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We report nanoindentation response of indium tin oxide (ITO) film deposited by reactive direct current (DC) magnetron sputtering. The phase pure ITO film showed dense and needle-like nanostructure. Detailed mechanical characterization of ITO film was carried out by the nanoindentation technique. The average nanohardness and elastic modulus were evaluated as about 17.5 GPa and 189 GPa, respectively. Comparatively higher nanohardness and modulus values were found in the present work, which is possibly linked with the dense nanostructure of the ITO film. Further, fracture toughness was measured as ~0.56 MPa m0.5. The corresponding projected area of indentation, elastic energy and plastic energy were also reported.
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Author's Accepted Manuscript
Nanoindentation response of ITO film
Maurya Sandeep Pradeepkumar, K.P. Sibin, Niharika
Swain, N. Sridhara, Arjun Dey, Har ish C. Barshilia,
Anand Kumar Sharma
PII: S0272-8842(15)00309-0
DOI: http://dx.doi.org/10.1016/j.ceramint.2015.02.090
Reference: CERI10012
To appear in: Ceramics International
Received date: 27 November 2014
Revised date: 30 January 2015
Accepted date: 15 February 2015
Cite this article as: Maurya Sandeep Pradeepkuma r, K.P. Sib in, Niharika Swain, N.
Sridhara, Arjun Dey, Harish C. Barshilia, Anand Kumar Sharma, Nanoindentation
response of ITO film, Ceramics International, http://dx.doi.org/10.1016/j.cera-
mint.2015.02.090
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1
Nanoindentation response of ITO film
Maurya Sandeep Pradeepkumar
a,c
, K. P. Sibin
b
, Niharika Swain
b
, N. Sridhara
a
, Arjun Dey
a
,
Harish C. Barshilia
b
and Anand Kumar Sharma
a
a
Thermal System Group, ISRO Satellite Centre, Bangalore 560017, India
b
Nanomaterials Research Lab, Surface Engineering Division, CSIR-National Aerospace
Laboratories, HAL Airport Road, Kodihalli, Bangalore - 560 017, India
c
Department of Mettalurgical and Materils Engineering, Natioal Institute of Technology,
Warangal-506004, Andra Pradesh, India
Abstract
We report nanoindentation response of indium tin oxide (ITO) film deposited by reactive
direct current (DC) magnetron sputtering. The phase pure ITO film showed dense and needle-
like nanostructure. Detailed mechanical characterization of ITO film was carried out by the
nanoindentation technique. The average nanohardness and elastic modulus were evaluated as
about 17.5 GPa and 189 GPa, respectively. Comparatively higher nanohardness and modulus
values were found in the present work, which is possibly linked with the dense nanostructure
of the ITO film. Further, fracture toughness was measured as ~0.56 MPa.m
0.5
. The
corresponding projected area of indentation, elastic energy and plastic energy were also
reported.
Keywords: Indium tin oxide; nanoindentation; nanohardness; elastic modulus; fracture
toughness; nanostructure
*Corresponding author. Tel.: +91 80 2508 3214; fax: +91 80 2508 3203.
E-mail addresses: arjundey@isac.gov.in, arjun_dey@rediffmail.com (A. Dey).
2
Introduction
Transparent conducting oxides such as indium tin oxide (lTO), zinc oxide (ZnO) etc.
with or without doping constitute a unique class of materials which combine two physical
properties together - high optical transparency and high electrical conductivity [1, 2]. The
ITO films possess potential candidature in state-of–the-art application domain, e.g., solar cell
for energy harvesting application, optical solar reflector for spacecraft application,
transparent thin film transistors and electrode for optoelectronic and control panel displays,
antireflection coating for wind shield of aircraft, transparent electromagnetic shield, low
emissivity optical window, etc. [3-5]. However, due to its poor mechanical properties it is
restricted to many of the applications in particular for flexible optoelectronic device
applications such as display panels, light emitting diodes, solar cells etc. where choice of ITO
is limited because of tendency of cracking either during deposition process due to mismatch
of thermal expansion of ITO ceramic film and polymeric substrates or in-service condition
due to bending or stretching which ultimately can lead to deteriorate its mechanical integrity
and electrical properties as well [6-9].
