22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Electron excitation dynamics at the ignition phase in pulsed capacitively coupled
plasmas
Y.X. Liu, G.H. Liu, D.Q. Wen, F. Gao 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, 116024 Dalian, P.R. China
Abstract: Electron excitation dynamics at the ignition phase in pulsed capacitively
coupled plasmas are studied by using phase resolved optical emission spectroscopy. Two
emission intensity maxima were found at the ignition phase. The former is due to the
ignition of the plasma at the electrode edge, while the latter is ascribed to the delayed
ignition at the electrode center.
Keywords:
emission intensity, ignition phase, phase resolved optical emission
spectroscopy, pulse modulation, 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]. As device feature size shrinks, leading to
higher density of devices on the wafers, processing
demands are becoming more severe [1-2].
In order to meet these demands, it is necessary to
independently control plasma properties, such as electron
temperature and density, ion flux and energy, radical and
neutral fluxes, and negative ion density. Unfortunately, in
conventional plasma sources, achieving this flexible
independent control of plasma parameters is difficult
since most of these plasma parameters are strongly
coupled (such as ion flux and ion energy). Hence, meeting
the desired on-wafer ultra-small device feature size might
be limited. Thus, there is a need for wider and more
flexible ranges of plasma operating conditions to improve
etch process control [3].
Pulsed discharge is a subject of great interest for many
plasma processing applications [4-6] mainly because its
ability to control the plasma density, the mean ion energy
and the time-averaged electron temperature T e , which
overall governs the fluxes bombarding a processing
substrate. The reaction-rates of complex molecular
species [6] in the discharge depend on the electron energy
distribution function which can be efficiently modified by
operating the discharge in pulsed-mode.
The initial stage of a pulse period is critical for the
process, because in this stage the plasma parameters
change dramatically, for example, a bias voltage is
formed on the blocking capacitor [7], and the electron
temperature rapidly increases above the steady state value
[8, 9]. Therefore, a more systematic investigation,
especially at the initial stage of a pulsed discharge, is
needed to understand the discharge dynamics and
temporal evolution of plasma parameters. This can not
only provide a better physical insight, but it can also give
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information on how to optimize the operating conditions.
However, in spite of the fact that pulsed plasmas are
widely investigated, most of these studies on pulsed
plasma are focused on a macroscopic time scale (i.e., a
time resolution on the pulsed period (~μs)), while a
temporal and spatial resolved study with a high resolution
has not been found, especially at the ignition phase of
pulsed plasma.
In this work, we used phase resolved optical emission
spectroscopy to detect the spatiotemporal dynamics of the
energetic electrons at the ignition phase of pulsed
discharge operating in neon and reveal the underlying
physics process when the plasma is ignited.
2. Experiment setup
Fig. 1. Schematic diagram of the CCP chamber,
supplemented with the hairpin probe and the phase
resolved optical emission spectroscopy diagnostic
systems.
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 in
neon between two parallel circular electrodes made of
stainless steel with an equal diameter of 6 cm and an
electrode spacing of 1.5 cm, and both electrodes are
surrounded by Taflon. The rf power had a frequency of
12.5 MHz, and was generated by a two-channel function
1
generator (Tektronix AFG 3252C). This power was
modulated by the pulse (square wave) generated by the
same device. In this experiment, the pulse repetition
frequency and duty cycle are fixed at 1 K and 50 %,
respectively. The same function generator synchronously
generates a square wave signal (1K) from the other
channel to trigger the pulse delay generator for the
synchronous measurement of PROES. The pulsemodulated sinusoidal rf signal is amplified by a power
amplifier (AR, Model 1000A225) and then applied on the
upper electrode through the matching networks (see fig.
2). The lower electrode and the chamber wall are
grounded.
can be obtained from the measured spatiotemporal
emission intensity by using the method described in [10].
3. Results and discussion
Fig. 3 shows a typical waveform over two pulsed
periods measured at the experimental conditions: rf
frequency of 12.5 MHz, pulsed repetition frequency of 1
K, duty cycle of 50%, input power of 30 W, working
pressure of 50 Pa. One can see that when the plasma is
initially ignited, the power electrode is charged negatively
by the electrons due to geometrical asymmetry of the
chamber, and then a negative dc bias is built up across the
sheath region close to the power electrode. At stable stage
of the discharge, the dc bias stays constant at -700V. At
the beginning of the afterglow, the negative charge in the
power electrode starts to be neutralized by the ions in the
discharge zone, so the dc bias gradually diminishes and
approaches zero at the end of the afterglow period.
Fig. 2. Schematic diagrams of the network circuit and the
measurement of the voltage waveform on the upper
electrode.
