pulse times, in turn, is usually not acceptable
because of increasing deposition time.
The data of QCM measurements demonstrate that at the
substrate temperature of 2158C, Dm0 as a function of the
J. Aarik et al. / Thin Solid Films 370 (2000) 163±172
167
growth rate, depends on the substrate temperature. Higher
source temperature is needed at T s ˆ 3508C where the
saturation is achieved at higher Dm0 values than at
T s ˆ 2158C (Fig. 4). This kind of behavior is related to
the processes causing the delay between switching on the
precursor pulse and the following mass sensor response
(Fig. 1). The delay increases with the decrease in concentration of precursor molecules in the carrier gas [18]. The
precursor concentration, in turn, decreases with decreasing
source temperature. Thus, at a certain source temperature,
the delay reaches the pulse length. Naturally, no saturation
can be obtained at this and lower source temperatures, any
more. The delay also increases with adsorption capacity of
the surface [18] i.e. with the increase of the Dm0 values at
which the saturation is obtained. For this reason it is not
surprising that at higher saturation levels of Dm0, higher
source temperatures are needed to keep the delay time
shorter than the pulse time.
3.2. Properties of thin ®lms
Fig. 3. Dm0 as a function of (a) H2O pressure and (b) H2O pulse time
recorded at substrate temperatures of 100, 215 and 3508C. Titanium ethoxide source temperature and pulse time are 788C and 2 s, respectively.
titanium ethoxide source temperature saturates when the
latter reaches 65±708C (Fig. 4). However, the saturation is
not complete when the substrate temperature is increased up
to 3508C. This result is not surprising because, as mentioned
above, a clearly observable unsaturated deposition occurs at
this substrate temperature (Fig. 1a). Nevertheless, the variation of Dm0 is about 25%, only, when the titanium ethoxide
source temperature rises from 65 to 858C (Fig. 4). An explanation for the weak dependence of Dm0 on the source
temperature is that the unsaturated growth, obviously
caused by decomposition of titanium ethoxide, is a
surface-controlled process [13]. Indeed, if decomposition
is slow compared with the adsorption of precursor molecules from the gas phase, then the surface becomes completely covered with precursor and in the ®rst approximation,
the decomposition rate is determined by the substrate
temperature rather than by the pressure of the precursor in
the gas phase.
An interesting feature shown in Fig. 4 is that the source
temperature, at which the saturation effects appear in the
The thin ®lms for post-growth studies were grown with a
set-up determined from the results of QCM measurements.
The pulse and purge times were chosen equal to 2 s. The
titanium ethoxide source temperature was set at 788C and
the partial pressure of water vapor was kept at 20 Pa in the
reactor during the water pulse. The set-up enabled us to
grow ®lms with rather uniform thickness independently of
the substrate temperature. Even at the temperature of 3508C,
the thickness gradient did not exceed 10% on the 60-mm
long substrates although no saturation of the titanium ethoxide adsorption with increasing pulse time was obtained at
this temperature. This fact enables us to conclude that the
unsaturated adsorption shown in Fig. 1a is due to the
surface-controlled decomposition of titanium ethoxide
rather than due to the gas phase reaction between titanium
Fig. 4. Dm0 as a function of Ti(OCH2CH3)4 source temperature recorded at
substrate temperatures of 215 and 3508C. Pulse times and purge times are
equal to 2 s.
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J. Aarik et al. / Thin Solid Films 370 (2000) 163±172
ethoxide and water vapor residues. Indeed, the amounts of
water that might reside in the carrier gas and/or desorb from
the reactor walls are rather small. Therefore they can unlikely support constant pressure along the reactor and to cause
uniform vapor-phase deposition on the whole substrate
under the conditions where the titanium ethoxide concentration is much higher than that of the water vapor.
The growth rate of the ®lms determined from thickness
measurements, is 0.06 and 0.08 nm per cycle at substrate
temperatures of 100 and 3508C, respectively. The increase
of the growth rate at 3508C is in a good agreement with the
dependence of Dm0 on the reactor temperature (Fig. 2a)
measured with QCM. However, the growth rate of 300 nm
thick ®lms grown at 2008C was 0.08 nm per cycle, i.e. about
30% higher than the value predicted from the growth rate
values measured at 100 and 3508C, and from the temperature dependence of Dm0 (Fig. 2a). This difference is
evidently connected with the surface roughness of the
®lms and will be discussed below.
