Prévia do material em texto

Evaluation and Comparison of Novel Precursors for Atomic Layer
Deposition of Nb2O5 Thin Films
Timothee Blanquart,†,* Jaakko Niinistö,† Mikko Heikkila,̈† Timo Sajavaara,‡ Kaupo Kukli,†
Esa Puukilainen,† Chongying Xu,§ William Hunks,§ Mikko Ritala,† and Markku Leskela ̈†
†Department of Chemistry, P.O. Box 55, University of Helsinki, FI-00014 Helsinki, Finland
‡Department of Physics, P.O. Box 35, University of Jyvas̈kyla,̈ FI-40014 Jyvas̈kyla,̈ Finland
§ATMI, 7 Commerce Drive, Danbury, Connecticut 06810, United States
ABSTRACT: Atomic layer deposition (ALD) of Nb2O5 thin films was studied using
three novel precursors, namely, tBuNNb(NEt2)3,
tBuNNb(NMeEt)3, and
tamylN
Nb(OtBu)3. These precursors are liquid at room temperature, present good volatility,
and are reactive toward both water and ozone as the oxygen sources. The deposition
temperature was varied from 150 to 375 °C. ALD-type saturative growth modes were
confirmed at 275 °C for tBuNNb(NEt2)3 and
tBuNNb(NMeEt)3 together with
both oxygen sources. Constant growth rate was observed between a temperature
regions of 150 and 325 °C. By contrast, amylNNb(OtBu)3 exhibited limited
thermal stability and thus a saturative growth mode was not achieved. All films were
amorphous in the as-deposited state and crystallized between 525−575 °C, re-
gardless of the applied precursor and oxygen source. Time-of-flight elastic recoil
detection analysis (TOF-ERDA) demonstrated the high purity of the films. Atomic
force microscopy (AFM) revealed that the films were smooth and uniform. The films exhibited promising dielectric
characteristics with permittivity values up to 60.
KEYWORDS: ALD, niobium oxide thin film, high-k, niobium imido-amido
■ INTRODUCTION
Nb2O5 is a wide band gap (3.6 eV) dielectric material with a
high index of refraction (n = 2.4) and permittivity (29 to 200
depending of the crystalline phase1). Due to its interesting op-
toelectronic properties, Nb2O5 has a wide range of applications
such as capacitor dielectrics2−5 and as catalyst-supporting oxide
materials.6−8 In particular, in dynamic random access memories
(DRAMs), where the dielectric should possess permittivity
exceeding 40, Nb2O5 has been explored as an alternative to the
more widely studied SrTiO3 and the rutile phase TiO2 doped
with Al. This is because the formation of high permittivity rutile
phase of TiO2 requires matching oxidized ruthenium electrode
material9 which can be a cost issue. In the case of the ternary
SrTiO3 films the dielectric properties are sensitive to the stoi-
chiometry which is difficult to control accurately.10
Nb2O5 thin films have mainly been deposited using physical
vapor deposition techniques such as electron beam evaporation
and magnetron sputtering11,12 and less frequently by pulsed laser
deposition.13 Nb2O5 thin films have also been deposited using
chemical techniques such as pyrolysis,14 sol−gel process,15 and
chemical vapor deposition (CVD).16
ALD is an advanced variant of the CVD method, where the
substrate surface is alternately exposed to the vaporized pre-
cursor fluxes.17 The reactant pulses are separated by purging
periods to eliminate gas-phase reactions and remove reaction
byproducts. In the case of metal oxide film growth, for instance,
the complete ALD growth cycle consists of metal precursor
exposure, the first purging period, oxygen precursor exposure,
and the second purging period. The main characteristic feature
of the ALD growth is the self-limited adsorption of the pre-
cursor on the substrate surface, providing inherent control of
the film thickness and excellent repeatability. The stepwise
growth via the self-limited adsorption processes makes ALD an
excellent method to deposit conformal and pinhole-free films
with superior uniformity.18−20
Tantalum has similar physical and chemical properties as
niobium and a large amount of work has been devoted to the
development of tantalum precursors for ALD of Ta2O5 and
TaNx. The most studied ALD precursors for Ta2O5 growth are
the halides21−23 and Ta(OEt)5.
