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A new TiO2 film deposition process in a supercritical fluid
Surface and Coatings Technology, 70 (1994) 73—78
73
A new Ti02 film deposition process in a supercritical fluid J. F. Bocquet, K. Chhor and C. Pommier Laboratoire d’Ingénierie des Matdriaux et des Hautes Pressions (CNRS 1311), Universitd Paris XIII, Avenue Jean-Baptiste Clement, 93430-Villetaneuse (France) (Received January 21, 1994; accepted in final form March 4, 1994)
Abstract A new process for Ti02 film formation with a high deposition rate is proposed. The thermal decomposition of titanium isopropoxide dissolved in a supercritical C02-isopropanol mixture is carried out below 300°Con alumina substrates. High solute concentrations, compared with chemical vapor deposition processes, allow one to obtain homogeneous anatase film with around 5 ~smthickness in less than 10 mm.
1. Introduction Thin metal oxide films are used in various fields such as optics, electronics or material science. Because of their particular physical properties, hO2 films have numerous applications such as antireflection coatings [1], high temperature optical filters [2]. protective layers on optical fibers [3], the dielectric in memory cell capacitors [4], semiconductor electrode material in photoassisted electrolytic process [5.6], the transparent electrode for solar cells [7] or the active component in exhaust gas oxygen gauges [8]. Properties required for a given application are strongly related to the film microstructure, crystalline form and thickness, all characteristics depending on the deposition process. A large number of formation routes have been proposed. Sol—gel processing of powders, based on controlled hydrolysis and polymerization of metal alkoxides, has been extended to film formation [3,5,9—11]. In a first step, the substrate can be coated by dipping, spinning or spraying techniques allowing large surface areas, up to some square meters, to be covered. In the second step, the gel coating is transformed into a dense oxide film by thermal treatments, and temperatures from 400 to 800 °Care needed in order to obtain crystallization of the solid. At this stage, problems related to film cracking and surface smoothness often occur [11]. Physical vapor deposition techniques, such as reactive electron beam evaporation, in beam sputtering or ionassisted deposition have been proposed but suffer from high cost and limitations in the size, shape and thickness of the obtainable films [2]. The most widely used chemical vapor deposition (CVD) overcomes these disadvantages. In most cases, low pressure evaporation of
0257—8972/94/$7.00 SSDI 0257-8972(94)0228l-T
a metal organic precursor, such as an alkoxide, is performed and followed by its thermal decomposition on the substrate surface [2,6,12—14]. Modified processes have been proposed in order to increase the film formation rate; the precursor molecules are usually carried on the substrate surface in a flow of an inert gas or, in some processes, into aerosol particles generated by ultrasonic atomization of a solution [15, 16]. Supercritical fluids have already been used in film deposition. The method proposed first was based on the rapid expansion of supercritical solutions through a nozzle into a supersonic jet where cooling and solute nucleation occur. Inorganic films can be obtained after dissolving metal oxides (i.e. Si02 or Al,03) in supercritical water at a high temperature (above 450 °C)and a high pressure (greater than 300 bar) [17,18]. Because of the very short time scale of the process, the solid formed is usually amorphous. A precursor compound can be dissolved in a supercritical fluid and is able to react or decompose after expansion of the solution. Metal organic derivatives such as metal ~ diketonates dissolved in organic solvents are used [19,20]. Metallic films are formed on substrate surface heated at 600—800°C and can be further transformed into oxides in an oxygen plasma. In the present paper, we proposed a new method where the precursor, dissolved in a supercritical solvent, is decomposed straight into oxide on the heated substrate surface, without expansion of the solution. The expected advantage of such a process is that thin homogeneous and crystallized Ti02 films will be obtained at a low temperature (below 300°C) with a higher deposition rate than from classical CVD techniques. This work is an extension of a previous study on submicron oxide
© 1994
—
Elsevier Science S.A. All rights reserved
J. F. Bocquet et a!. / New Ti0
74
2 film deposition process in supercritical fluid
powder synthesis from thermalfluid decomposition of metal alkoxides in a supercritical [21,22]. Here, the reagent is titanium tetraisopropoxide (TTIP) dissolved in a supercritical C0 2—isopropanol mixture.
2. Experimental details The experimental device used for Ti02 film synthesis is shown in figure 1. The films were deposited on an alumina substrate inside a stainless steel high pressure reaction3. The vesselsubstrate with anis internal of around fastened volume on a stainless steel 130 cmattached to one of the two heads of the high plate pressure cylindrical tube. The other head is fitted with pressure and temperature measurement devices as well as with tubes allowing a flow of the supercritical fluid. Heat pulses can be applied on the plate with two heating cartridges. Two high pressure pumps (LDC CP3000 and Varian 9001) allow carbon dioxide and pure isopropanol (or TTIP solution) to be introduced into the system. Experiments have been conducted either starting with the closed vessel filled with a given amount of TTTP supercritical solution (“static” mode) or sweeping the reactor with a constant flow of such a solution (“dynamic” mode). In typical experiments, the C0 2—iC3H1OH mixture used had a CO2 molar fraction X~0,= 0.5. The temperature and pressure inside the reactor were kept constant at TR°= 240°C and PR = 17 MPa respectively. In dynamic mode, the fluid mass flow rate through the system is controlled by the two high pressure pumps that deliver set constant flows of liquid TTIP alcoholic solution and carbon dioxide (the gaseous CO2 is condensed before entering the pump). The pressure inside the system can be modified using the outlet regulating valve. 3Amin’ typicalcorresponding flow rate for the to asupercritical liquid CO fluid, was 14 cm 2 flow rate
II
I
P
B difference
v a
v C
-i
H
K
U
Fig. 1. Experimental device for Ti02 film synthesis. A, CO2 tank, B, isopropanol (or TTIP solution) stock; C. cooling bath; D, high pressure pump; E, mixing chamber; F. heating; G. oven; H. reactor, K, heat pulse generator; L. stainless steel plate; p, pressure gauge; T. thermocouple; V, high pressure valve.
