Surface reactions in Al2O3 growth from trimethylaluminium and water by atomic layer epitaxy

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applied surface science ELSEVIER

Applied Surface Science 107 (1996) 107-115

Surface reactions in A 1 2 0 3 growth from trimethylaluminium and water by atomic layer epitaxy E-L. Lakomaa a,*, A. Root b, T. Suntola

a

a Microchemistry Ltd., P.O. Box 45, FIN-02151 Espoo, Finland b Neste Oy., Analytical Research, P.O. Box 310, FIN-06101 Porvoo, Finland

Received 12 October 1995; accepted 23 December 1995

Abstract

The layer by layer growth of A1203 from trimethylaluminium (TMA) and water vapour by atomic layer epitaxy (ALE) has been studied on porous large surface area silica. The use of this support enables the study of each reaction sequence brought to surface saturation by using elemental determinations, Fourier transform infrared (FTIR) and solid state nuclear magnetic resonance (NMR) spectroscopy. The chemisorption of TMA as a function of the preheat temperature of silica shows that bonding takes place to isolated OH groups and siloxane bridges. Increasing the preheat temperature of silica creates an increased amount of siloxane bridges. Increasing the chemisorption temperature of TMA leads to the reaction of double and triple siloxane bridges as measured by 298i NMR. This type of reaction has not been presented earlier. The first five reaction cycles of TMA and water on silica show that, after the first TMA reaction, 2.8 A1/nm 2 are present whereas the following reaction cycles increase A1 by 0.5 and 0.3 A1/nm 2 alternatively in the cycles 2-5. Water removes A1-CH 3 groups already at 120°C, but Si-CH 3 groups are not affected so much and are responsible for the slow progress of growth for the next layers.

1. I n t r o d u c t i o n

There is widespread interest in the use of alumina as passivating layers or insulators in different applications, and a high purity is required for this. Alumina thin films were first grown about 20 years ago for electroluminescent applications by atomic layer epitaxy (ALE) b y using A1C13 and H 2 0 as reactants [1]. However, the total removal of chlorides requires temperatures around 500°C and such high temperatures can cause damage to the structures to be cov-

* Corresponding author.

ered. Different A1 compounds for growing A120 3 at lower temperatures such as alkoxides have also been studied [2]. Some organometallic compounds are widely used in A L E growth [3], and Higashi and Fleming [4] have presented the growth of A120 3 on hydrogen passivated Si from T M A and water at 450°C, obtaining a growth rate o f about 0.11 n m / c y c l e . They could not explain the reaction mechanism for this growth rate, which is about half of the monolayer growth, 0.217 nm, calculated for sapphire. The growth rate is in good agreement with that presented by Kumagai et al. [5] on H F treated Si(100), Fan and T o y o d a [6] on G a A s and our own experiments with glass substrates and Si [7]. Dillon

0169-4332/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S0169-4332(96)005 13-2

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et al. [8] and George et al. [9] used a porous -y-alumina membrane and Si(100). The study of the reaction mechanisms of ALE reactions by using thin film substrates is difficult because of the low concentration of species to be analyzed after one reaction sequence. We have developed means to study the reactions layer by layer by using high surface area porous substrates commonly used for catalysts [10]. This type of experimental set-up allows the possibility to quantitate separately each reaction sequence. Infrared spectroscopy (FTIR) has been a good tool to study the chemisorption of TMA on porous silica. Yates et al. [11] described already in 1969 the basic reactions of TMA on silica forming A I - C H 3 and S i - C H 3 species as shown in Fig. 1. The silica surface consists of isolated, H bonded and geminal OH groups, and siloxane bridges. TMA reacts with equal probability with isolated OH groups and siloxane bridges [12]. This has been confirmed in several

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studies [11-17] and summarized in Ref. [18]. The number of methyl groups still bound to A1 has not been totally resolved and cannot be found from FTIR spectra. In addition to the main reactions Low et al. [13] have shown that S i - O - C H 3 can exist on the surface in addition to the S i - C H 3 groups originating from the reaction with siloxane bridges. The presence of various surfaces species should thus be expected. Although much research has been carried out by bringing TMA at room temperature to the surface, reaction mechanism studies at elevated temperatures are scarce. To provide a better understanding of the ALE growth, which takes place at temperatures where physisorption of any reactants is eliminated, reactions of TMA were studied at 50 to 350°C. In this paper we have combined the use of higher reaction temperatures, different preheat temperatures of the support and a wider selection of analysis methods to clarify the reaction mechanism and the growth during the first five reaction cycles with TMA and water pulsed sequentially to surface saturation. The growth was followed in the first five layers by combining element determinations, solid state NMR and FTIR on a well characterized silica surface.

