High-k dielectric stack-ellipsometry and electron diffraction
measurements of interfacial oxides
Kisik Choi? Harlan R. Harris, Sergey Nikishin, Shubhra Gangopadhyay, and
Henryk Temkin
NanoTECH Center, Texas Tech University
Lubbock, TX, 79409
Abstract. With the silicon interface becoming increasingly scrutinized in high dielectric constant materials for SiO2
replacement, fine distinctions in the quality of silicon cleaning can have a large impact on MOS parameters. One of the
cleaning schemes that have potential to replace the industry standard RCA clean with HF/H2O etch is a modified version
of the Shiraki clean. The evolution of Si (100) surface cleaned by the modified Shiraki method has been investigated by
a conventional, single-wave length ellipsometer. Using Low Energy Electron Diffraction (LEED), we have calibrated
the ellipsometric measurement for the as-cleaned silicon surface. It was found that a lower baseline of 0.7-0.9 nm from
ellipsometric measurements could be established as equivalent to a clean, hydrogen passivated surface. To verify the
effect of the interfacial oxide thickness on the dielectric constant of the high-k gate stack, thickness of the thin-oxide
grown under high vacuum condition was measured and correlated with the dielectric constant of the HfO2 gate dielectric
layer.
thus suitable for in-line, real time monitoring of pregate dielectric processes such as cleaning, a calibration
is still required for the initial stage of the native oxide
regrowth on hydrogen terminated surface. The native
oxide is expected to include the hydrogen terminated
layer, at least partially, since the outermost hydrogen
bonds remain until all four bonds of a surface silicon
atom react with oxygen atoms13. High vacuum based
electron diffraction tools such as Low Energy Electron
Diffraction (LEED) can be used to verify the growth
of one or two ad-layers by observing the evolution of
surface reconstruction patterns14 and provide a "clean"
surface calibration for ellipsometric measurements. In
this paper, we establish a correlation between electron
diffraction and ellipsometry for the purpose of
calibration of hydrogen terminated surface and the
subsequent native oxide regrowth.
INTRODUCTION
Decreasing thickness of the high-k gate dielectric
increases the impact of the interfacial silicon oxide on
the gate stack EOT1. Native oxide regrowth before
dielectric deposition is one of the sources of interfacial
oxide2. The other is the reaction between oxidants and
silicon atoms during deposition or the subsequent high
temperature heat treatment3. The silicon substrate
should be protected against native oxide regrowth or
contamination, under atmospheric conditions, after
pre-deposition gate cleaning. Hydrogen passivation of
silicon surface resulting from the HF/H2O etch is the
usual way to achieve this goal4. However, the
efficiency of HF/H2O cleaning in terms of suppression
of the native oxide regrowth varies with many
parameters5"11. Ellipsometric studies of the native
oxide regrowth on hydrogen passivated surfaces show
that it can reach about 0.2 nm12 within a period of one
hour. This is 10% of a 2 nm EOT gate dielectric. A
modified version of Shiraki cleaning was reported to
result in lower regrowth rate of native oxide, compared
to RCA cleaning due to atomically smoother surface
and more complete hydrogen passivation2'12. Though
the ellipsometric measurement is quick and simple and
EXPERIMENTAL
Modified Shiraki cleaning sequence12 such as
HNO3 boiling followed with 3:1:1 Ha:H2O:H2O2 and
1:3 HF/Methanol was applied to Si (100) /?-type
wafers of 1-10 Q, -cm resistivity. Deionized water with
CP683, Characterization and Metrology for VLSI Technology: 2003 International Conference,
edited by D. G. Seiler, A. C. Diebold, T. J. Shaffner, R. McDonald, S. Zollner, R. P. Khosla, and E. M. Secula
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186
the resistivity of 18 Q -cm and semiconductor grade
methanol (99.9 %) were used. In the final etch step,
the specimen was immersed for 1 minute in a Teflon
vessel. After etching the wafer was dried with N2 gas,
without the final water rinse. The etched wafers were
measured with an ellipsometer, in air, assuming a
fixed refractive index of n=1.46, a He-Ne laser
(A,=632.8 nm), and an incident angle of 70°. Each
wafer was measured at three points and the results
averaged to minimize the measurement error. For
LEED measurements, wafers were placed on a holder
and introduced into the vacuum chamber, through a
load-lock, immediately after etching. The vacuum in
the main and LEED chambers, isolated from each
other with a gate valve, was maintained at IxlO'9 Torr
and IxlO" 10 Torr, respectively. The sample was then
heated at 700°C for several minutes to desorb
hydrogen atoms from the silicon surface. The
oxidation process was carried out on hydrogen-free
samples, at 300°C. An oxygen beam with beam
equivalent pressure of 1.2 x 10~4 was focused on the
surface during oxidation. The process time was varied
from 30 seconds to 15 minutes. LEED images were
observed on as-cleaned surface, after hydrogen
desorption, and after the oxidation.
