Real-Time Monitoring in Atomic Layer Deposition of TiO2 from TiI4

Sep 23, 2000 - University of Tartu, Institute of Experimental Physics and Technology and Institute of Materials Science, Tähe 4, 51010 Tartu, Estonia...
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Real-Time Monitoring in Atomic Layer Deposition of TiO2 from TiI4 and H2O-H2O2 Kaupo Kukli,*,†,‡ Aleks Aidla,§ Jaan Aarik,§ Mikael Schuisky,| Anders Hårsta,| Mikko Ritala,‡ and Markku Leskela¨‡ University of Tartu, Institute of Experimental Physics and Technology and Institute of Materials Science, Ta¨ he 4, 51010 Tartu, Estonia, University of Helsinki, Department of Chemistry, P.O. Box 55, FIN-00014 Helsinki, Finland, and Uppsala University, Inorganic Chemistry, The Ångstro¨ m Laboratory, P.O. Box 538, SE-75121 Uppsala, Sweden Received March 23, 2000. In Final Form: July 11, 2000 Atomic layer deposition of TiO2 films from alternate pulses of TiI4 and H2O-H2O2 vapors was studied with a quartz crystal microbalance in real time. The film formation mechanism did not depend remarkably on the time parameters of growth cycles or precursor doses but was dominantly determined by the growth temperature. The reaction of H2O-H2O2 with the TiIx-terminated surface was a self-limited process. The adsorption of TiI4 was not entirely saturative but proceeded via partial decomposition of TiI4, as the adsorbed mass increased continuously during the TiI4 pulse. Changes in the growth mechanism and an increasing contribution of precursor decomposition were observed at temperatures between 200 and 300 °C.

Introduction Both the dielectric performance and the catalytic activity of TiO2 depend on its crystallographic phase. Rutile films have exhibited higher breakdown fields, higher resistivities, and higher dielectric constants than those possessed by anatase.1 Anatase is recognized as a phase of high catalytic activity.2 Anatase is usually formed at low temperatures and preferred for processing microelectronic devices, while rutile is formed at relatively high temperatures. Precursors and deposition techniques additionally contribute to the phase formation. Therefore, investigations of new precursor systems and precursor supply modes and their effect on the film growth rate are of importance. Uniform thickness in TiO2 films can be achieved by the atomic layer deposition (ALD) technique. In ALD of TiO2, the precursors (e.g., TiI4 and H2O-H2O2)3 are introduced in the pulsed mode, alternately one at a time. The film grows as a result of repeated deposition cycles, where each cycle comprises surface reactions between adsorbed molecular layers of precursors. The precursor fluxes are clearly separated in time by purge periods. Thus, a single deposition cycle comprises the TiI4 pulse with length t1, the first purge period, t2, the H2O-H2O2 pulse, t3, and the second purge period, t4. Commonly, one or less than one atomic layer of metal is deposited during each cycle. The mass of species adsorbed on the surface can be conveniently measured either by gravimetry2,4 or by quartz * To whom correspondence should be addressed. E-mail: kaupok@ ut.ee. † University of Tartu, Institute of Experimental Physics and Technology. ‡ University of Helsinki, Department of Chemistry. § University of Tartu, Institute of Materials Science. | Uppsala University, Inorganic Chemistry, The Ångstro ¨m Laboratory. (1) Ha, H.-K.; Yoshimoto, M.; Koinuma, H.; Moon, B.-K.; Ishiwara, H. Appl. Phys. Lett. 1996, 68, 2965. (2) Primet, M.; Basset, J.; Mathieu, J. V.; Prettre, M. J. Phys. Chem. 1970, 74, 2868. (3) Kukli, K.; Ritala, M.; Schuisky, M.; Leskela¨, M.; Sajavaara, T.; Keinonen, J.; Uustare, T.; Hårsta, A. Chem. Vap. Deposition, in press. (4) Boddenberg, B.; Horstmann, W. Ber. Bunsen-Ges. Phys. Chem. 1988, 92, 525.

