Langmuir 1998, 14, 1367-1370
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Monolayer Stabilization on Hydroxylated Aluminum Surfaces Steven L. Bernasek* and Jeffrey Schwartz Department of Chemistry, Princeton University, Princeton, New Jersey 08544 Received July 3, 1997. In Final Form: October 13, 1997 Thin layers with interesting electronic materials application can be synthesized via the adsorption of discrete organometallic complexes on appropriately activated metal, semiconductor, and insulator surfaces. The extent of hydroxylation of these surfaces can affect both the coverage and reaction chemistry for the adsorption of organometallics, and provides a convenient, controlable route to the formation of a number of thin film materials. Ambient condition and ultrahigh vacuum methods have been used to characterize these interactions and to develop synthetic strategies for the covalent attachment of organometallic species that can stabilize organic monolayers on these surfaces.
Introduction Organometallic chemical vapor deposition (OMCVD) is a viable route to the formation of a number of technically interesting thin film materials.1 Discrete organometallic complexes can be precursors to metallization,2 the formation of compound semiconductor thin films,3 the deposition of oxide- or nitride-insulating layers,4 or the stabilization of organic thin films with interesting electronic, optical, mechanical, or corrosion-resistance properties. Previous work has shown that the chemistry of interaction of organometallic complexes with oxophilic metallic substrates is controlled by the degree of hydroxylation of the metallic substrate.5 Both the final coverage of the organometallic complex and the stoichiometry and details of reaction chemistry are affected by the coverage of hydroxyl species on the surface. This approach can be used to deposit complexes with known bonding, geometric structure, and stability. Over the past several years, we have used a combination of ambient condition methods and ultrahigh vacuum (UHV) spectroscopic approaches to probe the interaction and reaction of organometallic species with metallic, semiconductor, and insulator surfaces. Our goal has been to obtain the broad range of structural, compositional, and morphological information needed to better exploit the use of organometallic complexes for the formation of thin films with technologically interesting properties. We discuss here one aspect of these studies, the stabilization of organic monolayers of alkanoic acids, which may be of use as packaging and corrosion-inhibition layers on aluminum surfaces. Alkanoic acids are attractive candidates in this application because of their wide accessibility, relative ease of synthesis, and potential for incorporation of reactive functionality in monolayers of easily varied hydrocarbon chain length and packing properties. To exploit this flexibility, a clear understanding of the structure and bonding of these overlayers must be available, and methods for covalent attachment for overlayer stabilization must be devised. The strategy of our investigations addressing these (1) Gandhi, S. K.; Bhat, I. B. MRS Bulletin 1988, XIII, 37. (2) Binstead, N.; Evans, J.; Greaves, G. N.; Price, R. J. Organometal 1989, 8, 613. (3) Manasevit, H. M. Appl. Phys. Lett. 1968, 12, 158. (4) Liu, H.; Bertolet, D. C.; Rogers, J. W., Jr. Surface Sci. 1994, 320, 145. (5) Miller, J. B. Ph.D Dissertation, Princeton University, 1992.
