Suppression of parasitic Si substrate oxidation in HfO2- ultrathin-Al2O3-Si structures prepared by atomic layer deposition

Article abstract of DOI:10.1063/1.1944206

We investigated the effects of Al₂O₃ thickness on the suppression of parasitic substrate oxidation in HfO₂-ultrathin- Al₂O₃-Si structures. The use of H₂O as oxidizing agent in the atomic la

Full text of DOI:10.1063/1.1944206

Suppression of parasitic Si substrate oxidation in Hf O 2 –ultrathin- Al 2 O 3 – Si
structures prepared by atomic layer deposition
Myungjin Park, Jaehyoung Koo, Jinwoo Kim, Hyeongtag Jeon, Choelhwyi Bae, and Cristiano Krug
Citation: Applied Physics Letters 86, 252110 (2005); doi: 10.1063/1.1944206
View online: http://dx.doi.org/10.1063/1.1944206
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APPLIED PHYSICS LETTERS 86, 252110 ͑2005͒
Suppression of parasitic Si substrate oxidation in HfO –ultrathin-Al O –Si
2
2
3
structures prepared by atomic layer deposition
a͒
Myungjin Park, Jaehyoung Koo, Jinwoo Kim, and Hyeongtag Jeon
Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, Korea
b͒
Choelhwyi Bae and Cristiano Krug
Department of Physics, North Carolina State University, Raleigh, North Carolina 27695
͑
Received 11 January 2005; accepted 9 May 2005; published online 17 June 2005͒
We investigated the effects of Al O thickness on the suppression of parasitic substrate oxidation in
2
3
HfO –ultrathin-Al O –Si structures. The use of H O as oxidizing agent in the atomic layer
2
2
3
2
deposition ͑ALD͒ chemistry is considered key to preventing the formation of an SiO interlayer
x
during oxide deposition. An Al O layer prepared with 10 cycles of atomic layer deposition ͑ALD,
2
3
ϳ0.74 nm͒ effectively suppressed substrate oxidation during rapid thermal annealing in N for 10
2
s below 800 °C. Parasitic oxidation was observed at 600 °C for samples with only five cycles or
without Al O . Ultrathin Al O films can be relevant for the integration of HfO as gate dielectric
2
3
2
3
2
in silicon technology. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.1944206͔
Excessive tunneling of charge carriers through SiO N
lected samples and directly on Si for control purposes. The
hot wall quartz reactor was kept at 300 °C. Trimethylalumi-
num ͑TMA, naturally vaporized͒ and hafnium tetrachloride
͑HfCl , kept at 150 °C and carried by N ͒ were the metal
x
y
thin films presently limits the miniaturization of metal–
oxide–silicon field effect transistors. Replacement of SiO N
x
y
by a material of higher dielectric constant ͑kϾ9͒ has been
4
2
1
,2
proposed. Other conditions being met, such a high-k ma-
terial would allow increasing the thickness of the dielectric
layer by a factor above k/7.5—therefore, reducing tunneling
currents—without significantly altering device characteris-
precursors, and H O was the oxidizing agent. A reactor purge
2
