Journal of Metals, Materials and Minerals. Vol.18 No.2 pp.137-141, 2008
Removal of Carbon Contamination on Silicon Wafer Surfaces
by Microwave Oxygen Plasma
Amorn THEDSAKHULWONG and Warawoot THOWLADDA
Laser and Surface Physics Research Laboratory, Department of Applied Physics,
Faculty of Science, King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520
Abstract
Received Nov. 25, 2008
Accepted Feb. 11, 2009
In this research, we report our experimental result on the removal of carbon contaminants existing
on silicon wafer surfaces by a dry cleaning method using micro-wave induced oxygen plasma. In this
cleaning process, the silicon wafer was placed at desired positions of 20 to 50 mm away from the open end
of the plasma tube by means of a linear translator and the plasma exposure time was varied. Carbon
contaminants could be removed through evaporation mechanism by chemical reactions with excited oxygen
atoms. The plasma density and electron temperature have been investigated by Langmuir probe. After
cleaning process, the carbon contaminants removal efficiency was evaluated by comparing the initial and
final Auger electron signals of silicon (92 eV), carbon (272 eV) and oxygen (508 eV). The higher silicon
signal with the lower carbon signal indicates the efficiency of carbon contaminants removal. In the oxygen
plasma discharge generated at pressure of 4.0 x 10-1 mbar and microwave power of 300 W, the plasma
density and electron temperature are 2.73 x 1010 cm-3 and 3.32 eV respectively. The magnitude of Auger
signal of silicon was increased more than two times and magnitude of Auger signal of carbon was decreased
to 20% of the initial signal within one minute of the plasma exposure time.
Key words : Carbon contamination, Silicon wafer, Micro-wave, Plasma
Introduction
It is well known in the semiconductor
industry that the organic contamination of silicon
wafer surfaces can lead to serious difficulties
in their production. Therefore, it is extremely
important to remove organic contaminants from
silicon surfaces prior to the fabrication process.
The methods adopted in wafer cleaning technology,
wet chemical cleaning techniques based on RCA
cleaning(2) have been widely used to remove
contaminants on wafer surfaces. However, the cost
of high-purity chemicals and DI water, problems
with waste disposal, and safety issues are the main
reasons for rapidly growing interest in dry wafer
cleaning technology. The representative dry cleaning
methods(3) are thermally enhanced cleaning, gas
phase cleaning, photochemically enhanced cleaning
and plasma enhanced cleaning and so on. Among
them, plasma enhanced cleaning(1, 4, 5) is one of the
most effective dry cleaning techniques in removing
organic contamination on silicon wafer surfaces. In the
plasma enhanced cleaning, glow discharge generated
reactive species which react with contaminated
molecules on wafer surfaces, then produced CO2 CO2
and H2O which are desorbed from the surfaces.
Direct plasma systems having the wafer in
the glow discharge are not suitable for wafer cleaning
due to the energetic interaction between the plasma
and the wafer. Mostly, ion bombardment results in
surface contamination, particularly with metals
including alkali ions, are being knocked into the
silicon substrate. Therefore, to alleviate problems
associated with direct plasma exposure, a remote
plasma-processing mode has become a preferred
method in dry wafer cleaning applications.
In this paper, a dry cleaning system was
developed and used for removing organic
contaminants from the silicon wafer surfaces using
microwave induced oxygen plasma. The efficiency
of carbon contaminants removal characterized by
Auger electron spectroscopy, here we report.
Experimental
System Configuration
Cleaning system using microwave induced
oxygen plasma is shown in Figure 1. The microwave
induced oxygen plasma discharge was produced in
a cylindrical quartz tube of 10 mm diameter. This
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138
THEDSAKHULWONG, A. and THOWLADDA, W.
quartz tube was inserted through the WR340
rectangular microwave resonant cavity, perpendicular
to its wide wall, into the cylindrical-glass process
chamber of 150 mm diameter. The rectangular
resonant cavity was operated in TE106 mode at 2.45
GHz using 800 W magnetron. The sample was
placed in the cylindrical-glass process chamber and
its positions were varied between 20 and 50 mm
away from the open end of the plasma tube by
means of linear translator. The amount of gas
flowed into the chamber from the other end of the
plasma tube was controlled by a needle valve. The
developed cleaning system was operated by means
of a remote plasma system. The process was
dominated by the chemical reaction between the
excited atomic species and the sample being
processed, without charge particles bombardment
on the sample surface.
electron temperature have been investigated by
Langmuir probe. The real time relative density of
the free radicals in the plasma were observed using
OES. The contamination on the silicon wafer
surface at the initial and final cleaning process was
characterized by Auger electron spectroscopy
(AES) shown in Figure 2.
