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Langmuir 2002, 18, 10046-10048
Reaction Mechanism Studies on the Atomic Layer Deposition of ZrxTiyOz Using the Novel Metal Halide-Metal Alkoxide Approach Antti Rahtu* and Mikko Ritala Laboratory of Inorganic Chemistry, Department of Chemistry, P.O. Box 55, FIN-00014, University of Helsinki, Finland Received August 5, 2002. In Final Form: October 4, 2002
1. Introduction The semiconductor industry is actively looking for new materials to be used in the next generation of devices.1,2 The continuous downscaling of the metal oxide semiconductor field effect transistors, in particular, sets increasing demands, especially on the properties of the gate oxide because of the high tunneling currents through ultrathin insulators. The so far dominant silicon dioxide based gate oxides are soon to be replaced by thicker layers of materials with a permittivity higher than that of SiO2. For obtaining equivalent oxide thicknesses below 1 nm, that is, a capacitance density higher than 1 nm of SiO2 would give, the gate oxide should be deposited in a way that the easily forming interfacial SiO2 layer would be, at a maximum, two molecular layers thick. This is a very challenging task due to the strong affinity of silicon toward oxygen. In addition, the gate oxides should be uniformly deposited over large wafers and their thickness must be controlled at an atomic level. Atomic layer deposition (ALD)3 has recently gained vast interest in the semiconductor industry as a deposition method for future generation gate oxides.4 ALD is based on alternate saturative surface reactions.3,5 Each precursor vapor is pulsed to the reaction chamber alternately, one at a time, and the pulses are separated by 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.3,6-8 Recently, new chemistry without strong oxidizers was introduced for ALD of metal oxides.9-11 By pulsing alternately metal halides and metal alkoxides on silicon, at least Al2O3 could be deposited without the interfacial SiO2 layer. The new chemistry was found to be versatile.9 However, no experimental results about the reaction * E-mail:
[email protected]. Fax: +358-9-19150198. http:// www.helsinki.fi/∼eorkm_ww. (1) Semiconductor Industry Association, National Technology Road map for Semiconductors: Technology Needs (SEMATECH, Austin, TX, 2001). See also public.itrs.net/. (2) Kingon, A. I.; Maria, J.-P.; Streiffer, S. K. Nature 2000, 406, 1032. (3) Ritala, M.; Leskela¨, M. Atomic Layer Deposition. In Handbook of Thin Film Materials; Nalwa, H. S., Ed.; Academic Press: San Diego, CA, 2001; Vol. 1, Chapter 2. (4) Gusev, E. P.; Copel, M.; Cartier, E.; Baumvol, I. J. R.; Krug, C.; Gribelyuk, M. A. Appl. Phys. Lett. 2000, 76, 176. (5) Niinisto¨, L.; Ritala, M.; Leskela¨, M. Mater. Sci. Eng. 1996, B41, 23. (6) Leskela¨, M.; Ritala, M. Thin Solid Films 2002, 409, 138. (7) Ritala, M.; Leskela¨, M.; Dekker, J.-P.; Soininen, P. J.; Skarp, J. Chem. Vap. Deposition 1999, 5, 7. (8) Ritala, M. Appl. Surf. Sci. 1997, 112, 223. (9) Ritala, M.; Kukli, K.; Rahtu, A.; Ra¨isa¨nen, P. I.; Leskela¨, M.; Sajavaara, T.; Keinonen, J. Science 2000, 288, 319. (10) Rahtu, A.; Ritala, M.; Leskela¨, M. Chem. Mater. 2001, 13, 1528. (11) Kukli, K.; Ritala, M.; Leskela¨, M. Chem. Mater. 2000, 12, 1914.
mechanism are published so far. The knowledge about the reaction mechanism would assist further development of the method. The idea of using metal alkoxides as oxygen sources in reactions with metal chlorides to form metal oxides had been employed earlier in nonhydrolytic solgel processes.12-15 In those processes the main reaction byproducts were alkyl chlorides. The alkyl chlorides have been speculated to be the reaction byproduct also in the ALD of ZrxTiyOz from Ti(OCH(CH3)2)4 and ZrCl4.10 The purpose of this study is to examine the reaction mechanism by using a quartz crystal microbalance (QCM) and quadrupole mass spectrometery (QMS). The QCM gives detailed information of the surface mass changes down to a submonolayer level while QMS probes the volatile reaction byproducts. Thus, the combination of these two techniques forms a powerful tool for studying the ALD reaction mechanisms.16-20 2. Experimental Section Experiments were carried out with a specially modified16,18,20 commercial flow type F-120 ALD reactor (ASM Microchemistry Ltd.) at 300 °C. The pressure in the reaction chamber was about 2 mbar. The gas composition was measured with a Hiden HAL/ 3F 501 RC quadrupole mass spectrometer using an ionization energy of 70 eV. The sampling for QMS was accomplished by differential pumping through a 50 µm orifice.17-20 The pressure in the QMS chamber was below 10-6 mbar. The surface mass studies were done with a Maxtek TM-400 quartz crystal microbalance. Ti(OCH(CH3)2)4 (Aldrich, 97%) and ZrCl4 (Aldrich, 97%) vapors were pulsed to the substrates alternately from open boats which were inside the reactor at 45 and 165 °C, respectively. The Ti(OCH(CH3)2)4 and ZrCl4 pulse lengths were 1 and 3 s, respectively. The precursor pulse lengths were optimized with a QCM to obtain the maximum growth rate per cycle. The purge time was 6 s after each precursor pulse. Before each experimental set, about 2-5 nm of TiO2 was grown on the glass substrates and on the QCM crystal. Argon (99.99%) was used as a purging and carrier gas.
