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In Situ Reaction Mechanism Studies on the Atomic Layer Deposition of Al2O3 from (CH3)2AlCl and Water Raija Matero,* Antti Rahtu,† and Mikko Ritala Laboratory of Inorganic Chemistry, Department of Chemistry, P.O. Box 55, University of Helsinki, Helsinki FI-00014, Finland Received November 19, 2004. In Final Form: February 4, 2005 Reaction mechanisms between dimethylaluminum chloride and deuterated water in the atomic layer deposition (ALD) of Al2O3 were studied at 150-400 °C using a quartz crystal microbalance (QCM) and a quadrupole mass spectrometer (QMS). The observed reaction byproducts were DCl and CH3D. QMS showed that about one-third of the chlorine, and half of the methyl ligands were released during the (CH3)2AlCl pulse. The growth rate deduced from the QMS and QCM data was in qualitative agreement with the previously published growth rate from ALD film growth experiments.
Introduction (ALD)1
is a chemical gas phase Atomic layer deposition thin film deposition method based on alternate surface reactions. The precursors are pulsed into the reactor one at a time, and the pulses are separated by inert gas purging periods. With properly chosen growth conditions, the reactions are saturated and the film growth is thereby self-limiting, which ensures that uniform films with excellent conformality can be deposited.2 Aluminum oxide thin films have been grown by ALD using AlCl3,3,4 (CH3)3Al,5-8 (CH3)2AlCl,9 (CH3)2Al(OiPr),10 Al(OEt)3,3 and Al(OPr)3.3 (CH3)2AlCl combines the characteristics of (CH3)3Al and AlCl3. As a liquid, transportation into the ALD reactor is more convenient than with solid AlCl3. The vapor pressure of (CH3)2AlCl is comparable to that of (CH3)3Al, and the exchange of one methyl ligand to chlorine may improve thermal stability. Reaction mechanisms of ALD Al2O3 processes have been studied with a quartz crystal microbalance (QCM)11-13 and quadrupole mass spectrometer (QMS).12,14,15 These two methods allow reaction mechanism studies under * To whom correspondence should be addressed. E-mail:
[email protected]. † Present address: ASM Microchemistry, Ltd., Ha ¨ meentie 135 A, FI-00560 Helsinki, Finland. (1) Suntola, T. Thin Solid Films 1992, 216, 84. (2) Ritala, M.; Leskela¨, M.; Dekker, J.-P.; Mutsaers, C.; Soininen, P. J.; Skarp, J. Chem. Vap. Deposition 1997, 5, 7. (3) Hiltunen, L.; Kattelus, H.; Leskela¨, M.; Ma¨kela¨, M.; Niinisto¨, L.; Nyka¨nen, E.; Soininen, P.; Tiitta, M. Mater. Chem. Phys. 1991, 28, 379. (4) Ritala, M.; Saloniemi, H.; Leskela¨, M.; Prohaska, T.; Friedbacher, G.; Grassenbauer, M. Thin Solid Films 1996, 286, 54. (5) Higashi, G. S.; Fleming, C. G. Appl. Phys. Lett. 1989, 55, 1963. (6) Ott, A. W.; McCarley, K. C.; Klaus, J. W.; Way, J. D.; George, S. M. Appl. Surf. Sci. 1996, 107, 128. (7) Groner, M. D.; Fabreguette, F. H.; Elam, J. W.; George, S. M. Chem. Mater. 2004, 16, 639. (8) Yun, S. J.; Lee, K.-H.; Skarp, J.; Kim, H. R.; Naim, K.-S. J. Vac. Sci. Technol., A 1997, 15, 2993. (9) Kukli, K.; Ritala, M.; Leskela¨, M.; Jokinen, J. J. Vac. Sci. Technol., A 1997, 15, 2214. (10) Cho, W.; Sung, K.; An, K.-S.; Lee, S. S.; Chung, T.-M.; Kim, Y. J. Vac. Sci. Technol., A 2003, 21, 1366. (11) Aarik, J.; Aidla, A.; Kukli, K. Appl. Surf. Sci. 1994, 75, 180. (12) Rahtu, A.; Alaranta, T.; Ritala, M. Langmuir 2001, 17, 6506. (13) Elam, J. W.; Groner, M. D.; George, S. M. Rev. Sci. Instrum. 2002, 73, 2981. (14) Ritala, M.; Juppo, M.; Rahtu, A.; Leskela¨, M. J. Phys. IV 1999, 9, Pr8-1021. (15) Juppo, M.; Rahtu, A.; Ritala, M.; Leskela¨, M. Langmuir 2000, 16, 4034.