Further, transparent and conducting ITO coating are exposed to the space
environment (i.e. as a top layer) deposited on both rigid (e.g. quartz glass or ceria doped
borosilicate glass) and flexible (Kapton, Teflon, fluorinated ethylene propylene or FEP etc.)
substrates used as reflectors and insulating blankets for spacecraft application. However,
often mechanical disintegration of ITO top layer is observed even minimal handling of these
components due to severe fragile or brittle nature of ITO [10, 11]. In addition, in low earth
orbit (LEO) spacecraft, ITO coated top layer on flexible polymeric surfaces have been eroded
primarily due to the strike of atomic oxygen (AO) along with the existence of UV radiation,
charged species and micrometeoroid particulates [12, 13].
3
Aforesaid discussion undoubtedly proves that the mechanical durability [14] of ITO
films has turn out to be a prime issue as the application fields pertinent to the ITO film has
expanded many folds in recent era. Therefore, hard, wear resistant and yet tough ITO
coatings will ensure the reliability in ‘in-servicecondition. It is also an important concern to
note that in spite of their emerging and multifaceted applications possibility, the reports on
mechanical behaviour on ITO coatings are scarce in literature [15-23] especially at the scale
of microstructure where actually material’s failure occurs.
To measure the nanomechanical properties such as hardness, modulus and fracture
toughness of ITO film by utilizing nanoindentation technique, the most of the researchers
[15, 18, 19, 22, 23] were not strictly followed the recommended limit of depth of penetration
in comparison to the film thickness. In general, the depth of penetration around 10% of the
film thickness are recommended in any nanoindentation experiments or else, the mechanical
properties of the substrate may perhaps influence the measured nanoindentation data of the
thin film or coating [24] .The influence of the substrate can be negligible from 20% to 30% of
the films thickness as well i.e. investigated both experimentally [25, 26] and finite element
simulation [27] when the deposited film was much softer or ductile than the substrate.
However, in the present case, it is well known that ITO shows lack of ductility as it is brittle
ceramic. Therefore, the reported data of hardness, modulus and fracture toughness of ITO
films are might be ambiguous and anomalous as the limit of depth of penetration was always
remain in the range of ~90% to 100% [15, 18, 19, 22, 23] and sometimes exceeded 100%
[17] as well. There was only a single literature found who had taken care of the limit of
penetration depth and the maximum penetration depth had been kept up to 280 nm whereas
the thickness of ITO was ~1500 nm [20].
It is very clear from the abovementioned argument that the reported mechanical
properties data ITO film are cynical in several cases [15, 17-19, 22, 23] plausibly due to the
4
indentation depth has been chosen irrationally which ultimately put a question about the
validation of measurement itself. Therefore, in the present work, the indentation depth has
been maintained cautiously thereby substrate effect can be avoided and thus a valid
nanoindentation measurement can be performed.
In the present communication, ITO thin films were grown on silicon substrate by
direct current (DC) reactive magnetron sputtering technique. The phase analysis was studied
by X-ray diffraction (XRD) technique. The surface morphology and microstructure were
characterised by atomic force microscopy (AFM) and field emission scanning electron
microscopy (FESEM). The composition of the deposited thin films was investigated by X-ray
photoelectron spectroscopy (XPS). Further, in-depth nanoindentation investigation had been
carried out at three different loads, viz. 1, 2 and 5 mN to evaluate nanohardness, Young’s
modulus and fracture toughness (K
1c
)
of ITO coating.
Material and methods
The ITO film was deposited on silicon (111) substrate by reactive sputtering using a
DC unbalanced magnetron sputtering system. A 3” diameter and 6 mm thick sputtering target
of composition (In:Sn-90:10 wt.%, 99.99% purity) supplied by ACI Alloys, USA was
utilized. The substrate was fixed directly below the target with a target-to-substrate distance
of 5.2 cm and substrate was heated to140˚C. The vacuum chamber was evacuated down to a
base pressure 6.4 x 10
-6
mbar prior to the deposition using a turbo molecular pump backed by
a rotary pump. The depositions were performed at pulse DC of 60 W and at pressure 5.8 x 10
-
2
mbar. The sputtering gas was a mixture of argon and oxygen, which were introduced into
the sputtering chamber by mass flow controller in ratio 14:3.45 (sccm). Prior to the
deposition, target was cleaned in argon plasma for 3 min.