with N preceded substrate exposure to each of the reactants.
2
The samples were submitted to rapid thermal annealing in N2
for 10 s at 600 to 800 °C. Physical characterization before
and after annealing was accomplished using x-ray photoelec-
tron spectroscopy ͑XPS͒ and cross-sectional high-resolution
transmission electron microscopy ͑HRTEM͒.
tics. Among high-k dielectric candidates, HfO is under in-
2
tense investigation. In addition to high dielectric constant
3
4,5
͑
kϳ22–25͒, it exhibits relatively large band gap ͑Eg
6
ϳ5.6 eV͒ and high free energy of reaction with Si. Never-
We used XPS to identify chemical bonding in the
1
0,11
theless, most HfO –Si structures reported to date show an
samples and to determine the thickness
of the Al O lay-
2
2 3
7
interfacial layer, probably formed due to oxidizing species.
ers. Figure 1͑a͒ schematically presents three samples used in
The interfacial layer generally forms during HfO deposition
2
and thickens upon thermal annealing, which is inherent to
device fabrication. While such a layer can reduce the density
of interface defects and improve the reliability of the HfO2
3
film, a critical disadvantage is the reduced effective dielec-
tric constant of the resulting dielectric stack. This prompts
for an engineered interface between HfO and Si.
2
Al O presents a moderate dielectric constant ͑kϳ9͒,
2
3
wide band gap, and large band offset energies with respect to
Si. It has also been shown to remain amorphous on Si after
8
9
annealing above 800 °C. Gusev et al. reported the deposi-
tion of Al O on Si by atomic layer deposition ͑ALD͒ with-
2
3
out the formation of an interfacial layer. We are thus inves-
tigating ultrathin Al O layers on Si to prevent interfacial
2
3
layer formation during HfO deposition and thermal anneal-
2
ing.
In this letter, we report the effects of Al O thickness on
2
3
the suppression of parasitic substrate oxidation in
HfO –ultrathin-Al O –Si structures. Ultrathin Al O films of
2
2
3
2
3
different thicknesses ͑Ͻ1 nm͒ were deposited on HF-last
Si͑100͒ by ALD in an F-120 system ͑ASM Microchemistry͒.
Atomic layer deposition of HfO ͑ϳ4.5 nm͒ followed on se-
2
FIG. 1. ͑a͒ Schematic illustration of three structures used for Al O thick-
2
3
a͒Author to whom correspondence should be addressed; electronic mail:
hjeon@hanyang.ac.kr
Present address: System LSI division, Samsung Electronics Co., Ltd., Yon-
ness determination by XPS: Clean Si substrate, thick Al O film on Si, and
2 3
thin Al O film on Si ͑sample of interest͒; ͑b͒ Si 2p and ͑c͒ Al 2p XPS data
2
3
b͒
from various thicknesses of as-deposited Al O on Si ͑“cy” refers to ALD
2 3
gin Gyeonggido 449-711, Korea.
cycles͒.
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003-6951/2005/86͑25͒/252110/3/$22.50 86, 252110-1 © 2005 American Institute of Physics
55.33.16.124 On: Wed, 26 Nov 2014 16:21:15
0
1

