Microwave-induced non-equilibrium plasmas by insertion ofsubstrate at low and atmospheric pressures

Kazutoshi Kiyokawa, Kazuo Sugiyama*, Manabu Tomimatsu,Hideki Kurokawa, Hiroshi Miura

Department of Applied Chemistry, Saitama University, Shimo-Okubo 255, Urawa, Saitama 338-8570, Japan

Received 29 July 1999; accepted 2 February 2000

Abstract

We found that microwave induced discharge or microwave plasma could be easily produced with a perovskite-type oxide

substrate even at atmospheric pressure. We investigated the plasma conditions in order to understand the plasma generation

mechanism. When pressure in the reactor was gradually increased, the plasma mode was changed from a diffused glow to a

®lamentary glow at a power of about 5:3� 104 Pa. The electron density of the plasma produced using a substrate was higher

than that of the ordinary microwave plasma produced without the substrate. It was considered that the electron emissions from

the substrate enhanced the plasma. # 2001 Elsevier Science B.V. All rights reserved.

Keywords: Microwave plasma; Perovskite-type oxide; Optical emission spectroscopy; Electron density; Atmospheric pressure

1. Introduction

Microwave induced discharge or microwave

plasma has been widely investigated and used as

one type of non-equilibrium plasma process. A glow

plasma is a common type of non-equilibrium plasma

from the viewpoint of cold plasma processing. It is

said that the glow plasma state can be divided into two

types; one a diffused glow (general glow) and the

other a ®lamentary glow [1].

In order to produce microwave plasma, we placed a

La0.8Sr0.2CoO3 substrate, a typical provskite-type

oxide into the reactor. As a result, we found that a

non-equilibrium plasma could be easily produced

even at atmospheric pressure. La1ÿxSrxCoO3 has been

investigated as an electron-emitter, a catalyst for NOx,

etc. [2,3]. Therefore, this rouse our interests to inves-

tigate the plasma. We have, so far, reported several

application examples using the plasma [4±6]. How-

ever, the details of the plasma was not fully under-

stood. Hence, in this experiment, the plasma

conditions were investigated for the purpose of under-

standing the plasma generation mechanism.

2. Experimental

The experimental apparatus used for plasma gen-

eration is shown in Fig. 1. This apparatus consists

essentially of a microwave generator, a waveguide, a

quartz-tube reactor, a gas control unit, and a rotary

pump. After a lump substrate of La0.8Sr0.2CoO3

(7 mm across) was placed in the quartz tube, argon

gas was introduced, and microwave power (2.45 GHz)

Applied Surface Science 169±170 (2001) 599±602

* Corresponding author. Tel.: �81-48-858-3505;

fax: �81-48-858-3505.

E-mail address: sugi@apc.saitama-u.ac.jp (K. Sugiyama).

0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 9 - 4 3 3 2 ( 0 0 ) 0 0 7 9 6 - 0

was applied (maximum 500 W). The substrate was

electrically ¯oated in the reactor in this experiment.

The work function of the La0.8Sr0.2CoO3 substrate is

about 4.8 eV [7]. The pressure was adjusted by vary-

ing a conduction valve that was placed between the

reactor and the rotary pump. The quartz tube wall was

partially dented so that the La0.8Sr0.2CoO3 substrate

could be supported. As a fundamental plasma para-

meter, the electron density was measured using a

triple-probe method, and optical emission was also

measured.

3. Results and discussion

Fig. 2 shows the change in optical emission inten-

sity by varying the pressure in the reactor as a function

of the existence of a La0.8Sr0.2CoO3; (a) with, and (b)

without. The observation point was at a distance of

about 10 mm from the center of a waveguide. In each

case, the state of the plasma was a diffused glow at low

pressure, and the optical emission intensity gradually

dropped with increasing pressure. The plasma went

out at a pressure of 5:3� 103 Pa in the case of (b),

while the plasma could be produced even at atmo-

spheric pressure when using the substrate. However, in

the case of (a), the plasma state was transferred from a

diffused glow to a ®lamentary glow plasma at about

5:3� 104 Pa. This transition would be a cause of the

high rise in the emission intensity at a pressure of

1:0� 105 Pa. The OES method detects the emission

species at a point, so the difference in the plasma

density would affect the high rise in the optical

emission.

