Fluorine effects on the dipole structures of the Al2 O3 thin films and characterization by spectroscopic ellipsometry

Article abstract of DOI:10.1063/1.2730581

Fluorine-terminated chemical bonds and dipole structures within an Al2 O3 layer were investigated in this study. Spectroscopic ellipsometry (SE) was employed to estimate the characteristics of fluorinated Al2 O3. The parameters S0 and λ0 extracted from SE were analyzed to detect the influence of F-terminated bonding. Al-O bonds were replaced by the Al-F bonds after fluorine was implanted on the silicon substrate. A physical model of dipole structures was proposed explaining the F incorporation phenomenon.

Full text of DOI:10.1063/1.2730581

Fluorine effects on the dipole structures of the Al 2 O 3 thin films and characterization
by spectroscopic ellipsometry
Chao Sung Lai, Kung Ming Fan, Hsing Kan Peng, Shian Jyh Lin, Chung Yuan Lee, and Chi Fong Ai
Citation: Applied Physics Letters 90, 172904 (2007); doi: 10.1063/1.2730581
View online: http://dx.doi.org/10.1063/1.2730581
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APPLIED PHYSICS LETTERS 90, 172904 ͑2007͒
Fluorine effects on the dipole structures of the Al O thin films
2
3
and characterization by spectroscopic ellipsometry
a͒
Chao Sung Lai, Kung Ming Fan, and Hsing Kan Peng
Department of Electronic Engineering, Chang Gung University, 259 Wen-Hwa 1st Rd., Kwei-Shan,
Tao-Yuan 333, Taiwan
Shian Jyh Lin and Chung Yuan Lee
Nanya Technology Corporation, Hwa-Ya Technology Park, 669 Fu-Hsing 3rd Rd., Kwei-Shan,
Tao-Yuan 333, Taiwan
Chi Fong Ai
Institute of Nuclear Energy Research, Atomic Energy Council, No. 1000, Wenhua Rd., Lungtan, Taoyuan
3
25, Taiwan
Received 26 December 2006; accepted 23 March 2007; published online 23 April 2007͒
Fluorine-terminated chemical bonds and dipole structures within an Al O layer were investigated
͑
2
3
in this study. Spectroscopic ellipsometry ͑SE͒ was employed to estimate the characteristics of
fluorinated Al O . The parameters S and ␭ extracted from SE were analyzed to detect the influence
2
3
0
0
of F-terminated bonding. Al–O bonds were replaced by the Al–F bonds after fluorine was implanted
on the silicon substrate. A physical model of dipole structures was proposed explaining the F
incorporation phenomenon. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2730581͔
For many years, silicon dioxide and oxynitride have
been used as gate dielectrics in complementary metal-oxide-
semiconductor or dynamic random access memory applica-
tions. However, as devices are scaling down, these dielectrics
suffer from several issues, including high gate leakage cur-
rent, degraded oxide reliability, and low gate capacitance.
F19+ species followed by standard RCA cleaning and rapid
thermal nitridation. Al O with 4.5 nm physical thickness
was deposited by atomic layer deposition with precursors of
2
3
trimethylaluminum and O . For comparison, fluorine doped
3
on bulk Al O was created by a plasma enhanced chemical
2
3
vapor deposition apparatus at a substrate temperature of
1
–3
High-k gate dielectrics, e.g., HfO , Al O , etc.,
which
2
2
3
300 °C. The rf power was 50 W with a CF plasma exposure
4
have been studied in recent decades, offer several advan-
tages, including higher dielectric constant, lower leakage
current at the same electrical thickness, and higher current
drive ability. Although the improvement in the reliability,
thermal stability, and channel mobility of high-k dielectric
materials has been researched extensively, problems such as
interface trap charges, oxide trap charges, and capacitance
frequency dispersion still persist. Many approaches have
been introduced to improve gate oxide integrity, such as
nitrogen/fluorine incorporation in bulk high-k films or on the
time of 5 min. A TiN/Ti stacked gate electrode of 150 nm
thickness was deposited by reactive sputtering with mixed
gases Ar and N . Post-metal-anneal ͑PMA͒ was carried out at
2
4
00 °C for 30 min in N ambient by furnace annealing sys-
2
tem. Electron spectroscopy for chemical analysis ͑ESCA͒
was used to monitor the F bonding by a Mg K␣ radiation
source. The distribution of F atoms was analyzed by second-
ary ion mass spectroscopy ͑SIMS͒. Refractive index disper-
sion was performed by spectroscopic ellipsometry.
Figure 1͑a͒ shows the fluorine SIMS depth profile of the
MOS capacitor structure with various F-incorporated meth-
ods. It is obvious that the F intensity is highest for both the
4
silicon substrate surface. In our previous work, we demon-
strated that hysteresis and the oxide reliability of Al O films
2
3
could be improved by fluorine incorporation. Several articles
reported that the capacitance equivalent thickness ͑CET͒ of
insulators will degrade as F atoms are implanted, which was
F-implantation and the CF plasma processes. In addition,
4
the CF plasma process incorporated F more efficiently than
4
5
,6
the conventional F-implantation process. The ESCA spectra
of F1s are shown in Fig. 1͑b͒, which reveals that the highest
intensity of F–Al bonding occurs at a binding energy of
687.5 eV for the sample treated by the F-implantation and
attributed to either the growth of an extra interface layer or
7
,8
to a lower dielectric constant. However, there are no ap-
propriate ways to investigate the mechanism by which fluo-
rine or nitrogen affects dielectric film characteristics.
In our present work we used two different F-doped ap-
CF plasma approaches. This implies that F atoms could be
4
proaches, including CF plasma treatment and conventional
incorporated and doped into bulk Al O by a conventional
4
2
3
F implantation, to study the effects of F-terminated dielectric
films. We introduced the Sellmeier formula to verify the ex-
istence of the Al–F bond structure. Furthermore, it was dem-
implanter and by CF plasma treatment. However, the peak
4
values of F intensity would slightly decrease after the PMA
process as shown in Fig. 1͑b͒. This could be attributed to the
outdiffusion of F species from the silicon surface or bulk-
Al O dielectric when samples are treated at an elevated tem-
onstrated that F treated Al O samples could influence the
2
3
dipole oscillator strength and oscillator position.
2
3
In the present study, P-type Si ͑100͒ substrates were fab-
ricated as metal-oxide-semiconductor ͑MOS͒ capacitors. Se-
lected samples were first implanted on the silicon surface by
perature. Despite the outdiffused F atoms, the residual F in
the Al O still dominates the characteristics of the dielectric
2
3
thin films. To evaluate the effects of F on the films, the
dipole structures of F-terminated Al O were examined by
2
3
a͒Electronic mail: cslai@mail.cgu.edu.tw
spectroscopic ellipsometry ͑SE͒ analysis.
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003-6951/2007/90͑17͒/172904/3/$23.00 90, 172904-1 © 2007 American Institute of Physics
29.49.23.145 On: Thu, 18 Dec 2014 05:23:41
0
1

