Jpn. J. Appl. Phys. Vol. 42 (2003) pp. 5759–5764
Part 1, No. 9A, September 2003
#2003 The Japan Society of Applied Physics
Comparative Studies of Perfluorocarbon Alternative Gas Plasmas for Contact Hole Etch
Shingo N AKAMURA, Mitsushi ITANO, Hirokazu A OYAMA, Kentaro S HIBAHARA1 ,
Shin Y OKOYAMA1 and Masataka H IROSE2
Chemical Division, DAIKIN Industries, Ltd., 1-1 Nishi Hitotsuya, Settsu, Osaka 566-8585, Japan
1
Research Center for Nanodevices and Systems, Hiroshima University, 1-4-2, Kagamiyama, Higashi-hiroshima, Hiroshima 739-8527, Japan
2
Advanced Semiconductor Research Center, National Institute of Advanced Industrial Science and Technology,
Central 4, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8562, Japan
(Received December 27, 2002; accepted for publication May 14, 2003)
Saturated perfluorocarbons (PFCs) such as CF4 , C2 F6 , C3 F8 and c-C4 F8 are used as dry-etch gases in the semiconductor
industry. They have a significant greenhouse effect. Unsaturated fluorocarbons can be alternated with these PFCs because of
their easy decomposition in the atmosphere. The authors have diagnosed the plasmas generated from straight-chain
unsaturated gases such as C3 F6 , C4 F6 , C4 F8 and C5 F8 in an inductively coupled plasma reactor and have compared their etch
properties. It was found that high selectivity has been obtained in a C4 F6 or C5 F8 plasma without mixing any specific gases.
Fine contact holes of approximately 100 nm in diameter also have been obtained using C4 F6 or C5 F8 with or without adding
Ar or O2 . These good etch properties of C4 F6 and C5 F8 have been achieved as a consequence of the appropriate balance
between the lower density of fluorocarbon polymers and the dominant etching species CFþ with lower etching efficiency. It
can be concluded that C4 F6 and C5 F8 can be used as PFC replacements for the dry-etch gas.
[DOI: 10.1143/JJAP.42.5759]
KEYWORDS: greenhouse effect, PFC alternative gas, straight-chain unsaturated fluorocarbon, inductively coupled plasma (ICP),
SiO2 etching, contact hole etch
1.
Introduction
Saturated perfluorocarbons (PFCs) such as CF4 , C2 F6 ,
C3 F8 and c-C4 F8 are used as dry-etch gases in the fabrication
of ultralarge-scale integrated circuits (ULSIs) in the semiconductor industry. These gases have long atmosphere
lifetimes and strong absorptions of infrared radiation,
exhibiting high global warming potentials (GWPs) and so
promoting the green house effect. Recently, the replacement,
decomposition or recycling of PFCs used for dry-etch gases
has been attempted to reduce the greenhouse effect.1) PFC
such as octafluorocyclopropane (c-C4 F8 ) are still widely
used to etch interlayer dielectrics and fabricate fine contact
holes in ULSIs, while it will be difficult to achieve a contact
hole etch with a high aspect ratio in the sub-65 nm range.2)
Table I summarizes the GWP100 and atmospheric lifetime of
PFC alternative candidates together with c-C4 F8 . The
GWP100 is defined as a relative GWP value calculated for
the period of one hundred years against CO2 .3,4) It should be
noted that C3 F6 and C4 F6 have much lower GWP100 values
and atmospheric lifetimes than c-C4 F8 . These alternative
candidates with a double bond in their molecules are subject
Table I. Molecular structure, global warming potential (GWP100 ) and
atmospheric lifetime for octafluorocyclopropane(c-C4 F8 ), hexafluoropropene (C3 F6 ), hexafluorobutadiene (C4 F6 ), octafluoro-2-butene (C4 F8 ) and
octafluoropentadiene (C5 F8 ).
