Download Reaction of Hydrogen-Desorbed Si(100) Surfaces with Water during

Japanese Journal of Applied Physics
Vol. 43, No. 12, 2004, pp. 8242–8247
#2004 The Japan Society of Applied Physics
Reaction of Hydrogen-Desorbed Si(100) Surfaces with Water during Heating and Cooling
Shinichi U RABE, Kazuo N ISHIMURA, Satoru M ORITA and Mizuho M ORITA
Department of Precision Science and Technology, Graduate School of Engineering, Osaka University,
2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
(Received June 11, 2004; accepted September 10, 2004; published December 9, 2004)
The reaction of hydrogen-terminated Si(100) surfaces with water during heating is analyzed using a combination of heating
and cooling in thermal desorption spectroscopy. The reaction of the Si surface with water after hydrogen desorption is
observed even at about 400 C at a low concentration of water molecules. An estimation method of surface hydrogen coverage
by the combined measurement is proposed and the surface coverage as a function of temperature is estimated for quantitative
understanding of hydrogen desorption and the subsequent reaction during heating. The combined measurement in thermal
desorption spectroscopy is useful for revealing the reaction after hydrogen desorption during heating and for estimating the
surface coverage. [DOI: 10.1143/JJAP.43.8242]
KEYWORDS: silicon, surface, hydrogen, water, reaction, thermal desorption spectroscopy, temperature-programmed desorption
Introduction
Ultrathin silicon dioxide (SiO2 ) films with high electrical
insulating performance and high reliability are continuously
demanded to realize ultrasmall metal-oxide-semiconductor
field-effect transistors for gigascale integrated chips. It is
essential to control the silicon (Si) surface condition during
the temperature ramp-up process in order to form highperformance ultrathin SiO2 films.1–3) The dielectric breakdown characteristics of SiO2 films have been reported to
depend on the heating condition.4–6) Therefore, it is very
important to analyze the Si surface during the temperature
ramp-up process or Si surfaces at elevated temperatures
immediately before thermal oxidation.
Thermal desorption spectroscopy (TDS) (or temperatureprogrammed desorption) has been widely used to analyze
surface conditions of Si wafers after wet cleaning.7–10) TDS
also is useful as an in situ characterization tool of the
chemical reactions of Si surfaces with oxygen or water.9)
TDS has been applied to analyze oxidation reactions of
hydrogen-terminated Si(100) surfaces in both heating and
cooling processes after hydrogen desorption.11)
In this paper, we discuss the reaction of hydrogenterminated Si(100) surfaces with water at elevated temperatures based on the TDS results obtained by a combination
of heating and cooling. We also propose a method of
estimating the surface hydrogen coverage from the desorption spectra obtained by combined measurements and
actually estimate the surface coverage as a function of
temperature from the TDS results.
2.
temperature and 1000 C with a linear change of the sample
temperature with time at a heating and cooling rate of
0.5 C/s (30 C/min). The base pressure was less than 4 107 Pa (3 109 Torr) immediately before the TDS measurement. The temperature was calibrated with a thermocouple-instrumented Si chip (SensArray).14) The pumping speed
(S) of the turbomolecular pump and the volume (V) of the
analysis chamber in the TDS system are 400 l/s and about
7 l, respectively. The characteristic pumping time ( ¼ V=S)
is calculated to be about 0.02 s. For example, the total sweep
time from 50 to 1000 C at the heating rate of 0.5 C/s is
1900 s. The total sweep time is much longer than the
characteristic pumping time under this TDS measurement
condition. Thus, it can be approximated that the desorption
rate is proportional to pressure.15)
3.
Figure 1 shows the thermal desorption spectra of hydrogen and water from the HF-cleaned Si(100) surface during
heating to 1000 C and subsequent cooling. The ion
intensities of atomic and molecular oxygen were of the
order of 1013 A or lower than 1013 A under this TDS
measurement condition. The hydrogen spectrum has peaks at
about 400 and 510 C during heating. This indicates that a
hydrogen-terminated Si(100) surface was prepared as the
Experimental
The wafers used in these experiments were Czochralski
growth, boron-doped p-type Si(100) wafers with a resistivity
of 8–10 cm. Wafers with a thickness of 0.625 mm were
cut to the area of 10 mm 10 mm. The samples were
chemically cleaned with an H2 SO4 /H2 O2 solution and
ultrapure water with 5 ppm ozone,12) etched with diluted
HF solution, rinsed with ultrapure water, and then dried by
blowing nitrogen gas. The sample was loaded into the TDS
system (ESCO, EMD-WA1000S) within 1 min after blowing
nitrogen gas.13)
The thermal desorption spectra of hydrogen (mass number
M=e ¼ 2) and water (18) were measured between room
Results
ION INTENSITY (A)
1.