18
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1.0
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1.5
2.0
2.5
Index (n)
FIGURE 1. Variation of the thickness values as a function
of the refractive index input to the ellipsometer.
unreconstructed surface was observed at an
accelerating voltage of 25 V on as-cleaned samples,
Fig, 2a. This pattern is indicative of a clean and
ordered surface2. For the as-cleaned sample, the
diffraction patterns from the crystalline layers located
several layers deeper from the surface appear as the
accelerating voltage increases. Fig. 2b-2d represent
the patterns observed at 63 V, 144 V, and 220 V
respectively. In general, the number of diffraction
spots increases as the electron beam penetrates deeper
into the silicon crystal and thus the number of
diffracting atoms increases. Before the oxidation
experiment, LEED images after hydrogen desorption
were acquired. The first appearance of spots was
detected, again at -25 V. Fig. 3a shows the Si(lOO) (2
x 1)(1 x 2) surface reconstruction at 65 V. The image
RESULTS AND DISCUSSION
Ellipsometric
measurements
of
hydrogen
terminated silicon surface, assuming a refractive index
of silicon dioxide (n=1.46), resulted in values ranging
between 0.7 and 0.9 nm. The native oxide regrowth
generally appears as additional thickness, above these
values. Therefore, the thickness of the hydrogen
termination layer described by the refractive index of
silicon dioxide should be considered an "effective
thickness" 12'15, The thickness of hydrogen passivation
layer was calculated to be in the range of 0.5-0.8 nm
by Yao et a/.15, in a study based on spectroscopic
ellipsometry. Dependence of the calculated thickness
of the hydrogen terminated silicon surface on the
assumed refractive index for a single layer/substrate
model is presented in Figure 1. The thickness curve
has a minimum around n -2.0 with the corresponding
thickness value of -0.7 nm. This agrees with the value
suggested by Ref. 9, which relied on spectroscopic
eilipsometry to measure the passivation layer of a
cleaned sample. Thus the calibration of our null
detection ellipsometer can be considered sufficient for
monolayer oxidation experiments,
LEED measurements were used to verify the
assumption of complete surface passivation with
hydrogen. A clear pattern of the Si(lOO) 1 x 1
FIGURE 2. LEED image of (a) as-cleaned, hydrogen
terminated silicon surface measured at 25 V (b) measured at
63 V (c) measured at 144 V (d) measured at 220 V.
187
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1
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Q
200
400
600
800
1000
Oxidation time (sec)
FIGURE 4. Thickness variation as a function of the
oxidation time,
low and no continuous oxide layer is expected on
samples exposed to oxygen for short times (30 to 180
sec). The initial island formation would generate
atomically rough, irregularly oxidized surface. This
model is supported by relatively large standard
deviations seen in these samples, consistent with
scattering of the laser beam. It is known that the
scattering of polarized beam can be a source of
thickness deviation in ellipsometry15. A discrepancy
between the thickness values from the hydrogenterminated surface (-0.8 nm) and hydrogen-free
surface (-1.4 nm) at t = 0 can be partially explained by
this model. The reduction of both the thickness and
standard deviation for 180 sec sample provides
additional support. On the other hand, the saturation of
thickness at longer oxidation times is likely to indicate
the formation of uniformly grown, passivating oxide
layer.
In-situ HfO2 film deposition was conducted on
another set of samples oxidized under conditions
shown in Fig. 4. A hafnium source was focused on the
sample in addition to the oxygen source to facilitate
growth of amorphous HfO2. The dielectric constant
was measured by C-V analysis to verify the effect of
the SiO2 thickness. Experimental details of the Metal
Insulator Semiconductor structure fabrication with
HfO2 film were reported elsewhere2'16. Fig. 5 shows
the dielectric constant of HfO2 plotted versus oxidation
time. The dielectric constant increases from -20 to
about 26 with increasing oxidation time. While the
correlation with the data of Fig. 4 appears very strong,
it should be analyzed in terms of chemical reactions of
the Hf/O/Si system. We believe, that for the lower
oxidation times, the incomplete surface oxidation
allows interaction of the Hf7O/Si to form an interlayer
that is significant compared to the overall stack.
