538 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 33, NO. 2, APRIL 2005
Simulation of Ion Beam Transport in an Ion Gun for
Sung Jin Kim, Soon Jung Wang, Jae Koo Lee, Senior Member, IEEE, and Geun Young Yeom
Abstract—Ion guns with several grids, which control ion energy
from a plasma source, are widely used in semiconductor manufac-
turing process. We report on particle-in-cell simulations of the ion
gun to which an electron beam and a magnetic field are applied in
order to obtain higher ion fluxes. The electron beam in the energy
range from 100 to 1000 eV is emitted in axial direction and the uni-
form magnetic field of 100 G perpendicular to the electron beam is
used. It is revealed from the simulations that an ion flux in the ion
gun with the electron beam and the magnetic field is twice larger
than that in the conventional ion guns.
Index Terms—Electron beam, ion gun, magnetized plasma, par-
ductor manufacturing process such as deposition, implantation,
tems for neutral beam etching, in order to reduce a charge-up
damage . In a neutral beam source, the ion beam is neutral-
ized by collisions withreflectors. Plasma is created byan induc-
tively coupled plasma (ICP) source. The ion gun is composed
of several grids controlled by an external voltage. In the ion gun
with two grids, the first grid is applied to positive voltage and
the second grid is grounded. Only positive ions are ejected by
mines the sheath structure around the aperture, which is closely
connected to the ion flux and the angle distribution . The ion
flux and the ion angle distribution, which depend on the ion gun
parameters such as grid voltage, grid width, grid interval, and
aperture size, are important factors for high performance pro-
Although the ion energy is easily controlled and estimated in
the ion gun with two grids, the ion flux decreases with a low-en-
ergy ion beam because the ion flux is proportional to electric
field between grids. An ion gun with three grids is used for a
low-energy and a high-flux ion beam . In the three-grid ion
first grid and the second grid. The potential difference between
OW-ENERGY ion beam sources from a few electronvolts
to hundreds of electronvolts are widely used in semicon-
Manuscript received June 29, 2004; revised October 11, 2004. This work was
Development Program of the Korea Ministry of Science and Technology.
S.J.Kim, S.J.Wang, andJ.K. Leearewiththe DepartmentofElectronicand
Electrical Engineering, Pohang University of Science and Technology, Pohang
790-784, Korea (e-mail: Medic@postech.ac.kr; firstname.lastname@example.org).
G. Y. Yeom is with the Department of Materials Engineering, Sungjyunkwan
University, Suwon 440-746, Korea.
Digital Object Identifier 10.1109/TPS.2005.845138
the first grid and the third grid has influence upon the ion en-
ergy. The three-grid ion gun allows independent control of ion
flux and ion energy, preventing a reduction of ion flux for low
energy ions. If a positive voltage is applied to the third grid, it
prevents ions from dispersing toward the grid. This improves
the directionality of ions, making it easy to control the incident
We have performed ion-gun simulations by the two-dimen-
sional object oriented particle-in-cell (OOPIC) code for the op-
timization of ion beam source . The ion flux, the energy and
the angle distribution are calculated in our simulations. In our
three-grid ion gun, 300, 0, and 200 V are applied to the grids.
The aperture size, the grid width, and the interval between the
grids are 4, 1.2, and 0.9 mm, respectively. A low pressure of
0.1 mtorr is used in order to obtain a narrow ion angle distribu-
tion. Ar gas is used for obtaining the fundamental characteris-
tics of positive ion beam. Since the ion flux is proportional to a
plasma density, an electron beam is utilized in order to increase
the plasma density. The current density of the electron beam in-
jected toward the aperture is 66.7 mA/cm
is applied. This magnetic field cannot affect ion angle distribu-
The electron beam and the magnetic field perpendicular to the
electron beam result in the increase of the plasma density and
the ion flux.
Fig. 1 is a snapshot of a particle distribution. We obtain data
from OOPIC and postprocessing the data has been done by a
commercial graphic package, SURFER. Density distributions
of ions and electrons in the ion gun without an electron beam
and amagnetic field areshown inFig.1(a) and (b).Fig.1(c)and
(d) are density distributions of ions and electrons in the ion gun
electron beam is 1000 eV in Fig. 1(c) and (d). We have changed
the energy of the electron beam from 100 to 1000 eV. The in-
crease of the energy of the electron beam raises the propagation
of the electron beam. This propagation depends on the inten-
sity of the magnetic field and the energy of the electron beam.
The magnetic field perpendicular to the electron beam prevents
high-energy electrons from being ejected between grids. While,
in the ion gun with low pressure of 0.1 mtorr and small aperture
of 4 mm, an ion flux is
cmsin the ion gun with the electron beam of 400 eV and
the uniform magnetic field of 100 G and
in the ion gun with the electron beam of 1000 eV. The ion flux
. The uniform mag-
cms , it is
0093-3813/$20.00 © 2005 IEEE
KIM et al.: SIMULATION OF ION BEAM TRANSPORT IN AN ION GUN FOR MATERIALS PROCESSING539
Fig. 1.Snapshotofaparticle densitydistribution byPICsimulation.(a)Ionand (b)electrondensitydistributions in theconventional iongun.(c)Ion and(d)electron
density distributions in the ion gun with the electron beam of 1000 eV and the magnetic field of 100 G. Magnetic field is perpendicular to the electron beam.
in the ion gun with the electron beam and the magnetic field is
twice as large as that in the conventional ion gun. The shape
and the size of the plasma sheath are important to decide on the
ion flux. A high-energy electron beam causes the sheath size to
decrease. The reduction of sheath size results in the increase of
the ion flux. At low pressure, the electron beam, higher than a
few kiloelectronvolts, reduces the ion flux because of the loss of
high-energy electrons ejected fast between grids.
In conclusion, we have simulated the ion gun, which is ap-
plied to an electron beam and a magnetic field in order to ob-
tain a high ion flux by PIC code. The plasma density and the
sheath with ion gun parameters have decisive influence on the
ion flux. The electron beam and the magnetic field result in
higher ion flux.
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