Spectroscopic ellipsometry characterization of high-k dielectric HfO2 thin
films and the high-temperature annealing effects on their optical properties
Yong Jai Cho, N. V. Nguyen, C. A. Richter, J. R. Ehrstein, Byoung Hun Lee et al.
Citation: Appl. Phys. Lett. 80, 1249 (2002); doi: 10.1063/1.1448384
View online: http://dx.doi.org/10.1063/1.1448384
View Table of Contents: http://apl.aip.org/resource/1/APPLAB/v80/i7
Published by the American Institute of Physics.
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APPLIED PHYSICS LETTERS
VOLUME 80, NUMBER 7
18 FEBRUARY 2002
Spectroscopic ellipsometry characterization of high-k dielectric
HfO2 thin films and the high-temperature annealing effects on
their optical properties
Yong Jai Cho,a) N. V. Nguyen, C. A. Richter, and J. R. Ehrstein
Semiconductor Electronics Division, National Institute of Standards and Technology, Gaithersburg,
Maryland 20899
Byoung Hun Lee and Jack C. Lee
Microelectronics Research Center, R9950, The University of Texas at Austin, Austin, Texas 78758
共Received 31 July 2001; accepted for publication 5 December 2001兲
The optical properties of a set of high-k dielectric HfO2 films annealed at various high temperatures
were determined by spectroscopic ellipsometry. The results show that the characteristics of the
dielectric functions of these films are strongly affected by high temperature annealing. For a sample
annealed at 600 °C, the film becomes polycrystalline, and its dielectric function displays a
distinctive peak at 5.9 eV. On the other hand, the film remains amorphous without the 5.9 eV feature
after 500 °C annealing. To model the dielectric functions, the Tauc–Lorentz dispersion was
successfully adopted for these amorphous and polycrystalline films. The absorption edge was
observed to shift to a higher energy at a high temperature annealing. Defects in the films were shown
to relate to the appearance of a band tail above the absorption edge, and they appear to diminish with
high temperature annealing. © 2002 American Institute of Physics. 关DOI: 10.1063/1.1448384兴
and
Due to the ever-decreasing dimensions used in modern
semiconductor device fabrication, a gate dielectric material is
needed to replace the traditional silicon dioxide which approaches its operational limit.1 Among many candidates of
the dielectric materials such as Ta2 O5 , TiO2 , and ZrO2 ,
HfO2 is one of the most stable oxides on Si.2 In spite of the
promising properties of HfO2 , little has been known about
its optical properties.3 In this work, we have applied spectroscopic ellipsometry 共SE兲 to investigate the optical properties
of a set of HfO2 films, and we will show that their dielectric
functions change as a function of annealing temperatures.
These changes are indicative of polycrystallization and defect reduction which occurs when the films are annealed at
high temperatures.
Reactive dc magnetron sputtering was employed to deposit nominally a 16 nm thick HfO2 film on a p-type Si
wafer at room temperature.4 The wafer was then diced into
three pieces 共samples S5, S6, and S7兲 which were annealed
at temperatures of 500, 600, and 700 °C, respectively, in oxygen ambient. SE data 共␺ and ⌬兲 for these samples were measured on a high-accuracy custom-made spectroscopic ellipsometer.
We adopt the Tauc–Lorentz 共TL兲 dispersion function5 to
characterize the dielectric function ( ⑀ ⫽ ⑀ 1 ⫹i ⑀ 2 ) of the HfO2
films as expressed in the following:
再
AE 0 C 共 E⫺E g 兲 2 1
,
2 2
2
2 2
⑀ 2 共 E 兲 ⫽ 共 E ⫺E 0 兲 ⫹C E E
0, 共 E⭐E g 兲
共 E⬎E g 兲 ,
⑀ 1共 E 兲 ⫽ ⑀ ⬁⫹
冕 ␰␰ ⑀
⬁
Eg
2
2共 ␰ 兲
d␰.
⫺E 2
共2兲
Equations 共1兲 and 共2兲 as functions of the photon energy E are
uniquely defined by four parameters A 共transition matrix element兲, C 共broadening term兲, E 0 共peak transition energy兲,
and E g 共band gap兲.