conditions prevailing in flow-type reactors. In ALD, the oxide film growth usually proceeds via surface exchange reactions where the metal precursor interacts with the surface -OH groups releasing some of the ligands attached to the metal. The subsequent water pulse releases the rest of the ligands and recreates an -OH terminated surface. In (CH3)2AlCl there are two kinds of ligands which raise the question whether the methyl groups or the chloride ion interact preferentially with the -OH groups during adsorption. This paper presents results from reaction mechanism studies on the (CH3)2AlCl and D2O ALD process. The studies were carried out using QCM and QMS. Experimental Section QMS-QCM-ALD Setup. The experiments were made in a specially modified commercial flow-type ALD reactor manufactured by ASM Microchemistry, Ltd. The equipment has been described in detail elsewhere.12 The gas composition was measured with a Hiden HAL/3F 501 RC QMS which has a mass range of 1-510 amu. A Faraday cup detector was used. The ionization energy was 70 eV. The pressure in the ALD reaction chamber was about 2 mbar, and it was reduced to about 10-7 mbar in the QMS chamber by differential pumping through an orifice of 200 µm in diameter. The mass balance studies were made using a Maxtek TM 400 QCM. The operating frequency of the crystal was 6 MHz; the sampling rate was 20 times per second. Deposition Parameters. The precursors were (CH3)2AlCl (Witco GmbH) and D2O (Euriso-top, 99.9% D). D2O is used instead of H2O to better distinguish the reaction byproducts from the species forming in the QMS ionizator.12 The precursors were held at room temperature and led into the reactor through needle and solenoid valves. The deposition temperature was varied between 150 and 400 °C. The QCM was used only up to 350 °C, however, because above that temperature even a small temperature instability caused large frequency variation and made the signal unreliable. Argon (99.99%, flow rate 150 sccm) was used as the carrier and purging gas. The reaction chamber (V ) 550 cm3) was loaded with soda lime glass substrates so that the total surface area was about 3500 cm2.
Results and Discusion The m/z values that were measured by QMS are listed in Table 1. The only reaction byproducts observed were DCl and CH3D. DCl was detected at m/z values of 37 and 39. D35Cl+ (m/z ) 37) was more intense, so it was chosen for the more detailed studies. The other byproduct, CH3D+,
10.1021/la047153a CCC: $30.25 © 2005 American Chemical Society Published on Web 03/17/2005
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Table 1. Studied m/z Values and the Corresponding Ions m/z 15 16 17 27 29 30 31 32 35 37 39 42
ion +
CH3 CH4+, O+ CH3D+ Al+, C2H3+ AlH2+, C2H5+ AlH3+, C2H6+ C2H5D+ C2H4D2+ Cl+ 37Cl+, D35Cl+ D37Cl+ AlCH3+
observed yes yes yes yes yes yes no no no yes yes yes
m/z 50 51 52 57 58 60 70 74 77 80 92 169
ion CH3Cl+
CH2DCl+ CHD3Cl+ Al(CH3)2+ AlOCH3+ Al(OD)CH3+ 35Cl + 2 37Cl + 2 AlCH3Cl+ Al(OD)Cl+ Al(CH3)2Cl+ Al2(CH3)2Cl2+
observed no no no yes no no no no yes no yes yes
is detected at m/z ) 17. The other ions presented in Table 1 are produced by fragmentation.16 A part of the QMS and QCM data is shown in Figure 1; the measurements have been made simultaneously. It can be seen that the byproducts are released during both the (CH3)2AlCl and the D2O pulses (Figure 1a). The QCM data is shown in Figure 1b; the weight increase during the adsorption of (CH3)2AlCl is labeled with m1. During the subsequent water pulse the adsorbed -CH3 and -Cl ligands are replaced by -OD groups or oxide ions. When the heavier -Cl is replaced by lighter -OD groups or oxide ions (one oxide for two chlorides), a mass decrease occurs; the mass is slightly increased when the lighter -CH3 groups are replaced with heavier hydroxyls. The mass increment during one complete ALD cycle is m0, which is related to the growth rate. The precursor