172904-2
Lai et al.
Appl. Phys. Lett. 90, 172904 ͑2007͒
FIG. 2. ͑a͒ Dispersion refractive index curves of fluorinated Al O films. ͑b͒
2
3
FIG. 1. ͑a͒ SIMS depth profiles of F atoms for different F incorporation
2
2
Plot of 1/͑n −1͒ vs 1/␭ for fluorinated Al O films. The S and ␭ can be
1
4
−2
2
3
0
0
methods. F-implantation dosage is 2ϫ10 cm and the exposure time of
the CF plasma is 5 min. ͑b͒ ESCA spectra for F peaks of different process
2
extracted from the slope of the fitted curve and the intercept of the 1/͑n
1͒ axis, respectively.
4
1s
−
conditions ͑F implantation and CF4 plasma͒. PMA was carried out by N2
RTA at 400 °C.
Ne2
͑␻͒ = 1 + ␧
fj
2
␧
,
͑2͒
Refractive index ͑n͒ as a function of incident light wave-
length ͑␭͒ for different F-implantation dosage samples was
measured by ellipsometry, as shown in Fig. 2͑a͒. The wave-
lengths of incident light varied from 417 to 751 nm. It can
be observed that the refractive index decreases as the sample
fluorine dosage increases. It is well known that the relative
dielectric constant ␧ is the square of the refractive index for
͚
2
0m0 j ͑␻ − ␻ − i␥ ␻͒
0j
j
where ␧, N, e, m , and ␧ are the relative dielectric constant,
number of atoms per unit volume, electron charge, mass of
light atoms, and permittivity in vacuum, respectively. ␻0j is
one of the resonant frequencies of a particular dipole. f is
0
0
j
the oscillator strength of the jth oscillator. ␥ is the damping
j
9
a sample of negligible absorption coefficient. In other
rate of a particular resonance. We assume that only one os-
cillator dominates the dispersion ͑j=1͒ at the closest reso-
nant frequency in the Al O films, so that we are able to
simplify Eq. ͑2͒ to Eq. ͑1͒. Comparing Eqs. ͑1͒ and ͑2͒ in the
region far from resonant frequency, we obtain
words, a sample with a higher F concentration has a lower
optical dielectric constant. In order to extract the change in
the dielectric dipole structure from the SE analysis data, we
should transform Fig. 2͑a͒ into another expression. Figure
2
3
2
1
͑b͒ shows the dispersion of the refractive index, plotted as
Ne2fj
2 2
/͑n −1͒ vs 1/␭ , from which S and ␭ are extracted from
0
0
S0 =
.
͑3͒
2
9
–11
␧0
m ͑2␲c͒
0
the fitted straight line of the Sellmeier formula:
S ␭2
The average oscillator strengths for the F-implanted dosages
0
o
n2 − 1 =
,
͑1͒
1
3
14
−2
14
− ͑␭ /␭͒2
of 5ϫ10 and 2ϫ10 cm are 2.36ϫ10 and 1.04
1
o
14
−2
ϫ10 m , respectively. S decreases with increasing
0
where n is the refractive index, S is the average oscillator
F-implanted atoms, as demonstrated in Fig. 2͑b͒. The values
0
strength, and ␭ is the average oscillator position. Equation
of the parameters in Eq. ͑3͒ are independent of the F con-
0
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͑1͒ is extracted from
centrations, except for m . In other words, Al–F instead of
0
1
29.49.23.145 On: Thu, 18 Dec 2014 05:23:41