crystal microbalance (QCM) measurements. The QCM oscillation period increases together with the mass adsorbing on the surface until all of the adsorption sites have become fully occupied, providing the stabilization of the QCM output signal.5 In the case of irreversible chemisorption, the QCM signal remains invariant against further changes in source gas flow. Also easily detectable is, when present, the linear weight increase due to the thermal decomposition of a metal precursor.5,6 The QCM has enabled the monitoring of an ion-beam assisted etching process at a metallic Ti surface upon the addition of molecular chlorine and the accompanying formation of volatile TiClx species.7 The oxidation state of the titanium film and the related film stoichiometry have also been detected with a QCM by measuring the total oxygen uptake in the anodization process.8 The QCM technique has been applied to different ALD oxide processes, such as Al2O3 growth from AlCl3 and H2O,9 Ta2O5 growth from Ta(OC2H5)5 and H2O6 or from TaCl5 and H2O,10 HfO2 growth from HfCl4 and H2O,11 or TiO2 growth from TiCl4 and H2O12 or from alkoxides and H2O.13 The studies on chloride ALD processes have revealed a monotonic decrease of the film growth rate with increasing reactor temperature in the case of Al2O39 and HfO2,11 as well as TiO2.12 Lower growth rates have been mainly attributed to the decreased surface density of active OH (5) Chiang, T. P.; Savin, H. H.; Thompson, C. V. J. Vac. Sci. Technol., A 1997, 15, 2677; J. Vac. Sci. Technol., A 1997, 15, 3104. (6) Kukli, K.; Aarik, J.; Aidla, A.; Siimon, H.; Ritala, M.; Leskela¨, M. Appl. Surf. Sci. 1997, 112, 236. (7) O’Brien, W. L.; Rodin, T. N.; Rathbun, L. C. J. Chem. Phys. 1988, 89, 5264. (8) Burrell, M. C.; Armstrong, N. R. J. Vac. Sci. Technol., A 1983, 1, 1831. (9) Aarik, J.; Aidla, A.; Jaek, A.; Kiisler, A.-A.; Tammik, A.-A. Acta Polytech. Scand., Chem. Technol. Metal. Ser. 1990, 195, 201. (10) Aarik, A.; Kukli, K.; Aidla, A.; Pung, L. Appl. Surf. Sci. 1996, 103, 331. (11) Aarik, J.; Aidla, A.; Kiisler, A.-A.; Uustare, T.; Sammelselg, V. Thin Solid Films 1999, 340, 110. (12) Aarik, J.; Aidla, A.; Sammelselg, V.; Siimon, H.; Uustare, T. J. Cryst. Growth 1996, 169, 496. (13) Aarik, J.; Aidla, A.; Sammelselg, V.; Uustare, T.; Ritala, M.; Leskela¨, M. Thin Solid Films 2000, 370, 163; Aarik, J.; Aidla, A.; Uustare, T.; Ritala, M.; Leskela¨, M. Appl. Surf. Sci. 2000, 161, 385.

10.1021/la0004451 CCC: $19.00 © 2000 American Chemical Society Published on Web 09/23/2000

Atomic Layer Deposition of TiO2

groups created on the surface with a water pulse. Especially in the case of metal halide processes, the QCM measurements have supported a proposed growth mechanism where the intermediate formation of an OHterminated surface enables chemisorption and strong binding of adsorbed halide molecules to the substrate surface. Higher H2O doses increase the densities of the OH groups and, concurrently, the growth rate of TiO2.12 For comparison, in the ALD of TiO213 or Ta2O5,6 where alkoxides were used as metal precursors, the thermal decomposition of the alkoxide precursor obviously increased the growth rate above a certain threshold substrate temperature. In the previous study of TiO2 in the TiI4-H2O2 system,3 the film growth rate determined from optical thickness measurements increased with growth temperature and TiI4 pulse length. At the same time, the reaction between H2O2 and surface terminating (TiIx) species should be a self-limiting process, as the film growth rate did not increase with the H2O2 pulse length. Titanium dioxide phases were obtained in the H2O2-assisted process, while the use of water could result in the formation of suboxides. This study has been aimed at the real-time investigation of the behavior of TiI4 and H2O-H2O2 in subsequent adsorption processes. As a result of the present study, the influence of the TiI4 pulse length, the TiI4 evaporation temperature, the H2O-H2O2 dose, and the purge period after the TiI4 pulse will be described.

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Figure 1. QCM signal as a function of time, recorded during a long deposition cycle: ∆m1, mass increase during TiI4 exposure time t1; t2, the first purge period; ∆mp1, mass decrease during the first purge period; ∆m2, mass decrease in the reaction between surface and H2O2 during H2O2 exposure time t3; ∆m0, mass of titanium oxide layer deposited in one ALD cycle; t4, the second purge period.

