22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Heating mode transition in capacitively coupled CF 4 discharges
G.H. Liu, Y.X. Liu, D.Q. Wen and Y.N. Wang
Key Laboratory of Materials Modification by Laser, Ion, and Electron Beams (Ministry of Education), School of
Physics and Optoelectronic Technology, Dalian University of Technology, CN-116024 Dalian, P.R. China
Abstract: A heating mode transition between the α- and drift-ambipolar mode in
capacitively coupled CF 4 discharges was studied by using a combined method of
experimental diagnostic and Particle-in-Cell/Monte Carlo collision simulation. The
transitions between the two modes were observed when changing driving frequency.
Keywords: heating mode transition, phase resolved optical emission spectroscopy, particle
in cell simulations, capacitive coupled plasmas
1. Introduction
Capacitively coupled plasmas (CCPs) have been widely
employed in material processing such as plasma etching
or plasma enhanced chemical vapor deposition in
semiconductor manufacture and flat panel display
industries [1]. In processing plasmas, complex mixtures
of reactive, often electronegative gases, such as CF 4 , are
widely used to etch silicon and silicon dioxide [1]. In
contrast to electropositive discharges, a different heating
mode was observed in time-averaged results of simulation
studies of CF 4 discharges. This mode is characterized by
a high ionization and electron mean energy in the plasma
bulk [2-6]. The bulk heating mode was found to be
caused by high electric field in the bulk, which accelerates
electrons to high energies, causing significant ionization
or excitation in the discharge center.
Schulze et al. [5] observed a novel operation mode, i.e.,
so-called Drift-Ambipolar (DA) mode in electronegative
CF 4 capacitive discharges, which is characterized by the
ionization maxima inside the bulk and at the collapsing
sheath edges at distinct times within the rf period.
However, the bulk heating mode, as a typical
characteristic of electronegative plasmas, was found not
to be always significant in the electronegative discharges.
By using the Particle-in-Cell/Monte Carlo collision
(PIC/MCC) model, Denpoh et al. [2, 3] and Proshina et
al. [4], observed a typical transition of electron heating
mode from the bulk heating mode into classical electron
heating (α-mode) when decreasing the pressure and/or
increasing the driving voltage in CF 4 capacitive
discharges. By using the same model, Yan et al. [7, 8]
compared the differences of the pressure, the power and
the driving frequency effects on the spatiotemporal
profiles of the electron heating rate and ionization
dynamics in electropositive argon and electronegative
silane capacitive discharges, in particular, they observed a
phase shift of the electric field in the bulk plasma with
respect to that in the sheath region. Liu et al. [9, 10]
found that the phase relation between bulk electric field
and the sheath field could significantly affect the electron
bounce resonance heating when the discharge is driven at
different frequencies.
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Although the simulation investigations on the transition
of electron heating mode in electronegative discharges
have been more performed [2-5], most of these studies
concentrate on the discharges driven at 13.56 MHz. The
different electron heating modes induced by the driving
frequency have not yet been investigated, especially a
study based on the comparison between the experiments
and simulations.
In this work, we focus on a comparison investigation of
the heating mode transitions between the experiments and
simulations in capacitive coupled CF 4 discharges driven
at various frequencies. All the experimental results are
compared to PIC/MCC simulations.
2. Experiment setup
The schematic of the CCP reactor and the diagnostic
systems used for the measurements of phase resolved
optical emission spectroscopy (PROES) and electron
density are shown in Fig. 1. The plasma is produced
between two parallel circular electrodes made of stainless
steel with an equal diameter of 10 cm and an electrode
spacing of 1.5 cm, and both electrodes are surrounded by
Taflon. A sinusoidal rf signal is generated by a twochannel function generator (Tektronix AFG 3252C),
which synchronously generates a square signal from the
other channel to trigger the pulse delay generator for the
synchronous measurement of PROES. The sinusoidal rf
signal is amplified by a power amplifier (AR, Model
1000A225) and then applied on the upper electrode
through the matching networks. The lower electrode and
the chamber wall are grounded. 10% neon is added into
working gas, CF 4 , as the tracer gas used for the timeresolved emission measurement.