10−10
H2
H2O
10
−11
0
200
400
600
800
1000
TEMPERATURE (°C)
Fig. 1. Thermal desorption spectra of hydrogen and water from hydrogenterminated Si(100) surface during heating to 1000 C and subsequent
cooling. The solid and broken lines denote the spectra measured during
heating and cooling, respectively.
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Jpn. J. Appl. Phys., Vol. 43, No. 12 (2004)
S. URABE et al.
ION INTENSITY (A)
−10
10
H2
−11
10
H2O
0
200
400
600
800
1000
TEMPERATURE (°C)
Fig. 2. Thermal desorption spectra of hydrogen and water from the inner
wall of the TDS analysis chamber during heating and subsequent cooling.
The solid and broken lines denote the spectra measured during heating
and cooling, respectively.
sample.9) The water spectrum has humps at about 370 and
740 C during heating. This may indicate that the reaction of
the hydrogen-terminated Si(100) surface with water begins
to occur at those temperatures.9)
Figure 2 shows the thermal desorption spectra of hydrogen and water from the inner wall of the TDS analysis
chamber. These spectra were measured under the condition
that no Si sample was set on the quartz sample stage in the
chamber. In the TDS system, the infrared lamp for heating
the sample on the stage was positioned below the stage and
the stage temperature was read with the thermocouple
mounted in the sample stage. The horizontal axis of Fig. 2
indicates the apparent temperature that was calculated from
the stage temperature using the calibration data obtained
from a thermocouple-instrumental Si chip. The maximum
apparent temperature does not reach 1000 C in Fig. 2,
although the Si sample temperature calculated using the
same stage temperature reaches about 1000 C in Fig. 1. It is
considered that the lamp power became the maximum
because the light intensity under the no-sample condition
must be higher than that under the sample-set condition for
the same sample stage temperature in the TDS system.
Under the no-sample condition, one zone in the top flange of
the stainless-steel chamber was heated by irradiating light
through the quartz sample stage from the lamp. Thus the
spectra shown in Fig. 2 are not precisely the background of
the spectra in Fig. 1, because the temperature of the lightirradiated zone of the chamber under the no-sample
condition might be higher than that under the sample-set
condition, and the light intensity might be different between
the apparent temperature under the no-sample condition and
the Si sample temperature under the sample-set condition for
the same sample stage temperature. However, the spectra in
Fig. 2 are helpful for understanding the desorption of
hydrogen and water from the inner wall.
In Fig. 2, the hydrogen and water ion intensities during
heating begin to increase at about 800 C. The increase is
also observed in Fig. 1. The hydrogen intensity during
cooling first decreases, begins to increase at about 900 C,
and then decreases at around 600 C, although the hydrogen
intensity monotonically decreases under the sample-set
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condition in Fig. 1. The water intensity during cooling first
increases and then begins to decrease at about 950 C under
the no-sample condition and at about 900 C under the
sample-set condition in Fig. 1. The increases of the hydrogen and water intensities during cooling are considered to be
due to the inner wall, which is positioned outside the lightirradiated zone, being slowly heated by thermal conduction
from that zone. The hydrogen and water intensities during
cooling do not coincide with those during heating, even at
about 100 C in Figs. 1 and 2. It is considered that the inner
wall did not cooled down quickly. Thus the contribution of
hydrogen and water desorption from the inner wall to the
spectra in Fig. 1 can be elucidated from the result in Fig. 2.