Using a simple three-phase model consisting of
substrate/film/ambient, we fit the data to obtain the film
thickness and the parameters of the TL dispersion for each
film. Due to high temperature annealing, a suboxide might be
formed at the interface.2 However, we have shown that the
existence of this interface does not adversely effect our conclusion about the impact of high temperature annealing.6
With the simplified three-phase model, the results are shown
in Table I where the fitting residual ␴ is the goodness-of-fit
parameter. The uncertainties of the film thicknesses listed in
Table I were obtained with a 90% confidence limit. Figure
1共a兲 depicts the differences between the fitting and experimental data, which show that the TL dispersion reproduces
almost identically the experimental data except in the spectral region above ⬃5.1 eV. It is noticed that samples S6 and
S7 exhibit substantial oscillatory discrepancies in this spectral range. For the sample S5, no such oscillatory features are
observed. Accordingly, the values of ␴ as seen in Table I for
samples S6 and S7 are larger than that of S5. To eliminate
these oscillations, we will show below that an additional TL
dispersion is needed to model samples S6 and S7.
To verify that the TL dispersion is in fact a correct dispersion for HfO2 , at least in our experimental spectral range
共1.5– 6.3 eV兲, we used data inversion to obtain its pseudodielectric function 具⑀典. In this inversion technique, 具 ⑀ 1 典 and
具 ⑀ 2 典 of the films are calculated from the measured ␺ and ⌬ at
each photon energy using the film thickness obtained from
共1兲
a兲
Guest researcher at NIST and postdoctoral associate of Korea Science and
Engineering Foundation, Taejon, Korea; present address: Korea Research
Institute of Standards and Science, Taejon, Korea; electronic mail:
[email protected]
0003-6951/2002/80(7)/1249/3/$19.00
2
P
␲
1249
© 2002 American Institute of Physics
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1250
Cho et al.
Appl. Phys. Lett., Vol. 80, No. 7, 18 February 2002
TABLE I. Compilation of the fitting results for all samples using one and/or two TL dispersions. A i , E gi , E 0i ,
and C i have units of eV. ⑀ ⬁ and ␴ are dimensionless.
Sample
Dispersion
t ox 共Å兲
⑀⬁
A1
E g1
E 01
C1
A2
E g2
E 02
C2
␴a
S5
S6
S7
S6
S7
One TL
One TL
One TL
Two TL
Two TL
158.8⫾0.7
160.6⫾1.1
165.9⫾0.9
160.0⫾1.1
167.2⫾0.7
3.04
2.53
1.30
1.98
1.72
48.3
105
250
1.22
0.66
4.19
4.94
5.33
3.47
3.28
7.08
7.43
7.64
5.94
5.92
0.95
1.79
4.70
0.56
0.40
¯
¯
¯
257
251
¯
¯
¯
5.65
5.56
¯
¯
¯
7.22
7.36
¯
¯
¯
2.47
2.83
0.53
1.02
0.85
0.79
0.51
a
A smaller value indicates a better fit.
the TL fitting 共see Table I兲. The results are shown by the
symbols in Fig. 2 with the overall uncertainty of 2%, deduced from the uncertainty of the fitting thickness. Since the
ellipsometric equation used in the inversion is the simplified
three-phase model, 具 ⑀ 1 典 and 具 ⑀ 2 典 might contain artificial
structures belonging to the silicon substrate dielectric function, as seen in our case as the small peaks at ⬃3.4 and ⬃4.2
eV. Therefore, these features are ignored in the analysis and
discussions of the HfO2 film dielectric functions. Comparing
with a solid curve generated from the TL dispersion obtained
from the fitting described above, we conclude that TL dispersion is the appropriate analytical dielectric function for
sample S5. However, the inversion 具 ⑀ 1 典 and 具 ⑀ 2 典 spectra of
samples S6 and S7 show a discernible shoulder at ⬃5.9 eV.
