Mechanism of Pyridine-Catalyzed SiO2 Atomic Layer Deposition

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J. Phys. Chem. C 2007, 111, 219-226

219

Mechanism of Pyridine-Catalyzed SiO2 Atomic Layer Deposition Studied by Fourier Transform Infrared Spectroscopy Y. Du,† X. Du,‡ and S. M. George*,†,‡ Department of Chemistry and Biochemistry, and Department of Chemical and Biological Engineering, UniVersity of Colorado, Boulder, Colorado 80309 ReceiVed: June 20, 2006; In Final Form: October 12, 2006

Fourier transform infrared (FTIR) investigations were performed to study the mechanism of catalytic SiO2 atomic layer deposition (ALD) using pyridine as the catalyst. Pyridine adsorption on hydroxylated SiO2 surface was examined by monitoring both the changes to the O-H stretching vibrations and the appearance of pyridine molecular vibrations. The strong hydrogen bonding of pyridine to the isolated hydroxyl groups with a desorption energy of 9 ( 2 kcal/mol is believed to make oxygen a stronger nucleophile for nucleophillic attack on the SiCl4 reactant. The SiCl4 reaction with the hydroxylated SiO2 surface was then investigated by monitoring the disappearance of O-H stretching vibrations and appearance of Si-Cl stretching vibrations. These FTIR results revealed that the SiCl4 reaction completely removed the isolated hydroxyl species and left a small fraction of hydrogen-bonded hydroxyl species. The H2O reaction was studied by observing the disappearance of the Si-Cl stretching vibration and the appearance of the O-H stretching vibration. To understand the role of pyridine during the H2O reaction, the Si-Cl stretching vibration of the SiClx surface species was monitored during pyridine adsorption. The weak interaction between pyridine and the SiClx surface species suggested that the pyridine catalyzes the H2O reaction by hydrogen bonding to the incoming H2O reactant. The FTIR spectra also revealed that a pyridinium salt was left behind on the SiO2 surface at lower temperatures after both the SiCl4 and H2O reactions. The pyridinium salt can desorb from the SiO2 surface and no pyridinium salt was observed for surface temperatures >340 K after either the SiCl4 or H2O reactions.

I. Introduction Atomic layer deposition (ALD) is a thin film growth technique based on sequential, self-limiting surface reactions.1,2 ALD can achieve atomic layer controlled and conformal thin film growth. Low-temperature ALD has recently facilitated the deposition of inorganic films on polymeric and low thermal stability substrates.3-5 One of the lowest temperature ALD processes is catalyzed SiO2 ALD.6-8 Catalyzed SiO2 ALD can deposit SiO2 thin films at room temperature with SiCl4 and H2O as the reactants and pyridine as the catalyst.7-9 The two sequential, self-limiting surface half-reactions for SiO2 ALD can be expressed as:6-10

(A) SiOH* + SiCl4 f SiO-SiCl3* + HCl

(1)

(B) SiCl* + H2O f Si-OH* + HCl

(2)

The SiCl4 and H2O half-reactions are performed in an ABAB... binary sequence to grow SiO2. Without the catalyst, both halfreactions proceed at temperatures above 600 K and reactant exposures on the order of 109 Langmuir (1 Langmuir ) 10-6 Torr s).10 The reaction mechanism during uncatalyzed SiO2 ALD can be understood by theoretical simulations.11 With the catalyst, the SiO2 deposition can be performed at room temperature and reactant exposures as low as 104 L.6-8 SiO2 growth rates of 1.3-2.1 Å per AB cycle can be obtained at room temperature.6-9 The growth rate is lower when using a viscous flow ALD reactor9 compared with static reactant exposures.7,8 † ‡

Department of Chemistry and Biochemistry. Department of Chemical and Biological Engineering.

