Reduction of effective dielectric constant of gate insulator by low-resistivity electrodes

Article abstract of DOI:10.1063/1.1519736

On metal-oxide-semiconductor capacitors, the effective dielectric constant (k_eff) values extracted from high-frequency capacitance-voltage measurements were found to decrease when gate electrodes of very low resistivity were used. The equivalent-oxide thickness increase reaches about 1 nm with the low-resistivity electrodes. We examined gate insulators of SiO₂, Al₂O₃, and HfO₂ and gate electrodes of Al, TiN, Au, Cr, and TaN. The equivalent-oxide thickness increase can be prevented by inserting a high-resistivity metal film only 0.3 nm thick between the very low-resistivity metal and the insulator. The present results suggest that k _eff is reduced by the screening of ionic insulators with free electrons of the metal due to a quantum effect.

Full text of DOI:10.1063/1.1519736

Reduction of effective dielectric constant of gate insulator by low-resistivity electrodes
Kunio Saito, Yoshito Jin, and Masaru Shimada
Citation: Applied Physics Letters 81, 3582 (2002); doi: 10.1063/1.1519736
View online: http://dx.doi.org/10.1063/1.1519736
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APPLIED PHYSICS LETTERS
VOLUME 81, NUMBER 19
4 NOVEMBER 2002
Reduction of effective dielectric constant of gate insulator by low-
resistivity electrodes
Kunio Saito,a) Yoshito Jin, and Masaru Shimada
NTT Microsystem Integration Laboratories, 3-1, Morinosato Wakamiya, Atsugi-shi, Kanagawa,
243-0198 Japan
͑Received 15 July 2002; accepted 14 September 2002͒
On metal–oxide–semiconductor capacitors, the effective dielectric constant (keff) values extracted
from high-frequency capacitance–voltage measurements were found to decrease when gate
electrodes of very low resistivity were used. The equivalent-oxide thickness increase reaches about
1 nm with the low-resistivity electrodes. We examined gate insulators of SiO2 , Al2O3 , and HfO2
and gate electrodes of Al, TiN, Au, Cr, and TaN. The equivalent-oxide thickness increase can be
prevented by inserting a high-resistivity metal film only 0.3 nm thick between the very
low-resistivity metal and the insulator. The present results suggest that keff is reduced by the
screening of ionic insulators with free electrons of the metal due to a quantum effect. © 2002
American Institute of Physics. ͓DOI: 10.1063/1.1519736͔
In downsized nanometer-scale devices in ultralarge scale
integration, quantum effects appear in device operation. For
example, the gate capacitance of metal–oxide–
semiconductor ͑MOS͒ is reduced by the wave function of
carriers in a thinned inversion layer or accumulation layer.1–3
As is well known, direct tunneling currents appear through
nanometer-scale insulators. To reduce these currents, high-k
͓k: relative dielectric constant (ϵ␧r)] gate insulators are
now being intensively investigated.4–6 In spite of the use of
high-k materials, semiconductor technology7 requires thick-
nesses of a few nanometers. On this background, we have
found a correlation between the effective dielectric constant
(keff) and resistivity of the gate electrodes. To the best of our
knowledge, this is the first report of clarified keff-reduction
phenomenon caused by gate electrodes. The equivalent-
more exact value when relatively high-resistivity substrates
are used.
The wafers were ͑100͒-oriented p-type 3–5 ⍀ cm Si and
n-type 0.002–0.003 ⍀ cm Si. The wafers were cleaned with
a H2SO4 /H2O2 mixture, a dilute HF, and a deionized water.
SiO2 films were formed by thermal oxidization at 800 °C in a
dry oxygen atmosphere. High-k materials of Al2O3 and HfO2
were deposited on the substrates using an electron cyclotron
resonance ͑ECR͒ sputtering system with Al and Hf targets
and Ar/O2 gases. The ECR sputtering system can provide
high-quality high-k gate insulator films.12 Postdeposition an-
nealing was performed in a vacuum (1–3ϫ10Ϫ4 Torr and
1–2ϫ10Ϫ6 Torr) at about 600 °C for all samples except that
for the temperature-variation study. TiN films were deposited
by reactive sputtering with a magnetron sputtering system.
By choosing appropriate sputtering conditions, two types of
TiN films, gold ͑G͒-TiN and dark brown ͑DB͒-TiN, were
formed. The resistivities of the G- and DB-TiN films were
about 80 and 750 ␮⍀ cm, respectively. In the DB-TiN film
deposition, a little oxygen was added at a very low flow rate.