Experimental Section The studies were carried out in a hot-wall horizontal flowtype ALD reactor. The details of the reactor and QCM measurement system have been described elsewhere.14 TiI4 (Strem Chemicals Inc., 99.99%) was evaporated from a silica boat inside the reactor. H2O2 (30% aq solution) was evaporated at 20 °C from a special reservoir outside the reactor. The H2O-H2O2 dose was controlled by a needle valve in the supply line. N2 (99.999%) was used as a carrier and purging gas. The total pressure in the ALD reactor was ∼250 Pa. For the real-time monitoring, a quartz crystal oscillator has been applied as the mass sensor to detect the adsorption of the precursor at the monolayer level. The working frequency of the oscillation circuit based on an AT-cut crystal was 30 MHz. During the real-time studies, the sensor was located in the reaction zone instead of the substrate. To avoid reactions between TiI4 and the Ag electrodes on the quartz crystal, prior to the studies, the sensor was covered with an inert buffer layer of a few nanometers of TiO2 deposited from Ti(OC2H5)4 and H2O. After the deposition of the buffer layer, regular ALD deposition cycles were recorded in the TiI4-H2O2 precursor system. The thickness of the films, with which the QCM measurements were performed, ranged from 2 to 20 nm. Sample films were grown onto (100)-oriented Si substrates at temperatures ranging from 230 to 375 °C. The crystalline structure of the films grown was checked by a Siemens D 5000 diffractometer. Results of the structural and compositional analyses of the films grown in a similar process3,15 have been published elsewhere.

Results Surface Mass Load during a Single Growth Cycle. An increase in the surface mass upon TiI4 adsorption manifested itself as an increasing QCM oscillation period during the TiI4 pulse, t1, tending to saturate at a certain level, ∆m1 (Figure 1). The saturation was more obvious at temperatures below 200 °C, while, at higher temper(14) Aarik, J.; Aidla, A.; Uustare, T.; Sammelselg, V. J. Cryst. Growth 1995, 148, 268. (15) Schuisky, M.; Hårsta, A.; Aidla, A.; Kukli, K.; Kiisler, A.-A.; Aarik, J. J. Electrochem. Soc. 2000, 147, 3319.

Figure 2. QCM signal as a function of time, recorded in the cyclic ALD film growth process. For label key, see Figure 1.

atures, ∆m1 increased continuously during t1. During the first purge period, t2, a mass decrease, ∆mp1, became apparent. This indicated desorption of some species from the surface. At the beginning of the subsequent H2OH2O2 pulse time, t3, the mass was rather abruptly decreased by ∆m2. Obviously, a certain amount of the material adsorbed during t1 was released or replaced in this reaction step. Most likely, oxygen and/or OH groups replaced the heavy iodide ions and terminated the surface after the H2O-H2O2 pulse. Eventually, the mass of the TiO2 layer deposited in a completed cycle was measured as the difference, ∆m0, between the initial and final levels of the QCM signal. It is noteworthy that, in the TiI4-H2O-H2O2 precursor system, both the ∆m1 and ∆m2 were rather large compared with ∆m0. This is because of the large difference in the atomic masses of oxygen (15.999) and iodine (126.90). The difference is significantly larger than that observed earlier in the chloride ALD processes.9,10,12 Figure 1 also shows that, at 200 °C, the stabilization of the QCM output signal during t3 was a relatively slow process. However, the stabilization was considerably faster at higher growth temperatures and, as exemplified by Figure 2, only a few seconds was needed to complete the surface reactions at 275 °C. To characterize the deposition rate as a function of process parameters, average mass changes corresponding

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Figure 3. Mass increments caused by TiI4 pulse, ∆m1, and completed growth cycle, ∆m0, as functions of TiI4 evaporation temperature. TiI4 exposure times are indicated in the inset. The first purge time, H2O2 exposure, and the second purge time were chosen at 2, 5, and 5 s, respectively.

Figure 4. Surface mass increments during TiI4 adsorption, ∆m1, and after complete growth cycle, ∆m0, as functions of TiI4 pulse time at 300 °C.

