Effect of annealing temperature on microstructural evolution and electrical properties of sol-gel processed ZrO2/Si films

Article abstract of DOI:10.1063/1.3541784

High-permittivity (k) ZrO₂/Si(100) films were fabricated by a sol-gel technique and the microstructural evolution with the annealing temperature (T_a) was correlated with the variation of their electrical performance. With increasing T_a, the ZrO₂ films crystallized into a tetragonal (t) phase which was maintained until 700 ° C at nanoscale thicknesses. Although the formation of the t-ZrO₂ phase obviously enhanced the k value of the ZrO₂ dielectric layer, the maximum capacitance in accumulation was decreased by the growth of a low-k interfacial layer (IL) between ZrO₂ and Si with increasing T _a. On the other hand, the gate leakage current was remarkably depressed with increasing T_a probably due to the combined effects of the increased IL thickness, optical band gap of ZrO₂, and density of ZrO₂ and decreased remnant organic components.

Full text of DOI:10.1063/1.3541784

Effect of annealing temperature on microstructural evolution and electrical
properties of sol-gel processed films
,
Soo Min Hwang, Seung Muk Lee, Kyung Park, Myung Soo Lee, Jinho Joo , Jun Hyung Lim, Hyoungsub Kim,
Jae Jin Yoon, and Young Dong Kim
Citation: Appl. Phys. Lett. 98, 022903 (2011); doi: 10.1063/1.3541784
View online: http://dx.doi.org/10.1063/1.3541784
View Table of Contents: http://aip.scitation.org/toc/apl/98/2
Published by the American Institute of Physics

APPLIED PHYSICS LETTERS 98, 022903 ͑2011͒
Effect of annealing temperature on microstructural evolution and electrical
properties of sol-gel processed ZrO /Si films
2
1
1
1
1
1,a͒
Soo Min Hwang, Seung Muk Lee, Kyung Park, Myung Soo Lee, Jinho Joo,
1
1
2
2
Jun Hyung Lim, Hyoungsub Kim, Jae Jin Yoon, and Young Dong Kim
1
School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746,
Republic of Korea
Nano-Optical Property Laboratory and Department of Physics, Kyung Hee University, Seoul 130-701,
2
Republic of Korea
͑
Received 20 October 2010; accepted 23 December 2010; published online 11 January 2011͒
High-permittivity ͑k͒ ZrO /Si͑100͒ films were fabricated by a sol-gel technique and the
2
microstructural evolution with the annealing temperature ͑T ͒ was correlated with the variation of
a
their electrical performance. With increasing T , the ZrO films crystallized into a tetragonal ͑t͒
a
2
phase which was maintained until 700 °C at nanoscale thicknesses. Although the formation of the
t-ZrO phase obviously enhanced the k value of the ZrO dielectric layer, the maximum capacitance
2
2
in accumulation was decreased by the growth of a low-k interfacial layer ͑IL͒ between ZrO and Si
2
with increasing T . On the other hand, the gate leakage current was remarkably depressed with
a
increasing T probably due to the combined effects of the increased IL thickness, optical band gap
a
of ZrO , and density of ZrO and decreased remnant organic components. © 2011 American
2
2
Institute of Physics. ͓doi:10.1063/1.3541784͔
In order to achieve the continuous downscaling and im-
proved performance of complementary metal oxide semicon-
ductor ͑MOS͒ devices, a number of works have been dedi-
cated to the search for high-permittivity ͑k͒ dielectric
ZrO2 films with two different thickness scales ͑ϳ7
and ϳ13 nm͒ were fabricated by the sequential steps of
solution coating, drying, and annealing. The ZrO2 pre-
cursor sol ͑0.1M͒ was prepared by dissolving Zr acety-
materials to replace the conventional SiO with its excessive
lacetonate ͓Zr͑C H O ͒ ͔ in
a
solvent mixed with
2
5
7
2 4
gate leakage current due to direct tunneling at thicknesses
N,N-dimethylformamide ͑C H NO͒ and ethanolamine
3
7
1
below ϳ1.2 nm. Among the various high-k dielectrics,
͑
C H NO͒ at a molar ratio of 2:1. The solution was spin-
2 7
ZrO has been considered as a promising alternative, to-
2
coated on dilute HF-treated p-type Si͑100͒ substrates at 4000
rpm, followed by drying on a hot plate at 200 °C. The thick-
gether with HfO , because of its moderately high k ͑ϳ20͒
2
1
,2
and wide band gap ͑ϳ5.8 eV͒. Moreover, it is a well-
ness of the ZrO layer was controlled by repeating the coat-
2
known polymorph having three crystal structures with differ-
ing and drying steps, one time for ϳ7 nm and two times for
ϳ13 nm. The gel films were subjected to annealing at
400–700 °C for 1 h in an air ambient.