There are many possible applications of catalyzed SiO2 ALD. Catalyzed SiO2 ALD could be employed to deposit SiO2 films on polymers at low temperatures.9 These SiO2 ALD films on polymers may serve as effective gas diffusion barriers.12 The extremely low temperature of catalyzed SiO2 ALD may also facilitate the deposition of SiO2 films on biological samples. In addition, other investigations have utilized catalyzed SiO2 ALD to reduce the pore size of gas separation membranes.13,14 By varying the type of Lewis base catalyst, the potential exists to reduce pore sizes down to a particular diameter that is defined by the size of the Lewis base catalyst. The mechanism of catalyzed SiO2 ALD has been explained by the hydrogen bonding interaction between the nitrogen lonepair electrons on the pyridine and either the surface hydroxyls during the SiCl4 reaction or the H2O reactant during the H2O reaction.7,8 This interaction makes the oxygen a better nucleophile for nucleophillic attack on the SiCl4 or the SiClx* surface species.15-17 The catalyst dramatically lowers the SiO2 deposition temperatures and reduces the precursor exposures by orders of magnitude.7,8 Previous theoretical investigations support this role of pyridine in catalyzed SiO2 ALD.18 However, the detailed mechanism of catalyzed SiO2 ALD remains elusive and additional studies are needed to clarify the role of the Lewis base catalyst. The FTIR investigations presented in this paper were performed to understand the mechanism of catalyzed SiO2 ALD. FTIR studies were employed to evaluate the interaction of pyridine on the hydroxylated SiO2 surface after the H2O reaction and on the chlorinated SiO2 surface after the SiCl4 reaction. The SiCl4 and H2O reactions were also monitored by following

10.1021/jp0638484 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/05/2006

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the SiO-H and Si-Cl stretching vibrations. These studies observed the self-limiting SiO2 ALD surface chemistry and identified the specific hydroxyl groups that react with SiCl4. The FTIR spectra also revealed a pyridinium salt reaction product on the SiO2 surface at low temperatures following the SiCl4 and H2O reactions. The removal of this pyridinium salt is necessary to prevent the poisoning of the surface reaction and impurities in the SiO2 ALD film. II. Experimental Section The experiments were performed in a vacuum chamber equipped with a Nicolet 670 FTIR spectrometer with a MCT/B infrared detector. This experimental apparatus has been described earlier.9 High surface area samples are required for the transmission FTIR investigations.19 These studies employed SiO2 powders with a particle size of 7 nm and surface area of 380 m2/g (Adrich, fumed silica). These SiO2 powders were pressed into a tungsten grid.19,20 These samples typically provided a surface area of >500 cm2 in the FTIR beam. The SiO2 powder sample was first annealed at 600 K for 3 h to remove organic contamination and a fraction of the hydrogen-bonded hydroxyl groups.21 After annealing, the sample was cooled to 305 K in a high-purity N2 flow. At least 3 AB cycles of catalyzed SiO2 ALD film growth were then performed to provide a reproducible surface. The SiCl4 reactant exposures were 108-109 L. The H2O reactant exposures were 107-108 L. A SiCl4 exposure of 104 L removes nearly all of the isolated hydroxyl groups on the SiO2 surface. Larger SiCl4 exposures were used to react all the isolated hydroxyl groups and a fraction of the hydrogen-bonded hydroxyl groups. For pyridine adsorption experiments, the chamber was first pumped in the absence of a N2 flow for several minutes. The gate valves that isolate the chamber from the CsI salt windows were then opened to allow the IR beam to pass through the SiO2 powder sample. The chamber was then closed from the pump by using a gate valve. Pyridine was introduced into the chamber through a solenoid valve. The pyridine pressure was adjusted by monitoring the Baratron and opening and closing the leak valve. Approximately 3 min was required to stabilize the pyridine pressure before acquiring the FTIR spectra. The FTIR spectrum was recorded in ∼100 s in the range from 400 to 4000 cm-1 with a resolution of 4 cm-1. If the pyridine pressure decreased, additional pyridine was added to the chamber by opening the solenoid valve. Pyridine pressure in the chamber was controlled with an accuracy of (0.01 Torr. At a particular temperature, pyridine adsorption experiments were performed by recording FTIR spectra from low to high pyridine pressures. The pyridine adsorption experiments were performed from high to low temperatures. The background spectrum displays almost no change versus sample temperature. However, a new background spectrum was used for each adsorption temperature. The temperature accuracy was about (2 °C during the pyridine adsorption experiments. D2O (deuterium oxide) was employed to determine if SiO-H stretching vibrations correspond to hydroxyl groups on the SiO2 surface that are isotopically exchangeable. D2O was introduced into the chamber with the gate valves closed to isolate the chamber from the CsI salt windows. FTIR spectra were collected after pumping out the chamber and purging the chamber with a N2 flow for at least 3 min. Typical D2O exposure pressures ranged from 1 to 5 Torr with exposure times from 3 to 10 min. These D2O exchange experiments revealed that the unreacted hydrogen-bonded hydroxyls after the SiCl4 reaction and a subsequent anneal were surface species.

