Materials Transactions, Vol. 46, No. 11 (2005) pp. 2536 to 2540
#2005 The Japan Institute of Metals
EXPRESS REGULAR ARTICLE
Ag–Ti Alloy Used in ITO–Metal–ITO Transparency Conductive Thin Film
with Good Durability against Moisture
Shi-Wei Chen1; *1 , Chun-Hao Koo1; *2 , Hsin-Erh Huang2 and Chia-Hua Chen2
1
Department of Materials Science and Engineering, National Taiwan University,
No. 1, Sec. 4, Roosevelt Road, Taipei, Taiwan 106, R. O. China
2
Metallurgy Division, Materials & Electro-Optics Research Division, Ghung-Shan Institute of Science & Technology,
Lungtan, P. O. Box 90008, Lung-Tan, Tao-Yuan, Taiwan, R. O. China
This study investigates the characteristics of IMI (ITO–Metal–ITO) transparent conductive thin films, with an Ag–Ti alloy intermediate
layer. Multi-layers were deposited by sputtering. ITO–AgTi–ITO films have better transmittance than ITO–Ag–ITO films in the visible
wavelength range. The maximum transparency is 94% after being annealed at 573 K in a vacuum. Although the resistivity of ITO–AgTi–ITO
films is slightly higher than that of ITO–Ag–ITO films, the former apparently exhibit greater thermal stability and durability. After exposure to
the environment at 323 K with a relative humidity of 90% for 144 h, corrosive spots appear on ITO–Ag–ITO films but not on ITO–AgTi–ITO
films.
(Received August 18, 2005; Accepted September 26, 2005; Published November 15, 2005)
Keywords: indium tin oxide–Metal–indium tin oxide transparent conductive thin film, silver–titanium alloy, resistivity, transparency,
durability
1.
Introduction
Transparent conductive oxide (TCO) thin films, e.g. ITO,
with good conductivity and transmittance in the wavelength
range of 400–700 nm have been widely used as the transparent conductor for opto-electronic devices.1–3) Presently,
along with the progress of display technology, TCOs for high
performance flat panel displays (FPD) require even lower
resistivity for high response rate, lower power consumption
and driving voltage. ITO with higher resistivity seems not
good enough to meet the requirement.
In recent years, the demand and application of TCOs with
excellent performance increase continuously. There are
many researches in advancing the properties of TCOs films
and improved materials.4–6) But most of these researches
without ITO can’t surpass ITO in conductivity. The simplest
and most effective way is to use ITO–Metal–ITO (IMI)
multilayer structures.7–10) IMI structures have quit low sheet
resistance and relatively lower thickness than single-layer
TCO films, e.g. ITO, although the transmittance is slight
lower than ITO.
The properties of IMI films are strongly dependent on the
intermediate layer.2,8) Silver is the best choice as the
intermediate layer of IMI films, because it owns both greatest
transmittance and conductivity than other metals when the
thickness of films gets very thin.11)
Pure silver films are unstable and easy to agglomerate and
corrode at high temperature, the conductivity and transmittance will be reduced by the increase of surface roughness
and corrosion or oxidation.9,10,12–14) IMI films, with the Ag
intermediate layer, will produce spots under environment
with high temperature and moisture for a long time. The
generation of spots is related with the corrosion of Ag. It was
reported10) that if silver layers become stable by alloying with
doping precious metal, e.g. Ag–Pd alloy, the corrosion and
*1Graduate
Student, National Taiwan University
author, E-mail: [email protected]
*2Corresponding
oxidation will be restrained. However, palladium is a rare
metal and very expensive.
This investigation develops Ag–Ti alloy to replace pure
Ag as the intermediate layer of IMI structures, by which the
resistivity, transmittance and crystalline after being annealed
in air and vacuum are measured. It is also to test the durability
in order to realize the difference in stability between pure
silver and silver–titanium alloy as the intermediate layer of
IMI films.
2.
Experimental
IMI multilayer films were deposited on glass substrates
(coring 7059) by continuous magnetron sputtering without
vacuum breaking. The thickness of the films was 45–10–
45 nm (ITO–Ag or Ag alloy–ITO). ITO films were deposited
from a sintered ITO target with high density and a RF power
source. Ag or Ag–Ti alloy was applied as metal targets. A DC
power source was supplied. The targets of Ag and Ag–Ti
alloy were produced in a vacuum arc furnace. The chamber,
which was equipped with a cyro pump, had a base pressure of
6:67 10 4 Pa. Deposition was conducted at a pressure of
6:67 10 1 Pa in an atmosphere of extra pure Ar. Films after
deposition were annealed in a vacuum (6:67 10 3 Pa) and
in air atmosphere for one hour at a temperature range from
573 to 773 K.
