Download Fragmentation and electronegativity of C4F8 plasmas in both

st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Fragmentation and electronegativity of C4F8 plasmas in both inductive and capacitive discharges
S. X. Zhao1,2, F. Gao1, Y. N. Wang1, A. Bogaerts2
1
PSEG team, Physics and Optoelectronic College, Dalian University of Technology, China
2
PLASMANT group, Chemistry Department, University of Antwerp, Belgium
Abstract: The statuses of dissociation, ionization and attachment of fluorocarbon plasmas are
very important to the silicon-material-based etching process, since they microscopically modulate the realistic pattern feature and etching profile by means of, i.e., ratio of ions to neutrals
concentration as well as species density and incident flux profiles characteristics. These properties strongly depends on reactor types, discharge conditions and ratios of inertial gases and
reactive gases etc. In this paper, the widely-used complex fluorocarbon plasma C4F8 was
studied by the hybrid plasma equipment model (HPEM) and the evolutions of the plasma
properties in both inductive and capacitive discharge modes are investigated under separately
typical applied power, radio frequency and gas pressure values. The obvious difference in the
dissociation, ionization and attachment processes caused by the external factors was observed
and the intrinsic mechanisms were discussed.
Keywords: Dissociation, Ionization, Attachment, C4F8 plasma, HPEM simulation
1. Introduction
To save fabricating costs and to improve the chip
properties, the micro-electronics industry is constantly
confronted with extremely high criteria for the Si-based
etching process with fluorocarbon plasma sources, such as
high etching rate, selectivity, anisotropy and high-aspect
ratio [1-4]. To achieve this goal, the fundamental fluorocarbon plasma properties need to be explored, including
the discharge principles and its interaction with surface
materials, for different discharge conditions and reactor
configurations.
An inductively coupled plasma (ICP) reactor is
characterized by a coil, separated from the reactor by a
dielectric window. Radiofrequency (RF) current is flowing through the coil, inducing an electromagnetic field in
the plasma reactor. It has many advantages over other
reactors, i.e., a simple structure, low pressure and the possibility of separate control over the bulk plasma density
(and ion flux) and the ion energy bombarding the wafer
[5]. Especially the low-pressure makes the ICP reactor the
first choice for the Si-based etching industry, since it can
generate a highly anisotropic ion flux towards the wafer
surface, which results in high-aspect ratio etching.
A capacitively coupled plasma (CCP) reactor, on the
other hand, is created by applying a (RF) potential difference between two parallel electrodes in a reactor. It is
characterized by a uniform radial profile of the plasma
density, at least when the frequency of the applied RF
power is not very high[6]. Nowadays, to save manufac-
turing costs, very-large-dimension (VLD) wafers are
proposed. This trend makes CCP reactors also promising
for Si-based etching, due to its superiority in spatial uniformity[7].
The main difference between an ICP and CCP is that
the plasma in an ICP is sustained by an azimuthal electric
field, while in a CCP the plasma is generated by a radial
and an axial electric field; see Fig. 1. Hence, even at the
same discharge conditions, the fc plasma properties generated in ICP and CCP reactors may be different, and this
will influence the etch process.
The electronegative property is very important for
the industrial reactive plasmas with working gases like
CF4, C4F8, SF6 and Cl2 etc. It is closely related to the ebehavior and chemical radicals in the bulk plasma and
therefore has essential influence on the wafer etching
characteristics, especially when pulse sources are used
and the negative ions can arrive at and interact with the
wafer surface[8].
Therefore, in this paper, the widely-used C4F8 plasma were selected and the hybrid plasma equipment model
(HPEM) was used to investigate the fragmentation and
electronegativity properties of the plasma sources.
2. Model description
For the simulations we made use of the HPEM, developed by Kushner and co-workers [9,10]. In this mod
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21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
The C4F8 reaction set is much more complicated,
since one C4F8 molecule has 4 C atoms and 8 F atoms.
The fragmentation of C4F8 can be diverse, and hence the
C4F8 plasma contains light (i.e., CFx and CFx+), middle
(i.e., C2Fx or C2Fx+) and heavy (i.e., C4Fx or C4Fx+) fc
radicals and ions. The species considered in the model for
the C4F8 plasma are presented in Table 1. Again, for all
these species the corresponding surface reaction coefficients and the collision processes in the plasma were
specified. The complex reaction set can be referred in
Ref.[11].
3. Results and discussion
In this section, the dissociation, ionization and attachment mechanisms of a C4F8 plasma and its dependence on the discharge conditions and reactor configuration will be presented.