Fig. 3 A typical waveform measured at frequency of 12.5
MHz, pulsed repetition frequency of 1 K, duty cycle of
50%, input power of 30W, pressure of 50 Pa.
The voltage waveform is measured by a high-voltage
probe and acquired with a digitizing oscilloscope (LeCroy
Waverunner) (see fig. 2). An intensified charge-coupled
device (ICCD) camera (Andor iStar DH734) is
synchronized with pulsed signal to obtain the timeresolved emission intensity. Due to the neglectable
cascade processes, the emission spectrum of Ne2p1-state,
whose excitation threshold energy is 19 eV, at 585.5 nm,
is detected through a narrow-band filter centered on the
desired optical transition with a bandwidth of 10 nm. The
lifetime of Ne2p1-state, i.e., 14.5 ns, is short enough to
resolve electron dynamics within one RF period at the
driving frequencies (12.5 MHz) used in this work. The
spatiotemporal distribution of electron impact excitation
2
Fig. 4. Spatiotemporal plots of the light emission intensity
at (a) the early glow of a pulsed period and (b) the rapid
changes in the first 8 μs of the ignition phase in a
discharge driven at 30 W and 20 Pa. Note that in (a) the
step size is 1 μs, and in (b) the step size is 80 ns (i.e.,
exactly one rf period). The color scales are given in units
of arbitrary unit (excitation rate).
Fig. 4 shows the spatiotemporal evolution of the light
emission intensity at (a) the early glow of a pulsed period
and (b) the rapid changes in the first 8 μs of the ignition
phase in a discharge driven at 30 W and 20 Pa. It can be
clearly seen that at the very early phase of the phase
period, two emission intensity maxima occur successively,
i.e., the first one at t1 ≈ 0.5 μs and the second one at t4 ≈
3.5 μs. The reason for the two emission intensity maxima
can be understood by the analysis of the fig. 5, which
shows the pictures taken by the ICCD camera at fours
typical time points, marked in fig. 4(b). At the phase t1,
one sees from fig. 5(a) the plasma is initially ignited at the
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edge of the electrode and close to the power electrode.
And then the energetic electrons spread toward the ground
electrode, but still concentrate at the electrode edge, until
the phase t4 (see fig. 5(d)), when plasma is again ignited
at the center and close to the power electrode. The second
ignition of the plasma at the electrode center gives rise to
the emission intensity maximum at t4, as shown in fig.
4(b).
[5] V. Šamara, M. D. Bowden and N. St. J. Braithwaite, J.
Phys. D: Appl. Phys., 43, 124017 (2010).
[6] V. I. Demidov, C. A. DeJoseph and A. A.
Kudryavtsev, IEEE Trans. Plasma Sci., 34, 825 (2006).
[7] H. B. Smith, C. Charles, R. W. Boswell, and H.
Kuwahara, J. Appl. Phys., 82, 561 (1997).
[8] A. Mishra, M. H. Jeon, K. N. Kim and G. Y. Yeom, J.
Appl. Phys., 21, 055006 (2012).
[9] V. Samara, M. D. Bowden and N. St. J. Braithwaite, J.
Phys. D: Appl. Phys., 43, 124017 (2010).
[10] T. Gans, der Gathen V Schulz-von and H. F. Döbele,
Contrib. Plasma Phys., 44, 523 (2004).
Fig. 5. The 2D pictures taken by the ICCD camera
through the quartz window at the sidewall four typical
time points, t1~t4 , which are marked in fig 4(b). The
experimental conditions are same to fig. 4. Note that
every picture is obtained with an gate width of 80 ns, i.e.,
one rf period.
4. Conclusion
Electron excitation dynamics at the ignition phase in
pulsed capacitively coupled plasmas are studied by using
phase resolved optical emission spectroscopy. Two
emission intensity maxima were found at the ignition
phase. The former is due to the ignition of the plasma at
the electrode edge, while the latter is ascribed to the
delayed ignition at the electrode center. Due to the
geometrical asymmetry of the chamber and blocking
capacitor in series with the power electrode, a negative dc
bias is built up during pulse on and the negative charge is
neutralized by the ions from the discharge zone after the
pulse is off.
5. 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. 2011ZX02403001).
6. Reference
[1] M. A. Lieberman and A. J. Lichtenberg, Principles of
Plasma Discharges and Materials Processing 2nd edn
(New York: Wiley) (2005).
[2] P. Chabert and N. St. J. Braithwaite. Physics of radiofrequency plasmas. (Cambridge University Press) (2011).
[3] S. Banna et al., IEEE Transactions on Plasma Science
37, 1730 (2009).
[4] D. J. Economou, J. Phys. D: Appl. Phys., 47 303001
(2014).
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