The voltage waveform is measured by a high-voltage
probe and acquired with a digitizing oscilloscope (LeCroy
Waverunner). The measured voltage waveforms are
adopted as the potential boundary in PIC/MCC
simulations. The electron density is measured by utilizing
a hairpin probe [11]. The measured electron density is
then corrected using the fluid model described by Piejak
et al. [12]. An intensified charge-coupled device (ICCD)
camera (Andor iStar DH734) is synchronized with rf
1
Fig. 1.
Schematic diagram of the CCP chamber,
supplemented with the hairpin probe and the phase
resolved optical emission spectroscopydiagnostic
systems.
signal to obtain the time-resolved emission intensity. The
spatiotemporal distribution of electron impact excitation
can be obtained from the measured spatiotemporal
emission intensity by using the method described in [13].
3. PIC/MCC simulation
A 1D3v electrostatic PIC/MCC model, which is a
modul of the multi-physics analysis of plasma sources
(MAPS) platform developed by our group [14, 15], is
employed to simulate the CF 4 capacitive discharge in this
work. In the simulation, we neglect the effect of the
addition of 10% neon gas and simplify the calculation by
considering pure CF 4 as the working gas. The discharge
is assumed to be produced between two infinite parallelplate electrodes. One electrode at x = 1.5 cm is grounded
and the other at x = 0 is driven by a single-frequency
voltage source, whose waveforms are obtained from the
experimental measurements. Four types of charged
species, i.e., CF 3 +, CF 3 −, F− ions and the electron, are
traced in the code. In the Monte Carlo collision part, we
take into account the same collision processes as in [16].
To make further simplification, the secondary electron
emission coefficient and reflection coefficient from the
electrodes are assumed to be zero, because the discharge
is driven at very low rf power and γ-model is beyond the
scope of our work.
An explicit scheme is used in all simulations. The
initial ion and electron temperatures are set as 0.026 eV
and 3 eV, respectively. Typically, the codes run for over
10000 rf cycles to reach equilibrium. After the system
reaches equilibrium, the plasma parameters, including the
spatiotemporal distribution of the CF 4 ionization rate, the
densities of various charged species and EEPF, are
calculated.
4. Results and discussion
Fig. 2 shows the spatiotemporal evolution of the
excitation rate (top row), ionization rate (middle row) and
electric field (bottom row) during two rf cycles in
discharges driven at various frequencies of 13.56 MHz
(left column), 27.12 MHz (middle column) and 40 MHz
(right column), at a fixed power of 20 W and pressure of
2
50 Pa. Analogous to the case of the heating mode
transition induced by the power and pressure, the
variation of the driving frequency can induce the
transition of electron heating mode. As is seen from the
spatiotemporal evolutions of the excitation and ionization
rates in the top and middle rows in Fig. 2, we can observe
that by decreasing the driving frequency at a fixed power
of 20 W, a hybrid heating mode (α and DA modes) at
40 MHz (reference case) transitions into a DA mode,
more strictly, an ambipolar field dominated heating mode
at 13.56 MHz, and simultaneously both the drift field in
the bulk and the ambipolar electric field at the collapsing
sheath edge are significantly enhanced. Especially at
13.56 MHz, the ambipolar electric field far exceeds the
drift field, which explains the prominent maximum of
excitation and ionization rates at the collapsing sheath
edge at 13.56 MHz in Fig. 2(a1) and 2(b1).
Fig. 2. Spatiotemporal plots of the electron impact
excitation rate (top row, experimental results), ionization
rate (middle row, simulation results), and corresponding
electric field (bottom row, simulation results) in
discharges driven at 20 W and 50 Pa. Left column:
f = 13.56 MHz. Middle column: f = 27.12 MHz. Right
column: f = 40.68 MHz. The color scales are given in
units of arbitrary unit (excitation rate), 1021 m-3s-1
(ionization rate) and 103 Vm-1 (electric field).
Although in experiment at 60 MHz, we cannot
satisfactorily resolve time-dependent electron dynamics
within one rf cycle by PROES, as the lifetime of Ne2p1
state (i.e., 14.5 ns) is comparable to one rf period (i.e.,
16.7 ns), a tendency can be well predicted by inspecting
the variation of several charged species and the
electronegativity with the driving frequency, as shown in
Fig. 3. By increasing the driving frequency from
13.56 MHz to 60 MHz, the densities of all charged
species are obviously raised, despite that at a fixed rf
power of 20 W the corresponding measured rf voltage
between the electrodes decreases from 255 V to 150 V.