3. Results and Discussion Several possible reaction byproducts such as TiClx(OH)y (x + y ) 4), O(CH(CH3)2)2, Cl2, and TiOCl2 were looked for with QMS but not found. The only detected reaction byproducts were HCl+ (m/z ) 36), CH2CHCH3+ (m/z ) 42), and (CH3)2CHCl+ (m/z ) 78) and its fragment CHCH3Cl+ (m/z ) 63). The ratios between the intensities at m/z ) 68 and 78 for 1-chloropropane and 2-chloropropane are 0.9 and 1.5, respectively.21 The observed ratio between the intensities at m/z ) 63 and 78 was 1.4, and therefore the reaction byproduct at m/z ) 78 was concluded to be 2-chloropropane. Figure 1 shows typical QMS and QCM data obtained. During the Ti(OCH(CH3)2)4 adsorption, a small mass increment (m1) is seen (Figure 1a). At the same time, 2-chloropropane is observed (Figure 1b). Also HCl and CH2CHCH3 were detected at this time, though in lower amounts (not shown). (12) Vioux, A. Chem. Mater. 1997, 9, 2292. (13) Corriu, R. J. P.; Leclercq, D.; Lefe`vre, P.; Mutin, P. H.; Vioux, A. J. Mater. Chem. 1992, 2, 673. (14) Livage, J.; Sanchez, C. J. Non-Cryst. Solids 1992, 145, 11. (15) Corriu, R. J. P.; Leclercq, D. Angew. Chem., Int. Ed. Engl. 1996, 35, 1421. (16) Rahtu, A.; Alaranta, T.; Ritala, M. Langmuir 2001, 17, 6506. (17) Matero, R.; Rahtu, A.; Ritala, M. Chem. Mater. 2001, 13, 4506. (18) Rahtu A.; Ritala, M. Chem. Vap. Deposition 2002, 8, 21. (19) Rahtu, A.; Ritala, M. J. Mater. Chem. 2002, 12, 1484. (20) Rahtu, A.; Ritala, M. Appl. Phys. Lett. 2002, 80, 521. (21) National Institute for Standards and Technology, mass spectra database, NIST98.
10.1021/la026357t CCC: $22.00 © 2002 American Chemical Society Published on Web 11/12/2002
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an important effect on the reaction mechanism. The more stabilized the cleaving carbonium ion is, that is, the more branched the alkoxide is, the more facile the reaction is. The effect of the alkoxide group was indeed observed in the earlier growth study,9 where the Ti(OCH(CH3)2)4ZrCl4 process resulted in a growth rate of 1.2 Å/cycle at 250 °C,10 but even at 300 °C no growth took place with the precursor combination Ti(OC2H5)4-ZrCl4. The reaction which leads to the release of HCl and CH2CHCH3
-ZrCl3(s)+ Ti(OCH(CH3)2)4(g) f -Zr(-O-)3TiOCH(CH3)2(s) + 3CH2CHCH3(g) + 3HCl(g) (2)
Figure 1. (a) QCM mass change and (b) amount of reaction byproducts detected by QMS during the ALD of ZrxTiyOz at 300 °C. m0 is the mass change during one complete ALD cycle, and m1 is the mass increment during the Ti(OCH(CH3)2)4 pulse. The Ti(OCH(CH3)2)4 and ZrCl4 pulse lengths were 1 and 3 s, respectively. The purge time was 6 s after each precursor pulse. Scheme 1. Proposed Transition States between the -ZrCl Surface Group and Ti(OCH(CH3)2)4
One should keep in mind that, at the same time as the titanium species are adsorbing, the previously existing chloride surface groups are released. Therefore, the mass increment during the titanium pulse (m1) is relatively small. During the ZrCl4 pulse the mass increment is larger. The mass change per one complete ALD cycle is labeled as m0 in Figure 1a. According to the QMS results, three out of four isopropyl groups of Ti(OCH(CH3)2)4 were released during the titanium precursor pulse. The reaction which leads to the release of 2-chloropropane during the titanium isopropoxide pulse involves surface chlorines and can be written as
-ZrCl3(s) + Ti(OCH(CH3)2)4(g) f -Zr(-O-)3TiOCH(CH3)2(s) + 3(CH3)2CHCl(g) (1) A reaction mechanism can be proposed where the oxygen atom coordinates to the zirconium and the chlorine atom attacks the central carbon of the alkoxide group.13 An intermediate state is formed (Scheme 1a), which leads to the dissociation of the bonds between zirconium and chloride, and carbon and oxygen. The alkoxide group has
