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Mar 28, 2007 - ... et de Physico-Chimie des Matériaux et des Interfaces (LEPMI) and Science et Ingénierie, MAtériaux, Procédés (SIMAP), ENSEEG-IN...
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5708

J. Phys. Chem. C 2007, 111, 5708-5714

Elaboration of Ta2O5 Thin Films Using Electrostatic Spray Deposition for Microelectronic Applications A. Lintanf,†,‡ A. Mantoux,‡ E. Blanquet,‡ and E. Djurado† Laboratoire d’Electrochimie et de Physico-Chimie des Mate´ riaux et des Interfaces (LEPMI) and Science et Inge´ nierie, MAte´ riaux, Proce´ de´ s (SIMAP), ENSEEG-INPG/UJF.CNRS, Domaine UniVersitaire, BP 75, 1130 rue de la Piscine, 38402 St Martin d’Heres Cedex, France ReceiVed: NoVember 17, 2006; In Final Form: February 27, 2007

The deposition of tantalum oxide thin films on SiOxNy/Si substrates using electrostatic spray deposition was investigated. Different microstructures (dense, reticulated, porous) depending on the process parameters such as the nature of the precursor solution, substrate temperature, nozzle-to-substrate distance, and precursor solution flow rate were obtained. The evolution of film thickness and morphology versus deposition time was discussed. The films deposited at temperatures ranging from 100 to 210 °C were found amorphous. A pure pseudo hexagonal δ-Ta2O5 single phase was identified by X-ray diffraction and Raman spectrocopy after annealing from 650 °C and above.

1. Introduction The evolution of many technologies toward nanometric scale engages new challenges in materials science in terms of elaboration and characterization. In microelectronic, the dimensions and architectures of devices, such as metal oxide semiconductor (MOS) transistors, metal insulator metal capacitors, and interconnexions in air gap technology, at low temperature (e 400 °C) lead to elaborate ultrathin layers with the need of excellent conformity (i.e., a constant thickness for whatever is the topography). Thin films of a large variety of transition metal oxides (TiO2, HfO2, Ta2O5, WO3) or nitrides (TiN, AlN, TaN) are usually prepared by conventional chemical vapor deposition (CVD), low-pressure CVD,1 metal organic CVD,2 derived laser-pulsed deposition,3 and atomic layer deposition4 but also by sol gel method, radio frequency sputtering,5 laser ablation,6 and others. Actually, TaN and TiN, for example, are used as a diffusion gate between copper and dielectrics or in damascene technology. Ta2O5, like TiO2, is considered as a dielectric for applications as the gate oxide of Si or SiC devices.7 Among these quite high dielectric constant (high K) materials, which can replace silicon dioxide as gate dielectric and MOS capacitors, Ta2O5 seems to be of considerable interest due to its dielectric constant generally being greater than 20. Moreover this oxide can be deposited at low temperature using conventional methods.8 The present work is focused on the deposition of Ta2O5 thin films on Si/SiOxNy with control of microstructure and thickness. The objective in this study is mainly to obtain rather dense and very thin homogeneous layers. A novel technique called electrostatic spray deposition (ESD) has been chosen for its very good suitability for a good control of film microstructures. This method has been successfully tested on many simple and multiple oxides9-12 and it has good reproducibility, homogene* To whom correspondence should be addressed. E-mail address: [email protected]. Phone: +33-4-7682-6684. Fax: +33-47682-6777. † Laboratoire d’Electrochimie et de Physico-Chimie des Mate ´ riaux et des Interfaces (LEPMI). ‡ Science et Inge ´ nierie, MAte´riaux, Proce´de´s (SIMAP).

