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Langmuir 2008, 24, 8610-8619
Gas-Surface Reactions between Pentakis(dimethylamido)tantalum and Surface Grown Hyperbranched Polyglycidol Films Manish Sharma, Abhishek Dube, Kevin J. Hughes, and James R. Engstrom* School of Chemical and Biomolecular Engineering, Cornell UniVersity, Ithaca, New York 14853 ReceiVed March 12, 2008. ReVised Manuscript ReceiVed May 16, 2008 We have investigated the growth of hyperbranched polyglycidol films, and their subsequent reaction with a transition metal coordination complex, pentakis(dimethylamido)tantalum, Ta[N(CH3)2]5 using ellipsometry, contact angle measurements, atomic force microscopy and X-ray photoelectron spectroscopy (XPS). Up to thicknesses of approximately 150 Å, the growth of polyglycidol is approximately linear with reaction time for growth activated using either sodium methoxide or an organic superbase. The reaction of Ta[N(CH3)2]5 at room temperature with these layers depends strongly on their thicknesssthe amount of uptake of Ta by the surface increases with the thickness of the organic layer, and thicker films also lead to more extensive ligand exchange reactions (with the R-OH groups), with as many as 4 ligands being lost on the thicker organic films. Ta penetrates the surface of all films examined (thicknesses 30-84 Å), but the average depth of the penetration is nearly independent of the thickness of the organic film, and it is ∼15-25 Å. Modification of the polyglycidol with an aminoalkoxysilane introduces a significant fraction of -NH2 termination in the organic layer. Reactions of this layer with the Ta complex are quite different than those on an unmodified layersnow on average only a single ligand exchange reaction occurs, while on the unmodified surface as many as four ligands are exchanged.
I. Introduction As semiconductor devices continue to become denser with each generation and approach molecular-scale dimensions, attempts are being made to use small organic molecules as active components in electronic circuitry.1–3 Organic molecules are attractive owing to their unique material properties, and their processing properties, such as their tendency toward selfassembly.4 However, to use them as conductors, insulators, and ultimately in transistors,5 a significant challenge is to integrate them with inorganic materialsswhich includes making reliable and robust inorganic-organic interfaces. The technological importance of inorganic-organic interfaces6 is beginning to be recognized in several areas, including organic light emitting diodes,7 molecular electronics,8–10 and microelectronic interconnects,11,12 and yet, effective strategies to control the nucleation and growth of these dissimilar materials at these interfaces remain elusive. Regarding organic-on-inorganic interfaces, self-assembly has been effectively used for making largely defect free (over several nanometers) organic monolayer films on metallic and semiconductor substrates.13 Some of the most studied organicon-inorganic interfaces include self-assembled monolayers * To whom correspondence should be addressed. E-mail: jre7@ cornell.edu. (1) Yan, L.; Gao, Y. Thin Solid Films 2002, 417, 101–106. (2) Joachim, C.; Ratner, M. A. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 8801– 8808. (3) James, D. K.; Tour, J. M. Top. Curr. Chem. 2005, 257, 33–62. (4) Ulman, A. An introduction to Ultrathin Organic Films: From LangmuirBlodgett to self-assembly; Academic Press: New York, 1991. (5) Kagan, C. R.; Afzali, A.; Martel, R.; Gignac, L. M.; Solomon, P. M.; Schrott, A. G.; Ek, B. Nano Lett. 2003, 3, 119–124. (6) Ishii, H.; Sugiyama, K.; Ito, E.; Seki, K. AdV. Mater. 1999, 11, 605–625. (7) Tanaka, D.; Sasabe, H.; Li, Y. J.; Su, S. J.; Takeda, T.; Kido, J. Jpn. J. Appl. Phys. 2 2007, 46, L10–L12. (8) Aviram, A. Angew. Chem., Int. Ed. Engl. 1989, 28, 520–521. (9) Tans, S. J.; Verschueren, A. R. M.; Dekker, C. Nature 1998, 393, 49–52. (10) Zhong, Z. H.; Wang, D. L.; Cui, Y.; Bockrath, M. W.; Lieber, C. M. Science 2003, 302, 1377–1379. (11) Rossnagel, S. M. J. Vac. Sci. Technol. B 2002, 20, 2328–2336. (12) Elam, J. W.; Wilson, C. A.; Schuisky, M.; Sechrist, Z. A.; George, S. M. J. Vac. Sci. Technol. A 2003, 21, 1099–1107. (13) Ulman, A. Chem. ReV. 1996, 96, 1533–1554.