ent theoretical k values: monoclinic ͑m, kϳ20͒, tetragonal
3
͑
t, kϳ47͒, and cubic ͑kϳ37͒ phases, while showing a k
4
value of ϳ22 for the amorphous ͑a͒ phase. Although t-ZrO
The microstructure was observed by transmission elec-
tron microscopy ͑TEM͒ and the chemical bonding state and
interfacial structure were characterized by x-ray photoelec-
tron spectroscopy ͑XPS͒ using an Mg K␣ ͑h␯=1253.6 eV͒
x-ray. All of the spectra were referenced to the Si 2p peak
͑99.3 eV͒ corresponding to bulk Si. Top gate electrodes
consisting of an Al ͑10 nm͒/TaN ͑50 nm͒ stack with a diam-
eter of 100 ␮m were deposited by sputtering and patterning
via a lift-off process to fabricate simple MOS capacitors.
Capacitance-voltage ͑C-V͒ and gate leakage current density–
2
5
is stable in bulk form at temperatures above ϳ1100 °C,
t-ZrO films with high k can be formed at lower temperatures
2
6
,7
by reducing the film thickness or grain size ,9or by doping
8
impurities such as Ge and La into the lattice.
Such t-ZrO dielectric films have been fabricated mostly
2
by vacuum-based processes but rarely studied by a chemical
solution deposition. The solution deposition process is a
simple and cost-effective way of producing large area films
with diverse compositions and is applicable to flexible and
transparent electronic device systems. Recently, the effects of
the processing variables on the chemical composition, inter-
voltage ͑J -V͒ properties were estimated using an Agilent
g
E4980A LCR meter and a HP4145B semiconductor param-
eter analyzer, respectively.
facial reactions, and dielectric properties of ZrO /Si films
2
1
0–12
deposited by a sol-gel route have been reported.
How-
Figures 1͑a͒–1͑e͒ show the cross-sectional TEM images
ever, systematic studies on the correlation between the
t-phase evolution and electrical properties have not yet been
of the sol-gel processed ZrO films annealed at different tem-
2
peratures. All of the films had a moderately flat and smooth
performed. In this work, we fabricated ZrO dielectric films
2
surface and an interfacial layer ͑IL͒ between ZrO and Si.
2
by a sol-gel deposition method on Si substrates and explored
The thickness of the ZrO layer fabricated by one time pro-
2
the effects of the annealing temperature ͑T ͒ on the t-phase
a
cessing was found to be 6.9͑Ϯ0.13͒ nm ͓Figs. 1͑a͒–1͑d͔͒.
formation, microstructure, and resultant electrical perfor-
mances.
As can be seen in the figures, the structure of the ZrO layer
2
was amorphous for the samples annealed at 400 ͑a͒ and
5
00 °C ͑b͒. In contrast, a crystalline phase appeared in the
a͒Author to whom correspondence should be addressed. FAX: ϩ82-31-299-
amorphous matrix at 600 °C ͑c͒ and the crystallinity in-
creased with increasing T . This implies that the 6.9-nm-
4749. Tel.: ϩ82-31-290-7358, 7385. Electronic mail: jinho@skku.edu.
a
0003-6951/2011/98͑2͒/022903/3/$30.00
98, 022903-1
© 2011 American Institute of Physics

022903-2
Hwang et al.