Figure 2. Auger Electron Spectrometer at Laser and
Surface Physics Research Laboratory.
Results and Discussion
Plasma Properties
Figure 1. The cleaning system using microwave induced
oxygen plasma.
The density and electron temperature of
the oxygen plasma operated at microwave power
of 300 W measured by Langmuir probe are shown
in Figure 3. The plasma density and electron
temperature are 2.73 x 1010 cm-3 and 3.32 eV
respectively at the pressure of 0.4 mbar and
decreased to 1.4 x 1010 cm-3 and 2.93 eV
respectively as the pressure increased to 1 mbar.
Cleaning Process
The wafer used in this study is B-doped ptype silicon with dimensions of 15 mm x 15 mm.
The wafer was placed on the sample holder in
the process chamber at 20 mm away from the open
end of the plasma tube. The oxygen plasma was
generated at different oxygen pressures and
microwave powers. The cleaning time was varied
from one to five minutes. The plasma density and
3.5
3.5
3.0
3.0
2.5
2.5
2.0
2.0
1.5
1.5
1.0
1.0
Plasma density
0.5
0.5
Electron temperature
0.0
0.0
0.2
0.4
E lectron tem perature (eV )
The oxygen plasma at each operating
conditions was characterized by measuring of
plasma density and electron temperature using
Langmuir probe. The optical emission spectroscopy
(OES) was also used to observe the relative density
of the free radicals in the plasma.
Plasm a density (×10 10 cm -3 )
Measurement of Plasma Properties
0.0
0.6
0.8
1.0
1.2
Pressure (mbar)
Figure 3. The plasma density and electron temperature
of the oxygen plasma at microwave power of
300 W with different pressures.
139
Removal of Carbon Contamination on Silicon Wafer Surfaces
by Microwave Oxygen Plasma
50
0
AES Intensity (a.u.)
The OES spectra of the oxygen plasma are
shown in Figure 4. The three main emission lines
of oxygen radical were found at the wavelength of
616.3 nm, 777.2 nm and 844.7 nm. The emission
line of 616.3 nm is related to the electron energy
transition between the energy states of 2p23s 4So
and 2p34s of the oxygen ion. The emission lines of
777.2 nm and 844.7 nm are related to the energy
transitions of 3p5P to 3s 5So and 3p3P to 3s3So of
the atomic oxygen respectively.(7) The OES
spectrum of oxygen plasma at pressure of 0.4 mbar
showed the emission lines of higher intensity than
at pressure of 1 mbar. This result indicated that
density of oxygen free radical at pressure of 0.4
mbar was higher than at a pressure of 1 mbar. That
was consistent with the measurement result by the
Langmuir probe.
-50
-100
-150
-200
Before cleaning
-250
After cleaning
-300
0
100
200
300
400
500
600
700
Electron energy (eV)
Figure 5. AES signals of silicon wafer at microwave
power of 300 W with pressure of 0.4 mbar.
Figure 6. Shows a change in the AES
signals over the oxygen plasma cleaning time from
one to five minutes. The measurement results show
that the intensity of carbon contamination signal
decreased rapidly within one minute of cleaning
time and remained virtually unchanged for five
minutes of cleaning time. The silicon AES signal
was changed in opposite direction, and meanwhile
the oxygen AES signal was almost unchanged.
140
120
40
Silicon
Carbon
Oxygen
35
30
100
25
80
20
60
15
40
10
20
5
C & O AES Intensity (a.u.)
Si AES Intensity (a.u.)
160
0
0
-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
Figure 4. The optical emission spectroscopy of oxygen
plasma at microwave power of 300 W with
pressure of 0.4 mbar and 1.0 mbar.
