Absorption Measurement of Sputtered Atom Density during ICP-Assisted
Sputter-Deposition of Al-doped ZnO Thin Films
R. Shindo1, A. Hirashima1, M. Shinohara1, Y. Matsuda2, K. Sakai3, T. Okada4
1
Graduate School of Science and Technology, Nagasaki University, Bunkyo 1-14, Nagasaki 852-8521, Japan
2
Graduate School of Engineering, Nagasaki University, Bunkyo 1-14, Nagasaki 852-8521, Japan
3
Cooperative Research Center, University of Miyazaki, 1-1, Gakuen Kibanadai Nishi, Miyazaki 889-2192, Japan
4
Graduate School of ISEE, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan
Abstract: Deposition of transparent conducting aluminum-doped zinc oxide (AZO) films using
inductively coupled plasma (ICP) assisted sputtering has been investigated. The metal atom
densities were measured by absorption spectroscopy using two different absorption systems:
one consists of a hollow cathode lamp (HCL), monochromator and photomultiplier tube
(HCL-M-PMT) and the other consists of a light emitting diode (LED), spectrometer, and CCD
detector (LED-S-CCD). Measured density ratio of Al to Zn atom density during ICP assisted
sputtering with AZO target using HCL-M-PMT was estimated to be two orders of magnitude
smaller than the elemental ratio in both the AZO target and deposited films. The evaluated
absolute Al density was confirmed to be highly reliable, since both Al densities measured with
HCL-M-PMT and LED-S-CCD agreed well in a DC planar magnetron discharge with Al target.
Keywords: Sputtering, zinc oxide, ICP, transparent conductive films, absorption spectroscopy
1. Introduction
Transparent conducting oxide (TCO) films have
been widely used as transparent conducting
electrodes of various optoelectronic devices such as
solar cells, flat panel displays, etc. Some TCO
materials such as tin oxide and indium oxide have
already been put to practical use in industry. In
particular, tin-doped indium oxide (ITO) has been
mainly used so far due to its high transmittance in the
visible region, high chemical stability and low
resistivity. For the last ten years, however,
aluminum-doped zinc oxide (AZO) received
attention as one of the alternatives to the ITO.
Though the AZO has lower inherent electric
conductivity than ITO, it has advantages over ITO in
environment resistance and resource cost.
To actually replace ITO with AZO, however, a
reproducible and highly-reliable fabrication process
of good quality AZO thin films has to be developed.
Thus, we have been investigating AZO film
deposition process by using inductively coupled
plasma (ICP) assisted sputtering. [1-6] The
advantages of ICP-assisted sputtering are as follows:
1) the target is sputtered with low target voltage and
high target current, 2) the usage efficiency of the
target is much improved due to the expansion of
erosion area, 3) ionization and excitation of the
sputtered particles are enhanced in the ICP and the
enhanced ion fluxes to the substrate promote the
crystallinity of thin films without intentional
substrate heating, and 4) lateral homogeneity of the
deposited film is much improved. In fact, we have
succeeded in depositing high quality AZO thin films
with resistivity of around 10-3Ωcm by using this
technique so far. [7, 8]
To obtain better quality AZO thin films, it is
essential to understand the fundamental plasma
surface interaction in this process. For the purpose of
estimating the particle flux to the substrate, we have
measured metal atom densities by absorption
spectroscopy using two different absorption systems:
one consists of a hollow cathode lamp (HCL),
monochromator
and
photomultiplier
tube
(HCL-M-PMT) and the other consists of a light
emitting diode (LED), spectrometer, and CCD
detector (LED-S-CCD). This paper describes the
theoretical calculation of experimental absorbance
against optical thickness and the experimental
determination of metal atom density for both
absorption systems.
2. Principles of Absorption Spectroscopy
In this work, densities of Al atoms and Zn atoms
were measured by HCL-M-PMT and LED-S-CCD
absorption spectroscopy. Principle of this technique
is briefly summarized here. Integral of absorption
coefficient profile k(ν) over the entire frequency
range is theoretically given by the following equation
∫ k (ν )dν =
λ2 g u AN
8πg l
(1)
where λ is the wavelength of absorption line, gu and
gl : statistical weights of upper and lower levels, A:
transition probability, and N: number density of
absorber. Absorbance α is experimentally determined
by incident power Iin and transmitted power Iout, and
is given by
α≡
I in − I out
= 1−
I in
∫ f (ν ) exp [− k f (ν )l ]dν
∫ f (ν )dν
s
0
a
(2)
s
where fs(ν) and fa(ν) are line profiles of light source
and absorber, k0: absorption coefficient at line center,
i.e., k(ν) = k0 fa(ν).
When using HCL-M-PMT in low pressure
sputtering condition, line profiles are approximated
by Gaussian form with Doppler width ∆νD.