Appl. Phys. Lett. 98, 022903 ͑2011͒
Δ = 2.4 eV
d5/2
o
700 C
o
(a) Zr 3d
700 C (b) Si 2p
3
3d3/2
o
o
6
00 C
600 C
o
o
5
00 C
Δ = 3.6 eV
Δ = 3.3 eV
500 C
o
o
4
00 C
400 C
0
Before anneal
Si
Bare Si
+
Δ = 4.2 eV
4
2+
Si
Si
180
182
184
186
98
100
102
104
106
Binding energy [eV]
Binding energy [eV]
1
2
0
8
6
4
2
0
(
f)
Total (ZrO + IL)
2
1
FIG. 2. ͑Color online͒ XPS ͑a͒ Zr 3d and ͑b͒ Si 2p spectra of ϳ6.9-nm-thick
ZrO /Si samples annealed at different temperatures. The dotted curves in-
2
dicate the deconvoluted spectra obtained by curve fitting.
ZrO2
IL
4
00
500
600
700
thickness was also observed for the 13-nm-thick samples:
o
Ta [ C]
2
.0, 2.1, 2.5, and 4.3 nm at 400, 500, 600, and 700 °C,
respectively.
FIG. 1. ͑Color online͒ Cross-sectional TEM images of ϳ6.9-nm-thick ZrO2
films annealed at ͑a͒ 400, ͑b͒ 500, ͑c͒ 600, and ͑d͒ 700 °C. ͑e͒ The corre-
sponding image of the ϳ13-nm-thick film annealed at 400 °C. ͑f͒ Variations
Figure 2 exhibits the XPS Zr 3d ͑a͒ and Si 2p ͑b͒ spectra
of the 6.9-nm-thick ZrO samples annealed at different tem-
peratures. For comparison, the Zr 3d signal from the ZrO2
gel film before annealing, i.e., after drying, and Si 2p signal
from the bare Si substrate are presented. All of the samples
showed the typical Zr 3d spectra with spin-orbit doublets
2
of the thickness of the layers with respect to T . All of the scale bars and
a
white arrows indicate 5 nm and a lattice spacing of ϳ3.0 Å corresponding
to t-ZrO ͑101͒, respectively.
2
͑
d5/2 and d3/2͒ separated by ϳ2.4 eV, as indicated in Fig.
thick ZrO started to crystallize at temperatures between 500
2
2
͑a͒. For the gel film, the Zr 3d5/2 peak associated with
and 600 °C. On the other hand, the 13.0͑Ϯ0.24͒-nm-thick
Zr-O bonds was located at 181.7 eV and was gradually
shifted from 182.1 to 182.2 eV with increasing T , indicating
that the Zr-O bonds in ZrO became more ionic due to the
annealing.
ZrO samples ͑two-time processing͒ were already crystal-
2
lized at temperatures below 400 °C, as shown in Fig. 1͑e͒,
indicating that the crystallization temperature decreased as
the thickness increased. It is notable that the critical thick-
a
2
1
5
No signal corresponding to Zr-Si bonds
1
6
͑
ϳ179.1 eV͒ due to Zr silicide was detected.
ness for crystallization was between 6.9 and 13 nm at a T of
a
In Fig. 2͑b͒, all of the annealed samples revealed Si 2p
400–500 °C and it became smaller than 6.9 nm at higher
signals having a different binding energy from that of the
temperatures ͑Ͼ500 °C͒. This behavior probably originates
0
Si substrate ͑Si , 99.3 eV͒. The bare Si, 600 and 700 °C,
from the difference in the surface energies of the polymorphs
1
3
samples commonly showed a chemical shift ͑⌬E͒ of Si 2p
of ZrO at the nanoscale. As a thinner film has a larger
2
0
by ϳ4.2 eV with respect to Si , indicating that their ILs
surface/volume ratio, the amorphous phase is thermodynami-
cally preferred over the crystalline one in order to lower its
total Gibbs free energy because the latter phase introduces
interfacial energy due to the evolution of the grain boundary.
The lattice spacing of all of the crystallized films ͑indi-
cated by white arrows͒ was measured as ϳ3.0 Å based on
additional identifications of the Fourier-transformed diffrac-
tograms extracted from the crystallized areas ͑not shown
1
4
consisted of stoichiometric SiO . On the other hand, ⌬E
2
values of ϳ3.3 and ϳ3.6 eV were found for the 400 and
5
were probably attributed to the IL containing either Zr sili-
cate ͑Si-O-Zr bonds͒ or thermally grown Si suboxide
00 °C samples, respectively. These lower values of ⌬E
7
,14,16
͑
SiO , 1ϽxϽ2͒,
consequently affecting electrical
x
performances of the ZrO /IL dielectric stacks.