GWP100 aÞ
Atmospheric lifetime (years)
10000
3200
CF3 CF=CFCF3
CF3 CF=CF2
—
—
—
10
C4 F6
CF2 =CFCF=CF2
290
0.003
C5 F8
CF3 CF=CFCF=CF2
—
—
Molecular Structure
c-C4 F8
F2 C-CF2
j j
F2 C-CF2
C4 F8
C3 F6
a) GWP100 (global warming potential for one hundred years)
to decomposition in the atmosphere through their reaction
with hydroxyl radicals. This fragmentation mechanism leads
to the much shorter atmospheric lifetime and much lower
effective GWP100 for the replacement candidates.5) In
addition, these alternative candidates have the potential of
good etch properties because the double bond is broken
selectively and the ratio of particular radicals and ions is
predominantly controlled. Thus it is important to investigate
the PFC alternative etch gases such as hexafluoropropene
(C3 F6 , CF3 CF=CF2 ), hexafluorobutadiene (C4 F6 , CF2 =
CFCF=CF2 ), octafluoro-2-butene (C4 F8 , CF3 CF=CFCF3 )
and octafluoropentadiene (C5 F8 , CF3 CF=CFCF=CF2 ).
These are straight-chain unsaturated compounds with the
double bond in the molecules. In the present study, we have
diagnosed these gas plasmas and examined the SiO2 etching
ability to evaluate its potential as an alternative etching gas.
2.
Experimental
The schematic diagram of an inductively coupled plasma
(ICP) reactor used in the present study is shown in Fig. 1.
Etching gases were introduced from the outlets set at eight
points on an inner wall. A single-turn antenna of 140 mm in
diameter generates plasmas through a 9-mm-thick quart
plate. A 2 inch Si wafer was clamped using an electrostatic
chuck (ESC) assembly on a chiller-cooled stage kept at
11 C. A quadrupole mass spectrometer and a Langmuir
probe were set on a chamber sidewall to diagnose the
plasmas. The Langmuir probe used to measure electron
densities and electron temperatures was set 20 mm above the
center of the wafer. The probe tip was heated to prevent
fluorocarbon polymer deposition. The relative amount of
positive ions was evaluated by quadrupole mass spectrometry, and the density of CFx (x ¼ 1{3) fluorocarbon radicals
was measured by appearance mass spectrometry (AMS).6)
The morphology of fluorocarbon polymers deposited on the
wafer was measured by atomic force microscopy (AFM).
Chemical bonding features were also analyzed by Fourier
transform infrared (FT-IR) spectroscopy. A 2-mm-thick SiO2
5759
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Jpn. J. Appl. Phys. Vol. 42 (2003) Pt. 1, No. 9A
Single Turn Antenna
Matching Box
200mm
QMS
130mm
100mm
Electron Temperature (eV)
13.56MHz
~
Quartz Plate
100mm
S. NAKAMURA et al.
0
2
4
6
8
5.2
5.1
5.3
c-C4F8
C3F6
C4F8
C4F6
C5F8
10
Electron
Temperature
6.6
6.0
20mm
Electrostatic
Chuck
Cb
Matching Box
Inner Wall
2 Inch Si Wafer
Langmuir Probe
400kHz ~
Electron
Density
1.07
1.08
c-C4F8
C3F6
C4F8
C4F6
C5F8
1.45
0.87
0.97
0
1
2
11
-3
Electron Density (×10 cm )
Fig. 1. Schematic diagram of an inductively coupled plasma(ICP) reactor.
Frequencies of source power and bias power are 13.56 MHz and 400 kHz,
respectively. The bottom of the inner wall (200 mm in diameter) is
located at a distance of 130 mm from the quartz plate. The gap from the
single-turn antenna of the ICP to the electrostatic chuck stage is 100 mm.
Heating was not used in this study while the cylindrical inner wall with
the heating function is set in an etching chamber.
layer deposited by atmospheric pressure chemical vapor
deposition (APCVD) was used as the substrate. The positive
chemical amplification electron-beam resist used mainly
consisted of highly sensitive novolak resin, which was
manufactured for the trial by Hitachi Chemical Co, Ltd.. The
electron beam resist with a thickness of 950 nm was
patterned to form contact holes with 180 nm–4.5 mm diameters. The SiO2 layer was etched with C3 F6 , C4 F6 , C4 F8 , cC4 F8 and C5 F8 plasmas under almost the same etching
conditions. Typical conditions were as follows: a pressure of
3 mTorr, a gas flow rate of 5–7 sccm, an ICP power of 600 W
and a bias power of 200 W.
