Characterization of radio frequency plasma using Langmuir probe
and optical emission spectroscopy
M. Nisha, K. J. Saji, R. S. Ajimsha, N. V. Joshy, and M. K. Jayaraj
a
Optoelectronics Devices Laboratory, Department of Physics, Cochin University of Science
and Technology, Kochi-682 022, Kerala, India
Received 21 February 2005; accepted 4 January 2006; published online 15 February 2006
The radio frequency plasma generated during the sputtering of Indium Tin Oxide target using Argon
was analyzed by Langmuir probe and optical-emission spectroscopy. The basic plasma parameters
such as electron temperature and ion density were evaluated. These studies were carried out by
varying the RF power from 20 to 50 W. A linear increase in ion density and an exponential decrease
in electron temperature with rf power were observed. The measured plasma parameters were then
correlated with the properties of ITO thin films deposited under similar plasma conditions. © 2006
American Institute of Physics. DOI: 10.1063/1.2171777
I. INTRODUCTION
Magnetron sputtering deposition is widely used to pro-
duce thin films and hard coatings because of its high depo-
sition rate, ease of scaling, and the quality of the deposited
films.
1
Among a variety of oxides, tin-doped indium oxide or
Indium Tin Oxide ITO thin film is one of the most widely
used material for microelectronic applications.
2
ITO conduc-
tive films have been used as transparent electrodes in opto-
electronic devices such as liquid-crystal displays and solar
cells.
3
The properties of ITO films have a strong relationship
with their processing plasma characteristics.
Plasma diagnostics is widely being used to analyze
plasma during the sputtering. Plasma is a gas ionized suffi-
ciently so that the charge separation which can take place in
it is small compared to its microscopic charge density. On a
macroscopic scale, therefore, plasma is approximately neu-
tral, although its principal constituents are charged ions and
electrons. Information about fundamental plasma parameters,
such as electron temperature, electron density, etc., are es-
sential in order to evaluate the energy transport into the
plasma. There are several diagnostic techniques employed
for the determination of electron density and temperature
which includes plasma spectroscopy,
4
Langmuir probe,
5
microwave and laser interferometries, and Thomson
scattering.
6“8
In this paper we report the studies on radio frequency
rf plasma using Langmuir probe and optical-emission spec-
troscopy OES . The plasma parameters such as ion density
and electron temperature were determined and their depen-
dence on properties of thin film deposited under similar
plasma conditions were studied. Plasma parameters were de-
termined for different rf powers keeping the distance from
the target a constant.
II. EXPERIMENTAL SETUP
The vacuum chamber used for the rf plasma analysis
work was pumped down to a pressure of
10
-5
mbar by
means of a diffusion pump backed by rotary pump. An inert
background gas argon was introduced into the chamber via
a mass flow controller MFC up to a pressure of 0.01 mbar.
The target used in the present study was a 2-in-diam ITO
sintered disk containing 95 wt % of In
2
O
3
and 5 wt % of
SnO
2
.
Langmuir probe is one of the simplest techniques for
obtaining information about the ions in plasma. A rf compen-
sated Langmuir probe was used for plasma diagnostics Fig.
1 . The Langmuir probe assembly consists of a Tungsten
wire, 0.5 mm in diameter and 5 mm in length, supported by
a glass sleeve along with the rf compensating circuit. Probe
current is measured for bias voltages in the range of
-60 to +60 V. The probe voltage-current V-I characteris-
tics are plotted for different rf powers 20“50 W . Great care
was taken to prevent the probe feed wires being exposed to
the plasma since this will contribute to the measured probe
current. To ensure a clean probe surface, the probe wire was
replaced frequently.
a
Author to whom correspondence should be addressed: FAX:
91 484
2577595; electronic mail: mkj[at]cusat.ac.in
FIG. 1. Experimental setup for Langmuir probe and optical emission spec-
troscopic measurements. R is resistance across which the output voltage
Vo is measured. Inset shows the rf-compensated probe. L1, and L2 are
inductors, while C, C1, and C2 are capacitor.
JOURNAL OF APPLIED PHYSICS 99, 033304 2006
0021-8979/2006/99 3 /033304/4/$23.00
© 2006 American Institute of Physics
99, 033304-1
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To investigate the ionic species present in the rf plasma
in detail OES of plasma plume generated by rf sputtering of
ITO target was recorded using a 0.32 m monochromator and
a charge-coupled device CCD detector. The OES were re-
corded through the side window of the chamber. The emis-
sion spectral data were collected from the plasma at a dis-
tance 4 cm from the target at various rf powers.
III. RESULTS AND DISCUSSION
A. Langmuir probe studies
A typical Langmuir probe I-V characteristics for a rf
power of 20 W is shown in Fig. 2. The floating potential V
f
and plasma potential V
p
are determined from the Langmuir
probe I-V plot. The difference between plasma potential and
floating potential V
p
-V
f
gives a measure of energy of the
sputtered particles bombarding the substrate.
9
V
p
-V
f
is plot-
ted in Fig. 3 as a function of rf power. In the present inves-
tigation V
p
-V
f
shows a slight decrease with increasing rf
power.
The ion saturation portion of the characteristics is used
to determine the ion density.
10
The ion current drawn by the
probe is given by the equation
I
i
=
Ae
3/2
N
2
3T
e
m
i
1/2
1 -
V - V
p
T
e
1/2
,
1
where A is the surface area of the probe, N is the plasma
density, m
i
is the ion mass, and T
e
is the electron temperature
in eV. Taking the derivative of I
i
2
with respect to V and
rearranging we get
N
i
2
= -
4 m
i
3A
2
e
3
I
i
2
V
.
