22P048-P
Study on a Production Control of Microwave-Excited Plasmas
by an Antenna-Cavity with the Specific Structure
構造を有するアンテナキャビティを用いたマイクロ波励起プラズマの生成制御に関する研究
Keisuke Ota, Yasuyoshi Yasaka, Naoki Tobita
大田啓介，八坂保能，飛田直樹
Dept. Electrical and Electronic Engineering, Kobe University
1-1,Rokkodai, Nada,Kobe 657-8501, Japan神戸大学大学院電気電子工学専攻 〒657-8501 神戸市灘区六甲台町1-1
The purpose of this research is to control the distribution of plasmas produced by a cylindricalmicrowave-excited plasma device with a multi-slotted planar (MSP) antenna.From the past research, it isknown that modes of an electric fields in the plasma chamber is similar to ones in a cavity which is part of
the antenna on the space. Therefor, it is comparatively easy to control the plasma distribution by changing thecavity structure. In this study, we developed an antenna-cavity with the specific structure,and revealed that thecavity with a metal circular plate may be able to control the plasma distribution and generate the plasma withgood uniformity.

1. Introduction
Plasma processing is used in various stage ofsemiconductor manufacturing process.A microwave-excited plasma is one source used for plasma
processing. Microwave-excited plasmas sustainedby surface waves have a long history, and a large
stock of knowledge has accumulated. The spatial
resonance arises under a certain condition of thedevice structure and the electron density, and it is
known to cause mode jumps and hysteresis
behavior, so it is not easy to control the
microwave-excited plasmas. On the other hand, theplasma resonance is a field enhancement
phenomenon at the position of a certain electron
density in nonuniform plasmas. To improve control,
it is desirable to suppress the spatial resonance andto actively use the plasma resonance, and have tocreate such conditions. To achieve the above
purposes, we create plasmas by the cylindricalmicrowave-excited plasma device with the MSP
antenna. The distribution of the electric fields in theplasma chamber is similar to ones in theantenna-cavity. Consequently, it is considered that
we comparatively easy to control the plasma
distribution by changing the cavity structure. In thispaper, we present some experimental results andview the availability of the antenna-cavity with thespecific structure we developed .
2. Experimental detail
Figure 1 shows the schematic illustration of theexperimental setup. The setup consists of themicrowave source, the directional coupler, theFig.1. Schematic illustration of the experimental setup

power monitor, the circular polarization converter,
the three-stub auto tuner, the TE-TM converter,
the antenna cavity, the metal circular plate, theMSP antenna, the quartz the window and theplasma chamber. Making the plasma from argon
gas. We use two types of Langmuir probes, which
one moves in radial direction, the other in azimuthaldirection to investigate the distribution of plasmas
and evaluate the uniformity of plasmas.
3. Results and Discussion
Figure 2 shows the azimuthal distribution ofplasmas produced in (a) 10 mTorr and (b)100 mTorr.
The plasma production conditions are that thepower is 1.0 kW, the antenna position ha is 55 mm,
and the metal circular plate is 200 mm in radius.The
left figures of Fig.2 shows the ratio of the azimuthal

amode number m. The percentage of the mcomponents is calculated by function expansions.
As a sample for the explanation, function
expansions for a(r, ) are explained below.
The Fourier expansion, which is used as thetransform in the azimuthal direction, is written as
follow:b(1)f(m, r) is calculated by the Fourier transform:(2)

Fig.2. The azimuthal distribution of plasmas; (a) Ar gas
pressure: 10 mTorr, (b) Ar gas pressure: 100 mTorr

a50 mTorr
40 mTorr
30 mTorr

Jis uniformity (%)

302010020304050
antenna height ha (mm)60b50 mTorr
40 mTorr
30 mTorr

Jis uniformity (%)

302010020304050
antenna height ha (mm)60Fig.3. The dependence of the plasma uniformity in theradial direction on the antenna position ha;The metal
circular plate is (a) 200 mm, (b) 210 mm in radius.

The ratio of the m = 0 component is more than 80
percent in either case. The m = 0 component is theuniform component in the azimuthal direction. In
brief, the plasma with relatively good uniformity inthe azimuthal direction can be produced in wide gaspressure range. The right figures of Fig.2 show the2D distribution of the ion saturation current density
Jis. The peak of Jis appears circularly in either case.
The aim of the metal circular plate is to adjust theradial distribution of the electric fields, so theseresults reflect the effect of the metal circular plate.
Figure 3 shows the dependence of the plasma
uniformity under the radius of 150 mm in the radial
direction on the antenna position ha. The metal
circular plate is (a) 200 mm, and (b) 210 mm inradius. The uniformity is calculated by (nmax nmin)=(2nave) × 100 %, where nmax, nmin, and nave
represent the maximum, minimum, and averagevalues, respectively. The plasma production
conditions are that the power is 1.2 kW and Ar gas
pressure is 30 or 40 or 50 mTorr. It is found thatthe radial plasma uniformity depends on the radiusof the metal circular plate. Additionally, as theantenna position ha changes, good uniformity point(≤5.0 %) moves. In summary, the radial plasma
distribution can be changed, and plasmas with good
uniformity in the radial direction can be producedby changing the cavity structure.
4. Conclusions
Using the cavity with the metal circular plate
which takes a part as the physical boundary andadjust the radial distribution of the electric fields
can improve the plasma uniformity in the radialdirection. And then, the cavity is available in wide
gas pressure range. They show that the plasma withgood uniformity can be produced in various
conditions by the antenna-cavity we developed, andthe plasma distribution can be controlled bychanging the radius of the metal circular plate andadjusting the antenna position ha.