ity, and adhesion. The principle consists in the formation of an aerosol by the application of an electrostatic field between a nozzle, containing a precursor solution, and a heated substrate. The outward electrostatic pressure of the solution, which forms the spray, is caused by the induced surface charge.13 This pressure is opposite to the inward-directed pressure caused by surface tension, and consequently surface instabilities are generated. Taylor or multijet cones are formed, according to the applied voltage and the solution conductivity. The microstructure of the films is controlled by both physical and chemical process parameters, as reported in previous studies11-13 This work is focused on the investigation of the influence of the precursor solution, the substrate temperature, the nozzle-tosubstrate distance, the precursor solution flow rate, and the deposition time on the ESD-deposited Ta2O5 film microstructure. 2. Experimental Section The tantalum oxide thin films were deposited on Si (100) wafers passivated by a SiOxNy layer, using the vertical ESD setup described previously.13 The ESD process consists of the formation of an aerosol from the atomization of a precursor solution by application of an electrical field. The resultant spray is used for film deposition on a heated substrate. Previously, the substrates were cleaned by dipping them in a sulfuric acid/sodium peroxydisulfate [H2SO4/Na2S2O8] (0.5 M) solution for 15 min and then were washed several times with deionized water. They were finally dried two times in 2-propanol vapors.14 Tantalum ethoxide Ta(OC2H5)5 (Strems Chemical, 99.99+% Ta) was used as the precursor salt for all experiments. This precursor is a solid at a temperature lower than 21 °C and a liquid above. The precursor solutions were prepared by dissolving the precursor in either ethanol (EtOH) (C2H5OH, Prolabo, 99%) or a mixture of diethylene glycol monobutyl ether (often called butyl carbitol (CH3(CH2)3(OC2H2)2OH, Acros Organics, 99+%) referred to as BC. The total concentration of the salt in the solution was varied from 0.00625 to 0.025 M. Solution conductivities were measured with a CDRV 62 conductimeter.

10.1021/jp0676585 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/28/2007

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Figure 2. TG/DTA analyses of Ta(OC2H5)5 precursor (heating rate: 10 °C/min).

Figure 1. SEM micrographs of ESD films starting from a 0.025 M solution of Ta(OC2H5)5 containing (a) 100 vol % EtOH and (b) 33 vol % EtOH and 67 vol % BC. Deposition time: 60 min. Substrate temperature: 100 °C. Nozzle-to-substrate distance: 20 mm. Flow rate: 0.49 mL/h.

A syringe pump (Sage M361) was used to control the ESD solution flow rate in the range of 0.49 to 1 mL/h. The precursor solution was atomized using a positive high voltage of 4 to 6 kV, connected to a stainless steel needle (0.6 mm in diameter, bevel in shape). The high tension was adjusted to keep the conejet stable. The substrate temperature was in the range from 100 to 210 °C. The nozzle-to-substrate distance was selected to be 20 and 30 mm. The deposition time was varied from 5 to 60 min. Thermogravimetric/differential thermal analyses (TG/DTA) were performed in Ar (75 mL/min) under atmospheric pressure at a 10 °C/min heating rate from 20 to 350 °C using a Netzsch simultaneous thermal analyzer STA 409 instrument. Surface morphologies and cross sections were analyzed using scanning electron microscopy (SEM) (LEO S440) and high-resolution field emission SEM (FE-SEM) (ZEISS ULTRA 55). X-ray diffraction was carried out using a Philips θ/2 θ diffractometer in the Bragg Brentano geometry (10 to 65° 2θ range, 0.04° 2θ step, 5 s counting time) with Cu anticathode (λ ) 0.15406 nm). Phases were identified using Philips X’Pert HighScore software. Transmission electron microscopy (TEM) samples were prepared by cross-sectional methods. A microscope (JEOL 3010), operating at 300 keV with a ponctual resolution of 0.25 nm, was used for the TEM imaging. Raman spectroscopy was used to determine the temperature of crystallization of Ta2O5 film (Renishaw InVia). The excitation wavelength was 514.53 nm (green laser Argon, Ar +). The laser power was reduced to 2.5 W to avoid the film deterioration. The exposition time was 100 s. The wavenumber domain was in the range from 100 to 3800 cm-1. 3. Results and Discussion In a first part, the microstructure of Ta2O5 coatings was investigated by varying four ESD parameters, such as the precursor solution, the substrate temperature, the nozzle-tosubstrate distance, and the precursor solution flow rate. The