XRD and RHEED studies showed that the ®lms grown at
substrate temperatures of 100±1508C were amorphous while
those deposited at 1808C and higher substrate temperatures
contained crystallites of the anatase structure (Fig. 5).
According to RHEED patterns (Fig. 5a) the crystallites
were randomly oriented in the ®lms grown at 1808C. Intense
background indicated a rather high amount of the amorphous phase in these ®lms. Expectedly, the background
intensity decreased with increasing growth temperature
Fig. 5. RHEED patterns of ®lms grown (a) by applying 1800 cycles at
substrate temperature of 1808C and (b) by applying 3600 cycles at substrate
temperature of 2758C.
and ®lm thickness. Moreover, the ®lms deposited at 2758C
(Fig. 5b) and higher substrate temperatures showed a rather
well developed preferential orientation. In these ®lms, the
(110) plane of crystallites was preferentially parallel to the
substrate surface. The sizes of crystallites depended on the
deposition temperature, ®lm thickness and orientation of
crystallites. In the ®lms of 200±300 nm thicknesses, the
average sizes of crystallites ranged from 20 to 80 nm.
A signi®cant amount of carbon (6±12 at.%) was recorded
with AES and EPMA methods in the ®lms grown at 1008C.
Also the O/Ti ratio was about 7% lower than that recorded
for the ®lms grown at 1508C and higher substrate temperatures. These results indicated that the exchange reactions
between water vapor and surface intermediate species
were not complete at so low temperature. However, the
carbon concentration rapidly decreased with increasing
deposition temperature. According to AES data the carbon
contamination did not exceed 0.5±0.7 at.% in the ®lms
grown at T s ˆ 150±2008C while the concentration of 0.7
at.% was measured with the EPMA method for a ®lm deposited at 2008C.
Optical measurements revealed that the refractive index of
the ®lms grown at 1008C reached 2.3 and the extinction coef®cient was as low as 1 £ 10 23 at the wavelength of 580 nm. The
®lms grown at 3508C possessed the refractive index values of
2.4±2.5 and the extinction coef®cient about 8 £ 10 23. With
decreasing ®lm thickness the value of refractive index somewhat increased and that of extinction coef®cient decreased.
This kind of thickness dependence of optical constants indicated that the surface roughness probably increased with the
®lm thickness and in¯uenced the material parameters determined from the transmission spectra.
The AFM measurements performed for 240±290 nm
thick ®lms revealed that the root-mean-square value of the
surface roughness was about 10 nm in the ®lms grown at
3508C (Fig. 6a). The surface roughness monotonously
increased up to 25 nm when the growth temperature
decreased to 2008C (Fig. 6b). With further decrease in the
growth temperature, however, the roughness abruptly
decreased and did not exceed 1 nm in the ®lms grown at
100±1508C (Fig. 6c). Therefore the surface roughness as a
function of substrate temperature (Fig. 7) had a maximum at
2008C, that is very close to the lowest temperature, at which
the crystallization appears. A very similar result has been
observed earlier in case of TiO2 thin ®lms grown from TiCl4
and H2O [5]. High surface roughness explains why the
growth rate determined for the deposition temperature of
2008C from the thickness of relatively thick ®lms is greater
than the growth rate estimated from the QCM data. On one
hand the ®lm density obviously decreases with increasing
surface roughness. For this reason the same mass increase
results in higher thickness increase. On the other hand the
effective surface area also increases with surface roughness.
Thus, rougher surface adsorbs greater amounts of precursors. As this effect increases with ®lm thickness, it has
insigni®cant effect on the QCM measurements performed
J. Aarik et al. / Thin Solid Films 370 (2000) 163±172
169
at the ®lm thickness of 2±20 nm. However, surface roughening accelerates the growth of much thicker ®lms.
Fig. 7. Surface roughness as a function of substrate temperature used for
thin ®lm growth. Film thicknesses range from 240 to 290 nm.