172904-3
Lai et al.
Appl. Phys. Lett. 90, 172904 ͑2007͒
tively. M− can represent the oxygen or fluorine atomic
masses in our experiment because they are negatively
charged ions. The decrease in ␻ could be the reason for the
0
increase in M, which implies that M− is changed from a
heavier to a lighter mass atom. In other words, lower reso-
nant frequency Al–F dipoles are substituted for higher reso-
nant frequency of Al–O dipoles. Therefore, the extracted S0
values are consistent with the ␭ , which also implies that
0
Al–F dipole structures are exchanged for Al–O dipoles. Fig-
ure 3 is a schematic of the dipole structures of Al–O and
Al–F bonds with specific resonant frequencies ␻ and ␻ ,
1
2
respectively, as defined in Eq. ͑4͒ or ͑5͒. ⌬X is the displace-
ment of O or F atoms from their equilibrium position.
9
F atoms have a lower ⌬X because of their heavier mass;
therefore, the dipole moment is smaller for F bonding than
the O bonding. The F-incorporated films would induce low-
ering of the dielectric constant or increase the CET ͑not
shown͒, as a result of the decrease in the dielectric dipole
moment for F-terminated thin films.
FIG. 3. ͑Color online͒ Schematic model of Al–O and Al–F oscillator di-
poles. Al–F dipoles have lower polarizability because of their smaller dis-
placement ⌬X.
In conclusion, the characteristics of Al O films with F
2
3
Al–O chemical bonds are formed by fluorine incorporation,
implanted on silicon substrates were studied by spectro-
which causes larger m for the Al–F dipoles than the Al–O
scopic ellipsometry. Extracted S and ␭ parameters from
0
0
0
dipoles because the mass of the F atom is larger than that of
the O atom. Therefore, the Al–O bonds are gradually re-
placed by Al–F bonds with increasing fluorine concentra-
tions, which can be determined from the fitted data of the
optical ellipsometry analysis. From the point of view of the
the fitted Sellmeier model demonstrated that Al–O bonds
were replaced by Al–F bonds during fluorine incorporation.
The average oscillator strength and position represent the
behavior of dipoles within the Al O films. This approach
was found to be suitable for estimation of future gate oxide
applications.
2
3
extracted parameter ␭ ,␭0 is largest for the highest
0
F-implantation dosage. The average oscillator positions, ␭ ,
0
1
for the F-implanted dosages of 5ϫ1013 and 2ϫ10 cm
14
−2
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␻0
instead of ␭ . The resonant frequency ␻ is related to ␭
0
0
0
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␲c
␻0
=
,
͑4͒
7
6.
␭0
4
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where c is the velocity of light in free space. A lower oscil-
lator ␻ for a sample of heavier F-implantation dose was
0
5
6
7
8
9
0
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1
deduced from the values of ␭ and Eq. ͑4͒. This is because
the resonant frequency ␻ of dielectric dipoles is inversely
proportional to ␭ . With regard to oscillator vibration, the
dipoles such as Al–O or Al–F are assumed to act like springs,
as shown in Fig. 3, which have a resonant frequency
0
0
0
k
1
1
1
␻0
=
ͱ
,
=
+
,
͑5͒
M
M
M− M+
1
where k is the spring constant. M, M , and M are the re-
−
+
1
duced mass and negative and positive ion masses, respec-
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1