Interfacial reaction depending on the stack structure of Al 2O3 and HfO2 during film growth and postannealing

Article abstract of DOI:10.1063/1.1807968

Interfacial reactions as a function of the stack structure of Al ₂O₃ and HfO₂ grown on Si by atomic-layer deposition were examined by various physical and electrical measurements. In the case of an Al₂O₃ film with a buffer layer of HfO ₂, reactions between the Al₂O₃ and Si layers were suppressed, while a HfO₂ film with an Al₂O ₃ buffer layer on the Si readily interacted with Si, forming a Hf-Al-Si-O compound. The thickness of the interfacial layer increased dramatically after an annealing treatment in which a buffer layer of Al ₂O₃ was used, while no change in thickness was observed in the film in which a HfO₂ buffer layer was used. Moreover, the stoichiometric change caused by a different reaction process altered the chemical state of the films, which affected charge trapping and the interfacial trap density.

Full text of DOI:10.1063/1.1807968

Interfacial reaction depending on the stack structure of Al 2 O 3 and Hf O 2 during film
growth and postannealing
M.-H. Cho, K. B. Chung, H. S. Chang, D. W. Moon, S. A. Park, Y. K. Kim, K. Jeong, C. N. Whang, D. W. Lee, D.-
H. Ko, S. J. Doh, J. H. Lee, and N. I. Lee
Citation: Applied Physics Letters 85, 4115 (2004); doi: 10.1063/1.1807968
View online: http://dx.doi.org/10.1063/1.1807968
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APPLIED PHYSICS LETTERS
VOLUME 85, NUMBER 18
1 NOVEMBER 2004
Interfacial reaction depending on the stack structure of Al2O3
and HfO2 during film growth and postannealing
M.-H. Cho,a) K. B. Chung, H. S. Chang, and D. W. Moon
Nano Surface Group, Korea Research Institute of Standards and Science, DaeJeon 305-600, Korea
S. A. Park, Y. K. Kim, K. Jeong, and C. N. Whang
IPAP, Yonsei University, Seoul, 120-749, Korea
D. W. Lee and D.-H. Ko
Department of Ceramic Engineering, Yonsei University, Seoul 120-749, Korea
S. J. Doh, J. H. Lee, and N. I. Lee
System LSI Division, Samsung Electronics Co. Yongin-Si, Kyunggi-Do, 449-711, Korea
(Received 9 March 2004; accepted 30 August 2004)
Interfacial reactions as a function of the stack structure of Al2O3 and HfO2 grown on Si by
atomic-layer deposition were examined by various physical and electrical measurements. In the case
of an Al2O3 film with a buffer layer of HfO2, reactions between the Al2O3 and Si layers were
suppressed, while a HfO2 film with an Al2O3 buffer layer on the Si readily interacted with Si,
forming a Hf–Al–Si–O compound. The thickness of the interfacial layer increased dramatically after
an annealing treatment in which a buffer layer of Al2O3 was used, while no change in thickness was
observed in the film in which a HfO2 buffer layer was used. Moreover, the stoichiometric change
caused by a different reaction process altered the chemical state of the films, which affected charge
trapping and the interfacial trap density. © 2004 American Institute of Physics.
[DOI: 10.1063/1.1807968]
High-permittivity dielectrics have been widely investi-
gated as alternative gate insulating layers in advanced MOS
devices because high dielectric oxides permit the use of
thicker films, compared to other oxides, such as SiO2, which
have a lower dielectric constant. The possible use of HfO2
based high dielectrics, in particular, have attracted consider-
able attention as a possible replacement for SiO2 in gate
oxides of ULSI because of their thermal stability and high
dielectricity.1,2 HfO2 has a relatively high dielectric constant
of 25–30, a low leakage current, and is highly reliable al-
though the structural transition from amorphous to crystal-
line phase occurs at a low temperature and the material has a
relatively small band gap ͑ϳ5.5 eV͒.3
A number of reports have focused on the use of compos-
ites of two dielectric oxides or multilayers of dielectrics for
the purpose of a achieving comparable dielectrics and to en-
hance thermal stability.4,5 Mixed Al–Hf oxides have attracted
particular attention because the transition temperature from
amorphous to polycrystalline is increased, compared to pure
HfO2 and Al2O3.6 Johnson et al. reported that the flatband
voltage shift of the Hf–Aluminate alloy is caused by electron
trapping.7 However, interfacial reactions between the high-k
oxide and the Si substrate were not considered, although the
interfacial reaction at the contact of the dielectric with Si can
affect intermixing, resulting in a significant change in ther-
mal stability and flatband voltage shift.
