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In Situ Quartz Crystal Microbalance and Quadrupole Mass Spectrometry Studies of Atomic Layer Deposition of Aluminum Oxide from Trimethylaluminum and Water Antti Rahtu,* Teemu Alaranta, and Mikko Ritala Laboratory of Inorganic Chemistry, University of Helsinki, P.O. Box 55, FIN-00014 Helsinki, Finland Received January 18, 2001. In Final Form: July 10, 2001 Reaction mechanisms in the atomic layer deposition of Al2O3 from Al(CH3)3 and water were studied with a quartz crystal microbalance at 150-350 °C and with a quadrupole mass spectrometer at 150-400 °C. The growth rate was the highest at 250 °C. At lower temperatures the growth was limited due to kinetic reasons and at higher temperatures due to lower amount of surface -OH groups. About half of the ligands were released during the Al(CH3)3 pulse and the other half during the water pulse. The reaction temperature had no marked effect on the growth mechanisms, in the temperature range studied.
Introduction Aluminum oxide (Al2O3) thin films have high resistivity, breakdown voltage, and chemical and thermal stability.1 Ion diffusion in Al2O3 is also virtually negligible.2 Due to these excellent properties, Al2O3 thin films have been used and examined in thin film electroluminescent displays and dynamic random access memory (DRAM) applications as dielectric layers and diffusion barriers.1,2 One advantage of Al2O3 is that it can be deposited on silicon without an interfacial silicon dioxide (SiO2) layer 3-7 which often easily forms and reduces the total capacitance. This is especially important when very small SiO2 equivalent oxide thicknesses are needed. However, in the future generation DRAM and metal oxide semiconductor field effect transistor applications, pure Al2O3 hardly can be used due to its moderate permittivity (r ∼ 9). More probably it is going to be used as a thin buffer layer between a higher permittivity oxide and silicon. One interesting method for depositing future gate oxides is atomic layer deposition (ALD), also called as atomic layer epitaxy. It is a gas-phase method for depositing highquality thin films.8-12 ALD is based on alternate saturative surface reactions. Each precursor is pulsed to the reaction chamber, one at a time, and the pulses are separated by * To whom correspondence may be addressed. E-mail:
[email protected]. (1) Leskela¨, M.; Ritala, M. J. Phys. IV 1995, 5, C5-937. (2) Barron, A. R. In CVD of nonmetals; Rees, W. S., Jr., Eds.; VCH: Weinheim, 1996; p 282. (3) Ritala, M.; Kukli, K.; Rahtu, A.; Ra¨isa¨nen, P. I.; Leskela¨, M.; Sajavaara, T.; Keinonen, J. Science 2000, 288, 319. (4) Gusev, E. P.; Copel, M.; Cartier, E.; Baumvol, I. J. R.; Krug, C.; Gribelyuk, M. A. Appl. Phys. Lett. 2000, 76, 176. (5) Gusev, E. P.; Copel, M.; Cartier, E.; Buchanan, D.; Okorn-Schmidt, H.; Gribelyuk, M.; Falcon, D.; Murphy, R.; Molis, S.; Baumvol, I. J. R.; Krug, C.; Jussila, M.; Tuominen, M.; Haukka, S. Electrochem. Soc. Proc. 2000, 2, 477. (6) Jung, Y.-C.; Miura, H.; Ishida, M. Jpn. J. Appl. Phys 1999, 38, 2333. (7) Hubbard, K. J.; Schlom, D. G. J. Mater. Res. 1996, 11, 2757. (8) Suntola, T.; Antson, J. US Patent No. 4 058 430, 1977. (9) Suntola, T.; Antson, J.; Pakkala, A.; Lindfors, S. SID 80 Digest 1980, 108. (10) Suntola, T. Mater. Sci. Rep. 1989, 4, 261. (11) Suntola, T. In Handbook of Crystal Growth 3, Thin Films and Epitaxy, Part B: Growth Mechanisms and Dynamics; Hurley, D. T. J., Eds.; Elsevier: Amsterdam, 1994; p 601. (12) Niinisto¨, L.; Ritala, M.; Leskela¨, M. Mater. Sci. Eng. 1996, B41, 23.
