22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Study of the growth mechanism of TiO 2 thin films by reactive cathodic arc
F. El-Ghoulbzouri and M.-P. Delplancke
4MAT, School of engineering, Université Libre de Bruxelles,50 Avenue Franklin Roosevelt, BE-1050 Brussels, Belgium
Abstract: The phase of the TiO 2 films deposited by pulsed filtered cathodic arc technique
could be tuned by controlling the normalized energy flux. A mixture of anatase and rutile
phase has been obtained at a normalized energy flux of 99 eV/atom and a pure rutile phase
was grown above 376 eV/atom. We could also correlate the refractive index value with
phase of TiO 2 coatings.
Keywords: ion energy flux, phase formation, pulsed filtered cathodic arc technique
1. Introduction
Titanium oxide, TiO 2 , thin films were deposited
by pulsed filtered cathodic arc (PFCA) under
different deposition conditions. To study the effect
of the current density and ion energy on the phase
formation, TiO 2 thin films were deposited at
different values of the applied arc current and
substrate bias voltage, respectively, while keeping
the other deposition parameters constant. The
optical properties and chemical composition of the
elaborated coatings were also characterized. The
deposition conditions are summarized in Table 1.
Table 1. Deposition conditions for Sample 1-4 at
different arc current and for Sample 2, 5, 6 and 7 at
different substrate bias voltage, respectively,
keeping the other parameters constant.
Deposition
Sample
Sample
parameters
1, 2, 3, 4
2, 5, 6, 7
Substrate
300
300
temperature (°C)
Base pressure (mbar)
2.10-5
2.10-5
-3
O 2 partial pressure
10
10-3
(mbar)
Arc current (A)
200
100, 200,
300, 400
Substrate bias
0
0, -100,
voltage (V)
-200, -400
Pulse frequency (Hz)
2,5
2,5
Deposition time (h)
4
4
Pulse duration (ms)
1,5
1,5
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Fig. 1. Experimental setup used in this work.
A schematic diagram of the experimental set-up
used in this work is presented in Fig. 1. The cathode
is a 99,5 % pure titanium rod and all films were
grown on Si (100) substrates. The plasma chemistry
was characterized by residual gas analyzer (RGA).
The structure of the deposited coatings was
determined by X-ray diffraction (XRD) at grazing –
incidence (ω-2θ, ω = 0,5°) mode.
The composition and optical properties of the
films were characterized by X-ray photoelectron
spectroscopy
(XPS)
and
variable
angle
spectroscopic ellipsometry (VASE), respectively.
2. Plasma characterization
The plasma composition was analysed by RGA
as a function of the arc current. The mass spectra,
presented in Figure 2(a) were acquired by
deactivating the ionizer part of the RGA and at an
oxygen partial pressure of 6.10-4 mbar. The pressure
of 6.10-4 mbar, which is less than the pressure in
Table.1 was fixed by the requirements of the RGA
instrument. The presence of Ti2+ ions is detected at
all applied arc currents under study. The intensity
of Ti2+ line increases and additional peaks
corresponding to O+/Ti3+, Ti+ and O 2 + appear for arc
current of 266 A and 400 A.
1
a)
Fig. 3. The current density as a function of the
substrate bias potential at an arc current of 200 A
and an oxygen partial pressure of 10-3 mbar.
b)
3. Thin film characterization
The structure evolution of the TiO 2 films as a
function of the arc current and substrate bias is
illustrated in Fig. 4.
a)
Fig. 2. (a) Evolution of the mass spectra and (b)
current density as a function of the arc current at an
oxygen partial pressure of 6.10-4 mbar and 10-3
mbar, respectively.
Presence of more positive ions can be attributed
to the increased electron emission efficiency from
the cathode, when the applied arc current increases.
