Surface chemistry of plasma-assisted atomic layer deposition of Al 2O3 studied by infrared spectroscopy

Article abstract of DOI:10.1063/1.2940598

The surface groups created during plasma-assisted atomic layer deposition (ALD) of Al₂O₃ were studied by infrared spectroscopy. For temperatures in the range of 25-150 °C,-CH₃ and -OH were unveiled as dominant surface grou

Full text of DOI:10.1063/1.2940598

Surface chemistry of plasma-assisted atomic layer deposition of Al 2 O 3 studied by
infrared spectroscopy
E. Langereis, J. Keijmel, M. C. M. van de Sanden, and W. M. M. Kessels
Citation: Applied Physics Letters 92, 231904 (2008); doi: 10.1063/1.2940598
View online: http://dx.doi.org/10.1063/1.2940598
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APPLIED PHYSICS LETTERS 92, 231904 ͑2008͒
Surface chemistry of plasma-assisted atomic layer deposition of Al2O3
studied by infrared spectroscopy
E. Langereis,a͒ J. Keijmel, M. C. M. van de Sanden, and W. M. M. Kesselsb͒
Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven,
The Netherlands
͑Received 18 April 2008; accepted 17 May 2008; published online 10 June 2008͒
The surface groups created during plasma-assisted atomic layer deposition ͑ALD͒ of Al2O3 were
studied by infrared spectroscopy. For temperatures in the range of 25–150 °C, –CH3 and –OH were
unveiled as dominant surface groups after the Al͑CH3͒3 precursor and O2 plasma half-cycles,
respectively. At lower temperatures more –OH and C-related impurities were found to be
incorporated in the Al2O3 film, but the impurity level could be reduced by prolonging the plasma
exposure. The results demonstrate that –OH surface groups rule the surface chemistry of the Al2O3
process and likely that of plasma-assisted ALD of metal oxides from organometallic precursors in
general. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2940598͔
Atomic layer deposition ͑ALD͒ is the method of choice
for the deposition of ultrathin and conformal high-k metal
oxide films as required in next-generation memory and tran-
sistor devices. To extend the applications of ALD, processes
using alternative oxidant sources, such as O3 and O2 plasma,
are actively researched. Using these oxidant sources, addi-
tional reactivity is supplied to the surface chemistry and this
potentially allows for deposition at lower temperatures
͑Ͻ150 °C͒ without compromising film quality.1–3
Ͼ100 °C, the growth per cycle of Al2O3 monotonically de-
creases with increasing deposition temperature for all three
oxidant sources and the differences are relatively small. Only
below 100 °C, the growth per cycle significantly differs
which has been attributed to difficulties to achieve saturation
when dosing H2O.9,10 For the H2O-based process, the de-
crease in growth per cycle with increasing temperature has
been related to the thermal stability of the –OH surface
groups which are involved in the Al͑CH3͒3 chemisorption
reactions.5,10 The O2 plasma-based process shows a similar
decrease but for the complete temperature range
͑25–400 °C͒. This resemblance might indicate some simi-
larities between the surface groups involved in plasma-
assisted and thermal ALD of Al2O3. To better understand the
dependence of the growth per cycle on the deposition tem-
perature, however, insight into the surface chemistry of
plasma-assisted ALD of Al2O3 is necessary.
The primary objective of this letter is to elucidate the
surface chemistry of plasma-assisted ALD of Al2O3 through
detection of the surface groups generated in the half-cycles
by means of transmission infrared spectroscopy measure-
ments. The main result of this study is the observation that
–CH3 and –OH surface groups are predominantly formed
after the Al͑CH3͒3 and O2 plasma half-cycles, respectively.