Figure 1. FTIR spectra in the SiO-H stretching vibration region of hydroxylated SiO2 surface with different pyridine pressures at 328 K. Spectra have been displaced for clarity in presentation.

Pyridinium salt was detected by the FTIR spectra after either the SiCl4 or H2O half-reaction at lower temperatures. The desorption of the pyridinium salt occurred slowly under a N2 flow at room temperature. Heating greatly accelerated the pyridinium salt desorption rate. Heating was performed in the chamber under a N2 flow. The surface after the H2O reaction and a subsequent anneal to 600 K for 30 min defined the “hydroxylated” surface. The surface after the SiCl4 reaction and a subsequent anneal to 600 K for 30 min defined the “chlorinated” surface. The difference between the surface after the H2O reaction and the hydroxylated surface is largely the pyridinium salt. The difference between the surface after the SiCl4 reaction and the chlorinated surface is mostly the pyridinium salt. III. Results and Discussion 1. Pyridine Adsorption on Hydroxylated SiO2 Surfaces. Pyridine adsorption on metal oxide surfaces has been widely studied and used to identify surface sites.22-26 Pyridine can adsorb on oxide surfaces at three sites: the hydroxyl, Brønsted, and Lewis acid sites. Figure 1 shows the spectrum of the isolated SiO-H stretching vibrations with a peak absorbance at ∼3746 cm-1 on the hydroxylated SiO2 surface for different pyridine pressures at 328 K. Pyridine strongly interacts with the isolated hydroxyl groups and reduces the peak intensities of the isolated hydroxyl groups and red-shifts the absorbance to lower frequencies. Similar behavior has been observed for NH3 interacting with hydroxyl groups on the SiO2 surface.27 Similar pyridine adsorption experiments were performed at different temperatures. To compare all of these results, a pyridine coverage of unity was assumed on the isolated hydroxyl groups when the isolated hydroxyl peak totally disappeared from the FTIR spectrum. No pyridine coverage is measured by the full absorbance of the isolated SiO-H stretching vibrations. The pyridine coverage is defined by (1 - A), where A is the normalized integrated absorbance of the isolated SiO-H stretching vibration. This integrated absorbance was determined between 3724 and 3760 cm-1 using a constant baseline. The pyridine coverage on the isolated hydroxyl groups versus pyridine pressure at different temperatures is presented in Figure 2. Interaction of pyridine with isolated hydroxyl groups was completely reversible. Vibrational modes from the adsorbed pyridine molecules were also observed in the FTIR spectrum. The assignment of the vibrational frequencies has been extensively studied by using

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Figure 4. Pyridine coverage on hydroxyl groups of hydroxylated surface determined by integrated absorbance of ν19b (H-bonded) molecular vibration versus pyridine pressure and temperature. Figure 2. Pyridine coverage on isolated hydroxyl groups on the hydroxylated SiO2 surface determined by integrated absorbance of the SiO-H stretching vibration for isolated hydroxyl groups versus pressure and temperature.

Lewis acid sites were not observed on the initial SiO2 powder samples. However, the Lewis acid sites became evident after annealing at 300 °C for 3 h and subsequently performing several AB cycles of catalyzed SiO2 ALD. These Lewis acid sites may result from strained Si-O-Si bridge species formed by catalyzed SiO2 ALD. Figure 3 shows that the hydrogen-bonded pyridine increased with pyridine pressure. However, the pyridine on Lewis acid sites did not change noticeably with pyridine pressure. The hydrogen-bonded pyridine was deconvoluted by using Original Professional 6.0 software. Results for the integrated absorbance of hydrogen-bonded pyridine as measured by the ν19b (Hbonded) molecular mode at different temperatures are given in Figure 4. There are similarities between the pyridine coverage measured by using the isolated SiO-H stretching vibrations in Figure 2 and the pyridine molecular vibrations in Figure 4. The desorption energies of pyridine from the hydroxyl groups on the isolated hydroxylated SiO2 surfaces can be obtained by using the Langmuir isotherm equation:

θ) Figure 3. FTIR spectra of pyridine molecular vibrations on hydroxyl groups (ν8a and ν19b (H-bonded)) and Lewis acid sites (ν19b (L)) for different pyridine pressures at 328 K. Spectra have been displaced for clarity in presentation.

theoretical simulations.28,29 These molecular vibrational modes are shown in Figure 3 for pyridine adsorption at 328 K. The ν8a modes arise only from hydrogen-bonded pyridine on hydroxyl groups.30,31 The ν19b vibrational mode results from pyridine on both hydroxyl groups and Lewis acid sites.30 Pyridine on hydroxyl groups and Lewis acid sites can be distinguished from the different ν19b peak positions. The peak position is at 1440 to 1447 cm-1 for pyridine on the hydroxyl groups.30 The peak position is at 1447 to 1460 cm-1 for pyridine on Lewis acid sites.26,30,31 Lewis acid sites are typically not observed on SiO2 surfaces unless the SiO2 sample has been annealed to create strained Si-O-Si bridge species.32,33 In addition, electropositive Lewis acid binding sites on SiO2 surfaces can be created by adding dopant cations or electronegative species bound to silicon.34

Φτ 1 + Φτ

(3)

In this equation, θ is pyridine coverage, Φ is the pyridine flux to the surface, and τ is the pyridine residence time. The residence times, τ, for pyridine adsorption were obtained at different temperatures by fitting the data with use of the Langmuir isotherm equation. The desorption rate, ν, is defined by ν ) 1/τ. The desorption rate is also proportional to exp[Ed/RT], where Ed is the desorption energy. The desorption energy can be obtained by plotting ln(ν) vs 1/T in the Arrhenius plot shown in Figure 5. The pyridine desorption energy is Ed ) 9 ( 2 kcal/mol on the isolated hydroxyl groups. Figure 5 shows that the pyridine desorption energy is nearly the same when measured by adsorption on isolated hydroxyl groups using the isolated SiO-H stretching vibrations or by pyridine adsorption on hydrogen-bonding sites using the ν19b vibrational mode of pyridine. The desorption energy of Ed ) 9 ( 2 kcal/mol for pyridine desorption from isolated hydroxyl groups is in reasonable agreement with previous studies of pyridine on hydroxylated SiO2 surfaces. Earlier spectroscopic ellipsometry studies with

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Figure 5. Arrenhius analysis of pyridine desorption rates obtained from the Langmuir adsorption isotherms. Desorption rates were derived from the FTIR spectra of isolated hydroxyl groups and ν19b (H-bonded) molecular vibrations for pyridine.

BET analysis measured a pyridine adsorption energy of 12.3 ( 0.9 kcal/mol on a hydroxylated silanol surface.7,8 Related ellipsometry investigations also obtained an adsorption energy of 9.4 ( 0.8 kcal/mol for NH3 adsorption on a hydroxylated silanol surface.6 Other chromatographic measurements of pyridine adsorption on Vydac silica have indicated that the majority of the adsorption energies for pyridine are e10.5 kcal/mol.35 The hydrogen bonding of pyridine on the hydroxyl groups weakens the O-H bond and causes a red-shift in the O-H stretching vibration. This interaction also makes the oxygen a much stronger nucleophile for nucleophillic attack.15-17 Previous studies have demonstrated that the attachment of either chlorosilanes16,17 or alkoxysilanes15 on hydroxylated silica surfaces is facilitated by pyridine. These silane attachment reactions are very similar to the SiCl4 reaction during catalyzed SiO2 ALD. 2. SiCl4 Reaction with Hydroxylated Surface. The hydroxylated SiO2 surface was characterized in the SiO-H stretching vibration region by Figure 6a(1) and in the Si-Cl stretching vibration region by Figure 6b(1). The hydroxylated surface was then exposed to a SiCl4 and pyridine vapor mixture for the catalyzed SiCl4 half-reaction. The vapor pressure was 1.6 Torr for SiCl4 and 0.2 Torr for pyridine. The exposure time was 5 min. After the catalyzed SiCl4 half-reaction, the chamber was evacuated and a new FTIR spectrum was collected as shown by Figure 6a(2) and Figure 6b(2). The isolated hydroxyl peak in the SiO-H stretching vibration region was totally eliminated from the FTIR spectrum as shown by Figure 6a(2). However, a substantial amount of hydrogenbonded hydroxyls are still present as observed in Figure 6a(2). These remaining hydroxyl groups were confirmed as surface species by using the D/H exchange experiments. Earlier studies with various chlorosilanes and other chlorinated molecules have also observed that isolated hydroxyls are much more reactive than hydrogen-bonded hydroxyls.36,37 The reaction of SiCl4 with surface hydroxyl groups is confirmed by the appearance of Si-Cl stretching vibrations on the SiO2 surface after the SiCl4 reaction in Figure 6b(2). The Si-Cl stretching mode appeared at ∼620 cm-1 and is consistent with a SiCl2 or SiCl3 surface species.38 Some additional small absorption features are observed in Figure 6b(2) at ∼680 and ∼750 cm-1. Larger absorption features at the same locations