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Fig. 1. Growth model of A1203 on silica during the first reaction cycle of (TMA + H 2 0 ) . An increase in reaction temperature from 80 to 200°C increases the amount of Si(CH3) 3 species,

Silica, EP 10 (Crosfield Ltd.), surface area 300 m2/g, pore volume 1.75 c m 3 / g and mean particle size of 100 /xm, was used as a substrate. Silica was preheated in air for 16 h at 200, 300, 600, 800 and 900°C for controlling the number and nature of bonding sites. Before the reaction the preheated silica was further stabilized at the reaction temperature concerned for 3 h in a nitrogen flow in case of reaction temperatures < 250°C and at 450°C for 3 h in a nitrogen flow at a pressure of 6 - 8 kPa in the case of silica batches preheated at temperatures over 600°C. This stabilizing was used for assuring the total absence of physisorbed water. TMA, 99.99% (Epichem Ltd.) was vaporized at room temperature and deionized water at 50°C. Preheated silica (5-12 g) was brought to the reaction temperature selected in a reaction chamber of quartz

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E.-L. Lakomaa et al. /Applied Surface Science 107 (1996) 107-115

at a pressure o f 6 - 1 0 kPa in a flow of nitrogen. Each reactant was led separately to the reaction chamber at a selected temperature. The dose of the reactant was calculated to exceed the number of available bonding sites by a factor of at least 2. Reaction times o f 2 - 6 h were used for T M A and 2 h for water. Each reactant pulse was followed by a nitrogen purge o f 1 - 2 h at the reaction temperature concerned. The samples were cooled in a nitrogen flow and transferred inertly for analysis. Reaction temperatures between 50°C and 350°C were studied with TMA. W a t e r vapor treatment at 120-300°C was used, and at 120°C when growing several reaction sequences. The layer by layer growth was studied on silica preheated at 200 or 600°C using 120°C for the reaction of T M A and water sequentially. A1 and C were determined after each T M A pulse, and C was determined again after water treatment. A1 was determined by X R F or instrumental neutron activation analysis ( I N A A ) and C with a Leco carbon analyzer. Pure silica was used in I N A A determinations to correct the error caused by the fast neutrons from 28Si(n, p) to ZSA1. The accuracy of the determination was confirmed by dissolution o f some samples and analysis by ICP-MS. F T I R spectra were measured either with a Galaxy Series 6020 spectrometer or Nicolet Impact 400 equipped with a diffuse reflectance accessory. A n inert atmosphere was maintained throughout the measurement. The spectra were recorded between 4000 and 650 c m - 1 directly from the surface using a spectral resolution of 2 c m -~. Two hundred scans with the Galaxy equipment and 50 scans with the Nicolet equipment were recorded. The a3C and 29Si C P M A S N M R measurements were carried out using a J E O L G S X 270 N M R instrument as described earlier [19].

3. Results 3. l. R e a c t i o n o f T M A on silica

The effect of the reaction temperature o f T M A as well as the effect of the preheat temperature o f the silica on the reaction at 80 and 200°C were studied. Also the growth o f 1 - 5 reaction cycles o f T M A +

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water will be presented in the following. The F T I R and M A S - N M R results were used to shed light on the surface species present at each step of the alumina growth. Increasing the preheat temperature decreases the saturation density of A1 from 3.2 to 1.0 A 1 / n m 2, while the C : A 1 ratio remains almost constant (Fig. 2a). The effect of the reaction temperature on the binding of T M A on silica preheated at 600°C was studied between 50 and 280°C. Fig. 2b shows the saturation level of A1 as a function of reaction temperature. The saturation level decreased from 2.2 A 1 / n m 2 at 50°C to 1.9-2.0 A 1 / n m 2 when reaction temperatures between 150 and 280°C were used. The C : A 1 ratio increases with increasing reaction temperature, but remains below 2. The increase is mainly

