Improvement in electrical characteristics of high- k Al2O 3 gate dielectric by field-assisted nitric oxidation

Article abstract of DOI:10.1063/1.2402215

The authors perform a simple technique for the improvements in both current density reliability of high- k Al2 O3 gate dielectrics. An Al2 O3 was prepared by chemical oxidation at an appropriate electrical field in nitric acid at room temperature then furnace annealed at 650 °C in N2. The interface trap-induced capacitance was used to investigate the interfacial property between the gate dielectric Si substrate. On the other hand the stress induced leakage current (SILC) was measured for characterizing the property of bulk oxide. It was found that the electrical characteristics of bulk oxide including leakage breakdown field SILC, were much improved without sacrificing interfacial property. The improvement can be ascribed to the compensation oxidation process.

Full text of DOI:10.1063/1.2402215

Improvement in electrical characteristics of high- k Al 2 O 3 gate dielectric by field-
assisted nitric oxidation
Kai-Chieh Chuang and Jenn-Gwo Hwu
Citation: Applied Physics Letters 89, 232903 (2006); doi: 10.1063/1.2402215
View online: http://dx.doi.org/10.1063/1.2402215
View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/89/23?ver=pdfcov
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APPLIED PHYSICS LETTERS 89, 232903 ͑2006͒
Improvement in electrical characteristics of high-k Al O gate dielectric by
2
3
field-assisted nitric oxidation
a͒
Kai-Chieh Chuang and Jenn-Gwo Hwu
EE-II, R446, Department of Electrical Engineering and Graduate Institute of Electronics Engineering,
National Taiwan University, Taipei 10617, Taiwan, Republic of China
͑
Received 10 May 2006; accepted 19 October 2006; published online 6 December 2006͒
The authors perform a simple technique for the improvements in both current density reliability of
high-k Al O gate dielectrics. An Al O was prepared by chemical oxidation at an appropriate
2
3
2
3
electrical field in nitric acid at room temperature then furnace annealed at 650 °C in N . The
2
interface trap-induced capacitance was used to investigate the interfacial property between the gate
dielectric Si substrate. On the other hand the stress induced leakage current ͑SILC͒ was measured
for characterizing the property of bulk oxide. It was found that the electrical characteristics of bulk
oxide including leakage breakdown field SILC, were much improved without sacrificing interfacial
property. The improvement can be ascribed to the compensation oxidation process. © 2006
American Institute of Physics. ͓DOI: 10.1063/1.2402215͔
High-k dielectrics are being considered as a possible re-
after native oxides were removed. Group II was fabricated in
placement for SiO as conventional oxide thickness scaled
the same condition but without applying electrical field dur-
ing nitric oxidation step.
2
into a regime where direct tunneling current dominates the
1
–4
gate leakage. However, some issues associated with high-
k gate dielectrics, such as flatband voltage shift and stress-
induced leakage current ͑SILC͒, are still hurdles for reliable
I-V and C-V measurements were performed by an
HP4140B picoampere meter and an HP 4284 precision im-
pedance meter, respectively. Equivalent oxide thickness
͑EOT͒ was extracted by fitting the 1 kHz C-V curve under
5
high-k gate dielectrics. Several methods were proposed to
1
2
improve high-k gate dielectrics such as N plasma
the consideration of a quantum mechanical effect.
2
6
7
8
treatment, fluorine incorporation, and multiple gate stacks.
Figure 1 displays the 1 MHz and 1 kHz C-V curves of
all samples. The trap densities calculated by the 1 MHz C
Here, we introduce a simple technique to improve high-k
1
0
11
11
−2
Al O gate dielectrics. Al O is a widely investigated gate
-V curves are 2.02ϫ10 , 2.62ϫ10 , and 1.34ϫ10 cm
2
3
2
3
9
dielectric with benefits in many aspects and can be formed
in nitric acid at room temperature without oxidizing
silicon. In this work, metal-oxide semiconductor ͑MOS͒
capacitors with Al O prepared on Si substrate by field-
of FA-HNO3, HNO3, and RTO samples, respectively. Theo-
retical simulation curves are also demonstrated to extract
EOT. The EOTs of FA-HNO3 and HNO3 samples are 2.5
and 2.7 nm, respectively. This means that applying electrical
field makes Al2O3 thin film more compact without increasing
interfacial layer. On the other hand, the 1 kHz CV curve of
1
0
2
3
assisted HNO3 oxidation ͑FA-HNO ͒ were investigated.
3
MOS capacitors with Al O prepared by only nitric oxida-
2
3
HNO sample distorted at large negative bias because of
tion and SiO prepared by rapid thermal oxidation ͑RTO͒
3
2
oxide traps at different energy levels. The hump of the 1 kHz
curves in the depletion region originates from the interface
states that provides interface trap induced capacitance con-
nected with oxide capacitance Cox in series. By examining
the C-V curves in the depletion region, it can be seen that all
were also fabricated for comparison. The results show that
samples prepared by field-assisted oxidation with HNO ex-
3
hibit excellent electrical properties and reliability character-
istics. It is believed that electrical field enhances oxidation
process, and thus reduces weak spots and imperfect
11
bondings.
p-type ͑100͒-oriented Si wafers were treated by modified
RCA clean. Ultrathin SiO was grown by RTO. The thick-
2
ness of RTO SiO was about 8–10 Å as measured by an
2
ellipsometer. 99.9999% pure Al was thermally evaporated
onto the silicon surface with thicknesses of 13–16 Å. These
samples were divided into group I ͑FA-HNO ͒ and group II
3
͑
͑
HNO3͒. Group
I was immersed in diluted HNO3
HNO :H O=1:19͒ with an applied electrical field of
3
2
5
V/cm for 60 s. Platinum plates were used as the elec-
trodes. Postoxidation annealing was implemented in the fur-
nace at 650 °C in N for 60 s. After Al deposition, the
2
−
4
2
2
.25ϫ10 cm Al gate was defined by conventional photo-
lithography and wet etching. The underside of each wafer
was coated with an aluminum film to form MOS capacitors
FIG. 1. C-V curves of ͑a͒ field-assisted nitric oxidation Al O /SiO ͑FA
2
3
2
-
HNO ͒ and ͑b͒ nitric oxidation Al O /SiO ͑HNO ͒ and ͑c͒ RTO samples
3 2 3 2 3
a͒Author to whom correspondence should be addressed; electronic mail:
hwu@cc.ee.ntu.edu.tw
at 1 MHz and 1 KHz. Theoretical curves considering quantum mechanical
effect are also shown.
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003-6951/2006/89͑23͒/232903/3/$23.00 89, 232903-1 © 2006 American Institute of Physics
On: Fri, 12 Dec 2014 08:15:44
0