to the complete deposition cycles were measured. Henceforth, multiple growth cycles were recorded at particular growth conditions (Figure 2) while the cycle time parameters were kept short enough (2-6 s) to provide a reasonable growth rate. In this case, however, entire saturation of the surface and the maximum in the growth per cycle were not necessarily attained. Effect of TiI4 Dose. The TiI4 evaporation temperature was varied between 70 and 130 °C, while keeping the pulse length constant. The film growth rate became measurable when the TiI4 evaporation temperature exceeded 75 °C (Figure 3). With a further increase in the evaporation temperature, both ∆m1 and ∆m0 rapidly increased and then, at still higher temperatures, continued to increase at a much lower rate. Longer TiI4 pulse times enabled this kind of saturation effect at lower evaporation temperatures. The mass added during one cycle, ∆m0, weakly depended on the TiI4 pulse length (Figure 3), regardless of the TiI4 evaporation temperature. The effect of the TiI4 pulse length, t1, was studied in more detail at an evaporation temperature of 115 °C. At the substrate temperature of 300 °C, the increase in both ∆m1 and ∆m0 tended to stabilize when the TiI4 pulse length exceeded 2 s (Figure 4). Influence of Oxygen Precursor Dose. The dose of the H2O-H2O2 mixture was varied at substrate temperatures of 195, 275, and 325 °C. At each temperature, three

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Figure 5. Dependence of surface mass exchange ratio, ∆m0/ ∆m1, on H2O-H2O2 dose: ∆m1, changes in adsorbed mass densities during the TiI4 pulse; ∆m0, mass grown after a complete ALD cycle.

Figure 6. Mass changes during TiI4 adsorption, ∆m1, during H2O-H2O2 exposure, ∆m2, and after a completed growth cycle, ∆m0, versus the length of the purge period between TiI4 and H2O-H2O2 pulses.

different growth cycle sequences, 2-2-2-5, 10-2-2-5, and 10-2-6-5 s, were used, corresponding to the notation t1-t2-t3-t4 for the cycle time parameters. At 275 and 325 °C, even with the longest TiI4 and the shortest H2O-H2O2 pulse lengths used, the H2O-H2O2 dose had a minor influence on ∆m0 (Figure 5). At 195 °C, the increase of ∆m0 with the H2O-H2O2 dose was more pronounced. At the same time, some increase in ∆m1 with increasing H2OH2O2 dose was still observable at other temperatures as well. For this reason, the H2O-H2O2 partial pressure was kept constant at about 8 Pa in the experiments described below. Effect of Purge Period. The effect of the first purge period, t2, was studied with different TiI4 pulse lengths. During t2, some mass decrease, ∆mp1, is noticeable (Figure 1), while the value of ∆mp1 increases with t2. At the same time, the mass released during the H2O-H2O2 pulse, ∆m2, clearly decreased with the increase in t2 (Figure 6). As a result, the value of ∆m0 was rather insensitive to the changes in t2. This was valid even when the TiI4 pulse length, t1, was increased up to 10 s and the growth temperature increased up to 300 °C. Effect of Substrate Temperature. The effect of the substrate temperature on the growth rate was studied with different TiI4 exposure times. It can be seen in Figure 7 that both ∆m1 and ∆m0 increase about 3-4 times when the temperature increases from 200 to 300 °C. No changes in the growth rate with further increase in the temperature were observed up to 350 °C. At all temperatures studied,

Atomic Layer Deposition of TiO2

Figure 7. Surface mass increments during TiI4 adsorption, ∆m1, and after a complete growth cycle, ∆m0, as functions of growth temperature. Cycle time parameters are given in the inset.

Figure 8. Typical X-ray diffraction patterns from TiO2 films deposited onto Si(100) substrates at different temperatures. The TiI4 evaporation temperature was 115 °C.

longer TiI4 pulse times resulted in somewhat higher deposition rates. Film Growth. Reference films were grown on Si(100) substrates in order to confirm the formation of TiO2. Representative X-ray diffraction patterns are shown in Figure 8. Distinct anatase and rutile diffraction peaks are observable, verifying the growth of TiO2. It can be seen that the films formed at temperatures below 300 °C contain only anatase. Films with mixed anatase and rutile structure were formed at 305 °C. Only rutile reflections can be observed in the films grown at 375 °C and higher. Thus, the growth on single-crystal Si resembles that on polycrystalline Si and amorphous glass studied earlier.3 It is to be noted that amorphous SiO2 between TiO2 and Si may hinder epitaxial growth that can still be observed on Si substrates16 at significantly higher temperatures than those used in the present work. The phase-boundary temperatures are similar for any substrate not supporting heteroepitaxial growth. By contrast, on MgO(100) substrates, phase-pure anatase has been grown at temperatures as high as 375 °C, while the minimum temperature allowing rutile growth can be lowered down to 275 °C by using sapphire substrates.15 Discussion Growth Rate. The dependence of the growth rate on the deposition temperature, measured by the QCM (16) Dai, Z.; Naramoto, H.; Narumi, K.; Yamaoto, S. J. Phys.: Condens. Matter 1999, 11, 8511.