concerns has been to obtain structural information about the interaction of organometallic species with a surface using well-characterized single-crystal substrates in UHV, and to determine kinetics of complex surface reactions and electronic properties of surface-bound species using more realistic, polycrystalline substrates. In what follows, we will first describe this experimental approach in more detail. We will then describe studies of the interaction of zirconium organometallic compounds with hydroxylated aluminum surfaces that suggest that ligand kinetic basicity controls the reactivity of organometallic species with oxophilic metal surfaces.6 Studies of the effect of hydroxyl coverage on the final stoichiometry of the adsorbed complex using X-ray photoelectron spectroscopy (XPS) and thermal desorption spectroscopy will be described. The elaboration of this chemistry, which can allow the formation of an interface to covalently join a metal and an organic film such as would exist in a stabilized device packaging layer,7 will then be discussed. This combination of approaches successfully provides the information needed to design and implement these stabilized organic monolayers on hydroxylated aluminum. Experimental Section The quartz crystal microbalance (QCM) provides a method for monitoring the kinetics of surface reactions gravimetrically.8 In the studies described here, a QCM electrode was fashioned by vapor deposition of 2000-2500 Å of polycrystalline aluminum on a 5.0 MHz AT-cut overtone polished quartz crystal (ValpeyFisher). The QCM was driven by an oscillator, and the frequency was monitored as described previously.6 The polycrystalline aluminum substrate was hydroxylated by exposure to water vapor, and the extent of hydroxylation was controlled by the temperature and duration of water vapor exposure. Vapor phase exposure of the hydroxylated aluminum surface to organometallic species or alkanoic acids was accomplished in a small reaction cell, also described in detail previously.6 The reaction of these species with the hydroxylated surface was monitored by following the change in frequency of the oscillating crystal. The gravimetrically measured extent of deposition is related to the oscillator frequency by the Sauerbrey relationship.9 Diffuse reflectance infrared spectroscopy was used to monitor the (6) Miller, J. B.; Schwartz, J.; Bernasek, S. L. J. Am. Chem. Soc. 1993, 115, 8239. (7) Aronoff, Y. G.; Chen, B.; Lu, G.; Seto, C.; Schwartz, J.; Bernasek, S. L. J. Am. Chem. Soc. 1997, 119, 259. (8) Miller, J. B.; Schwartz, J. J. Inorg. Chem. 1990, 29, 4579. (9) Sauerbrey, G. Z. Phys. 1959, 155, 223.
S0743-7463(97)00710-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/22/1998
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Figure 1. Change in QCM frequency for the exposure of hydroxylated aluminum electrodes to tetra-neo-pentyl zirconium. hydroxylation and depostion reaction on high surface area aluminum powder samples prepared similarly. Information on the structure, composition, and reactivity of surface adsorbed organometallic complexes was obtained by a variety of methods on a single-crystal Al(110) substrate. The surface was prepared as described previously,10 by ion bombardment and annealing. Water (HPLC grade) and D2O (Cambridge Isotope Laboratories, 99.9%) was degassed by several freezepump-thaw cycles prior to use in these measurements. Vacuumdistilled zirconium alkyl and alkoxyzirconium complexes were dosed directly onto the hydroxylated surface from a doser located 1.5 cm from the sample surface. Normal octanoic acid (Aldrich) degassed by freeze-pump-thaw cycles, was also dosed directly onto the surface from the vapor. The variously prepared overlayers were characterized in UHV using Auger electron spectroscopy, XPS, Fourier transform reflection absorption infrared spectroscopy (FT-RAIRS), and thermal desorption methods. XPS provides information about the chemical identity and extent of hydroxylation of the water exposed aluminum substrate and, upon reaction of the substrate with organometallic species, information about the chemistry and stoichiometry of subsequent reaction products. Thermal desorption spectroscopy was used to obtain information about thermal stability of the reacted overlayers and to elucidate decomposition reaction mechanisms. Together with vibrational spectroscopic identification of adsorbed intermediates by FT-RAIRS, a powerful tool exists for the investigation of surface reaction mechanisms.
Figure 2. O(1s) XPS peak area for OH coverage vs. the number of dose/desorb cycles.
