22P027-O
aA01
Influence of trench shape of ring-shaped hollow electrode on high-density
capacitive coupled plasma
高密度容量結合型プラズマに及ぼすリング状ホロー電極溝形状の影響
Yasunori Ohtsu, Yoshiki Yahata, Johji Kagami*, Yasushi Kawashimo*, Tatsuya Takeuchi* andTatsuya Misawa
大津康徳, 矢羽田祥貴，加々見丈二*，川下安司*，竹内達也*，三沢達也
Department of Electrical and Electronic Eng., Graduate School of Science and Engineering, Saga University
1, Honjo-machi, Saga 840-8502, Japan
佐賀大学大学院工学系研究科電気電子工学専攻 〒840-8502 佐賀市本庄町1*
Shinko Seiki Co., Ltd.
*
神港精機株式会社
Influence of trench shape of ring-shaped hollow electrode on high-density capacitive coupled plasma has
been investigated at a wide range of argon gas pressure and various input powers. The trench shapes
selected are well-typed, taper-typed, and step-typed ones. It is seen at the lower pressure less than200mTorr that plasma density for well-typed shape is the highest among three shapes, while for step-typed
shape, is the lowest. For the higher pressure more than 200mTorr, plasma density for step-typed shape has
the highest value. It was revealed that plasma density for all typed shapes was almost proportional to input
power.

1. Introduction
In the microelectronic industry, the parallel-plateradio frequency (RF) plasma source operating at
13.56 MHz is widely used [1]. This is because its
geometry is very simple. In especial, capacitively
coupled plasma (CCP) source is frequently utilized
for dry etching, plasma enhanced chemical vapor
deposition and sputtering processes. However, there
are the following problems; (1) density and energyof ions can be independently not controlled byexternal parameters such as power for the dryetching and sputtering process and (2) plasma
density is less than 109cm-3 so that the rates of
etching and deposition are very low. This is main
problem for RF capacitively coupled plasma. In
order to improve this problem, some solutions
based on (1) the proposal of dual frequency withhigh and low frequencies [2] and (2) the highdriving frequency [3] such as VHF region havebeen proposed.
For microelectronics and flat panel displays
industries, major uniformity problems in plasma
processing are occurred under the plasma excited at
higher frequencies. Recently, the large-areacapacitive discharge at frequencies higher than theusual industrial frequency of 13.56MHz has beenstudied numerically [4] and experimentally [5].
From these studies at higher frequency operation, it
has been pointed out that standing wave and skineffects can be important limitations for plasma
processing uniformity, which cannot be describedby conventional electrostatic theory. Since theconventional driving frequency is 13.56 MHz, it
will be required to develop high-density anduniform large plasma under the operation of theconventional driving frequency and the simple
configuration of geometry.
On the other hand, in our previous studies onplasma source, we have developed the high-density
plasma with ring-shaped hollow electrode ofwell-typed shape[6].
In this paper, influence of trench shape on thehigh-density capacitive discharge plasma has been
investigated in ring-shaped hollow cathodedischarge. The plasma parameters of density andtemperature of electrons are measured by Langmuirprobe at various external conditions of gas pressure
and RF input power.
2. Experimental setup
Figure 1 shows a schematic diagram ofaluminum cylindrical vacuum vessel with 160 mm
in diameter and 200 mm in length. The ring-shaped
hollow electrode is mounted as a top electrodealong the central axis in the vessel. In theexperiment, a base pressure in the vessel is less than
10-6 Torr. Ar gas is introduced from 40 to 400mTorr. The RF power supply of 13.56 MHz wasinput through a matching box between the RF
powered electrode and the grounded vessel. The
backside of the electrode is covered by thegrounded metal enclosure so as to avoid anadditional discharge between the electrode and thegrounded chamber. The RF input power is fixed atproduction has been investigated in ring-shaped hollow
cathode RF discharge. It is found that for the lower gas
pressure the well-typed trench is the highest value ofplasma density, while for the higher gas pressure thestep-typed trench is attained to be high-density plasma.

Acknowledgments
This work was partially supported by Grant-in-Aid for
Scientific Research (C) (23540577).
References[1] S. J. Fonash: IBM J. Res. Develop. 43 (1999)103.
[2] T. Makabe and Z. Lj. Petrovic: Appl.Surf.Sci., 192(2002) 88.
[3] M. Tanda, M. Kondo, A. Matsuda: Thin Solid Films

10 mm

5 mm

5 mm

10 mm

10 mm

(a) Well-typed(b) Taper-typed

5 mm
2 mm
5 mm
10 mm

(c) Step-typed

Fig.2. Structures of three typed trenches selected, where(a) the size for the well-typed trench is 5 mm width and
10 mm depth, (b) for the taper-typed trench, the widthsof the inlet and the bottom are 10 and 5 mm, respectively
and the depth is 10 mm, (c) the step-typed trench has alarge trench with 5 mm width and 5 mm depth and asmall trench with 2 mm width and 5 mm depth.
10-34. Conclusions
The influence of trench-shape on high-density plasma

Fig.1. A schematic diagram of experimental setup.

10

3. Results and discussion
Figure 3 shows plasma density as a function ofAr gas pressure at RF input power of 50 W. The
measured position is at r = 37.5 mm and z = 12 mm
under an inlet of trench. It is seen that for thewell-typed trench plasma density increases withincreasing gas pressure p for p < 200 mTorr andthen decreases for p < 400 mTorr. For thetaper-typed trench, plasma density raises with gaspressure for p < 200mTorr and then saturates. On
the other hand, plasma density increases withincreasing gas pressure in the case of the step-typed
trench. Therefore, it is found that the well-typed
trench has the highest value of plasma density for p< 200 mTorr, while for p > 200 mTorr thestep-typed trench gets its maximum value.
The hollow cathode effect is satisfied by therelation[6] of 2ds < W < λen, where ds, W and λen are
sheath thickness in the trench, trench width andelectron-neutral mean free path, respectively. The
influence of trench-shape on plasma density as afunction of gas pressure is mainly ascribed by therelation between W and λen. This is because themean free path is inversely proportional to gas
pressure.
It was also found that plasma density increased
proportionally with increasing RF input power for
all typed trenches. This results indicates that thering-shaped hollow cathode RF discharge proposedis very useful for the stable plasma processing
sources.

427 (2003)33.
[4] M. A. Lieberman, J. P. Booth, P. Chabert, J. M. Raxand M. M. Turner: Plasma Sources Sci. Technol. 11(2002)283.
[5] S Rauf, K Bera and K Collins: Plasma Sources Sci.
Technol. 17 (2008)035003.
[6] Y.Ohtsuand
H.Urasaki:
PlasmaSources
Sci.Technol. 19(2010) 045012.

Plasma density ne (×10 cm )50 W corresponding to the RF power density of0.64 W /cm2.
Figure 2 shows cross sectional images for three
typed shapes selected from well-typed, taper-typed
and step-typed trenches.

1z=12mm

Well-type
Taper-type
Step-type

0.1
0100
200
300
400
Ar gas pressure p (mTorr)

500

Fig.3. Plasma density as a function of Ar gas pressure at
fixed RF input power of 50 W.