Extended Abstracts of the 2004 International Conference on Solid State Devices and Materials, Tokyo, 2004, pp. 462-463
P3-9
Adsorption Behavior of Various Fluorocarbon Gases on the Silicon Wafer Surface
Atsushi Hidaka1, Satoru Yamashita2, Takeyoshi Kato2, Yasuyuki Shirai2 and Tadahiro Ohmi2
1
University of Tohoku, Department of Electronic Engineering, Graduate School of Engineering
10, Aza-Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8579, Japan
Phone: +81-22-217-3977 Fax: +81-22-217-3986 E-mail: [email protected]
2
University of Tohoku, New Industry Creation Hatchery Center (NICHe)
10, Aza-Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8579, Japan
1. Introduction
The improvement of ULSI performance is required to
make device structure miniaturize. The microfabrication in
the etching process is one of the indispensable technologies
for miniaturization of the device structure. Therefore, the
etching technology of high aspect ratio is required. Reactive ion etching (RIE) process is required to inhibit the micro loading effect and to make the selectivity between SiO2
and photo-resist as high as possible. Up to now, many researchers have been studying about these subjects [1,2]. In
RIE process, the etching reaction is promoted by assist of
the high-energy ion irradiated to etching gases which are
adsorbed to the substrate surface [3,4]. Therefore, one of
the factors that determine the etching characteristic is the
adsorption behavior of etching gas to the substrate surface.
In order to realize the etching technology of high aspect
ratio, the etching gas that has high adsorption capability
(high selectivity) to the SiO2 surface is required. Thus, it
becomes important to clarify the adsorption behavior of
etching gas to the substrate surface. The purpose of this
paper is to clarify the adsorption behavior (surface selectivity) of etching gases to the various substrate surfaces.
2. Experiment
Fig. 1 shows the molecular structure and the binding
energy of used fluorocarbon gases. (a) C4F8, (b) cyclic C5F8
and (c) advanced C5F8 were evaluated as fluorocarbon
etching gases (CxFy gases). Fig. 2 shows the schematic diagram of experimental setup. Atmospheric pressure ionization mass spectrometer (APIMS) was used as measuring
equipment. The adsorption behavior of CxFy gases was
examined by measuring the adsorption amount to various
surfaces. SiO2, Si, and photo-resist were prepared on the
inner surface of the electro polished stainless-steel tube and
used as test surfaces instead of silicon wafer surface. The
adsorption behavior to the wafer surface can be artificially
examined by using theses sample tubes. The measurement
procedure of the adsorption amount is as follows. At first,
the sample tube was installed in the predetermined position
of an experimental setup shown in a fig. 2. Then, the sample tube was maintained at predetermined temperature (25,
60, 100, 150 and 200 degrees C). And argon (Ar) gas contained 100ppb of CxFy gas was introduced to the sample
tube. The experiment was carried out using ultra-high purity Ar gas as a carrier gas. The impurity concentration in
Ar gas was below 0.2 ppb. As CxFy gas was adsorbed on
the inner surface of sample tube to some extent, we meas-
ured detected time for each CxFy gas by API-MS.
3. Results and Discussion
Fig. 3 shows the adsorption behavior model of CxFy gases
introduced to the various sample tubes. The total number
(QT) of adsorbed molecules on the inner surface of the sample tube can be obtained from the formula shown as follows;
Q T = Concentration × Flow Rate × Detection Time
Fig. 4 shows the adsorption behavior of various CxFy gases
on the SiO2 surface at 25 degrees C. The vertical axis shows
concentration of CxFy gas and horizontal axis shows detection time. Advanced C5F8 took the longest time to be detected. It was found that the easiest to adsorb gas on the
SiO2 surface is advanced C5F8. The ratio of the adsorption
amount of various CxFy gases was as follows;
C4F8: cyclic C5F8: advanced C5F8 = 1: 31: 96.
Fig. 5 (Arrhenius plot) shows the activation energy (Ea) of
various CxFy gases adsorption on the SiO2 surface. The vertical axis shows the total number of adsorbed CxFy gas
molecules per unit area, and horizontal axis shows reciprocal of temperature. Advanced C5F8 took the largest adsorption Ea value (temperature dependency is the largest), that is
to say, it adsorbed easier in case of the lower temperature. In
the low temperature etching process, it can be guessed that
the advanced C5F8 gas is better than the others. Fig. 6 shows
the adsorption behavior of advanced C5F8 gas on various
surfaces at 25 degrees C. The adsorption amount on the SiO2
surface was the largest. It was found that the adsorption
amount of advanced C5F8 gas was greatly influenced by
surfaces. The ratio of the adsorption amount on various surfaces was as follows;
photo-resist: Si: SiO2 = 1: 5.5: 100.