The electron density at the discharge center is measured
by a hairpin probe and compared to the calculated one.
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We find that the calculated results are approximately
double of the measured one, and more importantly a
generally qualitative agreement is obtained (see both
black solid and hollow squares in Fig. 3). On the contrary,
the electronegativity α decreases drastically from 88 to 36,
due to a much faster increase in the electron density than
other ions. This explains the weakened DA field as well
as the mode transition upon increasing the driving
frequency. Note that at 60 MHz, the discharge is still
quite electronegative (α = 36), and thus its EEPF still
exhibits a Druyvesteyn-like distribution, which is quite
similar to those at low driving frequencies (see Fig. 4).
This is because that the discharge is operated at a very
low power (20 W). So, we expect that a complete
transition of heating mode from the DA mode to α mode
induced by the driving frequency be seen either when the
rf power is fixed at a higher value (e.g., 100 W), or when
the upper limit of the driving frequency is extended to a
higher value (e.g., f > 60 MHz).
Fig. 4. Normalized EEPFs resulting from the PIC/MCC
simulations for the pure CF 4 discharge for different
driving frequencies: f = 13.56 MHz, 27.12 MHz,
46.68 MHz, and 60 MHz, and other conditions are the
same to these in Fig. 2.
silane [7, 8] and pure oxygen [10], in which the PIC/MCC
simulation results show that at 13.56 MHz, the bulk
electric field is in phase with the sheath field, which is
consistent with our results in Fig. 2(c1). When increasing
the driving frequency, the phase of the bulk electric field
will delay with respect to the sheath field. Especially, at
60 MHz the bulk electric field becomes out of phase with
the sheath field, which is quite different from our results,
e.g., these in Fig. 2(c2) and 2(c3).
Fig. 3. The densities of various charged species versus
driving frequency in discharges with the same parameters
to Fig. 2. Note that the black hollow square is the
electron density measured by a hairpin probe, while the
others are obtained from PIC/MCC simulations. Note
that the electron densities from both the experiment and
simulation were multiplied by a factor of 10.
In addition, the driving frequency significantly affects
the electronegativity, and thus the mode transition points
induced by the power and pressure. Similar studies show
that at a lower frequency, i.e., 27.12 or 13.56 MHz, the
transitions can also take place when changing the gas
pressure or rf power and the results are very similar to
that of 40 MHz. However, the difference is that as the
driving frequency is increased, the gas pressure, at which
the heating mode transition takes place, is increased,
while the rf power, at which the heating mode transition
takes place, is decreased.
It should be mentioned that at different driving
frequencies, our simulation results (Fig. 2c) show that the
phase difference between the bulk electric field (DA field)
and the sheath field is a constant. However, different
results are found in capacitive discharges operated in pure
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5. Conclusion
The electron heating mode transitions in capacitively
coupled CF 4 discharge were studied by synergistically
using two diagnostic methods in combination with
PIC/MCC simulations. Based on the method of PROES
of trace rare gas, the spatiotemporal evolutions of
energetic electrons (> 20 eV) were presented. The
time-average electron density at the discharge center was
measured by using a hairpin probe. All the experimental
results were compared to those obtained from PIC/MCC
simulations. Two different electron heating modes were
observed depending on the discharge conditions: 1) the α
mode, in which the electron heating maximum mainly
occurs near the sheath boundary, due to the electrons
energized during sheath expansion phase, 2) the driftambipolar (DA) mode, in which the electron heating
maxima occur throughout the entire bulk plasma and near
the collapsing sheath edge, due to the electrons
accelerated by a drift electric field in the bulk region and
an ambipolar field at the sheath edges during its
collapsing phase. The driving frequency is found to
significantly affect the electronegativity, i.e., as the
driving frequency increases, the discharge becomes more
electropositive, and the sheath heating (α mode)
dominates. Furthermore, we could conclude that as the
driving frequency is increased, the gas pressure, at which
the mode transition takes place, is increased, while the rf
3
power, at which the mode transition takes place, is
decreased.
6. Acknowledgements
This work was supported by the National Natural
Science Foundation of China (NSFC) (Grant No.
11335004) and (Grant No.11405018), the International
Science & Technology Cooperation Program of China
(Grant No. 2012DFG02150) and the Important National
Science and Technology Specific Project (Grant No.
2011ZX02403-001).
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