can be suggested to proceed through a six member ring transition state (Scheme 1b) which is formed between the zirconium chlorine surface group and titanium isopropoxide. In principle, HCl and CH2CHCH3 could also come from decomposition of 2-chloropropane. However, this is not likely in this process because 2-chloropropane was observed during both precursor pulses but CH2CHCH3 was detected almost only (95%) during the titanium precursor pulse. At 300 °C Ti(OCH(CH3)2)4 decomposes already to some extent.18 Therefore, those isopropoxide ligands which have not reacted with surface chlorides during adsorption can decompose, for example, through β-elimination producing surface hydroxyl groups:
-TiOCH(CH3)2(s) f -TiOH(s) + CH2CHCH3(g) (3) Because the temperature was relatively low, this reaction remains slow18 and therefore it occurs mainly during the purge period. This is seen in the QCM data (Figure 1a), where the mass decreases slowly during the purge time after the titanium isopropoxide pulse. By contrast, during the purge time after the ZrCl4 pulse, the mass stays at a constant level, that is, the surface is stable. As a result of the reactions proposed above, the surface left after the titanium isopropoxide pulse contains isopropoxide and hydroxyl groups. When ZrCl4 is pulsed on this surface, it reacts with the isopropoxide groups, releasing 2-chloropropane (Figure 1b)
-Zr(-O-)3TiOCH(CH3)(s) + ZrCl4(g) f -Zr(-O-)3TiOZrCl3(s) + (CH3)2CHCl(g) (4) At the same time, part of the ZrCl4 molecules can also react with the hydroxyl groups, releasing hydrogen chloride, as observed
-TiOH(s) + ZrCl4(g) f -TiOZrCl3(s) + HCl(g)
(5)
Both reactions convert the surface to chlorine terminated, that is, to the same state as that before the ALD cycle, and thus it is ready for the next titanium isopropoxide pulse. The relative amount of ligands released during the titanium precursor pulse can be calculated from the QCM data by comparing the relative mass changes after the first precursor pulse (m1) to those of the complete ALD cycle (m0).16-18 The QCM results verify the above QMS results that during the titanium isopropoxide pulse three out of four isopropyl groups are leaving the surface per one chemisorbing precursor molecule. The mass changes in reactions 1 and 2 are the same, and therefore, it is impossible to distinguish their relative importance by
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QCM. On the other hand, also from the QMS data it is quite hard to estimate the relative importance between these two reactions: because the fragmentation pattern of 2-chloropropane is complex, the relative sensitivities of the molecules are probably different and the QMS was not absolutely calibrated.
Notes Scheme 2. Schematic Overview of the Reaction Mechanisms in the ALD of ZrxTiyOz from ZrCl4 and Ti(OCH(CH3)2)4 (R ) (CH3)2CH, R′ ) CH3CHCH2)
4. Conclusion Despite the above problems, experimental results give us three facts: (i) According to QCM, three out of four isopropyl groups and three surface chlorines are released per each adsorbing Ti(OCH(CH3)2)4 molecule during the titanium precursor pulse. (ii) According to QMS results, 70-85% (of the total amount detected) of the 2-chloropropane (Figure 1b), 60% of the hydrogen chloride (not shown), and 95% of the propene (not shown) were released during the titanium precursor pulse. (iii) It was estimated that 20% of the total growth occurs through reactions which release HCl. This estimation was done by comparing the total amount of HCl detected in the present ZrxTiyOz process to that found in the ZrCl4-H2O process where all the chlorines are released as HCl.19 On the basis of these results, an overview of the reaction mechanism can be sketched (Scheme 2). The estimated relative importance of the reaction pathways is also presented in Scheme 2. Though other reaction pathways were found too, most of
the growth occurs through the release of 2-chloropropane, analogous to the sol-gel processes.12-15 Acknowledgment. This work was supported by the Academy of Finland. LA026357T