24−26 In addition, alkylamide pre-
cursors such as Ta(NMe2)5 have also been applied.27 However,
these precursors have several drawbacks such as chlorine contam-
ination from TaCl5 and limited thermal stability of Ta(OEt)5
and the alkylamides. Thus, in the recent years research efforts
have been devoted to the development of new halogen-free
precursor with enhanced thermal stability, such as Ta(NtBu)-
(3,5-di-tert-butylpyrazolate)3,
28 Ta(NtBu)(iPrNC(Me)-
NiPr)2(NMe2),
29 and Ta(NEt)(NEt2)3,
30,31 although often
with a lowered growth rate. In addition, tBuNTa(NEt2)3
Received: September 8, 2011
Revised: December 2, 2011
Published: February 8, 2012
Article
pubs.acs.org/cm
© 2012 American Chemical Society 975 dx.doi.org/10.1021/cm2026812 | Chem. Mater. 2012, 24, 975−980
pubs.acs.org/cm
has been investigated in plasma-enhanced ALD of TaC,32
TaNx,
33 TaCN,33 and Ta2O5.
34
Different from Ta2O5, few successful Nb2O5 ALD processes
have been reported. The first reference in the literature was an
attempt to deposit Nb2O5 from NbCl5 and water.35 However,
this process failed, reportedly due to etching caused by the
formation of gaseous NbOCl3.
36 The only successful Nb2O5
ALD processes reported so far use Nb(OEt)5/water,
37 NbI5/
O2,
38and NbF5
39 with either water or a combination of water
and ozone. In these processes deposition temperatures were
varied, respectively, between 150 and 350 °C and 400 and
600 °C and limited to 225 °C in the case of the NbF5
processes. The ALD window of these processes, if present,
was narrow. In the Nb(OEt)5/water process, the maximum
growth temperature at which the self-limiting ALD-type growth
was confirmed was 230 °C with a low growth rate of 0.28 Å/
cycle. Furthermore, Nb(OEt)5 exists as a dimer with low
volatility and decomposes when held at the >100 °C bubbler
temperature needed for volatilization. With NbF5, the upper
limit of the deposition temperature was restricted to 225 °C,
because at higher temperatures NbF5 started to etch the
growing film leading to no growth or nonuniform thickness and
composition of the films. Moreover, no evidence of the self-
limiting growth in the NbI5/O2 and NbF5/H2O processes was
reported. According to our knowledge, no Nb2O5 ALD
processes using only ozone as the oxygen source have been
reported in the literature. The availability of an ozone process is
important, especially for applications where the slow desorption
of water would lead to problems in the form of excessively long
purge times, like with high aspect ratio structures and at low
deposition temperatures.
Clearly, novel Nb2O5 ALD processes are needed, especially
with niobium precursors having enhanced thermal stability
and reactivity toward both water and ozone. In the present
paper, we report a comparative study on the ALD growth of
Nb2O5 thin films using three novel precursors, namely, tBuN
Nb(NEt2)3 (tert-butylimido)tris(diethylamido)niobium),
tBuNNb(NMeEt)3 (tert-butylimido)tris(ethylmethylamido)-
niobium), and tamylNNb(OtBu)3 ((1,1-dimethylpro-
pylimido)tris(tert-butoxide)niobium) with either ozone or
water as the oxygen source. While new to ALD deposition of
Nb2O5, both tBuNNb(NEt2)3 and tBuNNb(NMeEt)3
have been investigated in plasma-enhanced ALD of NbNx for
gate electrode.40
■ EXPERIMENTAL SECTION
Nb2O5 Film Deposition. The precursors evaluated for the ALD
deposition of Nb2O5 thin films were tBuNNb(NEt2)3,
tBuN
Nb(NMeEt)3, and
tamylNNb(OtBu)3 (ATMI, USA). Thermogravi-
metric analysis of the precursor was performed on a Netzsch STA449C
instrument operating inside a nitrogen drybox. The measurements
were performed under argon in open pan Pt/Rh crucibles with 5−
10 mg of sample and a heating rate of 10 K/min. Nb2O5 thin films
were grown on 5 × 5 cm2 Si(100) substrates (Okmetic, Finland) in a
hot-wall flow type F-120 ALD reactor (ASM Microchemistry Ltd.).
