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Thin Solid Films 502
Large area VHF plasma sources
T. Takagi a,*, M. Ueda a, N. Ito a, Y. Watabe a, H. Sato b, K. Sawaya b
a Ishikawajima-Harima Heavy Industries Co., Ltd., High Energy System Department, Yokohama, Japanb Tohoku University, Department of Electrical and Communication Engineering, Sendai, Japan
Available online 15 August 2005
Abstract
Array antenna, a novel plasma source, was developed to realize uniform deposition of silicon thin films such as amorphous silicon and
microcrystalline silicon on large area substrates with power frequency in the VHF band. It consists of plural U-shaped loop antenna type
electrodes, and silicon thin films are uniformly deposited on over >1 m2 substrates by introducing VHF power at 85 MHz with anti-phase.
The VHF PCVD system with array antenna is suitable for the mass-production of silicon thin film solar cells due to its high throughput
achieved by double-sided, multi-zone deposition.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Plasma processing and deposition; Glow discharge; CVD; Silicon
1. Introduction
PCVD (Plasma-enhanced Chemical Vapor Deposition)
systems consisted of parallel plate type planar electrodes
with power frequency of RF (Radio Frequency: 13.56 MHz)
are mostly used to fabricate silicon thin films such as
amorphous silicon, microcrystalline silicon and silicon
nitride for TFTs (Thin Film Transistors) and solar cells
application. Meanwhile, low cost PCVD systems capable to
fabricate high quality silicon thin films with high throughput
are demanded as the initial and running costs of PCVD
systems have a majority in the total production cost of the
devices in mass production. The throughput may be
improved by increasing the deposition rate, substrate size
and the number of the substrates to be handled at a time,
however, film quality and non-uniformity tends to deterio-
rate with those factors.
Meanwhile, power frequency in the VHF band (Very
High Frequency: 30–300 MHz) is noticed for its features in
PCVD application, such as lower electron temperature,
higher plasma density, lower ion bombardment energy and
lower powder formation compared to conventional RF,
0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.tsf.2005.07.235
* Corresponding author.
E-mail address: [email protected] (T. Takagi).
resulting in the formation of higher quality silicon thin films
at higher deposition rates [1,2]. However, as the frequency
increases, the wavelength k approaches the dimension of the
electrode, e.g. k =3 m at 100 MHz, which brings standing
waves into existence on the electrode surface in large area
deposition systems. Also as the skin depth decreases with the
increase in frequency, the power loss in the electrical lines
increases. Those effects result on inhomogeneous distribu-
tion of the deposition rate and characteristics of films over
the substrate. For example, with power frequency of 80
MHz, the limit in the size of parallel plate type electrodes to
obtain uniform deposition is reported to be 400 mm�400
mm [3]. It is also important to keep the whole anode at
equally grounded potential, flatness and temperature in order
to achieve uniform deposition in parallel plate type planar
electrodes, however, it becomes difficult as the size of the
anode increases.
Many attempts have been made by researchers to
overcome the above-mentioned problems to realize large
area uniform deposition. Conventional planar electrodes
are used with careful design in power feeding system to
give uniform deposition at 40 MHz over 0.8 m2 substrate
with non-uniformity of T5.6% [4]. Also novel plasma
sources have been developed to realize low cost, large
area, VHF PCVD systems for the deposition of silicon thin
(2006) 50 – 54
Fig. 2. Schematic side view of VHF PCVD system with an array antenna.
T. Takagi et al. / Thin Solid Films 502 (2006) 50–54 51
films on square substrates >1 m2. Those are the linear
source developed by Applied Films (Germany) [5], the
ladder-shaped electrode by Mitsubishi Heavy Industries
(Japan) [6] and our array antenna [7,8]. In all cases, special
care in the power feeding system is carried out to
introduce VHF power efficiently and uniformly to the
plasma sources.
The linear source was designed to decrease the active
electrode area in one spatial dimension. Uniform plasma
with high density is obtained in the longitudinal direction
by feeding power from multiple points, and deposition on
large area substrates is achieved by moving the substrate in
the direction perpendicular to the longitudinal dimension of
the electrode. The ladder-shaped electrode consists of a
plurality of parallel longitudinal rods and two parallel
lateral rods arranged into a form of lattice. Uniform plasma
is generated, by controlling the frequency, the number and
the positions of the power feeding points introduced to the
electrode. The array antenna consists of plural U-shaped
electrodes where uniform deposition is realized by control-
ling the power and the phase of VHF power introduced to
the electrodes. In this paper, the details of the array antenna
will be described.