An interesting example of the approach just outlined is the interaction of zirconium organometallic complexes with hydroxylated aluminum surfaces.6 Figure 1 shows the change in QCM frequency when variously hydroxylated aluminum electrodes are exposed to tetra-neo- pentyl zirconium from the gas phase. A plot of the natural logarithm of the frequency change versus time gives the observed rate constant for the deposition process on the three surfaces. The observed rate constants are identical, although the three surfaces have very different initial hydroxyl coverages and exhibit different final zirconium complex loadings. Comparison of the rate of tetra-neopentyl zirconium deposition on the aluminum surface treated with H2O steam to that for the aluminum surface treated with D2O steam shows a large kinetic isotope effect. This result suggests that the rate-determining step for the deposition of the zirconium alkyl complex is proton transfer to the CsZr σ bond, and that the surface coordination of the complex is reversible. In contrast, the reaction of the H2O or D2O steam-treated aluminum substrate with tetra-tert-butoxy zirconium shows only a
small kinetic isotope effect in the QCM measurements. This result suggests that lone pairs on oxygen or ZrsO π bonds provide an alternate, kinetically favorable site for proton transfer to the metal complex compared with the σ bond of the tetraalkyl. The observed rate for the deposition of the alkoxyzirconium species is an order of magnitude greater than that measured for the alkylzirconium reaction with the hydroxylated surface. This suggested mechanistic behavior is illustrated in Scheme 1. When the same reaction is examined by UHV methodology, the difference in ligand cleavage reactivity is apparent.11 In these experiments, the Al(110) surface is cleaned and annealed in UHV, and the clean surface is hydroxylated by exposure to water according to two different protocols. The extent of hydroxylation is monitored by XPS of the O(1s) region, and by vibrational spectroscopy. On exposure of the surface at 170 K to water, a thick layer of ice is formed as observed by FT-RAIRS. Analysis by XPS shows a strong water O(1s) peak but no Al substrate signal. Heating the substrate to 300 K results in desorption of molecular water, leaving hydroxylated aluminum oxide on the metal surface. The FT-RAIRS spectrum of this surface shows a strong band at 3733 cm-1, which is characteristic of the OsH stretch of the isolated hydroxyl group. In the second procedure, a multiple water dosing and desorption sequence is used. XPS analysis after each dose/desorb cycle showed surface hydroxyl concentration saturation after six cycles, and a saturation hydroxyl content that is about four times greater than that obtained by a single water dose/desorb cycle (Figure 2). Under UHV conditions, the tetra-neo-pentylzirconium complex does not react appreciably with the hydroxylated Al(110) surface.11 Even after several thousand langmuirs exposure of the hydroxylated surface to the neo-pentyl complex, no evidence of reaction was obtained. In contrast, the fully hydroxylated surface reacted readily on exposure to tetra-tert-butoxy zirconium and warming, via irreversible protolytic deposition to give a thermally stable ditert-butoxy zirconium species bound covalently to oxygen on the aluminum surface. This reaction process was monitored in UHV by XPS and FT-RAIRS, providing
(10) Miller, J. B.; Bernasek, S. L.; Schwartz, J. Langmuir 1994, 10, 2629.
(11) Miller, J. B.; Bernasek, S. L.; Schwartz, J. J. Am. Chem. Soc. 1995, 117, 4037.
Results and Discussion
Monolayer Stabilization on Hydroxylated Al Surfaces
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Scheme 1. Intereaction of Zr alkyl and Zr alkoxy species with hydroxylated aluminum surfaces.
Scheme 2. Irreversible adsorption of n-octanoic acid onto the Zr alkoxide-treated hydroxylated aluminum surface by formation of a Zr η2-carboxylate.
spectroscopic support for the reaction mechanisms inferred from the QCM studies on hydroxylated polycrystalline substrates. Protolytic deposition of the tert-butoxy complex but not the alkyl complex indicates the importance of ligand kinetic basicity (as opposed to thermodynamic basicity) in the design of organometallic chemical vapor deposition processes.6 When the reaction was carried out with the lightly hydroxylated aluminum surface, XPS and vibrational spectroscopy confirmed the formation of a covalently bound tri-tert-butoxy zirconium species that was formed by protolytic loss of only one tert-butoxy ligand. Heating this surface to 350 K results in total decomposition of the organometallic complex to form ZrO2, in contrast to the formation of the thermally stable di-tert-butoxy species on the fully hydroxylated surface at the same temperature. Deposition and thermolysis of the organometallic complex on the fully hydroxylated surface resulted in a coverage of zirconium-containing species about four times higher than the zirconium coverage following deposition and thermolysis on the lightly hydroxylated surface. Clearly, initial hydroxyl coverage determines not only the gross surface loading of the metallic complex, but also the course of thermolysis of the surface bound species and the resulting final stoichiometry of the adsorbed complex.12 Covalent attachment of the di-tert-butoxy zirconium organometallic complex just described provides a site for chemical elaboration of the the surface by a range of well(12) Bernasek, S. L.; Schwartz, J.; Miller, J.; Aronoff, Y.; Lu, G. Proceedings of the 50th Biennial Meeting of the Czech and Slovak Chemical Societies, Chem. Listy 1997, 91, 621.