In order to fully exploit the benefits of the ALD tech-
nique, a fundamental understanding of the underlying sur-
face chemistry is of vital importance. To this end the ALD
process of Al2O3 has been studied in great detail, since it
shares generic features to equivalent metal oxide ALD pro-
cesses. The conventional, thermal ALD process of Al2O3 us-
ing Al͑CH3͒3 precursor and H2O was found to be ruled by
the formation of –CH3 and –OH surface groups after the
precursor and oxidant half-cycles, respectively, with the for-
mation of volatile CH4 in both half-reactions.4–6 In the
O3-based ALD process, the formation of –CH3 surface
groups, and volatile CH4 after Al͑CH3͒3 adsorption were
found to be similar to the H2O-based process. During the O3
half-cycle, however, the formation of CH4 and C2H4 have
been reported1,2 and it is still debated whether –OH or for-
mate ͓–O͑–O͒CH͔ groups are the dominant surface species
produced by the O3 reactions.2,7 In our previous work on
plasma-assisted ALD of Al2O3, we observed the formation of
CH4 after Al͑CH3͒3 adsorption, while mainly CO, CO2, and
H2O were formed in the O2 plasma half-cycle through com-
bustionlike surface reactions.3,8 The surface chemistry during
this plasma-assisted ALD process could not be fully resolved
yet, because no measurements on the surface groups were
available.
Despite the different oxidant sources employed, it is
intriguing to note that the dependence of growth per cycle
on the deposition temperature is quite similar for the differ-
ent oxidant sources for ALD of Al2O3, as shown in Fig. 1
for H2O,2,9–13 O3,2,13 and O2 plasma.9 For temperatures
FIG. 1. ͑Color online͒ Literature data on the growth per cycle of Al2O3 as a
function of deposition temperature for ALD processes using Al͑CH3͒ as
3
precursor and H2O, O3, or O2 plasma as oxidant source. The data presented
were corrected for differences in mass density and represent the growth per
cycle of a film with a mass density of 3.0 g cm−3 as typically reported for
ALD Al2O3 films.
a͒
Electronic mail: e.langereis@tue.nl.
Author to whom correspondence should be addressed. Electronic mail:
w.m.m.kessels@tue.nl.
b͒
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0003-6951/2008/92͑23͒/231904/3/$23.00 92, 231904-1 © 2008 American Institute of Physics
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231904-2
Langereis et al.
Appl. Phys. Lett. 92, 231904 ͑2008͒
This observation implies that –OH surface groups rule the
surface chemistry during plasma-assisted ALD of Al2O3
similar to the H2O-based process. Also the influence of the
deposition temperature ͑25–150 °C͒, the incorporation of
impurities, and a direct comparison with the thermal ALD
process at 150 °C are addressed. The results will facilitate
the design of improved low temperature ͑Ͻ150 °C͒ ALD
processes and it is expected that they have generic implica-
tions for plasma-assisted ALD processes of other metal oxide
films.
Plasma-assisted and thermal ALD processes of Al2O3
were studied in a wall-heated reactor equipped with an
O2-operated inductively coupled plasma source and bubblers
with Al͑CH3͒3 ͑99.9999%, Akzo Nobel͒ and H2O ͑Ultra-
pur®, Ͼ99.9999%, Merck͒. The cycles consisted typically of
25 ms of Al͑CH3͒3 dosing followed by 2 s O2 plasma expo-
sure or 2ϫ240 ms H2O dosing in the plasma-assisted and
thermal ALD process, respectively. The Al2O3 films were
deposited onto KBr flats that also served as infrared view-
ports in the reactor. It was verified that the growth per cycle
and Al2O3 film quality were similar to results obtained on
substrates positioned on the substrate holder.9,14 In between
the ALD half-reactions, transmission infrared spectra ͑2
ϫ256 scans͒ were collected by an infrared interferometer
͑Bruker, Tensor 27͒ with an external liquid-N2 cooled
HgCdTe detector ͑Bruker, D313͒. The absorbance spectra
were averaged over 40 ALD cycles to reduce the noise level
to ϳ5ϫ10−6.
The surface species created during the plasma-assisted
ALD half-cycles of Al2O3 were studied at a deposition tem-
perature of 150 °C and were directly compared to the results
of the thermal ALD process carried out at the same tempera-
ture. From qualitative comparison of the differential vibra-
tional spectra shown in Fig. 2͑a͒, it is clear that the same
types of surface species are created in both ALD processes.
