and P. ane el la'. Dipartimento di Chimica Inorganica, Metallorganica ed Analitica, Centro di Studio sulla Stabilita e Reattivita dei Composti di Coordinazione del ...
J. Phys. IVFrance 11 (2001) O EDP Sciences, Les Ulis
AI2O3grovvfh optimisation using aluminium dimethylisopropoxide as precursor as a function of reaction conditions and reacting gases D. Barreca, G.A. and P. ane el la'
att ti st on', G. carta1, R. ~erbasi',G. ~ossettol,E. Tondello
Dipartimento di Chimica Inorganica, Metallorganica ed Analitica, Centro di Studio sulla Stabilita e Reattivita dei Composti di Coordinazione del CNR, Universita di Padova, Via Marzolo 1, 35131 Padova, ltaly lstitufo di Chimica e Tecnologie lnorganiche e dei Materiali Avanzati del CNR, Corso Stati Uniti 4, 35127 Padova, ltaly Abstract. Aluminum oxide films were grown in a hot-wall low-pressure metal organic chemical vapor deposition (MOCVD) system using aluminum dimethylisopropoxide as precursor. Experimental reaction conditions and the reacting gas (02, H20, N20) have been systematically varied with the aim to decrease the deposition temperature and obtain transparent, dense and carbon-fiee films. Changes in the gas phase composition were studied by FT-IR spectroscopy using an in-line cell. The reactor temperature ranged fiom 230 "C to 380 "C. The microstructure of the films was investigated by X-ray Difiaction, while the surface chemical composition was studied by X-ray Photoelectron. Atomic Force Microscopy was employed to analyze the surface morphology of the films as a function of reaction conditions and reacting gases. The best performances have been obtained using dry oxygen at 1000 Pa and oxygen mixed with water vapor at 100 Pa. High growth rates such as 140 nm min" have been obtained at 270 OC in the latter case. Different reaction mechanisms have been proposed in the two cases.
1. INTRODUCTION Aluminium oxide films are widely used in a variety of applications in ceramics, catalysis, integrated circuit manufacture [I-21 thanks to their excellent electrical properties as insulating layers and their superior chemical stability and mechanical hardness. Chemical Vapor Deposition (CVD) of alumina thin films has been carried out by using various derivatives of aluminium as precursors. However the use of a highly volatile non-pyrophoric liquid precursor is advisable to make the whole process more controllable and safer. In this context excellent results were obtained using aluminium dimethylisopropoxide, a nonpyrophoric liquid compound which gives transparent, stoichiometric and contamination-free films as reported in a recent work [3]. The only drawback was the relatively high deposition temperature range (540-600°C), which is undesirable for thermally sensitive substrates or for large scale applications. The aim of the present paper is to investigate the best experimental conditions to decrease the film growth temperature, without signilicantly altering the good qualities of the previously obtained coatings.
2. EXPERIMENTAL Aluminium dimethylisopropoxide was synthesised starting from Al(01C3H7)3and Al(CH3)3(Aldrich), molar ratio 1:2 [4]. All solvents were dried and distilled under nitrogen before use. All manipulations were carried out in a dry-box.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jp4:2001368
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The depositions were achieved in a low-pressure hot wall reactor described elsewhere [5], equipped with an in-line FT-IR spectrometer to monitor 'exhaust gases. The reactor was provided with a Pyrex pipe, inner diameter 4.8 cm, 31 cm long; 20 sccm nitrogen was employed as carrier gas; 50 sccm 02, 150 sccm O2,50 sccm N20, or 200 sccm H20plus 50 sccm 0 2 were in turn used as reactant gas; the total pressure was varied in the range 100 - 1000 Pa. Alumina film thickness (200-1200 nrn) and refractive index were estimated by interference f i g e analysis as reported elsewhere [6]. Soda-lime glass substrates were cleaned prior to the introduction into the reactor; they were immersed in soaped water, washed with distilled water, rinsed in acetone and dried in air. The FT-IR data were collected by an AT1 Mattson Genesis Series instrument with an MCT detector used in the range 3200- 400 cm-I with a 4 cm-' resolution. XRD patterns were recorded using a Philips PW 1830 &actometer employing a copper anode Xray tube at 40 kV and 30 mA. XPS analyses were carried out in a Perkin Elmer @ 5600ci spectrometer, using a standard AlKa radiation (1486.6 eV), with a working pressure ~ 1 . 