九州大学大学院総合理工学府報告第 35 巻 第 1 号 6-11 頁 平成 25 年 9 月

Engineering Sciences Reports, Kyushu University
Vol. 35 No. 1, pp. 6-11, SEP., 2013

Investigation of VHF Argon Plasma at High Pressure by
Balanced Power Feeding Using Laser Thomson
Scattering
Weiting CHEN*1,† Kohei OGIWARA*2 Koichiro KOGE*1Kentaro TOMITA*3 Kiichiro UCHINO*3 and Yoshinobu KAWAI*3
E-mail of corresponding author: wtchen0311@gmail.com†（Received July 12, 2013, accepted July 16, 2013）
The dependences of the VHF plasma parameters on the gas pressure and power were examinedby using the Laser Thomson scattering method, where the VHF plasma was produced in the highpressure region by the balanced power feeding method. It was found that the balanced power feeding
method provides a high electron density plasma with low electron temperature at high pressures.
This characteristics were confirmed by calculations using a 2-dimensional simulation code.
Key words: VHF plasma, laser Thomson scattering, balanced power feeding, 2-dimensional

simulation

1.

Introduction

A very high frequency (VHF) plasma iswidely
usedforthefabricationofmicrocrystalline silicon thin film solar cells1,2)
because it provides high deposition rates.
Recently the high pressure depletion
method3,4) has been proposed to increase thedeposition rate. In this case, the VHF plasma
is produced with the narrow discharge gap athigh pressures. Now faster depositions of
microcrystalline silicon are needed for further
cost reduction of solar cells. The deposition
rate is proportional to the electron density, sothat the production of a higher electron
density plasma is required from industry.
A VHF plasma is characterized by electron
trapping, that is, the electron displacement δxshould be shorter than a spacing gap between
discharge electrodes for ω << υm5):

x = qE0/(me m) << d/2(1)

Here d is a spacing gap between discharge
electrodes,  and m is the angular frequency
of VHF power source and electron collision
Department of Applied Science for Electronics andMaterials, Graduate Student*2 Graduate School of Information Science andElectrical Engineering*3 Department of Electrical and Material Science*1frequency, respectively, and q, me and E0 iselectron charge, electron mass and theamplitude of the VHF electric field. Electrontrapping effect provides better confinement ofelectrons and as a result the electron density
becomes high. Eq. (1) indicates that E0 and mare important parameters in the VHF plasma,that is, the VHF power and pressure are key
parameters in the VHF plasma characteristics.
When VHF powers are increased, theamplitude of the VHF electric field E0increases and as a result the condition forelectron trapping, x << d/2, is not valid.
Therefore, to increase m by increasing thepressure is required for VHF plasma discharge
at high powers. Thus, it is an important
subject in the production of microcrystalline
siliconsolarcells
toexaminethecharacteristics of the VHF plasma producedwith a short gap discharge.
The Langmuir probe method is mostly used
to investigate the characteristics of the VHF
plasma such as the electron density (ne) andthe electron temperature (Te). However, theplasma parameters with the narrow gap
discharge will be seriously disturbed when theprobe is inserted. In addition, the probe cannot
be used at high pressure because the ion mean
free path is much shorter than the sheathlength, leading to the overestimation of the iondensity6).

平成 25 年度

九州大学大学院総合理工学報告In the previous paper7), we examined how
the photo-ionization of metastable argon atoms
affects the Thomson scattering spectrum and
at
which
laser
powerdensity
thephoto-ionization becomes a problem for thelaser Thomson scattering (LTS) diagnostics ofargon plasmas. We succeeded in removing theinfluence of the photo-ionization of themetastable argon atoms on the Thomson
scattering spectrum from the VHF argon
plasma by reducing the power density of theYAG laser below 1 × 1013 W/m2. Thus, weconsider that the LTS can be a powerful
diagnostic method under such conditions.
In this paper, we examined the dependenceof the VHF plasma parameters on the pressure
and power by using the LTS method, where aVHF plasma was produced by a balanced
power feeding method8) in addition to theconventional power feeding method. Inaddition, we tried to simulate the VHF plasmausing the Plasma Hybrid Module (PHM) ofPEGASUS software.

2.

第1号

7and another electrode was connected to theground. This kind of power feeding system has
been commonly applied for many studies onplasma
processes.
However,abnormal
discharges may happen between the power
feeding cable and the chamber wall in the VHF
range. In order to avoid such abnormal
discharges, we used a balanced power feeding
method9). Then the plasma was mainly
produced in the region between the twoelectrodes.