inert gas purging periods. With properly chosen growth conditions, the reactions are saturative and the film growth is thereby self-limiting. This offers a lot of practical advantages, such as excellent conformality, accurate and simple thickness control, and large area uniformity.13,14 One of the most ideal ALD process is the growth of Al2O3 from Al(CH3)3 and water. In this process, full saturation of the surface reactions during each precursor pulse is observed. Therefore, equal film thicknesses and compositions over large areas and deep trenches are obtained.13 For these reasons the reaction mechanism of this process has been examined quite thoroughly especially on high surface area substrates.15-17 However, when high surface area substrates are used, very long pulse lengths are needed. The difference between the pulse times in high surface area processes and thin film growth complicates a direct comparison of the reaction mechanisms, because very slow reactions are not affecting the thin film growth. Recently the growth mechanism in the ALD of Al2O3 from Al(CH3)3 and water has been studied under conditions more relevant to the actual thin film growth with a quadrupole mass spectrometer (QMS).18 After that study we have added also a quartz crystal microbalance (QCM) to the QMS-ALD system.19 QCM is a powerful method for examining ALD processes, and it has been used often but not with the Al(CH3)3 and water process.20-25 (13) Ritala, M.; Leskela¨, M.; Dekker: J.-P.; Soininen, P. J.; Skarp, J. Chem. Vap. Deposition 1999, 5, 7. (14) Ritala, M. Appl. Surf. Sci. 1997, 112, 223. (15) Ott, A. W.; McCarley, K. C.; Klaus, J. W.; Way, J. D.; George, S. M. Appl. Surf. Sci. 1996, 107, 128. (16) Ott, A. W.; Klause, J. W.; Johnson, J. M.; George, S. M. Thin Solid Films 1997, 292, 13. (17) Lakomaa, E.-L.; Root, A.; Suntola, T. Appl. Surf. Sci. 1996, 107, 107. (18) Juppo, M.; Rahtu, A.; Ritala, M.; Leskela¨, M. Langmuir 2000, 16, 4034. (19) Rahtu, A.; Ritala, M. Electrochem. Soc. Proc. 2000, 2000-13, 105. (20) Aarik, J.; Kukli, K.; Aidla, A. Appl. Surf. Sci. 1996, 103, 331. (21) Aarik, J.; Aidla, A.; Kukli, K.; Uustare, T. J. Cryst. Growth 1994, 144, 116. (22) Aarik, J.; Aidla, A.; Sammelselg, V.; Siimon, H.; Uustare, T. J. Cryst. Growth 1996, 169, 496. (23) Aarik, J.; Aidla, A.; Uustare, T.; Ritala, M.; Leskela¨, M. Appl. Surf. Sci. 2000, 161, 385. (24) Kukli, K.; Aarik, J.; Aidla, A.; Siimon, H.; Ritala, M.; Leskela¨, M. Appl. Surf. Sci. 1997, 112, 236.
10.1021/la010103a CCC: $20.00 © 2001 American Chemical Society Published on Web 09/18/2001
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With this new QCM-QMS-ALD system, we can get even more detailed information on the process under study. In the simplest case, the overall reaction between the Al(CH3)3 and water can be divided to two parts:
n-OH(s) + Al(CH3)3(g) f (-O-)nAl(CH3)3-n(s) + nCH4(g) (1a) (-O-)nAl(CH3)3-n(s) + 3/2H2O(g) f -AlO3/2(OH)n(s) + (3 - n)CH4(g) (1b) During the Al(CH3)3 pulse, the precursor reacts with the surface -OH groups (reaction 1a). In this stage, n methyl groups are released. The rest of the methyl groups are released during the water pulse (reaction 1b). After the water pulse, the surface is again -OH terminated. However, the mechanism proposed above is oversimplified because the Al2O3 surface after the Al(CH3)3 pulse can have both methyl surface groups and coordinatively unsaturated surface (cus) sites.16,18,26-28 Therefore, two kinds of reactions can be suggested to occur during the subsequent water (deuterated water in the present study) pulse:
Al-CH3(s) + D2O(g) f Al-OD(s) + CH3D(g) (2) Al-O-Al (s) + D2O(g) f 2Al-OD(s)
(3)
The methyl groups bound to aluminum are very reactive toward water, and reaction 2 is most probably very fast with only a minor effect from the water dose. Also the cus sites react with water forming -OD groups, but with a slower rate (reaction 3). Therefore, the rate of reaction 3 can be increased by increasing the water dose. This was observed in a recent study29 where the growth rate appeared to saturate to different levels with large and small water doses. In the previous QMS study,18 the precursor doses were chosen according to the QMS data in a way that a single precursor pulse was long enough to react saturatively with the functional surface groups, e.g., Al-CH3 in reaction 2, as verified by the absence of CH3D release during the second pulse of the same precursor. However, there is always a possibility