3 min’ and TTIP liquid solution flow rate of of 3.7 5.0 cm3cmmin1 from the two high pressure feed pumps. Heat pulses were applied on the substrate in order to raise its surface temperature to around T5 = 290 °Cin less than 2 mm. In the experimental conditions of this study, no safety problems arise because of the small size of the reactor. However, using larger systems, it would be necessary to pay attention to the consequences of possible alcohol leaks outside the reactor, in the oven. An oxygen-free atmosphere should be maintained in the oven and a warning addedstudies detecting presence alcohol.to be Preliminary on of thethe choice of theofsolvent used were made by visual observations, heating a high pressure sapphire cell with an internal volume of around 9 cm3. The isopropanol and TTIP used are commercial products (Prolabo RP and Aldrich Chemicals respectively) with purity higher than 99 mol%. The carbon dioxide (purity, N45) was provided by Air Liquide (France~.The alumina substrates are small plates. about 20 mm x 10 mm x 1 mm in size, either commercial (Al 23 quality; Degussa, France) or provided by CRITT, Maubeuge (France). They differ in their surface smoothnesses, their roughness coefficients being 2.2 ~.tmand 0.3 I.tm respectively. However, for the first, cavities as large as 5—10 ~m have been observed. Characterization of the deposited films was made from X-ray diffraction studies (Siemens diffractometer), scanning electron microscopy (Cambridge 5360) and roughness measurements on a laboratory-developed apparatus using a laser sensor (Microfocus, Ettlingen, RFA).
3. Results 3.1. Preliminary Sf udies 3.1.1. Choice ofthe solvent Previous work has shown that TTIP begins to decompose in supercritical ethanol [21] or isopropanol [23] at temperatures Td of around 260°Cand 250°Crespectively, i.e. slightly above the solvent critical points (T~= 241 °C and 235 °C respectively). In these systems, the Td— T~is rather small and, because of convective movements of the supercritical fluid inside the reactor when the plate is heated, it is likely that the temperature in the vicinity of the substrate will be raised above Td. Then solute decomposition in the reactor volume could occur. Other organic liquids in which TTIP can be dissolved have even higher critical points, so that the only possibility of lowering the critical temperature of the solution is the use of a cosolvent. Carbon dioxide was chosen because of its low T~value (31 °C)and because it is safe and easy to handle. The
J. F. Bocquet et a!.
/
New Ti0
2 film deposition process in supercritical fluid
behavior of such mixtures has been studied in the transparent high pressure sapphire cell. Using an ethanol—CO2 mixture as solvent with X~02= 0.35, the critical point can be lowered by 60°C(T~= 181 °C) compared with pure ethanol. However, solid particle formation occurs at around 155°C(in the liquid phase) on heating a TTIP solution. This behavior can be explained as follows. In ethanol, TTIP gives rise to exchange reactions and transforms into ethoxide which is known to associate into polymerized species, which are more difficult to decompose than the monomeric molecule. It is likely that CO2 added as cosolvent can destroy these associations and then facilitatethe alkoxide decomposition. In order to avoid these exchange reactions, isopropanol was chosen as organic solvent. For an isopropanol—C02 mixture with X~02= 0.50 the critical point is around T~=180°C,P~=9.8MPa, and TTIP decomposition occurs above about 240 °C.
75
360
—
~ ~
320
i—
~ 280
260
3.1.2. Heat transfer in the system In order to determine the experimental conditions for heating the substrate surface, preliminary studies were performed following the temperatures inside the vessel (TR) and on the substrate surface (T5) when a heat pulse is applied. The initial TR°value and the regulated plate temperature T were chosen in order (1) to allow rapid heating of the substrate up to a temperature where TFIP decomposition is fast and (2) to avoid (or minimize) solute decomposition in the reactor volume. Figure 2 shows the result of a typical static experiment. The initial temperature in the system was TR°= 240 °C and the plate was heated to 340°Cin 2mm and then kept at this temperature for 3 mm (this last parameter was taken as the pulse time tn). In these conditions, the substrate surface temperature is raised to around 290 °C in 2 mm and maintained there for 3 mm. The temperature inside the vessel increases much more slowly and reaches 260°Cafter 5 mm, when the heating is turned off. With the same starting conditions but with a much longer pulse time (30 or 45 mm), the maximum T~and TR values reached were 310°Cand 265 °Crespectively.
240 -200
0
200
400
~O0
800
TI M E (s) Fig. 2. Heat transfers dunng a 3 mm heat pulse on the substrate: ~, inside the heated plate, T; Y, on the substrate surface, T5
•, inside
the reactor,
TR.
In the dynamic mode, with t~=l0min, the maximum TR value was around 265 °C. 3.2. Ti02 filmformation The experimental parameters expected to influence the Ti02 deposit formation are essentially the substrate surface temperature T5, the heat pulse length t~,and the solute concentration XTTIP in the supercritical fluid. Table 1 reports the conditions and results of some representative experiments.
TABLE 1. Conditions and main results of representative dynamic (D) and static (S) experiments in Ti02 film formation Run
1(D) 2(D) 3(S) 4(S) 5(S)
XTTIP
2.5 x iQ~ 3 2.5 x i0~ 55 x103 x10 2.5x103
7, (°C)
t~,
(°C)
(mm)
2’~ (°C)
240 240 240 240 240
340 340 340 340
10 453 3 3
290 310 290 290 290
TR°
R 5, substrate roughness; Rd, deposit roughness; e, film thickness.