different film stoichiometry as a result of interactions be-
tween the films and the Si substrate. The change in film
thickness after annealing was also dependent on the stack
structure, resulting in a drastic increase in total film thickness
when a buffer layer of Al2O3 was used, while it was sup-
pressed in the case of an Al2O3 film in which the buffer layer
was HfO2. As a result, we conclude that interfacial reactions
and intermixing are suppressed in a film in which a buffer
layer of HfO2 is used, compared to a film with a buffer layer
of Al2O3.
The p-type Si substrate with 2–5 ⍀ cm was cleaned us-
ing the RCA method and then preprocessed to include a
ϳ1.0 nm SiO2 layer to stabilize and reduce the interfacial
state. The metal oxides were grown by means of an ALD
system, which has a vertical warm wall reactor with a show-
erhead and a heated susceptor. Al2O3 and HfO2 films were
each grown at temperatures below 300 °C using Al͑CH3͒3
and HfCl4 as a precursors. H2O vapor served as the oxygen
source and N2 was supplied as the purge and carrier gas.
Bilayer structures with HfO2͑15 Å͒/Al2O3͑30 Å͒/Si and
Al2O3͑30 Å͒/HfO2͑15 Å͒/Si were fabricated. In order to
use the high thermal stability of Al2O3 and compensate the
negative trap charge in Al2O3 with HfO2, the Al2O3 layer
was thicker than the HfO2. The films were annealed using a
rapid thermal process in an ambient of N2 from
750 to 900 °C for 5 min.
In this study, we focused on the characteristics of bilayer
HfO2–Al2O3 films grown on Si using an atomic layer depo-
sition (ALD) system. In particular, physical and electrical
characteristics as a function of stack structure, whether Si is
in contact with HfO2 or Al2O3, were investigated. The find-
ings show that intermixing between HfO2 and Al2O3 was
significantly affected by the stack structure, resulting in a
The chemical state and stoichiometry of the films were
investigated using x-ray photoelectron spectroscopy and me-
dium energy ion scattering (MEIS) measurements. A MEIS
analysis was accomplished with a 100 keV proton beam in a
double alignment so as to reduce contributions from the crys-
talline Si substrate, allowing the deconvolution of the spectra
into contributions from the Al2O3 layer and Si signals. Pt
dots with a 0.01 cm radius for a MOS diode were evaporated
onto the oxide surface as electrodes. C–V measurements
a)Electronic mail: mhcho@kriss.re.kr
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4116
Appl. Phys. Lett., Vol. 85, No. 18, 1 November 2004
Cho et al.
FIG. 1. MEIS spectra of (a) an HfO2 film ͑15 Å͒ with a buffer layer of
Al2O3͑30 Å͒ on an oxidized Si and (b) Al2O3͑30 Å͒ film with a buffer layer
of HfO2 film ͑15 Å͒ on an oxidized Si substrate. MEIS spectra of annealed
films at 750 and 900 °C for 5 min in a N2 ambient are also shown in (a) and
(b).
FIG. 2. HRTEM images of (a) an as-grown Al2O3͑30 Å͒ film with a buffer
layer of HfO2 film ͑15 Å͒ on an oxidized Si and (b) an as-grown HfO2 film
͑15 Å͒ with a buffer layer of Al2O3͑30 Å͒ on an oxidized Si. Images of the
films annealed at 900 °C for 5 min in an N2 ambient: (c) an Al2O3͑30 Å͒
film with a buffer layer of HfO2 film ͑10 Å͒ on an oxidized Si and (d) a
HfO2 film ͑10 Å͒ with a buffer layer of Al2O3͑30 Å͒ on an oxidized Si.
were performed in a probe station using an HP 4284A instru-
ment.