3.1 Fragmentation and electronegativity of C4F8 plasma in
ICP reactors
Fig. 1. Different ways of sustaining the plasma by the
electric field, an ICP and CCP reactors
Table 1. Species included in the model for the C4F8 gas, besides
the electrons
Type
Species
C species
C, C+
F species
F, F-, F+, F2, F2+
CFx species
CF, CF2, CF3, CF+, CF2+, CF3+
CxFy species
C2F3, C2F4, C2F5, C2F6, C3F5, C3F6,
C3F7, C4F8, C4F8-, C4F8*-, C2F3+,
C2F4+, C2F5+, C3F5+, C3F6+, C3F7+,
C4F7+
15
z (cm)
el, the electromagnetic field is calculated based on the
Maxwell equations in the so-called electromagnetics
module. Subsequently, the field is transferred to the electron Monte Carlo module, which describes the electron
dynamics. The rate coefficients of electron impact reactions and the electron temperature are obtained by integrating the calculated electron energy distribution function (EEDF). These quantities are then used as input in
the so-called fluid kinetic simulation to generate the densities of charged particles and various radicals by means
of continuity equations, as well as the electric field distribution, obtained from the Poisson equation. The electron conductivity, as a function of electron density, is inserted into the electromagnetics module to update the
fields. These three modules are solved iteratively until
convergence is obtained.
10
5
0
0
5
10
15
20
r (cm)
Fig. 2 Schematic picture of the ICP reactor assumed in the model. A two-turn coil is used. The C4F8 gas is pumped into the
chamber by the nuzzle along the edge ring near the sidewall.
The coil power and gas pressure are 500 W and 10 mTorr. The
frequency of the power source is 13.56MHz.
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
Fig. 3. Density of various neutrals, i.e., CFx, C2Fy, C3Fz, C4F7, C,
F, F2, in a C4F8 ICP reactor, averaged over the reactor geometry.
The discharge conditions are presented in Fig. 2.
the sum of C3F5+, C3F6+ and C3F7+. It can be seen from Fig.
19(a) that the CFα+ ions are again the most important species at the conditions under study. This behavior corresponds to experimental data [12] under similar discharge
conditions. As for the negative ions, C4F8-, which is generated by direct attachment of electrons with C4F8, is
found to be the most important type of negative ions.
3.2 Fragmentation and electronegativity of C4F8 plasma in
CCP reactors
z(cm)
4
(a)
2
0
0
5
10
15
20
25
r(cm)
Fig. 5. Schematic picture of the CCP reactor assumed in the
model. A shower head is fixed underneath the top electrode. The
pump is fixed at the bottom edge of the chamber. The bottom
electrode is powered by a 27.12MHz source. The applied power
and the gas pressure are 500 W and 10 mTorr, respectively.
(b)
Fig. 4 Densities of various positive ions (a) i.e., CFα+, C2Fβ+,
C3Fγ+, C4F7+, and negative ions (b) , i.e., C4F8-*, C4F8-, CF3-, F-,
in a C4F8 ICP reactor, averaged over the reactor geometry. The
discharge conditions are presented in Fig. 2.
The ICP reactor configuration and discharge conditions are specified in Fig. 2. In Figs. 3 and 4, the densities
of various neutral species and (positive and negative) ions
are presented, illustrating the total dissociation, ionization
and attachment status of the C4F8 ICP. The discharge conditions are as follows. The coil power and the gas pressure are 500 W and 10 mTorr. The frequency of the power
source is 13.56MHz. The main neutrals are classified into
7 types, i.e., CFx, C2Fy, C3Fz, C4F7, C, F, F2. Here CFx
means the sum of CF, CF2 and CF3. C2Fy means the sum
of C2F3, C2F4 , C2F5, and C2F6. C3Fz means the sum of
C3F5, C3F6 , and C3F7. It can be seen from Fig. 3 that the
CFx radicals have the highest density in the plasma, at the
conditions under study.
In fc plasmas, positive ions are generated mainly by
electron-impact ionization or dissociative ionization, and
the negative ions are generated mainly by electron attachment or dissociative attachment. In Fig. 4, both positive and negative ions are plotted to examine the ionization and attachment mechanisms in the C4F8 ICP reactor.
Again, the main positive ions are lumped as CFα+, C2Fβ+,
C3Fγ+, C4F7+, where CFα+ means the sum of CF+, CF2+ and
CF3+, C2Fβ+ is the sum of C2F3+ , C2F4+ and C2F5+, C3Fγ+ is
Fig. 6 Densities of various neutrals, i.e., CFx, C2Fy, C3Fz, C4F7,
C, F, F2, in the C4F8 CCP reactor, averaged over the reactor geometry. The discharge conditions are specified in Fig. 5.
