Surface Reaction Mechanisms during Ozone-Based Atomic Layer

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2008, 112, 9552–9554 Published on Web 06/10/2008

Surface Reaction Mechanisms during Ozone-Based Atomic Layer Deposition of Titanium Dioxide Vikrant R. Rai and Sumit Agarwal* Department of Chemical Engineering, Colorado School of Mines, Golden, Colorado 80401 ReceiVed: April 02, 2008; ReVised Manuscript ReceiVed: May 09, 2008

We have investigated the surface reaction mechanisms during the atomic layer deposition (ALD) of TiO2 using titanium tetraisopropoxide (TTIP) and ozone using in situ attenuated total reflection Fourier transform infrared spectroscopy. The ozone reaction mechanism is fundamentally different than water-based ALD of metal oxides, where -OH groups are the reactive sites for metal alkoxides. In ozone-based ALD, the isopropoxy-ligand-terminated surface, generated after TTIP exposure, reacts with ozone, forming different surface carbonates. Upon subsequent TTIP exposure, the isopropoxy ligands chemisorb on the surface carbonates, releasing CO2. The growth rate, ∼0.52 Å/cycle, is almost independent of the substrate temperature between 150 and 250 °C. TiO2 is a technologically important material due to its high dielectric constant and refractive index, good transparency in the visible and the infrared region, and high photocorrosion resistance.1 TiO2 thin films are commonly deposited using thermal or plasma-assisted chemical vapor deposition (CVD) techniques using metal halides and metal alkoxides as precursors. Among the CVD techniques, atomic layer deposition (ALD) is an attractive method for depositing conformal films with abrupt interfaces on high aspect ratio nanostructures.2–4 Titanium tetraisopropoxide (TTIP, Ti[OCH(CH3)2]4)5,6 and TiCl47,8 are the most commonly used precursors in TiO2 ALD, with H2O as the oxidizing agent.8–12 In some studies, H2O25 and O radicals13 have also been employed as oxidizers. More recently, O3 has been explored as an alternative14 to H2O in metal-oxide ALD to lower the deposition temperature and to reduce the purge time in cold-wall reactors. The self-limiting surface reactions that occur during each half cycle in an ALD process determine the film’s composition and its properties. However, thus far, the detailed surface reaction chemistry of a relatively small set of ALD processes has been explored.15 For TiO2 ALD, surface reactions have been reported using techniques such as infrared (IR) spectroscopy,9 quartz crystal microbalance,10,11 and quadrupole mass spectrometry.10,11 In H2O-based and O2-plasma-based ALD processes, hydroxyl groups have generally been reported as the reactive sites for the chemisorption of the metal precursor,3,15 including TiO2.11,16 In this paper, the authors report the surface reaction mechanism during the ALD of TiO2 from TTIP and O3, which involves a completely different mechanism in which surface carbonates are identified as the reactive sites for TTIP chemisorption. TiO2 films were deposited in a cold-wall ALD reactor equipped with an in situ attenuated total internal reflection Fourier transform infrared (ATR-FTIR) spectroscopy setup.17 Deposition was done on ZnSe internal reflection crystals (IRCs) * To whom correspondence [email protected].

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with dimensions of 50 mm × 20 mm × 1 mm; the short faces of the crystal were beveled at 45°. IR radiation from a spectrometer (Nicolet 6700) was directed with a series of mirrors and focused by a lens through a KBr window onto the beveled edge of the IRC. In our geometry, the IR beam has 25 reflections on each flat face of the crystal before it emerges from the opposite beveled edge and is focused onto a HgCdTe (MCT A) detector. We chose ZnSe as the IRC material since its refractive index is closely matched to that of amorphous TiO2, and therefore, total internal reflection occurs at the vacuum-film interface.18 Consequently, the beam passes through the growing film 50 times, greatly enhancing the signal-to-noise ratio, which was further improved by averaging each IR spectrum over 200 scans. Submonolayer sensitivity for surface adsorbates was obtained for each half reaction cycle. The sensitivity of this setup in reflectance, R, is ∆R/R ≈ 10-4 per reflection. All IR data were collected as difference spectra, that is, a new background spectrum was recorded after each half reaction cycle. Therefore, the change in absorbance in these spectra represents changes in the surface chemical composition that occur only due to the reactions of the precursor introduced in that cycle. To ensure that we were probing the reactions with only a TiO2 surface, the ZnSe IRC was coated with 50-100 nm of the oxide film prior to these measurements. The IR data were used to determine the minimum precursor dosage required for surface saturation during both the TTIP and O3 cycles. TTIP was delivered into the vacuum chamber using N2 as the carrier gas. The bubbler containing TTIP and the vapor delivery lines were maintained at a temperature of 76-78 °C to prevent precursor condensation. O3 was delivered by passing O2 through an inline, corona-discharge-based O3 generator, which produced ∼6 wt % O3. TTIP flow was pulsed into the chamber using a solenoid valve, while O3 was pulsed using a mass flow controller. N2 was used as the purge gas between each half reaction cycle. TTIP was pulsed for 1 s followed by a 15 s purge with N2 at a flow rate of 300 sccm, whereas O3 was pulsed for 30 s followed by a 20 s purge with N2 also at  2008 American Chemical Society

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Figure 1. IR difference spectra collected during TTIP exposure of a TiO2 surface at a substrate temperature of 150 °C. The background was collected after O3 exposure.