that only the saturation of reaction 2 but not that of reaction 3 was detected, because in reaction 3 no gas-phase species are formed. In this study we confirmed the saturation of the growth rate with QCM which gives the mass increment of the film during each cycle. Experimental Section Experiments were made in a specially modified flow type ALD reactor F-120 manufactured by ASM Microchemistry Ltd. (Espoo, Finland) (Figure 1). The reaction chamber was enlarged as described elsewhere and was loaded with glass substrates so that these formed narrow flow channels.19,30 The total area of glass substrates was about 3500 cm2. The gas composition was measured with a Hiden HAL/3F 501 RC mass spectrometer using (25) Yousfi, E. B.; Fouache, J.; Lincot, D. Appl. Surf. Sci. 1999, 153, 223. (26) George, S. M.; Ott, A. W.; Klaus, J. W. J. Phys. Chem. 1996, 100, 13121. (27) Dillon, A. C.; Ott, A. W.; Way, J. D.; George, S. M. Surf. Sci. 1995, 322, 230. (28) Puurunen, R. L.; Root, A.; Lindblad, M.; Krause, A. O. I. Phys. Chem. Chem. Phys. 2001, 3, 1093. (29) Matero, R.; Rahtu, A.; Ritala, M.; Leskela¨, M.; Sajavaara, T. Thin Solid Films 2000, 368, 1. (30) Matero, R., Rahtu, A., Ritala, M. Submitted for publication in Chem. Mater.
Figure 1. A schematic view of the reactor. The precursors are transported with the carrier gas to the reaction chamber from the right and are pumped by the mechanical pump (MP). A small part of the total flow is pumped by the turbomolecular pump (TP) through the sampling orifice and the QMS chamber. an electron multiplier detector and ionization energy of 70 eV. The sampling and pressure reduction were accomplished through a 50 µm orifice. The pressure in the ALD reactor is about 1 mbar and that in the QMS chamber below 1 × 10-6 mbar. The surface mass studies were done with a Maxtek TM-400 QCM. The crystal operating frequency was 6 MHz, and the sampling rate 20 times per second. At 250 °C the growth rate is usually about 1 Å/cycle. The observed frequency change during one complete ALD cycle was about 10 Hz giving a sensitivity of 0.1 Å/Hz. As the resolution of the QCM is 0.1 Hz, it can be roughly estimated that the smallest observable thickness is 0.01 Å. However, conversion to thickness is complicated because the QCM signal is affected not only by the weight change but also by viscosity of gas, stress in forming film, pressure, and temperature.31 Therefore, we preferred to use primary data (frequency) which are adequate for the mechanism studies. A more detailed description of the experimental setup has been given elsewhere.19,30 Al(CH3)3 (Witco Gmbh) and D2O (Euriso-top, 99.9% D) were held at room temperature (20-22 °C) outside the reactor. The pulsing of the precursors was accomplished by solenoid valves, and the precursor flows were controlled by needle valves. Deuterated water was used to distinguish the species coming from the surface reactions from those coming directly from the aluminum precursor. Argon (99.99%) was used as a purging and carrier gas. The reaction temperature was varied between 150 and 400 °C. At 400 °C the experiments were made only with the QMS because above 350 °C the temperature dependence of the resonant frequency of the QCM is very significant. Therefore, even a small temperature instability causes large frequency variation.
Results and Discussion The different gas-phase species coming from the Al(CH3)3-D2O process were studied earlier.18 When only Al(CH3)3 was pulsed into the reactor, the species CH3+ (m/z ) 15), CH4+ (m/z ) 16), and Al(CH3)2+ (m/z ) 57) were detected. During the D2O pulses, the species O+ (m/z ) 16), OD+ (m/z )18), and D2O+ (m/z ) 20) were detected. Unfortunately m/z ) 17 was also detected when D2O was pulsed on an -OD surface.18 However, this background coming directly from the precursor could be subtracted from the data as discussed earlier.32 In this study, we concentrated only on the reaction byproduct (CH3D+, m/z ) 17). CH3D was released during both D2O and Al(CH3)3 pulses (Figure 2a). Simultaneous with the QMS measurement, weight changes were monitored with the QCM (Figure 2b). The weight increase during the Al(CH3)3 adsorption is labeled in Figure 2 as m1. During the D2O pulse the mass increases because the -CH3 surface groups are replaced by the slightly heavier -OD groups. A small hump during the (31) Mecea, V. M.; Carlsson, J. O.; Heszler, P.; Baˆrtan, M. Vacuum 1995, 46, 691. (32) Rahtu, A.; Kukli, K.; Ritala, M. Chem. Mater. 2001, 13, 817.