3.
Results and Discussion
3.1 Plasma diagnostics
The electron density and temperature of the c-C4 F8 , C3 F6 ,
C4 F8 , C4 F6 and C5 F8 plasmas are compared in Fig. 2. The
electron temperatures of C4 F6 and C5 F8 are slightly higher
than those of C3 F6 , C4 F8 and c-C4 F8 while the electron
densities of C4 F6 and C5 F8 are almost the same as those of
C3 F6 and c-C4 F8 . In addition, the electron density of C4 F8 is
the highest in these plasmas. The amount of positive ion
components in each plasma is shown in Fig. 3. A CFþ ion
with low etching ability is the main ionic species and a CFþ
2
is a minor species in every plasma. It is suggested for C3 F6
and C4 F8 that a lot of CFþ
3 ions with high etching ability are
efficiently produced from the CF3 CF fragment generated by
the break of the double bond. This is consistent with the fact
that the fraction of the CFþ
3 ion in the C5 F8 plasma is higher
than in the C4 F6 plasma. A large amount of Fþ ions in the
C4 F6 and C5 F8 plasmas are attributed to high electron
temperature, as shown in Fig. 2. Fluorocarbon plasma with
high electron temperature in general tends to contain highþ
density Fþ ions and F radicals. Since the CFþ
2 /CF ratio is
about 0.2 and is nearly equal for these plasmas, the CFþ
3/
CFþ ratio can be utilized as an index of etching efficiency.
Fig. 2. Electron temperature and electron density at 3 mTorr and 600 W
source power.
+
F
+
C+
CF +
CF2
+
CF3
CF2-CF2
c-C4F8
CF
CF3
CF2-CF2
+
+
+
F
C
+
CF +
CF2+
CF3
+
C3F6 (CF3CF=CF2)
CF
+
+
CF3
+
F
+
C+
CF +
CF2+
CF3
C4F8 (CF3CF=CFCF3)
CF
+
+
CF3
+
+
F
F+
C
+
CF +
CF2+
CF3
C4F6 (CF2=CFCF=CF2)
+
CF
+
CF3
+
F
F
+
C+
CF
+
CF2+
CF3
+
C5F8 (CF3CF=CFCF=CF2)
+
CF
+
CF3
0
20
40
60
80
Ion Count (%)
Fig. 3. Relative amount of positive ions at 3 mTorr and 600 W source
power. These ions were detected by quadrupole mass spectrometry.
þ
The CFþ
3 /CF ratio in each plasma is in the following order:
C3 F6 (0.81) > C4 F8 (0.68) > c-C4 F8 (0.33) > C5 F8 (0.30) >
C4 F6 (0.10). Figure 4 shows the total CFx (x ¼ 1{3) radical
densities of these plasmas. The total densities of CFx
(x ¼ 1{3) radicals are highest in the c-C4 F8 plasma and
lowest in the C4 F6 plasma. The density of the CF3 radical is
the highest of all these plasmas and that of the CF radical is
the lowest. Figure 5 shows the average roughness Ra
measured by AFM and the deposition rate rdepo of the
fluorocarbon polymer films deposited on a Si substrate with
each plasma without RF bias power. The observations in
Fig. 5 show that the larger molecule of parent gas results in a
higher deposition rate and a rougher surface of the polymer,
except for C4 F8 . This implies that the polymeric radicals are
Jpn. J. Appl. Phys. Vol. 42 (2003) Pt. 1, No. 9A
CF3
CF2
S. NAKAMURA et al.
3
CF
c-C4F8
C3F6
CF3CF=CF2
C4F8
CF3CF=CFCF3
C4F6
CF2=CFCF=CF2
C5F8
CF3CF=CFCF=CF2
0.1
2.1
4.1
6.1
Normalized Absorbance (arb.units)
CF2-CF2
I
I
CF2-CF2
8.1
C4F8
C3F6
2
C4F6
1
v(CF3)
1360-1170cm-1
0
2000
0
500
[nm]
C3F6
0 40 [nm]
0 10 [nm]
[nm]
C5F8
0
0
500
500
500
0 10 [nm]
Ra(nm) rdepo(nm/min)
c-C4F8
0
500
500
[nm]
500
[nm]
[nm]
C4F8
C3F6
c-C4F8
C4F6
C5F8
0.8
1.4
1.8
2.8
6.1
1500
1000
500
285
243
259
262
317
Fig. 5. The surface roughness and deposition rate of fluorocarbon polymer
films. The fluorocarbon polymer films were deposited at 3 mTorr, 600 W
source power and electrostatic chuck temperature of 11 C. Ra is
average roughness measured by AFM and rdepo is the deposition rate of
the fluorocarbon polymer films.
the origins of surface roughness. Figure 6 shows the FT-IR
spectra of the fluorocarbon polymer films deposited for
1.5 min on Si substrates in each plasma without RF bias
power. Absorbance is normalized by the fluorocarbonpolymer film thickness measured by scanning electron
microscopy (SEM). The absorbances for the C3 F6 , C4 F8
and c-C4 F8 plasmas are higher than those for the C5 F8 and
C4 F6 plasmas, which indicates that C3 F6 , C4 F8 and c-C4 F8
plasmas deposit higher-density fluorocarbon-polymer films.
This fact is consistent with the results in Fig. 5. A rough
surface generally indicates a porous structure. It is also
noteworthy that both C4 F8 and C3 F6 with the CF3 CF
fragment deposit high-density fluorocarbon polymers.
The results obtained by a series of plasma diagnostics of
Fig. 6. Fourier transform infrared (FT-IR) spectroscopy of fluorocarbon
polymer films. These films were deposited at 3 mTorr, 600 W source
power and electrostatic chuck temperature of 11 C. The absorbance
normalized by a film thickness indicates the density of the fluorocarbon
polymer films. v(CF3 ), v(C–F) and v(–CF2 –) show the vibration
absorption range.
these candidates (C3 F6 , C4 F6 , C4 F8 , C5 F8 ) and the conventional etching gas (c-C4 F8 ) are summarized in Fig. 7.
Figure 8 shows the dissociation and etching model of these
fluorocarbon gases in the plasma. The total CFx (x ¼ 1{3)
radical density of the c-C4 F8 plasma is the highest, however
the c-C4 F8 plasma deposits a lower-density fluorocarbonpolymer film than the C4 F8 and C3 F6 plasmas do. The
fluorocarbon polymer surface of the c-C4 F8 plasma was
rougher than those for the C4 F8 and C3 F6 plasma as shown
in Fig. 5. These facts indicate that polymeric radicals such as
a Cx Fy (x = 3, y = 5) deposit the rough and porous films of
fluorocarbon polymers in the c-C4 F8 plasma while a large
number of CF2 CF2 radicals rearranged from a CF3 CF
fragment in the C4 F8 and C3 F6 plasmas deposit high-density
fluorocarbon polymers in a similar manner to the CF2
radical, as illustrated in Fig. 8. The C4 F6 plasma had the
Density of Fluorocarbon Polymer (arb.units)
0 20 [nm]
[nm]
0 5
C4F6
500
v(C-F)
1400-1000cm-1
Wave Number (cm )
0
500
v(-CF2-)
1250-1050cm-1
-1
Fig. 4. CFx (x ¼ 1{3) radical densities at 3 mTorr and 600 W source
power. These radical densities were measured by appearance mass
spectrometry (AMS).
500
c-C4F8
C5F8
Total Radical Density (×10 12cm-3)
C4F8
5761
:Total CFx(x=1-3) Radical Density
3.0
C4F8 (CF3CF=CFCF3)
C3F6 (CF3CF=CF2)
2.5
c-C4F8
C5F8 (CF3CF=CFCF=CF2)
2.0
C4F6 (CF2=CFCF=CF2)
1.5
0
2
4
6
Surface Roughness Ra (nm)
8
Fig. 7. A summary of fluorocarbon polymer properties and radical
densities. X-axis indicates a surface roughness Ra shown in Fig. 5. Y
axis indicates the fluorocarbon polymer densities estimated by the
normalized FT-IR spectrum shown in Fig. 6. Diameters for each circle
correspond to the total CFx (x ¼ 1–3) radical densities shown in Fig. 4.