Figure 6. FTIR spectra of (1) hydroxylated SiO2 surface and (2) surface after SiCl4 reaction at 305 K in (a) the SiO-H stretching vibration region and (b) the Si-Cl stretching vibration region. Spectra have been displaced for clarity in presentation.

are observed with pyridine partial pressures. These features are identified as ν6b and ν4 pyridine vibrations, respectively.29,39 Consequently, the small features in Figure 6b(2) are attributed to residual pyridine on the SiO2 surface or possibly trapped in the SiO2 film. Pyridine was also used as a surface probe to quantify the surface hydroxyl groups. Pyridine adsorption experiments were performed on both the hydroxylated and chlorinated surfaces. FTIR difference spectra in the SiO-H stretching vibrational region are displayed in Figure 7a. For the hydroxylated surface shown in Figure 7a(1), pyridine adsorption at 0.2 Torr and 305 K resulted in a pronounced decrease of the isolated hydroxyl peak. In contrast, after the SiCl4 reaction on the chlorinated surface, only a small absorbance decrease at the frequencies for the isolated hydrogen-bonded hydroxyl groups was observed in Figure 7a(2). This decrease indicates that only a small amount of isolated hydroxyl groups does not react with SiCl4. Pyridine adsorption can also be measured by monitoring the pyridine molecular vibrations. The appearance of the ν19b hydrogen-bonded pyridine peak on the hydroxylated surface is observed in Figure 7b(1). This ν19b vibration at 1440-1447 cm-1 is attributed to pyridine adsorption on surface hydroxyl groups.30 After the SiCl4 reaction, the ν19b hydrogen-bonded pyridine peak is significantly decreased as shown in Figure 7b(2). However, the finite amount of hydrogen-bonded pyridine confirms that some hydroxyl groups remain after the SiCl4 reaction. An estimate indicates that the unreacted hydroxyl groups occupy up to 25% of the total hydroxyl groups on the original hydroxylated surface after exposure to a mixture of pyridine at 0.2 Torr and SiCl4 at 1.6 Torr for a reaction time of 5 min at 305 K. The reactant exposures for both SiCl4 and pyridine were increased to attempt to react more of the hydrogen-bonded hydroxyl groups. The absorbance for the hydrogen-bonded pyridine peak (ν19b) decreased slightly (∼10%) and the

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Figure 8. FTIR spectra in the SiO-H stretching vibration region revealing evidence for pyridinium salt on the SiO2 surface after the SiCl4 reaction at lower substrate temperatures. Spectra have been displaced for clarity in presentation.

Figure 7. FTIR difference spectra of the effect of a constant pyridine pressure of 0.2 Torr on (1) hydroxylated SiO2 surface and (2) surface after SiCl4 reaction at 305 K in (a) the SiO-H stretching vibration region and (b) the pyridine molecular vibration region. Spectra have been displaced for clarity in presentation.