AFM (atomic force microscopy) was used to measure the
thickness of the films and estimate the sputtering rate. Low
angle XRD was used to explore the crystallization, and the
thickness of films used for analysis was 45–30–45 nm (ITO–
Ag or Ag alloy–ITO). The four point probe method was used
to measure the resistivity. UV–visible spectroscopy was used
to analyze the transmittance.
The durability test was conducted at 323 K in a testing box
with a relative humidity of 90%. After the samples had been
exposed for 144 h, optical microscopy was employed to
observe the surface and the resistivity and transmittance were
measured.
Ag–Ti Alloy Used in ITO–Metal–ITO Transparency Conductive Thin Film with Good Durability against Moisture
Resistivity of the Ag, Ag–Ti and IMI films as deposition at 298 K.
Resistivity, =cm
3.
Ag film
4:5 10
6
Ag–Ti film
6:5 10
6
ITO–Ag–ITO film
3:4 10
5
ITO–AgTi–ITO film
4:3 10
5
Result and Discussion
3.1 Resistivity
3.1.1 As deposition
Table 1 presents the resistivity of the Ag film, Ag–Ti film
and IMI films with different intermediate layer deposited at
room temperature. The resistivity of the Ag–Ti film exceeds
that of the Ag film, according to the theory of the free
electron model,15) that is because of the increase of the
probability that carriers are scattered by impurities. Table 1
also shows that the resistivity of the ITO–AgTi–ITO film
exceeds that of the ITO–Ag–ITO film. The conductive
properties of IMI films are dominated by the intermediate
layer,2,8) which indicates that the resistivity of AgTi alloy
exceeding that of pure Ag causes the resistivity of ITO–
AgTi–ITO films to exceed that of ITO–Ag–ITO films.
3.1.2 Heat treatment in air atmosphere
Figure 1 presents the resistivity of IMI films after being
annealed in air atmosphere or in a vacuum. The resistivity of
ITO–Ag–ITO and ITO–AgTi–ITO films increase slightly
after being annealed at 573–673 K in air atmosphere. It seems
possible for the following referred reasons. (1) identifying
with the gas sensor, the surface resistance increases along
with the adsorption of oxygen.16) (2) oxygen in air diffuses
into the ITO layer at the top of the multi-layer, the resistivity
of the ITO film increases as the number of conductive carriers
is reduced by the decrease of the number of oxygen
300
300
500
ITO - AgTi - ITO
ITO - AgTi - ITO
ITO - Ag - ITO
ITO - Ag - ITO
250
Resistivity, ρ/ Ω · cm
400
200
600
700
in air
in a vacuum
in air
in a vacuum
3.2 Transmittance
3.2.1 Heat treatment in air atmosphere
Figure 2 shows the result of X-ray diffraction of ITO–
AgTi–ITO films as deposition and annealed at 473–573 K in
air atmosphere. ITO film is amorphous as depositing at room
temperature, but changes to crystalline after being annealed
in air atmosphere at above 573 K. Meanwhile, defects
decrease and the transmittance of ITO will increase according to the decrease of light scattering by defects, as proposed
by Kikuchi et al.17–19)
800
300
ITO (222)
250
200
150
6
150
6
vacancies.17–19) (3) oxygen atoms diffuse through the ITO
layer and oxidize the metal layer inside.10)
When the temperature exceeds 673 K, the resistivity of
ITO–Ag–ITO and ITO–AgTi–ITO films raise sharply. Jung
et al.10) presented similar results too. It is mainly and
probably because the inter-diffusion of atoms becomes
severer.20) Silver atoms diffuse into ITO layers or oxygen
atoms diffuse through ITO and react with intermediate layers,
causing the oxidation of the metal layers, so the conductivity
of both the metal layer and the IMI films decline.
The resistivity of the ITO–AgTi–ITO film is lower than
that of the ITO–Ag–ITO film after being annealed at 773 K in
air atmosphere, it apparently reveals that Ag–Ti alloy is more
resistant than pure silver to oxidation.
3.1.3 Heat treatment in a vacuum
Figure 1 shows that IMI films annealed in a vacuum can
reduce the resistivity below that obtained by annealing in air
atmosphere. It is suggested that, during annealing in a
vacuum, more rare oxygen atoms are adsorbed onto the
surface of the films or diffuse into ITO, a fact that will not
increase the resistivity of ITO as well as the IMI films.
Moreover, the Ag or Ag–Ti layer is not oxidized, by which
the resistivity of IMI films will not decline. Heat treatment
reduces the concentration of defects, so the resistivity of IMI
films decreases according to the decrease of the number of
carriers scattered by the defects.