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technique, was identical to that determined from the thicknesses of the films grown on soda lime glass substrates.3 Both QCM (Figure 7) and optical thickness measurements3 revealed a considerable increase in the deposition rate between 230 and 300 °C and an almost constant growth rate at 300-400 °C. This indicates that the growth mechanism studied by the QCM technique, in the case of very thin (2-20 nm) films (i.e., at the initial stage of growth), is similar to that dominating in the growth of much thicker films grown on soda lime glass. The increase of ∆m1 and ∆m0 with t1 at 300 °C (Figure 4) and above (Figure 7), as well as with increasing TiI4 evaporation temperature (Figure 3), could be due to a partial thermal decomposition of TiI4 molecules and/or (TiIx) surface species,17 resulting in the release of I2. This process should result in the decrease of the size of the surface species and thereby adsorption of larger amounts of titanium. The degree of decomposition naturally increases with temperature. The partial decompositiondissociation of the TiI4 is probably enhanced by the surface, rather than being initiated in the gas phase. The mass decrease observed during the first purge time (Figure 1) refers to the desorption of iodine, while titanium remains anchored to the surface and will be oxidized during the next H2O-H2O2 pulse. This is confirmed by the fact that ∆m0 is almost independent of t2 (Figure 6). Higher densities of metal atoms in the adsorbate layer may also be a result of better removal of iodine in the hydroxylation step, especially at lower substrate temperatures. Growth Mechanism. Possible growth mechanisms can be described in the form of two-step surface-reaction schemes. Such reactions comprise the adsorption of the TiI4 layer on the oxide surface and subsequent reaction of the oncoming water-peroxide pulse with the terminating iodo ligands. For instance, the adsorption of the halide may proceed via the reaction with hydroxyl groups18 created in the preceding H2O2-H2O pulse:

Ti-OH + TiI4(g) f Ti-O-TiI3 + HI(g)

(1a)

The surface hydroxyl groups are recreated in the hydrolysis during the following exposure to H2O2-H2O:

Ti-O-TiI3 + H2O2(g) f Ti-O-Ti-O(OH) + HI(g) + I2(g) (1b) Ti-O-TiI3 + 2H2O(g) f Ti-O-Ti-O(OH) + 3HI(g) (1c) whereas the surface OH groups saturate the coordination spheres of surface Ti atoms.19,20 It is generally known that the density of OH groups is high on the surface of ionic materials21 but rapidly decreasing with increasing temperature.22 OH groups can also be created during the dissociation of molecular water, most effectively on the (110) rutile surface.4 The molecular water can be removed completely at 300-400 °C from the rutile surface23,24 and at 200 °C from the anatase surface2 while OH groups are partially retained, even after evacuation at 400 °C.18 (17) Ritala, M.; Leskela¨, M.; Rauhala, E.; Jokinen, J. J. Electrochem. Soc. 1998, 145, 2194. (18) Parfitt, G. D.; Ramsbotham, J.; Rochester, C. H. J. Chem. Soc., Faraday Trans. 1 1972, 1, 17. (19) Boehm, H.-P. Angew. Chem. 1966, 78, 617. (20) Lindan, P. J. D.; Harrison, N. M.; Holender, J. M.; Gillan, M. J. Chem. Phys. Lett. 1996, 261, 246. (21) Takeda, S.; Fukawa, M.; Hayashi, Y.; Matsumoto, K. Thin Solid Films 1999, 339, 220. (22) Bourgeois, S.; Jomard, F.; Perdereau, M. Surf. Sci. 1992, 279, 349.

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Complete removal of hydrogen-containing species during heat treatment and evacuation is a time-consuming process, lasting several hours.18,23 Indeed, IR studies on the ALD of oxides, such as ZrO225 and TiO2,26 on porous substrates at around 300 °C indicate the intermediate formation of OH groups after water pulses. These data allow one to consider the contribution of OH groups to the H2O- and/or H2O2-assisted growth processes at all temperatures examined in the present study. The extra energy supplied at higher growth temperatures may, expectedly, increase the contribution of bifunctional reactions,26 where TiI4 reacts with two OH groups and becomes bound to two TiO2 surface oxygens: Figure 9. Dependence of ∆m0/∆m1 ratio on growth temperature. Cycle time parameters are given in the inset. Lines indicate the growth reaction schemes discussed in the text. Numbered labels denote the amount of iodo ligands per Ti atom adsorbed and retained on the surface during the TiI4 pulse.

