22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Beam extraction characteristics of an ICP ion source with a three-electrode
aperture
D.U. Kim1 and J.-H. Seo2
1
2
Korea Accelerator and Plasma Research Association, Choerwon-Gun, Gangwon-do, South Korea
Department of Quantum System Engineering, Chonbuk National University, Jeonju-city, Jeollabuk-do, South Korea
Abstract: An ICP (inductively coupled plasma) ion source has been constructed for
studying beam extraction characteristics with the operation conditions. Ion beam of the RF
plasma is extracted through a three-electrode (plasma electrode – extractor electrode –
ground electrode) system. A voltage divider circuit, which consists of eight resistors
(10 mega-Ohm each) connected in series, is adopted to adjust the configuration of
equipotential lines on the one hand, and to remove the charges which accumulate on the
surface of the middle electrode on the other hand. A copper disk of 40 mm in diameter is
placed 40 mm below the ground electrode in order to measure the bema current.
Measurements of the total ion beam current were made to obtain the performance of the
ICP ion source.
Keywords: ion source, ICP, beam extraction, three-electrode
1. Introduction
Various kinds of ion sources are widely used in the area
of charged particle application which covers not only
small devices such as microscopy, focused ion beam, ion
milling and ion implanter but large facilities like
accelerators. ICP ion source consists of RF power supply
and matching box and has relatively simple configuration
compared to other types of ion sources. For example, a
DuoPlasmatron has three components, filament to emit
thermal electrons, solenoid to trap electrons and increase
electron density emitted from the filament and some
electrodes to generate arc through the collision between
the electrons and neutral gas of target ion species.
Therefore, three electric power suppliers are required and
the configuration of the ion source becomes complicated.
In this study, development of a low pressure ICP ion
source with three-electrode aperture and its beam
extraction characteristics are presented.
2. Design and fabrication of ICP ion source
Firstly, the radius of the RF discharge chamber R was
determined using equation for skin depth as follows [1, 2].
δ p ≈ c / ωpe , ωpe =
ne 2
me
(1)
For 13.56 MHz RF oscillator, δ p is about 1 ~ 2 cm and
the radius of the RF discharge chamber R was set to 3 cm.
Secondly, electrode gap, d g and aperture diameter, 2a
were determined using the equation for breakdown
voltage and equation of the space charge limit current
density given in equation (2) and (3), respectively [3].
Vb = 6 × 10 4 ⋅ d g1/ 2 [V]
1/ 2
J si =
4  2Ze  Va

ε0 
9  mi  d s2
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(2)
3/2
(3)
The electrode gap, d g is related to the maximum
extraction voltage and the aperture diameter, 2a is related
to the beam current. In this experiment, d g was given as
2.4 mm and the maximum extraction voltage is expected
to be above 30 kV. The aperture diameter, 2a was set to
1.6 mm and 1 mA of beam current is expected for Ar
plasma at extracting voltage of 10 kV.
Lastly, the electrode thickness, d 0 is determined to be
0.35 mm. In design of apertures, there are some
considerations such as beam optics, mechanical strength
of the aperture and heat dissipation of the aperture etc. In
addition, the relation of 2a > 3d 0 between aperture
diameter and aperture thickness is recommended in some
reference [3].
3. Experimental Setup
The configuration of the ICP ion source is shown in fig.
1. RF plasma chamber and three-electrode aperture
structure are fabricated with the geometry specified in
previous section 2. Gas feeding system with mass
flowrate controller, RF power supply with matching box,
vacuum system with rotary pump and turbo molecular
pump are installed. Beam extraction voltage is applied to
aperture structure using high voltage power supply which
is isolated from other electrical components via insulating
transformer.
Three apertures correspond to plasma electrode, bias
electrode and ground electrode from the upper side of the
structure, respectively. Plasma electrode is connected to
the positive high voltage of HVPS and the ground
electrode is connected to ground. A voltage divider
circuit, which consists of eight resistors (10 mega-Ohm
each) connected in series, is adopted to adjust the
configuration of equipotential lines on the one hand, and
1
to remove the charges which accumulate on the surface of
the middle electrode on the other hand. Each of three
terminals of the voltage divider is connected to the plasma
electrode, bias electrode and ground electrode,
respectively.