On the other hand, when using LED-S-CCD,
absorbance α is given by
∫
α = 1−
= 1−
∆ν w
1
∆ν w
exp [− k 0 f a (ν )l ]dν
∫
∫
∆ν w
∆ν w
1dν
exp [− k 0 f a (ν )l ]dν
(3)
by setting fs(ν) of Eq. (2) is constant, since LED has
continuous spectrum. Here, ∆νD is the bandwidth of
spectrometer.
Fig.1 shows the calculated results of absorbance α
against the target plasma’s optical thickness k0l, for
HCL-M-PMT and LED-S-CCD (∆νD = 0.15nm). The
absorbance for LED-S-CCD was two orders of
magnitude smaller than that for HCL-M-PMT, but
the measurable density range was evaluated to be
almost the same because the signal-to-noise ratio of
LED-S-CCD is two orders of magnitude better than
that of HCL-M-PMT.
10 0
Absorbance α
10 -1
10 -2
distance from the RF coil to the substrate were set
both 40mm.
For the measurement of sputtered Al and Zn atoms
from the AZO target, absorption measurement has
been done by using HCL-M-PMT. [9] Optical
emission from a HCL (Hamamatsu Photonics,
L233-30NQ (Zn) or L233-13NB (Al)) was chopped
by an optical chopper (NF, 5584A), and guided to a
monochromator (JASCO, CT25) through the
ICP-assisted sputtering chamber by lens, prism and
optical fiber optics. The time modulated output signal
from the PMT was monitored and averaged for 1024
times on a digital oscilloscope. By comparing the
difference in the modulated amplitude of PMT output
between plasma ON and OFF phases, absorbance
was measured. The sputtered atom density was
obtained by comparing the experimental absorbance
and the theoretical absorbance that was calculated
using assumed gas temperatures (400K) in light
source and plasma reactor and assumed optical path
length (0.3m). The absorption measurements were
done for Zn with 307.6 nm (4s2 1S0-4s5p 3P1o) and for
Al with 396.15 nm (3s23p 2P3/2o-3s24s 2S1/2),
respectively.
ICP RF power dependence of Al and Zn atom
densities was investigated. In the experiments, ICP
RF power (= input power to the ICP coil) was varied
0-300W, and the target discharge power (= input
power to the planar magnetron) and argon gas
pressure were kept constant at 40W and 30mTorr,
respectively.
10 -3
10 -4
HCL-M-PMT
LED-S-CCD
10 -5
10 -6
10 -3
10 -2
10 -1
10 0
10 1
10 2
10 3
Optical thickness k0l
Fig.1 The calculated results of absorbance α against
the target plasma’s optical thickness k0l, for
HCL-M-PMT and LED-S-CCD (∆νD = 0.15nm).
3. Experimental
The experimental setup of HCL-M-PMT is shown
in Fig.2. A vacuum chamber attached with a 3 inch
DC planar magnetron, argon gas supply system, and
pumping system (turbo molecular pump and rotary
pump combination) was used in the experiment. A
disk target of ZnO: Al2O3 (2wt%) of 60 mm diameter
and 6mm thick was used as target, and glass
substrates were set on a earthed substrate holder with
a gap length of 80 mm. A single turn coil antenna of
100 mm diameter was installed between these
electrodes, and used for the production of 13.56MHz
inductively coupled plasma. The antenna was
covered with insulator and water-cooled. The
distance from the target to the RF coil and the
Fig.2 Setup for absorption measurement of sputtered
atom density using HCL-M-PMT during
ICP-assisted sputter-deposition of AZO thin films.
The experimental setup of LED-S-CCD is shown
in Fig.3. Absorption measurement of Al atoms was
done by using LED-S-CCD in a DC planar
magnetron discharge with Al target at 2Pa. The
absorption measurements were done for Al with
394.0 nm (3s23p 2P1/2o-3s24s 2S1/2) and 396.15 nm
(3s23p 2P3/2o-3s24s 2S1/2).
Optical emission from a LED (THORLABS,
LED405E Ultra Bright Violet LED) was guided to a
spectrometer with CCD (Spex, SPEX270M) through
the sputtering chamber by lens, prism and optical
fiber optics. The transmitted continuous spectrum
was obtained by multichannel measurement. As
shown in Table 1, four combinatorial states of LED
and plasma were arranged and the signal obtained for
each state was named from I1 to I4. Absorption
spectrum can be obtained by A = (I1 – I2.) / (I3 – I4).