The two OH groups, if created in step 2b, may be attached to the same Ti atom or to neighboring Ti atoms. To correlate the QCM data obtained with possible changes in the growth mechanism, surface mass exchange ratios, ∆m0/∆m1, can be calculated for any hypothetical reaction schemes.9,11 In the case of the metal oxide growth process, this ratio expresses the average mass of the oxide layer, ∆m0, formed in a completed deposition cycle (1b), in relation to the mass change, ∆m1, caused by the adsorption of the metal precursor during its exposure time, t1 (1a). Thus, for the eqs 1a and 1b and eqs 1a-1c, with three iodine atoms per one Ti atom bound to its surface, the ∆m0/∆m1 ratio is 0.187. The ∆m0/∆m1 ratio 0.267 characterizes the sequence in eqs 2a and 2b. It should be emphasized, however, that the nature of the layer-by-layer growth does not require the presence of the OH groups. The surface-controlled reaction mechanism may also be retained in the case of the molecular adsorption of TiI4, provided that no iodine is released in this reaction step and that TiO2 is formed on the surface during the H2O2-H2O pulse (the ∆m0/∆m1 ratio is 0.144). In the case of TiI4 decomposition on the film surface, iodine may leave the surface as I2, and the ratios are 0.186, 0.265, and 0.457 when three, two, and one iodine atom(s) per Ti atom are adsorbed on the surface during the TiI4 pulse. The experimental ∆m0/∆m1 ratios calculated from the QCM records (Figures 1 and 2) are shown in Figure 9. Therefore, when comparing the experimental ∆m0/∆m1 values with those calculated from different reaction models, one can find the most probable reaction mechanism. As shown above, the ∆m0/∆m1 ratio is sensitive to the I/Ti ratio in the adsorbate layer. At the same time, the ratio is almost independent of whether this ratio has been obtained in the exchange reactions with hydroxyl groups or in the TiI4 decomposition process. In addition, desorption of titanium-containing surfaceintermediate species and incompleteness of exchange reactions might influence the experimental ∆m0/∆m1 ratio. However, the value of ∆m0 is independent of purge times. Consequently, desorption of surface species containing (23) Jackson, P.; Parfitt, G. D. Trans. Faraday Soc. 1971, 67, 2469. (24) Jones, P.; Hockey, J. A. J. Chem. Soc., Faraday Trans. 1 1972, 1, 907. (25) Kyto¨kivi, A.; Lakomaa, E.-L.; Root, A. Langmuir 1996, 12, 4395; Kyto¨kivi, A.; Lakomaa, E.-L.; Root, A.; O ¨ sterholm, H.; Jacobs, J.-P.; Brongersma, H. H. Langmuir 1997, 13, 2717. (26) Haukka, S.; Lakomaa, E.-L.; Jylha¨, O.; Vilhunen, J.; Hornytzkyj, S. Langmuir 1993, 9, 3497.

titanium is negligible. Furthermore, no remarkable hydrogen or iodine contamination has been observed by time-of-flight elastic recoil detection analysis.3 The hydrogen content did not exceed 1.2 atom %, while the iodine content was below the detection level of the analysis method. These results confirm that the surface reactions were complete. Formation of crystalline anatase and/or rutile phases, in turn, indicated that stoichiometric TiO2 was dominating in the films. Therefore, the main factor influencing the experimental ∆m0/∆m1 ratio is the I/Ti ratio in the adsorbate layer formed during the TiI4 pulse. The experimental value of ∆m0/∆m1 increases with the temperature in the range 200-300 °C (Figure 9). At the lowest temperatures, the mass exchange ratio is fairly close to that predicted by eqs 1a, 1b, and 1c. At all temperatures, the ratio increases with TiI4 pulse length. As discussed above, a nearly linear increase in ∆m0 and ∆m1 can be observed in the t1 range 2-20 s (Figure 4). This increase was attributed to the thermal decomposition of TiIx proceeding with a constant but markedly lower rate than that of the exchange reactions. Therefore, the ratios describing the fast surface exchange reactions in the beginning of the TiI4 pulse can be obtained by extrapolating the slowly increasing linear part of the ∆m0/ ∆m1 versus t1 curve (Figure 4) to t1 ) 1 s, where the TiI4 adsorption starts. The results of the extrapolation (Figure 9) satisfactorily describe the reaction mechanisms, in which the TiI4 adsorbs by loosing just one iodo ligand. At 200-230 °C, even better agreement with the experimental results can be achieved by assuming that TiI4 adsorbs without any ligand release and all the iodo ligands are exhanged during the oxygen precursor pulse (the lowest horizontal line in Figure 9). In the substrate temperature range 280-350 °C, the prolongation of the TiI4 pulse results in an obvious increase in the ∆m0/∆m1 ratio up to 0.3-0.4 (Figure 9), referring to the increasing role of further dissociation of the TiIx species on the surface. The data still indicate that less than three iodo ligands per Ti atom, on average, are released from an adsorbed TiI4 molecule, even during a 10 s long pulse time. Nevertheless, this thermally enhanced release of iodine likely enables further adsorption of TiI4. Therefore, more Ti becomes available for the following oxidation and, as a result, ∆m0 is increased in comparison with that measured in the case of shorter TiI4 exposures (Figure 7) or at lower processing temperatures. The decrease in the surface mass by ∆mp1, observed during t2 (Figure 1), is directly related to desorption of volatile decomposition or reaction products. Desorption

