Change in the chemical state and thermal stability of HfO2 by the incorporation of Al2O3

Article abstract of DOI:10.1063/1.1633976

The investigation of Al₂O₃ incorporated HfO ₂ films grown by atomic layer deposition using various measurement tools was discussed. It was observed that the accumulation capacitance of the Al₂O₃ incorporated into HfO₂ film increases as the postannealing temperature increases. It was found that the incorporation of Al₂O₃ into the HfO₂ has no effect on silicate formation at the interface between the film and Si, while the ionic bonding characteristics and hybridization effects were enhanced compared to a pure HfO₂ film. It was shown that dissociated Al₂O₃ on the film surface was removed by a vacuum annealing treatment.

Full text of DOI:10.1063/1.1633976

Change in the chemical state and thermal stability of HfO 2 by the incorporation of Al 2
O 3
M.-H. Cho, H. S. Chang, Y. J. Cho, D. W. Moon, K.-H. Min, R. Sinclair, S. K. Kang, D.-H. Ko, J. H. Lee, J. H. Gu,
and N. I. Lee
Citation: Applied Physics Letters 84, 571 (2004); doi: 10.1063/1.1633976
View online: http://dx.doi.org/10.1063/1.1633976
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APPLIED PHYSICS LETTERS
VOLUME 84, NUMBER 4
26 JANUARY 2004
Change in the chemical state and thermal stability of HfO2
by the incorporation of Al O
2
3
M.-H. Cho,a) H. S. Chang, Y. J. Cho, and D. W. Moon
Korea Research Institute of Standards and Science, Nano-surface group, Daejon 305-600, Korea
K.-H. Min and R. Sinclair
Department of Materials Science and Engineering, Stanford University, Stanford, California 94305
S. K. Kang and D.-H. Ko
Department of Ceramics Engineering, Yonsei University, Seoul, 120-749 Korea
J. H. Lee, J. H. Gu, and N. I. Lee
Process Development Team, Samsung Electronics Company, Limited, San No. 24, Yongin-Si, Kyunggi-Do,
4
49-711, Korea
Received 27 May 2003; accepted 22 October 2003͒
Al O incorporated HfO films grown by atomic layer deposition were investigated using various
͑
2
3
2
measurement tools. The accumulation capacitance of the Al O3 incorporated into HfO2 film
2
increases as the postannealing temperature increases because of changes in interfacial and upper
layer thickness and in interfacial stoichiometry. The core-level energy state of a 15 Å thick film
shows a shift to higher binding energy, as the result of silicate formation and Al O incorporation.
2
3
The incorporation of Al O into the HfO film has no effect on silicate formation at the interface
2
3
2
between the film and Si, while the ionic bonding characteristics and hybridization effects are
enhanced compared to a pure HfO film. Any dissociated Al O on the film surface is completely
2
2
3
removed by a vacuum annealing treatment over 850 °C, while HfO contributes to Hf silicide
2
formation on the surface of the film. © 2004 American Institute of Physics.
͓
DOI: 10.1063/1.1633976͔
As device sizes become scaled down, future ultralarge-
scale integration devices will require a gate dielectric capaci-
conduction band of Hf–Al oxide is largely derived from the
state of the Al ions. This indicates that the thermal stability
of the Hf–Al oxide might be related to a change in the
chemical state owing to the incorporation of Al O . Renault
tance equivalent to a SiO thickness of Ͻ15 Å in order to
2
1
,2
achieve the required high speed and low power. However,
the current in an ultrathin oxide layer suffers from direct
tunneling, and the high current of the direct tunneling regime
degrades the reliability of the oxide film. As a result, a vari-
2
3
8
et al. reported on changes in the chemical state due to the
interfacial layer of Hf silicate between HfO /SiO , as evi-
2
2
denced by synchrotron radiation x-ray photoelectron spec-
troscopy.
ety of efforts to replace the SiO with high dielectric oxides
2
have been reported.
In this study, we focused on the characteristics of the
Al O incorporated HfO films grown on Si using an atomic
Unfortunately, the high dielectric oxide films investi-
gated to date do not show sufficient thermal stability because
the structure of the oxide films are easily converted from
amorphous to polycrystalline and react with the Si substrate.
Some studies have recently focused on silicate and oxide
alloys in an attempt to overcome the poor thermal stability.3
In particular, the nanolamnate structure composed of two
high dielectric oxides represents a promising solution for en-
hancing thermal stability to sustain high dielectricity.5
2
3
2
layer deposition ͑ALD͒ system. We investigated the change
in the chemical state related to the structural change of the
films as a function of annealing temperature using high-
resolution transmission electron microscopy ͑HRTEM͒, syn-
chrotron radiation x-ray photoelectron spectroscopy
,4
͑
͑
SRXPS͒, and near-edge x-ray absorption fine structure
NEXAFS͒. The dielectric change was also assessed by
,6
capacitance-voltage (C–V) measurements. We observed the
Hf 4f shift to a higher binding state, which is caused by the
formation of a silicate layer and Al-oxide incorporation. The
Among the nanolaminates, HfO –Al O shows good ther-
mal stability and high dielectricity even after the high an-
nealing temperature used in the process. However, no mi-
2
2
3
6
dissociation of pure HfO occurs at a temperature of 700 °C
2
croscopic model or in depth characterization of the
enhancement in thermal stability has been reported because
most studies have focused on interfacial reactions through
investigations of structural changes of the films, which do
not provide information on the chemical states.
under ultrahigh vacuum conditions while that of Hf–Al–O
occurs at around 850 °C. The dissociation of Al in the Hf–
Al–O occurs at a temperature of 850 °C which is similar to
the dissociation temperature of a pure Al O layer. The dis-
sociated Al-oxide is removed by the vacuum treatment, while
Hf is formed in the silicide layer on the film surface.
A p-type Si substrate with a resistivity of 2–5 ⍀ cm was
cleaned using the Radio Corporation of America method and
2
3
7
Recently, Afanas’ev et al. reported that the energy band
diagram of an Si/Hf–Al oxide interface appears to be very
close to that of the Si/Al O interface and proposed that the
2
3
then preprocessed to include a ϳ12 Å layer of SiO to sta-
bilize and reduce the interfacial state. The metal oxides were
2
a͒Electronic mail: mhcho@kriss.re.kr
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32.203.227.62 On: Wed, 09 Jul 2014 11:26:52
0
1

