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Journal of Sol-Gel Science and Technology 8, 347–351 (1997) c 1997 Kluwer Academic Publishers. Manufactured in The Netherlands. °
Influence of the Deposition Parameters on the Characteristics of Aerosol-Gel Deposited Thin Films M. LANGLET AND C. VAUTEY Laboratoire des Mat´eriaux et du G´enie Physique, URA 1109 CNRS, ENSPG, BP46, 38402 Saint Martin d’H`eres, France
Abstract. The aerosol-gel process is a thin film deposition process based on the sol-gel polymerisation of a liquid film deposited from an ultrasonically sprayed aerosol. This process offers an attractive alternative for the deposition of sol-gel thin films. The effects of the aerosol deposition route on the film characteristics have been investigated with regard to sol-gel chemistry. TEOS solutions have been studied by viscosimetry and FTIR spectroscopy using an ATR device. Silica xerogel coatings have been studied by transmission FTIR and optical microscopy. Film morphology and uniformity depend closely on the aerosol deposition conditions. The film growth is controlled by a droplet coalescence surface phenomenon. Keywords: ultrasonic pulverization, aerosol, thin film deposition, silica, sol-gel chemistry
Introduction The aerosol-gel process is a new sol-gel deposition technique based on implementation of an aerosol obtained by ultrasonic pulverization of a sol [1–2]. It was previously shown that a large range of solutions and deposition conditions could be flexibly used for the deposition of homogeneous and dense SiO2 thin films [2]. However, in the course of our investigations some experimental conditions were also observed to lead to undesirable morphology features such as film roughness or local thickness inhomogeneities. As no information concerning such features were found in the sol-gel thin film literature, it is believed that these effects are specifically related to our deposition process. The aim of this paper is to examine to what extent the aerosol deposition route is affected by the sol-gel chemistry. Experimental TEOS-water-HCl-ethanol solutions were tested with regard to the water to TEOS ratio (rw ), TEOS concentration (C) and pH. The solution preparation and
deposition device were described elsewhere [2]. Deposition experiments were carried out using a horizontal deposition chamber. The whole system was regulated at a constant temperature. The ultrasonically sprayed aerosol was transported in the deposition chamber by an air flow previously bubbled in a thermostated ethanol bath. The aerosol coated the substrate (fixed parallel to the aerosol flow) leading to a liquid film. After a given deposition duration, the aerosol flow was cut off and the solvent was allowed to vaporize from the liquid film leading to the formation of a xerogel thin coating. Before deposition, the solution viscosity was systematically studied using the following procedure. An open bottle containing 100 cc of a solution was placed in a water bath thermally regulated at 60◦ C. A volume of 5 cc was regularly taken from the bottle, rapidly cooled down to room temperature and its viscosity was measured using a rotating cylinder viscosimeter. The sample was then poured back in the heated bottle. This procedure enabled comparison of the gelification kinetics of various solutions. The polymerization kinetics of the solutions was also studied by FTIR using an Attenuated Total Reflectance
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(ATR) device. A liquid drop from a given solution (about 0.01 cc in volume) was deposited on a high refractive index prism (ZnSe) and allowed to spread and vaporize. The IR beam propagated through the liquid by reflection at the liquid-crystal interface. Using this technique, it was possible to collect a FTIR spectrum at any time of the film transformation from the liquid state to the xerogel state. Xerogel thin coatings deposited on silicon wafers using the same procedure were also studied by transmission FTIR. Results and Discussion Figures 1(a) and (b) show the typical surface of two xerogel films deposited by aerosol-gel process. The film presented on Fig. 1(a) was deposited from a solution with rw = 2.2 and C = 0.4 mole/l. The film was ˚ thick and the surface was very smooth and ho2000 A mogeneous. The film shown on Fig. 1(b) was deposited from a solution with rw = 2.2 and C = 1.5 mole/1. In this case, the substrate was fully coated but the coating surface was affected by colored dots due to optical interference effects related to local thickness variations. The dots were roughly circular and more or less connected. An estimation of the local film thickness could be obtained from interference color observation under optical microscope assuming a refractive index value of 1.45 [2]. The clear dots shown on Fig. 1(b) corre˚ while the darker spond to a thickness of about 2000 A ˚ thick. The circular dot diameter areas are about 1500 A was about 500 µm. Figure 1(c) shows the typical aspect of the substrate surface at the very first stage of the deposition process (1 sec. deposition time). The substrate was coated with very small circular isolated dots whose diameter ranged from 10 microns or less to about 40 microns. If we assume that the observed dots are disk shaped, from the diameter D and thickness t of each dot we can calculate the diameter φ of the corresponding spherical liquid drop (before solvent evaporation) by φ = (1.5D 2 td/MC)1/3 where d and M are respectively the silicon oxide density (d = 2.6 g/cc) and molecular weight (M = 60 g) and C is the concentration of TEOS in the solution (C ≈ 1 mole/l). The dot thickness estimated from interference color observation ranged from 0.10 to 0.30
Figure 1. Typical surface for xerogel coatings ultrasonically deposited from a solution rw = 2.2, pH = 3.5 and C = 0.4 mole/l (a) or C = 1.5 mole/l (b) and for a coating at the very first stage of the ultrasonic deposition (c).