second part is focused on the influence of the deposition time on the film growth in terms of microstructure and thickness. A correlation between the process parameters was discussed on the basis of these experimental data. 3.1. Influence of the Precursor Solution. Two precursor solutions based on Ta(OC2H5)5 salt dissolved in (i) EtOH and (ii) a mixture of EtOH/BC, 33:67 vol %, were used to show the influence of the solvent composition on the microstructure. We have selected the following ESD parameters: 100 °C as the substrate temperature, 20 mm as the nozzle-to-substrate distance, 0.49 mL/h as the precursor solution flow rate, and 60 min as the deposition time. SEM morphology of Ta2O5 starting from EtOH solution is shown in Figure 1a. A porous microstructure was observed. It can be related to the physicochemical properties of the precursor solution (i.e., boiling point, conductivity, and solution concentration). Indeed, at 100 °C the boiling of EtOH has occurred. The particles that impact the substrate are almost dried and cannot spread due to the absence of the liquid phase. A very slight evaporation of Ta(OC2H5)5 might occur at 100 °C, as shown by an endothermic band in the DTA curve (Figure 2), but we did not detect any significant weight loss by TGA at atmospheric pressure (Figure 2). The vapor pressure of Ta(OC2H5)5 at 100 °C is certainly far below 1 atm.15 The complete evaporation of the Ta(OC2H5)5 precursor was found at around 260 °C as reported by Koyama et al.15 and is at the origin of an endothermic peak with a weight loss of about 90% around this higher temperature. Figure 2 shows simultaneously an endothermic peak and a weight loss in the TGA and DTA curves,respectively. This microstructure as “cauliflower” is favored by the preferential landing, a typical phenomenon previously described by Chen et al., which takes place when charged dried particles are attracted on the top of the first particles that have already impacted the substrate.13 A dense morphology was obtained when BC was added (EtOH/BC, 33:67 vol %) to the previous precursor solution (Figure 1b). On one hand, BC is characterized by a much higher boiling point (231 °C) compared to EtOH (78 °C). On the other hand, the presence of EtOH in the complete solution results in a decrease of the boiling point of BC from 231 to about 195 °C (Figure 3). Consequently, the droplets based on EtOH/BC contain a larger quantity of liquid when impacting the substrate heated at 100 °C because the solvents are then not fully evaporated. Moreover, when BC was used the conductivity σ of the solution was decreased from 72 to 1.21 µS cm-1. With the surface tension γ and the density F being equal to 0.022 N m-1 and 0.789 g cm-3 for EtOH and 0.030 N m-1 and 0.955 g cm-3

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Figure 3. TG/DTA analyses of the precursor solution: Ta(OC2H5)5 dissolved in EtOH/BC, 33:67, vol %. Concentration: 0.025 M. (Heating rate: 10 °C/min.)

for BC, respectively, we can estimate an increase of the droplets’ size of about 1500% when BC is used, following the Kelvin’s relationship (eq 1)

x 3

d≈

γ02

(1)

σ2F

where 0 is the electrical permittivity of vacuum (8.85 × 10-12 F/m). The concentration of the precursor solution was decreased from 0.025, 0.0125, to 0.00625 M to investigate the influence of the precursor solution concentration on the film microstructure. The substrate temperature, nozzle-to-substrate distance, solution flow rate, and deposition time were fixed at 100 °C, 20 mm, 0.49 mL/h, and 60 min, respectively. When the concentration was decreased, the spreading of the droplets increased slightly (Figure 4a,b) until cracks disappeared for the lowest concentration (Figure 4c). Indeed, the droplets incoming on the substrate were larger when the solution conductivity was decreased from 1.21 µS cm-1 for the more concentrated solution (0.025 M) to 1.10 and 0.86 µS cm-1 for the other ones (0.0125 and 0.00625 M). These observations are explained by the relationships between the conductivity σ and the diameter d of droplets (eq 2). For constant solution flow rate, Q, and electrical permittivity, r, the droplet size is only dependent on the precursor solution conductivity

d ≈ r1/6 ×

(Qσ )

1/3

(2)

However, larger droplets lead to fluid accumulation on the substrate. When the precursor solution concentration is decreased from 0.025 to 0.00625 M, a larger density of matter is present in the droplets leading to a lower-coating growth with all other ESD parameters being constant. The film thickness, measured by FE-SEM, was decreased from 280 to 100 nm. Consequently, the cracking of the films is certainly caused by the thermal expansion coefficient mismatch between SiO2 (0.5 × 10-6/K) and Ta2O5 (3.6 × 10-6/K)with the latter being more pronounced in thicker films. Actually, cracks disappeared for the lower concentration due to the deposition of a thinner layer on the substrate. Consequently, the spreading can be enhanced starting from the lower concentration of the precursor solution, the objective being to obtain dense and thin, crack-free films. 3.2. Influence of the Substrate Temperature. This part describes the effect of the substrate temperature on the morphology of Ta2O5 films prepared by ESD in the range from 100 to 210 °C for two nozzle-to-substrate distances of 20 and 30 mm