The surface morphology and microstructures of the deposited films were investigated
using AFM (MTS, USA) and FESEM (Supra VP 40, Carl Zeiss, Germany) techniques. The
5
thicknesses of the deposited ITO film were measured by using a surface profilometer
(Nanomap 500 LS 3D, USA). The phase analysis of the deposited ITO film was carried out
by X-ray diffraction (D8 Advance X-ray diffractometer, Bruker) technique using a Cu KĮ
radiation at Ȝ = 0.154060 nm. Further, XPS (SPECS spectrometer, Germany, non-
monochromatic Al Kα radiation, 1486.6 eV) and X-ray source run at 150 W (12 kV, 12.5
mA) were carried out to examine electronic structure and oxidation state of the deposited
films.
The nanomechanical properties e.g., nanohardness and Young’s modulus of the ITO
film were evaluated by the nanoindentation (CSEM Nano-hardness tester) technique utilizing
a Berkovich indenter at three different loads e.g. 1, 2 and 5 mN. The nanohardness and
Young’s modulus were evaluated from the load versus depth of penetration (P-h) plots using
the well-established Oliver and Pharr model [28]. Further, K
1c
of the ITO film was measured
by the nanoindentation technique with the same Berkovich indenter as mentioned above
using the following relation [29]:
(1)
where P is the applied load and C is the crack length measured from the centre of the
nanoindent while E and H are the Young’s modulus and hardness of the ITO coating,
respectively and Į is the geometric constant taken as 0.016 [29].
Results and discussion
The AFM morphology of ITO film is shown in Fig. 1a. A dense, uniform, smooth and
nanostructured grainy morphology was observed. The root mean square and average surface
roughness was 7.16 nm and 5.82 nm, respectively. Figure 1b shows a typical line profile
across a step of the ITO film deposited on silicon substrate. The thickness of the ITO film
was measured as ~1.25 µm.
6
The FESEM micrographs of ITO film prior and after the nanoindentation test are
shown in Fig. 2 (a-b), respectively. A dense needle-like nanostructure is depicted as was also
reported by Jung et al. [30]. Several needle-like nanostructures formed a colony with a
specific growth orientation. Further, several colonies were observed with random growth
orientations. Figure 2b depicted the residual impression of a typical nanoindent made at 5 mN
load. The residual indentation impression area showed no sign of severe damage and
chipped-up. Only, the tiny cracks (marked with dotted ellipses in Fig. 2b) were formed from
three corner of the indent.
The well crystalline XRD pattern of the deposited film is shown in Fig. 3. The pattern
was thoroughly indexed and matched with ICDD (collection ID: 50848), which confirmed
only the presence of pure ITO phase. The average crystallite size was calculated as ~69 nm
using Debye Scherrer formula.
The composition and oxidation state of deposited film investigated by XPS shown in
Fig. 4 (a-c). The characteristic peaks of In 3d
3/2
/ 3d
5/2
and Sn 3d
3/2
/ 3d
5/2
and corresponding
binding energies are shown in Fig. 4a, b, respectively. The binding energies of the In 3d
5/2
and In 3d
3/2
were found as 444.3 and 452.05 eV, respectively while the binding energies of Sn
d
3/2
and Sn 3d
5/2
were observed as 486.45 and 494.85 eV, respectively. These data are in well
agreement with the reported literature. The oxygen 1s peaks were curve-fitted and the data is
shown in Fig. 4c. The two peaks could be resolved at binding energies ~530.3 and ~532.4 eV,
which correspond to lattice oxygen in ITO phase and oxygen atoms in hydroxide or oxy-
hydroxide phase, respectively [31].
Typical load vs. depth (P-h) plots of ITO film at 1, 2 and 5 mN load are shown in Fig.