300 sccm. The chamber pressure during the pulse duration was not constant; the highest pressures recorded during TTIP and O3 cycles were 800 and 370 mTorr, respectively. These TiO2 films were also simultaneously deposited on Si wafers at the same temperature as the IRC, and their thickness and refractive index were measured using ex situ spectroscopic ellipsometry (Woollam M-44). The ellipsometry data was fitted over the wavelength range of 400-1300 nm using the Cauchy model.19 For ex situ ellipsometry, films were deposited up to a total thickness of ∼15-20 nm. Films were grown over a temperature range of 150-250 °C; however, the IR data reported here are for films deposited at 150 °C. Figure 1 shows the temporal evolution of IR difference spectra during TTIP exposure of a TiO2 surface at a substrate temperature of 150 °C. The background spectrum was collected after O3 exposure. Ligand-exchange reactions can be directly observed in the spectra in Figure 1; an increase in absorbance is due to freshly adsorbed surface species, and any decrease in absorbance is due to the reaction of previously adsorbed species with the incoming precursor. Upon TTIP exposure, an increase in absorbance for fingerprint spectral regions of the isopropoxy ligands was observed (see Figure 1). These include CHx (x ) 1,3) stretching modes in the 2800-3000 cm-1 region, CO stretching modes in the 1000-1100 cm-1 region, and weak CHx (x ) 1,3) bending modes in the 1300-1450 cm-1 region.20,21 Vibrational frequencies were assigned to different structural units. CH3 antisymmetric stretches were not doubly degenerate22 and were centered at 2972 and 2977 cm-1, while the symmetric stretching mode was at 2885 cm-1.21–23 The vibrational mode at 2935 cm-1 was assigned to the CH stretching mode.21,23 The vibrational bands at 1008 and 1124 cm-1 correspond to the symmetric and antisymmetric stretching modes, respectively, of C-O in the isopropoxy ligand.20,21,23 Since no change in absorption was observed in the 3200-3800 cm-1 region, we conclude that there were no hydroxyl groups present on the surface after O3 exposure. Therefore, other functional groups on the surface must be the reactive sites for TTIP chemisorption, and absorption due to these sites must decrease in the difference spectra in Figure 1. Indeed, in Figure 1, in the 1300-1700 cm-1 region, there is a decrease in absorbance due to several overlapping bands. These bands have been assigned in the literature to metal bicarbonates (HCO3-),24–29 carbonates (CO32-),24–29 and formates (HCOO-).25,26 The carbonate-like bands in some cases are believed to be strongly perturbed CO2 molecules on the surface, called carboxylates.25 Bicarbonates and carbonates are formed due to the simultaneous reaction of CO2 and H2O with

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Figure 2. IR difference spectra collected during the TTIP (a and b) and O3 (c and d) cycles. In spectra (b) and (d), the chamber was isolated from the vacuum pump to accumulate the reaction products. An increase in absorbance due to gas-phase CO2 was observed only in these cycles, indicating that CO2 is a surface reaction product during both half cycles. With the pumping port open (a and c), CO2 was removed from the chamber as it was generated and, therefore, not observed in the IR spectra.