3.3. Growth mechanism
The data of composition studies demonstrate that TiO2
thin ®lms with rather low concentration of impurities can
be grown from titanium ethoxide and water at substrate
temperatures 150±3508C. This fact together with the data
of QCM measurements enables us to describe the growth
mechanism in more detail. For instance, using the molar
mass of TiO2 and the mass ratio Dm0/Dm1 (Fig. 8) where
Dm1 ˆ Dm0 1 Dm2 (Fig. 1a), one can estimate the molar
mass of surface intermediate species utilized in formation of
each TiO2 unit during the water pulse. Furthermore, as will
be demonstrated below, the Dm0/Dm1 ratio and its dependence on the process parameters permits to propose a model
of surface reactions describing the ALD-type growth of
TiO2 from titanium ethoxide and water at substrate temperatures where the self-limited growth is dominating.
Some possible mechanisms of surface exchange reactions
in ALD growth of TiO2 from titanium ethoxide and H2O
have been discussed in an earlier paper [13]. Provided that
the surface is at least partially hydroxyl-terminated, the
Fig. 6. AFM images of thin ®lms grown at substrate temperatures of (a) 350,
(b) 200 and (c) 1508C.
Fig. 8. Dm0/Dm1 as a function of substrate temperature. Titanium ethoxide
source temperature is 718C. Partial pressure of H2O is 6 Pa.
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J. Aarik et al. / Thin Solid Films 370 (2000) 163±172
chemisorption of titanium ethoxide can be described as
n…2OH† …s† 1 Ti…OCH2 CH3 †4 …g†
! …2O2†n Ti…OCH2 CH3 †42n …s† 1 nCH3 CH2 OH …g† …1†
where (g) and (s) denote gas phase and surface species,
respectively and n is the average number of surface hydroxyls reacting with a titanium ethoxide molecule in the
adsorption process. In the case of hydroxylated surface
0 , n # 4. The following H2O pulse should result in the
hydrolysis of the surface intermediates and recover the
concentration of the hydroxyl groups on the ®lm surface.
The reaction can be written as
…2O2†n Ti…OCH2 CH3 †42n …s† 1 …4 2 n†H2 O …g†
! …2O2†n Ti…OH†42n …s† 1 …4 2 n†CH3 CH2 OH …g†
…2†
when all the ligands are replaced by hydroxyl groups. One
can easily see from Eqs. (1) and (2) that the surface concentration of hydroxyl groups should continuously increase or
decrease when the number of them consumed during the
titanium ethoxide pulse (Eq. (1)) is not equal to those
formed during H2O pulse (Eq. (2)). Therefore in case of a
stable ALD growth when the growth rate does not depend
on the number of cycles applied, and no hydroxyl groups
remain in the ®lms, the value of n must be equal to two.
Using the molar masses of substances, one can ®nd from
Eqs. (1) and (2) that Dm0 =Dm1 ˆ 0:59 at n ˆ 2. This is
signi®cantly higher than the experimental values (Fig. 8)
measured for the substrate temperatures of 100±2508C, at
which no decomposition of titanium ethoxide appears. The
Dm0/Dm1 value can be lower than that calculated from Eqs.
(1) and (2), if some titanium-containing surface intermediates formed during the titanium ethoxide pulse desorb
before the water exposure. However, as mentioned above,
no dependence of the growth rate on the purge length was
observed. Therefore the contribution of this kind of desorption is insigni®cant.
Another explanation for lower Dm0/Dm1 values is that less
than two hydroxyls are involved in the chemisorption reaction described by Eq. (1). It is possible that titanium ethoxide is adsorbed on a completely dehydroxylated oxide
surface as well [13]
Ti…OCH2 CH3 †4 …g† ! Ti…OCH2 CH3 †4 …s†
…3†
In this case the following hydrolysis reaction should
again result in a completely hydroxyl-free surface [13]
Ti…OCH2 CH3 †4 …s† 1 2H2 O …g†
! TiO2 …s† 1 4CH3 CH2 OH …g†
…4†
One can ®nd that the Dm0/Dm1 value calculated from Eqs.