8 ~ 1 0mbar. - 9 The spectrometer was calibrated by assuming the binding energy (BE) of the Au 4f7j2 line at 84.0 eV with respect to the Fermi level. Charging effects (=6 eV) were corrected assigning to the Cls signal of adventitious carbon the BE value of 284.8 eV [7]. After a Shirley-type background subtraction, the raw spectra were fitted using a nonlinear least-square fitting program adopting Gaussian-Lorentzian shapes for all the peaks. The atomic compositions were evaluated using sensitivity factors as provided by (lj V5.4A software. Sputtering + beam at 3.0 kV, 0.4 p~.cm-2beam current density with an treatments were carried out by an ~ r ion argon partial pressure of 4x10-* mbar. AFM images were taken using a Park Autoprobe CP instrument operating in contact mode in air. The background was subtracted from the images using the ProScan 1.3 software from Park Scientific. 3. RESULTS AND DISCUSSION
3.1 Precursor decomposition The aim of decreasing the precursor decomposition temperatures was pursued by performing controlled variations of fundamental parameters as total pressure, supplementary oxygen sources such as NzO or H20, and eventually their combined effect. Preliminary results are sumrnarised in fig. 1,
temperature ("C)
Figure 1. Precursor consumption measured by FT-IR in line at the end of the reactor as a function of reactor temperature: a) 50 sccm O2 or 50 sccm N20, 100 Pa; b) 50 sccm 02, 500 Pa; c) 150 sccm 02, 1000 Pa; d) 200 sccm HzO and 50 sccm 02, 100 Pa
where the precursor decomposition trends are plotted against the reactor temperature, on the basis of
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the inbred precursor band intensities collected by the FT-IR apparatus in line to the reactor. At low temperatures the curves are almost constant, indicating that no precursor consumption occurs. As the temperature is increased, there is a different range for each curve, in which the precursor concentration is decreasing but is not completely annulled: this is the range more useful for the MOCVD procedure. Finally, each curve approaches zero, meaning that all the precursor is consumed along the reactor but surely not in a uniform way: this is not a useful situation for MOCVD-based uniform coatings. Some combinations resulted ineffective in order to decrease the deposition temperature, as it can be observed fiom the curve obtained using 50 sccm N2O at 100 Pa (fig. 1, line a): in fact, this curve resulted perfectly coincident to the one obtained in ref. 5 using 50 sccm 0 2 as reactant gas. The curve obtained with 50 sccm 0 2 at 500 Pa (fig. 1, line b) decreased the decomposition temperature, but a more pronounced effect resulted fiom the use of 150 sccm 0 2 at 1000 Pa (fig. 1, line c). The greatest lowering of the deposition temperature was achieved by the use of 200 sccm Hz0 at 100 Pa (fig. 1, line dl. On this basis, we decided to investigate in greater detail the following two cases: i) total pressure of 1000 Pa, temperature range 320-380°C, in 0 2 atmosphere; ii) total pressure of 100 Pa, temperature range 230-270°C, in 02+H20 atmosphere. The different decomposition mechanisms involved in the two experimental conditions were studied through the FT-IR analysis of the exhaust gases as a b c t i o n of the reactor temperature. Idared gas spectra are shown in figure 2, where the band assignment and a tentative identification of
A
021H20at I 0 0 Pa v C-H
d \
k
230 'C /
280 'C \ -
-
L
*
A +
--,
CH4
~sopropanol (reference)
O2 at 1000 Pa
d
.-
A
& A " -
CH4
CH4
acetone (reference)
,Ak-
3200 3000 2800 2600 2400 2200 2000 1800 1600 1400 1200 1000
800
600
400
wavenumber (cm-') Figure 2. FT-IR gas phase absorbance spectra at the reported temperatures: A) in presence of 200 sccm Hz0 at 100 Pa; B) in presence of 150 scan O2 at 1000 Pa.
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decomposition products (gases at the end of the MOCVD reactor) are reported. In 02+H20 atmosphere, the observed decomposition products result to be methane and isopropanol (see fig. 2, A); conversely, in a pure Oz atmosphere methane and acetone are found (fig. 2, B). On the basis of these observations a mechanism of the decomposition processes, can be hypothesised as reported in Scheme 1.
glass substrate
Scheme 1. Proposed reaction decomposition mechanisms of aluminum dimethylisopropoxide either in the presence of oxygen mixed with water vapour or only dry oxygen.