Experimental Setup

Figure 1 shows the schematic diagram of theLTS system on the VHF plasma. The chamber(diameter 200 mm, length 400 mm) wasdesigned
specificallyfortheLTS
measurements. The chamber was equippedwith baffles, two Brewster windows, a beam
dump and a triple grating spectrometer (TGS).
The structure and the function of TGS havebeen reported in Ref. 8). This TGS made us
possible to detect the Thomson scattering
spectrum at 1nm away from the laser
wavelength without the problem of the straylight. Finally, the scattered light signals
passed through the TGS were detected by an
ICCD
camera(Princeton
Instruments,PI-MAX Ⅲ). The quantum efficiency of thecamera was ~50 % at  = 532 nm. The photon
counting method was used to enable reliablemeasurements of small scattered signals.
The VHF plasma of 60 MHz was sustained
between two parallel plate electrodes (60 mm× 60 mm × 8 mm) made of stainless steel andthe distance between two electrodes wasalways kept at 10 mm. The working gas wasargon, the gas pressure used in this study wasranged from 100 mTorr to 1000 mTorr and theVHF power was in the range between 20 W to80 W. In the case of the conventional power
feeding method, one electrode was connectedto the power supply through the matching box

第 35 巻(a) Side view(b) Top viewFig. 1 Schematic diagram of the experimental setup.

3.

Results and Discussion

In order to examine the effect of theconventional power feeding and balanced
power feeding method on the plasma
parameters, we measured ne and Te using theLTS system as a function of VHF power. As
seen in Fig. 2, when the VHF power isincreased from 20 watt to 80 watt, ne increases
from 3.9 × 1016 m-3 to 6.3 × 1016 m-3 for the caseof balanced power feeding method. ne values
obtained for the case of the balanced power
feeding method were higher about 40 % thanthose obtained for the conventional power

8Investigation of VHF Argon Plasmas at High Pressure by Balanced Power Feeding Using Laser Thomson Scattering

Power (W)Electron temperature (eV)(a)

increases from 4.6 × 1016 m-3 to 1.2 × 1017 m-3
when the conventional power feeding method
is used. The average ne ratio of the balanced
power feeding method is 20 % lower than thatby conventional power feeding method when
the gas pressure is higher than 800 mTorr.
This may be explained by the fact that theVHF discharge tends to be localized when thepressure is increased and, as a result, theplasma region moves to the electrode side.
Figure 3 (b) shows that Te is always kept
around 1.8 eV independent of the pressure
when the balanced power feeing method isused. In the case of the conventional power
feeding method, Te is around 3.0 eV except at
100 mTorr. Note that the balanced power
feeding method provides lower electron
temperature plasma that is favorable forplasma processes.
Electron density ( × 1016 m-3)

Electron density ( × 1016 m-3)

feeding method. Thus, we conclude that thebalanced power feeding method provides
higher electron density. On the other hand, as
seen in Fig.2 (b), Te decreases with increasing
the power independent of the power feeding
method.

Pressure (mTorr)(a)Power (W)

Then we measured the dependence of theplasma parameters on the gas pressure.
Figure 3 (a) shows that when the pressure isincreased, the electron density increases. Thisis understood by electron trapping effectdescribed in Sec. 1. Looking at carefully Fig. 3(a), the results can be divided into two parts inthe case of using the balanced power feeding
method. In the first part, ne increases from 6.3× 1016 m-3 to 1.0 × 1017 m-3 when the gaspressure is increased from 100 mTorr to 800
mTorr. In the second part, when the gaspressure is increased from 800 mTorr to 1000mTorr, ne decreases slightly from 1.0 × 1017 m-3
to 9.8 × 1016 m-3. On the other hand, neElectron temperature (eV)(b)
Fig. 2 Dependences of the electron density (a) and theelectron temperature (b) on the VHF power arecompared for the case of the balanced power
feeding method and the case of the conventional
feeding method. Here the pressure was 100 mTorr.

Pressure (mTorr)(b)
Fig. 3 Dependences of the electron density (a) and theelectron temperature (b) on the gas pressure arecompared for the case of the balanced power
feeding method and the case of the conventional
feeding method. Here the VHF power was fixed at80 W.

平成 25 年度

九州大学大学院総合理工学報告第 35 巻第1号

9Electron density ( × 1016 m-3)

probe cannot be used at the higher gas
pressure, as described in Sec. 1. Thus, weconclude that the LTS is a reliable diagnostic
method at high gas pressures.

Pressure (mTorr)

Electron temperature (eV)(a)

Pressure (mTorr)(b)
Fig. 4 Electron densities (a) and electron temperatures (b)measured by the LTS method and Langmuir probe
method for different gas pressures. Here the VHF
power was fixed at 80 W.