5a. The P-h plots showed elastic-plastic behaviour as expected for brittle ITO film. In the
maximum load, e.g., 5 mN, the maximum depth was reached up to 150 nm which is 12% of
the film thickness (i.e., 1.25 µm) and thus the possibility of substrate effect was avoided. The
7
ratio of final depth and maximum depth of penetration (h
f
/h
max
), projected area, elastic energy
(W
e
) and plastic energy (W
p
)
during indentation at various loads are summarized in Table 1.
The average values of h
f
/h
max
ratio for the ITO coating was in the range of 0.58 to 0.62 which
is well below 0.7. The present data justified the applicability of Oliver-Pharr model to
analyze the nanoindentation data of the present ITO coating [27]. The projected area of
indentation was increased as the indentation load increased, as expected. The value of W
e
was
almost equal with W
p
up to 2 mN load. However, at 5 mN load, the value of W
p
was much
higher than W
e
. The variation of nanohardness and Young’s modulus as a function of load is
shown in Fig. 5b. The nanohardness was marginally decreased from 17.82 GPa to 17.6 GPa
as load was increased from 1 mN to 5 mN while the average Young’s modulus was measured
as 189 GPa. The present nanohardness and Young’s modulus data reported here are the
average data of the same taken at ten unbiased chosen different locations of the
microstructure of ITO film.
The mechanical properties such as hardness, elastic modulus and fracture toughness
of ITO film, reported occasionally in the literature are summarized in Table 2 [15-23]. For
instance, ITO film of thickness ~1.5 µm grown on Polyethylene terephthalate (PET) substrate
showed very low values of both hardness (0.25-1.5 GPa) and elastic modulus (116 GPa)
measured by nanoindentation technique [19, 20]. Similarly, ITO film of thickness 50-300 nm
on polycarbonate (PC) showed very low hardness of 16.5 Hk to 110 Hk [16, 21]. Further,
moderate hardness of ITO film was reported e.g. 8.1-4.9 GPa for 250 nm ITO film on glass
[15] and 12 GPa for 100-400 nm ITO film on soda lime glass [18, 22, 23] while the
corresponding Young’s modulus were reported as 113.4-86.2 GPa [15] and 133 [18] and 141
GPa [22, 23] measured by nanoindentation technique with a Berkovich [6, 13, 14] and cube
corner [18] tip. The present data of nanohardness and Young’s modulus of ITO film were
8
much superior in comparison to the reported data [15, 16, 18-23], plausibly linked with dense
nanostructure of the ITO.
It is reported that the microhardness of ZnO film is significantly increased after
annealing due to evolution of needle-like nanostructure [32]. Highly transparent sputtered
piezoelectric ceramic i.e. nanocrystalline (K
0.34
Na
0.65
)NbO
3.01
[33] as well as sputtered
nanocrystalline TiC [34] films show significant increase in both hardness and elastic modulus
also due to their grain refinement.
Diminishing grain size shows the increase in mechanical properties according to Hall-
Petch relation and that’s why nanostructured or fine grain materials always show superior
mechanical properties than that of coarse grain structure.
Beside the films and coatings, bulk materials also often follow Hall-Petch criteria and
as a result they show the increase in hardness with decrease in grain size; reported for various
bulk ceramics such as yttria (Y
2
O
3
) and ceria (CeO
2
) stabilized tetragonal zirconia (ZrO
2
)
[35,36], ZrO
2
-Y
2
O
3
[37], WC [38], TiO
2
[39], alumina [40], Pb(Zr,Ti)O
3
[41] and
magnesium aluminate spinel [42]. The grain size of these different ceramics [36-43] varies
from few microns to sub-micron/nanometric range and the hardness value always shows
significantly higher side in fine grain microstructure than that of coarse grain microstructure
of the same ceramic.
Here, sputtered ITO film showed needle-like dense fine grain structure and thereby
achieved higher hardness and modulus. Further, it is very important to know the mechanical
properties of coarse grain ITO film which might be achieved by controlled heat treatment.
This important aspect should be addressed in future as this is beyond the scope of the present
work.