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Table 1. Possible Reaction Mechanisms in the Al(CH3)3-D2O Process and the Corresponding m0/m1 Ratiosa n
reactions
m0/m1
0
Al(CH3)3(g) f Al(CH3)3(s) Al(CH3)3(s) + 1.5D2O(g) f -AlO1.5(s) + 3CH3D(g)
0.71
1
-OD(s) + Al(CH3)3(g) f -O-Al(CH3)2(s) + CH3D(g) -O-Al(CH3)2(s) + 1.5D2O(g) f (-O-)1.5Al(OD)(s) + 2CH3D(g)
0.93
2
2-OD(s) + Al(CH3)3(g) f (-O-)2Al(CH3)(s) + 2CH3D(g) (-O-)2Al(CH3)(s) + 1.5D2O(g) f (-O-)1.5Al(OD)2(s) + CH3D(g)
1.34
3
3-OD(s) + Al(CH3)3(g) f (-O-)3Al(s) + 3CH3D(g) (-O-)3Al(s) + 1.5D2O(g) f (-O-)1.5Al(OD)3(s)
2.43
a The letter n corresponds to the number of ligands released during the Al(CH ) pulse. The m /m values are calculated according to 3 3 0 1 the following equation: m0/m1 ) M(AlO3/2)/(M(Al(CH3)3) - nM(CH3D)).
Figure 2. (a) QMS data and (b) QCM mass change in two complete ALD cycles at 250 °C. m0 is the frequency change during one complete ALD cycle, and m1 is the mass increment during the Al(CH3)3 pulse. Al(CH3)3 and D2O pulse lengths were 3 s, and purge times were 10 s.
D2O pulse can be observed. This is most likely an effect of D2O adsorption and desorption. The mass increment during a complete ALD cycle is m0. Because these masses are related to the adsorbate Al(CH3)3-n (m1) (cf. eq 1a) and Al2O3 (m0), their ratio gives an estimate of how large fractions (n/3) of ligands are released during the Al(CH3)3 pulse. Table 1 shows four possible reaction mechanisms and the corresponding m0/m1 ratios. Similar n/3 values can be calculated also from the QMS data by dividing the amount of reaction byproducts released during the Al(CH3)3 pulse by the total amount of reaction byproducts released during one cycle. At each temperature studied, the precursor doses were optimized using the QCM data (Figure 3). The precursor dose is sufficient when the growth rate (m0) and the fraction of ligands released during the Al(CH3)3 pulse settle to a constant level. The proper precursor pulse lengths were found to be 3 s for both Al(CH3)3 and D2O. The temperature had no effect on the precursor doses needed. The precursor doses (i.e., flow rates and pulse lengths) were much larger than those used in the earlier study18 where the saturation could be examined only with QMS. As will be seen, this can have an effect on the growth rate and mechanism. The saturation of m0 as a function of D2O pulse length (Figure 3b) verifies that both reactions 2 and
Figure 3. The mass change during one complete ALD cycle (m0), and the fraction of ligands released during the Al(CH3)3 pulse as a function of (a) Al(CH3)3 and (b) D2O pulse length at 250 °C.
3 are saturated. In other words, all the methyl groups become consumed (reaction 2) and the surface becomes as fully -OD terminated as possible at a given temperature. It must be noted that the saturation of -OD coverage does not mean a full monolayer coverage because such high coverages are not stable against dehydroxylation. Instead, the saturation coverage should equal that which is characteristic of an amorphous Al2O3 surface at each temperature. The actual surface densities of -OD groups cannot be measured with QMS and QCM, however, so their saturation can be observed only indirectly through saturation of m0. The earlier QMS study18 showed that the total amount of reaction byproducts increased as a function of temperature. Also here such an increase was observed with QMS from 150 to 200 °C, but otherwise our new results are quite different (Figure 4). At 150 °C there is a difference between the QCM and the QMS results. The total mass increment (m0) during the cycle is high, but quite low amounts of reaction byproducts are released. Earlier growth experiments have shown33 that the films grown at 150 °C contained 5 atom % hydrogen and 0.8 atom % carbon. These impurity contents are quite high when they
Atomic Layer Deposition of Aluminum Oxide
Figure 4. The weight change during one complete ALD cycle (m0) and the total amount of reaction byproducts (CH3D, m/z ) 17) as a function of reaction temperature. The growth rates measured in the earlier studies optically29 (crosses) are shown also.
