Physicochemical Study of Ta2O5 and SiO2-Ta2O5 Film Formation from Film-Forming Solutions

Article abstract of DOI:10.1023/A:1012763009684

In order to study physicochemical processes of formation of Ta₂O₅ and SiO₂-Ta₂O₅ films from film-forming solutions, the properties of these solutions, and also the thermal-oxidative breakdown and the properties of the obtained films, were studied.

Full text of DOI:10.1023/A:1012763009684

Russian Journal of Applied Chemistry, Vol. 74, No. 1, 2001, pp. 18 20. Translated from Zhurnal Prikladnoi Khimii, Vol. 74, No. 1,
2001, pp. 18 21.
Original Russian Text Copyright
2001 by Gryaznov, Borilo, Kozik, Mal’chik.
INORGANIC SYNTHESIS
AND INDUSTRIAL INORGANIC CHEMISTRY
Physicochemical Study of Ta2O5 and SiO2 Ta2O5 Film
Formation from Film-Forming Solutions
R. V. Gryaznov, L. P. Borilo, V. V. Kozik, and A. G. Mal’chik
Tomsk State University, Tomsk, Russia
Received March 23, 2000; in final form, July 2000
Abstract In order to study physicochemical processes of formation of Ta2O5 and SiO2 Ta2O5 films from
film-forming solutions, the properties of these solutions, and also the thermal-oxidative breakdown and the
properties of the obtained films, were studied.
At present, particular attention is given to physico-
chemical properties of thin films, since the develop-
ment of new fields of modern technology assigned a
prominent place to the application of various materials
in the thin-film state. The broadest application in
various fields of electronics is characteristic of di-
electric films based on complex oxides [1]. For thin-
film materials to be successfully used, a relationship
should be established between their physicochemical
and service properties, composition, and preparation
conditions.
a substrate and decompose to oxides upon solvent
evaporation at elevated temperature [2]. The temporal
stability of film-forming solutions is a factor im-
portant in technological regard, and, therefore, the
relationship between the viscosity of solutions, their
storage time, and the possibility of their use to obtain
films was studied experimentally. When TaCl is dis-
5
solved in ethanol, alkoxy derivatives are formed [3]:
TaCl5 + 3C2H5OH
TaCl2(OC2H5)3 + 3HCl.
A study of the film-forming ability of alcoholic
TaCl solutions demonstrated that the obtained solu-
5
EXPERIMENTAL
tions possess a viscosity (Fig. 1, curve 1) sufficient
for obtaining films, and the film-forming ability is
preserved for a long time since the chloride ion, being
a ligand of -donor type, forms cluster compounds
with tantalum. With account taken of published data
on cluster compounds of tantalum [3], it may be as-
sumed that chloride ions serve as bridges in the for-
mation of stable polynuclear structures of tantalum
alkoxy derivatives
Film-forming solutions (FFSs) were prepared from
tetraethoxysilane (TEOS), ethanol, and tantalum pen-
tachloride. The viscosity was measured with a VPZh-
2 or VPZh-4 glass capillary viscometer. The decom-
position of deposits obtained from FFSs was studied
on a Q-1500 derivatograph. The films were formed on
glass or single-crystal silicon substrates with the sur-
2
face area of 1 1.5 cm by centrifugation and drawing,
at a speed of centrifuge rotation of 1000 5000 rpm
and drawing velocity of 1 5 mm s . The uniformity
OC2H5
OC2H5
OC2H5
Ta
OC2 H5
1
Cl
Cl
Cl
Ta
OC2 H5
Ta
OC2 H5
of the films formed across their thickness was moni-
tored by determining the optical thickness at 6 8
points for each sample. The film adhesion to the sub-
strate was determined sclerometrically, and the refrac-
tive index and thickness, with an LEF-3M laser ellip-
someter. Films 120 140 nm thick were obtained and
studied. IR transmission spectra of the films were re-
Cl
Thus, the film-forming ability of an alcoholic solu-
, cP
1
corded in the 4000 400 cm range on IR-75, M-40,
and SF-20 instruments. As known, the film-forming
ability is exhibited by the substances that can form
macromolecules or associates in solution, which are
attached to the surface when the solution is applied to
, days
of TaCl -containing
Fig. 1. Variation of the viscosity
FFS with time
5
.
1070-4272/01/7401-0018$25.00 2001 MAIK Nauka/Interperiodica