doses were optimized according to the QCM measurements, that is, the pulse lengths were optimized by observing a saturation of m0, the growth rate. Figure 2 a,b shows how m0 behaved when the precursor pulse length was elongated from 0.5 to 5.0 s while the pulse length of the other precursor was kept constant at 3.0 s. It can be seen in Figure 2a that, in the case of the D2O pulse, m0 increases very rapidly at first but especially at the highest temperature settles to a constant level at 3.0 s. At 150-300 °C there is a slight increase in m0 between 3.0 and 5.0 s, but this is not significant; 3.0 s was chosen as the pulse length for D2O. For (CH3)2AlCl, m0 settled to a constant level already with 1-2 s pulses at 300-350 °C. At lower temperatures there was a rapid increase in m0 until the pulse length of 3.0 s was reached, so 3.0 s was chosen also for (CH3)2AlCl. Growth Rate. The growth rates according to QMS and QCM are depicted in Figure 3. The growth rate obtained from the QMS data is the total amount of reaction byproducts, that is, the sum of the amounts of m/z ) 17 and m/z ) 37 released during both precursor pulses in one ALD cycle. The m0 obtained from the QCM measurement presents the growth rate. Optically determined growth rates from an earlier film deposition study9 are included for comparison. The data are normalized to unity at 250 °C. The results are in good accordance with each other. At 150 °C the value obtained from QMS measurements is low compared to those from QCM and ALD results. It has been shown that the films deposited below 200 °C contain relatively high amounts of hydrogen, chlorine, and carbon impurities.9 These impurities are incorporated into the film during the film growth because of incomplete surface reactions; that is, the ligands are not removed completely. The QMS measures gaseous byproducts originating from the surface reactions; when these reactions are not completed, the amount of byproducts is lower than would be expected on the basis of the (16) Tanaka, J.; Smith, S. R. Inorg. Chem. 1969, 8, 265.
Figure 1. (a) QMS data and (b) QCM mass change in two complete ALD cycles at 250 °C. m0 is the mass increment during one complete ALD cycle, and m1 is the mass increment during the (CH3)2AlCl pulse. The (CH3)2AlCl and D2O pulse lengths were 3.0 s, and the purge length was 10 s.
growth rate. Above 250 °C the growth rate decreases slowly, which is usually attributed to the decreasing hydroxyl group density on the surface; hydroxyl groups are known as the reactive sites in oxide ALD.17,18 Reaction Mechanism. Because there are two different ligands attached to the metal, there are two different reaction byproducts and, hence, several possible reaction mechanisms (introduced in Table 2). These reactions may of course also occur simultaneously. In addition to those reactions, one should take into account the possibility of reaction with coordinatively unsaturated surface sites and the readsorption of DCl.12,19,20 The amounts of reaction byproducts m/z ) 17 (CH3D+) and m/z ) 37 (DCl+) are shown in Figure 4a,b. The amount of CH3D released during the (CH3)2AlCl pulse first increases between 150 and 200 °C, then it settles to about a constant level until 300 °C, and then it starts to decrease slowly. The amount released during the water pulse follows the same trend. The released amount is somewhat larger during the (CH3)2AlCl pulse at 150-350 °C; at 350-400 °C about the same amount is released during both precursor pulses. Most of the other byproduct, DCl, is released during the water pulse, but it is detected also (17) George, S. M.; Ott, A. W.; Klaus, J. W. J. Phys. Chem. 1996, 100, 13121. (18) Haukka, S.; Lakomaa, E.-L.; Root, A. J. Phys. Chem. 1996, 97, 5085. (19) Puurunen, R.; Lindblad, M.; Root, A.; Krause, O. Phys. Chem. Chem. Phys. 2001, 3, 1093. (20) Kyto¨kivi, A.; Lindblad, M.; Root, A. J. Chem. Soc., Faraday Trans. 1995, 91, 941.