Figures 3(a) to 3(d) show thermal desorption spectra of
hydrogen and water from the hydrogen-terminated Si(100)
surface during heating to 300, 400, 500 and 600 C,
respectively, followed by cooling to 100 C, and subsequent
heating to 1000 C. The thinner lines denote the spectra
measured during simple heating to 1000 C (as shown in
Fig. 1). In Fig. 3(a), the hydrogen spectrum for the heating/
cooling combination nearly overlaps that of simple heating.
The water intensity between 100 and 300 C during cooling
and the second heating stage is slightly lower than that
during the first heating stage. Comparison of this result with
the water desorption spectrum measured during heating to
about 300 C followed by cooling to about 100 C and
subsequent heating to about 1000 C under the no-sample
condition could not clarify whether the reaction of the
Si(100) surface with water took place after hydrogen
desorption even at 300 C where the increase of hydrogen
intensity during heating was small. A decrease of water
intensity during cooling in the spectral result measured
during the cycle of heating to 200 C, cooling to 100 C and
subsequent heating to 1000 C, was not observed under this
TDS measurement condition.
In Fig. 3(b), the hydrogen spectrum in the first heating
stage of the combined method overlaps with that for simple
heating. The height of the hydrogen peak at about 400 C in
the second heating stage is lower than that in simple heating.
The water intensity between 100 and 400 C during cooling
and the second heating stage is lower than that in the first
heating stage. These suggest that the Si surface sites after
hydrogen desorption react with water molecules. It is
considered that some hydrogen-desorbed sites in the Si
surface remain after the first heating stage and so the
reaction continues during cooling and the second heating
stage to about 450 C, because the concentration of water
molecules is low in the analysis chamber.11)
The water intensity between 100 and 500 C during
cooling and the second heating stage is much lower than
that during the first heating stage, as shown in Fig. 3(c). In
the second heating stage, a hydrogen peak is not observed at
about 400 C and the hydrogen peak height at about 510 C is
lower than that during simple heating. The hydrogen peak
height at about 510 C in the second heating stage when the
first heating stage was to 600 C is lower than that when the
first heating stage was to 500 C, as shown in Figs. 3(c) and
3(d). These results suggest that the surface concentration of
hydrogen-desorbed sites that can react with water is higher
when the sample is heated to higher temperatures in the first
heating stage.
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−10
ION INTENSITY (A)
ION INTENSITY (A)
−10
10
H2
−11
10
H2O
0
200
400
600
800
10
H2
−11
10
H2O
1000
0
200
TEMPERATURE (°C)
400
(a)
ION INTENSITY (A)
ION INTENSITY (A)
−10
H2
−11
10
H2O
200
400
800
1000
(b)
10
0
600
TEMPERATURE (°C)
600
800
−10
10
H2
−11
10
H2O
1000
0
200
TEMPERATURE (°C)
400
600
800
1000
TEMPERATURE (°C)
(c)
(d)
4.
Discussion
Figure 4 shows the thermal desorption spectrum of
hydrogen from the hydrogen-terminated Si(100) surface
during heating to 500 C, cooling to 100 C and subsequent
heating to 1000 C. The hydrogen intensity is plotted on a
linear scale. This spectrum was obtained by subtracting the
minimum intensity from the hydrogen spectrum shown in
Fig. 3(c). The removal of the horizontal baseline is a crude
and convenient technique for visual improvement of a
spectrum. The first heating curve h1 contains two overlapping peaks, while the second curve h2 has one peak. This
suggests that the data may be analyzed by subtracting one
heating curve from the other in the spectrum. The proper
application of subtraction requires that the curves be
adjusted such that the intensities are the same at appropriate
temperature. A normalization factor is defined as the ratio of
the curve h1 intensity to the curve h2 intensity at the
1.0
-10
In Figs. 3(b) to 3(d), the water intensity begins to decrease
at about 370 C during first heating stage and continually
decreases during cooling. Therefore, it can be concluded that
the combination method is effective for revealing the
reaction of the Si surface with water at a low concentration
of water molecules.
ION INTENSITY ( 10 A)
Fig. 3. Thermal desorption spectra of hydrogen and water from hydrogen-terminated Si(100) surfaces during heating to (a) 300 C,
(b) 400 C, (c) 500 C and (d) 600 C, followed by cooling to 100 C and subsequent heating to 1000 C. The thinner lines denote the
spectra measured during simple heating to 1000 C.