In the thickness-variation study for metal, ECR-sputtered
TaN films were used because of their high resistivity and
easy controllability in ECR sputtering. TaN films were de-
posited with a Ta target and Ar/N2 gases. The resistivity of a
thickly deposited TaN film was 2400 ␮⍀ cm. The TiN and
TaN films were patterned by chemical wet etching with an
evaporated Al mask. Gate electrodes of Al, Au, and Cr were
formed by vacuum evaporation with a tungsten-heater boat
and a stencil mask. All surfaces of the evaporated films were
optically flat. Measured resistivities of these metal films were
7, 5.5, and 130 ␮⍀ cm, respectively. All of the gate elec-
trodes were deposited after postannealing, except for the
samples for the trap study. Finally, an Al film was deposited
on the back of all samples to form ohmic contacts.
oxide-thickness
EOTϵk(SiO )d͑insulator͒/k͑insulator͒,
͓
2
where d is the physical thickness͔ increase of about 1 nm
was observed when very low-resistivity gate electrodes were
used, but EOT of less than 1 nm will be required in the
future. We believe that this phenomenon is caused by a quan-
tum effect that appears with the progress of the semiconduc-
tor technology trend. Because the metal gate offers several
advantages for device performance,7,8 the keff-change phe-
nomenon caused by it must be taken into account in future
MOS-device technology.
The gate capacitance extracted from the accumulation
region of the capacitance–voltage (C–V) curve9 does not
show the exact insulator–film capacitance because it in-
cludes a reduction due to the interfacial layer of low dielec-
tric constant;10 an influence from the series resistance, which
causes a voltage loss due to sharing between the resistance of
the insulator and that of other parts;11 and an influence from
substrate resistivity, which gives a gate-area dependence of
the EOT. While the reason for the last factor is unknown, we
have found that gate-area dependence disappears when a
very low-resistivity silicon substrate is used and clarified that
the EOT with a smaller gate area (Ͻ1ϫ10Ϫ4 cm2) is the
High-frequency C–V curves were obtained at 1 MHz
with a root-mean-square voltage of 20 mV and back-and-
forth bias sweep. Areas of all gate electrodes were measured
using microscope photographs to normalize the capacitance
by gate area. The quantum effect due to the accumulation
a͒
Electronic mail: ksaito@aecl.ntt.co.jp
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Appl. Phys. Lett., Vol. 81, No. 19, 4 November 2002
Saito, Jin, and Shimada
3583
FIG. 2. EOT for vacuum-evaporated Al, Au, and Cr gate electrodes as a
function of HfO2 thickness. Resistivities of these electrodes were about 7,
5.5, and 130 ␮⍀ cm, respectively. Very-low-resistivity substrates were used.
FIG. 1. EOT versus resistivity of gate electrodes obtained from metal/
insulator/p-Si MOS capacitors. Gate electrodes of Al, G TiN, and DB TiN
and gate insulators of thermally grown SiO2 and ECR-sputtered Al2O3 were
employed.
evaporation with a resistance-heater boat causes hardly any
damage to the sample surface, this result apparently indicates
that the keff of the capacitors depends on the resistivity of the
gate electrodes. It further shows that the EOT change is
caused at all insulator thicknesses.
layer was not taken into account in calibrating the measured
capacitances. The film resistivity was determined for thickly
deposited films ͑Ͼ50 nm͒.
Figure 1 shows the EOT versus the resistivity of gate
electrodes for metal/insulator/p-Si MOS capacitors. The EOT
changes with different gate electrode and it decreases with
increasing resistivity of the gate electrodes. The results for
the thermally oxidized SiO2 films show that the gate–
electrode dependence of EOT is not a special phenomenon in
ECR-sputtered insulator films. The lack of data for the DB
TiN/SiO2 is due to poor adherence of the DB-TiN films. The
large EOT values for all the ECR-Al2O3 plots are due to
interfacial-oxide layers formed by annealing in the lower
vacuum.
Recently, it has been reported that the Al electrode very
easily reacts with high-k insulators, such as hafnium silicate
and zirconium silicate but the Au electrode does not.13 How-
ever, we have not observed such an indication for Al2O3 and
HfO2 . Table I summarizes the EOTs we have obtained with
Al, Au, and Al/G-TiN electrodes under various conditions.
These data include the samples annealed after G-TiN depo-
sition but the tendency is the same. Furthermore, we have
usually obtained very small hysteresis ͑Ͻ20 mV͒ with the Al
electrode, suggesting little reaction with the insulators.
In order to eliminate the slight uncertainty regarding the
possibility of damage infliction by the plasma for TiN in the
aforementioned experiments, the EOT-change phenomenon
was further examined using vacuum-evaporated electrodes of
different resistivities. Figure 2 shows the EOT versus HfO2
thickness. Almost the same EOT was obtained for Al and Au,
and much smaller values were obtained for Cr. Since vacuum
The thickness of the high-resistivity metal layer required
in order to reduce the EOT values was studied using a
stacked electrode comprising a low-resistivity metal layer
͑Al͒/high-resistivity metal layer ͑ECR-sputtered TaN͒. The
result is shown in Fig. 3. It is surprising that TaN film only
0.3 nm thick decreases the EOT. This limits the location of
the origin of the EOT increase to near the metal/insulator
interface. Additionally, it shows that the ECR-sputtered TaN
film was deposited with a uniform thickness on an atomic
scale, as suggested from previous reports.12,14 Moreover, it
means that a thick low-resistivity metal film can be used in
future MOS devices.