572
Appl. Phys. Lett., Vol. 84, No. 4, 26 January 2004
Cho et al.
FIG. 1. HRTEM images for ͑a͒ as-grown and annealed 80 Å thick Hf–Al–O
films at the annealing temperatures of ͑b͒ 850 °C, and ͑c͒ 950 °C for 5 min.
͑
e͒ NEXAFS spectra of O K edge features of HfO , Al O , and Hf–Al–O
2 2 3
films. ͑d͒ C–V data as-grown and annealed 80 Å thick Hf–Al–O films at
the annealing temperatures of 750 °C, 850 °C, and 950 °C for 5 min in N2
ambient. The measurement was performed at 1 MHz and room temperature.
FIG. 2. XPS peak spectra of Si 2p ͑a͒ in pure SiO2 12 Å and ͑b͒ Hf–Al–
O͑12 Å͒ on SiO (12 Å)/Si. The inset is the Hf 4f peaks corresponding to
2
͑
b͒. Hf 4f peaks are obtained two different take-off angles at 90° and 30°
with respect to the film surface.
grown using an ALD system, which has a vertical warm-wall
reactor with a showerhead and a heated susceptor. Al O and
2
3
HfO films were each grown at temperatures below 300 °C
to the oxygen p-projected density of states, which consists of
four unoccupied hybridized orbitals, Hf 5dϩO 2p␲, Hf 5d
ϩO 2p␴, Hf 6sϩO 2p, Hf 6pϩO 2p, under an assumption
of an octahedral symmetry. The spectrum of Al O shows a
2
by using the precursors, Al(CH ) and HfCl , respectively.
3
3
4
H O vapor served as the oxygen source and N was supplied
2
2
as the purge and carrier gas.
2
3
Figure 1 shows HRTEM images of 80 Å thick Hf–Al–O
oxide with annealing temperatures at 850 °C and 950 °C. It is
noteworthy that the film maintains a nearly amorphous struc-
ture up to the annealing temperature of 850 °C, while our
previous study showed that pure 80 Å thick as-grown HfO2
film was converted into a polycrystalline structure even at an
double band caused by O p states hybridized with Al 3sp
states. The incorporation of Al O into HfO results in the
2
3
2
superposition of the spectra of both Al O and HfO . Al-
2
3
2
though the ratio of atomic concentration of Hf to Al in the
HfAlO film is close to 1.8:1, as confirmed by the x-ray pho-
toelectron spectroscopy ͑XPS͒ intensity, the intensity of the
NEXAFS spectra of the HfAlO film indicates that the con-
tribution from the AluO bond is more than that from the
HfuO bond. The results clearly show the existence of
strong electronic interactions at HfO –Al O : i.e., incorpo-
9
annealing temperature of ϳ700 °C. The fringes of the crys-
tallite in the TEM image result not from crystalline Al O3
2
but from crystalline HfO , because the diffraction data are in
2
agreement with monoclinic or tetragonal HfO ͑diffraction
2
2
2
3
not shown here͒. Thus, crystallization is strongly dependent
rated Al O significantly changes the molecular orbitals of
2 3
on the HfO structure. The segregation of the Al O or HfO
HfO2 .
2
2
3
2
due to phase separation is not observed in the film annealed
at 950 °C, indicating that the fine Al O grains are embedded
Figure 2 shows Si 2p core-level spectra of a pure 1.2 nm
thick SiO layer before and after Hf–Al-oxide growth and
2
3
2
in the film. The characteristics of the C–V curve in the 80 Å
thick films in Fig. 1͑d͒ show that the dielectric constant cal-
culated from the accumulation capacitance of the films
gradually increases from 14 to 17 as the annealing tempera-
ture increases. The changes in stoichiometry of the interfa-
cial layer and in the thickness of the interfacial layer and
upper film indicate that the change in accumulation capaci-
tance is the result of the changes in the two layers. However,
a great change in the upper film thickness indicates that the
Hf 4f core-level spectra of the 1.5 nm sample on the SiO2
layer at take-off angles of 90° and 30°. Before deposition of
xϩ
the Hf–Al-oxide, four oxidation states (Si ,xϭ1,2,3,4) can
be found with chemical shifts, while, after deposition, an
ϩ4
additional oxidation state Si* appears between Si
and
ϩ3
Si , shifted by 0.6 eV to lower binding energy relative to
ϩ4 12
Si
.