ITO (400)
Intensity
Table 1
2537
5
5
4
4
3
3
300°C
2
800
200°C
2
300
400
500
600
700
Ag (111)
Ag (200)
25°C
Temperature, T/K
Fig. 1 Resistivity of IMI films after being annealed. (a) ITO–AgTi–ITO
films annealed in air atmosphere ( ). (b) ITO–AgTi–ITO films
annealed in a vacuum ( ). (c) ITO–Ag–ITO films annealed in
air atmosphere (— —). (d) ITO–Ag–ITO films annealed in a vacuum
(— —).
30
35
40
45
two theta, 2θ
50
Fig. 2 XRD spectra of ITO–AgTi–ITO films annealed at a temperature
range from 298 to 573 K in air atmosphere.
2538
S.-W. Chen, C.-H. Koo, H.-E. Huang and C.-H. Chen
100
(e)
(d)
80
(a)
(a):298K
(b):573K
(c):673K
(d):723K
(e):773K
60
(b)
(c)
40
350
Transparency (%)
Transparency (%)
80
450 550 650 750
Wavelength, λ/nm
250
(b)
(c)
(d)
350
450 550 650 750
Wavelength, λ/nm
850
Fig. 5 Transmittance curves of the ITO–Ag–ITO films annealed at a
temperature range from 298 to 773 K in a vacuum.
95
100
(a)
(e)
(c)
90
90
(b)
(d)
(a)
85
(a):298K
(b):573K
(c):673K
(d):723K
(e):773K
80
(b)
(c)
Transparency (%)
Transparency (%)
(a):298K
(b):573K
(c):673K
(d):773K
40
20
850
Fig. 3 Transmittance curves of the ITO–Ag–ITO films annealed in air
atmosphere.
(a)
60
(d)
80
(a):298K
(b):573K
(c):673K
(d):773K
70
60
75
350
450 550 650 750
Wavelength, λ/nm
850
Fig. 4 Transmittance curves of the ITO–AgTi–ITO films annealed in air
atmosphere.
Figure 3 plots transmittance curves of ITO–Ag–ITO films
annealed in air atmosphere at various temperatures. The
transmittance declines when the film is annealed in air
atmosphere at 673 K. Annealing at over 673 K in air
atmosphere might cause severe inter-diffusion of atoms
between silver and ITO layers.10,20) Therefore, Ag atoms
diffuse into ITO and reduce the transmittance of the ITO
films as well as the IMI films. Under the same conditions, the
transmittance of ITO–AgTi–ITO films does not decline, as
plotted in Fig. 4, probably because Ag–Ti alloy is more
stable than pure Ag, and inter-diffusion could not occur easily
between Ag–Ti alloy and ITO.
Furthermore, as shown in Figs. 3 and 4, when ITO–Ag–
ITO films are annealed in air atmosphere at above 723 K
(Fig. 3) and ITO–AgTi–ITO films at above 773 K (Fig. 4),
300
400
500
600
700
Wavelength, λ/nm
800
Fig. 6 Transmittance curves of the ITO–AgTi–ITO films annealed at 298
to 773 K in a vacuum.
transmittance curves change and have the similar shape as
ITO single layer with the same thickness (100 nm). It is
suggested that because ITO and silver (or silver alloy) interdiffused fully, films might change from sandwich structures
to single layers of the mixture of Ag and ITO, and the optical
properties for single layer are presented.
Meanwhile, the temperature at which the transmittance
curves of ITO–AgTi–ITO films changed is higher than that of
ITO–Ag–ITO films also because of the better stability of Ag–
Ti alloy.
3.2.2 Heat treatment in a vacuum
Figure 5 pots the transmittance curves of ITO–Ag–ITO
films annealed in a vacuum. The curves shift towards the
direction of shorter wavelength. The maximum transparency
is approximately 82% after being annealed at 573 K, and the
Ag–Ti Alloy Used in ITO–Metal–ITO Transparency Conductive Thin Film with Good Durability against Moisture
2539
(c)
(a)
100 µm
50 µm
(b)
50 µm
transmittance declines during annealing at over 673. Kim et
al.13) proposed that silver films agglomerated when they were
heat-treated at over 673 K in a vacuum. It is suggested that
the agglomeration of silver layers, as mentioned above, also
occurs herein, scattering light, which thereby reduces the
transmittance.