After the Al͑CH3͒3 precursor adsorption, –CH3 surface
groups appear as observed in stretching ͑asymmetric
ϳ2937 cm−1, symmetric ϳ2896 cm−1, and bending overtone
ϳ2831 cm−1͒, deformation ͑ϳ1208 cm−1͒, and rocking
͑ϳ750 cm−1͒ modes, while –OH surface groups disappear as
observed in the broad stretching ͑3750–2770 cm−1͒
mode.4,5,15 This broad –OH stretching feature originates from
various types of –OH groups on the Al2O3 surface, ranging
from unassociated –OH ͑3800–3700 cm−1͒ to associated
–OH ͑3700–2700 cm−1͒ modes.5,16–18 For both ALD pro-
cesses, the differential spectra clearly show that –OH surface
groups are created after the reaction of the O2 plasma and the
H2O oxidant with the –CH3 surface groups.
FIG. 2. ͑Color online͒ Differential infrared spectra after the Al͑CH3͒ pre-
3
cursor and oxidation half-cycles showing appearance and disappearance
͑with respect to the baseline͒ of surface groups in the two ALD half-
reactions. ͑a͒ Comparison between the surface groups involved in plasma-
assisted and thermal ALD of Al2O3 at a deposition temperature of 150 °C.
͑b͒ Surface groups involved in plasma-assisted ALD at deposition tempera-
tures of 25, 100, and 150 °C. The relevant peak positions of the –CH3
groups, –OH groups, and CO-containing impurities ͑with CvO or COO
bonds͒ are indicated.
addition, an unassigned broad feature around ϳ900 cm−1 is
observed for the films deposited by plasma-assisted ALD.
The feature is most-likely not related to the broad Al–O pho-
non mode ͑ϳ954 cm−1 in Ref. 15͒, since its magnitude in-
creased with increasing temperature opposite to the trend ob-
served for the growth per cycle of the Al2O3 films ͑Fig. 1͒.
Insight into the type and relative amount of impurities
incorporated into the Al2O3 films can be deduced from a
differential spectrum obtained after a full ALD cycle ͑i.e.,
basically adding the differential spectra for the two half-
cycles͒. From such differential spectra, it is concluded that
–OH groups were incorporated in the Al2O3 bulk with the
amount of –OH incorporated increasing when going to lower
deposition temperatures. This observation is in line with pre-
vious reports,5,9,14 as well as with the compositional data of
Al2O3 films deposited in the temperature range studied, i.e.,
x=͓O͔/͓Al͔=1.6 with ϳ4 at. % H at 150 °C, x=1.7 with
ϳ7 at. % H at 100 °C, and x=2.1 with ϳ15 at. % H at
25 °C ͑all using 2 s O2 plasma exposure͒.9 From the spectra,
no evidence is found for the incorporation of –CHx groups
into the bulk Al2O3 film at the studied temperatures. Inter-
estingly for the film deposited at 25 °C, carbon-related im-
purities appeared ͑ϳ1662 cm−1͒ after the O2 plasma step,
which changed into a broad feature ͑1600–1450 cm−1͒ after
the Al͑CH3͒3 dosing. The peaks observed can be attributed
to impurities containing CvO or COO bonds indicating in-
complete removal of the carbon from the surface during the
2 s O2 plasma step at 25 °C. The incorporation of these sur-
face features into the Al2O3 bulk after Al͑CH3͒3 precursor
For the O2 plasma-based process, the influence of depo-
sition temperature on the surface groups was studied for
films deposited at 25, 100, and 150 °C, as shown in Fig.
2͑b͒. From the similarities in the differential vibrational
spectra, it is concluded that the surface chemistry at lower
temperatures also proceeds predominantly via the formation
of –CH3 and –OH surface species. At lower temperatures,
the amount of –OH surface groups created by the O2 plasma
strongly increased and a broadening of the –OH stretching
modes was observed. The broadening at lower deposition
temperatures can be attributed to the dipole interaction ͑in
particular, hydrogen bonding͒ between the densely packed
–OH surface groups. Moreover, the presence of physisorbed
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H2O on the Al2O3 surface can also be of influence.19,20 In
adsorption is expected to account for the shift and broaden-
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231904-3
Langereis et al.