(SAMs) of organothiols on Au14 and long chain trichloro- and trimethoxy silanes covalently bound to silicon oxide.15,16 In contrast, if we consider the formation of these interfaces in reverse order, i.e., inorganic-on-organic, this is an area that is still rather immature. To date, fabrication of inorganic-onorganic interfaces has been carried out primarily using elemental evaporation in vacuum of a metal on top of the organic film. Evaporative deposition of metals including Cu,17,18 Cr,19,20 and Ag21,22 on alkanethiol SAMs self-assembled on Au and having different terminal groups, -CH3,19 -OH,17 -COOH,21 -COOCH3,20 and -CN23 has been studied. Evaporative deposition of titanium on functionalized alkanethiol SAMs has also been reported.24 Summarizing this early work, in the case of less reactive metals (e.g., Ag, Cu), significant penetration of the SAM was implicated and the metal is deposited primarily at the SAMsubstrate interface.18,21 For more reactive metals (e.g., Ti), the metal reacts rather indiscriminately with the entire organic layer, thereby forming a composite metal-organic film.24 More recently this work concerning the evaporative deposition of elemental metals on organothiols has been revisited,25–30 including the adsorption of Ti and Au on oligo(phenylene-ethynylene)thiols,25 (14) Laibinis, P. E.; Nuzzo, R. G.; Whitesides, G. M. J. Phys. Chem. 1992, 96, 5097–5105. (15) Ulman, A. AdV. Mater. 1990, 2, 573–582. (16) Wasserman, S. R.; Tao, Y. T.; Whitesides, G. M. Langmuir 1989, 5, 1074–1087. (17) Jung, D. R.; King, D. E.; Czanderna, A. W. Appl. Surf. Sci. 1993, 70-1, 127–132. (18) Jung, D. R.; King, D. E.; Czanderna, A. W. J. Vac. Sci. Technol. A 1993, 11, 2382–2386. (19) Jung, D. R.; Czanderna, A. W. J. Vac. Sci. Technol. A 1994, 12, 2402– 2409. (20) Jung, D. R.; Czanderna, A. W. Appl. Surf. Sci. 1996, 99, 161–168. (21) Herdt, G. C.; Jung, D. R.; Czanderna, A. W. Prog. Surf. Sci. 1995, 50, 103–129. (22) Jung, D. R.; Czanderna, A. W.; Herdt, G. C. J. Vac. Sci. Technol. A 1996, 14, 1779–1787. (23) Herdt, G. C.; Jung, D. R.; Czanderna, A. W. J. Adhesion 1997, 60, 197– 222. (24) Konstadinidis, K.; Zhang, P.; Opila, R. L.; Allara, D. L. Surf. Sci. 1995, 338, 300–312. (25) Walker, A. V.; Tighe, T. B.; Stapleton, J.; Haynie, B. C.; Upilli, S.; Allara, D. L.; Winograd, N. Appl. Phys. Lett. 2004, 84, 4008–4010.
10.1021/la800790u CCC: $40.75 2008 American Chemical Society Published on Web 07/18/2008
Gas-Surface Reactions
and the adsorption of Ti,26 Ca,26,30 Mg,27,29,30 and Cu28 on functionalized alkanethiols. In order to better control the reactions of metal containing species with organic films, over the past few years we and others have been examining an approach that involves the use of chemical vapor deposition (CVD) or atomic layer deposition (ALD).31,32 In perhaps the first studies using an approach based on CVD, Fischer and co-workers have examined the thin film deposition of Al, Au and Pd on functionalized self-assembled monolayers using the precursors trimethylamine alane, [N(CH3)3]AlH3,33 trimethylphosphinemethylgold, [(CH3)3P]AuCH3.34,35 and cyclopentadienyl-allyl-palladium, [Cp(allyl)Pd].35 More recently Sung and co-workers have investigated the ALD thin film