Figure 4. SEM micrographs of ESD films from Ta(OC2H5)5 dissolved in EtOH/BC, 33:67, vol % with different concentrations. Substrate temperature: 100 °C. (a) 0.025 M; (b) 0.0125 M; (c) 0.00625 M. Flow rate: 0.49 mL/h. Nozzle-to-substrate distance: 20 mm. Deposition time: 60 min.

for 60 min and 0.49 mL/h flow rate starting from a solution containing Ta(OC2H5)5 in EtOH/BC, 33:67, vol %, 0.025 M in concentration. From 100 °C (Figure 5a) to 125 °C (Figure 5b), we observed a dense film with cracks. At this low temperature, a large quantity of BC was still present in the droplets. Indeed, Figure 3 shows two endothermic peaks corresponding to the complete evaporation of EtOH and BC at 70 °C and 195 °C, respectively. The lower the temperature, the larger the liquid amount is present due to the low evaporation rate of solvent during the transport of the droplets. Consequently, a large volume change is expected due to the presence of solvent excess at the end of the coating. This fast drying step11 leads to the development of stresses because the coating is not able to shrink freely due to the adhesion to the substrate. The drying stresses disappeared when the substrate temperature was increased up to 150 °C (Figure 5c) due to the evaporation of a larger part of the solution. At around 175 °C (Figure 5d), the formation of a reticulated morphology was observed. This microstructure is probably due to the simultaneous boiling and drying of the precursor solution as shown by thermal analyses (Figure 3). This phenomenon occurs when the substrate temperature and the solution boiling point are closed.11 Above 175 °C and up to 200 °C (Figure 5e), the morphology was changed. For a higher temperature, the quantity of liquid in the droplet was decreased and the boiling and drying steps did not occur according to Princivalle et al.,11 certainly due to the deficiency of the liquid for boiling. When the substrate temperature was increased, this phenomenon was exacerbated. Indeed, at 210 °C the deposition of dried particles was observed (Figure 5f) because the solution was completely evaporated. As far as the process continues, the

Ta2O5 Thin Films Using ESD

Figure 5. SEM micrographs of ESD films at different substrate temperatures: (a) 100 °C, (b) 125 °C, (c) 150 °C, (d) 175 °C, (e) 200 °C, and (f) 210 °C. Solution: Ta(OC2H5)5 in EtOH/BC, 33:67, vol %. Concentration: 0.025 M. Flow rate: 0.49 mL/h. Nozzle-to-substrate distance: 20 mm. Deposition time: 60 min.

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Figure 6. SEM micrographs of ESD films at different substrate temperatures: (a) 100 °C, (b) 125 °C, (c) 150 °C, (d) 175 °C, (e) 190 °C, and (f) 200 °C. Solution: Ta(OC2H5)5 in EtOH/BC, 33:67, vol %. Concentration: 0.025 M. Flow rate: 0.49 mL/h. Nozzle-to-substrate distance: 30 mm. Deposition time: 60 min.

incoming solid particles and droplets preferentially land on the top of already existing solid particles, because the latter ones act as concentrators of the electrical field lines at the surface of the substrate.16 These dried particles were connected in a tridimensional porous microstructure (Figure 5f) due to this preferential landing. Consequently, an increased roughness was observed. A schematic temperature dependence representation of the evolution of film morphology is shown as the following:

3.3. Influence of the Nozzle-to-Substrate Distance. In this part, the evolution of the film microstructure is discussed as a function of the substrate temperature for two nozzle-to-substrate distances of 20 and 30 mm. Other parameters such as deposition time and flow rate were fixed to 60 min and 0.49 mL/h, respectively. The solution consists of Ta(OC2H5)5 in EtOH/BC, 33:67, vol %, 0.025 M in concentration. For the various nozzle-to-substrate distances, we observed the same evolution of the morphology as the previous one versus substrate temperature (Figure 5 and 6). Indeed, films were found dense and cracked for the smallest nozzle-to-substrate distance and at low substrate temperatures from 100 to 125 °C (Figure 5a,b). When the nozzle-to-substrate distance was increased from 20 to 30 mm at 125 °C, these cracks disappeared (Figure 6b) in comparison to Figure 5b. This microstructural evolution toward a crack-free coating was the signature of the presence of drier particles that resulted from either a longer flight from the nozzle toward the substrate (Figure 6b) or a higher substrate temperature (Figure 5c). When the substrate temperature was increased up to 150 °C (Figure 6c) for 30 mm, we observed a reticulated morphology that was observed previously at 175 °C

Figure 7. SEM micrographs of ESD films at different precursor solution flow rates: (a) 0.49 mL/h; (b) 1 mL/h. Solution: Ta(OC2H5)5 in EtOH/BC, 33:67, vol %. Concentration: 0.025 M. Substrate temperature: 200 °C. Nozzle-to-substrate distance: 30 mm. Deposition time: 60 min.

for 20 mm (Figure 5d). It is clear that when the nozzle-tosubstrate distance is increased, it acts in the same way as the substrate temperature increase on the evaporation of the droplet. At 175 °C and 30 mm (Figure 6d), the reticulation was also reduced compared to the film obtained at 20 mm (Figure 5d). This was mainly due to a smaller quantity of liquid impacting the heated substrate. At substrate temperatures such as 190 and 200 °C (Figure 6e,f) for 30 mm, we retrieved the porous microstructure previously found only at 200 and 210 °C (Figure

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Figure 8. FE-SEM (surfaces (a-d) and cross sections (e-h), micrographs of Ta2O5 films deposited during (a,e) 5 min, (b,f) 15 min, (c,g) 30 min, and (d,h) 60 min. Substrate temperature: 150 °C. Nozzleto-substrate distance: 20 mm. Solution: Ta(OC2H5)5 in EtOH/BC, 33:67, vol %. Concentration: 0.025 M. Flow rate: 0.49 mL/h.

Figure 9. Evolution of Ta2O5 film thickness as a function of deposition time

5e,f) for 20 mm. Thus, in conclusion we observed a clear shift of the morphology evolution (dense with cracks, dense, reticulated, granular), toward the lowest substrate temperatures, when the nozzle-to-substrate distance was increased because the two parameters were correlated.12 3.4. Influence of the Precursor Solution Flow Rate. Films were prepared starting from the previous precursor solution (Ta(OC2H5)5 in EtOH/BC, 33:67, vol %, 0.025 M in concentra-

Lintanf et al.

Figure 10. TEM micrographs of a film deposited during 5 min (a) as-deposited and (b,c) annealed at 800 °C. (b) Bright field. (c) Dark field. Substrate temperature: 150 °C. Solution: Ta(OC2H5)5 in EtOH/ BC, 33:67, vol %. Concentration: 0.025 M. Flow rate: 0.49 mL/h. Nozzle-to-substrate distance: 20 mm.

tion) with two different flow rates (0.49 and 1 mL/h). The substrate temperature, nozzle-to-substrate distance and deposition time were fixed at 200 °C, 30 mm, and 60 min, respectively. When the precursor solution flow rate was increased from 0.49 mL/h (Figure 7a) to 1 mL/h (Figure 7b), we observed some cracks in the film. The quantity of liquid that reaches the substrate is larger at a certain time. In addition, the diameter of droplets is also related to the precursor solution flow rate as described in eq 2. It follows that the larger the flow rate, the larger the droplet diameter. In our case, the solution flow rate was doubled, and the droplet size was increased by about 125%. Consequently, in these conditions the accumulation of liquid injected to the substrate surface was too large and mechanical stresses were generated by the drying of the coating giving rise to cracked films.17 As demonstrated in this work, the different ESD process parameters, such as substrate temperature, nozzle-to-substrate distance, and precursor solution flow rate are dependent parameters. Indeed, to obtain the same microstructure (e.g., dense and crack-free coating) it is necessary to decrease the substrate temperature or to decrease the nozzle-to-substrate distance or even to increase the precursor solution flow rate just enough to avoid the formation of cracks. All these actions