Changes in layer thickness and stoichometry were inves-
tigated by MEIS measurements, as shown in Fig. 1. The
calculated thickness of the as-grown Al2O3 and HfO2 films
from MEIS spectra is about 32 and 14 Å, respectively when
HfO2 is used as a buffer layer. A very thin ϳ12 Å SiO2 layer,
which was formed before film growth to limit interfacial
oxidation and to improve interfacial quality, was observed in
the film in which Al2O3 was used as a buffer layer, indicating
that no interfacial reactions occurred between Al2O3/Si.8
The interface between HfO2/Al2O3 is quite sharp and no
interdiffusion was observed. The most important finding is
that intermixing between HfO2 and Al2O3 is dependent on
which buffer layer is in contact with the Si. The depth profile
of an as-grown film in the stack structure of Al2O3/HfO2/Si
is almost the same as that for a film annealed at 750 °C,
while that of HfO2/Al2O3/Si was altered somewhat. Slight
changes in the peak heights of Al/Si and Hf indicate that
interdiffusion between HfO2 and Al2O3 starts at an annealing
temperature of 750 °C. The difference in the depth profile of
the annealed samples at 900 °C indicates that the HfO2 and
Al2O3 layers were completely intermixed in the stack struc-
ture of HfO2/Al2O3/Si, while two layers were intermixed
slightly in the stack structure of Al2O3/HfO2/Si. Another
noteworthy change is that small amounts of Si on the film
surface after a 900 °C annealing treatment is observed when
Al2O3 is used as a buffer layer. Thus, the diffusion of Si to
the upper film occurs, although silicate formation in an as-
grown film is effectively suppressed compared to the as-
grown Al2O3/HfO2/Si stack structure. The Si on film sur-
face indicates that Si diffusion is closely related to the drastic
interdiffusion of HfO2 and Al2O3, i.e., the diffusion of Si
from the bottom layer enhances interdiffusion. Moreover, a
difference in the peak width of O between two stack struc-
tures shows that the increase of the oxide layer thickness by
more than 10 Å occurs only in the HfO2/Al2O3/Si stack
structure annealed at 900 °C, which is caused by the oxida-
tion of Si layer at the film surface diffused from Si substrate.
Changes in the stack structure were also investigated us-
ing HRTEM, as shown in Fig. 2. The TEM image shows that
layers composed of HfO2 and Al2O3 have an amorphous
structure up to an annealing temperature of 900 °C. The as-
grown HfO2 and Al2O3 film thicknesses are consistent with
previously reported MEIS data. The boundary between
maintained after the annealing treatment, while that for
HfO2/Al2O3/Si is not clear and a gradual change in contrast
with the relative quantity of Hf and Al contents can be ob-
served. Moreover, a distinct increase in the film thickness in
the HfO2/Al2O3/Si stack structure after the annealing treat-
ment was observed as shown in MEIS data, although the
oxidized Si layer on the film surface cannot be clearly dis-
criminated. These results are in good agreement with the
MEIS data, which indicates intermixing between
Al2O3/HfO2 as well as the diffusion of Si (Fig. 3).
Changes in chemical state with stack structure were in-
vestigated using XPS. The binding energy of Hf in Hf sili-
cate formation is shifted to a higher position because the
more ionic Hf-oxide would be expected to become even
more ionic after the formation of the complex oxide, whereas
the Si of the more covalent oxide should experience a corre-
sponding covalence under reduced Madelung and relaxation
effects.9 Thus, the peak shift of Hf to higher binding energy
in the as-grown Al2O3/HfO2/Si structure indicates that the
Hf silicate layer is formed on the contact area between
HfO2/Si, while the formation of silicate layer is suppressed
in the as-grown HfO2/Al2O3/Si structure.10,11 The high
FIG. 3. XPS spectra of annealed films at 900 °C for 5 min in a N2 ambient:
(a) and (b) Hf 4f spectra of as-grown films with stack structures of
Al2O3 /HfO2 /Si and HfO2 /Al2O3 /Si͑30 Å͒, respectively. (e) and (f) Hf 4f
spectra of annealed films at 900 °C with stack structures of Al2O3 /HfO /Si
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Al2O3 and HfO2 in the Al2O3/HfO2/Si stack structure is
and HfO2 /Al2O3 /Si͑30 Å͒, respectively.
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Appl. Phys. Lett., Vol. 85, No. 18, 1 November 2004
Cho et al.