(3) and (4) is equal to 0.35. This value is rather close to but
somewhat lower, already, than the lowest experimental
points in Fig. 8. Therefore an exchange reaction should
still take place in the real growth process during titanium
ethoxide adsorption but less than two ligands, in average,
should be removed from every titanium ethoxide molecule
adsorbed. This kind of reaction can be described by Eq. (1)
where 0 , n , 2.
As mentioned above, a constant growth rate and deposition of the stoichiometric oxide can be achieved when the
amount of hydroxyl groups consumed during the titanium
ethoxide adsorption is equal to that formed during the water
pulse. In case of n , 2 this means that only a fraction of the
-OCH2CH3 ligands coordinated to titanium in the surface
intermediate layer (see Eq. (1)) are replaced by hydroxyl
groups in the hydrolysis process. The rest of the ligands are
substituted by oxygen bridging between adjacent titanium
atoms. As a result, the ®lm surface becomes partially hydroxyl-terminated at the beginning of the next ALD cycle. The
corresponding exchange reaction can be written as
…2O2†n Ti…OCH2 CH3 †42n …s† 1 2H2 O …g†
! …2O2†n TiO22n …OH†n …s† 1 …4 2 n†CH3 CH2 OH …g† …5†
where 0 # n # 2. The Dm0/Dm1 ratio calculated from Eqs.
(1) and (5) depends on n and can be expressed as
Dm0 =Dm1 ˆ 79:9=…227:9 2 46n†
…6†
Using the experimental data presented in Fig. 8 and Eq.
(6) one can estimate the average value of n for each
substrate temperature at which the reactions are self-limited.
Calculations show that n is equal to 0.6 at the substrate
temperatures of 225±2508C, at the highest temperatures
where the decomposition of titanium ethoxide is negligible.
At temperatures below 2258C, the decrease in Ts is accompanied with the increase in Dm0/Dm1 (Fig. 8). As a result, the
value of n calculated from Eq. (6), increases up to 1.2 and
1.5 at 200 and 1508C, respectively.
Unfortunately, at temperatures below 1508C, this kind of
estimation of the n value is not reliable any more because
the exchange reactions are not completed during the water
pulses and the ®lms contain a signi®cant amount of carbon.
Moreover, the decrease of the O/Ti ratio accompanying the
increase of carbon contamination in the ®lms grown at
1008C can not be explained by the deposition mechanisms
discussed. Thus, the reactions are obviously more complex
at so low temperature. Also, the model proposed does not
apply at the substrate temperatures where titanium ethoxide
decomposes and unsaturated deposition appears.
The observation that only 0.6±1.5 ligands, in average, are
released from each titanium ethoxide molecule during its
adsorption at substrate temperatures 150±2508C is in a
good agreement with in situ mass spectrometry studies of
the processes where metal ethoxides (Ti(OCH2CH3)4,
Ta(OCH2CH3)4 and Nb(OCH2CH3)4) and water have been
used as the precursors [20]. The data of these studies indicate that CH3CH2OH is almost exclusively released during
the water pulse.
The small amount of ligands released during adsorption
of metal precursor is one of the main reasons limiting the
J. Aarik et al. / Thin Solid Films 370 (2000) 163±172
growth rate. Indeed, if in average, 2.5±3.4 relatively large OCH2CH3 ligands are adsorbed together with each titanium
atom, then the concentration of titanium in the surface intermediate layer is determined by the size of the surface intermediate species rather than by the density of titanium sites
on the surface of TiO2.
Incomplete exchange of ligands in the low-temperature
hydrolysis process is another reason for low growth rate.
The ligands not removed during the water pulse stay on
the ®lm surface and diminish the number of adsorption
sites for titanium ethoxide in the next ALD cycle. On the
other hand, incomplete removal of the ligands from the ®lm
surface enhances the probability for the incorporation of the
ligand constituents into the ®lm. Therefore carbon contamination of the ®lms grown at the substrate temperature of
1008C con®rms that at this temperature, the water doses
used are really unable to remove all ligands from surface
intermediate layer.
