22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
FTIR study of zirconium tetra tert-butoxide dissociation process in correlation
with ZrO 2 thin film growth deposited in low pressure plasma
I. Martinko1, R. Verhoef1, Y. Engelmann1, J. Cornil2, S. Salah3, R. Snyders4,5, O. Antonin6 and P. Raynaud1,7
1
Université de Toulouse; UPS, CNRS; LAPLACE (Laboratory on Plasma and Conversion of Energy), 118 route de
Narbonne, FR-31062 Toulouse cedex 9, France
2
Laboratory for Chemistry of Novel Materials, University of Mons, Place du Parc 20, BE-7000 Mons, Belgium
3
Universités de Constantine, Laboratoire Microsystèmes et Instrumentation (LMI), Université Constantine 1,
Faculté des Sciences de la Technologie, Route de Ain El Bey, DZ-25017 Constantine, Algeria
4
Chimie des Interactions Plasma-Surface, Center of Innovation and Research in Materials & Polymers (CIRMAP),
Université de Mons - UMONS, Place du Parc 23, BE-7000 Mons, Belgium
5
Materia Nova Research Center, Parc Initialis, Avenue N. Copernic 1, BE-7000 Mons, Belgium
6
Institute for Applied Laser, Photonics and Surface Technologies, Berne University of Applied Science, 21 Quellgasse,
CH-2501 Bienne, Suisse
7
CNRS, Laboratoire LAPLACE – Matériaux et Procédés Plasmas, Université Paul Sabatier, 118, route de Narbonne,
FR-31062 Toulouse, France
Abstract: Deposition of metal-organic thin films was obtained in MPP-DECR plasma
using Zirconium Tetra tert-Butoxide (ZTB) as a precursor. To clarify the dissociation
process of the precursor Fourier Transform Infrared Spectroscopy (FTIR) and Density
Functional Theory (DFT) were employed for the examination of gas phase, while Scanning
Electron Microscope (SEM) was used for thin film investigation.
Keywords: PECVD, ZTB, metal-organic thin films, dissociation process, FTIR, DFT
1. Introduction
Zirconium oxide thin films are of great scientific and
technological interest. Their significance in both science
and technology is due to their desirable physical and
chemical properties. These properties include high
thermal stability, low thermal conductivity, high melting
point, chemical and corrosion resistance, large oxidation
resistance and high hardness. They are used as thermal
barrier coatings and oxygen gas sensors. High dielectric
constant and wide band gap makes them eligible for use
in microelectronic production. They are widely employed
as optical coatings because of their high transparency and
high refractive index. Biocompatibility makes zirconium
oxides of interest for biomedical and prosthetic use.
Zirconium oxide coatings have been synthesized using
various techniques such as Chemical Vapour Deposition
(CVD) and sol-gel processes. In this work coatings were
obtained by Plasma Enhanced CVD (PECVD). Zirconium
oxide thin films were deposited using Zirconium Tetra
tert-Butoxide (ZTB, C 16 H 36 O 4 Zr) as a precursor.
Plasma phase reactions resulting in the appearance of
different reactive species depend on the process
parameters and affect composition and characteristics of
deposited thin films. From the demand to achieve
desired thin film characteristics arises the need for
complete gas phase characterization, understanding the
dissociation process and finally connecting the latter with
thin film growth.
Fig. 1. Schematic diagram of the experimental setup.
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1
3. Results and analysis
Thin films deposited from ZTB/O 2 plasma at different
amounts of oxygen present in the gas mixture show
significant structural differences. Figure 2 presents cross
sectional SEM images of thin films deposited from 50%
ZTB/50% O 2 plasma (Fig. 2a) and 10% ZTB/90% O 2
plasma (Fig. 2b). Thin films obtained from plasma at
amounts of oxygen smaller than 80% are homogenous,
while those obtained at 80% of O 2 and higher show
columnar structure. The diameter of the columns is of the
order of tens of nanometers. Columns are perpendicular to
the surface of the substrate. Columnar growth of
zirconium oxide thin films deposited by PECVD has
already been reported, but its appearance has not been
2
Fig. 2. SEM images of thin films deposited from ZTB/O 2
plasma. (a)50% O 2 in ZTB/O 2 mixture, (b) 90% O 2 in
ZTB/O 2 mixture.
explained nor correlated with plasma phase composition
[5,6].
Figure 3 presents FTIR spectra of ZTB/O 2 plasma
phase obtained at different amounts of oxygen in the gas
mixture. Most of the peaks are present with more or less
equal intensities in all the spectra shown (spectra of
70%ZTB/30%O 2 , 50%ZTB/50%O 2 , 20%ZTB/80%O 2
and 10%ZTB/90%O 2 plasma). Yet some peaks show
substantial differences in size between ZTB/O 2 with
<80% of O 2 spectra and ZTB/O 2 with ≥80% of O 2
spectra. Peaks that significantly differ in intensities are
labelled in figure 4. Those peaks and their assignations
are listed in table 1. In 20%ZTB/80%O 2 plasma, the 667
cm-1 peak (L9) characteristic for CO 2 considerably
increases. Also, a decrease in amount of hydrocarbons is
detected. It is best visible for 729 cm-1 peak (L6) which
corresponds to C 2 H 4 .