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Figure 2. Deposition time dependence of the film thickness for a solution rw = 2.2, pH = 3.5 and C = 1.5 mole/l (∆), C = 0.8 mole/l (◦), C = 0.4 mole/l (+) or C = 0.2 mole/l (¤). The lined area shows the limit conditions for homogeneous film deposition.
microns. From these values, the diameter of the liquid droplets corresponding to each dot was calculated in a range 10 microns or less to about 30 microns. This size distribution fits closely with the typical size distribution of the droplets constituting the aerosol. It is believed therefore that each circular dot observed at the first stage of the deposition process is the sol-gel reaction product of one individual droplet deposited on the substrate. Some more elongated dots also observed on the substrate surface consisted of the juxtaposition of two or more circular dots and suggest that the liquid film growth mechanism is controlled by a droplet coalescence phenomenon taking place at the substrate surface. The same kind of feature was observed for all the solutions examined in this study. The effect of thickness on the film behaviours was investigated using a solution rw = 2.2, pH = 3.5 and for various TEOS concentrations. The films were deposited at T = 20◦ C. The film thickness (measured after drying) was observed to grow linearly with the deposition time (Fig. 2). The thinnest films were systematically affected by very small dots. The dots size increased continuously with the film thickness from a minimal size corresponding to the impact of the droplets on the substrate up to a limit where fully homogeneous films could be obtained. These data support the assumption of a liquid film growth mechanism controlled by a surface coalescence phenomenon taking place after droplet deposition and before solvent evaporation and film polymerization. A dotted film is
likely to be obtained only if the droplets are not able to spread and to coalesce fully on the substrate surface before polymerization. Coalescence in sol-gel systems is usually described in terms of surface tension driven viscous flow mechanism [3]. High initial viscosity and fast polymerization rate solutions can therefore favour the formation of dotted films. Conversely, when the viscosity and polymerization rate are not too high, the liquid surface tension favours the surface coalescence of sufficiently close droplets leading progressively to the formation of a planar liquid layer while the film is growing. For the greatest thicknesses, the liquid films were observed to flow during drying leading to inhomogeneous xerogel coatings. It was therefore possible to define for each solution a thickness range where homogeneous layers could be deposited. In the case of the solution presented in Fig. 2, we have deposited homogeneous ˚ or less to films with a thickness ranging from 400 A ˚ depending on the TEOS concentration about 4000 A in the solution. Influence of the solution composition on the film morphology is summarized in Table 1 for films deposited at T = 25◦ C. The deposition duration was ad˚ justed to obtain a constant film thickness of 1500 A for the different solutions. Table 1 shows that not only the appearance of dots but also the size of these dots was strongly dependent on the solution viscosity and gelification rate: the higher the viscosity and polymerization rate (obtained from high rw ratio or high TEOS
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Table 1. Solution composition, gelification behaviours and ˚ thick xerogel thin films ultrafilm surface aspect for 1500 A sonically deposited at 25◦ C. Symbols 0, +, − refer respectively to the absence of dots or the presence of large (more than 500 µm in diameter), or small dots (a few hundred µm in diameter). rw /pH/CTEOS (mole/l)
Solution viscosity (cP)
Gel time at 60◦ C (hrs)
Film aspect
2.2/1.5/1.5
2.50
67
+
2.2/3.5/1.5
2.25
67
+
2.2/3.5/0.8
1.30
85
0
2.2/3.5/0.4
1.20
92
0
5.5/1.5/1.5
6.85
13
−
5.5/1.5/0.8
2.70
55
±
5.5/1.5/0.4
1.75
80
0