integrated absorbance of the Si-Cl stretching vibrations increased slightly (∼15%) after exposure at higher SiCl4 pressures of 6.3 Torr. Similar progressive changes were observed for higher SiCl4 exposures at 13.0 Torr. Experiments at SiCl4 pressures of 13 Torr and larger pyridine pressures of 2 Torr observed no additional change in the absorbances for the hydrogen-bonded pyridine peak (ν19b) or the Si-Cl stretching vibration. These results suggest that the amount of pyridine that hydrogen bonds to the chlorinated SiO2 surface reaches a limiting value at higher pyridine pressures. Experiments that monitored the integrated absorbance of the ν19b(H-bonded) pyridine molecular vibration versus pyridine pressure at various temperatures revealed that pyridine pressures >0.05 Torr were sufficient for the catalyzed SiO2 ALD. This finite number of remaining hydroxyl groups after large SiCl4 exposures may be caused by steric hindrance between the -SiCl2 or -SiCl3 species on the surface. The average distance between two hydroxyls on the SiO2 surface is ∼4.6 Å based on a hydroxyl coverage of 4.6 × 1014 cm-2 at room temperature.21,40 The SiCl4 molecule has a center-to-corner distance of ∼2.0 Å.41 The pyridine molecule has a flat shape of 4-4.5 Å in width and 1-2 Å in thickness.31 The short distance between the surface hydroxyl groups and the relative large diameter of the -SiCl2 or -SiCl3 surface species may lead to steric hindrance as the SiCl4 reactant approaches the hydrogen-bonded hydroxyl groups. According to the proposed mechanism for the catalyzed SiCl4 half-reaction,7,8 a pyridine molecule hydrogen bonds to a surface hydroxyl group and a SiCl4 molecule is adjacent to the hydroxyl group in advance of the nucleophillic attack. After the isolated hydroxyl groups are replaced by silicon chloride species, the remaining hydrogen-bonded hydroxyl groups will probably be congested by the neighboring chlorine ligands. Pyridine may

Figure 9. FTIR spectra in the pyridine molecular vibrational region showing pyridine on Brønsted acid sites consistent with pyridinium salt after SiCl4 reaction at lower substrate temperatures. Spectra have been displaced for clarity in presentation.

still be able to interact with the remaining hydrogen-bonded hydroxyl groups. However, the SiCl4 molecules may have difficulty reaching these hydrogen-bonded hydroxyl groups. After SiCl4 reaction pressures of 13 Torr, the unreacted hydroxyl groups account for ∼16% of the total hydroxyl groups on the SiO2 surface. Steric limitations may explain this fraction of unreacted hydroxyl groups. A pyridinium salt was formed on the SiO2 surface after the SiCl4 half-reaction at low temperature. This pyridinium salt results from pyridine interacting with the HCl reaction product. Evidence for the pyridinium salt is observed in the O-H vibrational stretching region in Figure 8 and in the pyridine molecular vibration region [ν19b(B)] in Figure 9. The amount of pyridinium salt is very dependent on temperature. At high temperatures, the salt is quickly removed and is eliminated at 347 K. The removal of pyridinium salt from the chlorinated SiO2 surface corresponds to desorption of pyridine from the HCl Brønsted acid sites. 3. H2O Reaction with Chlorinated SiO2 Surface. Pyridine may catalyze the H2O reaction with the chlorinated SiO2 surface by either interacting with the silicon Lewis acid sites or hydrogen bonding with the incoming H2O reactant.7 The

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Figure 10. FTIR spectra of the Si-Cl stretching vibration on chlorinated SiO2 surface at 328 K (1) before pyridine exposure, (2) under pyridine exposure of 0.2 Torr, and (3) after pyridine exposure. Spectra have been displaced slightly for clarity in presentation.

interaction of pyridine with silicon to form a pentacoordinated silicon was suggested earlier as the mechanism for pyridine catalysis of chlorosilane attachment reactions on silanol surfaces.42 However, earlier FTIR studies of the effect of pyridine on chlorosilane attachment reactions later explained the role of pyridine as hydrogen bonding to the surface hydroxyl groups rather than interacting with the silicon Lewis acid sites.16 To examine the catalysis mechanism for pyridine during the H2O reaction, the interaction of pyridine with the chlorinated surface was first examined in the absence of H2O at 328 K. Figure 10 shows that a very slight Si-Cl peak shift is observed for the chlorinated SiO2 surface with pyridine adsorption at 0.2 Torr at 328 K. This shift is reversible with pyridine adsorption and desorption. Figure 10(1) was obtained for a chlorinated SiO2 surface before pyridine adsorption. Figure 10(2) was obtained for the same chlorinated SiO2 surface with a pyridine pressure of 0.2 Torr. The Si-Cl peak with pyridine adsorption has shifted to slightly lower frequency positions compared with the Si-Cl peak without pyridine adsorption. When pyridine was pumped away and removed from the chlorinated SiO2 surface, the Si-Cl peak shifted back to its original position as shown in Figure 10(3). The slight red-shift of the Si-Cl peak was found to be proportional to the pyridine adsorbed on the hydrogen-bonded hydroxyl groups. This redshift of the Si-Cl peak with pyridine adsorption may result from attractive dispersion interactions between the silicon chloride surface species and the pyridine molecules that are hydrogen bonded to the few remaining hydroxyl groups. This weak interaction between pyridine and the SiCl surface species suggests that pyridine does not catalyze the H2O reaction by activating the silicon Lewis acid sites on the silicon chloride surface species. These results support the earlier conclusion on the role of pyridine during chlorosilane attachment reactions to silica. The pyridine does not promote the attachment reaction by coordinating with the silicon Lewis acid sites and making the silicon more susceptible to nucleophillic attack.42 Rather, the pyridine has been shown to hydrogen bond to surface hydroxyl groups and make the oxygen atom a stronger nucleophile.15,16 Likewise, for the H2O reaction, the pyridine is believed to catalyze the H2O reaction during catalyzed SiO2 ALD by hydrogen bonding to the incoming H2O reactant. The reaction of H2O with the chlorinated surface was