2. Experimental
2.1. U-shaped loop antenna type electrode
The array antenna consists of plural U-shaped loop
antenna type electrodes (hereafter ‘‘electrode’’) and is
designed to generate and maintain plasma at a specific
power frequency [7,8]. Fig. 1 shows a schematic diagram of
one electrode composed of U-shaped conductor, where one
end is connected to the chamber wall (grounded point) and
the other end to the power-feeding point.
The electrode used in this work consists of a metal pipe
and a metal rod, connected to each other at one end by a
conductor (folded point). The other end of the pipe was
Fig. 1. Schematic diagram of a U-shaped electrode.
grounded to the chamber wall, and VHF power was
introduced to the electrode from the other end of the rod
through a coaxial power-feeding port. The length of an
electrode from the grounded point to the power-feeding
point was approximately 3200 mm, designed for a power
frequency of 85 MHz. The rod was covered by an insulator,
which contributes to uniform deposition.
2.2. Array antenna
Twenty-five electrodes were arranged to form one array
of antenna where the rods and the pipes were positioned
alternately in this work. Fig. 2 shows a schematic side view
of an array antenna. The distance between each rod and pipe
was 35 mm. A pair of glass substrates was placed on both
sides of the array antenna, facing to each other, so that the
array antenna is placed in between two substrates.
The vacuum chamber was evacuated from the bottom,
and the source gas was introduced into the chamber
through the gas feeding openings on the pipe wall. This
realizes uniform gas feeding throughout the discharge
region. By introducing VHF power to all electrodes
simultaneously with a mixture of silane and hydrogen of
a certain pressure, silane plasma is generated around the
array antenna, and either amorphous silicon or micro-
crystalline silicon film is deposited on the surface of the
substrates depending on the discharge conditions.
There is no anode, i.e. no grounded backing plate at the
back of the substrate in this system. This is an advantage in
large size, making the substrate heating and handling
mechanism easy and simple.
Another feature is the simple VHF power feeding
system; once the plasma is generated, the reflected power
at the power feeding point becomes negligible. This allows
the power feeding system to avoid the commonly used
matching network between the VHF power generator and
the feeding ports, making the VHF power feeding system
more simple and the VHF power to be utilized efficiently to
maintain the discharge.
Fig. 3. The cross-sectional view of multi-zone type deposition system with
three discharge regions.
Fig. 4. The deposition rate distribution of amorphous silicon film along the
electrode for in-phase and anti-phase conditions. The non-uniformity within
1000 mm along the antenna was T42% for in-phase, and T4% for anti-
phase.
T. Takagi et al. / Thin Solid Films 502 (2006) 50–5452
2.3. Multi-zone deposition
With the use of the array antenna, multi-zone deposition
became possible by increasing the number of arrays. The
equipment used in this work was designed to simultane-
ously deposit on six square substrates of over 1 m2 in size,
by arranging three array antennas in parallel to each other,
forming three discharge regions. The cross-sectional view of
the deposition chamber with three discharge regions is
shown in Fig. 3.
The substrates arranged vertically on a substrate carrier
are transferred into the deposition chamber after being
heated to a required temperature in the heating chamber. The
deposition chamber is heated by a set of panel heaters and
sub-heaters placed at the top, bottom, and the sides of the
chamber (not shown), by which the temperature of six
substrates of 1200 mm�1600 mm was controlled to be
200T 9.7 -C.With uniform feeding of the source gas and VHF power,
uniform deposition of thin films over large area in multi-
zone was achieved. No specific interference among the three
discharge regions was observed during multi-zone deposi-
tion. The total deposition area can be further increased either
by increasing the number of the antennas in each array
antenna or by increasing the number of the discharge
regions.
2.4. Electric field strength simulation
To have an idea of the discharge mechanism of the array
antenna, the electric field strength was simulated using a
simple configuration of the electrodes with FDTD (Finite
Difference Time Domain) method.
The relationship between the deposition rate of
amorphous silicon film and the plasma density is almost
linear. Therefore, it can be assumed that there is a cor-
relation between the deposition rate and the electric field
strength. The electric field strength distribution simulated
by FDTD method, under the assumption of uniform
plasma density and by taking into account that plasma is
a strongly dispersive medium, showed a correlation with
the deposition rate distribution of amorphous silicon film
[9,10].
3. Results and discussion
The deposition conditions, such as the power density, gas
flow rate of silane and hydrogen, hydrogen dilution ratio
and the total pressure were varied to control the deposition
rates and the film properties of amorphous silicon. The
distance between the antenna and the surface of glass
substrates was kept at 45 mm and the substrate temperature
was kept at 200 -C.