4117
the negative direction as shown in Fig. 4(b), which indicates
that the positive trap changes are contained in the as-grown
film. The positive trap charges decrease with increasing an-
nealing temperature. For Al2O3 in contact with SiO2, the
negative charge is located at the interface between the Al2O3
and SiO2, while for the HfO2 film, the positive charge is in
the film.3,8 The two types of trap charges are compensated by
each of the other layers, resulting in a reduced flatband volt-
age shift. Thus, the decrease in positive trap charge in the
Al2O3/HfO2/Si film after an annealing treatment at 900 °C
is caused by the partially intermixed alloy with a negative
trap charge structure, which compensates for the positive
trap charge in the as-grown film. Moreover, since the quan-
tity of the trap charge is dependent on the depth position and
interfacial characteristics, the resulting flatband voltage shift
is influenced by the stack structure. In the HfO2/Al2O3/Si
structure, the shift in the as-grown film is insignificant. How-
ever, when the stack structure almost changes into an alloy
film after annealing treatment at 900 °C, a flatband voltage
is a little shifted to lower direction. Johnson et al. reported
that Hf d states of the alloy act as localized electron traps,
which is the opposite shift direction compared to our data.8
The difference can be explained by the fact that the intermix-
ing of Si, which is observed using MEIS data as shown in
Fig. 1(a), significantly affects the total effective charge of the
film.
FIG. 4. C–V curves of (a) a HfO2͑15 Å͒/Al2O3͑30 Å͒/Si and (b) an
Al2O3͑30 Å͒/HfO2͑15 Å͒/Si. The C–V curves for films annealed at
900 °C for 5 min in a N2 ambient are also provided in (a) and (b). The line
indicates a zero flatband voltage when a Pt electrode with a workfunction of
5.65 eV is used.
binding energy of ϳ18 eV for the Hf 4f7/2 of the mixed ox-
ide with HfO2, Al2O3, and SiO2, after the annealing treat-
ment, would be caused by the characteristic differences be-
tween ionic and covalent bonding. The broad Hf 4f peak in
the annealed samples is caused by various chemical states,
depending on the degree of intermixing among Hf͑Si͒Ox,
Al2O3, and SiOx. In particular, broadening of Hf 4f peak in
the annealed Al2O3/HfO2/Si structure is increased because
of incompletely intermixed layer of HfAlO between Al2O3
and Hf͑Si͒Ox in the depth direction as shown in the MEIS
spectra. On the other hand, completely mixed layer of HfAl-
SiO is uniformly formed in the annealed HfO2/Al2O3/Si
structure, resulting from the interdiffusion between
HfO2/Al2O3 and the diffusion of Si from Si substrate.
We also investigated changes in the dielectric character-
istics by measuring accumulation capacitances. The charac-
teristics of the C–V curve in the stack structures are shown
in Fig. 4. The maximum capacitance value of as-grown films
has almost the same value in both structures with different
buffer layers. Moreover, in the Al2O3/HfO2/Si structure, the
accumulation capacitance value is maintained after the an-
nealing treatment, although the thicknesses of the layers with
different dielectric constants are changed as shown in TEM
and MEIS data. The slight intermixing in the structure after
annealing treatment can result in a higher capacitance, as-
suming the resulting dielectric constant of the mixed oxide
lies on intermediate value between the pure oxide values.
This increase in capacitance can be offset by the slight in-
crease in interfacial SiO2 thickness seen in TEM data. Thus,
no change in accumulation capacitance value reflects above
two opposite results. It is noteworthy that the change in di-
electric constant after the annealing treatment shows a very
different tendency, i.e., the accumulation capacitance in the
HfO2/Al2O3/Si structure is drastically decreased after the
annealing treatment, but no change in the Al2O3/HfO2/Si
structure is detected. The most plausible possibility based on
the previous MEIS data is that the decrease of the accumu-
lation capacitance in the HfO2/Al2O3/Si structure resulted
from the SiO2 layer formation on the film surface caused by
the diffusion of Si from Si substrate and its oxidation during
the annealing process. Therefore, we conclude that the
change in stoichiometry in the film as a result of Si incorpo-
ration can affect the dielectric constant, resulting in a de-
crease in accumulation capacitance. The flatband voltage in
the as-grown film of Al2O3/HfO2/Si structure is shifted to
In summary, the thermal stability and the structural char-
acteristics of Al2O3–HfO2 bilayer films are demonstrated in
this study. Structural stability is maintained in the
Al2O3/HfO2/Si structure, but not in the HfO2/Al2O3/Si
structure. In particular, the diffusion of Si atoms from the Si
substrate has a major effect on thermal stability. The diffu-
sion of Si atoms also changes the dielectric constant as well
as the oxide trap charge density.
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.
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