Two possible reasons can be given for the EOT-change
phenomenon. One is the effect of traps near the interface and
the other is the screening effect of free electrons in the metal.
The screening effect is caused by the tail of the wave func-
tion of the free electrons because the tail must enter into the
insulator due to a quantum effect. To clarify the trap effect,
we experimentally obtained the annealing temperature de-
pendence
of
the
EOT
and
hysteresis
with
Al/G-TiN(3 nm)/HfO2(5 nm)/p-Si MOS capacitors. The re-
sults are shown in Table II. The hysteresis decreased with
increasing postanneal temperature, indicating effective defect
reduction by annealing. The EOT change was, however, very
small. We therefore believe that the EOT decrease is not due
to traps. This is reasonable considering the well-known fact
that the number of fast traps ͑1 MHz͒ is much smaller than
TABLE I. EOTs we have obtained with Al, Au, and Al/G-TiN electrodes under various conditions.
Si substrate
Insulator
EOT ͑nm͒
Gate electrode
Au
Measured
frequency
͑Hz͒
Resistivity
͑⍀ cm͒
Thickness
Type
Material
͑nm͒
Al
G-TiN
p
p
n
n
p
3–5
3–5
2–3 m
2–3 m
3–5
1 M
10 k
1 M
1 M
1 M
HfO2
HfO2
HfO2
HfO2
Al2O3
5
5
5
2.7
3
2.4
2.2
2.2
2.1
3.3
2.5
2.4
2.5
2.1
3.0
1.7
1.6
1.4
1.1
2.4
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3584
Appl. Phys. Lett., Vol. 81, No. 19, 4 November 2002
Saito, Jin, and Shimada
band-gap closure from the interface must coincide with the
T–F screening length in the metal ͑1/2 or 1 atom layer͒. The
narrowed band-gap energies of ultrathin Al2O3 films on
metal substrates18,19 and unchanged barrier energy of ultra-
thin SiO2 films on a silicon substrate20 have recently been
measured and reported. The reported values for metal sub-
strates were very small ͑3.2–ϳ5 eV for Ͻ0.9 nm thick
Al2O3 films and ϳ9 eV for a ϳ3 nm thick film͒,18,19 com-
pared to the values predicted by Inkson and Anderson ͑1–2
eV reduction͒. Since our capacitance measurements do not
directly relate to the band-gap energy, the present results
paradoxically provide strong evidence of screening by the
tail of the wave function. Note that our results do not depend
on insulator thickness. In the frequency region higher than
infrared, the keff change may be different from the present
behavior because the dominant contribution would change
from ion movement to electron excitations.
FIG. 3. EOT versus TaN film thickness. The TaN films ͑2400 ␮⍀ cm͒ were
formed between Al and HfO2 by ECR sputtering.
that of slow traps in low-frequency ͑Ͻ10 kHz͒ C–V mea-
surements or quasistatic C–V measurement.
In oxide insulators, a large dielectric constant in the fre-
quency range below infrared appears due to ionic bonds of
the metal atoms and oxygen. The metal (␦ϩ) and oxygen
(␦Ϫ) ions move in opposite directions for external field and
store large electrostatic energy. If the ions near the metal
could not store a large electrostatic energy due to a strong
screening effect, the capacitance of an ultrathin insulator
must be significantly reduced. For example, if keff of a 0.25
nm thick layer is reduced to 1, a 1 nm EOT increase must
result.
From the standpoint of the screening phenomenon, the
results in Fig. 3 mean that the wave function of the free
electrons of aluminum is effectively attenuated by the 0.3 nm
thick TaN film. It also means that the free electrons of the
TaN film can not effectively screen the ion pairs of HfO2 .
This coincides with the Thomas–Fermi ͑T–F͒ screening
length15 of metals. The T–F screening length can be roughly
estimated as ϳ0.05 nm for the Al film and ϳ0.4 nm for the
TaN film. Since the distance between Hf and O atoms in an
amorphous HfO2 film may be 0.1–0.2 nm, the ion pairs can
be effectively screened with Al but not with TaN. This pre-
sumption includes a basic hypothesis that the tail of the wave
function of the free electrons really shows the screening be-
havior.
In conclusion, a keff-reduction phenomenon caused by
low-resistivity gate electrodes is presented. The EOT in-
crease reaches about 1 nm. The resistivity dependence of the
EOT strongly suggests that the keff-reduction phenomenon is
due to a quantum effect appearing as a result of the progress
of the semiconductor technology trend.
The authors are grateful to Dr. T. Ono ͑NTT Afty Cor-
poration͒, Associate Professor S. Ohmi, and Professor T. Sa-
kai ͑Tokyo Institute of Technology͒, and Dr. Y. Murata for
their valuable discussions.
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Hysteresis
EOT
temperature ͑°C͒
͑mV͒
͑nm͒
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