5762
Jpn. J. Appl. Phys. Vol. 42 (2003) Pt. 1, No. 9A
S. NAKAMURA et al.
C4F8
0
C5F8
C3F6
CF3CF=CF2
c-C4F8
CF2CF2
I I
CF2CF2
C4F6
CF2=CFCF=CF2
CF3CF=CFCF=CF2
CF3CF:
CF3CF=CFCF:
. CF CF .
2
2
CF3+
CxFy (x 3,y 5)
CF3CxFy (x 2,y 0)
CFx (x=1-3)
plasma
CF3 CF+ CF2+
CF3
CF2CF2
COx,SiFx, CF +
3
COFx
:CFCF=CF2
plasma
CF2
CF3+
CF
CF
CF+
COx,SiFx, CF+
COFx
- CF2 - CF2 - CF2 -
CF2+
CF3
CxFy CF
- CxFy - CxFy - CxFy SiCxFyOz reactive layer
SiO2
Main Etching Species : CF ,
Etching Efficiency : High
Fluorocarbon polymer :
High Density and Fine Surface
8
10
Selectivity(SiO2/Si)
1.34
1.37
1.34
2.13
1.93
6.11
Selectivity(SiO2/Resist)
678 Etch Rtae
654
643
c-C4F8
C3F6
C4F8
C4F6
C5F8
507
588
0
200
400
600
800
1000
SiO2 Etch Rate (nm/min)
SiO2
CF3+
Selectivity
6
4.89
c-C4F8
C3F6
C4F8
C4F6
C5F8
+
SiCxFyOz reactive layer
+
4
2.47
2.68
2.29
c-C4F8
C3F6
C4F8
C4F6
C5F8
Main Etching Species : CF +
Etching Efficiency : Low
Fluorocarbon polymer :
Low Density and Rough Surface
Fig. 8. The image of dissociation and etching in the unsaturated
fluorocarbon gas plasmas. CFþ
3 ions with high etching ability impinge
on the high-density film of fluorocarbon polymers in the C3 F6 and C4 F8
plasmas. On the other hand, CFþ ions with low etching ability bombard
the porous and low-density films of fluorocarbon polymer in the C4 F6 and
C5 F8 plasmas. Etch reaction products such as CO2 , COF2 and SiF4 are
released from the Six Oy Fz reactive layer.
Fig. 9. SiO2 etch rate and selectivity at 3 mTorr, 600 W source power and
200 W bias power.
1.2
1.1
Normalized Etch Rate
CF3CF=CFCF3
2
1.0
0.9
c-C4F8
C3F6
C4F6
C5F8
C4F8
0.8
0.7
0.6
0.1
lowest CFx (x ¼ 1{3) radical densities, and its fluorocarbonpolymer density was the lowest. The deposition rate of
fluorocarbon polymers for the C5 F8 plasma is the highest,
and the surface is the roughest. Hence, it is probable that
more polymeric radical species other than CFx (x ¼ 1{3)
radicals are the main precursor for these two plasmas. We
speculate that the polymeric radicals are the large fragment
ones which has another double bond stabilized by the break
of one of the double bonds as illustrated in Fig. 8. Etching
efficiency was also explained as illustrated in Fig. 8 based on
the plasma diagnostics. The relative etching efficiency
þ
defined as the CFþ
3 /CF ratio is in the order C3 F6 > C4 F8
> c-C4 F8 > C5 F8 > C4 F6 , as described before. The C4 F8
plasma has the highest etching efficiency because of its high
ion density due to a high electron density. In the C3 F6 and
C4 F8 plasmas, CFþ
3 ions impinge on the high-density
fluorocarbon-polymer film. On the other hand, in the C4 F6
and C5 F8 plasmas, CFþ ions with low etching ability
bombard the porous and low-density films of fluorocarbon
polymers.
