Ion Beam Processing and Analysis of MgO Thin Films
Hiroshi Nomura a, Ari Ide-Ektessabi b†, Nobuto Yasui a, and Yuji Tsukuda c
a
b
Graduate School of Engineering, Kyoto University, Yoshidahonmachi, Sakyo-ku, Kyoto, 606-8501, Japan
International Innovation Center, Kyoto University, Yoshidahonmachi, Sakyo-ku, Kyoto, 606-8501, Japan
c
Sanwa Kenma Ltd., 22-1, Kaminoyama, Okubo-cho, Uji City, Kyoto, 611-0033, Japan
Abstract. Thin film of MgO is widely used as a protecting layer of plasma display panel (PDP). We
prepared the MgO thin films using ion beam assisted deposition (IBAD) technique with the aim of
controlling the crystal orientation of the films. Oxygen ion beam was utilized to irradiate the growing films.
The ion beam irradiation was performed by electron cyclotron resonance (ECR) type ion source. The flux of
evaporated MgO was produced using an electron gun. Energy and current density of the ion beam as well as
deposition rate were taken as the parameters to control the deposition. Composition and crystallinity of the
films were measured using Rutherford backscattering spectroscopy (RBS) and X-ray diffraction (XRD). In
this study, we discuss the effects of simultaneous irradiation of the films by oxygen ion beam during film
growth. Experimental results suggest that the ion beam irradiation during film growth strongly influences
the crystal properties of the films to have the best orientation for high efficiency secondary electron emission.
insulation characteristic results in the accumulation of
a large amount of wall charge [2-4]. Thus the
discharge occurs and sustains easily, resulting in a low
operating voltage.
Though it is known that MgO is a better material for
the protecting layer than other materials, some of the
properties of MgO, such as high secondary electron
emissivity, are still not well studied. Many researches
is now being carried out in order to find the optimum
conditions for an MgO thin film as a protecting layer
and the best methods for preparing the film.
The secondary electron emission coefficient γ is the
most important factor for the protecting layer and there
are many studies focused on it. It has been reported
that the value of γ is dependent on the crystal
orientation of MgO [4, 5]. But the reported results on
the best crystal orientation for the highest γ are
contradictory.
The density of the MgO thin film is another
significant factor that determines the performance of
the PDP [1]. A protecting layer with a high density
prevents the ambient moisture from being absorbed
into the film, resulting in a stability of optical
INTRODUCTION
Magnesium oxide (MgO) thin film has attracted
attention for applications to the protecting layer of ac
plasma display panels (ac PDPs). The protecting layer
plays an important role in preventing the energetic
plasma particles from bombarding the dielectric layer
over the electrodes on the front glass panel of a PDP.
The properties of the protecting layer exposed directly
to the discharge space have an influence on the
discharge characteristics and lifetime of a PDP. The
important requirements for the protecting layer are
high density, high secondary electron emission and
good transparency [1]. There has been a lot of research
on the development of good quality protecting layers
and so far MgO is known to be the best material for
the protecting layer [2]. An MgO thin film applied to
the protecting layer lowers the inception voltage and
the sustaining voltage of a PDP due to its high ioninduced secondary electron emission coefficient γ and
high insulation characteristic. The large secondary
electron emission provides the discharge cell with
sufficient electrons for quick response and the high
†
Corresponding author: Ari Ide-Ektessabi. E-mail: [email protected]
CP680, Application of Accelerators in Research and Industry: 17th Int'l. Conference, edited by J. L. Duggan and I. L. Morgan
© 2003 American Institute of Physics 0-7354-0149-7/03/$20.00
613
properties, such as the refractive index and
transmission [6, 7].
The ion beam assisted deposition (IBAD) technique
uses ion beam irradiation during the growth of thin
films deposited by conventional methods such as
electron beam evaporation. The ion bombardment of
the growing film causes considerable changes in the
principal properties of the films due to the effects of
ion mixing and ion sputtering. The independent
operation of an ion source and a vapor source make
the IBAD technique highly controllable, reproducible
and flexible. It is possible to grow films with the
required properties such as preferable orientations and
high densities by selecting the parameters of IBAD
appropriately [7-12].
In this study, we utilized the oxygen ion beam
assisted deposition technique in preparing the MgO
thin films. We investigated the influence of irradiation
with an oxygen ion beam during electron beam
deposition in order to obtain the optimum conditions
of IBAD for high film density and selective crystal
orientation.