growth of TiO2 [using Ti(O-i-C3H7)4 and H2O] on alkyltrichlorosilane SAMs on SiO2.36 In another application, Bent and co-workers have used unreactive SAMs (e.g., with -CH3 termination) as masks for area selective ALD of HfO2, for example.37 Over the past few years we have been engaged in a systematic study of the reaction of Ti and Ta containing complexes with a variety of organic layers. These layers have included straightchain alkylsilanes with -CH3, -OH and -NH2 terminations on SiO2,38,39 oligo(phenylene-ethynylene)thiophenes with -NH(iC3H7) termination on Au,40 and, most recently, first generation polyamidoamine dendrons with -NH2 termination on SiO2.41 This work has been extended to the examination of the ALD of TiN thin films on top of a variety of organic layers,42,43 including these just listed. From these studies of both the chemisorption of the transition metal coordination complexes and subsequent thin film deposition on the organic layers considerable insight has been developed concerning the nature of inorganic-on-organic interface formation in these systems. For example, we find an excellent correlation between the initial rate of growth via ALD and the saturation density of the transition metal complex measured at room temperature.39 We also find that ALD growth on organic layers with reactive terminations produces uniform, smooth films, while growth on those organic layers that are generally unreactive produces very nonuniform and rough thin films.42,43 (26) Walker, A. V.; Tighe, T. B.; Haynie, B. C.; Uppili, S.; Winograd, N.; Allara, D. L. J. Phys. Chem. B 2005, 109, 11263–11272. (27) Zhou, C.; Nagy, G.; Walker, A. V. J. Am. Chem. Soc. 2005, 127, 12160– 12161. (28) Nagy, G.; Walker, A. V J. Phys. Chem. B 2006, 110, 12543–12554. (29) Walker, A. V.; Tighe, T. B.; Cabarcos, O.; Haynie, B. C.; Allara, D. L.; Winograd, N. J. Phys. Chem. C 2007, 111, 765–772. (30) Nagy, G.; Walker, A. V J. Phys. Chem. C 2007, 111, 8543–8556. (31) Suntola, T. Thin Solid Films 1992, 216, 84–89. (32) George, S. M.; Ott, A. W.; Klaus, J. W. J. Phys. Chem. 1996, 100, 13121– 13131. (33) Weiss, J.; Himmel, H. J.; Fischer, R. A.; Wo¨ll, C. Chem. Vapor Depos. 1998, 4, 17–21. (34) Winter, C.; Weckenmann, U.; Fischer, R. A.; Kashammer, J.; Scheumann, V.; Mittler, S. Chem. Vapor Depos. 2000, 6, 199–205. (35) Fischer, R. A.; Weckenmann, U.; Winter, C.; Kashammer, J.; Scheumann, V.; Mittler, S. J. Phys. IV 2001, 11, Pr3/1183-Pr3/1190. (36) Lee, J. P.; Jang, Y. J.; Sung, M. M. AdV. Funct. Mater. 2003, 13, 873. (37) Chen, R.; Kim, H.; McIntyre, P. C.; Bent, S. F. Chem. Mater. 2005, 17, 536–544. (38) Killampalli, A. S.; Ma, P. F.; Engstrom, J. R. J. Am. Chem. Soc. 2005, 127, 6300–6310. (39) Ma, P. F.; Dube, A.; Killampalli, A. S.; Engstrom, J. R. J. Chem. Phys. 2006, 125, 034706. (40) Dube, A.; Chadeayne, A. R.; Sharma, M.; Wolczanski, P. T.; Engstrom, J. R. J. Am. Chem. Soc. 2005, 127, 14299–14309. (41) Sharma, M.; Dube, A.; Engstrom, J. R. J. Am. Chem. Soc. 2007, 129, 15022–15033. (42) Dube, A.; Sharma, M.; Ma, P. F.; Engstrom, J. R. Appl. Phys. Lett. 2006, 89, 164108. (43) Dube, A.; Sharma, M.; Ma, P. F.; Ercius, P. A.; Muller, D. A.; Engstrom, J. R. J. Phys. Chem. C 2007, 111, 11045–11058.
