Voltage Waveform Tailoring in a RF-CCP reactor: Exploration

Voltage Waveform Tailoring in a RF-CCP reactor:
Exploration of self-bias, electron density and current waveform
Pierre-Alexandre Delattre1,2, S. Pouliquen1, E.V. Johnson2, J.P. Booth1
1 Laboratory of Plasma Physics, Palaiseau, France
2 Laboratory of Physics of Interfaces and Thin Films, Palaiseau, France
Introduction
Electron density
We have implemented Voltage Waveform Tailoring to
excite capacitive plasmas with arbitrary voltage
waveforms. In particular we have investigated peak
and valley waveforms (see figure 1). These lead to
electrical asymmetry [1], causing self-bias, even for a
geometrically symmetrical reactor. These waveforms
differ from standard sine waveforms by their fast
voltage rise-time (efficient for electron heating) and
their skewed duty cycle, which allows the average
sheath potential drop, and therefore the ion energy
hitting electrodes to be controlled.
The electron density in H2 plasma,
determined by hairpin quarter-wave
microwave resonator [3]
C
V
plasma
substrate
electron density (cm-3)
ground
Figure 1 : The RF-CCP reactor
used. Philix is geometrically
asymmetric (grounded /
powered area ratio≈ 2). The
substrate is on grounded
electrode for PECVD.
[1] Donko Z et al. J. Phys. D: Appl. Phys., 42(02):5205, 2009.
Electron density
Peaks > Valleys > Sine
Peaks
1E10
Valleys
1E9
Sine
100 mTorr
1E8
0
100 200 300 400 500 600 700
a)
Vpp (V)
Voltage Waveform Tailoring
electron density (cm-3)
Peaks
We create the desired voltage waveforms by superposing 4 harmonics
(fundamental at 15 MHz) and correcting for distortion in fourier space [2](Fig
3). The voltage is measured at the RF feedthrough.
0
-5
-100
-10
-200
Voltage (V)
0
-20
40
60
80
100
Time (ns)
Frequency Response
20
15
100
5
0
0
-5
-100
-10
Sine
Slopes are different
1E9
250 mTorr
1E8
100 200 300 400 500 600 700
b)
Vpp (V)
-15
-300
120
Valleys
0
10
-200
-15
-300
a)
200
10
5
20
V
selfbias
I
Current (A)
Voltage (V)
15
100
0
valleys
300
20
-20
0
20
b)
Capacitively Coupled
Plasma
no impedance
matching
40
60
80
100
120
Time (ns)
Figure 3 : Correction loop.
VTW optimise d waveforms
FG : function generator
RF : RF amplifier
C : decoupling capacitor
HV : high voltage probe
Derivative : Vigilant-VI
derivative probe
(www.solayl.com) probe
Note the absence of
impedance matching.
• Why do the complementary waveforms (peaks and
valley) give different electron density ?
The reactor is asymmetric. Geometrical asymmetry
and electrical asymmetry interact each other.
• How do you measure Vpp ?
Peak to peak voltage is measured at RF feedthrough
and not exactly at electrode. Therefore, there exists
some uncertainty on Vpp.
900 mTorr
1E10
Peaks
Valleys
1E9
Sine
Difference is
smaller at
higher pressure
1E8
0
100 200 300 400 500 600 700
c)
Vpp (V)
Figure 4 : Vpp is peak to peak voltage.
[3] Piejak R.B. J. Appl. Phys., 95(7):3785, 2004.
[2] Patterson et al. 2007 Plasma Sources Sci. Technol. 16 257
Discussion
electron density (cm-3)
V
selfbias
I
200
Correction (in fourier space)
signal
power
numerical
peaks
300
Current (A)
Figure 2 : Tailored
voltage waveform &
current waveforms.
The self-bias is a
combination of
geometrical and
electrical asymmetry
(a : Vpp = -80 - 100 =
-180 V, b : Vpp = -80 +
100 = +20 V). Duty
cycle are a) 35 % and
b) 65 %.
1E10
Conclusion
Perspective
• The peaks and valleys waveforms
lead to significantly higher
electron density than sine due to
faster slew rate for the same Vpp.
• The electrical asymmetry effect is
sensitive to geometrical
asymmetry of the reactor
• Use a more symmetric
reactor
• Measure voltage directly at
the electrode
• Investigate uniformity in a
larger reactor
• Measure ion energy
distribution function
Thanks : JPB, EVJ, SP & S. Dine Fundings : CNRS-PIE STEP-UP, ANR-CANASTA, Ecole Polytechnique Bourse de Thèse
[email protected]
+331 6933 5871
http://www.lpp.fr/?Pierre-Alexandre-Delattre