We note that the peak position of Hf 4f in Hf–Al–O
is shifted to a higher binding energy than that of the Hf-
silicate film.10 This excessive energy shift can be caused by
molar volume (V ), which affects the dielectric constant,
the incorporation of Al-oxide into the HfO , indicating that
m
2
also contributes to the increment since larger molar volume
of Al O can change the molar volume of the mixture of
charge changes are induced by incorporation of Al O with a
2
3
different covalence.9
,10
2
3
6
1
,10
HfO and Al O .
the film, ϳ4ϫ10 cm /eV is somewhat poor, compared to
The interfacial trap charge density of
In order to assess changes in thermal stability of the film
with annealing temperature, the chemical state as a function
of the annealing temperature at vacuum conditions was in-
vestigated using in situ XPS. The change in the chemical
state of the Hf–Al–O film starts at around an annealing tem-
2
2
3
1
Ϫ2
1
1
Ϫ2
that of pure HfO , ϳ2ϫ10 cm /eV.
2
The NEXAFS spectra of the O K edge clearly show the
change of the molecular structure between HfO /Hf–Al–O.
2
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The O K edge NEXAFS spectrum of HfO is directly related
perature of 850 °C, while HfO is dissociated at an annealing
2
2
1
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Appl. Phys. Lett., Vol. 84, No. 4, 26 January 2004
Cho et al.
573
FIG. 4. Change in the Al 2p XPS spectra of a Hf–Al–O ͑15 Å͒ film as a
function of annealing temperature at ͑a͒ as-grown, ͑b͒ 850 °C, and ͑c͒
900 °C for 5 min in the UHV circumstances. The reported binding energy
related to the Al silicate and Al O are located from 74.4 eV to 74.8 eV and
2
3
from 75.5 to 75.8, respectively.
gen in HfO is evacuated to form a silicide layer. The simple
2
dissociation process of the oxides can be written as ͑In this
FIG. 3. XPS peak spectra of Si 2p and Hf 4f after the Al–Hf–O ͑15 Å͒ film
was annealed at a temperature of 850 °C for 5 min in the UHV conditions.
Si 2p spectra and Hf 4f spectra ͑inset͒ were obtained at different take-off
angles of 90° and 30° with respect to film surface. Si and Si** are caused
by the Si surface reconstruction structure and the Hf silicide layer, respec-
tively.
process, SiO2 is included because silicate and interfacial
SiO should be considered.͒
2
s
HfO ϩSiO ϩSi→HfSi ϩSiO͑g͒ϩSi, Al O ϩSiO
2
2
x
2
3
2
ϩSi→AlO ͑g͒ϩSiO͑g͒ϩSi.
x
In summary, peak changes of Hf, Al, and Si indicate that
charge transfer is closely related to changes in the bonding
characteristics, suggesting that the thermal stability of Hf–
Al–O films is largely dependent on the incorporation of Al-
temperature of 700 °C ͑data not reported in this article͒.9
Figure 3 shows the changed chemical state of the film, i.e.,
the Si peaks related to the interfacial SiO and silicate are
2
reduced, and metallic Hf peaks appear. The peak difference
is dependent on the take-off angle, indicating that the disso-
ciation and outdiffusion process occur preferentially at the
surface region, and not at the interface. The Hf peak related
to the Hf silicate and Al O incorporation can be deconvo-
oxide into the HfO film. This suggests that the metal oxide
can be stabilized by changing the bonding characteristics via
the addition of a metal oxide as a dopant.
2
This work was supported by the National Program for
Tera-level Nanodevices of the Ministry of Science and Tech-
nology as one of the 21 Century Frontier Programs.
2
3
luted from the difference of the peak profile between the
surface and the interface. If the incorporation of Al O into
2
3
HfO has the same role as the SiO in HfO , the charge
1
2
3
2
2
2
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͑
1
2
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10
2
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3
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2
2
2
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