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Figure 11. XRD patterns of Ta2O5/substrate (a) as-deposited and annealed at (b) 600, (c) 650, and (d) 700 °C. Pseudohexagonal system.

result in a larger spreading of droplets impacting the heated substrate. A coherent picture of the process comes up with the temperature correlation with the nozzle-to-substrate distance and the precursor solution flow rate. These results agree well with data previously given by Neagu et al. in the case of YSZ coatings.17 3.5. Influence of the Deposition Time. The evolution of microstructure and thickness was studied as a function of deposition time in the range from 5 to 60 min starting from the atomization of the previous solution (Ta(OC2H5)5 in EtOH/BC, 33:67, vol %, 0.025 M in concentration) at 150 °C, with 0.49 mL/h flow rate and at a nozzle-to-substrate distance of 20 mm. The surface and cross-section morphologies were examined by FE-SEM (Figure 8). In these experimental conditions, as previously shown, a dense microstructure was obtained (Figure 5c). After 5 min (Figure 8a), a spreading of the droplets already occurred on the substrate. The splats of individual droplets were detected with the formation of disks with a crust on the border. This phenomenon was due to a fast spreading of the droplets followed by an evaporation of the solvents, especially on the border, where the quantity of liquid had decreased as shown by Neagu et al.17 When the deposition time was increased up to 15 min, we observed a linear growth with time because no morphological change was observed. From our microscopic observations (Figure 8a,b), we observed a complete recovering still leading to a dense coating. After 30 min (Figure 8c), a reticulated net was progressively developed. This morphology can be the consequence of simultaneous boiling and drying of the precursor solution that is possible if (i) the substrate surface temperature is close to the boiling point of the precursor solution and (ii) the quantity of solution is sufficient for boiling11,18 After 60 min, arriving liquid droplets in excess penetrated into formed pores in the sublayer and consequently densified the previous porous microstructure leading to a dense film. The reticulation disappeared (Figure 8d) and a decrease of film growth was observed (Figure 9). The reticulated sublayer was mainly recovered by the liquid droplets penetrating inside the net. The nonlinear growth behavior with time is certainly caused by a time-dependent morphological and density evolution of ESD coatings (Figure 8a-d) with all other ESD parameters being constant. We can distinguish two linear domains: the first one from 5 to 15 min, corresponding to the dense films obtained in Figure 8a,b, is characterized by a correlation parameter equal to 12; the second one from 30 to 60 min, corresponding to the disappearance of dense morphology and the development of a

Figure 12. Raman spectroscopy of (a) the substrate heated at 150 °C, (b) the as-deposited film, and annealed during 2 h in O2 at (c) 600 °C and (d) 700 °C.

reticular net (Figure 8c), leads to a slope modification with a correlation parameter equal to about 7. Then an interpenetration of this reticulated net occurred after 60 min, leading again to a recovering and to a dense film. This morphological evolution is probably at the origin of a nonlinear growth behavior. The thickness and the quality of the film deposited during the first 5 min as-deposited and after annealing at 800 °C for 2 h in air were reported by TEM cross-section films (Figure 10). A decrease of Ta2O5 thickness from 60 nm (Figure 10a) to 40 nm (Figure 10b) was observed after thermal treatment due to sintering step and crystallization of the film as confirmed by following X-ray diffration (XRD) and Raman spectroscopies. The grain size after annealing was characterized by highresolution TEM in dark field and was about 100 nm (Figure 10c). The as-deposited films were found amorphous after 60 min. Crystallization occurred after a thermal treatment at about 650 °C for 2 h in air and a pure δ-Ta2O5 pseudo hexagonal phase was clearly detected by XRD (Figure 11). Raman spectroscopy measurements (Figure 12) confirmed the crystallization temperature at about 700 °C. Indeed the Ta2O5 characteristic bands at 250, 500, 625, 725, and 850 cm-1 were detected.19 4. Conclusion We have successfully deposited Ta2O5 films starting from Ta(OC2H5)5 precursor with three different microstructures