R~ (jon) ‘
0.3 2.2 0.3 0.3 0.3
Rd (jan) 2 1 ~1 0.7
—
e (jim)
5—7
76
J. F. Bocquet er a!. / New Ti0
2 film deposition process in supercritica! fluid
In dynamic experiments, XTTIP, TR°and T5 were kept at constant values and t~,was varied from 3 to 10 mm. All the deposmts obtained had similar features, as shown in Figs. 3 and 4. On the rmght-hand side of Fmg. 3, the virgin substrate (hidden during the experiment) can be seen. On the left-hand side, a continuous film appears. formed by contiguous particles with diameter less than 0.5 Ii.m. Although no powder was found on the reactor wall, spherical particles (up to about 3 j.tm in diameter), probably formed near the substrate surface, have settled on the primary homogeneous film. The number of such particles as well as the ultimate film thickness are strongly dependent on the heat pulse length (Fig. 4). X-ray diffraction studies have shown that these films are titanium dioxide crystallized in the anatase structure. They stick strongly to the substrate surface and their roughness is of the order of the settled particle diameter (around 2 iim). However, it is rather difficult to control the film thickness in a reproducible manner by applying a given heat pulse length. Assuming that, in the dynamic mode, important convective movements around the heated plate enhance the formation of spherical particles in the neighbouring of the substrate surface, we investigated a static mode of operation. Runs 3 and 4 (Table 1) are typical static experiments performed by varying the heat pulse length and keeping all other parameters at constant values. For long pulses. all TTIP initially introduced was decomposed and much powder is recovered in the reactor. Figure 5 shows that a rugged substrate (Ra >2 lam) can then be homogeneously covered with a Ti02 film in the anatase structure. For short pulses, the decomposition in the reactor volume is minimized and the deposition rate on the substrate surface increases with increasing TTIP ~
*
.
.
. ,
. ~...
.
.
*
ta~
hi
ig. 4. Structure of the Ti02 films obtained in dynamic mode with 10 mm and b) a 3 mm heat pulse on the substrate surface (other conditions: Table I. runs I and 2 respectively). ii
concentration in the supercritical fluid (runs 4 and 5). With a solute molar fraction of around 2.5 x l0~, a 3 mm pulse is sufficient to obtain a continuous adherent film with about 1 ~am thickness. It is formed with somewhat sintered submicron particles (Fig. 6) and has a roughness below 1 )am. However, in such conditions, setting the other experimental parameters (t~ and T~) in order to obtain a given precise thickness is still difficult. A further reduction in the solute concentration
t 1-ig. 3. Boundary between alumina substrate (right) and Im02 him (left) obtained in dynamic mode (experimental conditions: Table I run i~.
would allow better control of the deposition rate in the formation of thin films. Nevertheless, a time pulse longer than about 10 mm is not advisable in order to avoid a significant heat diffusion from the substrate surface to the reactor volume where TTIP decomposition could occur.
J. F. Bocquet et a!.
/
Nest Ti0
2.film deposition process in supercritical fluid
~_—~
-—.—
_______ _____
1’
-
________ -.
-
-
______ ________
________
-
______ -
..
‘~ -
-
.
.
~‘‘.
~ .
~-
,
t
.
F
‘
Fie.5. Coverine of a rouch substrate (R5=2.iiml with Tb, film deposited in static mode (experimental conditions: Table 1, run 3). _____
_____
~
~
~ ~ ~
—-
--‘
~
~
Fig.6. Ti02 film obtained on a smooth substrate (R~=0.3(mm). static mode (experimental conditions: Table 1. run 5).
ifl
4. Discussion Various studies have reported the thermal decomposition of metal alkoxides, for oxide powder synthesis either in a gas-phase reactor or in CVD processes. For TTIP, the overall reaction can be written T.(O-iC3H7 )4—~TiO2+ 4C3H6 + 2H2O (1) However, the decomposition mechanism is much more complex. Various apparent reaction orders between 0 and 2 (integer and half integer values) have been reported from kinetics studies [12—14,24—26]. In many cases, Ti02 particles are not obtained by homogeneous nucleation at temperature lower than 300°C[26] but many workers emphasize the catalytic effects of the reactor walls; the reaction takes place more
77
rapidly, even at a lower temperature, when TiO2 particles have been previously deposited on the walls [24]. Competitive reactions take place, depending strongly on the experimental conditions. In CVD experiments, the proposed successive steps are (a) the activation of TTIP molecules by collisional excitation in the gas phase, (b) the adsorption of the activated species on the surface and (c) the decomposition of the adsorbed molecules [12]. However, in order to explain the kinetics results at temperatures higher than 350°C, the formation of Ti02 monomers or Ti02 particles in the gas phase has been considered in addition to the heterogeneous reaction [13]. Furthermore, it is well known that even a minute trace of water can be sufficient to initiate autocatalytic decomposition of alkoxides. Then, water molecules, as the reaction product in eqn, (I), can lead to a further undesirable homogeneous gas phase reaction subsequently to primary film formation [27]. In classical CVD processes, the film growth is limited either by mass transfer of the precursor molecules or by reaction kinetics. In both cases the deposition rate can be expressed as proportional to the partial pressure of the reagent above the substrate surface [2]. In the usual conditions, this pressure is low and attainable rates range from about 0.01 ~m h’ to some micrometres per hour. They can be slightly increased by increasing the temperature using an inert carrier gas or ultrasonic atomization of the reagent but, even in such conditions, the film growth is only 10—15 ~m h’. In the above experiments, the supercritical solution of TTIP allows a solute concentration above the substrate surface of around 2 x 10-2 mol 1~,i.e. about 1000—5000 times greater than in CVD. Furthermore, the fluid is homogeneous and single phase while atomized droplets in the derived processes are up to about 30 ~m in diameter [16]. The film formation can then be much more rapid, even at temperatures lower than 300 °C.We have shown that it is possible to find experimental conditions for which the reaction can be confined on the surface substrate or in a very limited layer above it. It is difficult to determine whether the observed spherical particles deposited on the film are the result of an homogeneous gas-phase reaction induced either by a thermal gradient above the substrate or by water molecules generated in the heterogeneous surface reaction. However, in static experiments, these particles are well incorporated with the Ti02 layer formed first and homogeneous films with a thickness of several micrometres, a low roughness and strongly adherent to the substrate are obtained. It has been reported that thin Ti02 films formed by the CVD process at low temperature (below 300°C)are amorphous [12]. With increasing temperature and film thickness, crystallized oxide is deposited, first in the
