Roles of nitrogen incorporation in HfAlOx „N… gate dielectrics
for suppression of boron penetration
Toshihide Nabatame,a) Kunihiko Iwamoto, Katsuhiko Yamamoto, Koji Tominaga,
Hirokazu Hisamatsu, Morifumi Ohno, Koji Akiyama, and Minoru Ikeda
MIRAI, Association of Super-Advanced Electronics Technology (ASET), AIST Tsukuba West 7,
Ibaraki 305-8569, Japan
Tomoaki Nishimura, Hiroyuki Ota, and Tsuyoshi Horikawa
MIRAI, Advanced Semiconductor Research Center (ASRC), National Institute of Advanced Industrial
Science and Technology (AIST), AIST Tsukuba Center 4, 1-1-1 Higashi, Tsukuba, Ibaraki 305-0046, Japan
Akira Toriumi
Department of Materials Science, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-8586, Japan
共Received 9 February 2004; accepted 3 April 2004; published 19 August 2004兲
We have investigated impacts of nitrogen incorporation into HfAlOx films on the gate leakage
currents and the flat band voltage V FB shifts. Also, a role of O–Hf–N bonding states in HfAlOx (N)
in suppression of boron penetration is discussed. The nitrogen concentration CN in HfAlOx (N) was
controlled by changing the NH3 annealing temperature at the step of the layer-by-layer deposition
and annealing process as well as the O2 annealing temperature of post-deposition annealing. The CN
over 10 at. % in HfAlOx (N) films effectively suppressed boron penetration as revealed by very slow
diffusion in Hf3 N4 films and the formation of boron-nitrogen remote complex in HfOx (N),
preserved the amorphous structure and reduced the V FB shift compared to the case of HfAlOx
without nitrogen incorporation, while CN exceeding 13 at. % led to a significant increase of the gate
leakage current. This suggests the excess O–Hf–N bond formation enhanced the leakage current.
© 2004 American Vacuum Society. 关DOI: 10.1116/1.1768526兴
I. INTRODUCTION
Several metal oxides have been investigated as candidates
for high-k 共high dielectric-constant兲 gate dielectrics to replace SiO2 in 65 nm complementary metal-oxidesemiconductor technology and beyond. Among the high-k
alternatives, HfO2 is also an excellent material due to its high
dielectric constant, high density, wide band gap and relevant
band offsets with respect to the Si band edge.1 However, a
relatively low crystallization temperature around 400 °C for
HfO2 might cause problems such as the surface roughness
enhancement and the grain boundary leakage in the gate
stack. The purpose of the nitrogen incorporation into high-k
materials is mainly to improve thermal stability and suppress
boron penetration. Nitrogen-incorporated HfO2 and HfSiOx
have increased the crystallization temperature up to about
950 °C.2– 4 Moreover, it has been reported that the boron penetration into HfOx (N) or HfSiOx (N) layer is closely related
to the film quality in terms of crystallization and phase
separation.2,4,5 On the other hand, there have been several
reports on using aluminum incorporation into HfO2 in order
to raise the crystallization temperature.6 – 8 We have also obtained a low leakage current density and a high dielectric
constant in homogeneous amorphous HfAlOx films.9 Recently, to obtain greater thermal stability at high temperature,
nitrogen-incorporated HfAlOx films have also been investigated and device properties have been reported.10–13 Nevera兲
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J. Vac. Sci. Technol. B 22„4…, JulÕAug 2004
theless, the roles of nitrogen incorporation into HfAlOx have
not been well understood, in connection with the electrical
properties and boron penetration behavior.
In this article, we have investigated impacts of nitrogen
incorporation into HfAlOx films on the gate leakage currents
and the flat-band voltage (V FB) shifts. We also discuss
whether the O–Hf–N bonding in HfAlOx (N) suppresses boron penetration during the activation annealing.
II. EXPERIMENT
We first grew a 1.5-nm-thick thermal SiO2 film as an interfacial layer on n-Si(100) through a rapid thermal annealing 共RTA兲 process in O2 ambient at 950 °C for 30 s.
HfAlOx (N) films were prepared by the layer-by-layer deposition and annealing 共LL-D&A兲 process,11,13 in which a sequence consisting of 0.7-nm-thick HfAlOx film growth by
the atomic layer deposition 共ALD兲 followed by an in situ
NH3 RTA process 共RTN兲 at 650 or 850 °C was repeated.
HfAlOx films with a Hf content of 60 at. % were deposited
on a SiO2 interfacial layer at 250 °C by ALD. The nitrogen
concentration (CN) in the HfAlOx (N) films was controlled
by changing the RTN and post-deposition annealing 共PDA兲
temperatures. We prepared HfAlOx (N) films with CN of 2,
10, and 13 at. % by combination of the RTN/PDA temperatures. Finally, HfAlOx (N) films with a thickness of 6 or 8 nm
were grown. For comparison, we also prepared HfAlOx films
without nitrogen incorporation through the conventional
ALD process followed by PDA at 650 °C in O2 ambient. A
150-nm-thick ␣-Si gate was deposited by low pressure-
1071-1023Õ2004Õ22„4…Õ2128Õ4Õ$19.00
©2004 American Vacuum Society
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Nabatame et al.: Roles of nitrogen incorporation in HfAlOx „N…
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FIG. 1. SIMS depth profiles of HfAlOx (N) films with nitrogen concentrations of 2, 10, and 13 at. %. 共a兲 For nitrogen and 共b兲 for oxygen profiles.