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E.-L. Lakomaa et al. /Applied Surface Science 107 (1996) 107-115

caused by increased binding of methyl groups to siloxane bridges as was found in the NMR measurements. The reactivity of TMA is high, and both isolated OH groups and siloxane groups are involved in the reactions. Reaction of TMA at 370°C caused decomposition of TMA on the surface resulting in a brown color of the sample. Diffuse reflectance FT1R measurements showed that reaction temperatures below about 150°C cause the appearance of five peaks in the spectra (2960, 2930-2940, 2900, 2850, 2823 cm-1). Yates et al. [11] have measured three peaks from TMA in solution, namely 2940, 2898 and around 2920 cm -1 The five peaks found in our samples are fairly consistent with those presented by Kunawicz et al. [14] and Kinney and Staley [15] for TMA on silica. The peaks 2930-2940, 2900 and 2823 cm-1 correspond to the C - H stretching of A1-CH 3, whereas S i - C H 3 gives 2900 and 2960 cm -~ C - H stretching peaks. At reaction temperatures below 150°C A1CH 3 peaks can be detected, whereas the 2930-2940 cm-1 peak forms only a shoulder when the reaction temperature increases to 200°C or over. The peak at 2823 cm-1 can still be seen at the reaction temperature of 280°C. The methoxy group proposed by Low et al. [13] could not be distinguished in our FTIR spectra, although in a sample with TMA reaction at 80°C on silica (Fig. 3a-C, D) and in another sample after a fourth reaction sequence of TMA in layer by layer growth, 13C NMR revealed the presence of this peak. Any systematic behavior in the appearance of the methoxy peak could, however, not be determined and was absent in most of the other samples. The quantitative elemental determinations do not support a high abundance of AI(CH3) 2 species, but suggest more the presence of A1-CH 3 in addition to S i - C H 3 species. The total exchange of methyl groups from TMA with OH groups leading straight to aluminum oxide species and CH 4 cannot be totally excluded as one reaction type. 13C CPMAS NMR measurements show that the peak of the S i - C H 3 species increases when the

111

preheating temperature of silica increases from 200 to 800°C and TMA is reacted at 80°C (Fig. 3a). Also a peak of S i - O C H 3 can be seen at the lower reaction temperature of 80°C, whereas at 200°C the methoxy peak is absent and both A1-CH 3 and S i - C H 3 are seen in the a3C NMR spectra. 298i MAS NMR spectra show an increased tendency to form OaSi(CH3) a and OSi(CH3) 3 species when the preheat temperature of silica is increased from 200 to 800°C with reaction at 80°C and especially when the reaction temperature of TMA is 200°C (Fig. 3b). 3.2. Reaction with water vapor on TMA / silica surface

The reaction of A1-CH 3 with water vapor took place at all temperatures studied and FTIR spectra revealed the disappearance of A1-CH 3 peaks. The reaction at room temperature may also take place in ambient air. The OSi(CH3) 3 species seemed to be unstable to the water vapor treatment, whereas O2Si(CH3) 2 and O3SiCH 3 were more stable. After TMA reaction both A I - C H 3 and S i - C H 3 groups are present in 13C NMR spectrum (Fig. 4A). Water treatment causes the disappearance of the A1-CH 3 peaks in the 13C NMR spectrum leaving two sharper peaks due to O2Si(CH3) 2 and O3SiCH 3 (Fig. 4B). The presence of these groups and the loss of the OSi(CH3) 3 group is confirmed in the 29Si spectrum (Fig. 4C). 3.3. Growth o f layers

The layer by layer growth during the five first reaction cycles at 120°C on silica preheated at 600°C is presented in Fig. 5. The corresponding FTIR and 13C NMR spectra recorded after each TMA pulse support the quantitative A1 determinations revealing a slow growth after the first reaction cycle. The second and the fourth cycle bring 0 . 5 - 0 . 6 A l / n m 2 whereas the third and fifth cycle increase A1 by 0.3 A 1 / n m 2. Whether this alternatively changing growth

Fig. 3. The effect of chemisorptiontemperatureof TMA on silica preheated at 800 (A, C) and 200°C (B, D). (a) 13C CPMAS NMR spectra, when TMA reacted at 200°C (A, B) and at 80°(2 (C, D), (b) 29SiCPMAS NMR spectra after chemisorptionof TMA at 200°C (A, B) and at 80°C (C, D), respectively.

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E . - L Lakomaa et al./Applied Surface Science 107 (1996) 107-115

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amounts on silica preheated at 200°C with reaction of TMA at 200°C. We propose that there are groups of siloxane bridges consisting of 2 - 3 S i - O - S i species close to each other. The OSi(CH3) 3 species is produced when surrounding TMA molecules react with a Si with three siloxanes attached to it. The OSi(CH3) 3 groups here are in a different chemical environment than, for example, the same type of groups produced by

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Fig. 5. Growth of AI203 at 120°C on silica preheated at 600°C as a function of the number of reaction cycles with TMA and H20. A1 determined by INAA. A surface area of 300 m2/g was used for the calculation of the surface density.

continues requires further studies. The 29Si CPMAS N M R spectra recorded after each T M A reaction of the five reaction cycles ( A - E ) show that the OSi(CH3) 3 peak disappears after the first reaction cycle, but the O2Si(CH3) 2 and O3SiCH 3 peaks remain and increase somewhat during the second and third cycle and are similar during the fourth and fifth cycle, Fig. 6.