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In addition to simple straight-chain self-assembled monolayers, organic layers that possess branched backbones have also attracted attention. In one set of studies, preformed solution synthesized molecules that possess a branched dendritic backbone44,45 have been immobilized on solid supports such as silica,46 gold47 and mica.48 In these cases,46–48 there was no formal attempt to bind these species to the surface covalently. In other work, however, covalent attachment of branched molecules to high surface area substrates such as Ultrafine silica and polymeric beads has been investigated. In these cases,49–51 growth of the branched molecules was initiated at the surface. While immobilization of preformed molecules with branched backbones has undoubtedly a number of applications, growth of such molecules from a single attachment point on a surface may have substantial advantages in some cases. For example, one might imagine that denser layers could be realized, as monomers may act to fill empty space better than a much larger preformed molecule. The roughness of the subsequent organic thin film may also be reduced for the same reason. In recent work, we have demonstrated the growth of first generation polyamidoamine dendrons with -NH2 termination on SiO2, and have reported on the reactivity of these layers toward a Ta complex, Ta[N(CH3)2]5.41 We have found that the branched nature of dendritic growth can be used to amplify the chemisorption capacity of the surface.41 In contrast to the step-by-step reaction strategy employed in the growth of polyamidoamine dendrons, in the present work we investigate an alternative strategy to form branched reactive organic thin films based on a single-pot synthesis. Namely, we examine here the growth of hyperbranched polyglycidol organic thin films on SiO2.52 Subsequent to synthesis and characterization of these layers, we consider further reactions. First we examine the chemisorption of pentakis(dimethylamido)tantalum, Ta[N(CH3)2]5, on these layers. Ta[N(CH3)2]5, for example, can be used for the ALD of TaN when used in combination with NH3.53,54 In this work we seek to make connections between the nature of the polyglycidol thin films (e.g, their thickness) and the nature and location of the chemisorbed Ta species that are formed. Second we react these layers with an organic modifier, 3-aminopropyldimethylethoxysilane, to change the -OH termination of the polyglycidol to one with -NH2 character. We also characterize the reactivity of these layers with the Ta complex. There are some similarities between the observations we make here, and those that we have made previously with self-assembled monolayers with simpler backbones, but also important differences that may be attributed to the density and distribution of reactive species in the polyglycidol layers. (44) Tomalia, D. A.; Baker, H.; Dewald, J.; Hall, M.; Kallos, G.; Martin, S.; Roeck, J.; Ryder, J.; Smith, P. Polym. J. 1985, 17, 117–132. (45) Wooley, K. L.; Hawker, C. J.; Frechet, J. M. J. J. Am. Chem. Soc. 1991, 113, 4252–4261. (46) Knecht, M. R.; Sewell, S. L.; Wright, D. W. Langmuir 2005, 21, 2058– 2061. (47) Hierlemann, A.; Campbell, J. K.; Baker, L. A.; Crooks, R. M.; Ricco, A. J. J. Am. Chem. Soc. 1998, 120, 5323–5324. (48) Hellmann, J.; Hamano, M.; Karthaus, O.; Ijiro, K.; Shimomura, M.; Irie, M. Jpn. J. Appl. Phys. 2 1998, 37, L816–L819. (49) Fromont, C.; Bradley, M. Chem. Commun. 2000, 283–284. (50) Reynhardt, J. P. K.; Alper, H. J. Org. Chem. 2003, 68, 8353–8360. (51) Tsubokawa, N.; Satoh, T.; Murota, M.; Sato, S.; Shimizu, H. Polym. AdVan. Technol. 2001, 12, 596–602. (52) Khan, M.; Huck, W. T. S. Macromolecules 2003, 36, 5088–5093. (53) Furuya, A.; Ohtsuka, N.; Misawa, K.; Shimada, M.; Ogawa, S. J. Appl. Phys. 2005, 98, 094902. (54) Wu, Y. Y.; Kohn, A.; Eizenberg, M. J. Appl. Phys. 2004, 95, 6167–6174.