Nitrogen concentration at depth of 1 nm is listed in the figure.
chemical vapor deposition using SiH4 at 550 °C. Subsequently, BF⫹
2 for pMOS capacitors was implanted at 35 keV
with a dose of 3⫻1015 cm⫺2 . Activation annealing was done
through RTA at 950 °C for 20 s in N2 ambient, followed by
forming gas annealing at 400 °C for 30 min in H2 ambient.
To reveal the behavior of nitrogen in the HfAlOx (N) films,
we also deposited 8-nm-thick Hf3 N4 and AlN films on
SiO2 (1.5 nm)/n-Si(100) by dc-magnetron sputtering.
Secondary ion mass spectroscopy 共SIMS兲 was used to
analyze the depth profiles of nitrogen, oxygen, and boron in
the poly-Si/HfAlOx (N兲/SiO2 /Si gate stacks. The morphology of the HfAlOx (N) films was characterized by transmission electron microscopy 共TEM兲.
III. RESULTS AND DISCUSSION
The nitrogen depth profiles of 2-nm-thick HfAlOx (N)
films with various RTN/PDA conditions are shown in Fig.
1共a兲. The CN of the HfAlOx (N) films was estimated from N1s
spectra by using x-ray photoemission spectroscopy analysis.
Therefore, each SIMS profile was fitted by each CN value at
a depth of 1 nm in the HfAlOx (N) films. The CN at a depth
of 1 nm in the HfAlOx (N) films changed by the combination
of RTN/PDA temperatures. The CN of the HfAlOx (N) films
clearly increased when the RTN temperature was increased
while holding the PDA temperature unchanged. Furthermore,
a higher PDA annealing temperature slightly reduced the CN
of the upper layer in the HfAlOx (N) films when the RTN
temperature was unchanged. This is because the oxygen
PDA replaces some of nitrogen bonds by oxygen bonds as
shown in Fig. 1共b兲, where the increase of oxygen concentration by 850 °C PDA corresponds to the decrease of nitrogen
in the HfAlOx (N) film 关Fig. 1共a兲兴. Nitrogen out-diffusion
from the surface layer and oxygen interdiffusion into the film
were observed, while nitrogen atoms remained unchanged in
the deeper layer of the HfAlOx (N) film even after PDA at
850 °C in O2 ambient. These results indicate that the effective nitrogen incorporation into a HfAlOx film can be controlled by changing the RTN and PDA annealing temperatures.
The J g -V g characteristics of p ⫹ poly-Si/HfAlOx (N兲/
SiO2 /n-Si capacitors are compared in Fig. 2. The thickness
of all HfAlOx (N) films was constant at about 8 nm. The gate
JVST B - Microelectronics and Nanometer Structures
FIG. 2.
Leakage current density—gate voltage curves for
p ⫹ poly-Si/HfAlOx (N兲/SiO2 (1.5 nm)/n-Si capacitors. The thickness of all
HfAlOx (N) films was about 8 nm and CN was 0, 2, 10, and 13 at. %.
leakage current was clearly increased as the nitrogen content
was increased. In particular, a high leakage current density of
about 1 A/cm2 was observed even at V g ⫽1.0 V for CN
⫽13 at. %. In contrast, the leakage current density for CN
⫽10 at. % is significantly reduced by five orders of magnitude compared to the 13 at. % case but keeps one order of
magnitude larger compared to the CN⫽2 at. % case at V g
⫽1.0 V. This suggests that an excess amount of nitrogen in
HfAlOx (N) creates the current leakage sites arising from excess O–Hf–N bonds.
Normalized C – V curves for different nitrogen contents
are compared in Fig. 3. The C – V curve for the CN
⫽13 at. % was not measurable because of the high leakage
current. An anomalous, positive V FB shift for CN⫽0 at. % is
well suppressed by nitrogen incorporation because of
boron penetration. In fact, boron depth profiles for
HfAlOx (N兲/SiO2 structures are shown in Fig. 4 for different
nitrogen concentrations, by using backside SIMS. Boron
penetration into a Si substrate through HfAlOx (N) strongly
depended on the nitrogen concentration in the HfAlOx (N)
films. Noticeable boron penetration through the HfAlOx (N)
films was observed with CN of 0 and 2 at. %, while it was
FIG. 3. Normalized C – V curves of the p ⫹ poly-Si/HfAlOx (N兲/
SiO2 (1.5 nm)/n-Si capacitors measured for various nitrogen concentrations.