For selected samples, TiN covered Si substrates were also used. The
operating pressure of the reactor was 5 to 10 mbarduring the de-
position. As the oxygen source, either O3, produced from >99.999%
O2 in an ozone generator (Wedeco Ozomatic modular 4 HC Lab
Ozone, ozone concentration 100 g/m3), or water was used. Nitrogen
(>99.999%) generated with Nitrox UHPN 3000−1 nitrogen generator
was used as a carrier and purge gas. The air and moisture sensitive
Nb2O5 precursors were handled in a glovebox and inserted into the
reactor in sealed boats. Precursors were evaporated from open boats
inside the reactor. Evaporation temperatures were 65, 55, and 60 °C
for tBuNNb(NEt2)3,
tBuNNb(NMeEt)3, and tamylNNb-
(OtBu)3, respectively. The growth rate as a function of deposition
temperature was studied in the temperature range of 150−375 °C
using a pulsing sequence of 0.7/1.0/1.0/1.5 s (metal precursor pulse/
purge/oxygen precursor pulse/purge). Self-limited growth of Nb2O5
was determined by studying the growth rate as a function of the metal
precursor pulse length, that is, by varying x in the pulsing sequence x/x
+ 0.5/1.0/1.5 s. Some of the films were annealed at 600 °C for 20 min
in a tube furnace under nitrogen.
Film Characterization. The thickness and crystallinity of the
Nb2O5 thin films were evaluated by X-ray reflectivity (XRR) and X-ray
diffraction (XRD) using a Panalytical X̀Pert Pro MPD X-ray diffracto-
meter, and MAUD software was used for Rietveld refinements. High-
temperature XRD (HTXRD) measurements were performed under
nitrogen, 99.999%, further purified with Entegris 35KF-I-4R inert gas
purifier, at temperatures ranging from 25 to 1175 °C using an Anton-
Paar HTK1200N oven. Film composition was analyzed by time-of-
flight elastic recoil detection analysis (TOF-ERDA) using the 6.8 MeV
35Cl3+ beam from 1.7 MV Pelletron accelerators. Surface morphology
was examined with a MultiMode V atomic force microscope (AFM)
equipped with NanoScope V controller (Veeco Instrument) operated
in the tapping mode. Samples were measured with a scanning fre-
quency of 0.5 Hz. Several wide scan images (5 × 5 μm2) were
recorded from different parts of the samples to check their uniformity.
Final images were measured from a scanning area of 2 × 2 μm2.
Roughness values were calculated as root mean squares (rms).
Electrical characterization of the films was carried out on Al/
Nb2O5/TiN/p-Si(100)/Al capacitors with top electrodes consisting of
100−110 nm thick Al layers e-beam evaporated through a shadow
mask. Capacitance−voltage (C−V) curves were recorded using a
HP4284A precision LCR-meter in a two-element series circuit mode.
The stair-sweep voltage step was 0.05 V. The period between the volt-
age steps was 0.5 s. The AC voltage applied to the capacitor was 0.05 V
while the frequency of the AC signal was 1 kHz. The current−voltage
(I−V) curves were measured with a Keithley 2400 Source Meter in the
stair sweep voltage mode, while the voltage step was 0.05 V and the
top electrodes were biased negatively in relation to the TiN/Si
substrate; that is, electrons were injected from the top electrode. All
measurements were performed at room temperature on samples in the
as-deposited state and after annealing in N2 at 600 °C.