3.1. VHF power and phase control
The control of both the VHF power intensity and the
phase was an important factor to realize uniform deposition.
VHF power of equal intensity was introduced to all
electrodes simultaneously, (a) with equal phase to all
electrodes (in-phase), and (b) with opposite phase to the
electrodes next to each other (anti-phase). Fig. 4 shows the
thickness distribution of amorphous silicon films in the
direction along the electrode when the phase of the power
fed to the electrodes were controlled to be (a) in-phase and
(b) anti-phase. The deposition conditions were the same in
both cases except for the phase of VHF power. Uniform
thickness distribution was obtained in the anti-phase case,
while the deposition rate distribution showed M-shaped
along the electrode ( x-direction) in the in-phase case. The
poor uniformity in the in-phase case is suggested to be due
to the formation of standing waves in the direction along the
electrode. The non-uniformity of the film thickness within
1000 mm along the electrode was T42% for in-phase, and
T4% for anti-phase.
Fig. 6. The deposition rate distribution of amorphous silicon film deposited
by one array antenna. The non-uniformity over 1000 mm�1400 mm is
0.22 nm/sT15%.
T. Takagi et al. / Thin Solid Films 502 (2006) 50–54 53
The electromagnetic field strength around the array
antenna was simulated by FDTD method using a model of
the deposition system with nine electrodes [10]. Fig. 5
shows the calculated current intensity along the antenna
for (a) in-phase and (b) anti-phase conditions. The current
intensity for in-phase condition shows the existence
of standing wave, which correlates with the M-shape of
the deposition rate distribution shown in Fig. 4. On the
other hand, the current intensity for anti-phase decreases as
it propagates along the electrode, and at the end
diminishes. This correlates well with the negligible
reflected power detected at the power feeding point,
and the uniform deposition rate distribution is assumed
to be the sum of current intensity folded at the folded
point, BC.
Therefore, it is ascribed that the electromagnetic wave
propagates along the array antenna in anti-phase case
resulting in uniform field over the array.
3.2. Large area distribution
Amorphous silicon deposition over large area was carried
out using one array antenna under anti-phase power control.
Fig. 6 shows the deposition rate distribution of amorphous
silicon over 1200 mm�1600 mm substrate. The silane and
hydrogen flow rates were 500 sccm each, the pressure was
3.3 Pa and the power density was 7.5 mW/cm2. A
deposition rate of 0.22 nm/s was obtained with a non-
uniformity of T15% over 1000 mm�1400 mm. This
deposition rate distribution correlates well with the electric
field strength simulated by the FDTD method; in the regions
with relatively low deposition rate shown on the side edges
of the substrate in Fig. 6 was obtained as low intensity
regions in the simulated electric field strength distribution
[10].
Fig. 5. The current distribution along the electrode for (a) in-phase and (b)
anti-phase conditions, calculated by FDTD method. A and D relate to the
power feeding point and the grounded point, and BC the folded point of the
U-shaped electrode, respectively, shown in Fig. 1.
When simultaneous deposition on six substrates with
three discharge regions was carried out, the source gas flow
rates were increased to 1000 sccm each to obtain the same
deposition rate and film quality. The non-uniformity of the
deposition rate of amorphous silicon on six substrates was
identical for all substrates.
4. Conclusions
Array antenna developed to realize large area deposi-
tion on square glass substrates >1 m2 at 85 MHz was
introduced in this work. It consists of simple and easy to
handle U-shaped electrodes, which are arranged in one
plane to form an array, and uniform deposition is
achieved by introducing the VHF power simultaneously
to all electrodes at anti-phase. This method realized to
generate and to maintain the discharge with no backing
plate and no matching network, which are both essential
in conventional deposition systems. Apart from the basic
design, the construction of the apparatus introduced in this
work can be varied depending on the process needed. For
example, the distance between the electrode and the
substrates may vary to the films to be deposited, and the
optimum length of the electrodes differs to the frequency
used.
A high throughput is realized by double-sided and
multi-zone deposition. In this system, six substrates of
1200 mm�1600 mm can be handled, where both the size
(width) and the number of the substrate can be increased
by increasing both the number of the electrodes in one
array and the number of the array in one deposition
chamber.
The array antenna type VHF PCVD system is designed
to reduce the initial and the running cost of PCVD
systems by achieving high throughput and simple config-
uration of the components. These features are especially
required in the solar cell mass-production, where high
throughput and low cost equipment for large area
deposition is demanded, however, other applications are
also expected.
T. Takagi et al. / Thin Solid Films 502 (2006) 50–5454
Acknowledgement
This work was partially supported by the New Energy
and Industrial Technology Development Organization
(NEDO, Japan).
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