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Figure 2. QCM mass change during one complete ALD cycle (m0) as a function of (a) D2O and (b) (CH3)2AlCl pulse lengths at different temperatures.
the QMS data. It is obtained as follows separately for the both ligands (nCH3 and nCl): the amount of the respective byproduct released during the (CH3)2AlCl pulse is divided by the total amount of this byproduct and then multiplied by the number of the respective ligands in (CH3)2AlCl. So, as there are two -CH3 ligands and one -Cl ligand in (CH3)2AlCl, the values of nCH3 and nCl can vary from 0 to 2 and from 0 to 1, respectively. The separate values are then added together to give the total number of ligands (nCH3 + nCl) reacting during the metal precursor pulse (Figure 5). The average reaction can thus be depicted by eq 1:
(nCH3 + nCl) - OD(s) + (CH3)2AlCl f Figure 3. Growth rate in the (CH3)2AlCl-D2O process measured with QMS and QCM. Optically determined growth rate (labeled “ALD”) for the (CH3)2AlCl-H2O process is included for comparison (taken from ref 9). Table 2. Reaction Mechanism Suggestions for the (CH3)2AlCl-D2O ALD Process n
nCH3
nCl
0
0
0
(CH3)2AlCl(g) f (CH3)2AlCl(s) (CH3)2AlCl(s) + 1.5D2O(g) f -AlO1.5(s) + 2CH3D(g) + DCl(g)
reaction
m0/m1 0.55
1
1
0
-OD(s) + (CH3)2AlCl(g) f O-Al(CH3)Cl(s) + CH3D(g) -O-Al(CH3)Cl(s) + 1.5D2O(g) f (-O-)1.5AlOD(s) + CH3D(g) + DCl(g)
0.68
1
0
1
-OD(s) + (CH3)2AlCl(g) f O-Al(CH3)2(s) + DCl(g) -O-Al(CH3)2(s) + 1.5D2O(g) f (-O-)1.5AlOD(s) + 2CH3D(g)
0.93
2
1
1
2-OD(s) + (CH3)2AlCl(g) f (-O-)2Al(CH3)(s) + CH3D(g) + DCl(g) (-O-)2Al(CH3)(s) + 1.5D2O(g) f (-O-)1.5Al(OD)2(s) + CH3D(g)
1.34
2
2
0
2-OD(s) + (CH3)2AlCl(g) f (-O-)2AlCl(s) + 2CH3D(g) (-O-)2AlCl(s) + 1.5D2O(g) f (-O-)1.5Al(OD)2(s) + DCl(g)
0.87
3
2
1
3-OD(s) + (CH3)2AlCl(g) f (-O-)3Al(s) + 2CH3D(g) + DCl(g) (-O-)3AlO(s) + 1.5D2O(g) f (-O-)1.5Al(OD)3(s)
2.44
during the (CH3)2AlCl pulse. Thus, according to QMS, it appears that -CH3 ligands are more reactive than the chloride. An n value implying the number of ligands released per one Al during the (CH3)2AlCl pulse can be calculated from
(-O-)nCH3+nClAl(CH3)2-nCH3Cl1-nCl(s) + nCH3CH3D(g) + nClDCl(g) (1) According to the QMS results (Figure 5) only a small part (about 20-30%) of the chloride ligands is released during the (CH3)2AlCl pulse and most of them are released during the water pulse. It is also seen that at 150-350 °C only a slight majority (65-55%) of the methyl ligands is released during the (CH3)2AlCl pulse, and at 350-400 °C half of the methyl ligands are released during the (CH3)2AlCl pulse and the other half during the water pulse. The total n value (nCH3 + nCl) decreases with increasing temperature, indicating that the total number of ligands released during the (CH3)2AlCl pulse decreases as the deposition temperature is raised. For comparison, a study on AlCl3 has suggested that at low temperatures part of the chloride ligands are released already during the AlCl3 pulse, but above 300 °C all the ligands are released during the water pulse.21 A study on Al(CH3)3 revealed that about half of the methyl ligands is released during the Al(CH3)3 pulse; at higher temperatures this amount decreases only slightly.12 The reaction mechanism is usually evaluated from the QCM data by measuring m0/m1 ratios (Figure 1) and comparing these with those calculated for suggested reactions (Table 2). However, this procedure evidently assumes that only a single reaction occurs at a given temperature. The QMS data (Figures 4 and 5) shows that this assumption does not hold in this process. Therefore, the following analysis basing on QCM data should be considered only tentative. (21) Aarik, J.; Aidla, A.; Jaek, A.; Kiisler, A.-A.; Tammik, A.-A. Acta Polytech. Scand., Chem. Technol. Metall. Ser. 1990, 195, 201.