■ RESULTS AND DISCUSSIONS
Nb2O5 Film Growth. All the precursors are liquid at room
temperature and volatile enough to be evaporated at modest
temperatures (55−65 °C). As shown by the TG measure-
ments performed under 1 atm of argon (Figure 1), the three
precursors show the same behavior: evaporation occurs in a
single step with very low residual mass (2012, 24, 975−980977
film thickness and refractive index. For crystalline films, grain
growth is possible, which can lead to increased roughness. Even
the stoichiometry may change if the thermodynamically stable
composition is different at elevated temperatures.44
In the literature, the phase composition and phase transition
temperatures of Nb2O5 films differ depending on the applied
deposition technique, film thickness, and substrate. Even studies
on nominally similar processes, sample, and substrate have led to
different conclusions. However, there is generally agreement
that Nb2O5 thin films are amorphous when deposited below
400 °C.38,45 The crystallization temperatures reported are between
400 and 550 °C, and the phase has been identified as mono-
clinic,46 TT (pseudohexagonal),42,47 hexagonal,38 orthorhombic,38
or tetragonal.48 A second phase transition at temperatures between
500 and 770 °C has also been observed, the resulting phase being
identified as hexagonal, orthorhombic, or a mixed phase.38
According to XRD measurements on 30 to 50 nm thick films,
all the films deposited in this study were amorphous prior to
heat treatment. HTXRD measurements indicate that crystal-
lization occurs between 525 and 575 °C, and no more changes
in the phase composition are observed up to 1000 °C, regard-
less of the precursor (Figure 3). The crystalline phase appears
to be orthorhombic Nb2O5. Figure 4 depicts the experimental
data and the Rietveld refinement based on the orthorhombic
phase. The close correlation and the very low residual error
assert our conclusion. Refined cell parameters are 6.175,
29.298, and 3.923 Å for the a, b, and c axis, respectively. The
literature values in the same order are 6.175, 29.175, and 3.93 Å
(ICSD card 1840).
Roughness and film morphology was studied with AFM.
Figure 5 displays representative images of Nb2O5 thin films
deposited with tBuNNb(NEt2)3 and ozone at 275 °C, in the
as-deposited state and after annealing at 600 °C for 20 min in
N2. Films were found to be very smooth (0.2 to 0.6 nm rms
roughness for 20 to 40 nm thick films) when deposited at
275 °C from tBuNNb(NEt2)3 and tBuNNb(NMeEt)3,
regardless of the oxygen source. The striped structures
observed in the annealed films are attributed to the crystallite
growth of the previously determined orthorhombic phase and
cause a slight increase in the surface roughness. About 40 nm
thick films deposited by the tamylNNb(OtBu)3/ozone
process at 250 °C were found to have significantly rougher
surfaces (rms roughness of 4 nm), which is apparently due to
the thermal decomposition of the precursor.
The impurity contents in the Nb2O5 films were examined by
TOF-ERDA from films deposited onto Si(100) substrate at 175
to 275 °C (Table 1). As expected, TOF-ERDA showed high
contamination levels (C,H,N) in the films deposited from
tamylNNb(OtBu)3, most likely due to thermal decomposi-
tion of the precursor. In the Nb2O5 films grown from tBuN
Nb(NEt2)3 and tBuNNb(NMeEt)3 with water, very low
impurity contents were observed: the carbon and hydrogen
impurity contents remained below 1 atom %. By contrast, when
using ozone as the oxygen source, higher impurity contents
were observed. This is surprising, as in many cases ozone is a
strong oxidizer that efficiently burns the ligands during the
Figure 3. HTXRD patterns of 30 nm thick Nb2O5 films deposited at 275 °C using (a) tBuNNb(NEt2)3/ozone, (b)
tBuNNb(NMeEt)3/ozone,
and (c) tamylNNb(OtBu)3/water processes.