3.2 Etch properties
Figure 9 shows the SiO2 etch rate and selectivity for the
etch plasmas. The SiO2 etch rates for the C3 F6 , C4 F8 and cC4 F8 plasmas are slightly higher than those for the C4 F6 and
C5 F8 plasmas. The selectivity against resist or Si is smaller
in the C3 F6 , C4 F8 and c-C4 F8 plasmas than in the C4 F6 and
1.0
10.0
Contact Hole Size (µm)
Fig. 10. Microloading effect at 3 mTorr, 600 W source power and 200 W
bias power.
C5 F8 plasmas. Figure 10 shows the microloading effect of
SiO2 contact holes in these plasmas. The etch rates of the
contact holes are normalized by that of a 4.5-mm-diameter
hole. In the case of C4 F6 plasma, the normalized etch rate for
a 180 nm hole is about 0.8 and the microloading effect is not
so severe. The normalized etch rate for the holes ranging
from 500 nm to 2.5 mm is larger than 1.0 except for the C4 F6
plasma. This is a reverse tendency of the ordinary microloading effect. As shown in Fig. 11, the etched holes have
tapered shape except for in the case of the C4 F6 plasma. The
tapered shape gives rise to an ion-focusing effect at the
bottom. This ion-focusing effect and high-density fluorocarbon polymers are the origins of the reverse-microloading
effect in the plasmas except C4 F6 . As shown in Fig. 11, in
the case of the c-C4 F8 and C3 F6 plasmas, the widening at the
top and narrowing at the bottom of the contact holes are
significant. This is explained by the widening of the resist in
the shape of a facet because of the low etch selectivity for
resist of SiO2 . The nearly vertical etching profile is obtained
with the C4 F6 plasma. In this case, the SiO2 etch rate is
507 nm/min and the selectivity against resist and Si are 2.1
and 6.1, respectively. The diameter and depth of the hole,
Jpn. J. Appl. Phys. Vol. 42 (2003) Pt. 1, No. 9A
S. NAKAMURA et al.
c-C4F8
Resist
5763
C4F6
C4F6 / O2(10 %)
4mTorr , ICP400W, Bias150W
5mTorr , ICP400W, Bias200W
Resist
Resist
SiO2
1µ
µm
Si
SiO2
SiO2
1µ
µm
1µm
Si
(a) φ max 0.49µm , D1.04µm
Si
C3F6
C4F8
Resist
Resist
SiO2
SiO2
(b) φ d 0.2µm
( φ max 0.10µm, D 0.95µm)
C4F6 / Ar(25 %)
C5F8 / Ar(50%)
5mTorr , ICP400W, Bias200W
7mTorr , ICP600W, Bias200W
1µm
1µm
Resist
Resist
SiO2
SiO2
Si
Si
(b) φ max 0.55µm , D1.08µm
Resist
(a) φ d 0.18µm
( φ max 0.12µm, D 0.95µm)
(c) φ max 0.43µm, D1.04µm
C4F6
Resist
SiO2
Si
1µm
Si
Si
(d) φ max 0.20µm , D1.32µm
Si
(c) φ d 0.2µm
( φ max 0.13µm, D 1.11µm)
SiO2
1µm
1µm
1µm
C5F8
(f) φ max 0.22µm , D1.01µm
Fig. 11. Etch profiles of 0.2 mm contact holes at 3 mTorr, 600 W source
power and 200 W bias power. max and D represent the maximum
diameter and depth of etched contact holes, respectively.
shown in Fig. 11(d) for C4 F6 , are 200 nm and 1.32 mm,
respectively. The fluorocarbon polymer film deposited in the
C4 F6 plasma has a rough surface and the lowest density as
described previously. Hence, it seems that etch reaction
products are released easily and the SiO2 etch rate is
consequently in an acceptable range. The good etch profile is
due to the appropriate balance between the high-density CFþ
ions with a low etching efficiency and the low-density
fluorocarbon polymers. In the C5 F8 plasma, the etch profile
exhibits a slight side etching since the ions reflected at the
tapered resist are more than in the C4 F6 plasma. The
microloading effect is also higher than in the C4 F6 plasma
because the CFx (x ¼ 1{3) radical densities and the film
density of fluorocarbon polymers in the C5 F8 plasma are
higher than in the C4 F6 plasma. In the C4 F8 plasma, the etch
profile is not shrunk at the bottom of the contact holes. This
can be attributed to the higher electron density and the lower
ion-focusing effect of the C4 F8 plasma than those of the cC4 F8 and C3 F6 plasmas.