In the 8-nm-thick HfAlOx (N) films, the equivalent oxide thickness for CN of
0, 2, and 10 at. % 共estimated from the accumulation capacitance at V g
⫽V FB⫹2.5 V) was about 3.0, 2.3, and 3.3 nm, respectively.
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Nabatame et al.: Roles of nitrogen incorporation in HfAlOx „N…
FIG. 4. Boron depth profiles for different nitrogen concentrations measured
by backside SIMS.
significantly suppressed for CN⫽10 at. % or more. This boron penetration behavior is quite consistent with the observed V FB shifts shown in Fig. 3.
We used TEM to characterize the structural changes in the
HfAlOx (N) films after activation annealing at 950 °C for 20
s. Figures 5共a兲 and 5共b兲 show cross-sectional TEM images of
HfAlOx (N) with CN of 0 and 13 at. %, respectively. We observed that the HfAlOx (N) with CN of 13 at. % remained
amorphous even after the high-temperature activation. Furthermore, the thickness of film with CN of 13 at. % slightly
shrinks compared with that of the CN⫽0 at. % film. This
result corresponds to the densification of film by the annealing process. In fact, we confirmed that the film density prepared by LL-D&A process with RTN at 850 °C is increased
by about 20% compared to that of the conventional ALD
process with PDA at 650 °C, as revealed by x-ray reflection
analysis 共the data is not shown here兲. On the other hand, in
the HfAlOx films without nitrogen incorporation, polycrystalline phase with grain boundaries that would act as boron
diffusion paths appeared. Therefore, as shown in Fig. 3, the
large V FB shift of 0.95 V for polycrystalline HfAlOx is basically due to boron penetration.
To investigate how nitrogen in HfAlOx (N) affects boron
penetration, we prepared 8-nm-thick pure Hf3 N4 and AlN
films. Both films were deposited on SiO2 (1.5 nm)/n-Si by
dc-magnetron sputtering followed by ␣-Si deposition. This
was then followed by BF⫹
2 implantation and activation an-
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FIG. 5. Cross-sectional TEM images of a HfAlOx (N)(8 nm兲/SiO2 (1.5 nm)
stack structure after activation annealing at 950 °C for 20 s with a nitrogen
concentration of 共a兲 0 at. % and 共b兲 13 at. %.
nealing at 950 °C for 20 s or 1000 °C for 10 s. Figures 6共a兲
and 6共b兲 show boron depth profiles in the Hf3 N4 and AlN
films, respectively, after the activation annealing. It is obvious that boron penetration was stopped in Hf3 N4 films even
after high-temperature activation at 1000 °C, while considerable boron diffusion occurs into Si through the AlN film.
This indicates that the Hf–N bonds in the Hf3 N4 films apparently act as a boron diffusion barrier. Furthermore,
through a TEM electron-energy-loss spectroscopy analysis
we examined whether incorporated nitrogen bonded to Hf or
Al elements in HfO2 and Al2 O3 films nitrided by RTN treatment at 850 °C in NH3 ambient.14 We observed that the incorporated nitrogen mainly bonded to a Hf element rather
than an Al element during RTN treatment. Therefore, these
results suggest that the O–Hf–N bonding of HfAlOx (N)
films leads to effective protection against boron penetration.
Furthermore, it is thought from the first principle calculation
of boron diffusion in HfOx (N) 15 that the additional possibility of boron stop by nitrogen incorporation is due to the
formation of boron-nitrogen remote complex in the network.
IV. CONCLUSIONS
We have investigated the roles of nitrogen incorporation
into HfAlOx films in the electrical properties and the boron
penetration in pMOS capacitors. The incorporation of nitrogen into HfAlOx films suppressed boron penetration, while
FIG. 6. Boron depth profiles for 共a兲
Hf3 N4 (8 nm兲/SiO2 (1.5 nm) and 共b兲
AlN(8 nm兲/SiO2 (1.5 nm) stack structures annealed at 950 °C for 20 s and
1000 °C for 10 s as measured by backside SIMS.
J. Vac. Sci. Technol. B, Vol. 22, No. 4, JulÕAug 2004
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Nabatame et al.: Roles of nitrogen incorporation in HfAlOx „N…
an excess amount of nitrogen 共13 at. %兲 caused a high leakage current. This suggests that an excessive increase of
O–Hf–N bonds generates the leakage sites in the
HfAlOx (N). At the optimized nitrogen concentration of 10
at. %, the V FB shift due to boron penetration was significantly
suppressed. Observing the suppression of the boron diffusion
in Hf3 N4 films, it is suggested that O–Hf–N bonds in
HfAlOx (N) act as a boron-diffusion-barrier. Therefore, the
concentration and bonding configuration of nitrogen in
HfAlOx (N) gate dielectrics need to be precisely controlled in
poly-Si gate CMOS fabrication.
ACKNOWLEDGMENTS
This work was supported by New Energy and Industrial
Technology Development Organization 共NEDO兲. The authors wish to thank Mrs. Y. Tamura, T. Narui, S. Maeda, and
O. Satoh, for their support and encouragement.
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