H2
0.5
h1
c1
h2
0.0
0
200
400
600
800
1000
TEMPERATURE (°C)
Fig. 4. Thermal desorption spectrum of hydrogen from hydrogen-terminated Si(100) surface during heating to 500 C, cooling to 100 C and
subsequent heating to 1000 C. The broken line denotes the curve
measured during cooling. The hydrogen intensity is plotted on a linear
scale.
maximum temperature of curve h1, which in this case is
500 C. This normalization requires that all the intensities in
curve h2 is multiplied by the factor before the difference
spectrum is calculated.
1.0
S. URABE et al.
ION INTENSITY ( 10 -10A)
-10
ION INTENSITY ( 10 A)
Jpn. J. Appl. Phys., Vol. 43, No. 12 (2004)
H2
0.5
0.0
0
200
400
600
800
1000
TEMPERATURE (°C)
ION INTENSITY ( 10 -10 A)
1.0
H2
0.5
h1
c1
h2s
0
200
400
600
h2
800
H2
0.5
0.0
0
200
400
600
800
1000
TEMPERATURE (°C)
Fig. 5. Hydrogen desorption peak at about 400 C.
0.0
1.0
8245
1000
TEMPERATURE (°C)
Fig. 6. Thermal desorption spectrum of hydrogen from hydrogen-terminated Si(100) surface during heating to 800 C, cooling to 100 C and
subsequent heating to 1000 C. The broken line denotes the curve
measured during cooling. The thinner solid line denotes a fitted linear
background. The hydrogen intensity is plotted on a linear scale.
Figure 5 shows the hydrogen desorption peak at about
400 C. This is the difference spectrum obtained by the
subtraction of normalized curve h2 from curve h1 in Fig. 4.
The hydrogen intensity rises at about 370 C on the lowtemperature side of the peak. The difference spectrum
reflects only the marked increase of hydrogen intensity in the
simple heating spectrum shown in Fig. 1.
Figure 6 shows the thermal desorption spectrum of
hydrogen from the hydrogen-terminated Si(100) surface
during heating to 800 C, cooling to 100 C and subsequent
heating to 1000 C. This is the spectrum after the removal of
the horizontal baseline. The second heating curve h2 nearly
overlaps the first curve h1 at about 100 and 800 C. This
supports the idea that the second heating curve h2 can be
considered to be the background of the first curve h1 at any
point because the spectrum was continuously measured. No
hydrogen peak is observed at about 510 C during heating in
the spectra in Fig. 2, or was also observed in the second
heating curve of the spectrum measured during heating to
about 800 C, cooling to about 100 C, and subsequent
heating to about 1000 C under the no-sample condition.
These results indicate that the peak at about 510 C in curve
h2 in Fig. 6 is not due to the desorption of hydrogen from the
Fig. 7. Difference spectrum for hydrogen desorption.
inner wall of the analysis chamber. A small peak at about
510 C in curve h2 is suspected to be due to the desorption of
hydrogen remaining on the Si surface, hydrogen in nonvolatile silicon oxide formed by the reaction of Si with water
during the first heating stage and subsequent cooling, or
hydrogen from the hydrogen-terminated the Si surface
formed by the reaction of hydrogen-desorbed sites with
hydrogen or water molecules in the analysis chamber during
cooling. Thus curve h2 for the second heating stage cannot
be considered to be background of the first curve h1. The
hydrogen intensity during heating changes linearly with
temperature within the region around 510 C in Fig. 2. This
suggests that a linear background, drawn as a straight line
between the first and last sets of data points on the curve, is
more suitable for the background of the first curve h1. The
fitted linear background drawn between suitably chosen
points at 457 and 576 C is illustrated in Fig. 6. Therefore, it
can be considered that curve h2s synthesized with the linear
background and the second heating curve, except for the
peak at around 510 C, approximates to the background of
the first curve h1.
Figure 7 shows the difference spectrum for hydrogen
desorption. This is obtained by subtracting the synthesized
curve h2s from h1 in Fig. 6. From a comparison of the peaks
shown in Fig. 7 and the peak shown in Fig. 5, the desorption
peak at about 510 C can be expected to be obtained by the
subtraction method. The normalization factor can be
obtained as a ratio of the intensity in Fig. 7 to that in
Fig. 5, at or near the temperature that gives the maximum
intensity in Fig. 5.