Deposition rate monitor
Sample
holder
Substrate
O+
To vacuum pump
ECR ion source
MgO
Evaporation source
Electron gun
FIGURE 1. Schematic representation of the ion beam
assisted deposition system.
X-ray diffraction (XRD) was employed for the
investigation of the relationship between the crystal
orientation of the MgO films and the ion beam
irradiation. We used θ-2θ scanning with the Cu Kα
line. XRD measurements were performed on the MgO
films on silicon and glass substrates. For the
measurement of the density and the composition of the
film, Rutherford backscattering spectroscopy (RBS)
was performed. 4He2+ ions with energy of 2 MeV and a
total irradiation dose of 12µC were used. RBS
measurements were performed on the MgO films on
carbon substrates.
EXPERIMENTAL SET-UP AND
EQUIPMENT
A schematic illustration of the IBAD system is
shown in Figure 1. MgO pellets of 99.9% purity were
used as the source material. The oxygen ion (O+)
irradiation was carried out using an electron cyclotron
resonance (ECR) type ion source. The oxygen ion
beam for irradiating the growing film had a current
density of 10 µA/cm2 on the substrate. The incident
angle of the oxygen ion beam was 45˚. The distance
between the outlet of the ion beam and the substrate
was 260 mm. The IBAD chamber was evacuated to a
base pressure of 1 x 10-4 Pa. The working pressure was
3.5 x 10-2 Pa with the oxygen gas flow during
deposition. The energy of the ion beam and the
deposition rate of MgO were controlled to produce
different kinds of samples. The ion beam energy was
varied from 300 to 2000 eV and the deposition rate
was ranging from 0.5 to 2 nm/s. During deposition, the
thickness and the deposition rate of the films were
monitored with a quartz oscillator located close to the
substrate. For comparison, non-irradiated samples
were also deposited in an oxygen atmosphere. From
here on, “non-irradiated” or “without ion beam
irradiation” will be represented by the ion beam
energy of 0 eV. 500-nm-thick MgO films were
prepared on silicon (100) and glass substrates. These
samples were annealed at 400˚C for 2 hours after
deposition. Besides, 200-nm-thick MgO films were
prepared on polished carbon substrates.
EXPERIMENTAL RESULTS AND
DISCUSSION
Figure 2 and Figure 3 show the XRD spectra of the
annealed MgO thin films grown on Si substrates
(Figure 2a, 3a) and on glass substrates (Figure 2b, 3b)
at different deposition rates and assisting ion beam
energies.
FIGURE 2. XRD spectra of MgO thin films deposited by
IBAD using a 500 eV ion beam at various deposition rates
(a) on Si substrates and (b) on glass substrates. θ-2θ scanning
with the Cu Kα line was used for the measurements.
614
dominant, resulting in the prevention
crystallization of the evaporated MgO.
of
the
The XRD spectra for films prepared with the same
deposition rate (1 nm/s) varied with the assisting ion
beam energy (Figure 3). As mentioned above, for
IBAD, the ion beam energy is an important parameter.
The energy must be adjusted so that the energy is high
enough to produce collisional mixing, but must, on the
other hand, also be in a range that prevents sputtering
from dominating.
600
Ion beam energy: 300eV
Deposition rate: 1nm/s
Substrate: carbon
500
counts
FIGURE 3. XRD spectra of MgO thin films deposited by
IBAD using various ion beam energies at the deposition rate
of 1 nm/s (a) on Si substrates and (b) on glass substrates. θ2θ scanning with the Cu Kα line was used for the
measurements.
400
300
200
100
The preferred orientation of an MgO thin film
prepared with IBAD is (200), whereas MgO thin films
deposited by electron beam evaporation without an
assisting ion beam have (111) and (222) as the
preferred orientations. This tendency is observed
regardless of the kind of substrate on which the MgO
thin films were deposited. From these results, the
following conclusions can be drawn: First, it is
concluded that the (111) and (222) oriented MgO thin
films and the MgO films with the preferred orientation
of (200) can be prepared selectively by using electron
beam deposition and IBAD; Second, in the case where
the film thickness is 500 nm, as in our experiment, it is
observed that the crystal orientation of an MgO thin
film is not affected by the substrate.