a metal-oxide surface,30 but the proportion of CO2 adsorbing as carbonates increases with increasing temperature.27 In fact, for most metal-oxide surfaces, bicarbonates are not stable at a temperature g 150 °C and decompose to form carbonates.27,28 Formates are formed when CO reacts with a -OH-terminated metal-oxide surface.25 To our knowledge, the characteristic vibrations for the C-O symmetric and anitsymmetric stretching modes for these species on amorphous TiO2 surfaces have not been conclusively identified. However, these vibrational frequencies have been identified for anatase and rutile surfaces29 and other metal oxides similar to TiO2, such as ZrO2.25,27 Characteristic carbonate bands are reported for a ZrO2 surface at 1450 and 1430 cm-1 for polydentate (p-CO32-),26,27 1595 and 1315 cm-1 for bidentate (b-CO32-),26,27 and 1375 and 1355 cm-1 for monodentate (m-CO32-)26,27 carbonates. The formate groups show characteristic absorption bands on the TiO2 surface at 1580 and 1365 cm-1 for CO antisymmetric and symmetric stretches, respectively, and at 2870 cm-1 for the CH stretching mode.31 It is also been reported that carboxylate complexes can be formed due to the chemisorption of CO on rutile or anatase surfaces with vibrations at 1560 and 1360 cm-1.25 Given the similar positions of these bands, it was not possible to uniquely identify these species simply based on their IR absorption. To identify chemical species present on the surface, we observed the reaction products that were formed upon TTIP chemisorption. Since our IR measurements showed negligible carbon incorporation in the films, a possible elimination mechanism for the carbon incorporated into the surface after the O3 cycle may be the desorption of CO2 or CO from the metal carbonates, formates, or carboxylates into the gas phase upon TTIP chemisorption. Both molecules are IR active and can be detected by the IR beam, which has a ∼80 cm long path through the chamber. The antisymmetric stretching mode for CO2 is at 2350 cm-1, and the stretching mode for CO is at 2149 cm-1. However, all of the spectra in Figure 1 and spectrum a in Figure 2 show no IR signature of these molecules. This does not imply that no CO2 and/or CO are released in the reaction since their concentration may be below the detection limit of the setup. It is reasonable to expect a low concentration of these molecules in the chamber since there would be at most one monolayer equivalent desorbing from the surface, which would be rapidly pumped out of the chamber along with the excess precursor. To enhance the concentration of CO2 and/or

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Figure 3. Temporal evolution of the integrated absorbance change for the CHx (x ) 1,3) (0) and CO32- (O) stretching regions during TTIP exposure of a TiO2 surface at a substrate temperature of 150 °C. The first-order time constants for the exponential fits (s) for both CHx and CO32- are similar. The inset shows the growth per cycle and refractive index over the temperature range of 150-250 °C.

CO, during one of the TTIP exposure cycles, the chamber was isolated from the pumping port by a gate valve. Therefore, all reaction products were trapped in the chamber, and their concentration increased gradually. An IR spectrum was collected under these conditions, which is shown in Figure 2b. In this spectrum, gas-phase CO2 was unambiguously detected at ∼2350 cm-1, but no absorption band for CO was observed. It should be pointed out that H2O should also be released from the surface along with CO2. However, H2O has a very high sticking probability on stainless steel at room temperature and, therefore, is lost to the walls and not detected along with CO2 in the spectra in Figure 2. On the basis of this data, we conclude that the reactive sites for TTIP are surface carbonates, and CO2 is released from these sites upon TTIP chemisorption. Since no CO was detected and because there are several overlapping vibrational bands in the 1300-1700 cm-1 region in Figure 1, we also conclude that different types of metal carbonates, m-, b-, and p-CO32-, coexist on the surface. Wang et al.32 had also previously speculated that carboxylic acid and a nitro group with spectral fingerprints in the 1500-1700 cm-1 region were the reactive sites during the ALD of HfO2 from tetrakis(ethylmethyl amino hafnium) and O3. The carbonates species, which are the reactive sites for TTIP, must be formed due to the reaction of O3 with the isopropoxy ligands. We speculate that O3 combusts the hydrocarbon ligands, producing CO2 and H2O. Under these conditions, metal bicarbonates can be formed on the surface, which would rapidly decompose to form carbonates at a substrate temperature of 150 °C.27 The formation of these carbonates is apparent in the difference spectra in Figure 2c and d, where an increase in absorbance is observed in the 1300-1700 cm-1 region, while the CHx (x ) 1,3) stretching modes in the ∼3000 cm-1 region show a decrease in absorbance. If the reaction of isopropoxy groups with O3 produces CO2, it could be detected in the gas phase in the IR spectra. Again, in the difference spectrum in Figure 2c, recorded during O3 exposure of an isopropoxyterminated surface, no CO2 was detected. However, a clear CO2 absorption band was detected upon isolating the chamber from the pumping port (see Figure 2d). On the basis of the data in Figure 2c and d, we conclude that while most of the CO2 desorbs into the gas phase and gets pumped out, a small fraction reacts with the surface to form the carbonates. The reaction of TTIP with surface carbonates was further confirmed by examining the temporal evolution of the integrated