Figure 6 plots the transmittance curves of ITO–AgTi–ITO
films annealed in a vacuum at various temperatures. The
maximum transparency of ITO–AgTi–ITO films is approximately 94% after being annealed at 573 K. This excellent
result has seldom been obtained for IMI systems with the
same structure. ITO–AgTi–ITO films keep high transmittance after being annealed at a relatively higher temperature,
probably because Ag–Ti layers keep smooth surface by
which rare light is scattered. Silver layers are stable and keep
smooth surface at a relatively high temperature, because the
addition of Ti into Ag reduces the diffusivity and thereby
retards the agglomeration.
The transmittance curves of ITO–Ag–ITO films shift
towards the direction of shorter wavelength after being
Fig. 7 Surface observation of IMI films after exposure for 144 h at 323 K
with a relative humidity of 90% by OM. (a)–(b) image of the ITO–Ag–ITO
film. (c) image of the ITO–AgTi–ITO film.
annealed seems because of the decrease of the refractive
indices of ITO due to the relaxation of compressive stress as
proposed by Jung.10) The stress relaxation of ITO associates
with the intermediate layer (Ag or Ag alloy). Jeong et al.21)
proposed that Ag alloy (Ag–Pd–Cu) films started to relax the
compressive stress at a relatively higher temperature than
pure Ag films. It is suggested that the addition of Ti into Ag
can retard the compressive-stress relaxation of silver films, as
well as ITO layers. Thus, the refractive indices of ITO does
not decrease and the transmittance curves of ITO–AgTi–ITO
films do not shift after being annealed.
3.3 Durability
Deposited films exposed for 144 h at 323 K with 90%
relative humidity were tested for durability. Figures 7(a)–(b)
reveals that numerous spots form on the surfaces of the ITO–
Ag–ITO films. It was reported that the generation of the
defects is related with the corrosion of Ag.10)
As presented in Fig. 7(c), observations of the surface
reveal that no corrosive spot appears on the ITO–AgTi–ITO
2540
S.-W. Chen, C.-H. Koo, H.-E. Huang and C.-H. Chen
Table 2
Resistivity of IMI films before and after durability testing.
before durability test
(resistivity, cm)
ITO–Ag–ITO film
ITO–AgTi–ITO film
after durability test
(resistivity, cm)
3:4 10
5
1 10
4
4:3 10
5
5 10
5
film after testing. A fact indicates that ITO–AgTi–ITO films
are more durable than ITO–Ag–ITO films, and it seems
because Ag–Ti alloy is more resistant than pure silver to
corrosion. Wei et al.22) proposed that the addition of Ti into
Ag can reduce the activity, increasing the chemical stability
and resistance to corrosion.
Table 2 presents the resistivity of IMI films after testing.
The resistivity of ITO–Ag–ITO films increases and that of
ITO–AgTi–ITO films is almost unchanged after testing. It is
probably because Ag–Ti alloy is more resistant than pure Ag
to corrosion as mentioned above, the corrosion of Ag films
increases the resistivity of both Ag films and ITO–Ag–ITO
films.
Figure 8 reveals the transmittance of IMI films after
testing. ITO–AgTi–ITO films keeps high transmittance at a
wavelength of 550 nm after testing, but the transmittance of
ITO–Ag–ITO films declines apparently at the same wavelength due to the existence of sopts on the surface, scattering
light. However, no spot appears on the surface of ITO–AgTi–
ITO films after testing.
4.
Conclusion
The transparency of the ITO–AgTi–ITO film at a wavelength of 550 nm raises to 94% after being annealed at 573 K
in a vacuum, and the transparency exceeds that of the ITO–
100
Transparency (%)
a
80
b
c
d
60
a: ITO-AgTi-ITO after testing
b: ITO-AgTi-ITO before testing
c: ITO- Ag -ITO after testing
d: ITO- Ag -ITO before testing
40
20
300
400
500 600 700 800
Wavelength, λ/nm
900
Fig. 8 Transmittance curves of ITO–Ag–ITO films and ITO–AgTi–ITO
films after durability testing.
Ag–ITO film with the best transparency of only 82%. This
excellent result of transmittance has seldom been obtained
for IMI systems with the same structure. The results for
resistivity and transparency following annealing in air
atmosphere or exposing to moist environment, and observations of the surface of films after durability testing, all
indicate that using Ag–Ti alloy as the intermediate layer
increases the stability and durability of IMI films. The IMI
film with Ag–Ti alloy as the intermediate layer is a good
candidate for transparent conductive electrodes.
Acknowledgement
The authors would like to thank Metallurgy Division,
Materials & Electro-Optics Research Division, Ghung-Shan
Institute of Science & Technology, and also show their
appreciation to professor Kuo and Dr. Sun, Department of
Materials Science and Engineering, National Taiwan University for support of equipments.
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