232903-2
K.-C. Chuang and J.-G. Hwu
Appl. Phys. Lett. 89, 232903 ͑2006͒
samples had comparable interface state density qualitatively.
Furthermore, Fig. 2 shows the Weibull plots of the interface
trap induced capacitance at flatband voltage, which is calcu-
9
lated by modified high-low frequency method. Their inter-
facial properties are quite similar. This can be attributed to
the initial RTO oxide formation.
Figure 3 compares the gate injection current densities at
VG−VFB=−1 V as a function of EOT for RTO oxides, anod-
ization ͑ANO͒ oxides, FA-HNO3 Al O /SiO , and
2
3
2
HNO Al O /SiO . The leakage currents of both FA-HNO
3
2
3
2
3
Al O /SiO , and HNO Al O /SiO are at least one order of
2
3
2
3
2
3
2
magnitude lower than pure SiO oxides. This confirms the
2
basic characteristics of high-k dielectrics. On the other hand,
the leakage current of FA-HNO3 samples and HNO3
FIG. 2. Weibull plots of interface trap induced capacitance Cit at flatband
voltage for FA-HNO3 Al O /SiO , HNO Al O /SiO , and RTO SiO
2
3
2
3
2
3
2
2
samples.
samples are compatible while the FA-HNO samples are 2 Å
3
thinner than HNO samples. This result accounts for the bet-
3
ter bulk oxide properties of FA-HNO3 samples than the
other.
Figure 4 shows the time-zero dielectric breakdown
͑
TZDB͒ endurance of FA-HNO and HNO samples. The
3 3
J-V is also shown in the inset. It is noted that the equivalent
breakdown fields of stacked oxides prepared by FA-HNO3
with EOT=2.5 nm are better than those prepared by HNO3.
This can be ascribed to the better bulk oxide properties that
are consistent with the percolation model which is often used
to explain breakdown phenomenon.
Finally, charge trapping characteristic of Al O gate di-
2
3
electric is investigated by constant current stress as shown in
2
Fig. 5. The stressed current density is −10 mA/cm for
FIG. 3. Relationship between the gate injection current density at V −V
G
FB
1
20 s. The SILC is measured immediately after stress being
=
−1 V and equivalent oxide thickness for FA-HNO3 Al O /SiO , HNO
2 3 2 3
removed. Considerable increase is observed for HNO3
Al O /SiO , RTO SiO , and anodization ͑ANO͒ SiO oxides.
2
3
2
2
2
sample as shown in Fig. 5͑b͒ at about V =−1 V, which cor-
G
responds to the flatband voltage. This increase is probably
due to bulk oxide trap generation as caused by the electrical
1
3
stress. However, little increase is observed except for the
VFB around. This suggests that the stressed induced defects
are localized.
The field-assisted nitric oxidation can improve Al O
2
3
bulk property. This can be explained qualitatively. Chemical
oxidation is a fast, nonuniform process that often results in
1
4
oxygen vacancy and imperfect bonds. If we apply an elec-
trical field during the oxidation process, the weak spots will
be fixed automatically. That is, when the oxide forms, it be-
−
comes less conductive locally, and therefore most of the NO3
ions will impose elsewhere. Thus the insulator is more com-
pact and perfect while keeping minimized interface states.
In this work, a simple technique of applying electrical
field during chemical oxidation process is demonstrated. It is
found that the electrical characteristics of bulk oxide were
much improved while the interface states were kept mini-
mized. The improvement can be ascribed to the compensa-
tion effect during field-assisted oxidation process.
FIG. 4. Accumulative plots of oxide breakdown field of FA-HNO3
Al O /SiO and HNO Al O /SiO stacked oxides. Typical J-V curves for
2
3
2
3
2
3
2
two kinds of samples are shown in the inset.
This work was supported by the National Science Coun-
cil, ROC, under Contract No. NSC94-2215-E-002-004.
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