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II. Experimental Procedures The experimental procedures employed in the present work are described in complete detail in Supporting Information, and we give only a brief summary here of the elements most essential to this study. The synthesis of the organic layers was initiated by chemically growing a thin oxide on a silicon single crystalline substrate. This procedure consistently produces silicon dioxide with a thickness of 15-20 Å with the surface being fully wet by water (advancing and receding contact angles of 0°).55 This oxide has been reported to possess SiOH groups of a density ∼5 × 1014 cm-2 and is the “chemical oxide” referred to below.16 The hyperbranched polyglycidol films were grown on this substrate using anionic ring-opening multibranching polymerization of glycidol after activating the surface with Na+ ions in a solution of sodium methoxide. This method for growing -OH terminated hyperbranched oligo-/polymers is identical to that first shown by Khan and Huck52 and similar to the solvent phase anionic polymerization of glycidol using 1,1,1-tris(hydroxymethyl)propane as the initiator, a reaction that yields free-standing hyperbranched aliphatic polyethers.56 In a variation of this approach, we have also used Verkade superbase (VSB), 2,8,9-trimethyl-2,5,8,9tetraaza-1-phosphabicyclo(3.3.3)undecane, for surface activation, as a substitute for sodium methoxide. VSB, being a neutral organic base, permits milder synthesis conditions57 while eliminating subsequent sample charging due to absence of residual metal ions. Sodium containing compounds, importantly, are also forbidden from semiconductor device fabrication facilities, thus VSB provides an alternative for these applications. The polyglycidol modified substrates were taken out of the glycidol solution at the intervals of 15, 30, 45, and 60 min, followed by rinsing with methanol to stop the polymerization. After additional rinsing in deionized water and blow drying with N2, substrates were stored in a desiccator for further analysis. All polyglycidol films were characterized by contact angle measurements and ellipsometry before being transferred to a custom designed ultra high vacuum (UHV) chamber58 for analysis via X-ray photoelectron spectroscopy (XPS). In another set of experiments, we further modified the polyglycidol film by reacting it with 3-aminopropyldimethylethoxysilane, and this organic bilayer was also studied using XPS. The UHV chamber employed for performing XPS was also used for carrying out in situ gassurface reaction studies between the organic films and Ta[N(CH3)2]5. Precise exposures of the substrates to Ta[N(CH3)2]5 was made using a multicapillary array doser. XP spectra, both at a fixed take off angle, and variable (angle resolved XPS) were acquired in the relevant binding energy regions before and after exposing the films to Ta[N(CH3)2]5. Sample charging effects, associated with the relatively thick (insulating) polyglycidol layers, precluded precise identification of binding energies in some cases.
III. Results and Discussion A. Characterization of the Polyglycidol Thin Films. First, we present the results related to the characterization of the polyglycidol films using ellipsometry, contact angle measurements, atomic force microscopy (AFM), and XPS. In order to initiate the synthesis of polyglycidol, deprotonation of the surface silanol groups was carried out by treating the chemical oxide substrates with sodium methoxide or Verkade superbase in separate experiments. In Figure 1 we plot the film thickness measured using ellipsometry for growth in glycidol at 105 °C for both initiators used here. Also shown are the results reported by Khan and Huck52 for growth at 110 °C using sodium methoxide. We see that there is good agreement between all (55) Liebmann-Vinson, A.; Lander, L. M.; Foster, M. D.; Brittain, W. J.; Vogler, E. A.; Majkrzak, C. F.; Satija, S. Langmuir 1996, 12, 2256–2262. (56) Sunder, A.; Hanselmann, R.; Frey, H.; Mulhaupt, R. Macromolecules 1999, 32, 4240–4246. (57) Kovacevic, B.; Baric, D.; Maksic, Z. B. New J. Chem. 2004, 28, 284–288. (58) Xia, L. Q.; Jones, M. E.; Maity, N.; Engstrom, J. R. J. Vac. Sci. Technol. A 1995, 13, 2651–2664.
Figure 1. Ellipsometric thickness of polyglycidol films as a function of reaction time in glycidol at 105 °C, where the activator was (O) sodium methoxide, and (0) Verkade organic superbase. Also shown are results from Khan and Huck (1) obtained at 110 °C (ref 52). The inset shows the advancing (∆) and receding (9) contact angles for water on these same films.