5. Results
Figure 4 and 5 show the beam extraction characteristics
of two-electrode system. In two-electrode system,
extracted beam current increases in accordance with RF
input power, whereas it decreases with increment of
plasma gas flow rate. Both the RF input power and the
gas flow rate have effect on sheath distance – d s , distance
between the ground electrode and the plasma surface
where the ions are extracted – and thus they influence on
extracted beam current.
2.5
Fig. 1. Schematic of the ICP ion source.
4. Experiments
Experiments were carried out for two kinds of
extraction system. The one was two-electrode system
composed of plasma electrode and ground electrode
without the voltage dividing electrode. The other was
three-electrode system with plasma electrode, bias
electrode and ground electrode. For three-electrode
system, the configuration of the equipotential line was
controlled by changing the connection of the voltage
dividing circuit to the electrodes. Thus, homogeneous
equipotential line and inhomogeneous equipotential line
were considered as shown in fig. 2 and fig. 3.
Extraction Beam Current (mA)
250 W
200 W
2.0
150 W
1.5
1.0
0.5
0.0
0
2
4
6
8
10
12
Extraction Voltage (kV)
Fig. 4. RF power effect in two-electrode system.
2.0
G = 0.3 sccm
G = 0.4 sccm
G = 0.5 sccm)
G = 0.8 sccm
Beam Current (mA)
1.6
1.2
0.8
0.4
Fig. 2. Homogeneous equipotential line.
0.0
0
2
4
6
8
10
12
Extraction Voltage (kV)
Fig. 5. Gas flow rate effect in two-electrode system.
Fig. 3. Inhomogeneous equipotential line.
Operating conditions for performance tests were RF
power, P RF [W] and gas flow rate G [sccm].
2
Figure 6 and 7 show the beam extraction characteristics
of three-electrode system. In three-electrode system,
since a part of extracted beam I intercepted by the voltage
dividing electrode inserted between the plasma electrode
and the ground electrode, extracted beam current is
smaller than in two-electrode system for the same
operating conditions. The main difference between the
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Beam Current (mA)
1.2
RF = 150 W
RF = 200 W
RF = 250 W
1.4
Homogeneous Equipotential Line
Inhomogeneous Equipotential Line
1.2
1.0
0.8
0.6
0.4
0.2
0.8
0.0
0
2
4
6
8
10
12
Extraction Voltage (kV)
Fig. 8. Equipotential line effect in three-electrode system.
0.4
0.0
0
2
4
6
8
10
12
Extraction Voltage (kV)
Fig. 6. RF power effect in three-electrode system.
1.2
G = 0.8 sccm
G = 0.5 sccm
G = 0.4 sccm
Beam Current (mA)
Figure 8 shows the effect of the configuration of
equipotential line on extracted beam current. If only the
beam current is considered and beam optics is ignored,
homogeneous equipotential line is favorable.
Beam Current (mA)
characteristics of two-electrode and three-electrode is the
existence of threshold in extraction voltage. In twoelectrode system, ion beam is extracted the minute the
extracting voltage is applied and increases with the
applied voltage. In three-electrode system, on the other
hand, ion beam is extracted only after the extracting
voltage reaches some value. Once the beam is extracted,
the dependency of beam current on applied voltage is
almost same in two cases. Another big difference is the
effect of gas flow rate on the extracted beam current. The
existence of bias electrode seems to influence on the
beam trajectory. This should be dealt with in future work.
6. Conclusions
In this study, physics design for ICP ion source and
performance tests were carried out. On the whole, beam
extraction characteristics of the ICP ion source shows
similar values that are predicted in other references.
According to the application of ICP ion source, further
study on beam optics, beam control, material search (e.g.
appropriate material for electrode, vacuum or insulation
components etc.) and plasma diagnostics will be required.
7. References
[1] V. I. Kolobov and D. J. Economou, “The anomalous
skin effect in gas discharge plasmas,” Plasma Sources Sci.
Tech., 6, (1997)
[2] P. Y. Nabhiraj, R. Menon, R. Mohan, S. Mohan, and
R. K. Bhandari, “Characterization of compact ICP ion
source for focused ion beam applications,” Nuclear Inst.
and Methods in Physics Research A, V621, issue 1-3, pp
57-61
[3] Ishikawa, Ion Sources, Kyoto University
0.8
0.4
0.0
0
2
4
6
8
10
12
Extractin Voltage (kV)
Fig. 7. Gas flow rate effect in three-electrode system.
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