2
Figure 3͑a͒ shows the C-V characteristics of the 6.9-nm-
thick samples measured at 100 kHz. Good transition behav-
ior from the accumulation to inversion regions and a hyster-
esis width of less than 70 mV during the bidirectional
voltage sweep were found for all of the samples. The
here͒, revealing that the planes corresponded to t-ZrO
2
7
͑
101͒. Thus, we considered that the a-ZrO transformed into
2
t-ZrO2 during annealing at 600 °C for the 6.9-nm-thick
samples and at 400 °C for the 13-nm-thick samples, and the
t-phases were retained until 700 °C. Although m-ZrO is
2
capacitance/unit area at V =−3 V in the accumulation re-
gion ͑Cmax͒ was decreased slightly from ϳ0.87 ␮F/cm at
4
cantly decreased to 0.60 ␮F/cm at 700 °C. This behavior
was well matched with the increase of the total dielectric
thickness with increasing T observed in the TEM results,
and a similar observation was also made for the 13-nm-thick
samples. In order to investigate the effects of T on the k
value of ZrO ͑k ͒, the kZrO2 was calculated by using a
ZrO
2
series capacitance model, 1/Cmax=1/CZrO +1/CIL, based on
more stable than t-ZrO at low temperatures ͑Ͻ1100 °C͒,
g
2
2
the presence of the t-phase is probably related to its lower
1
3
2
00 and 500 °C to 0.81 ␮F/cm at 600 °C and then signifi-
surface energy than that of m-ZrO2.
2
As T increased, the IL was thickened, while the thick-
a
ness of the ZrO layer remained constant. The measured
2
thickness of the IL was ϳ1.9 nm at 400 °C and it increased
a
to ϳ2.2, ϳ2.6, and ϳ4.3 nm at 500, 600, and 700 °C, re-
spectively. It has been reported that the IL between ZrO and
a
2
7
,14
Si is either Zr silicate or Si oxide ͑SiOx͒.
The monotonic
2
increase in the thickness of the IL with increasing T resulted
a
2
in an increase of the total thickness of the ZrO /IL dielectric
the TEM and C-V results and was plotted in the inset of
2
stacks, as depicted in Fig. 1͑f͒. A similar variation of the IL
Fig. 3͑a͒. Although the ILs did not consist of SiO in all
2

022903-3
Hwang et al.
Appl. Phys. Lett. 98, 022903 ͑2011͒
1.0
0.8
0.6
0.4
0.2
0.0
28
-2
-3
-4
-5
(
a)
10
(b)
24
20
16
12
8
10
10
10
FIG. 3. ͑Color online͒ ͑a͒ C-V characteristics ͑100
kHz͒ of ϳ6.9-nm-thick samples and dependence of
͓inset in ͑a͔͒ kZrO2 value and ͑b͒ Jg value at Vg −VFB
=−1 V on Ta for ϳ6.9-nm- ͑circles͒ and ϳ13-nm-
͑squares͒ thick samples.
Circle: ~ 6.9 nm
Square: ~ 13 nm
-
6
-7
-8
-9
10
10
10
10
4
00 500 600 700
o
o
T [ C]
4
5
6
7
00 C
00 C
00 C
00 C
a
o
Circle: ~ 6.9 nm
Square: ~ 13 nm
o
o
-10
10
400
500
T [ C]
600
700
-3
-2
-1
0
1
2
3
o
Gate voltage [V]
a
of the samples, we approximated the k of the ILs to that of
SiO ͑3.9͒. The k value increased from 13.1 to 18.2 as the
Ta increased for the 6.9-nm-thick samples, while those of the
the crystallization temperature was different for the ϳ6.9-
and ϳ13-nm-thick scales. The high-temperature annealing
resulted in an increase of both the thickness of the low-k IL
2
ZrO
2
13-nm-thick samples ranged from 25.7 to 26.5, which are
and kZrO2 value due to t-ZrO phase evolution. In addition,
2
somewhat lower than the reported k values ͑ϳ30–43͒ of
the J was significantly suppressed with increasing T prob-
g
a
t-ZrO2 deposited by the conventional vacuum-based
ably due to the combinatorial effects of the increased IL
thickness, E of ZrO , and density of ZrO and the reduced
6
,8,9
method.