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Figure 4. Amounts of (a) m/z ) 17 (CH3D+) and (b) m/z ) 37 (DCl+) detected with QMS during (CH3)2AlCl and D2O pulses at different temperatures.
Figure 5. n values calculated from the QMS data for the (CH3)2AlCl-D2O process. n refers to the total number of ligands released per one Al during the (CH3)2AlCl pulse. It can be divided into nCH3 and nCl (see eq 1), which refers to the number of -CH3 and -Cl ligands, respectively, released during the (CH3)2AlCl pulse.
Figure 6. QCM mass change in two complete ALD cycles at different temperatures.
Figure 6 shows two complete ALD cycles measured with QCM at different temperatures. Some conclusions can be drawn from the shape of the signal. It seems that at 150 °C the chloride is released already during the (CH3)2AlCl pulse; otherwise, there should be a clear drop in the signal at the point of the water pulse, caused by the mass decrease when the heavier chlorine is replaced by a lighter hydroxyl or oxide ion. This is supported also by the measured m0/ m1 value (Figure 7) which matches the theoretical value
Figure 7. m0/m1 obtained from the QCM data at different temperatures (solid squares) and calculated using the n values from the QMS data (open squares). These values refer to possible reactions presented in Table 2. n refers to the total number of ligands released during the (CH3)2AlCl pulse. It can be divided into nCH3 and nCl (see eq 1), which refer to the number of -CH3 and -Cl ligands, respectively, released during the (CH3)2AlCl pulse per one Al.
for the reaction mechanism where the chloride leaves during the (CH3)2AlCl pulse (Table 2). In the signal measured at 250 °C a drop during the water pulse can be seen indicating that something heavier is replaced by something lighter (Cl f OD). The m0/m1 value, 0.89, also refers to the mechanism where the chloride is released during the water pulse. The results obtained at 350 °C also suggest clearly that the chloride is released during the water pulse. A small hump can be observed during the water pulse, especially at the high temperatures. It may be an effect of D2O adsorption and desorption, but it is also possible that the D2O pulse cools the crystal and changes its frequency.22 To investigate further the agreement of the results obtained with the QMS and the QCM, m0/m1 values were calculated using the nCH3 and nCl values obtained from the QMS data (Figure 5). Because m1 is the change in mass during the (CH3)2AlCl pulse, it corresponds to the change in surface species in eq 1 and can be calculated as follows: m1 ) M(Al) + (2 - nCH3)M(CH3) + (1 - nCl)M(Cl) - (nCH3 + nCl)M(D). m0 refers to the Al2O3 deposited; in this case, when only one Al is involved, it corresponds to M(AlO1.5). The calculated values are, indeed, reasonably close to those measured with the QCM at 200, 300, and 350 °C (Figure (22) Rahtu, A.; Ritala, M. Chem. Vap. Deposition 2002, 8, 21.
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7). There is some difference between the measured and calculated value at 250 °C; however, both values refer to a similar reaction mechanism (Table 2). Also at 150 °C, the experimental and calculated values are as close to each other as they are at the other temperatures. However, if they are interpreted by a single reaction (Table 2) different mechanisms are obtained. This shows the problematic nature of employing QCM in analyzing reactions that can proceed by multiple mechanisms. QMS obviously gives a better view of the average reaction mechanism. Conclusions ALD of Al2O3 from (CH3)2AlCl and D2O was studied with QMS and QCM. According to the QMS the main reaction products were CH3D and DCl. Because there are two different products, interpretation of the QCM data is
Matero et al.
complicated. The QMS data suggests that most of the -Cl ligands are released during the water pulse at all temperatures, 150-400 °C. The methyls, in turn, are released during both precursor pulses. The reaction mechanism deduced from the QMS results was crosschecked by calculating the QCM mass changes corresponding to this average reaction mechanism. A reasonably good agreement was observed with the measured QCM data. Acknowledgment. This work was financially supported in part by the Finnish National Technology Agency (TEKES) and the Academy of Finland. Teemu Alaranta and Kjell Knapas are thanked for carrying out part of the experimental work. LA047153A