9
The K
1c
was measured as 0.56 MPa.m
0.5
at 5 mN load. The average crack length was
found as 0.629 µm. No radial crack had been observed for the indentation made at 1 and 2
mN loads.
K
1c
of ITO film reported utilizing energy based model as in the range of 2.39- 2.79
MPa.m
0.05
on PET substrate for a thickness of 200 nm [17], 2.2 MPa.m
0.05
on soda lime glass
for a thickness of 400 nm [18] and 2.1-2.2 MPa.m
0.05
for a thickness of 240-400 nm [22, 23].
The indentation fracture toughness by direct crack measurement method was adopted in only
Ref. 22 and 23. They reported [22, 23] the increase in K
1c
of ITO film from 0.7 MPa.m
0.05
to
0.9 MPa.m
0.05
as the thickness of ITO layer decreased from 400 nm to 240 nm.
The present K
1c
value of ITO film was in lower side (0.56 MPa.m
0.5
vs 0.7-0.9
MPa.m
0.5
) in comparison to the reported literature [22, 23]. However, as discussed earlier, the
authors have not maintained the desired depth of penetration, which will be around 10% of
the total film thickness [22, 23]. Therefore, the comparatively higher K
1c
found in literature
due to the energy dissipation distributed possibly in multilayer stacking beneath the ITO layer
and the substrate. In contrast, in the present case, the depth of penetration was maintained
almost 10% of the total film thickness.
Conclusion
Phase pure, crystalline and dense, needle-like nanostructured ITO coating of thickness
1.25 µm was deposited by reactive DC magnetron sputtering on silicon (111) substrate. The
higher average nanohardness and elastic modulus were found about 17.5 GPa and 189 GPa,
respectively. The higher nanohardness and modulus were linked with the dense needle-like
nanostructure of the ITO coating. Indentation fracture toughness was measured as ~0.56
MPa.m
0.5
at 5 mN load. The cracks were not observed in both 1 and 2 mN loads.
10
Acknowledgements
The work carried out herein is supported by ISRO under RESPOND program (Project
# U-1-127). The authors thank Director, CSIR-NAL for permission to publish these results.
Praveen Kumar, Siju and G. Srinivas (all from CSIR-NAL) are thanked for various
measurements.
11
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16
Figure and Table Captions
Fig. 1: (a) Typical AFM surface morphology and (b) corresponding line profile across a step
showing the thickness of the ITO film.
Fig. 2: The FESEM photomicrographs of ITO film: (a) prior and (b) after the nanoindentation
test.
Fig. 3: Typical XRD pattern of ITO film.
Fig. 4: The binding energies of (a) In 3d
5/2
, In 3d
3/2
, (b) Sn d
3/2
, Sn 3d
5/2
and (c) curve-fitted
oxygen 1s peak.
Fig. 5: (a) Typical P-h plots of ITO film at 1, 2 and 5 mN loads and (b) the variation of
nanohardness hardness and Young’s modulus as a function of load.
Table 1: The values of h
f
/h
max
, projected area, W
e
and W
p
at various loads.
Table 2: Literature status of hardness, Young’s modulus and fracture toughness of ITO films
Table 1
Load
(mN)
h
f
/h
max
Projected area
(10
4
x nm
2
)
W
e
(nJ)
W
p
(nJ)
1 0.58
5.66 11.72 11.05
2 0.59
11.76
32.22 35.75
5 0.62
29.24 124.57
149.47
17
Coating/s
ubstrate
Proce
ssing
details
Thickne
ss (nm)
IFT
(MP
a.m
0.
5
)
EMFT
(MPa.
m
0.5
)
Har
dnes
s
(GP
a)
You
ng’s
mod
ulus
(GP
a)
Indenter
type/Ind
entation
load or
depth
h
f
/h
max
Refer
ences
ITO/glass RF
sputter
ing
250 - - 8.1-
4.9
113.