The higher electron flux facilitates the electron
impact excitation and ionization of the background
gas (O 2 and evaporated Ti atoms/ions) [1]. Fe is
evaporated from the stainless steel anode which is
heated by the arc and may. The mass spectrometry
measurements are confirmed by the current density
measured at the substrate (Fig. 2(b)). As it can be
seen, the current density increases when a stronger
arc current is applied. We have measured the
evolution of current density at the substrate position
as a function of the applied bias potential (relative
to the ground) at 200 A and an oxygen pressure of
10-3 mbar. The results are illustrated in Fig. 3. The
current density increases initially with bias voltage
and it stabilizes for a bias values greater than 40 V
in absolute value; where we are in the ion saturation
regime. The current density is principally due to the
incident ions on the collection plate and to the
secondary emission effect of electrons. We can
assume that a bias voltage greater than 50 V –in
absolute value- is enough to repel all electrons
originating from the incident plasma flux.
2
b)
Fig. 4. GIXRD patterns of TiO 2 thin films
deposited at different (a) arc current intensity and at
(b) several substrate bias voltages
A pure anatase structure with (101) preferential
orientation was obtained at an arc current of 100 A.
As the arc current increases, we observe a mixture
of anatase and rutile phases at 200 A and 300 A.
Finally, a pure rutile phase was deposited at 400 A.
The application of a substrate bias at fixed arc
current value induced a transition from a mixture of
phases to a pure rutile structure at -100 V.
Increasing the bias up to -400 V, did not influenced
greatly the obtained structure. Nevertheless, the
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Phase
Constitution
sample
s
elabora
ted at 200 V
and 400 V
exhibit
Bais
Voltage
(V)
Ion
current
density
(mA/cm²)
Ion
energy
[8]
(eV)
Energy
flux ( 𝚽)
(mJ/s.cm²)
φ norm
Anatase +
Rutile
Rutile
Rutile
0
1,8
100
110
Ion
deposition
flux (during
the pulse)
(atom/s.nm²)
57
-100
-200
4,7
4,7
300
500
705
1175
117
125
376
587
Rutile
-400
4,7
900
2115
108
1222
(eV/atom)
99
formati
on of
the
rutile
nuclei
through
the
migrati
Table. 2. The structure evolution of the TiO 2 coatings as a function of ion energy flux.
a (101) preferential texture.
The determination of the relevant parameter
responsible of the observed phase transition was a
subject of several papers [2-4]. Löbl and coworkers
[2] suggested that the phase transformation is
tailored by the energy of particles impinging on the
growing films. Whereas, Mráz and Schneider [5]
proposed that the ratio between the ion energy flux
and the deposition flux is the main parameter
responsible of the phase constitution of the
deposited films.
In the present study, the phase transitions from
anatase to a mixture of rutile and anatase and finally
to
rutile are observed when the arc current increases,
i.e. the ion flux at the substrate increases.
Therefore, the transition is found when the
deposition rate increases, similar to Ref [3]. It is
important to note that the measurements realized by
E. Byon et al. [4] revealed
that the ion energy distribution function does not
depend on the arc current value for plasma
generated by PFCA technique in vacuum. They
found that IEDF had the same shape and were
centred at the same energy regardless of the arc
current value. Therefore we can assume that the
deposited coatings at different
arc current intensity where all grown from ionic
species which have almost the same energy
distribution function. This first deposition set
suggests that ion current density plays an important
role in the determination of the crystalline phase of
the synthetized films. However, the measurements
have shown that the current density increases with
arc current. Thus, for a fixed pressure and a higher
arc current intensity the number of interaction of
ions with the oxygen background pressure becomes
significant and may affect their IEDF and their
charge state. The enhancement of the substrate
temperature with arc current could also be a
possible explanation for the observed phase
transformation. Therefore, for this first deposition
set, it is difficult to assess which parameter is
responsible for the phase transition.
However, when a bias on the substrate is applied,
a transition from a mixture of phases to a pure rutile
phase was observed, which suggests a possible
effect of the ion impact energy on the phase
transition. The bias voltage accelerates additionally
the impinging ions, which contribute to the
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on effect on the substrate surface [2].