Figure 4. Experimental and Rietveld refined XRD pattern of
crystallized orthorhombic 40 nm thick Nb2O5 thin film. The bottom
line indicates the residual error.
Figure 5. Representative AFM images of Nb2O5 films in the as-
deposited state (on the left: rms = 0.2) and after annealing at 600 °C
in nitrogen (on the right: rms = 0.4). The film was 30 nm thick and
deposited at 275 °C with the tBuNNb(NEt2)3/ozone process.
Chemistry of Materials Article
dx.doi.org/10.1021/cm2026812 | Chem. Mater. 2012, 24, 975−980978
process, leaving lower amounts of impurities. The impurity
levels decrease with increasing deposition temperature when
more thermal energy is available for accelerating the surface
reactions, but still the films deposited with ozone contain
clearly higher amounts of carbon and hydrogen compared to
the films deposited with water. Differences in reaction mech-
anisms between ozone and water-based processes are very
interesting, and in situ quadrupole mass spectrometry studies
are ongoing.
To investigate the electrical properties, capacitance and I−V
measurements were carried out on Al/Nb2O5/TiN/Si/Al ca-
pacitor structures with 45 nm thick Nb2O5 films deposited at
275 °C using the tBuNNb(NEt2)3/O3 process and annealed
at 600 °C in nitrogen. The films present high k-values around
60. These high k-values were reached with a reasonably low
leakage current density, which was at the noise level when
applying a bias voltage up to 0.75 V and below 10−5 A/cm2 at
1 V. The capacitance remains stable upon applying bias voltages
from −2 to 0.5 V (Figure 6). The dielectric properties of the
binary Nb2O5 films deposited by ALD here are clearly promis-
ing for future capacitor dielectrics, and further research toward
the optimization of the dielectric properties is ongoing.
■ CONCLUSIONS
In this study three novel precursors for the ALD growth of
Nb2O5 thin films have been evaluated. On the one end, tBuN
Nb(NEt2)3 and
tBuNNb(NMeEt)3 distinguish themselves as
well-behaving ALD precursors and high quality films were
achieved, with both water and ozone as the oxygen sources.
These are the first reported ALD Nb2O5 processes using ozone
as the oxygen source. The observed preferable deposition temper-
ature windows were wide. On the other hand, tamylNNb(OtBu)3
presents poor thermal stability, resulting in high thickness non-
uniformity, high surface roughness, and increased impurity levels.
However, its limited thermal stability might make tamylN
Nb(OtBu)3 worth studying as a CVD precursor. According to
compositional analysis, the films deposited using water as the
oxygen source process higher purity than those deposited with
ozone. Most interestingly, the preliminary evaluation of the dielec-
tric properties has been very promising as high permittivity values
with low leakage could be reached. The full dielectric char-
acterization of the Nb2O5 thin films obtained by the developed
processes will be the subject of a future paper.
According to this study, the novel imido-amido precursors of
niobium are well-behaving ALD precursors offering wide pos-
sibilities in processing high quality Nb2O5 films by ALD (see
Table 2 for summary of the ALD-related properties). In addi-
tion, this novel precursor family is most likely applicable in
ALD of other group five oxides, that is, Ta2O5 and V2O5.
■ AUTHOR INFORMATION
Corresponding Author
*E-mail: timothee.blanquart@helsinki.fi.
■ ACKNOWLEDGMENTS
The research leading to these results has received funding from
the European Community’s Seventh Framework Programme
(FP7/2007-2013) under Grant Agreement No. ENHANCE-
23840.
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tBuN
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physical state at ambient
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liquid liquid liquid
temperature of
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ALD window 150−325 °C 150−325 °C no
growth rate inside the
ALD window
0.4 Å/cycle 0.5 Å/cycle
saturation yes at 275 °C yes at 275 °C no
Chemistry of Materials Article
dx.doi.org/10.1021/cm2026812 | Chem. Mater. 2012, 24, 975−980979
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Chemistry of Materials Article
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