three sets of results. A fit to our data for sodium methoxide gives a growth rate of ∼1.1 Å · min-1, vs ∼1.6 Å · min-1 reported by Khan and Huck.52 The approximate linear behavior indicates the controlled nature of polymerization for both the sodium methoxide and VSB initiated processes. As our results agree quite well with the previous work of Khan and Huck, we reproduce here for convenience their most significant observations related to the mechanism of polymerization and the factors that might lead to deviations from linear growth. First, they find that there is a limiting thickness of about 150 Å. Growth can be reactivated by additional application(s) of initiators, and they reported thin films as thick as 700 Å after three applications. For the region where growth is linear (between ca. 30 and 140 Å) this indicates that the total number of active sites, e.g., -R-O-Na+, is approximately constant, and that they are all accessible. Factors leading to deviations include thermal autopolymerization, which is discounted for these reactions conditions by other work,59 and active site transfer to the bulk glycidol solution, the latter which may contribute to the limiting thickness. In any event, for the conditions and thicknesses examined here (30-140 Å) linear growth appears to prevail mostly. In the inset of Figure 1, we also report the contact angles measured for the polyglycidol films. There are two competing factors which influence the contact angle on the polyglycidol modified substrates. The increase in the density of -CH2- groups in the film backbone would tend to make the surface hydrophobic; while the simultaneous increase in the density of terminal -OH groups would tend to produce a contact angle closer to the value for chemical oxide, i.e., 0°. As may be seen in the inset, the receding contact angle was measured to be 10, thus we can make use of the 1:3 correspondence between the densities of the terminal -OH groups and the total density of carbon in these layers to make an estimate for the absolute densities of -OH as a function of film thickness. A thin film Au sample was used as an absolute standard, and the precise nature of the calculation is given in Supporting Information. In our calculation the -OH is assumed to be uniformly distributed throughout the film, as we only have knowledge of the total amount of carbon in the thin film. The calculated values are given in Table 1, and range from 0.92 ( 0.25 × 1015 cm-2 for the 30 Å film to 3.8 ( 1.2 × 1015 cm-2 for the 84 Å thin film. As this model assumes that the -OH are uniformly distributed, we can also calculate the 3D density using the film thickness, and these values are also given in Table 1. We see that these values range from 3.1 ( 0.5 × 1021 cm-3 for the 30 Å film, to 4.6 ( 0.8 × 1021 cm-3 for the 66 Å thin film. Interestingly, making use of the molecular weight of glycidol, we calculate mass densities of 0.38 to 0.57 g · cm-3. These densities are somewhat lower than what one expects for bulk polyglycidol (unreported, but probably ∼1 g · cm-3), which may reflect that our estimates for the density of -OH are low (perhaps by a factor of 2), or the films are of lower density (e.g., nanoporous) themselves. B. Gas-Surface Reaction between Ta[N(CH3)2]5 and Polyglycidol. The reactivity of polyglycidol films of different thicknesses with a coordination complex of tantalum, Ta[N(CH3)2]5, has been examined in ultrahigh vacuum. The substrates were exposed to the Ta complex at room temperature, and subsequently in situ XPS has been used to characterize the gas-surface reactions. In Figure 5 we display the density of Ta that adsorbs on polyglycidol as a function of the total exposure to Ta[N(CH3)2]5 for 30 and 84 Å thick films. Here the area under the Ta(4f) feature was used to calculate the absolute density of chemisorbed tantalum, after suitable correction for the contribution from the O(2s) feature and comparison to a Ta reference standard (see Supporting Information). The smooth curves represent fits to first-order Langmuirian kinetics, ns(dθ/dt) ) SR,0F(1 - θ), where ns is the total density of reactive sites (cm-2), θ is the fractional coverage of the chemisorbed Ta species, SR,0 is the initial reaction probability, and F is the incident flux of Ta[N(CH3)2]5 (molecules · cm-2 · s-1). As may be seen, first-order Langmuirian kinetics describes the data well in both cases. Using these data we calculate SR,0 to be 0.099 ( 0.005 and 0.060 ( 0.004 for the 30 and 84 Å thick polyglycidol films respectively. The absolute experimental uncertainty in these values is about 50% while the relative uncertainty between these values is less. The values obtained here are of similar order of magnitude to what we obtained earlier for adsorption of Ta[N(CH3)2]5 on straight chain and branched -NH2 terminated self-assembled monolayers, where SR,0 ) 0.028.41 Next, we move on to a comparison of the saturation densities of Ta adsorbed on polyglycidol films of different thicknesses. In this case we collected spectra in both the Ta(4f) and Ta(4d) regions. The spectra in the Ta(4f) region, while more intense, are complicated by the presence of a O(2s) peak at a similar energy, which we account for as described in the Supporting Information. In Figure 6a we show the Ta(4f) spectra after they have been stripped of their O(2s) components. It may be seen that there is a clear increase in the density of Ta adsorbed on the surface with an increase in the thickness of polyglycidol. In Figure S-3 in Supporting Information, we also plot the Ta(4d) spectra for these same films, and a similar trend is observed. We have utilized the areas under both Ta(4f) and Ta(4d) peaks to estimate independently the absolute density of Ta chemisorbed