78
J. F. Bocquet et a!.
/
New Ti0
2 film deposition process in supercriticalfluid
anatase structure and later in the rutile form [12,14]. In our experiments, we obtained anatase although the solute concentration and deposition rate are high, conditions which favor rutile formation in the CVD process [14]. The influence of the pressure there appears to be predominant; we have previously shown that Ti02 submicron powders obtained from TTIP decomposition in supercritical alcohol is anatase and that this structure is stabilized, i.e. transforms into rutile only on heating at 900°C[21]. The anatase and rutile have different electrical and electrochemical properties and it is of interest to have processes allowing formation of Ti02 films in these various structures.
5. Summary We have reported a new process for ceramic film formation by thermal decomposition of a metal organic derivative dissolved in a supercritical fluid. The proposed application is Ti02 deposition from TTIP dissolved in a CO2— isopropanol mixture. Because of the high reagent solubility in the supercritical fluid, a high concentration and a high deposition rate are attained. Homogeneous films, about 1—S ~m thick, built up from 200—400 nm particles, are obtained in less than 10 mm below 300 °C. However, in the conditions studied precise control of the film thickness below about 2—3 ~m is difficult. This control could be achieved by reducing the deposition rate, using a lower solute concentration. Using this process, relatively large surfaces can be treated and TiO2 is crystallized in a stabilized anatase structure. Crack formation under thermal treatment of the covered surface is then minimized. References 1 A. Yeung and K. W. Lam, Thin Solid Films, 109 (1983(169. 2 S. B. Desu, Mater. Chem. Phys., 31(1992) 341.
3 K. Jurek, M. Guglielni, G. Kuncova, 0. Rennet, F. Lukes. M. Navratil, E. Krousky, V. Vorlicek and K. Kokesova, J. Mater. Sd., 27 (1992) 2549. 4 W. D. Brown and W. W. Granneman, Solid-Stare Electron., 21 (1978) 837. 5 H. Cui, H. S. Shen, Y. M. Gao, K. Dwight and A. Wold, Mater. Res. Bull., 28 (1993) 195. 6 K. L. Hardee and A. J. Bard, J. Electrochem. Soc., 122 (1975) 739. 7 M. A. Butler and D. S. Gmniey, J. Mater. Sci., 15 (1980)19. 8 R. Jerisian, J. Gautron and J. P. Loup, J. Phvs. (Paris) III, 2 (1992) 679. 9 B. Samuneva, V. K. Zhukharov. Ch. Trapalis and R. Kranold. J. Mater. Sci., 28 (1993) 2353. 10 G. J. Exarhos and M. Aloi, Thin Solid Films, 193 (1990) 42. 11 G. Yi and M. Sayer, Ceram. Bull., 70(1991)1173. 12 W. G. Lai, K. L. Siefering and G. L. Griffin, in P. Vincenzini (ed(. High Performance Ceramic Films and Coatings, Eisevier, Amsterdam, 1991, p. 151. 13 H. Y. Lee and H. G. Kim, Thin Solid Films, 229 (1993) 187. 14 Y. Takahashi, H. Suzuki and M. Nasu, J. Chem. Soc., Faraday. Trans. 1, 81(1985) 3117. 15
W. W.
Xu. R. Kershaw, K. Dwight and A. Wold, Mater. Res. Bull..
16 25 M. (1990)1385. Langlet and J. C. Joubert, Chemistry of Advanced Materials, IUPAC, Blackwell, Oxford, 1992, p. 55. 17 R. C. Petersen, D. W. Matson and R. D. Smith, J. Am. Chem. Soc., 108 (1986) 2100. 18 J. I. Brand and D. R. Miller, Thin Solid Films, 166 (1988) 139. 19 B. N. Hansen, B. M. Hybertson, R. M. Barkiey and R. E. Sievers, Chem. Mater., 4 (1992) 749. 20 R. E. Sievers and B. N. Hansen, US Pat. 4,970,093, 1990. 21 K. Chhor, J. F. Bocquet and C. Pommier, Mater Chem. Ph vs.. 32 (1992) 249. 22 M. Barj, J. F. Bocquet, K. Chhor and C. Pommier. J. Mater., Sci., 27 (1992) 2187. 23 V. Gourinchas, K. Chhor, J. F. Bocquet and C. Pommier. unpublished results, 1993. 24 H. Komiyama, T. Kanai and H. Inoue, Chem. Lert., (1984)1283. 25 K. Okuyama, Y. Kousaka, N. Tohge, S. Yamamoto, J. J. Wu, R. C. Flagan and J. Seinfeid, AIChE J., 32 (1986) 2010. 26 K. Okuyama, J. T. Jeung, Y. Kousaka, H. V. Nguyen, J. J. Wu and R. C. Flagan, Chem. Eng. Sd., 44 (1989)1369. 27 G. J. M. Dormans, M. de Keijser, P. J. van \‘eldhoven. D. M. Frigo, J. E. Holewijn, G. P. M. van Mier and C. J. Smit, Chem. Mater.. 5 (1993) 448.