Figure 8 shows the hydrogen desorption peak at about
510 C. This is a residual peak after removing the normalized
curve in Fig. 5 from the curve in Fig. 7. Thus peak splitting
can be achieved by the subtraction of spectra.
From the separated peaks shown in Figs. 5 and 8, the
surface coverage can be estimated by calculating the area of
the peak. The normalized surface coverage is given by15)
Z tc
Z tc
¼
Ndt=
Ndt;
0
t
0
where is the surface coverage at temperature T, 0 is the
initial surface coverage, N is the rate of desorption from a
unit surface area, t is the time at T, and tc is the time when
desorption is essentially complete.
Jpn. J. Appl. Phys., Vol. 43, No. 12 (2004)
ION INTENSITY ( 10 -10 A)
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1.0
S. URABE et al.
H2
0.5
0.0
0
200
400
600
800
1000
TEMPERATURE (°C)
NORMALIZED COVERAGE
Fig. 8. Hydrogen desorption peak at about 510 C.
1.0
0.8
0.6
0.4
0.2
0.0
0
200
400
600
800
1000
TEMPERATURE (°C)
NORMALIZED UNCOVERAGE
1.0
0.8
0.6
0.4
0.2
0
200
400
600
800
5.
Conclusions
We proposed a combined method of heating and cooling
in thermal desorption spectroscopy to reveal the reaction of
the Si surface with water after hydrogen desorption during
heating and to estimate the surface coverage or uncoverage.
The successive decrease of water intensity in the spectrum,
which indicates the reaction of the Si surface with water
after hydrogen desorption, was observed even at about
400 C at a low concentration of water molecules. The
surface coverage or uncoverage as a function of temperature
was estimated from the hydrogen desorption spectra
obtained by combined measurements. The combined method
is useful for analyzing the hydrogen desorption and the
subsequent reaction during heating in various oxidation or
deposition processes.
(a)
0.0
where I is the desorption intensity, T0 is the temperature at
the start of the sweep, and Tc is the temperature when
desorption is essentially complete.
Figure 9 shows the normalized surface coverage and
uncoverage as a function of temperature. The surface
uncoverage is defined as 0 and the normalized one is
1 =0 . The coverage or uncoverage changes at around
400 and 510 C, which reflect only the marked increase of
hydrogen intensity in the simple heating spectrum, are
calculated from the peaks shown in Figs. 5 and 8, respectively. Thus the surface coverage of the desorbing phases
can be estimated from the desorption spectra measured with
the combination of heating and cooling. The surface
coverage change provides quantitative information to the
hydrogen desorption model proposed previously.9) Almost
half the hydrogen of SiH2 in the hydrogen-terminated
Si(100) surface desorbs and the surface changes to SiH at
around 400 C. Then the hydrogen of SiH desorbs at around
510 C. The surface uncoverage corresponds to the surface
concentration of hydrogen-desorbed sites. It can be inferred
from the surface uncoverage change shown in Fig. 9(b) that
the successive decrease of water intensity, which begins at
about 370 C in Figs. 3(b) to 3(d), is observed at a low
concentration of water molecules. The surface uncoverage
may provide the surface concentration of sites oxidized by
water or oxygen molecules at a high concentration of water
or oxygen molecules. This result can make an important
contribution to the understanding and control of the thermal
oxidation of the Si surface during heating.4–6)
1000
TEMPERATURE (°C)
(b)
Fig. 9. (a) Normalized surface coverage and (b) normalized surface
uncoverage as a function of temperature.
When the temperature is varied in a linear fashion with
time and the sweep duration is long compared with the
characteristic pumping time, the normalized surface coverage may be obtained from
Z Tc
Z Tc
¼
IdT=
IdT;
0
T
T0
Acknowledgments
This work was carried out at the Ultra Clean Room
of the Ultra Precision Machining Research Center, Osaka
University. This work was partially supported by Grants-inAid for Scientific Research (No. 14350166, No. 13555007,
No. 13875012, No. 08CE2004) from the Ministry of
Education, Culture, Sports, Science and Technology and
Japan Society for the Promotion of Science.
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