Appl. Phys. Lett. 92, 231904 ͑2008͒
ing of the peak. The appearance of these CvO/COO bonds
is most-likely characteristic for the strong oxidation power of
the O2 plasma and these bonds can be interpreted as inter-
mediate reaction states in the combustionlike reaction from
the –CH3 covered surface to the –OH covered surface after
the O2 plasma exposure. This hypothesis is corroborated by
the fact that the magnitude of the C-related absorbance could
be reduced by using a longer O2 plasma step in the ALD
cycle. Furthermore, in our previous work, compositional data
on the Al2O3 films deposited at 25 °C revealed that the
amount of O, C, and H impurities was substantially reduced
when using 4 s ͑instead of 2 s͒ of O2 plasma exposure.9
From the quantitative amount of surface groups ob-
served, in principle, conclusions can be drawn regarding the
ALD surface chemistry at the different temperatures. For ex-
ample, the increase in –OH surface groups involved in
plasma-assisted ALD when going to lower temperatures can
be related to the increase in growth per cycle. However, care
must be taken in such interpretation, since residual H2O ͑ei-
ther being remnants of the excessive H2O dosing during ther-
mal ALD or of the H2O reaction byproducts created during
plasma-assisted ALD3͒ is difficult to completely purge out of
the reactor, especially at the low temperatures employed. Re-
sidual H2O can lead to –CH3 consumption and –OH forma-
tion during the purge time and ͑relatively long͒ infrared mea-
surement time following the Al͑CH3͒3 precursor half-cycle.
Evidence for this effect was found by comparing infrared
measurements consecutively acquired during the purge time.
Therefore, differences in the experiment with respect to con-
ditions and timing can partly account for the different
amount of –CH3 groups observed in thermal and plasma-
assisted ALD at 150 °C ͓Fig. 2͑a͔͒.
Notwithstanding this influence of residual H2O and other
effects related to the experiment, the increase in –OH surface
groups when going to lower temperatures is remarkable
compared to the virtually temperature independent amount of
–CH3 surface groups ͓Fig. 2͑b͔͒. When linked to the increase
in growth per cycle ͑Fig. 1͒, these observations support a
more pronounced bifunctional adsorption of the precursor
when going to lower temperatures, as was also reported for
thermal ALD.21 Upon bifunctional adsorption, the Al͑CH3͒3
molecule splits off two –CH3 ligands consuming two –OH
surface groups and, thereby, releasing two CH4 molecules.
Consequently, after bifunctional adsorption only one –CH3
surface group remains per adsorbed Al͑CH3͒3 compared to
two –CH3 surface groups remaining after monofunctional
adsorption; a process that becomes more important at higher
temperatures when the amount of –OH surface groups be-
comes relatively low.21 In this respect, the virtually constant
amount of –CH3 surface groups observed in the infrared
spectra in Fig. 2͑b͒ combined with the increase in growth per
cycle confirms that a larger fraction of the Al͑CH3͒3
molecules bifunctionally adsorbs when going to lower
temperatures.
the O2 plasma species. Therefore the following surface reac-
tions are proposed for plasma-assisted ALD of Al2O3:
ء
*
AlOH + Al͑CH3͒3͑g͒ → AlOAl͑CH3͒2 + CH4͑g͒, ͑1͒
ء
*
AlCH3 + 4O͑g͒ → AlOH + CO2͑g͒ + H2O͑g͒,
͑2͒
where the asterisks designate the surface species and only the
case of monofunctional adsorption is considered for simplic-
ity. Moreover, the amount of –OH groups involved in the
surface reactions increased for lower deposition temperatures
which can be related to the higher growth per cycle at these
temperatures. It was also demonstrated that the high reactiv-
ity delivered by the O2 plasma allows for film deposition at
temperatures down to room temperature, but under these
conditions a longer plasma exposure time was required to
complete the surface reactions and reduce the impurity con-
tent of the Al2O3 films. Tuning the plasma reactivity in the
ALD process is thus key in obtaining high quality films at
low deposition temperatures. Because combustionlike reac-
tions of organic surface ligands by O2 plasma species have
been reported for more plasma-assisted ALD processes,22 it
is expected that the surface chemistry discussed is generic
for plasma-assisted ALD processes of high-k metal oxides
using ͑similar͒ metal-organic precursors and O2 plasma.
The authors thank Dr. A. C. R. Pipino for fruitful discus-
sions. The Dutch Technology Foundation STW is acknowl-
edged for their financial support.
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