73
A new Ti02 film deposition process in a supercritical fluid J. F. Bocquet, K. Chhor and C. Pommier Laboratoire d’Ingénierie des Matdriaux et des Hautes Pressions (CNRS 1311), Universitd Paris XIII, Avenue Jean-Baptiste Clement, 93430-Villetaneuse (France) (Received January 21, 1994; accepted in final form March 4, 1994)
Abstract A new process for Ti02 film formation with a high deposition rate is proposed. The thermal decomposition of titanium isopropoxide dissolved in a supercritical C02-isopropanol mixture is carried out below 300°Con alumina substrates. High solute concentrations, compared with chemical vapor deposition processes, allow one to obtain homogeneous anatase film with around 5 ~smthickness in less than 10 mm.
1. Introduction Thin metal oxide films are used in various fields such as optics, electronics or material science. Because of their particular physical properties, hO2 films have numerous applications such as antireflection coatings [1], high temperature optical filters [2]. protective layers on optical fibers [3], the dielectric in memory cell capacitors [4], semiconductor electrode material in photoassisted electrolytic process [5.6], the transparent electrode for solar cells [7] or the active component in exhaust gas oxygen gauges [8]. Properties required for a given application are strongly related to the film microstructure, crystalline form and thickness, all characteristics depending on the deposition process. A large number of formation routes have been proposed. Sol—gel processing of powders, based on controlled hydrolysis and polymerization of metal alkoxides, has been extended to film formation [3,5,9—11]. In a first step, the substrate can be coated by dipping, spinning or spraying techniques allowing large surface areas, up to some square meters, to be covered. In the second step, the gel coating is transformed into a dense oxide film by thermal treatments, and temperatures from 400 to 800 °Care needed in order to obtain crystallization of the solid. At this stage, problems related to film cracking and surface smoothness often occur [11]. Physical vapor deposition techniques, such as reactive electron beam evaporation, in beam sputtering or ionassisted deposition have been proposed but suffer from high cost and limitations in the size, shape and thickness of the obtainable films [2]. The most widely used chemical vapor deposition (CVD) overcomes these disadvantages. In most cases, low pressure evaporation of
0257—8972/94/$7.00 SSDI 0257-8972(94)0228l-T
a metal organic precursor, such as an alkoxide, is performed and followed by its thermal decomposition on the substrate surface [2,6,12—14]. Modified processes have been proposed in order to increase the film formation rate; the precursor molecules are usually carried on the substrate surface in a flow of an inert gas or, in some processes, into aerosol particles generated by ultrasonic atomization of a solution [15, 16]. Supercritical fluids have already been used in film deposition. The method proposed first was based on the rapid expansion of supercritical solutions through a nozzle into a supersonic jet where cooling and solute nucleation occur. Inorganic films can be obtained after dissolving metal oxides (i.e. Si02 or Al,03) in supercritical water at a high temperature (above 450 °C)and a high pressure (greater than 300 bar) [17,18]. Because of the very short time scale of the process, the solid formed is usually amorphous. A precursor compound can be dissolved in a supercritical fluid and is able to react or decompose after expansion of the solution. Metal organic derivatives such as metal ~ diketonates dissolved in organic solvents are used [19,20]. Metallic films are formed on substrate surface heated at 600—800°C and can be further transformed into oxides in an oxygen plasma. In the present paper, we proposed a new method where the precursor, dissolved in a supercritical solvent, is decomposed straight into oxide on the heated substrate surface, without expansion of the solution. The expected advantage of such a process is that thin homogeneous and crystallized Ti02 films will be obtained at a low temperature (below 300°C) with a higher deposition rate than from classical CVD techniques. This work is an extension of a previous study on submicron oxide
© 1994
—
Elsevier Science S.A. All rights reserved
J. F. Bocquet et a!. / New Ti0
74
2 film deposition process in supercritical fluid
powder synthesis from thermalfluid decomposition of metal alkoxides in a supercritical [21,22]. Here, the reagent is titanium tetraisopropoxide (TTIP) dissolved in a supercritical C0 2—isopropanol mixture.
2. Experimental details The experimental device used for Ti02 film synthesis is shown in figure 1. The films were deposited on an alumina substrate inside a stainless steel high pressure reaction3. The vesselsubstrate with anis internal of around fastened volume on a stainless steel 130 cmattached to one of the two heads of the high plate pressure cylindrical tube. The other head is fitted with pressure and temperature measurement devices as well as with tubes allowing a flow of the supercritical fluid. Heat pulses can be applied on the plate with two heating cartridges. Two high pressure pumps (LDC CP3000 and Varian 9001) allow carbon dioxide and pure isopropanol (or TTIP solution) to be introduced into the system. Experiments have been conducted either starting with the closed vessel filled with a given amount of TTTP supercritical solution (“static” mode) or sweeping the reactor with a constant flow of such a solution (“dynamic” mode). In typical experiments, the C0 2—iC3H1OH mixture used had a CO2 molar fraction X~0,= 0.5. The temperature and pressure inside the reactor were kept constant at TR°= 240°C and PR = 17 MPa respectively. In dynamic mode, the fluid mass flow rate through the system is controlled by the two high pressure pumps that deliver set constant flows of liquid TTIP alcoholic solution and carbon dioxide (the gaseous CO2 is condensed before entering the pump). The pressure inside the system can be modified using the outlet regulating valve. 3Amin’ typicalcorresponding flow rate for the to asupercritical liquid CO fluid, was 14 cm 2 flow rate
II
I
P
B difference
v a
v C
-i
H
K
U
Fig. 1. Experimental device for Ti02 film synthesis. A, CO2 tank, B, isopropanol (or TTIP solution) stock; C. cooling bath; D, high pressure pump; E, mixing chamber; F. heating; G. oven; H. reactor, K, heat pulse generator; L. stainless steel plate; p, pressure gauge; T. thermocouple; V, high pressure valve.