Gas-Surface Reactions
Langmuir, Vol. 24, No. 16, 2008 8615 Table 1. Properties of Polyglycidol Thin Films calculated -OH terminal density
reaction time (min) 15 30 45 60
ellipsometric thickness (Å) 29.9 ( 3.3 46.7 ( 8.8 65.9 ( 10.4 84.4 ( 11.2
contact angle, adv/rec (deg) 6.5/2.5 4/0 8.5/0 17/8.5
on each of the films. In order to achieve this, XP spectra for a polycrystalline Ta sheet was also obtained, which was used as a reference standard. The exact procedure is detailed in Supporting Information, and this first estimate assumes that there is no attenuation of the Ta photoelectrons. We find the saturation density of Ta[N(CH3)2]5 to be 2.9 ( 0.3 × 1014, 4.2 ( 0.4 × 1014, and 4.9 ( 0.4 × 1014 cm-2 on the 30, 66 and 84 Å thick polyglycidol films, respectively. As indicated in Table 2, these values are the average of the densities calculated using both the Ta(4f) and Ta(4d) spectra. These densities are within about a factor of 2 to what we have observed earlier for the ligand exchange reactions between Ta[N(CH3)2]5 and straight chain and branched -NH2 terminated SAMs (∼1-4 × 1014 cm-2),41 and also for the reaction of Ti[N(CH3)2]4 with both -OH and -NH2 terminated straight chain SAMs (∼1-3 × 1014 cm-2).38 It is clear from the above discussion that Ta[N(CH3)2]5 undergoes a gas-surface reaction with polyglycidol films of all thicknesses, and the amount of Ta deposited is best correlated with the thickness of the films. The next question to answer concerns the spatial extent of this reaction, i.e., does Ta penetrate the organic thin film? Before proceeding further, we note that it is likely that there is a significant density of -OH groups buried beneath the surface (indeed, we assumed -OH was distributed uniformly throughout the film). These sites, if they can be accessed, obviously would also be reactive toward Ta[N(CH3)2]5. To address the issue of the spatial extent of reaction between Ta[N(CH3)2]5 and polyglycidol, we have acquired angle resolved XP spectra for the Ta(4f) feature [stripped of any O(2s) component]. In Figure 6b we display the integrated intensities for the Ta(4f) feature for five photoelectron takeoff angles between 0° and 60°. As a first observation, it may be seen that for all three cases, there is an increase in the Ta(4f) peak areas with an increase
Figure 5. Coverage-exposure relationships for the reaction of Ta[N(CH3)2]5 with two polyglycidol thin films of different thicknesses. The solid curves represent a fit to first-order Langmuirian kinetics.
corrected, 2d (cm-2)
corrected, 3d (cm-3)
rms roughness from AFM (Å)
0.92 ( 0.25 × 1015 1.8 ( 0.6 × 1015 3.0 ( 1.0 × 1015 3.8 ( 1.2 × 1015
3.1 ( 0.5 × 1021 3.8 ( 0.6 × 1021 4.6 ( 0.8 × 1021 4.5 ( 0.8 × 1021
6.9 11.7
in the takeoff angle. This suggests that Ta is adsorbed near the polyglycidol-vacuum interface.38,40,41 To quantify the amount of penetration we will use the simplest models that are capable of describing the data. First, we will assume that the Ta forms a 2D film, buried at a distance dTa from the surface. In this case the functional form for the photoelectron intensity, I(θ), is given by I(θ) ) (I0 /cosθ) exp(-dTa/λTa cosθ), where θ is the take off angle and λTa is the inelastic mean free path of photoelectrons.38 These fits are given by the smooth curves in Figure 6(b). The dTa/λTa values we obtained are shown in Table 2. Using the value measured for the attenuation of the Si(2p) photoelectrons, λTa ) 48.3 Å [cf. Figure 3b, and Supporting Information], we make a small (4%) correction for the slightly different kinetic energy and find λTa ) 50.1 Å. This value may be used to estimate dTa in each case, and we find dTa ) 15 ( 4, 19 ( 4, and 16 ( 4 Å for 30, 66 and 84 Å thick polyglycidol films. These results would seem to suggest that the reaction of Ta[N(CH3)2]5 with polyglycidol is confined to the top ∼15-20 Å depth of the film, irrespective of the film thickness, at least for film thicknesses of 30 Å and greater. A second model we will use to fit the data is one that assumes the Ta reacts to form a uniformly modified organic thin film of thickness dTa,film. In this case, to fit the results from ARXPS
Figure 6. (a) XP spectra of the Ta(4f) feature for saturation exposures of Ta[N(CH3)2]5 to three polyglycidol thin films of different thicknesses. (b) Peak areas of the Ta(4f) feature for saturation exposures of Ta[N(CH3)2]5 to three polyglycidol thin films of different thicknesses plotted as a function of the photoelectron takeoff angle. The smooth curve in each case is a fit to the model described in the text.