PHYSICOCHEMICAL STUDY OF Ta2O5 AND SiO2 Ta2O5 FILM
19
Table 1. Assignment of bands in the IR spectra of powders at different temperatures
1
Vibration frequency, cm
FFS
T, K
H OH
stretching
OC2H5
stretching
H O H
bending
OC2H5
bending
Si O Si
O Si O Ta O
TaCl5
373
773
848
900
3600
1640
400
400
400
400
TaCl5 + TEOS
TEOS
373
600
633
3560
3600
1640
1640
1100
1100
1100
600 800
600
460 400
460 400
460 400
2900
2900
600
373
473
773
3660
3680
2935
2935
1640
1640
1095
1100
1100
600 800
800
460
460
460
800
tion of tantalum pentachloride is not limited to the
stages of its ripening and aging.
determine the conditions of film synthesis, the main
stages of formation of pure oxides and complex oxides
on their base were studied. On heating a powder ob-
tained by drying at 333 K an alcoholic solution of
To study the properties of film-forming solutions
in the system TEOS TaCl C H OH, kinetic curves
5
2 5
TaCl , the following processes occur according to
5
describing the variation of the solution viscosity
were taken at different salt concentrations (Fig. 1,
curves 2 4). More complex processes are observed in
these solutions. The solutions have no ripening range,
which is probably associated with processes of tan-
talum pentachloride dissolution in ethanol. However,
after 2 days the viscosity starts to grow steadily
because of the simultaneously occurring processes of
hydrolytic polycondensation of TEOS and alkoxy
derivatives of tantalum:
thermal analysis data (Fig. 2): partial evaporation of
physically adsorbed water and alcohol, reaction of
TaCl with atmospheric oxygen, and, finally, combus-
5
tion of oxochlorides and alkoxy derivatives of tan-
talum to form the oxide.
The IR data (Table 1) confirm the presence of
ethoxy groups at the oxide formation temperature of
800 840 K. At higher temperatures there are no
stretching or bending vibrations of ethoxy groups.
Complex oxides SiO Ta O are formed in four
2
2 5
Si(OC2H5)3OH + TaCl2(OC2H5)3
C2H5OH
stages (Fig. 3). The activation energy was calculated
for all the processes involved by the Horowitz
Metzger method (Table 2) [5].
Cl
+
Si
Ta
Ta
O
O
Si
Ta
O
In stage I, physically adsorbed water and alcohol
are removed, which is accompanied by an endo-
thermic effect at 393 K. The second endothermic peak
Cl
etc.
Therefore, the time during which stable film-form-
ing properties are preserved decreases to 30 days. On
m, mg
lowering the SiO concentration in a film to 10 20%,
2
DTA
the time of FFS serviceability is again extended to
48 days. This probably occurs because the forming
cluster compounds of tantalum hinder the hydrolytic
polycondensation of tetraethoxysilane.
TG
The oxide film formation consists in that the
chemical composition of the starting film-forming
compounds undergoes changes first in solution, then
at the instant of film formation on the substrate sur-
face, and, finally, during thermal treatment [4]. To
T, K
Fig. 2. Derivatograms of alcoholic TaCl solution dried
at 333 K. ( m) Weight loss and (T) temperature; the same
5
for Fig. 3.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 74 No. 1 2001

20
GRYAZNOV et al.
Table 2. Kinetic parameters of SiO2, Ta2O5, and SiO2 Ta2O5 film formation
SiO2
Ta2O5
SiO2 Ta2O5
Forma-
tion
stage
conver-
sion, %
Ea,
kJ mol
conver-
sion, %
Ea,
kJ mol
conver-
sion, %
Ea,
kJ mol
T, K
T, K
T, K
1
1
1
I
II
III
IV
298 473
473 623
823 973
33.0
29.0
37.5
41.4
51.8
68.5
298 473
473 573
573 923
51.7
15.0
33.3
44.1
32.1
107.8
298 473
473 623
623 763
763 973
50.0
34.5
3.4
40.5
32.6
60.9
12.0
117.4
Table 3. Physicochemical properties of SiO2 Ta2O5 films
Parameter value at indicated Ta2O5 content in film, %
Parameter
10
20
30
40
50
60
70
80
90
Refractive index
Dielectric constant
Adhesion, kg mm
1.51
4.6
0.93
1.61
5.1
0.95
1.63
7.4
0.91
1.66
9.2
0.96
1.69
11.0
0.97
1.72
12.5
0.94
1.75
14.3
0.96
1.83
15.1
0.94
1.91
16.5
0.95
2
(734 K) is due to evaporation of water released upon
condensation of silanol groups. The activation energy
calculated for this stage (Table 2) is in agreement with
the value characteristic of the condensation of hy-
droxyl-containing organosilicon compounds [6]. In the
final stage of the process (exothermic peak at 848 K)
the ethoxy groups contained in the products of FFS
solvation and hydrolysis are oxidized, and the forma-
tion of the SiO Ta O film is complete. The ac-
the obtained films exhibit good adhesion to various
substrates and contain no micropores.
CONCLUSION
An alcoholic solution of tantalum pentachloride
possesses film-forming ability, with unlimited durabil-
ity of stable film-forming properties. Gravimetry
and IR spectroscopy were applied to establish the
sequence of main stages of Ta O and SiO Ta O
2
2 5
2
5
2
2 5
tivation energy of this stage is typical of a chemical
reaction.
formation from film-forming solutions. The properties
of the obtained films were studied.
The firmness of film attachment to the surface, and
the optical and electrical properties of films, deter-
mine the possibilities of their practical use. The op-
tical and electrophysical properties, adhesion, and
porosity were studied for SiO Ta O films (120
REFERENCES
1. Borisenko, A.I., Novikov, V.V., Prikhod’ko, N.E.,
et al., Tonkie neorganicheskie plenki v mikroelektro-
nike (Thin Inorganic Films in Microelectronics), Le-
ningrad: Nauka, 1972.
2
2 5
140 nm, silicon substrate) (Table 3). It was found that
2. Mal’chik, A.G., Borilo, L.P., and Kozik, V.V., Zh.
Prikl. Khim., 1996, vol. 69, no. 2, pp. 224 227.
m, mg
3. Fairbrother, F., The Chemistry of Niobium and Tan-
talum, Amsterdam: Elsevier, 1967.
4. Tochitskii, E.I., Kristallizatsiya i termoobrabotka ton-
kikh plenok (Crystallization and Thermal Treatment of
Thin Films), Minsk: Nauka i Tekhnika, 1976.
DTA
5. Fialko, M.B., Neizotermicheskaya kinetika v termi-
cheskom analize (Nonisothermal Kinetics in Thermal
Analysis), Tomsk: Tomsk. Gos. Univ., 1981.
6. Voronkov, M.G., Mileshkevich, V.P., and Yuzhelev-
skii, Yu.A., Siloksanovaya svyaz’ (Siloxane Bond),
Novosibirsk: Nauka, 1976.
TG
T, K
Fig. 3. Derivatogram of film formation from FFS with
TaCl and Si(OC H ) .
5
2 5 4
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 74 No. 1 2001