5714 J. Phys. Chem. C, Vol. 111, No. 15, 2007 (dense, reticulated, porous) optimizing ESD process parameters such as precursor solution, substrate temperature, nozzle-tosubstrate distance, precursor solution flow rate, and deposition time. To conclude, the morphology of the coating is strongly dependent on the droplet size and physical properties of the incoming droplets, such as boiling point and spreading behavior on the substrate. A coherent correlation between substrate temperature, nozzle-to-substrate distance, and precursor solution flow rate was confirmed. The optimal conditions to obtain dense, free-cracks Ta2O5 films were reached for large droplets and large spreading. This microstructure was successfully obtained when a mixture of EtOH and BC was used as solvent with the latter being characterized by a low evaporation rate and a low concentration such as 0.00625 M. This desirable spreading is also favored at 150 °C, a low substrate temperature, at 20 mm for a short nozzle-to-substrate distance and for a low 0.49 mL/h as precursor solution flow rate to avoid an excess of liquid on the substrate and to get free-cracks coatings. For MOS applications, 5 min deposition time will be selected to obtain thin films. Deposited films were found amorphous and crystalline in the pseudohexagonal system after thermal treatment. Moreover, a nonlinear crystalline growth was underlined. The deposition time must be decreased to 5 min to obtain very thin continuous films. The role of microstructural properties and thickness of tantalum oxide films on electrical performances is in progress. Acknowledgment. The authors acknowledge assistance from P. Donnadieu and R. Martin on TEM and SEM, S. Brice-Profeta on DTA, and N. Sergent on Raman spectroscopy.

Lintanf et al. References and Notes (1) Briand, D.; Mondin, G.; Jenny, S.; Van der wal, P. D.; Jeanneret, S.; De Rooij, N. F.; Banakh, O.; Keppner, H. Thin Solid Films 2005, 493, 6-12. (2) Chiu, H. T.; Wang, C. N.; Chuang, S. H. Chem. Vap. Deposition 2000, 6, 223-225. (3) Huang, C. J. Thin Solid Films 2005, 478, 332-337. (4) Ale´n, P.; Vehkama¨ki, M.; Ritala, M.; Leskela¨, M. J. Electrochem. Soc. 2006, 153, G304-G308. (5) Gruger, H.; Kunath, Ch.; Kurth, E.; Sorge, S.; Pufe, W.; Pechstein, T. Thin Solid Films 2004, 447-448, 509-515. (6) Chen, M.; Wang, X.; Zhang, L.; Yu, M.; Qin, Q. Chem. Phys. 1999, 242, 81-90. (7) Holloway, K.; Fryer, P. M. Appl. Phys. Lett. 1990, 57, 1736-1738. (8) Chaneliere, C.; Autran, J. L.; Devine, R. A. B.; Balland, B. Mater. Sci. Eng. 1998, R22 (6), 269-322. (9) Nguyen, T.; Djurado, E. Solid State Ionics 2001, 138, 191-197. (10) Taniguchi, I.; van Landschoot, R. C.; Schooman, J. Solid State Ionics 2003, 160, 271-279. (11) Princivalle, A.; Perednis, D.; Neagu, R.; Djurado, E. Chem. Mater. 2005, 17, 1220-1227. (12) Neagu, R.; Perednis, D.; Princivalle, A.; Djurado, E. Surf. Coat. Technol. 2006, 200, 6815-6820. (13) Lintanf, A.; Neagu, R.; Djurado, E. Solid State Ionics 2007, 177, 3491-3499. (14) Semaltianos, N. G.; Pastol, J. L.; Doppelt, P. Surf. Sci. 2004, 562, 157-169. (15) Koyoma, H.; Tanimoto, S.; Kuroiwa, K.; Tarui, Y. J. Appl. Phys. 1994, 33, 6291-6298. (16) Neagu, R.; Perednis, D.; Princivalle, A.; Djurado, E. Solid State Ionics 2006, 177, 1451-1460. (17) Neagu, R.; Perednis, D.; Princivalle, A.; Djurado, E. Chem. Mater. 2005, 17, 902-910. (18) Princivalle, A.; Perednis, D.; Neagu, R.; Djurado, E. Chem. Mater. 2004, 16, 3733-3739. (19) Chan, H. Y. H.; Taloudis, C. G.; Weaver, M. J. J. Am. Chem. Soc. 1999, 121, 9219-9220.