The precursor decomposition could be initiated thanks to the hydroxyl groups always present on the substrate surface. The first step would consist in the precursor molecular adsorption on glass substrate, interaction with -OH, and consequently release of methane. Atter this stage, the mechanism in the two environments follows a different path. When only oxygen is present as reactant gas, the isopropoxyl group is oxidised giving rise to the formation of acetone. In the presence of an excess of water, the situation is different. The water participates directly to the reaction favouring the isopropanol formation, which is not converted into acetone in these experimental conditions. 3.2 Kinetics Either at a total pressure of 1000 Pa or in the presence of water vapour as reacting gas, the deposition rate increased with temperature suggesting a chemical kinetics controlled domain. Assuming that the reaction rate depends only on the precursor concentration, because either oxygen or water are present in large stoichiometric excess, a pseudo first-order kinetics can be used. The analysis was carried out only in the axial reactor direction on the basis of the high uniformity of film thickness obtained in the radial direction. The expression of precursor mass continuity here utilised, has been already reported [S]. The reaction parameters kgim and kopwd(rate constants for film and parasitic powder formation respectively), E~~~and ndd (activation energies for film and powder formation respectively) were determined by matching the experimental precursor concentrations at the end of the reactor evaluated by FT-IR measurements, and the experimental growth rates as a function of reactor temperature and position along the reactor. Three temperatures were taken into account: 230, 250 and 270 OC for experiments carried out with water vapour as reactant gas, whereas 320, 350 and 380 OC were considered at 1000 Pa in presence of only dry oxygen. Growth rates were measured in five points along
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-'
the reactor pipe and resulted in the range 60-140 nm in the presence of water at 100 Pa and 5 -20 nm min-' in presence of only oxygen at 1000 Pa (see fig. 3). ' 3.10'~ The values of E ~ " ,k$"", E pd and kfwd resulted 70 kJ mol-', 6 -lo6min-I, 157 kJ m o ~ and e'respectively in the presence of oxygen at 1000 Pa, and 40 kJ mol-I, 3.10~e l , 140 kJ m o ~ and ' 3.10'~min-' respectively in the presence of water at 100 Pa. Similar values of ko are commonly found for radicals or for many other centre activation states [8-111.
0
,
,
,
,
,
,
,
,
,
,
2
4
6
8
10
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22
0
,
,
,
,
,
,
,
,
,
,
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Reactor coordinate ( c m ) Figure 3. Growth rate at the reported temperatures as a function of reactor axial w-ordinate. Symbols refer to experimental data, lines to the calculated trends: A) in the presence of water vapour-oxygen at 100 Pa; B) in the presence of dry oxygen at 1000 Pa.
The activation energy when using dry oxygen at 1000 Pa is nearly halved with respect to that ' suggesting that the growth rate increases obtained at 100 Pa with 50 sccm of 02, Le. 130 kJ m o ~ [3], not only with concentration but also with collision frequency and therefore with total pressure [12]. The activation energy value obtained using water vapour as reactant gas is much lower compared to 130 kJ mol-' calculated using only oxygen at the same pressure, indicating that the presence of Hz0 greatly favours the precursor decomposition process. This effect may be related to the fact that the hydrolysis of Al-0 bonds is easier than the isopropoxyl-group oxidation to acetone, which requires a higher energy to be initiated. 3.3 Film characterisation
At 1000 Pa the film resulted transparent (80-90% transmittance), with reffactive index 1.6 and a density of about 3 g/cm3. In the presence of water vapour and oxygen, the films resulted highly transparent (>95%), with reffactiveindex 1.6 and a density of about 4 g/cm3. Irrespective of the preparation conditions, all aluminium oxide films resulted amorphous, as indicated by X-ray =action. This observation is in agreement with the results reported in [13], where the XRD patterns showed low intensity reflections of a-alumina only after annealing the films in air at temperatures greater than 900 "C. The composition of the outermost layers was investigated by XPS, yielding similar results for all the obtained samples. In all cases only aluminiium, oxygen and carbon signals were detected on the film surface (see fig. 4). The complete disappearance of the C signal after erosion indicated that it arose only f?om atmospheric contamination and confirmed that the adopted precursor had a clean decomposition pattern, either in the presence of 0 2 at 1000 Pa or in the presence of 02+H20at 100 Pa. This result further supports the reaction mechanisms proposed in scheme 1 on the basis of the FT-IR results. A detailed analysis of A12p photopeak revealed the presence of a single component centred at 74.3 eV (FWHM=1.8 eV). This is in excellent agreement with the literature values for M203 [14]. The
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Figure 4. XPS wide scan spectra of an N2O3film grown at 1000 Pa, T=370°C: (a) surface, @) after 20' sputtering. The initial carbon contamination equal to 31%, drops to noise level after erosion.
presence of this oxide is h t h e r confirmed by the O/Al surface ratio, which was always close to the expected value. For all samples, the 01s photopeak resulted broadened towards higher BEs and was decomposed in three distinct components (fig. 5): (A) -. 530.8 eV, attributed to lattice oxygen in 4 2 0 3 [15]; (B) -. 531.8 eV, related to the presence of hydroxyl species (-OH); (C) 533.0 eV, due to adsorbed water [7]. A comparison of the 01s signal before and aRer erosion reveals that no dramatic changes occur after the sputtering treatments, irrespective of the presence of water vapour. This indicates that -OH groups are probably incorporated in the oxide network and is consistent with the proposed reaction mechanisms (scheme 1). These findings point m t to the formation of an oxidehydroxide macromolecular network, which might be formulated as AIO,(OH),. The substitution of bridging 0 atoms with -OH groups in the alumina structure might be responsible of the amorphous
Figure 5. Decomposition of the 01s signal. Contributing bands (A), (B) and (C) are due to A1203,-OH and H20 species respectively.