2
The electron temperature is determined from the slope of the
ln I -V curve of the probe in the region between V
f
and V
p
by the equation
10
T
e
=
V
ln I
,
3
where I is the electron current.
Figures 4 and 5 show the dependence of ion density and
electron temperature on rf power. Ion density is found to
FIG. 2. Langmuir probe I-V characteristics for a probe distance of 4 cm
from the target and a rf power of 20 W.
FIG. 3. Variation of floating potential V
f
and V
p
-V
f
with rf power.
FIG. 4. Variation of ion density as a function of rf power.
FIG. 5. Variation of electron temperature as a function of rf power.
033304-2
Nisha et al.
J. Appl. Phys. 99, 033304 2006
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increase linearly with increase in rf power whereas electron
temperature decreases with rf power. The increase in ion
density is due to the greater ionization resulting from colli-
sions that may occur at higher rf power. The drop in electron
temperature and V
p
-V
f
is due to the increase in ion
density.
11
The ion velocity and electron velocity were calcu-
lated using the following equations:
12
e
=
8kT
e
m
e
1/2
,
4
i
=
8kT
e
m
i
1/2
,
5
where k is the Boltzmann constant, T
e
is the electron tem-
perature, m
e
is the electron mass, and m
i
is the ion mass.
Variation of ion and electron velocities is shown in Fig. 6.
Velocity of electrons is found to be of the order of 10
6
m s
-1
while that of ions is of the order of 10
3
m s
-1
.
B. Optical emission studies
The rf plasma generated during the sputtering of ITO
target was analyzed by recording the optical emission spectra
to identify the ionic species in the plume. The spectral analy-
sis revealed that the ionic species is mainly composed of
argon ions. The identified species essentially comprises ar-
gon neutrals Ar
I
, singly ionized argon Ar
II
, doubly ion-
ized argon Ar
III
, indium neutral In
I
, oxygen neutral
O
I
, and tin neutral Sn
I
.
13
Figure 7 gives a typical optical
emission spectrum taken at a rf power of 20 W. The spectral
data are collected from the plasma at a distance of 4 cm from
the target.
The optical emission spectral shows that the intensity of
emission lines increases with increase of rf power. The varia-
tion of integral intensity of argon
I
at a wavelength of
811.5 nm with rf power is given in the inset of Fig. 7. The
integral intensity is found to increase linearly with rf power.
Increase of rf power causes more ionization, which in turn
increases the population of various energy levels associated
with the ions leading to the increase in integral intensity.
In order to correlate the plasma parameters with the ob-
served film properties, Indium tin oxide thin films were de-
posited by rf magnetron sputtering at room temperature at a
target to substrate separation of 4 cm by varying rf power.
The films deposited at lower rf powers were polycrystalline
and do not show any preferred orientation as seen from the
x-ray-diffraction pattern Fig. 8 . These films show 222 and
440 diffraction peaks of indium oxide In
2
O
3
.
14
With in-
crease in rf power, the intensity of these peaks reduced and
the films oriented in the 100 direction, which was inferred
from the appearance of 400 diffraction peak in the x-ray-
diffraction pattern. The increase in ion density with rf power
enhances the deposition rate, which leads to an increase in
film thickness. The change in orientation of the grains may
be due to this increase in thickness of the film.
15
The film deposited at a rf power of 50 W was highly
conducting also. The conductivity increased from 6 to
170 S/cm when the rf power was increased from
20 to 50 W. The increase in mobility of the carriers and
FIG. 7. Optical Emission Spectrum of rf plasma generated with ITO target
at a rf power of 20 W and at a distance of 4 cm from the target. Inset shows
the Variation of integral intensity of Argon I at a wavelength of 811.5 nm
with rf power.
FIG. 8. X-ray-diffraction pattern of ITO thin films deposited at various rf
powers for a target to substrate spacing of 4 cm.
FIG. 6. Variation of electron velocity and ion velocity with rf power.
033304-3
Nisha et al.
J. Appl. Phys. 99, 033304 2006
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number of free carriers with increase in rf power increases
the conductivity of the films.
16
Figure 9 shows the variation
of resistivity of ITO thin films with rf power. The plasma
studies showed that the ion density is high and the electron
temperature is minimum at a rf power of 50 W, which re-
sulted in best film properties at that particular power. The
electron temperature and ion velocity attain a constant value
at about 40 W. The rate of film deposition also more or less
independent of the rf power above 40 W Fig. 10 .
IV. CONCLUSION
Langmuir probe and Optical Emission Spectroscopic
OES studies were done to determine the plasma param-
eters. Different ionic species in the plasma were identified
from the spectrum. The plasma parameters such as electron
temperature, electron velocity, ion velocity, and ion density
were determined by the Langmuir probe for various rf
power. The Langmuir probe and OES studies were carried
out for scaling the ion density with rf power. The observed
plasma parameters were correlated with the thin films depos-
ited under similar plasma conditions. The ion density and
electron temperature were the highest for a rf power of 50 W
and the best film properties were obtained at that particular rf
power.
ACKNOWLEDGMENTS
This work was supported by Department of Science and
Technology, Government of India. One of the authors
M.K.J. wishes to thank Kerala State Council for Science,
Technology and Environment for the financial assistance un-
der SARD programme. Another author N.M. thanks Coun-
cil of Scientific and Industrial Research for Junior research
fellowship.
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