The lower k was believed to be due to the lower
g
2
2
atomic density and hence smaller electronic and ionic polar-
izations induced by the possible remnant organic compo-
remnant organics. These results show the utility of crystalline
t-ZrO deposited by a sol-gel route for improving the gate
2
nents in the sol-gel processed ZrO . It was noted that the
2
dielectric performances, but additional studies are required to
increase the purity, atomic density, and reproducibility via
optimizing the process conditions.
kZrO2 was enhanced as the content of t-ZrO phase increased.
2
In addition, we partly attributed the higher overall kZrO2 of
the 13-nm-thick samples than those of the 6.9-nm-thick
samples to the thickness dependence of the effective k value
The authors would like to thank Dr. Byungha Shin for
helpful comments. This research was supported by Basic
Science Research Program through the National Research
Foundation of Korea ͑NRF͒ funded by the Ministry of Edu-
cation, Science and Technology ͑Grant No. 2010-0009997͒.
1
7
according to Lorentz’s local field theory. As the film thick-
ness increases, the contribution of the bulk dipole moment
becomes more dominant than that of the surface, resulting in
an increase of the effective k value. Although the kZrO2 value
was apparently increased by the formation of t-ZrO phase,
2
1
the contribution of kZrO2 to Cmax was screened by the IL
S. Guha and V. Narayanan, Annu. Rev. Mater. Res. 39, 181 ͑2009͒.
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2
growth with increasing T ͓Fig. 3͑a͔͒.
a
3
Figure 3͑b͒ shows the dependence of the gate leakage
current density ͑J ͒ at V −V =−1 V on the T and thick-
4
g
g
FB
a
5
ness of the ZrO layer. The J was considerably diminished
6
2
g
as the T and ZrO thickness increased, despite the evolution
a
2
͑2005͒.
7
of crystalline t-ZrO whose defective grain boundary gener-
S. K. Dey, C.-G. Wang, D. Tang, M. J. Kim, R. W. Carpenter, C.
2
1
8
ally serves as the predominant leakage current path. The
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Y.-H. Wu, M.-L. Wu, J.-R. Wu, and L.-L. Chen, Appl. Phys. Lett. 97,
8
suppression of J was mainly caused by the increase of the
g
0
43503 ͑2010͒.
total thickness of the ZrO /IL dielectric stacks due to the
2
9
L. Lamagna, C. Wiemer, S. Baldovino, A. Molle, M. Perego, S. Schamm-
growth of the IL with increasing T . However, in the com-
a
Chardon, P. E. Coulon, and M. Fanciulli, Appl. Phys. Lett. 95, 122902
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lar thicknesses, other factors were considered to affect the Jg.
From additional spectroscopic ellipsometry measurements
10͑
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with a rotating analyzer and an autoretarder ͑1.12–6.52 eV͒
1
9
11
12
13
based on the Tauc Lorentz model, we obtained the optical
band gap energies ͑E ͒ of ZrO for the 6.9-nm-thick samples.
g
2
The values were around 2.4, 4.9, 5.2, and 5.2 eV for the 400,
500, 600, and 700 °C samples, respectively. Thus, a larger
A. Navrotsky, J. Mater. Chem. 15, 1883 ͑2005͒.
1
4
E probably resulted in a smaller J since a large E generally
g
g
g
T. S. Jeon, J. M. White, and D. L. Kwong, Appl. Phys. Lett. 78, 368
͑2001͒.
15
corresponds to a large conduction band offset and high bar-
1
8
rier height against electron tunneling transport. In addition,
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1
25117 ͑2001͒.
the higher-temperature annealing may densify the ZrO layer
2
1
6
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082904 ͑2005͒.
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and lower the content of organics, such as C and H, in the
1
0
layer, thereby improving the leakage performance and k
17
18
value as shown in Fig. 3͑a͒.
In summary, the sol-gel deposited ZrO dielectrics trans-
2
1
9
formed from a- into t-ZrO phases as the T increased and
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2
a