It has been shown that the ion energy, E ion and
ion current density, j, are the main parameters,
which define the phase transition. The energy flux,
𝚽 can be expressed by those two quantities:
j
.E
Φ=
q. e ion
From the ion energy flux we can also define the
normalized energy flux as the ratio between the ion
energy flux and the deposition flux [5]. The
normalized energy flux also takes into account also
of the deposition rate.
It can be assumed that the average ion charge, q
equals to 2 because; the mass spectra (Fig. 3)
showed that Ti2+ ions are the dominant species in
the generated plasma. Table 2 presents the
calculated normalized energy flux for the
corresponding thin film phase as a function of the
substrate bias. The phase transition from a mixture
of phases to a pure rutile phase occur at a
normalized energy flux comprises between 99
eV/atom and 376 eV/atom.
The chemical composition of the samples was
measured by XPS. It is found that the stoichiometry
of the deposited thin films is not affected by the arc
current and substrate bias. All the deposited
coatings exhibit an elemental ratio of O /Ti close to
2. The spin-orbit splitting values, (Δ SOS = 5,7 eV)
were in agreement with values for Ti4+ [6].
Figure 5 illustrates the refractive indices as a
function of wavelength of TiO 2 coatings deposited
at different arc currents and substrate biases. The
refractive index increases with the arc current and
bias amplitude as the thin film phase changes from
anatase to pure rutile. The transition from anatase to
rutile phase leads to denser thin films, which
exhibits higher refractive index values. Indeed, both
phases have a tetragonal system, but the density of
rutile (4,24 g/cm³) is higher than that of the anatase
phase (3,83 g/cm³) [7]. As an example, the
refractive index at 550 nm wavelength, increases
3
from 2,61 at 100 A in anatase to around 2,78 at 400
A in rutile. It is worth noting that the refractive
index in the bulk phases of rutile and anatase are
equal to 2,95 and 2,57 respectively.
a)
[2] P. Löbl, M. Huppertz, D. Merge, Thin Solid
Films, 251, 72-79 (1994).
[3] A. Bendavid, P.J. Martin, E.W. Preston, Thin
Solid Films, 517, 494-499 (2008).
[4] E. Byon, A. Anders, J. Appl. Phys. 93, 1899
(2003)
[5] Stanislav Mráz and Jochen M.Schneider, J.
Appl. Phys, 109, 023512 (2011).
[6] Mark C. Biesinger, Leo W.M. Lau, Andrea R.
Gerson, Roger St.C. Smart, Applied surface
science, 257, 887-898 (2010).
[7] Ulrike Diebold, Surface Science Reports, 48,
53-229, (2003).
[8] C. Paternoster, I. Zhirkov and M.P. DelplanckeOgletree, Surafce & Coatings Technology, 227, 4247 (2013).
b)
Fig. 5.
Refractive indices as a function of
wavelength of TiO 2 films at (a) different arc current
intensities and (b) different substrate biases.
To conclude, TiO 2 thin films have been deposited
at different arc current and bias voltage values by
PFCA deposition technique. We have shown that
the phase of the TiO 2 films deposited by PFCA
technique is controlled by the normalized energy
flux. We have demonstrated that the transition from
a mixture of anatase and rutile phases to a pure
rutile phase can occur at a normalized energy flux
which is between 99 eV/atom and 376 eV/atom.
The chemical composition analysis shows that a
stoichiometric TiO 2 films could be grown under the
presented deposition conditions. Finally, we
observed that the refractive index increases as the
thin films structures evolve from anatase to a pure
rutile phase.
4. Acknowledgements
This work/research was carried out in the
framework of the network on Physical Chemistry of
Plasma-Surface Interactions - Interuniversity
Attraction Poles, phase VII (PSI-IAP7), and
supported by the Belgian Science Policy Office
(BELSPO).
5. References
[1] A. Anders, Springer, “Cathodic arcs : from
fractal spots to energetic condensation”, 50, (2009).
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