3.3 Fine contact hole
The Ar gas dilutes the fluorocarbon gas and the O2 gas
decomposes the polymeric radical to depress excess deposition. These gases generally prevent etch stop. The contact
holes of about 100 nm in diameter, which are smaller than
the mask resist patterns, are obtained in the C4 F6 , C5 F8 or a
mixture plasma with Ar or O2 . The hole-size reduction was
realized by the local deposition of fluorocarbon polymer
around the resist opening by increasing the pressure of the
etching atmosphere from 3 mTorr to 4–7 mTorr. Figure 12
(d) φ d 0.17µm
( φ max 0.13µm, D1.06µm)
Fig. 12. Etch profiles of the fine contact holes formed with C4 F6 , C4 F6 /
O2 (10%), C4 F6 /Ar (25%) and C5 F8 /Ar (50%) plasmas. d , max and D
represent designed diameter, etched maximum diameter and etched depth
of the contact holes, respectively.
shows the SEM crosssection of the fine contact holes etched
with C4 F6 , C4 F6 /O2 (10%), C4 F6 /Ar (25%) and C5 F8 /Ar
(50%) plasmas. Figure 13 shows the positive ion content in
the C4 F6 , C4 F6 /O2 (10%) and C4 F6 /Ar (25%) plasmas. The
CFþ is the dominant etching species and the others are less
significant in these plasmas. Figure 14 shows CFx (x ¼ 1{3)
radical densities in the C4 F6 /O2 (10%) and C4 F6 /Ar (25%)
plasmas. Certain radicals do not contribute to etching the
fine contact hole in both plasmas. It is possible that both the
polymeric radicals except CFx (x ¼ 1{3) and the CFþ ion
play an important role in etching the fine holes in the C4 F6 ,
C4 F6 /O2 (10%) and C4 F6 /Ar (25%) plasmas.
Ar+
CF2+
CF+
C4F6/Ar (25%)
5mTorr
ICP400W,Bias200W
C+
F+
O+
CF+
Ar+
CF2+
C4F6/O2 (10%)
5mTorr
ICP400W,Bias200W
CF+
C4F6
4mTorr
ICP400W,Bias150W
CF2+
C+
C+
F+
CF+
CF2+
0%
20%
40%
60%
Ion Count (%)
80%
100%
Fig. 13. The positive ion content in the C4 F6 , C4 F6 /O2 (10%) and C4 F6 /
Ar (25%) plasmas.
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Jpn. J. Appl. Phys. Vol. 42 (2003) Pt. 1, No. 9A
S. NAKAMURA et al.
result of the plasma diagnostics and the SiO2 etch properties
have shown that the C4 F6 and C5 F8 gas can be used as the
PFC alternatives, which can realize acceptable etch profiles
for contact holes. A fine hole of approximately 0.1 mm in
diameter can be formed in the C4 F6 , C5 F8 and the mixture
gas plasma with Ar or O2 . These results are obtained as a
consequence of the good balance between the low-density
film of fluorocarbon polymers and the many CFþ ions with
low etching efficiency. The straight-chain C4 F8 can also be a
promising dry-etch gas if etch conditions such as bias
voltage are optimized.
CF3
C4F6/O2 (10%)
5mTorr
ICP400W
Bias200W
CF2
CF1
CF3
C4F6/Ar (25%)
5mTorr
ICP400W
Bias200W
CF2
CF1
1010
1011
10 12
10 13
Radical Density (cm-3)
Fig. 14. CFx (x ¼ 1{3) radical densities in the C4 F6 /O2 (10%) and C4 F6 /
Ar (25%) plasmas.
4.
Conclusion
The straight-chain unsaturated fluorocarbon compounds
were compared with the conventional c-C4 F8 gas to evaluate
them as PFC alternative candidates for dry-etch gases. The
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