Atomic Layer Deposition of TiO2

of species that contain titanium can be excluded because the deposited mass of titanium oxide, ∆m0, did not decrease noticeably with increasing t2. The decrease of ∆m2 with the increase in t2 and ∆mp1 additionally indicates the decreased amount of iodine available for the subsequent reaction with the oncoming H2O-H2O2. In the case of fixed TiI4 pulse length, the growth temperature was the main factor influencing the deposition mechanism (Figures 7 and 9). It is noteworthy that the substrate temperature also determined the formation of different crystalline phases in the nonepitaxial films. Phase-pure anatase and rutile were formed below and above 300 °C, respectively (Figure 8). At the same time, equilibrium thermodynamics allows the preferred formation of rutile in the given precursor system in the whole temperature range studied.3 Therefore, it is possible that, besides the direct effect of temperature, the modification of adsorption sites and the decreased amount of iodo ligands in the surface species determine the kinetic limits for lattice densification required in the rutile formation. For comparison, Pedraza et al.29 have noticed that the exploitation of lower chlorides (TiCl3) instead of TiCl4 in the oxidation process enabled preparation of the rutile phase even at room temperature. Comparison with the TiCl4-Based ALD Process. When compared to the TiI4-H2O2 process investigated in the present study, the formation of rutile in the TiCl4H2O ALD process has not been that well-defined in the temperature range 300-375 °C. The critical temperature for the formation of rutile as the dominant phase has varied between 30030 and 350-375 °C,12,14 being evidently dependent on the reactor configuration and/or other process parameters. It is also noteworthy that the behavior of the growth rate, which is studied both in situ and ex situ, is substantially different in the chloride and iodide ALD processes. In the iodide process described in the present study, the growth rate was continuously increasing with the temperature. In the chloride process, on the contrary, the growth rate monotonically decreased with increasing growth temperature.14 In the temperature range 100-300 °C, the decrease was related to the diminishing contribution of chlorine release from TiCl4 during its adsorption. The amount of chlorine, released during the adsorption of TiCl4, decreased with increasing substrate temperature and approached zero at 300 °C.31 At the same time no evidence of the TiCl4 decomposition has been obtained in this temperature range. For this reason, the ligand release could be due to exchange reactions with OH groups, while the reactions had insignificant effects at temperatures above 300 °C. Furthermore, it has been noticed that the agglomeration and crystallization of ZrO2 and TiO2 in an ALD process on powders start at the temperatures where the surface becomes considerably dehydroxylated.25,32 Therefore, it might be suggested that the decrease in the amount of either molecularly or dissociatively adsorbed water can contribute to the formation of denser phases, such as rutile in this study. The results of this work demonstrate, however, that effective ligand release during the titanium precursor (27) Tolstikhina, A. L.; Sorokina, K. L. Crystallogr. Rep. 1996, 41, 320. (28) Plappert, E.-C.; Dahmen, K.-H.: Havert, R.; Ernst, K.-H. Chem. Vap. Deposition 1999, 5, 79. (29) Pedraza, F.; Vazquez, A. J. Phys. Chem. Solids 1999, 60, 445. (30) Matero, R.; Ritala, M.; Leskela¨, M.; Salo, T.; Aromaa, J.; Forsen, O. J. Phys. IV (France) 1999, 9, Pr8-493. (31) Siimon, H.; Aarik, J.; Uustare, T. Electrochem. Soc. Proc. 1997, 97-25, 131. (32) Lindblad, M.; Haukka, S.; Kyto¨kivi, A.; Lakomaa, E.-L.; Rautiainen, A.; Suntola, T. Appl. Surf. Sci. 1997, 121-122, 286.