Fig. 3 shows the electron density (Ne) of plasmas

measured by a triple-probe method at a pressure of

1:3� 103 Pa; (a) with the La0.8Sr0.2CoO3 substrate,

Fig. 1. Experimental apparatus for plasma generation. This

apparatus consists essentially of a microwave generator, wave-

guide, quartz-tube reactor, gas control unit, and rotary pump.

Fig. 2. Variation of optical emission intensity, I(p), with pressure

(p) as a function of the existence of a La0.8Sr0.2CoO3 substrate; (a)

with substrate, (b) without substrate. In each case the intensity at a

pressure of 133 Pa, I(p0) was used as a standard. The microwave

power was 400 W.

Fig. 3. Electron density of plasma as a function of microwave

power; (a) with substrate, (b) without substrate. In the case of (a),

the plasma could be maintained even at lower power. The pressure

in the reactor was 1:3� 103 Pa in each case.

600 K. Kiyokawa et al. / Applied Surface Science 169±170 (2001) 599±602

and (b) without substrate (b). The tip of the probe was

set at a distance of 20 mm from the center of the

waveguide. In this experiment, the plasma could be

generated at about 370±380 W in each case; (a) or (b).

The microwave power was gradually increased to

500 W, and then the power was decreased until the

plasma went out. As a result, the plasma could be

maintained even at a power of 170 W when using the

substrate (a), while the plasma went out at 350 W

without the substrate (b). This result indicates that Ne

was increased by insertion of the electron-emissive

materials. The cause of the difference in Ne between

(a) and (b) has not been fully understood yet. How-

ever, we consider that the electron emission from the

substrate would affect the electron density.

It has been, so far, discovered that the surface of the

substrate was locally heated by microwave irradiation,

and the surface temperature reached to more than

1000 K [7]. We consider that the heating is caused

by the microwave-induced current in the skin depth,

that is absorption of the microwave power [4,7]. It was

also found out that the high rise of the substrate

surface by microwave irradiation is the ®rst require-

ment for the plasma generation [7]. In short, the

substrate which did not absorb the microwave power

could not produce the plasma. Therefore, the plasma

generation would be triggered by electron emission

from the heated sites of the substrate. There is much

possibility that the emitted electron would have a

collision with an argon atom. The result is an excited

argon atom would be produced and a chain collision

would occur. Electron emission could also occur on

the surface of the substrate by the irradiation of a

photon or the collision of an excited argon atom or an

electron (secondary electron emission). There is every

possibility that the high temperature of the substrate's

surface assists the secondary electron emission. In

short, an electron increase would be assisted by the

existence of the substrate.

The distribution of activated argon atoms (811 nm)

in a reactor observed by OES is shown in Fig. 4. The

observation point, x, was gradually moved along the

quartz tube reactor from the center of a waveguide.

From the results, we can ®nd that the optical emission

intensity of the plasma with substrate was higher than

that of plasma without the substrate (1:3� 103 Pa),

and that the optical emission of an atmospheric pres-

sure plasma was much higher than that of vacuum

plasma. This would be because became ®lamentary

like a shower and the power density increased.

4. Conclusions

The results can be summarized as follows:

1. A microwave plasma could be easily produced

with a La0.8Sr0.2CoO3 substrate in the reactor even

at atmospheric pressure.

2. When the pressure in the reactor was gradually

increased, the plasma mode was changed from a

diffused glow to a ®lamentary glow at a power of

about 5:3� 104 Pa.

3. The electron density of the plasma produced with

the substrate was higher than the case without

substrate.

4. It was considered that an electron emission from

the surface of the substrate would maintain the

plasma even under dif®cult plasma producing

conditions.

Acknowledgements

TheauthorsaremuchindebtedtoProf.ShinichiKoba-

yashi for valuable discussion and useful suggestions.

Fig. 4. Distribution of optical emission intensity (I) of activated

argon atoms (811 nm) in a reactor; (a) with substrate at

1:3� 103 Pa, (b) without substrate at 1:3� 103 Pa, (c) with the

substrate at atmospheric pressure (1:0� 105 Pa). The observation

point (x) was gradually moved along the quartz tube reactor from

the center of a waveguide.

K. Kiyokawa et al. / Applied Surface Science 169±170 (2001) 599±602 601

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