3 min’ and TTIP liquid solution flow rate of of 3.7 5.0 cm3cmmin1 from the two high pressure feed pumps. Heat pulses were applied on the substrate in order to raise its surface temperature to around T5 = 290 °Cin less than 2 mm. In the experimental conditions of this study, no safety problems arise because of the small size of the reactor. However, using larger systems, it would be necessary to pay attention to the consequences of possible alcohol leaks outside the reactor, in the oven. An oxygen-free atmosphere should be maintained in the oven and a warning addedstudies detecting presence alcohol.to be Preliminary on of thethe choice of theofsolvent used were made by visual observations, heating a high pressure sapphire cell with an internal volume of around 9 cm3. The isopropanol and TTIP used are commercial products (Prolabo RP and Aldrich Chemicals respectively) with purity higher than 99 mol%. The carbon dioxide (purity, N45) was provided by Air Liquide (France~.The alumina substrates are small plates. about 20 mm x 10 mm x 1 mm in size, either commercial (Al 23 quality; Degussa, France) or provided by CRITT, Maubeuge (France). They differ in their surface smoothnesses, their roughness coefficients being 2.2 ~.tmand 0.3 I.tm respectively. However, for the first, cavities as large as 5—10 ~m have been observed. Characterization of the deposited films was made from X-ray diffraction studies (Siemens diffractometer), scanning electron microscopy (Cambridge 5360) and roughness measurements on a laboratory-developed apparatus using a laser sensor (Microfocus, Ettlingen, RFA).
3. Results 3.1. Preliminary Sf udies 3.1.1. Choice ofthe solvent Previous work has shown that TTIP begins to decompose in supercritical ethanol [21] or isopropanol [23] at temperatures Td of around 260°Cand 250°Crespectively, i.e. slightly above the solvent critical points (T~= 241 °C and 235 °C respectively). In these systems, the Td— T~is rather small and, because of convective movements of the supercritical fluid inside the reactor when the plate is heated, it is likely that the temperature in the vicinity of the substrate will be raised above Td. Then solute decomposition in the reactor volume could occur. Other organic liquids in which TTIP can be dissolved have even higher critical points, so that the only possibility of lowering the critical temperature of the solution is the use of a cosolvent. Carbon dioxide was chosen because of its low T~value (31 °C)and because it is safe and easy to handle. The
J. F. Bocquet et a!.
/
New Ti0
2 film deposition process in supercritical fluid
behavior of such mixtures has been studied in the transparent high pressure sapphire cell. Using an ethanol—CO2 mixture as solvent with X~02= 0.35, the critical point can be lowered by 60°C(T~= 181 °C) compared with pure ethanol. However, solid particle formation occurs at around 155°C(in the liquid phase) on heating a TTIP solution. This behavior can be explained as follows. In ethanol, TTIP gives rise to exchange reactions and transforms into ethoxide which is known to associate into polymerized species, which are more difficult to decompose than the monomeric molecule. It is likely that CO2 added as cosolvent can destroy these associations and then facilitatethe alkoxide decomposition. In order to avoid these exchange reactions, isopropanol was chosen as organic solvent. For an isopropanol—C02 mixture with X~02= 0.50 the critical point is around T~=180°C,P~=9.8MPa, and TTIP decomposition occurs above about 240 °C.
75
360
—
~ ~
320
i—
~ 280
260
3.1.2. Heat transfer in the system In order to determine the experimental conditions for heating the substrate surface, preliminary studies were performed following the temperatures inside the vessel (TR) and on the substrate surface (T5) when a heat pulse is applied. The initial TR°value and the regulated plate temperature T were chosen in order (1) to allow rapid heating of the substrate up to a temperature where TFIP decomposition is fast and (2) to avoid (or minimize) solute decomposition in the reactor volume. Figure 2 shows the result of a typical static experiment. The initial temperature in the system was TR°= 240 °C and the plate was heated to 340°Cin 2mm and then kept at this temperature for 3 mm (this last parameter was taken as the pulse time tn). In these conditions, the substrate surface temperature is raised to around 290 °C in 2 mm and maintained there for 3 mm. The temperature inside the vessel increases much more slowly and reaches 260°Cafter 5 mm, when the heating is turned off. With the same starting conditions but with a much longer pulse time (30 or 45 mm), the maximum T~and TR values reached were 310°Cand 265 °Crespectively.
240 -200
0
200
400
~O0
800
TI M E (s) Fig. 2. Heat transfers dunng a 3 mm heat pulse on the substrate: ~, inside the heated plate, T; Y, on the substrate surface, T5
•, inside
the reactor,
TR.
In the dynamic mode, with t~=l0min, the maximum TR value was around 265 °C. 3.2. Ti02 filmformation The experimental parameters expected to influence the Ti02 deposit formation are essentially the substrate surface temperature T5, the heat pulse length t~,and the solute concentration XTTIP in the supercritical fluid. Table 1 reports the conditions and results of some representative experiments.
TABLE 1. Conditions and main results of representative dynamic (D) and static (S) experiments in Ti02 film formation Run
1(D) 2(D) 3(S) 4(S) 5(S)
XTTIP
2.5 x iQ~ 3 2.5 x i0~ 55 x103 x10 2.5x103
7, (°C)
t~,
(°C)
(mm)
2’~ (°C)
240 240 240 240 240
340 340 340 340
10 453 3 3
290 310 290 290 290
TR°
R 5, substrate roughness; Rd, deposit roughness; e, film thickness.