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adsorption may support rutile growth. Therefore, the advantage of TiI4 in comparison to TiCl4 seems to be its relatively easy ligand release at temperatures above 300 °C. At these temperatures, the densities of OH groups should also be relatively low because of thermal dehydroxylation of the surface. Thus, mainly due to thermally assisted decomposition of TiI4, two preconditions for rutile growthslow areal density of hydroxyls and efficient release of ligands from the adsorbed titanium precursors are simultaneously fulfilled in the TiI4-H2O2 process. A difference between the chloride and iodide ALD may also arise from the role of secondary reactions during the adsorption process. Namely, it has been shown that the reaction product HCl, released in the adsorption of TiCl4 on the hydroxylated rutile surface, reacts with OH groups and releases, in turn, water.2,33 The readsorption of HCl has been studied and considered also in the TiO2 growth on OH-terminated porous silica.34 The readsorbed chlorine can block sites from TiCl4 adsorption. This results in thickness profiles in thin films, since the growth rate decreases with the distance measured from the leading edge of the substrate along the gas flow direction.31,35 When the gaseous reaction products have high sticking probability, the most pronounced thickness gradient appears at the leading edge of the substrate followed by remarkably smaller gradients at the central part and trailing edge. These kinds of calculated thickness profiles31,35 are qualitatively consistent with those measured experimentally for the TiCl4-H2O process.36 It has also been predicted that low readsorption probability (i.e., low reactivity of gaseous reaction products) results in essentially smaller and more linear thickness profiles over the whole substrate length.31,35 Such thickness profiles have been observed, for instance, in the Ta2O5 films grown from Ta(OC2H5)5 and H2O,6 as well as in the TiO2 films grown from TiI4 and H2O2.3 This fact indirectly indicates that, similar to ethanol molecules, HI is probably not an actively readsorbing reaction product compared to HCl. Finally, our results demonstrate that changes in the H2O-H2O2 pulse, dose, and purge times weakly influence the growth mechanism and growth rate. Although the effect of the TiI4 pulse length is markedly stronger, the growth temperature is the dominant factor responsible for the changes in the growth rate and mechanism and thin film structure. This is consistent with the results obtained for the TiO2 films grown on soda lime glass,3 where the variation of substrate temperature had a much stronger influence on the crystal structure and growth rate than the variation of cycle times. Summary A quartz crystal microbalance method enabled monitoring of the TiI4 and H2O2 chemisorption processes during atomic layer deposition of TiO2 thin films. At 170-200 °C, the film growth rate was insensitive to deposition temperature, and the adsorption processes of both precursors tended to saturate. The film growth rate increased with the increase of the substrate temperature from 200 to 300 °C. At temperatures above 300 °C, however, it was again almost invariant. According to the XRD data, the anatase phase could be grown at temperatures below 300 °C. At 305 °C, the rutile (33) Parfitt, G. D.; Ramsbotham, J.; Rochester, C. H. Trans. Faraday Soc. 1971, 67, 3100. (34) Haukka, S.; Lakomaa, E.-L.; Root, A. J. Phys. Chem. 1993, 97, 5085. (35) Siimon, H.; Aarik, J. J. Phys. D.: Appl. Phys. 1997, 30, 1725. (36) Ritala, M.; Leskela¨, M.; Nyka¨nen, E.; Soininen, P.; Niinisto¨, L. Thin Solid Films 1993, 225, 288.

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phase was observed in the films while, at even higher temperatures, this phase became the dominant one. The QCM data allowed us to conclude that the growth temperature influenced the mechanism of surface reactions. At 200-230 °C, TiI4 adsorbed without essential loss of the iodo ligands. The increase in the growth rate observed at higher temperatures was possibly attributed to the increase in the probabilities for polyfunctional surface reactions and decomposition of the titanium precursor. At these temperatures, some ligands were already released in the initial stage of TiI4 adsorption, and a marked decomposition of species, which had reactively adsorbed on the thin film surface, was observed during the TiI4 pulse and following purge time. Thus, a

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significant loss of iodo ligands and the consequent increase in the areal density of Ti atoms in the surface layer appeared at temperatures above 300 °C, where the rutile phase was formed. Acknowledgment. The authors are indebted to Ms. Alma-Asta Kiisler for the technical assistance. The study has been partially supported by the Estonian Science Foundation (Grant No. 4205), the Swedish Institute, the Academy of Finland, and the Finnish National Technology Development Agency TEKES. LA0004451