Figure 11. FTIR spectra of (1) chlorinated SiO2 surface and (2) surface after H2O reaction at 305 K in (a) the SiO-H stretching vibration region and (b) the Si-Cl stretching vibration region. Spectra have been displaced for clarity in presentation.

monitored by the reappearance of the isolated hydroxyl groups and the disappearance of the -SiCl2 and -SiCl3 species on the SiO2 surface. The H2O pressure was 2.0 Torr and the pyridine pressure was 0.2 Torr. The exposure time was 5 min. Figure 11a(1) shows the FTIR spectrum for the chlorinated surface with a small number of hydrogen-bonded hydroxyl groups. The isolated hydroxyl groups reappear after the H2O reaction as shown in Figure 11a(2). The Si-Cl stretching mode from surface chloride species is observed at ∼620 cm-1 on the chlorinated surface as shown in Figure 11b(1). The reaction of H2O with these surface chloride species is confirmed by the disappearance of Si-Cl stretching vibrations in Figure 11b(2). A pyridinium salt is also formed on the SiO2 surface after the H2O half-reaction at low temperatures. Evidence for pyridine adsorption on Brønsted acid sites and a pyridinium salt is shown in Figures 12 and 13. Figure 12 shows the SiO-H stretching vibration region and Figure 13 shows the molecular pyridine vibrational region. These spectra reveal that the pyridinium salt is unstable at higher surface temperatures. At T > 340 K, almost all the pyridinium salt was removed from the surface. 4. Prospects for Catalyzed ALD. Catalyzed SiO2 ALD has been well-established by using pyridine and NH3 Lewis base catalysts. The catalysis is effective with either chlorosilanes6-9 or alkoxysilanes27 as the silicon precursor. Other Lewis bases such as diethylamine and triethylamine are also effective as catalysts.43 Catalyzed SiO2 chemical vapor deposition (CVD) has also been demonstrated with SiCl4 and H2O as reactants and NH3 as the Lewis base catalyst.44 In addition, the catalysis effect of Lewis bases has been extended to TiO2 CVD by using titanium tetraisopropoxide and H2O as reactants and NH3 as the catalyst.45 Lewis bases should be effective as catalysts for the ALD or CVD of metal oxides for metal oxide surfaces that have acidic

Pyridine-Catalyzed SiO2 Atomic Layer Deposition

Figure 12. FTIR spectra in the SiO-H stretching vibration region revealing evidence for pyridinium salt on the SiO2 surface after H2O reaction at lower substrate temperatures. Spectra have been displaced for clarity in presentation.

J. Phys. Chem. C, Vol. 111, No. 1, 2007 225 formation of a pyridinium or ammonium salt on the surface after either the SiCl4 or H2O reaction.6-9 Changing from chlorosilane to alkoxysilane can eliminate the HCl reaction product that forms the salt.27 However, the reaction efficiencies are much lower for alkoxysilanes compared with chlorosilanes.27 The catalyzed SiO2 ALD reactions can be run at higher temperatures to desorb the salt more easily. However, the Lewis base catalyst is also desorbed from the surface at higher temperatures and the loss of the Lewis base coverage leads to a reduction in the SiO2 ALD growth rate.9 The optimum temperature for catalyzed SiO2 ALD results from a temperature that is low enough to maintain the coverage of the Lewis base catalyst but high enough to desorb the salt. Compared with the earlier work on flat surfaces with a viscous flow carrier gas,9 the pyridinium salt formation is more severe during these experiments on high surface area SiO2 powder samples with static reactant exposures. The high surface area SiO2 powders have a much lower conductance. The static exposures also do not allow for the removal of the pyridinium salt until after the reactant exposures. Previous studies on flat substrates observed reliable catalyzed SiO2 ALD growth at temperatures from 305 to 360 K.9 Only reactant micropulsing was required to avoid the buildup of pyridinium salt formation on the SiO2 surface at lower temperatures. IV. Conclusions