R~ (jon) ‘
0.3 2.2 0.3 0.3 0.3
Rd (jan) 2 1 ~1 0.7
—
e (jim)
5—7
76
J. F. Bocquet er a!. / New Ti0
2 film deposition process in supercritica! fluid
In dynamic experiments, XTTIP, TR°and T5 were kept at constant values and t~,was varied from 3 to 10 mm. All the deposmts obtained had similar features, as shown in Figs. 3 and 4. On the rmght-hand side of Fmg. 3, the virgin substrate (hidden during the experiment) can be seen. On the left-hand side, a continuous film appears. formed by contiguous particles with diameter less than 0.5 Ii.m. Although no powder was found on the reactor wall, spherical particles (up to about 3 j.tm in diameter), probably formed near the substrate surface, have settled on the primary homogeneous film. The number of such particles as well as the ultimate film thickness are strongly dependent on the heat pulse length (Fig. 4). X-ray diffraction studies have shown that these films are titanium dioxide crystallized in the anatase structure. They stick strongly to the substrate surface and their roughness is of the order of the settled particle diameter (around 2 iim). However, it is rather difficult to control the film thickness in a reproducible manner by applying a given heat pulse length. Assuming that, in the dynamic mode, important convective movements around the heated plate enhance the formation of spherical particles in the neighbouring of the substrate surface, we investigated a static mode of operation. Runs 3 and 4 (Table 1) are typical static experiments performed by varying the heat pulse length and keeping all other parameters at constant values. For long pulses. all TTIP initially introduced was decomposed and much powder is recovered in the reactor. Figure 5 shows that a rugged substrate (Ra >2 lam) can then be homogeneously covered with a Ti02 film in the anatase structure. For short pulses, the decomposition in the reactor volume is minimized and the deposition rate on the substrate surface increases with increasing TTIP ~
*
.
.
. ,
. ~...
.
.
*
ta~
hi
ig. 4. Structure of the Ti02 films obtained in dynamic mode with 10 mm and b) a 3 mm heat pulse on the substrate surface (other conditions: Table I. runs I and 2 respectively). ii
concentration in the supercritical fluid (runs 4 and 5). With a solute molar fraction of around 2.5 x l0~, a 3 mm pulse is sufficient to obtain a continuous adherent film with about 1 ~am thickness. It is formed with somewhat sintered submicron particles (Fig. 6) and has a roughness below 1 )am. However, in such conditions, setting the other experimental parameters (t~ and T~) in order to obtain a given precise thickness is still difficult. A further reduction in the solute concentration
t 1-ig. 3. Boundary between alumina substrate (right) and Im02 him (left) obtained in dynamic mode (experimental conditions: Table I run i~.
would allow better control of the deposition rate in the formation of thin films. Nevertheless, a time pulse longer than about 10 mm is not advisable in order to avoid a significant heat diffusion from the substrate surface to the reactor volume where TTIP decomposition could occur.
J. F. Bocquet et a!.
/
Nest Ti0
2.film deposition process in supercritical fluid
~_—~
-—.—
_______ _____
1’
-
________ -.
-
-
______ ________
________
-
______ -
..
‘~ -
-
.
.
~‘‘.
~ .
~-
,
t
.
F
‘
Fie.5. Coverine of a rouch substrate (R5=2.iiml with Tb, film deposited in static mode (experimental conditions: Table 1, run 3). _____
_____
~
~
~ ~ ~
—-
--‘
~
~
Fig.6. Ti02 film obtained on a smooth substrate (R~=0.3(mm). static mode (experimental conditions: Table 1. run 5).
ifl
4. Discussion Various studies have reported the thermal decomposition of metal alkoxides, for oxide powder synthesis either in a gas-phase reactor or in CVD processes. For TTIP, the overall reaction can be written T.(O-iC3H7 )4—~TiO2+ 4C3H6 + 2H2O (1) However, the decomposition mechanism is much more complex. Various apparent reaction orders between 0 and 2 (integer and half integer values) have been reported from kinetics studies [12—14,24—26]. In many cases, Ti02 particles are not obtained by homogeneous nucleation at temperature lower than 300°C[26] but many workers emphasize the catalytic effects of the reactor walls; the reaction takes place more
77
rapidly, even at a lower temperature, when TiO2 particles have been previously deposited on the walls [24]. Competitive reactions take place, depending strongly on the experimental conditions. In CVD experiments, the proposed successive steps are (a) the activation of TTIP molecules by collisional excitation in the gas phase, (b) the adsorption of the activated species on the surface and (c) the decomposition of the adsorbed molecules [12]. However, in order to explain the kinetics results at temperatures higher than 350°C, the formation of Ti02 monomers or Ti02 particles in the gas phase has been considered in addition to the heterogeneous reaction [13]. Furthermore, it is well known that even a minute trace of water can be sufficient to initiate autocatalytic decomposition of alkoxides. Then, water molecules, as the reaction product in eqn, (I), can lead to a further undesirable homogeneous gas phase reaction subsequently to primary film formation [27]. In classical CVD processes, the film growth is limited either by mass transfer of the precursor molecules or by reaction kinetics. In both cases the deposition rate can be expressed as proportional to the partial pressure of the reagent above the substrate surface [2]. In the usual conditions, this pressure is low and attainable rates range from about 0.01 ~m h’ to some micrometres per hour. They can be slightly increased by increasing the temperature using an inert carrier gas or ultrasonic atomization of the reagent but, even in such conditions, the film growth is only 10—15 ~m h’. In the above experiments, the supercritical solution of TTIP allows a solute concentration above the substrate surface of around 2 x 10-2 mol 1~,i.e. about 1000—5000 times greater than in CVD. Furthermore, the fluid is homogeneous and single phase while atomized droplets in the derived processes are up to about 30 ~m in diameter [16]. The film formation can then be much more rapid, even at temperatures lower than 300 °C.We have shown that it is possible to find experimental conditions for which the reaction can be confined on the surface substrate or in a very limited layer above it. It is difficult to determine whether the observed spherical particles deposited on the film are the result of an homogeneous gas-phase reaction induced either by a thermal gradient above the substrate or by water molecules generated in the heterogeneous surface reaction. However, in static experiments, these particles are well incorporated with the Ti02 layer formed first and homogeneous films with a thickness of several micrometres, a low roughness and strongly adherent to the substrate are obtained. It has been reported that thin Ti02 films formed by the CVD process at low temperature (below 300°C)are amorphous [12]. With increasing temperature and film thickness, crystallized oxide is deposited, first in the