252110-2
Park et al.
Appl. Phys. Lett. 86, 252110 ͑2005͒
FIG. 2. Si 2p XPS data for samples featuring ͑a͒ zero, ͑b͒ 5, and ͑c͒ 10 ALD
cycles of Al O between the Si substrate and the HfO2 overlayer, with
2
3
annealing temperature as parameter; Si–Si bonding appears at 99 eV, and
Si–O at 103 eV.
the thickness determination process: Clean Si substrate, thick
Al O layer on Si, and thin Al O layer on Si ͑sample of
2
3
2
3
interest͒. Figures 1͑b͒ and 1͑c͒ show actual Si 2p and Al 2p
XPS spectra acquired from various samples featuring Al O
2
3
layers of different thicknesses. The Si 2p and Al 2p data
evidence only Si–Si and Al–O bonding, respectively, indicat-
ing that there is no parasitic oxidation of the substrate during
Al O deposition and that the overlayer is fully oxidized. In
2
3
the thickness calculations we use 2.9 and 2.8 nm, respec-
tively, as the attenuation lengths for Si 2p and Al 2p photo-
1
2
electrons in Al O . These values are reasonably close to
2
3
1
3
those reported by Bender et al.: 2.65 and 2.70 nm. The
Al O thicknesses determined are 0.27 and 0.74 nm for 5 and
2
3
10 ALD cycles, respectively. A linear fit to the full set of
thickness versus number of ALD cycles data ͑not shown͒
indicates about three cycles of incubation, i.e., no actual ox-
ide deposition during the first three ALD cycles. Gosset et
FIG. 3. Cross-sectional HRTEM images of HfO –ultrathin-Al O –Si struc-
2
2
3
1
4
tures as-deposited ͓͑a͒–͑c͔͒ and after thermal annealing in N2 for 10 s at
00 °C; the number of Al O ALD cycles increases from zero to 5 to 10 in
al. also reported a nucleation retardation of four deposition
cycles for Al O on HF-last Si. The retardation has been
7
2
3
2
3
the sequences ͑a͒–͑c͒ and ͑d͒–͑f͒. “IL” stands for interfacial layer.
understood as due to the low reactivity of hydrogen-
terminated silicon towards the ALD precursors.
To discuss unintentional interfacial layer formation dur-
bonding was inferred from XPS for all as-deposited samples.
Figures 3͑a͒ shows a transition layer that is only one to two
atomic layers-thick, while Figs. 3͑b͒ and 3͑c͒ show a layer of
thickness approaching 1 nm. Regarding Fig. 3͑b͒, it is rea-
sonable to question if 5 ALD cycles produce a qualified,
continguous layer of Al O ; in this sense, the thickness re-
ing HfO deposition and thermal annealing, we consider the
2
Si 2p XPS data in Fig. 2, acquired from the HfO –ultrathin-
2
Al O –Si structures as-prepared and after annealing at 600 to
2
3
8
00 °C. Data from as-deposited samples evidences only
Si–Si bonding, indicating the absence of interfacial SiO ir-
x
2
3
respective of Al O layer presence or thickness. We attribute
ported from XPS ͑0.27 nm͒ should be interpreted as an ap-
2
3
that to the use of H O as oxidizing agent in the ALD process.
proximate average. Based on XPS, the thickness of Al O in
2
2
3
1
5
The alternative chemistry with O oxidizes the substrate.
Fig. 3͑c͒ is more than twice that in Fig. 3͑b͒. We, therefore,
3
Si–O bonding becomes evident at 103 eV in Figs. 2͑a͒ and
state that most of the lighter area between Si and HfO in
2
2
͑b͒ after annealing at 600 °C, indicating low thermal stabil-
Fig. 3͑b͒ corresponds to the Si substrate. Concerning the
annealed samples, Figs. 3͑d͒ and 3͑e͒ clearly indicates the
growth of an interfacial layer, as opposed to Fig. 3͑f͒, in
which the interface is stable with reference to the as-
deposited sample in Fig. 3͑c͒. HRTEM, therefore, confirms
the result provided by XPS, namely that 10 ALD cycles of
Al O between Si and HfO prevent the formation of an
ity of the HfO –Si structure ͓Fig. 2͑a͔͒ and no beneficial
effect of 5 ALD cycles of Al O between HfO and Si ͓Fig.
2
2
3
2
2
͑b͔͒. In contrast, the Si–O XPS component is absent from
Fig. 2͑c͒ until the annealing is performed at 800 °C, indicat-
ing enhanced thermal stability for the sample incorporating
1
0 ALD cycles of Al O .
2 3
2
3
2
Figure 3 shows cross-sectional HRTEM images of the
HfO –ultrathin-Al O –Si structures as-deposited ͓Figs.
unintentional interfacial layer during thermal annealing at
700 °C. We finally note that the presence of Al O does not
2
2
3
2
3
3
͑a͒–3͑c͔͒ and after annealing at 700 °C ͓Figs. 3͑d͒–3͑f͔͒; the
prevent crystallization of the HfO overlayer, which is evi-
2
number of Al O ALD cycles increases from zero to 5 to 10
dent in Fig. 3 for all annealed samples.
2
3
in Figs. 3͑a͒–3͑c͒ and Figs. 3͑d͒–3͑f͒. Images from as-
In summary, the suppression of parasitic substrate oxida-
tion in HfO –ultrathin-Al O –Si structures was investigated.
deposited samples show both HfO and Al O films as amor-
2
2
3
2
2
3
phous; the thickness labels indicate upper limits for the layer
Ten ALD cycles of Al O ͑0.74 nm͒ yielded structures pre-
2 3
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between HfO and Si. We recall that the absence of Si–O
senting thermal stability during rapid thermal annealing in
2
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Appl. Phys. Lett. 86, 252110 ͑2005͒
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