However, for the samples where complete oxygen
passivation has occurred, the interaction is prevented
FIGURE 3. LEED images of (a) Si(lOO) (2xl)(lx2)
reconstruction after hydrogen desorption measured at 65 V
(b) after 30 seconds of oxidation measured at 40 V (c) after 2
minutes of oxidation measured at 120 V (d) after 15 minutes
of oxidation measured at 240 V.
indicates four primary cells, with the largest
diffraction spots correlating to the diffraction spots in
figure 2a. The half order spots between the primary
spots result from buckled dimers in the [-110]
direction. The lowest accelerating voltage (Vs) at
which the first diffraction spot appears increases as the
surface disorder increases. Such is the case when oxide
is adsorbed on the surface since the coherent
diffraction form surface structure is attenuated by the
foreign adsorbent. The accelerating voltage increased
to Vs=40 V for a sample oxidized for 30 seconds as
shown in Fig. 3b and farther increased to 120 V for a
sample oxidized for 2 minutes as shown in Fig. 3c. In
addition, the background haziness enlarges as the
inelastic scattering from amorphous layer on the
surface increases. After 30 minutes of oxidation, only
a faint spot could be detected from very hazy
background image even at 240 V, Fig. 3d. Therefore,
we conclude that Vs-25 V corresponds to the oxidefree hydrogen terminated surface which, under
ellipsometry, produces the effective oxide thickness of
0.7 -0.9 nm.
Fig. 4 plots the oxide thickness as a function of the
oxidation time. Note that the oxidation was carried out
after thermal desorption of hydrogen atoms from the
surface. It is found that the thickness decreases
initially as the oxidation time increases, before
saturating at 0,9 - LO nm. The higher ellipsometric
thickness at short oxidation times can be rationalized
by the formation of low-density oxide islands. Given
the low oxygen partial pressure and low substrate
temperature, the oxide island formation rate should be
188
4. W. Kern, Handbook of Semiconductor Wafer
Cleaning Technology, W. Kern, Ed., Noyes
Publications, Park Ridge, NJ. 1993
30
5.
G. Mende, J. Finster, D. Flamm, and D. Schulze,
Surf. Sci. 128 169 (1983)
6.
D. Graf, M. Grundner, and R. Schulz, J. AppL
Phys. 68(10), 5155(1990)
B 25
S
8
•8
ts
•S
20
7. M. Niwano, J. Kageyama, and N. Miayamoto, J.
Appl. Phys. 76 (4) 2157 (1994)
2
20
40
60
80
100
Oxidation time (min)
8. T. Miura, M. Niwano, D. Shoji, and N. Miyamoto,
Appl. Surf. Sci. 100/101 454 (1996)
FIGURE 5. Variation of the dielectric constant of
HfO2/SiO2 MIS structure as a function of oxidation time for
interfacial oxide.
9. T. Hattori, T. Aiba, E. lijima, Y. Okube, and M.
Katayama, Appl. Surf. Sci. 104/105 323 (1996)
so that the interlayer is minimized. Examination of the
bulk and interfacial properties of the HfO2 films
deposited from atomic sources will be reported in
another communication.
In conclusion, we have used LEED to verify
hydrogen termination of a clean silicon surface and
used it to establish the baseline effective thickness for
ellipsometric measurements. We show that the
regrowth of native oxide on the silicon surface can be
monitored by ellipsometry, assuming a fixed refractive
index, making it a satisfactory method of evaluating
the pre-gate cleaning processes. Thickness of the oxide
grown under high vacuum condition was measured
and correlated with the dielectric constant of the HfO2
gate dielectric layer.
10. H. Ikeda, Y. Nakagawa, S. Zaima, Y. Ishibashi,
and Y. Yasuda, Jpn. J. AppL Phys. Vol 38, 3422
(1999)
11. X. Zhang, E. Grafunkel, Y. J. Chabal, S. B.
Christman, and E. E. Chaban, AppL Phys. Lett. 79
(24), 4051 (2001)
12. K. Choi, H. Harris, S. Gangopadhyay, and H.
Temkin, Proceedings of MRS Conference, Spring
2003
13. T. Miura, M. Niwano, D. Shoji, andN. Miyamoto,
Appl. Surf. Sci. 100/101 454 (1996)
14. J. Dabrowski, H. J, Mussig, Silicon Surfaces and
Formation of Interfaces, World Scientific, 2000
ACKNOWLEDGMENTS
15. H. Yao, J. A. Wollam and S. A. Alterovitz, AppL
Phys. Lett. 62, 3324 (1993)
This work was supported by the National Science
Foundation (under grant ECS0223109) and the J. F
Maddox Foundation.
16. H. Harris, K. Choi, S. Gangopadhyay, and H.
Temkin, AppL Phys. Lett. 81, 1065 (2002)
REFERENCES
1. D. A. Buchanan, IBM J. Res. Develop. Vol. 43, No.
3,245(1999)
2. K. Choi9 H. Harris, S. Gangopadhyay, and H.
Temkin, to be published to J. Vac. Sci. Technol. A.
21 3, (2003)
3. B. K. Park, J. Park, M. Cho, and C. S. Hwang,
AppL Phys. Lett. 80 2368 (2002)
189