Figure 13. FTIR spectra in the pyridine molecular vibrational region showing pyridine on Brønsted acid sites consistent with pyridinium salt after H2O reaction at lower substrate temperatures. Spectra have been displaced for clarity in presentation.

hydroxyl groups. The acidity of hydroxyl groups can be characterized by their isoelectric point in liquid water. Acidic hydroxyl groups have a negative surface charge in liquid water at pH 7.0 because the acidic proton is transferred to the water and MO- surface species are left behind on the surface. The pH has to be lowered to obtain a neutral surface at the isoelectric point. SiO2 has a very acidic proton and has an isoelectric point of pH ∼2.46 Other metal oxides with acidic hydroxyl groups include SnO2, ZrO2, and TiO2 with isoelectric points of pH ∼45, ∼4-6, and ∼4-6, respectively.46 Metal oxides that have basic hydroxyl groups should not be susceptible to catalyzed ALD or CVD by Lewis bases. Basic hydroxy groups would rather accept a proton from water and become protonated as MOH2+. These surfaces have a positive surface charge at pH 7.0 and the pH has to be raised to obtain a neutral surface. Metal oxides with basic hydroxyl groups include Al2O3, ZnO, and MgO with isoelectric points of pH ∼8-9, ∼9, and ∼12, respectively.46 Hydroxyl groups on these metal oxides will not hydrogen bond effectively with the Lewis base. In support of these ideas, catalyzed SiO2 cannot be initiated on hydroxylated Al2O3 ALD surfaces.47 One of the main limitations of catalyzed SiO2 ALD is the

Fourier transform infrared (FTIR) investigations were performed to study catalytic SiO2 ALD at room temperature with SiCl4 and H2O as the reactants and pyridine as the Lewis base catalyst. Pyridine adsorbed strongly on the hydroxylated SiO2 surface as revealed by changes to the O-H stretching vibrations and the appearance of pyridine molecular vibrations during pyridine adsorption. Arrhenius analysis of these results yielded a pyridine desorption energy of 9 ( 2 kcal/mol on the isolated hydroxyl groups of the hydroxylated SiO2 surface. The strong hydrogen bonding of pyridine to the isolated hydroxyl groups makes the oxygen a stronger nucleophile for nucleophillic attack on the SiCl4 reactant. The SiCl4 reaction with the hydroxylated SiO2 surface was monitored by the disappearance of O-H stretching vibrations and appearance of Si-Cl stretching vibrations. The SiCl4 reaction completely removed the isolated hydroxyl species and left behind a small fraction of hydrogen-bonded hydroxyl species. The remaining hydroxyl species were quantified by using pyridine as a surface probe to measure the hydroxyl species. The H2O reaction with the chlorinated SiO2 surface was observed by the disappearance of the Si-Cl stretching vibration and the appearance of the O-H stretching vibration. Monitoring the Si-Cl stretching vibration during pyridine adsorption revealed only a weak interaction between pyridine and the SiCl surface species. These results suggested that the pyridine catalyzes the H2O reaction by hydrogen bonding to the incoming H2O reactant. The pyridine and HCl reaction product can react to form a pyridinium salt on the SiO2 surface at lower temperatures after both the SiCl4 and H2O reactions. The pyridinium salt can desorb from the SiO2 surface and its desorption rate is very dependent on surface temperature. Negligible pyridinium salt was monitored on the SiO2 surface at >340 K after either the SiCl4 or H2O reactions. The pyridinium salt complicates the use of catalyzed SiO2 ALD. The optimum temperature for catalyzed SiO2 ALD must be high enough to desorb the salt but low enough to maintain the coverage of the Lewis base catalyst.

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