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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: tomoko_takagi@ihi.co.jp (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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