are compared to the films grown at 250 °C, where the hydrogen and carbon contents were 1.3 and 0.3 atom %, respectively. Therefore, it could be suggested that at 150 °C the reactions do not go to completion. Therefore, the amount of reaction byproducts is low, but the mass increment is high. However, the amount of hydrogen was still quite low, and therefore possible incorporation of water residues cannot fully explain the large difference between the QCM and QMS data at 150 °C. When the temperature is increased from 200 to 250 °C, both the total mass increment during one cycle (m0) and the amount of reaction byproduct (CH3D, m/z ) 17) increase (Figure 4). One explanation could be that the surface consists of different kinds of sites. Some are more reactive than others. At low temperature, the precursor molecules do not have enough thermal energy to react with all the surface sites and therefore the growth rate is saturated to a lower level. In the temperature range of 250-400 °C, m0 and the total amount of reaction byproducts decrease. The number of -OH groups on the aluminum oxide surface is known to decrease smoothly as a function of temperature.34-36 This has been suggested in many earlier ALD oxide studies1,12,15-18,26 to be the reason for the decreasing growth rate. The QMS, QCM results and the growth rates determined earlier optically from 100 to 200 nm thick films29 show similar trends as a function of temperature (Figure 4). Therefore, in these new experiments both reactions 2 and 3 are saturated; i.e., the cus sites are also converted to -OD groups during the D2O pulse. About half of the ligands are released during the Al(CH3)3 pulse, as calculated from the QMS data. This is between the reaction mechanisms n ) 1 and n ) 2 in Table 1 (Figure 5). On the other hand, 60-70% of the ligands are released during the Al(CH3)3 pulse, when (33) Matero, R.; Sajavaara, T.; Ritala, M.; Leskela¨, M. To be submitted for publication. (34) Nelson, C. E.; Elam, J. W.; Cameron, M. A.; Tolbert, M. A.; George, S. M. Surf. Sci. 1998, 416, 341. (35) Peri, J. Phys. Chem. 1965, 69, 211. (36) Zamora, M.; Co´rdoba, A. J. Phys. Chem. 1978, 82, 584.
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Figure 5. Temperature dependence of the reaction byproducts released during the Al(CH3)3 pulse divided by the total amount of reaction byproducts.
calculated from the m0/m1 ratio of the QCM data (Figure 5). This is closer to the reaction mechanism n ) 2 in Table 1. There is a bit more scatter in the QCM data, though. Therefore, it can be concluded that both methods give about the same mechanism; i.e., about half of the ligands are released during the Al(CH3)3 pulse. The reaction temperature does not have any major effect on the reaction mechanism. This result is contradictory to the earlier results obtained with a small water dose18 where more and more ligands were released during the D2O pulse when the reaction temperature was increased from 150 to 400 °C. With a large water dose, the same kind of trend as in this study could be observed. Therefore, it seems that in the earlier study18 the water dose was most probably too low to also saturate reaction 3. Conclusions The ALD of Al2O3 from Al(CH3)3 and D2O was studied with QMS and QCM. According to QMS, the main reaction byproduct was CH3D. Its amount increased in the temperature range 150-250 °C. At higher temperatures the amount of reaction byproduct started to decrease. At lower temperatures the growth was limited due to kinetic reasons and at higher temperature due to lower amount of surface -OH groups. Two kinds of reactions were supposed to occur during the water pulse, the exchange reaction with the surface methyl groups and the reaction where coordinatively unsaturated surface sites are converted to -OD groups. When both of these reactions are saturated, about half of the ligands are released during the Al(CH3)3 pulse and another half during the water pulse. The reaction temperature had no marked effect on the growth mechanism. Acknowledgment. The valuable discussions with Ms. Riikka Puurunen, Mrs. Marika Juppo, and Professor Markku Leskela¨ are gratefully acknowledged. Financial supportance from the Academy of Finland and the Finnish National Technology Agency (TEKES), Helsinki, Finland, is gratefully acknowledged. LA010103A