Table.1 Four combinatorial states of LED and plasma
Signal
LED
Plasma
I1
On
On
I2
On
Off
I3
Off
On
seems to be in agreement with the dependence of
relative density of Al to Zn atoms in gas phase as
shown in Fig.4. However, the absolute density ratio
of Al to Zn in the gas phase was estimated to be two
orders of magnitude smaller than the elemental ratio
in both the AZO target and deposited films. The
cause of such a large gas-phase Zn atom density is
unclear, but is possibly due to the re-evaporation or
the re-sputtering of Zn atoms from inside the
chamber, or the overlapping of OH absorption.
I4
Off
Off
magnetron discharge with Al target
4. Results and Discussion
4.1 Absorption spectroscopy using HCL-M-PMT
As we have described in the experimental section,
number densities of sputtered atoms (Al and Zn)
were measured by absorption spectroscopy using
HCL-M-PMT. Fig.4 shows the experimental result
for ICP RF power dependence of metal atom
densities in gas phase. Both Zn and Al atom densities
increases with increasing ICP RF power, but there is
a difference in the ICP RF power dependence
between Zn and Al atom densities. Al atom density
monotonously increases with increasing ICP RF
power, while Zn atom density increases 4 times with
increasing ICP RF power from 0 to 100W, then
saturates for ICP RF power more than 100 W.
Fig.5 shows ICP RF power dependence of
elemental ratio of Al and resistivity in the film. The
former was obtained from the XPS analysis of Al 2p
signal. With increasing ICP RF power, the elemental
ratio of Al in the film increases. Accordingly, the
resistivity decreases as shown in Fig.5. The ICP RF
power dependence of elemental ratio of Al in the film
2.5
0.1
2
0.08
1.5
0.06
1
Elemental Ratio of Al
Resistivity
0.5
0
0.04
Resistivity [Ω·cm]
Fig.3 Setup for absorption measurement of sputtered
atom density using LED-S-CCD in a DC planar
Elemental Ratio of Al [Atomic%]
Fig.4 ICP RF power dependence of Al and Zn atom
densities during ICP assist magnetron sputtering at
the working pressure of 30mTorr.
0.02
0
50
100
150
200
250
0
300
ICP RF Power [W]
Fig.5 ICP RF power dependence of elemental ratio
of Al in the film and resistivity.
4.2 Absorption spectroscopy using LED-S-CCD
Absorption measurement of Al atoms was done by
using LED-S-CCD in a DC planar magnetron
discharge with Al target. An example of the results is
shown in Fig.6, indicating clear absorption lines at
394.40nm and 396.15nm of Al. The experimental
absorbance using LED-S-CCD was two order of
magnitude smaller than that using HCL-M-PMT
under the same conditions.
Fig.7 shows discharge power dependence of
sputtered Al atom density using HCL-M-PMT and
LED-S-CCD during magnetron sputtering. Both Al
densities measured with HCL-M-PMT and
LED-S-CCD agreed well. Thus, the evaluated
absolute Al density was confirmed to be highly
reliable.
0.998
Acknowledgment
This research was partially supported by a
Grant-in-Aid for Scientific Research (C) from the
Japan Society for the Promotion of Science (No.
20540485) and by Special Coordination Funds for
Promoting Science and Technology sponsored by
Japan Science and Technology Agency (JST).
Transmittance
0.996
LED current 10mA
Exposure time 200ms
Average 128
Power 10W
Pressure 2Pa
0.994
0.992
0.99
References
Al 394.4nm
(2P1/2→2S1/2)
0.988
390
392
Al 396.15nm
(2P3/2→2S1/2)
394
396
398
Wavelength [nm]
Fig.6 Absorption spectrum of Al atom measured
with LED-S-CCD in a DC planar magnetron
discharge.
Al Density (2P3/2)[×1010cm-3]
5
Ar Pressure: 2Pa
Al 396.15nm(2P3/2→2S1/2)
4
3
LED-S-CCD
HCL-M-PMT
2
1
0
0
2
4
6
8
10
12
Dischsrge Power [W]
Fig.7 Discharge power dependence of sputtered Al
atom density using HCL-M-PMT and LED-S-CCD
during magnetron sputtering at the working
pressure of 2Pa.
5. Conclusion
We have measured metal atom densities by
absorption spectroscopy using HCL-M-PMT and
LED-S-CCD, and evaluated the reliability of
measurements.
Both Zn and Al atom densities increases with
increasing ICP RF power, and the ICP RF power
dependence of relative density of Al to Zn atoms in
gas phase was in qualitative agreement with that of
elemental ratio of Al to Zn in the film. However, the
absolute density ratio of Al to Zn in the gas phase
was estimated to be two orders of magnitude smaller
than the elemental ratio in both the AZO target and
deposited films.
The evaluated absolute Al density was confirmed
to be highly reliable, since both Al densities
measured with HCL-M-PMT and LED-S-CCD
agreed well in a DC planar magnetron discharge with
Al target.
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