Despite possible limitations, the growth rate obtained in
the present work is remarkably higher than that observed for
a similar ALD process earlier [13]. At the substrate temperatures 150±3508C, the growth rate well compares to that
achieved for the TiCl4/H2O and TiCl4/H2O2 ALD processes
in the same temperature range [3,5,9,10]. Furthermore,
according to our preliminary studies, the structure and optical properties of the ®lms investigated in this work compare
to those of the thin ®lms deposited by ALD from TiCl4 and
H2O at similar substrate temperatures [5,8].
Finally it should be noted that the changes in the thin ®lm
structure appearing at substrate temperatures of about 1808C
evidently in¯uence the surface reactions as well. In this
connection, one should take into account that the real-time
QCM studies have been performed at a rather small thickness of the ®lm deposited onto the mass sensor. Typically
the thickness was below 20 nm. Furthermore, the deposition
conditions were signi®cantly varied in the growth process. It
is clear that no highly developed crystal structure can be
obtained under these conditions. For this reason the
mechanisms discussed above adequately describe the
growth of amorphous TiO2 and/or the initial stage of nonepitaxial growth of anatase. The increase of thickness
usually results in changes in surface roughness and crystallite orientation of polycrystalline ®lms [5] and, in this way,
may affect adsorption processes. Therefore the reaction
mechanisms may somewhat change in the growth process.
Nonetheless, a rather good agreement between the results of
real-time and post-growth measurements is obtained.
According to rough estimations the variation in growth
rate caused by crystallization and surface roughening do
not exceed 30% even if the ®lm thickness reaches 300 nm.
could be reproducibly deposited from these precursors at the
substrate temperatures ranging from 100 to 3508C. The
adsorption of titanium ethoxide was self-limited at the
substrate temperatures up to 225±2508C. Although the decomposition of titanium ethoxide signi®cantly contributed to the
®lm growth at temperatures above 2508C, the effect of titanium ethoxide source temperature on the growth rate was
weak and ®lms of uniform thickness were obtained even
under these conditions.
QCM studies revealed that at the substrate temperatures
of 225±2508C, low contribution of exchange reactions
during titanium ethoxide adsorption was a reason for limited
growth rate. According to our estimations less than one
ligand, in average, was liberated from each titanium ethoxide molecule adsorbed at these temperatures. Correspondingly, each surface intermediate complex consumed for
formation of one TiO2 unit during the following water
pulse, contained more than three ligands. For this reason
each of surface intermediate species was able to cover
several titanium sites on the oxide surface and the growth
rate was evidently limited by the steric hindrance.
At substrate temperatures below 1508C, the growth rate
was affected by the insuf®cient reactivity of H2O towards
the surface intermediate species formed during the titanium
ethoxide pulse. No complete saturation of the growth rate
with increasing water dose was achieved and the ®lms
contained carbon residues. Nevertheless, the QCM and optical thickness measurements showed that at suf®cient H2O
doses, the growth rate was about 0.06 nm per cycle at the
substrate temperatures of 1008C and did not change remarkably when the temperature was raised to 2508C.
The ®lms grown at 100±1508C were amorphous while
those deposited at 1808C and higher substrate temperatures
were polycrystalline. Anatase was the only crystalline phase
observed in the ®lms with X-ray diffraction. The surface
roughness was highest in case of ®lms grown at the substrate
temperature of 2008C, i.e. very close to the minimum
temperature at which the crystallization occurred. The
refractive index of amorphous ®lms grown at 1008C was
2.3 while the extinction coef®cient equaled to 1 £ 10 23 at
the wavelength of 580 nm. The refractive index of the polycrystalline ®lms grown at 3508C reached 2.5 and the extinction coef®cient was about 8 £ 10 23.
Acknowledgements
The authors are grateful to Alma-Asta Kiisler and Jelena
Asari for technical assistance, and to Hugo MaÈndar for XRD
measurements. The work was supported by Finnish
National Technology Agency (TEKES) and Estonian
Science Foundation (Research Grants No. 1878 and 3871).
4. Conclusions
Real-time studies of the TiO2 growth from titanium ethoxide and water demonstrated that titanium dioxide thin ®lms
171
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