concentration solutions), the finer the dots. The dot diameter increased strongly when low viscosity and low polymerization rate solutions were used. The elimination of dots was achieved only for solutions with the lowest viscosity and lowest polymerization rate (low rw ratio or TEOS concentration) as it could be expected from a viscous flow controlled coalescence process. Figure 3 shows ATR and transmission FTIR spectra collected after five minutes polymerization on xerogel coatings deposited from 0.01 cc of a slow polymerization rate solution (rw = 2.2, pH = 3.5, C = 0.4 mole/l) (Figs. 3(a), (b)) and a fast polymerization rate solution (rw = 5.5, pH = 1.5, C = 1.5 mole/l) (Figs. 3(c), (d)). The Si-O region spectra (1300–500 cm−1 ) exhibit the typical features of a silica xerogel obtained from low water (Figs. 3(a), (b)) or high water (Figs. 3(c), (d)) content solutions [4]. In addition, both ATR and transmission spectra presented on Figs. 3(a), (b) show typical vibration bands of TEOS (triplet at 3000–2800 cm−1 and quintuplet at 1500–1300 cm−1 ) and a rather weak Si-OH vibration band (960 cm−1 ) which are consistent with an incomplete TEOS hydrolysis promoted by a low water content solution. Compared to the transmission spectrum, the higher TEOS content seen on ATR spectrum (Fig. 3(b)) indicates a lower hydrolysis level. The transmission spectrum presented on Fig. 3(c) shows very few residual organic species and exhibits intense Si-OH and Si-O-Si vibration bands which characterize a complete TEOS hydrolysis and the development of a stiff xerogel network related to the
high water content of the corresponding solution. In comparison, the ATR spectrum for the same coating (Fig. 3(d)) shows the presence of a significant amount of ethanol (3000–2800 cm−1 region and shoulder at 880 cm−1 ) and water (3600–3000 cm−1 region) and a higher content of residual silanol species (960 cm−1 ) indicating a less developed xerogel network. ATR and transmission FTIR spectra are respectively representative of the coating-substrate interface and of the whole coating thickness. From the different observations, it can thus be concluded that the polymerization reaction proceeds primarily at the air-liquid interface (presumably promoted by solvent evaporation) and that the polymerization of the deeper layers is delayed. In the case of a low rw ratio, the delay is not very significant, i.e., the slow polymerization proceeds rather homogeneously through the whole film thickness. In the case of a higher rw ratio, the formation of a xerogel surface network is observed. Because of the high rw value, this surface network is dense [1, 2] and is likely to hinder the water and ethanol escape from the deeper layers and to cause a significant delay in the polymerization of silanol species. It is believed that these observations can apply to the understanding of the droplet coalescence mechanism. In the case of slow polymerization rate solution, the solgel reaction proceeds slowly through the whole core of the droplet leading to a slow viscosity increase which does not hinder significantly the coalescence mechanism. In the case of a fast polymerization solution, the reaction proceeds quickly at the droplet surface leading to the formation of a stiff surface xerogel network. In this case, the liquid coalescence process is prematurely stopped. The resulting limited droplet growth causes the formation of finer dots as shown on Table 1 for fast polymerization rate solutions. Conclusion From this study, it appears clearly that the deposition conditions of homogeneous films by aerosol-gel process are closely related to the solution composition and deposition conditions. Film behaviours are related to a viscous flow controlled coalescence process taking place at the substrate surface after droplet deposition and before film polymerization. The best layers are obtained from low rw and low TEOS concentration solutions. Depending on the TEOS concentration in
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Figure 3. ATR and transmission FTIR spectra for a xerogel coating deposited from 0.01 cc of a solution rw = 2.2, pH = 3.5, C = 0.4 mole/l: transmission (a), ATR (b) and rw = 5.5, pH = 1.5, C = 1.5 mole/l: transmission (c), ATR (d).
the solution, it is possible to adjust the film thickness in a large range of values. Acknowledgments The present research was performed under project number BE-7765 sponsored by the Brite Euram Programme 2.
References 1. M. Langlet, D. Walz, P. Marage, and J.C. Joubert, Thin Solid Films 221, 44 (1992). 2. P. Marage, M. Langlet, and J.C. Joubert, Thin Solid Films 238, 218 (1994). 3. C.J. Brinker and G.W. Scherer, Sol-Gel Science (Academic Press, San Diego, 1990), pp. 676–683. 4. T.M. Parill, J. Mat. Res. 7(8), 2230 (1992).