78
J. F. Bocquet et a!.
/
New Ti0
2 film deposition process in supercriticalfluid
anatase structure and later in the rutile form [12,14]. In our experiments, we obtained anatase although the solute concentration and deposition rate are high, conditions which favor rutile formation in the CVD process [14]. The influence of the pressure there appears to be predominant; we have previously shown that Ti02 submicron powders obtained from TTIP decomposition in supercritical alcohol is anatase and that this structure is stabilized, i.e. transforms into rutile only on heating at 900°C[21]. The anatase and rutile have different electrical and electrochemical properties and it is of interest to have processes allowing formation of Ti02 films in these various structures.
5. Summary We have reported a new process for ceramic film formation by thermal decomposition of a metal organic derivative dissolved in a supercritical fluid. The proposed application is Ti02 deposition from TTIP dissolved in a CO2— isopropanol mixture. Because of the high reagent solubility in the supercritical fluid, a high concentration and a high deposition rate are attained. Homogeneous films, about 1—S ~m thick, built up from 200—400 nm particles, are obtained in less than 10 mm below 300 °C. However, in the conditions studied precise control of the film thickness below about 2—3 ~m is difficult. This control could be achieved by reducing the deposition rate, using a lower solute concentration. Using this process, relatively large surfaces can be treated and TiO2 is crystallized in a stabilized anatase structure. Crack formation under thermal treatment of the covered surface is then minimized. References 1 A. Yeung and K. W. Lam, Thin Solid Films, 109 (1983(169. 2 S. B. Desu, Mater. Chem. Phys., 31(1992) 341.
3 K. Jurek, M. Guglielni, G. Kuncova, 0. Rennet, F. Lukes. M. Navratil, E. Krousky, V. Vorlicek and K. Kokesova, J. Mater. Sd., 27 (1992) 2549. 4 W. D. Brown and W. W. Granneman, Solid-Stare Electron., 21 (1978) 837. 5 H. Cui, H. S. Shen, Y. M. Gao, K. Dwight and A. Wold, Mater. Res. Bull., 28 (1993) 195. 6 K. L. Hardee and A. J. Bard, J. Electrochem. Soc., 122 (1975) 739. 7 M. A. Butler and D. S. Gmniey, J. Mater. Sci., 15 (1980)19. 8 R. Jerisian, J. Gautron and J. P. Loup, J. Phvs. (Paris) III, 2 (1992) 679. 9 B. Samuneva, V. K. Zhukharov. Ch. Trapalis and R. Kranold. J. Mater. Sci., 28 (1993) 2353. 10 G. J. Exarhos and M. Aloi, Thin Solid Films, 193 (1990) 42. 11 G. Yi and M. Sayer, Ceram. Bull., 70(1991)1173. 12 W. G. Lai, K. L. Siefering and G. L. Griffin, in P. Vincenzini (ed(. High Performance Ceramic Films and Coatings, Eisevier, Amsterdam, 1991, p. 151. 13 H. Y. Lee and H. G. Kim, Thin Solid Films, 229 (1993) 187. 14 Y. Takahashi, H. Suzuki and M. Nasu, J. Chem. Soc., Faraday. Trans. 1, 81(1985) 3117. 15
W. W.
Xu. R. Kershaw, K. Dwight and A. Wold, Mater. Res. Bull..
16 25 M. (1990)1385. Langlet and J. C. Joubert, Chemistry of Advanced Materials, IUPAC, Blackwell, Oxford, 1992, p. 55. 17 R. C. Petersen, D. W. Matson and R. D. Smith, J. Am. Chem. Soc., 108 (1986) 2100. 18 J. I. Brand and D. R. Miller, Thin Solid Films, 166 (1988) 139. 19 B. N. Hansen, B. M. Hybertson, R. M. Barkiey and R. E. Sievers, Chem. Mater., 4 (1992) 749. 20 R. E. Sievers and B. N. Hansen, US Pat. 4,970,093, 1990. 21 K. Chhor, J. F. Bocquet and C. Pommier, Mater Chem. Ph vs.. 32 (1992) 249. 22 M. Barj, J. F. Bocquet, K. Chhor and C. Pommier. J. Mater., Sci., 27 (1992) 2187. 23 V. Gourinchas, K. Chhor, J. F. Bocquet and C. Pommier. unpublished results, 1993. 24 H. Komiyama, T. Kanai and H. Inoue, Chem. Lert., (1984)1283. 25 K. Okuyama, Y. Kousaka, N. Tohge, S. Yamamoto, J. J. Wu, R. C. Flagan and J. Seinfeid, AIChE J., 32 (1986) 2010. 26 K. Okuyama, J. T. Jeung, Y. Kousaka, H. V. Nguyen, J. J. Wu and R. C. Flagan, Chem. Eng. Sd., 44 (1989)1369. 27 G. J. M. Dormans, M. de Keijser, P. J. van \‘eldhoven. D. M. Frigo, J. E. Holewijn, G. P. M. van Mier and C. J. Smit, Chem. Mater.. 5 (1993) 448.