understood reactions. One possible approach to surface passivation using this attached complex involves the bound zirconium species as an olefin polymerization catalyst, with the resultant passivation of the substrate by the formation of an impervious polymeric coating, grown in place. This approach, though promising, has yet to be implemented in our studies. Another, already successful approach, is to use ligand replacement chemistry of the tert-butoxy groups to stabilize self-assembled monolayer (SAM) molecules on the oxide surface. The “self-assembly” of long-chain organic molecules on native metal oxide surfaces provides an early example of SAM investigations.13 Carboxylic acids,14 hydroxamic acids,15 and alkyltrichloro- or alkyltrialkoxysilanes16 have all been used in studies of self assembly on oxide covered metal surfaces. Adsorption of the acids likely occurs by hydrogen bonding, leading to weak and reversible binding to the native oxide film. The alkyltrichloro- and alkyltrialkoxysilanes are of limited utility, as they are not readily available in wide variety. Carboxylic acids are attractive in this application because of their easy availability and the relative ease of functionalization, but their weak binding via hydrogen bond interactions or proton transfer processes limits their use for surface passivation of technically important metal surfaces. An (13) Laibinis, P. E.; Hickman, J. J.; Wrighton, M. S.; Whitesides, G. M. Science 1989, 245, 845. (14) Allara, D. L.; Nuzzo, R. G. Langmuir 1985, 1, 45. (15) Folkers, J. P.; Gorman, C. B.; Laibinis, P. E.; Buchholz, S.; Whitesides, G. M.; Nuzzo, R. G. Langmuir 1995, 11, 813. (16) Xia, Y.; Mrksich, M.; Kim, E.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117, 9576.
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species is quite robust thermally, with alkanoic acid desorption occurring only on heating above 500 K. Surface pretreatment with zirconium alkoxides serves to stabilize the alkanecarboxylate film on the oxide surface, providing a general route for the adsorption enhancement of carboxylic acids or similar reagents on hydroxylated oxide metal films. Conclusions
Figure 3. FT-RAIRS spectra of the interaction of n-octanoic acid with the covalently bound Zr complex. (a) monolayer ditert-butoxy Zr complex; (b) surface of (a) reacted with n-octanoic acid.
interface based on the covalent attachment of the zirconium tert-butoxy species just described can enable alkanecarboxylates to bind strongly to the native oxide of an oxophilic metal. Scheme 2 illustrates the chemistry suggested here. Reaction of the covalently bound zirconium tert-butoxy complex with n-octanoic acid has been monitored by QCM measurements.7 These measurements indicate an irreversible reaction of the octanoic acid with the zirconium complex, desorbing tert-butanol and forming a surface bound Zr η2-carboxylate. The carboxylate species is identified by its characteristic reflection-absorption infrared spectrum, which is shown in Figure 3. This
The reaction of discrete organometallic species with wellcharacterized hydroxylated single-crystal surfaces, as well as polycrystalline and high surface area powdered samples, provides a characterizable approach to the synthesis of modified surfaces and thin films with interesting materials application. The combination of detailed UHV spectroscopic information with kinetic and synthetic methods employed under ambient conditions provides a deeper understanding of this interesting chemistry than could be obtained by a single approach. Examples of this surface organometallic chemistry relevant to the passivation of the surfaces of oxophilic metals, such as aluminum, have been presented. Passivation of such surfaces is an important consideration in the packaging of integrated circuits and other electronic devices. Acknowledgment. We dedicate this paper to the memory of Professor Brian Bent, for his example in addressing complex questions of surface chemistry and for his many contributions to our understanding in this area. We also acknowledge the support of the National Science Foundation and the work of several undergraduate, graduate, and postdoctoral students, including Carwei Seto, John Miller, Yael Aronoff, Bo Chen, Kathleen Purvis, and Gang Lu. LA970710R