0,05
30% O2
80% O2
50% O2
90% O2
0,04
Intensity (a.u)
2. Experimental procedure
Deposition of thin films was achieved using microwave
multipolar plasma (MMP) excited by distributed electron
cyclotron resonance (DECR). MPP-DECR source allows
generating very low-pressure plasma, of the order of
millitorr, and is capable of producing plasma of high
electron density (ne ≈ 1011 cm-3) [1]. The metal-organic
coatings were obtained in a main chamber of the reactor
which consists of a stainless steel cylinder of 25 cm in
inner diameter and 30 cm in height. The chamber is
connected by an airlock to a transfer chamber. The
pumping is assured by a group consisting of a rotary vane
pump and a turbomolecular pump as well as a cold trap.
The gaseous precursors are injected and checked by series
of mass flow meters (MKS). There is also a setup
allowing injection of low vapour pressure precursors, as
ZTB. The reactor is equipped with 8 internal antennas
(injecting the microwave energy at 2.45 GHz) with
regard to 8 external samarium-cobalt magnets distributed
around the metallic cylinder composing the enclosure. A
Sairem microwave generator, coupled with an impedance
matching, allows injecting up to 800 watts. Metal-organic
thin films were deposited on Si (100) substrates. During
the deposition the total pressure was maintained at 1
mTorr.
Vibrational spectra were obtained using FTIR
absorption spectroscopy of the plasma phase achieved in a
multipass cell previously described [2]. The pressure was
not fixed, yet it varied from 2 to 4 mTorr. Density
functional theory (DFT) was used in order to support the
analysis of experimental data and therefore help getting a
clear image of the precursor dissociation process [3]. DFT
calculations were performed with the purpose of
obtaining IR spectra of precursor and fragment molecules
and relevant bond energies. The calculations include
optimization of the geometries of molecules and
computation of Gibbs free enthalpies (at 298K) of the
associated reactions. Regarding the computational details,
B3LYP hybrid exchange-correlation functional with the
6-311++g (3df, 3pd) basis set was used. For open shell
systems the unrestricted method (UB3LYP) was
employed. All calculations were performed using the
Gaussian09 package [4].
0,03
0,02
0,01
0,00
3500
3000
2500
2000
1500
1000
500
-1
Wavelength (cm )
Fig. 3. FTIR spectra of ZTB/O 2 plasma obtained at
different percentages of O 2 in the mixture
(P = 125W; p = 2 – 4 mTorr).
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30% O2
80% O2
0,03
L9
L7
L8
Intensity (a.u)
0,02
L6
L4
L2
0,01
L3
L5
L1
0,00
3500
3000
2500
2000
1500
1000
500
Wavelength (cm-1)
Fig. 4. FTIR spectra of ZTB/O 2 plasma obtained at 30%
and 80% of O 2 in the mixture
(P = 125W; p = 2 – 4 mTorr).
Table 1. Peaks labelled in figure 4 and their assignations.
Label
Wavelength (cm-1)
Assignation
L1
L2
L3
L4
L5
L6
3321
3016
1305
949
889
729
2359
2335
1747
667
C2H4
CH 4
CH 4
C2H4
C 2 H 5 OH?
C2H4
L7
L8
L9
CO
HOCCH 3
CO 2
4. Discussion
It is logical to conclude that there is a strong connection
between the plasma phase composition and thin film
structural growth. FTIR measurements show a clear
difference in composition between plasma that derives
deposition of homogenous thin films and the one that
results in deposition of films with columnar structure. For
plasmas with 80% and higher amounts of O 2 introduced
in the gas chamber, a significant increase in amount of
carbon oxides in the plasma can be observed, especially
of CO 2 (Fig. 4, peak L6). At the same time a substantial
decrease in ethylene (C 2 H 4 ) takes effect (Fig. 4, peak L9).
This is expected when considering the increase in oxygen
in the gas mixture, but there still remains the question of
explaining why the drastic change in plasma composition
and film structure happens for 20% ZTB/80% O 2 mixture.
Further study of plasma phase and thin film composition
is needed.
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Columnar growth was also reported in the case of thin
film deposition using titanium isopropoxide (TTIP) as a
precursor [7,8]. It has been shown that a small addition of
hexamethyldisiloxane (HMDSO) (R HMDSO ≥ 0.5%) to
TTIP/O 2 plasma causes disappearance of columnar
structure [9]. Deposited films are very dense and
and
TTIP
homogenous.
ZTB
(C 16 H 36 O 4 Zr)
(C 12 H 28 O 4 Ti) have a similar structure. They are both
composed of a central transition metal atom (zirconium
and titanium for ZTB and TTIP, respectively). Central
atom is surrounded with four oxygen atoms and each
oxygen is bonded to three hydrocarbon chains (C 4 H 9 for
ZTB and C 3 H 7 for TTIP). This suggests that adding a
small amount of HMDSO to ZTB/O 2 plasma may result
in removal of columnar growth and deposition of
homogenous thin films. Thus, current work focuses on the
use of ZTB/HMDSO/O 2 mixture. Also, a novel multipass
optical system for FTIR spectroscopy (100 passes) will be
used in future experiments. It will provide high sensitivity
needed for further investigation of plasma phase and ZTB
dissociation process.
5. Acknowledgements
The financial support of Midi Pyrénées Regional
Council is gratefully acknowledged.
6. References
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