Plasma Cleaning
The AES signals from the silicon wafer for
initial and after one minute plasma cleaning
process are shown in Figure 5. The AES signal at
92 eV indicates the silicon atom on the silicon
wafer. The signals at 272 eV and 508 eV describe
the existent of carbon and oxygen contaminations
on the silicon wafer surface. The intensity of these
AES signals associated with the degree of
contaminants. The Figure 5 shows significantly
higher signal of silicon wafer and the signal of
carbon contamination was almost absent from the
surface. The AES signal of oxygen contamination
was almost unchanged.
Exposure Time (min)
Figure 6. Si, C and O AES signals of silicon wafer as a
function of cleaning time at microwave
power of 300 W with pressure of 0.4 mbar.
To realize the effect of the microwave
power on the contamination removal efficiency,
the ratio of AES signals intensity before cleaning
process to AES signals intensity at one minute of
plasma cleaning time were calculated. Figure 7
depicts the Si, C and O AES signals intensity ratios
at different microwave powers. At oxygen-plasma
pressure of 0.4 mbar, the AES signal ratios of
carbon and oxygen slightly decrease as the
microwave power is increased from 60 W to 300
W while the AES signal ratios of silicon increase.
This measurement results revealed that the
140
THEDSAKHULWONG, A. and THOWLADDA, W.
efficiency of carbon contamination removal was
improved by increasing the microwave power. As
shown in Figure.7 (b), the AES signals ratios of
carbon rarely changed at the pressure of 1 mbar.
The AES signal ratios of oxygen increased as the
microwave power was increased. The AES signal
ratios of silicon increase as the microwave power
was increased from 60 W to 150 W and were
almost unchanged at higher microwave power. At
higher oxygen-plasma pressure condition, not only
plasma etching was the most important process but
also plasma deposition process. The result showed
that at 1 mbar the plasma deposition was becoming
dominating process at higher microwave power.
Therefore, the efficiency of carbon contamination
removal was decreased. This could be because of
the fact that oxygen forms thin layer on the sample
surface. As a result, the cleanliness of silicon
surface tended to be lower when plasma cleaning
time is increased.
As oxygen radicals were generated by
microwave plasma, the remote plasma chemical
reaction between them and the carbon atoms and
its compounds on silicon surface being processed
produced CO, CO2 and H2O desorbed from the
surface. The experiment showed that the carbon
contamination was almost completely removed in
one minute which is indicated by the rapid
decrease of carbon AES signal down to 20% of the
initial signal within one minute of the cleaning
process. As the carbon contamination was
removed, the clean silicon surface was obtained
indicated by the increase of silicon AES signal to
greater than two times of the initial signal. The
oxygen plasma cleaning method is an efficient
method compare to the conventional RCA cleaning
method. The RCA method needs at least 10
minutes to clean the surface with a result of a lower
cleanliness.(6) The consistency of the oxygen AES
signal indicated that this process could not remove
the oxygen contamination from the surface.
2.5
AES Intensity ratio (a.u.)
Conclusion
2.0
1.5
1.0
Carbon
Silicon
0.5
Oxygen
0.0
0
50
100
150
200
250
300
350
Microwave Power (Watts)
(a)
AES Intensity ratio (a.u.)
2.5
2.0
1.5
Carbon
Silicon
Oxygen
1.0
0.5
The microwave induced oxygen plasma
was developed to remove the carbon contamination
on the silicon wafer surface. The Langmuir probe
and the OES were used to characterize the oxygen
plasma properties. The efficiency of the
contamination removal was characterized by Auger
electron spectroscopy. The oxygen plasma
generated by 300 W microwave power at pressure
of 0.4 mbar, the plasma density and electron
temperature were 2.73 x 1010 cm-3 and 3.32 eV,
respectively. The carbon contamination was
removed rapidly by this plasma in remote system
within one minute. The carbon and silicon AES
signals decrease to 20% and increase more than
two times of the initial signal respectively.
However, the oxygen AES signal indicated that
oxygen contamination on the surface could not be
removed by the oxygen plasma.
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0.0
0
50
100
150
200
250
Microwave Power (Watts)
300
350
(b)
Figure 7. Si, C and O AES signals intensity ratio of silicon
wafer as a function of microwave power at
cleaning time of 1 minute with pressure of
(a) 0.4 mbar and (b) 1.0 mbar.
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Removal of Carbon Contamination on Silicon Wafer Surfaces
by Microwave Oxygen Plasma
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