Fig. 5 illustrates the CCP reactor used in the simulations. Calculations were performed for the following conditions: The bottom electrode is powered by a 27.12MHz
source. The applied power and the gas pressure are 500 W
and 10 mTorr, respectively. Figs. 6 and 7 present the densities of the various radicals, positive and negative ions in
the C4F8 CCP reactor. The same species are listed as in the
ICP reactor. It can be seen from Fig. 6 that now the C2Fy
radicals are the most important species in the CCP reactor.
As for the ions, we can see from Fig. 7(a) that both C2Fβ+
and C3Fγ+ are now the most important positive ions. For
the negative ions, Fig. 7(b) demonstrates that both the
excited negative ions C4F8*- and the light F- ions (which
are generated by dissociative attachment) are the most
st
21 International Symposium on Plasma Chemistry (ISPC 21)
Sunday 4 August – Friday 9 August 2013
Cairns Convention Centre, Queensland, Australia
important species. When comparing these results with the
ICP model results, it can be concluded that the main
chemical components are greatly dependent on the reactor
configuration (ICP vs CCP). This information is very important for the micro-electronics industry, as the chemical
composition in the plasma will affect the etching characteristics.
(a)
[1]H. Rhee et al, J. Vac. Sci. Technol. B 27, 33 (2009)
[2]M. Sekine, Appl. Surf. Sci. 192, 270 (2002)
[3]H. Motomura, Thin Solid Films, 390, 134 (2001)
[4]A. Sankaran and M. J. Kushner, Appl. Phys. Lett. 82, 1824
(2003)
[5]A. Maresca et al, Phys. Rev. E, 65, 056405 (2002)
[6]A. V. Vasenkov and M. J. Kushner, J. Appl. Phys., 95, 834
(2004)
[7]K. Ellinas et al, Microelectronics Engineering, 88, 2547
(2011)
[8]Lieberman M A and Lichtenberg A J 2005 Principles of
Plasma Discharges and Materials Processing (New York:
Wiley-Interscience)
[9]M. J. Kushner, J. Phys. D: Appl. Phys., 42, 194013 (2009)
[10]Y. Yang and M. J. Kushner, Plasma Sources Sci. Technol.,
19, 055011 (2010)
[11]A. V. Vasenkov et al, J. Vac. Sci. Technol. A, 22, 511 (2004)
[12]X. Li et al, J.Vac. Sci. Technol. A, 22, 500 (2004)
Acknowledgements
(b)
Fig. 7 Densities of various positive ions (a) i.e., CFα+, C2Fβ+,
C3Fγ+, C4F7+, and negative ions (b) , i.e., C4F8-*, C4F8-, CF3-, F-,
in the C4F8 CCP reactor, averaged over the reactor geometry.
The discharge conditions are specified in Fig. 5.
4. Conclusion
In this work, the big difference in the ICP and CCP
C4F8 fragmentations and electronegativities were observed by means of the hybrid plasma equipment model.
In the ICP reactor with its typically used discharge condition in industry, the light species are more dominated
while in CCP reactor with its typically used conditions,
the middle heavy species, like C2Fy ions and radicals are
more important. As for electronegativity, in ICP reactor,
the parent-attached negative species such as C4F8-* and
C4F8- are more dominated, while in CCP reactor, both
C4F8-* and F- fractions are more significant. The distinct
features of plasma fragmentations and electronegativities
in the two reactors reveals the influence of reactor configuration (discharge modes) and discharge conditions on
the internal plasma properties.
5. References
The work was done when Dr. Zhao was a postdoc in
PLASMANT group and now she is a young assisted professor in Dalian University of Technology (DLUT). The
job was financially supported by a fellowship of the Belgian Federal Science Policy, cofinanced by the Marie
Curie Actions of the European Commission. Besides, the
work was also financially supported by the joint research
project within the framework of the agreement between
the Fund for Scientific Research Flanders (FWO) and
MOST and by the National Natural Science Foundation of
China (NSFC) (Grant No 11075029). The calculations
were carried out using the Turing HPC infrastructure at
the CalcUA core facility of the Universiteit Antwerpen, a
division of the Flemish Supercomputer Center VSC,
funded by the Hercules Foundation, the Flemish Government (department EWI) and the Universiteit Antwerpen.