We found that to reproduce this peak an additional TL oscillator is needed, which can be implemented by adding a second identical term to the ⑀ 2 expression in Eq. 共1兲. Thus, a
total of nine parameters is needed for two TL dispersions;
these are A 1 , E g1 , E 01 , and C 1 for the first one, A 2 , E g2 ,
E 02 , and C 2 for the second, and ⑀ ⴥ . The results from the
data fitting are listed in Table I. In Fig. 1共b兲, the large ⌬
discrepancies of the samples S6 and S7 have essentially vanished above 5.7 eV, and the goodness-of-fit 共␴兲 has also improved. To compare with the inversion data for S6 and S7,
the two TL dispersion data that were generated by using nine
fitting parameters are shown by the solid curves in Fig. 2. It
is clear that two TL dispersion essentially duplicates the in-
FIG. 1. Discrepancies (⌬ fit⫺⌬ exp) between the experimental (⌬ exp) and
fitted (⌬ fit) data for samples S5, S6, and S7 annealed at 500, 600, and
700 °C, respectively. The fitted curves were obtained by using the: 共a兲 one
TL and the 共b兲 two TL dispersion for the dielectric function of HfO2.
version data except near the absorption edge, which will be
discussed below.
The appearance of the shoulder at 5.9 eV in samples S6
and S7 is due to the change in film structures when annealed
at and above 600 °C. In general, such structures that are absent in its amorphous counterpart are the result of
singularities7 in the interband density of states of the materials. These singularities are caused by the presence of longrange order in materials; i.e., the material becomes polycrystalline or crystalline. Similar characteristics were observed in
Ta2 O5 films when annealed at high temperatures in our previously reported study.8 Therefore, we attribute the appearance of this shoulder to the polycrystallization of HfO2 films.
For sample S5, the film remains amorphous since no such
shoulder is observed at 5.9 eV in its dielectric function. Furthermore, the fact that the shape and magnitude at 5.9 eV of
the dielectric functions of the samples annealed at 600 and
700 °C are almost identical implies that, at these temperatures, the degree of polycrystallization in these films appears
to remain basically unchanged. Figure 3 displays the absorption coefficients near the band edge of all samples. It clearly
shows that the absorption edge shifts to a higher energy
FIG. 2. The real 具 ⑀ 1 典 and imaginary 具 ⑀ 2 典 part of the pseudodielectric functions of HfO2 films, annealed at 500 °C 共S5兲, 600 °C 共S6兲, and 700 °C 共S7兲,
obtained by the inversion technique 共symbols兲 and the best TL fitting 共solid
lines兲. The best fitting was achieved by one TL dispersion for S5 and two TL
dispersion functions for samples S6 and S7.
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Cho et al.
Appl. Phys. Lett., Vol. 80, No. 7, 18 February 2002
FIG. 3. The absorption coefficient, ␣ ⫽4 ␲ k/␭, near the absorption edge
where ␭ is the wavelength of a photon and k is the extinction coefficient
which is calculated from 具 ⑀ 1 典 and 具 ⑀ 2 典 of samples S5, S6, and S7.
when the annealing temperature increases. The same trend
has also been observed in other materials such as amorphous
silicon.9 In additon, in Figs. 2 and 3, in the spectral region
between 4.2 and 5.3 eV, all three samples display absorption
tails of similar shape but different magnitude, which are related to defects in the films. It is known that defects and
disorder have large effects on the optical properties of amorphous materials.10 Similarly, unspecified defects exist in
these HfO2 films, giving rise to the appearance of these tails.
Since the absorption tail becomes smaller with higher temperature annealing, it is reasonable to conclude that the defects were reduced in the films. Furthermore, defect reductions in HfO2 films under high temperature annealing is also
evidenced from a recent study2 by x-ray photoemission spectroscopy.
In summary, spectroscopic ellipsometry was used to investigate the optical properties of high-k dielectric HfO2
films. The results show that the characteristics of the dielec-
1251
tric functions of the films are strongly influenced by structural changes that occur in the films when annealed at high
temperatures. For samples annealed as low as 600 °C, the
film becomes polycrystalline. The resulting dielectric function displays a distinguishing feature at 5.9 eV and can be
modeled by using a two TL dispersion. On the other hand, at
500 °C annealing, the film remains amorphous with the absence of the 5.9 eV feature, and only one TL dispersion is
needed to describe the film’s dielectric function. The
annealing-temperature dependence of the absorption coefficients near the absorption edge of these films was also discussed and related to the film qualities. It was shown that the
absorption edge increases with annealing temperature. As an
application, SE herein was demonstrated to be an effective
technique to analytically characterize high-k gate dielectrics,
a technologically important and challenging class of materials.
This work was supported in part by the NIST Office of
Microelectronics programs and SEMATECH Contract No.
FEPZ001.
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