4. Discussion The binding of T M A on porous silica at room temperature has been thoroughly studied earlier using mainly FTIR [11-17]. In addition to FTIR we applied N M R to study the surface species formed. The FTIR spectra obtained in this work are in good agreement to those presented earlier [11-16]. In alumina growth the reaction with water vapor removes methyls from A 1 - C H 3 groups, but S i - C H 3 groups remain according to the FTIR results. N M R supports this, but in addition to S i - C H 3 also Si(CH3) n, where n is 2 - 3 , was detected. Low et al. [13] have also proposed the presence of Si-methoxy groups and in some of our samples the methoxy groups were found by NMR. The appearance of Si(CH3) 2 and Si(CH3) 3 species suggests a more complicated growth to the siloxane groups than proposed earlier. A higher tendency to form the Si(CH 3)3 species was found on silica preheated at 800°C. The Si(CH3) 3 species were also detected in minor

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E.-L Lakomaa et al. / Applied Surface Science 107 (1996) 107-115

the reaction of hexamethyldisilazane (HMDS) with silica [19]. In the former case the 29Si chemical shift is 24 p p m while in the latter it is only 12 ppm. In the H M D S reaction the OSi(CH3) 3 groups are sticking out from the surface whereas in the case here we assume that these same groups are ' b u r i e d ' somewhat into the surface. The AI(CH3) 2 peaks suggested b y Bertolet and Rogers [17] could not be confirmed in our samples and the determined C : A 1 ratio also favours the presence of A 1 - C H 3. The A1 determinations as a function of the number o f the reaction cycles showed that the binding o f T M A in the first cycle gives a surface saturation of 2.8 A 1 / n m 2. The next two cycles increase the amount of A1 by 0.5 a t o m s / n m 2 and 0.3 a t o m s / n m 2 during the following two cycles. The fourth and fifth cycles again repeat the growth of the second and third cycles. The preheat temperature of silica regulates the surface density of A1 during the first cycle as well as the number of S i - C H 3 species caused by the reaction to siloxane bridges. The reaction temperature of T M A does not have a drastic effect on A1 surface density, but more on the type of S i - C H 3 species formed as shown by the N M R measurements. The exchange of methyl groups to OH groups takes place at all temperatures studied from A 1 - C H 3, but the Si(CH3) 2 and S i - C H 3 groups are more resistant to water treatment.

5. Conclusions The reactions of T M A on porous silica at temperatures between 50 and 370°C and the growth of alumina from T M A and H 2 0 at 120°C were studied. Although the chemisorption of bare T M A on silica has been thoroughly studied by FTIR, the higher temperatures showed even more complicated mechanisms than was expected. The presence of Si(CH3) 2 and Si(CH3) 3 species as measured by 29Si N M R suggest high reactivity at elevated temperatures to several siloxane bridges. The removal of S i - C H 3 groups could not be made at low growth temperatures and led to slow growth during the 2 - 5 reaction cycles. The carbon contamination originating from the first T M A reaction is probably low enough to stay undetected by surface analysis techniques on films containing several layers of alumina hiding the

S i - C H 3 species. In thin films the use of growth temperatures exceeding 450°C removes the main part of S i - C H 3 groups leading to contamination free alumina growth. This could explain the good quality o f alumina thin films reported in [5]. On porous silica, however, as low a reaction temperature as 370°C led to partial decomposition of T M A and a brown color of the sample. Solid state N M R proved to be a good method to study the various surface species formed, since all of them could not be detected by F T I R only.

Acknowledgements Ms. Mirja Rissanen (lab. techn.) and Ms. P~iivi Jokimies (techn.) are thanked for the careful experimental work. The University of Joensuu, Department of Chemistry provided the means to measure F T I R spectra in an inert atmosphere, which is greatfully acknowledged. The study was partly supported by the A c a d e m y of Finland.

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Characterization and Chemical Modification of the Silica Surface. Studies in Surface Science and Catalysis, Vol. 93, Eds. B. Delmon and J.T. Yates (Elsevier, Amsterdam, 1995) p. 364. [19] S. Haukka and A. Root, J. Phys. Chem. 98 (1994) 1695.