Nishimiya9) suggested the advantage of thebalanced power feeding method, where theLangmuir probe was used as a diagnostic tool.
In our previous study7), we examined thedependence of ne and Te by the LTS andLangmuir probe on the VHF power at thepressure of 100 mTorr. Recently there is atendency operated at higher pressure inplasma processes to get higher deposition
rates. In this paper, we investigated thecharacteristic of LTS method and Langmuirprobe method at the gas pressrue higher than
100 mTorr. As seen in Fig. 4, when the gaspressure is 100 mTorr, ne values measured by
the LTS method and Langmuir probe method
are 6.3 × 1016 m-3 and 5.7 × 1016 m-3,respectively. The Te value measured by LTS
method is lower than that by probe method.
This tendency agrees with the result reportedin Refs. 10) and 11). When the gas pressure isincreased to 1000 mTorr, ne measured by LTS
method increased. On the contrary, nemeasured by the probe method decreases. Thisdifferent tendency may due to the fact that theWe have performed the simulation of a VHF
plasma by using the Plasma Hybrid Module(PHM) of PEGASUS software Inc.12,13). The
detail of the PHM was described in Ref. 14).
Figure 5 shows the Balanced Power Feeding(BPF) model which uses a cylindricalcoordinate system with axial symmetry. Here,
we briefly schematic the computational
procedure of the PHM. The density and thevelocity of electrons, which are utilized for thecalculation of the electron energy distributionfunctions (EEDFs) by the Monte Carlo method,
are calculated by the fluid model and theequation of the electron motion. A pair ofparallel plate electrode with the radius of 34
mm was set at the center of a cylindricalchamber. The gap between the electrodes wasfixed to 8 mm. The working gas was argon, thegas pressure used in this simulation was 100mTorr and the amplitude of the applied
voltages was in the range between 60 and 100
V. The argon gas was introduced from thecentered top position of the chamber. An
exhaust port was located on the bottom face ofthe chamber. In order to suppress the plasma
emission from the outside surfaces of theelectrodes, we covered it with insulators.
However, the leak currents of the electrode are
not considered in the model, thus, the effect of
the applied VHF voltages in the BPF model on
the VHF plasma with short-gap parallel
electrodes was investigated.

Fig. 5 Schematic diagram of BPF model.

10Investigation of VHF Argon Plasmas at High Pressure by Balanced Power Feeding Using Laser Thomson Scattering

Figure 6 shows the 2-dimensional images ofthe electron density and the electron
temperature in the BPF model. Here thefrequency of the VHF power source and theamplitude of the VHF voltage are 60 MHz and60 V, respectively. The gas pressure was set to100 mTorr. The ne and Te values at the centerof the electrodes in the BPF model are 1.5 ×1016 m−3 and 1 eV, respectively.
Figure 7 demonstrates the applied voltagedependence of the ne and Te distributions. Thedistributions are in the z direction and arecalculated at the radial position r = 20 mm.
Obviously the highest ne appears at the center
position between two electrodes. The central nevalues for the applied voltage of 60 V, 80 V and100 V are 1.5 × 1016 m-3, 2.6 × 1016 m-3 and 4.1
× 1016 m-3, respectively. In addition, Te is kept
around 1 eV at the center independent of theapplied voltage. Besides, Fig. 7 shows that the(a)Electron temperature (eV)Fig. 6 Spatial distributions of the electron density (a) andthe electron temperature (b) at the gas pressure of
100 mTorr. The images are 2-D in the balanced
power feeding model (Vrf = 60 V).

Electron density ( × 1016 m-3)

plasma is produced outside the discharge
electrode and the ne distribution outside theelectrode takes a peak at z = 18 mm, namely 8mm apart from the power electrode, and then,ne decreases with the distance from theelectrode. The VHF power of 20 W, 40 W, 60 W
and 80 W in our experiment correspond to theVrf of 28 V, 37 V, 45 V and 50 V, respectively.
Therefore, we compared the measure plasma
parameters obtained for the VHF power of 80
W with the calculated parameters by thesimulation setting the applied voltage of 60 V.
The ne values obtained by the LTS method andthe simulation model are 6.2 × 1016 m-3 and 1.5
× 1016 m-3. On the other hand, the Te value
obtained by the simulation model is 1 eV, and
it is slightly lower than that by the LTS
method. In any case, these results indicatethat the plasma parameters obtained by thesimulation are not so much different fromthose measured by the LTS method.

(b)
Fig. 7 Spatial distributions of the electron density (a) andthe electron temperature (b) for different applied
voltages. The gas pressure was 100 mTorr, thedistance of the electrodes was d = 8 mm. The
distributions are in the z direction and arecalculated at the radial position r = 20 mm.

平成 25 年度4.

九州大学大学院総合理工学報告第 35 巻第1号

11Conclusions

We examined the dependence of the VHF
plasma parameters on the gas pressure andthe VHF power by using the LTS method. Herethe VHF plasmas were generated by thebalancedpower
feeding
methodandconventional power feeding method. Comparedwith the conventional power feeding method,ne and Te by the balanced power feeding
method showed the outstanding performancenot only on the dependence of the pressure but
also on the power. In addition, we measured
the pressure dependence of the plasma
parameters by the LTS method and theLangmuir probe method at high pressure thatwas reported as an indispensable condition for
VHF plasma processes. It was found that as
the gas pressure is increased from 100-1000
mTorr, the ne values measured by theLangmuir probe method showed the different
tendency from the values measured by the LTS
method. This is considered that the Langmuirprobe method is not reliable for high pressuresat around 1 Torr.
We have successfully simulated the VHF
argon plasma using the PHM of PEGASUS
software. The 2-D spatial distributions of neand Te indicate that the plasma is produced
outside the discharge electrode in the balanced
power feeding model. The ne and Te values
inside the electrodes in the BPF model are 1.5
× 1016 m−3 and 1 eV, and these values are notso much different from those measured by theLTS method. Therefore, it is concluded thatboth LTS diagnostics and the simulation canbe powerful tools to study VHF plasmas.

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