Letters absorbance for CHx (x ) 1,3) and CO32- stretching modes, shown in Figure 3. During TTIP exposure, the CHx (x ) 1,3) modes showed an increase in integrated absorbance while the CO32- modes showed a decrease; the exponential fits to the data in Figure 3 revealed almost identical time constants. This is consistent with a pseudo-first-order reaction of TTIP with surface carbonate sites. The inset in Figure 3 shows the growth per cycle and the refractive index for films deposited over a temperature range of 150-250 °C, which are nearly independent of temperature over the range studied. Therefore, for TiO2 deposited from TTIP and O3, the ALD window exists at least over the above temperature range. The growth per cycle of ∼0.5 Å/cycle with a refractive index of ∼2.2 is comparable to previous work on H2O-based,6 O2-plasma-assisted,13 and O3based ALD of TiO2,14 suggesting that the growth rate may be limited by the stearic hindrance of the TTIP ligands and not by the type of growth sites created by the oxidizing agent. Acknowledgment. The authors would like to thank Dr. C. A. Wolden for several insightful discussions. References and Notes (1) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (2) Suntola, T. Thin Solid Films 1992, 216, 84. (3) George, S. M.; Ott, A. W.; Klaus, J. W. J. Phys. Chem. 1996, 100, 13121. (4) Leskela, M.; Ritala, M. Angew. Chem., Int. Ed. 2003, 42, 5548. (5) Aarik, J.; Aidla, A.; Uustare, T.; Ritala, M.; Leskela, M. Appl. Surf. Sci. 2000, 161, 385. (6) Ritala, M.; Leskela, M.; Niinisto, L.; Haussalo, P. Chem. Mater. 1993, 5, 1174. (7) Aarik, J.; Aidla, A.; Kiisler, A. A.; Uustare, T.; Sammelselg, V. Thin Solid Films 1997, 305, 270. (8) Ritala, M.; Leskela, M.; Nykanen, E.; Soininen, P.; Niinisto, L. Thin Solid Films 1993, 225, 288. (9) Gu, W.; Tripp, C. P. Langmuir 2005, 21, 211. (10) Matero, R.; Rahtu, A.; Ritala, M. Chem. Mater. 2001, 13, 4506. (11) Rahtu, A.; Ritala, M. Chem. Vap. Deposition 2002, 8, 21. (12) Ritala, M.; Leskela, M.; Johansson, L. S.; Niinisto, L. Thin Solid Films 1993, 228, 32. (13) Lim, J. W.; Yun, S. J.; Lee, J. H. Electrochem. Solid State Lett. 2004, 7, F73. (14) Kim, S. K.; Kim, W. D.; Kim, K. M.; Hwang, C. S.; Jeong, J. Appl. Phys. Lett. 2004, 85, 4112. (15) Puurunen, R. L. J. Appl. Phys. 2005, 97, 121301. (16) Heil, S. B. S.; Kudlacek, P.; Langereis, E.; Engeln, R.; van de Sanden, M. C. M.; Kessels, W. M. M. Appl. Phys. Lett. 2006, 89, 3. (17) Harrick, N. J. Internal Reflection Spectroscopy; John Wiley: New York, 1967. (18) Agarwal, S.; Hoex, B.; van de Sanden, M. C. M.; Maroudas, D.; Aydil, E. S. J. Vac. Sci. Technol., B 2004, 22, 2719. (19) Tompkins H. G., McGahan, W. A. Spectroscopic Ellipsometry and Reflectometry: A User’s Guide John Wiley: New York, 1999. (20) Lynch, C. T.; Mazdiyasni, K. S.; Crawford, W. J.; Smith, J. S. Anal. Chem. 1964, 36, 2332. (21) Moran, P. D.; Bowmaker, G. A.; Cooney, R. P.; Finnie, K. S.; Bartlett, J. R.; Woolfrey, J. L. Inorg. Chem. 1998, 37, 2741. (22) Colthup, N. B. Daly, L. H., Wiberly, S. E. Introduction to Infrared and Raman Spectroscopy, 3rd ed.; Academic Press: San Diego CA, 1990. (23) Burgos, M.; Langlet, M. Thin Solid Films 1999, 349, 19. (24) Baltrusaitis, J.; Jensen, J. H.; Grassian, V. H. J. Phys. Chem. B 2006, 110, 12005. (25) Busca, G.; Lorenzelli, V. Mater. Chem. 1982, 7, 89. (26) Cho, B. O.; Lao, S. X.; Chang, J. P. J. Appl. Phys. 2003, 93, 9345. (27) Pokrovski, K.; Jung, K. T.; Bell, A. T. Langmuir 2001, 17, 4297. (28) Primet, M.; Pichat, P.; Mathieu, M. V. J. Phys. Chem. 1971, 75, 1221. (29) Yates, D. J. C. J. Phys. Chem. 1961, 65, 746. (30) Henderson, M. A. Surf. Sci. 1998, 400, 203. (31) Munuera, G. J. Catal. 1970, 18, 19. (32) Wang, Y.; Dai, M.; Ho, M. T.; Wielunski, L. S.; Chabal, Y. J. Appl. Phys. Lett. 2007, 90, 3.

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