Surprisingly, the adsorption amount of advanced C5F8 on the
SiO2 surface was 100 times larger than the photo-resist surface. Fig. 7 shows the total number of adsorbed molecules
per unit area of various CxFy gases on various surfaces at 25
degrees C. There was no difference in the adsorption amount
of C4F8 gas on any surface. The ratio of the adsorption
amount of cyclic C5F8 gas on various surfaces was as follows;
photo-resist: Si: SiO2 = 1: 1.5: 40.
It was found that advanced C5F8 gas had great surface selectivity between SiO2 and photo-resist.
4. Conclusions
We clarified that the advanced C5F8 gas had surface
selectivity between SiO2 and photo-resist. This result shows
that the advanced C5F8 has the best etching ability for high
aspect ratio.
- 462 -
References
[1] J.W. Coburn, J. Appl. Phys. 50 (1979) 5210.
[2] K. Suzuki, K. Ninomiya, S. Nishimatsu and S. Okudaira, J.
Vac. Sci. Technol. B3 (1985) 1025.
[3] G.C. Schwartz, L.B. Zielinski and T. Schopen, in “Etching”,
M.J. Rand and. H.G. Hughes, eds, Electrochem. Soc. Symp.
Series, Princeton, N.J. (1976) 122.
[4] J.W.Coburn and H.F.Winters, J. Appl. Phys. 50 (1979) 3189.
CxFy Concentration [ppb]
100
6.07eV
F
C
C
F C
F
C
F
F
4.64eV
4.99eV
F
F
F
C
F 3.99eV
C
F
C
F
F
C
C
F
F
F
3.44eV
5.06eV
ng
i
nd
e
tP
n
te
a
P
(a) C4F8
(b) Cyclic C5F8
(c) Advanced C5F8
Boiling point ： －5.85℃
Vapor pressure
： 0.27MPa at 25℃
Boiling point ： 27℃
Vapor pressure
： 0.09MPa at 25℃
Boiling point ： 7℃
Vapor pressure
： 0.24MPa at 25℃
Fig. 1 Molecular structure and binding energy of various Fluorocarbon gases.
Purge Ar
CXFY /Ar
: 20 cm3/min.
MFC
CXFY Gas
Concentration
: 100 ppb
Heating Range of Sample Tube
: 25℃～200℃
Sample Tube
PID
: φ6.35mm - 1m
Inner Surface
Area ： 136.6cm2
Used Gases
・ C4F8
・ Cyclic C5F8
・ Advanced C5F8
Ar :
Needle Electrode
MFM
Exhaust
※ Two-compartment ion source is used
for the deposition prevention to electrode.
Fig. 2
API‐MS
600 cm3/min.
MFM
550 cm3/min
150
Exhaust
Schematic diagram of the experimental setup.
Temperature [℃]
100
150
200
1.0E+15
ΔCxFy(ppb)
0
Time (min)
CxFy
Sample Tube
Fluorocarbon gases begin to
adsorb from a tube entrance.
CxFy
Sample Tube
25
60
Advanced C5F8
Ea = 0.100 [eV]
1.0E+14
1.0E+13
Cyclic C5F8
Ea = 0.044 [eV]
1.0E+12
C4F8
Ea = 0.028 [eV]
1.0E+11
1.0E+10
2.5
3
3.5
Fig. 5 Activation energy of various fluorocarbon gases adsorption on the SiO2 surface.
100
75
Si Surface
50
SiO2 Surface
Photo-Resist Surface
25
0
0
50
100
150
200
250
300
Time [min.]
Sample Tube
When adsorption reached equilibrium,
fluorocarbon gases are detected with APIMS.
Number of Adsorbed
Molecules [molecules/cm2]
CxFy　concentration (ppb)
b)
a)
CxFy
300
Fig. 6 Adsorption behavior of advanced C5F8 gas on various
surfaces at 25 degrees C.
※ TD is time for the area of a) and b) to become the same.
QT
250
Fig. 4 Adsorption behavior of various fluorocarbon gases on
the SiO2 surface at 25 degrees C.
QT (The Amount of Absorption) ＝ Concentration × Flow Rate × TD (Detection Time)
TD(min)
200
3
Total
: 200 cm3/min.
MFC
100
1/ T [10 x1/K]
MFC
1000 cm3/min.
50
2
180 cm3/min.
Ar :
Inner Surface of Sample Tube
: SiO2 , Si and Photo-Resist
on SUS316L-EP Surface
Advanced C5F8
Time [min.]
Advanced C5F8 Concentration [ppb]
Exhaust
Cyclic C5F8
25
0
Number of Adsorbed
2
Molecules [molecules/cm ]
F
C4F8
50
0
5.49eV
2.56eV
75
1.0E+14
SiO2 Surface
Si Surface
Photo-Resist Surface
1.0E+13
1.0E+12
1.0E+11
C4F8
Cyclic C5F8
Advanced C5F8
Fig. 7 The total number of adsorbed molecules of various
fluorocarbon gases on various surfaces at 25 degrees C.
Fig. 3 Adsorption behavior model of fluorocarbon gases introduced to the various sample tubes.
- 463 -