0
0
200
channel
400
600
FIGURE 4. RBS spectrum of an MgO thin film deposited
on a carbon substrate by IBAD with an ion beam energy of
300 eV and a deposition rate of 1 nm/s. 4He2+ ions with an
energy of 2 MeV and a total irradiation dose of 12µC were
used for the RBS measurement.
Figure 4 shows a typical RBS spectrum of an MgO
thin film deposited on a carbon substrate in this study.
The relative atomic densities of oxygen and
magnesium in the MgO thin films were calculated
from the RBS spectra. Figure5 shows the relative
atomic densities normalized with the atomic density of
magnesium in the MgO film deposited at the
deposition rate of 1nm/s without ion beam irradiation.
Figure 6 and 7 show the compositions and the relative
mass densities of the deposited MgO. The chemical
formulae of the deposited MgO are represented as
MgOx and the compositions are described by using the
values of x. The mass densities are normalized with
the density of the MgO film deposited at the
deposition rate of 1nm/s without ion beam irradiation.
The following observations can be made from these
figures:
(i) The atomic densities of oxygen and magnesium in
the MgO films deposited at the deposition rate of 1
nm/s are similar regardless of the difference in the ion
beam energy. The composition and mass density also
do not fluctuate.
(ii) In contrast to the results mentioned at (i), the
atomic density of magnesium in the MgO films
deposited at the deposition rate of 0.5 nm/s decreases
when the ion beam energy increases. Consequently,
The XRD spectra of films prepared with the same
assisting ion beam energy (500 eV) is significantly
influenced by the deposition rates (Figure 2). There are
only weak peaks in the XRD spectra for the MgO thin
films deposited by 500 eV ion beam irradiation at the
deposition rate of 0.5 nm/s. This result suggests that
the MgO thin films deposited by IBAD become
amorphous when the deposition rates are small. The
effects of bombarding ions are mainly determined by
the transport ratio (the ratio of the number of
bombarding ions to that of arriving neutral particles on
the substrate, which is determined by the ion beam
current density and the deposition rate) and the energy
of ions [13]. In this experiment, the deposition rate of
0.5 nm/s was employed at an ion beam energy of 500
eV and ion beam current density of 10 µA/cm2. This
rate was small enough so that the effects of the
bombarding ions (such as sputtering and mixing) were
615
the value of x increases and the mass density decreases
according to an increase in the ion beam energy.
It is inferred that the magnesium atoms were
sputtered considerably when IBAD was conducted
with a slow deposition rate and a high ion beam
energy, whereas the oxygen atoms remained due to the
irradiation of the oxygen ion beam.
ACKNOWLEDGMENTS
The authors would like to thank Mr. Y. Tanaka at
Sanwa Kenma Ltd. where the sample preparations
were performed. We are also very grateful to Dr.
Moriyama and Dr. Murase for their assistance with the
XRD measurements that were conducted at the
Department of Material Engineering, Kyoto University.
RBS measurements in this paper were performed at
the Quantum Science Engineering Center, Graduate
School of Engineering, Kyoto University. The authors
would also like to thank Mr. K. Yoshida for his
collaboration during these experiments.
Atomic density (a.u.)
1.4
1.3
1.2
1.1
1.0
Mg (deposition rate:1nm/s)
O (deposition rate:1nm/s)
Mg (deposition rate:0.5nm/s)
O (deposition rate:0.5nm/s)
0.9
0.8
0.7
0
500
1000
REFERENCES
1500
1.
2000
Ion beam energy (eV)
2.
FIGURE 5. Atomic densities of O and Mg in the MgO films
deposited at the various conditions. They are normalized
with the atomic density of magnesium in the MgO film
deposited at the deposition rate of 1nm/s without ion beam
irradiation.
3.
4.
5.
Value of x
1.5
deposition rate:1nm/s
deposition rate:0.5nm/s
1.4
6.
7.
1.3
8.
1.2
9.
1.1
0
500
1000
1500
2000
10.
Ion beam energy (eV)
FIGURE 6. Values of x at MgOx deposited at the various
conditions.
11.
Mass density (a.u.)
12.
13.
1.10
1.05
1.00
0.95
deposition rate:1nm/s
deposition rate:0.5nm/s
0.90
0
500
1000
1500
2000
Ion beam energy (eV)
FIGURE 7. Mass densities of the MgO films deposited at
the various conditions. They are normalized with the density
of the MgO film deposited at the deposition rate of 1nm/s
without ion beam irradiation.
616
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