4 -
86.2
Berkovic
h/ 6 mN
(Depth:
0-200
nm)
150
nm /
225
nm
[15]
ITO/PC RF
sputter
ing
100/- - - 16.5
Hk
- Knoop/ - -
[16]
ITO/PET RF
sputter
ing
(50
W)
200/188
000
- 2.59
(t
s
-
25°C)
- - Berkovic
h/500 µN
24-30
nm /
196-
223
nm
[17]
2.51
(t
s
-
35°C)
2.44
(t
s
-
55°C)
2.39
(t
s
-
75°C)
ITO(h)
/P
ET (PET
temp.:
75°C)
RF
sputter
ing
(50
W)
000
-
2.70(t
s
-
25°C)
-
Berkovic
h/600 µN
20
-
26
nm /
166-
178
nm
[17]
2.69(t
s
-
35°C)
2.51(t
s
-
55°C)
2.52(t
s
-
75°C)
ITO/Cu/P
ET
RF
sputter
ing
(50
W)
200/10/1
88000
- 2.79(t
s
-
25°C)
- - Berkovic
h/700 µN
38-46
nm
/192-
222
nm
[17]
2.7(t
s
-
35°C)
2.63(t
s
-
55°C)
2.62(t
s
-
75°C)
ITO/soda
lime glass
sputter
ing
400 - 2.2 12 133 Cube
corner/
h
max
=4
00 nm
[18]
Depth:
40-400
nm
ITO/PET DC
magne
1459-
1508/18
- - - 116 -/10 mN 600
nm /
[19]
18
tron
sputter
ing
8000
1500
nm
ITO/PET
DC
magne
tron
sputter
ing
(100W
)
1459
-
1508/18
8000
-
-
0.25
– 1.5
-
-
/0.4
-
1.2
mN
h
max
=2
80 nm
[20]
ITO/PC RF
magne
tron
sputter
ing
50-300
nm
- - 20-
110
Hk
- Knoop/
1gf.
-
[21]
ITO/soda
lime glass
-
ITO top
layer
multilay
er stack
0.9
2.2
[22,23]
ITO/soda
lime glass
-
ITO top
layer
multilay
er stack
0.7
2.1
12
(100
-250
nm)
141
(100
-250
nm)
Berkovic
h and
cube
corner /
Depth:
40-400
nm
-
IFT: Indentation fracture toughness, EMFT: Fracture toughness evaluated by energy method, h
f
=Final depth of penetration, h
max
=Maximum
depth of penetration, PET= Polyethylene terephthalate, PC=Polycarbonate, t
s
=Temperature of nanoindenter sample stage
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
... 33 The XPS spectrum of In 3d displayed peaks at 445.3 eV and 453.0 eV which can be ascribed to In 3d 5/2 and In 3d 3/2 respectively present on ITO-PET substrate Fig. 9(e). 35 The XPS spectrum of Sn 3d indicated Sn 3d 5/2 and Sn 3d 3/2 peaks at binding energy values 487.4 eV and 496.3 eV respectively Fig. 9(f). 35 In Fig. 9(g), the XPS core level spectrum of O 1s displays two peaks at 530.1 eV and 530.3 eV corresponding to oxygen atoms in hydroxide or oxyhydroxide phase and lattice oxygen present in ITO-PET respectively. ...
... 35 The XPS spectrum of Sn 3d indicated Sn 3d 5/2 and Sn 3d 3/2 peaks at binding energy values 487.4 eV and 496.3 eV respectively Fig. 9(f). 35 In Fig. 9(g), the XPS core level spectrum of O 1s displays two peaks at 530.1 eV and 530.3 eV corresponding to oxygen atoms in hydroxide or oxyhydroxide phase and lattice oxygen present in ITO-PET respectively. 36 O bonded to In and/or Sn and In-O-H was seen at binding energy value 532.0 eV. ...
... The core level C 1s XPS spectrum displayed a peak at 284.6 eV belonging to C-C or C]C present in PPy and 4-(hydroxymethyl) pyridine in Fig. 10(a). [33][34][35][36][37][38] The peaks observed at 285.0 eV and 285.2 eV can be assigned to C-C of ITO-PET substrate. 34 C-N and C]N moieties present in 4-(hydroxymethyl) pyridine adsorbed on Pt-Pd/PPy/ITO used before oxidation were corroborated from peaks at 285.4 eV and 288.3 eV respectively. ...
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