microstructure detected for all the synthesized films. DSerently ffom the composition and microstructure, the surface morphology of the alumina layers is strongly influenced by the synthesis conditions, as deduced ffom the AFM analyses. Figure 6 reports
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Figure 6. AFM 3D surface micrographs (4pm x 4pm) for: (up) a film synthesised in 02+H20atmosphere at a total pressure of 100 Pa (T=230°C); (down) a film obtained in dry
4 atmosphere at a total pressure of 1000 Pa (T=370°C).
two representative micrographs of fjlms deposited in only dry oxygen at 1000 Pa and in the presence of water vapour at 100 Pa respectively. Films deposited at 1000 Pa show the presence of globular grains well interconnected between themselves, whose average size ranges fiom 210 to 290 nm on increasing the growth temperature (average roughness= 3 nm). Conversely, samples grown in the presence of Hz0 show a fine and regular texture where no big agglomerates are clearly detectable. In our opinion, this morphology difference can be related to the lower activation energy value obtained using water vapour as reactant gas. In fact, this would lead to a higher reaction rate and the consequent formation of more nucleation centres and thinner grains. However, this aspect is still under investigation. 4 CONCLUSIONS
The goal of this work was to decrease the growth temperature of aluminium oxide 61ms by using aluminum dimethylisopropoxide as precursor. The changing of the reaction conditions (such as temperature and total pressure) and of the reacting gases (such as 9 , H20, NzO) have been considered. The obtained results showed that the best performances are obtained with water vapour at 100 Pa and dry oxygen at 1000 Pa. Growth rates resulted 60-140 nm min-' in the presence of water at 100 Pa and temperature range 230-270°C, and 5 -20 nm min-'in O2atmosphere at 1000 Pa in the temperature range 320-380 "C. Different reaction mechanisms have been proposed for the precursor decomposition in the two
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cases. While in the presence of water the &oxide moiety is removed forming isopropanol, in oxygen atmosphere the isopropoxyl group is oxidised yielding acetone. Both mechanisms are consistent with the absence of carbon contamination in the films, as proved by XPS analyses. Moreover, the presence of water vapour corresponds to a smoother surface texture, which has been related to the precursor decomposition kinetics. References
Henrich V.E., Cox P.A., The Surface Science of Metal Oxides (Cambridge University Press, Cambridge, 1994) pp. 5 1,139. Hitchman M.L., Jensen K.F., Chemical Vapor Deposition. Principles and Applications (Academic Press, London, 1993) pp. 591-656. Barreca D., Battiston G.A., Gerbasi R. and Tondello E., J. Muter. Chem. lO(9) (2000) 21272130. Koh W., Ku S.-J. and Kim Y., Thin Solid Films 304 (1997) 222-224. Battiston G.A.. Gerbasi R., Porchia M. and Gasparotto A., Chem. Vap. Deposition 5 (1999) 13-20. Swanepoel R., J. Phys. E: Sci. Jnstrum. 16 (1983) 1214-1222. Moulder J.F., Stickle W.F., Sobol P.W. and Bonben K.D., Handbook of X-ray Photoelectron Spectroscopy (Perkin-Elmer, Eden Prairie, MN, 1992). Benson S. W., Thermochemical Kinetics (John Wiley & Sons, New York, 197) pp. 141-208. Teyssandier F., AUendorf M.D., Chemical Vapor Deposition, Proceedings of the XIV International Conference and EUROCVD11 (ECS, Pennington, 1997) pp. 15-22. Arndt J., Wahl G., Chemical Vapor Deposition, Proceedings of the XIV International Conference and EUROCVD 11 (ECS, Pennington, 1997) pp. 147- 154. Raghavan. R., Lee D.C., Conrad D.A. and Morrison P.W. Jr, The 197th Meeting of the ECS, (ECS, Toronto, 2000) p. 850. Smith D.L., Thin-Film Deposition. Principles & Practice (Mc Graw Hill, Boston, 1995) pp.336344. Barreca D., Battiston G.A., Gerbasi R and Tondello E., The Electrochemical Society Series, Toronto 2000, in press. (a) Taylor J.A., J. Vac. Sci. Technol. 20 (1982) 751; (b) Nefedov V.I., J Electron Spectrosc. Relat. Phenom 25 (1982) 29; (c) McGuire G.E., Schweitzer G.Kk. and Carlson T.A,. Inorg. Chem. 12 (1973) 245 1. (a)Wagner C.D., Passoja D.E., Hillery H.F ., Kinisky T.G., Six H.A., Jansen W. T., and Taylor J.A., J. Vac. Sci. Technol. 21 (1982) 933; (b) Barr T.L., Appl. Surf: Sci. 15 (1983) 1.