8616 Langmuir, Vol. 24, No. 16, 2008
Sharma et al. Table 2. Properties of Chemisorbed Layers Ta densitya (cm-2)
penetration, 2d model
penetration, thin film model
organic layer
uncorrected (cm )
corrected (cm )
dTa/λTa
dTa (Å)
dTa/λTa
〈dTa〉 (Å)
poly-G, 30 Å poly-G, 66 Å poly-G, 84 Å -NH2 modified poly-G, 84 Å
2.9 ( 0.3 × 1014 4.2 ( 0.4 × 1014 4.9 ( 0.4 × 1014 2.5 ( 0.2 × 1014
4.2 ( 0.9 × 1014 6.7 ( 1.2 × 1014 7.2 ( 1.8 × 1014 3.3 ( 0.8 × 1014
0.30 ( 0.08 0.37 ( 0.07 0.31 ( 0.09 0.20 ( 0.12
15 ( 4 19 ( 4 16 ( 4 10 ( 5
0.74 ( 0.25 1.00 ( 0.26 0.76 ( 0.28 0.20 ( 0.12
19 ( 6 25 ( 6 20 ( 7 10 ( 5
a
-2
Average values from Ta(4f) and Ta(4d).
b
-2
N/Ta 2.0 ( 0.4 0.8 ( 0.2 5.7 ( 1.4 b
Includes contributions from N of the modifying SAM.
we use I(θ) ) I0 [1 - exp(-dTa,film/λTa cosθ)]. This functional form also describes our data well, and the curves are actually indistinguishable from those of the 2D model (see Supporting Information). Again making use of λTa we find in this case that dTa ) 37 ( 12, 50 ( 13, and 38 ( 14 Å for the 30, 66 and 84 Å thick polyglycidol films. If we calculate the mean depth of penetration from this model we find values of ca. 19, 25 and 20 Å, and these values are ∼20-30% greater than those of the 2D model. Obviously, we cannot distinguish between the two models at this point, since they can both describe the data, but we can conclude that the mean depth of penetration of Ta implicated from both approaches is similar, and is about 15-25 Å, not depending strongly on the thickness of the polyglycidol layer. Given the results presented in Figure 6b we can now make a revised estimate for the saturation density of Ta, given the fact that dTa/λTa is not ,1. These values, corrected for attenuation, are also shown in Table 2. The estimates now lay in the range of 4.2-7.2 × 1014 cm-2 for the 30-84 Å thick polyglycidol films. Although these values are quoted in terms of an areal density, the Ta is likely present throughout the organic layer given the magnitude of these densities, and the two models considered above (2D layer vs uniform thin film) bracket the true situation. Thus, there is likely a distribution of Ta species in the layer, centered about a depth of ∼15-25 Å from the surface. A final issue we can address concerning the reaction of Ta[N(CH3)2]5 with these polyglycidol thin films is the stoichiometry of the reaction: what is the extent of ligand loss in the Ta complex, and does it depend upon the thickness of the polyglycidol thin film? In order to shed light on this issue we have acquired XP spectra in the range of the N(1s) and Ta(4p) regions. As there is no nitrogen in the starting chemical oxide substrate, nor the polyglycidol thin film, the N(1s) feature should be an unambiguous indicator of the presence of the N(CH3)2 ligands. Also, as the N retained is likely still bound to Ta attenuation affects for the two features should be of equivalent strength. As stated above, assigning an absolute chemical shift in this case (where perhaps the oxidation state, Tan+, could be deduced) is problematic due to sample charging effects. XP spectra for the binding energy region of 390-410 eV are shown in Figure 7 for three cases: saturation exposures of Ta[N(CH3)2]5 to chemical oxide, 30 and 84 Å thick polyglycidol films. Here in each case, the lower binding energy peak (∼398 eV) is for N(1s), while the higher binding energy peak (∼403 eV) is for Ta(4p) photoelectrons. Ta[N(CH3)2]5 can react with terminal -OH via ligand exchange reactions, producing volatile HN(CH3)2, in the same way as the comparable Ti precursor, Ti[N(CH3)2]4.38 As may be seen in Figure 7, the spectrum in each case has been fit to two peaks: the Ta(4p) at 403 eV, while the N(1s) peak positions are 398.1 eV for chemical oxide; 398.0 eV for 30 Å and 398.6 eV for 84 Å thick polyglycidol films. The binding energy of 398 eV for N(1s) is consistent with what has been reported in literature for sputter deposited TaNO films,54 and a ∼0.5 eV higher binding energy observed for N(1s) for the
Figure 7. XP spectra in the binding energy region corresponding to the N(1s) and Ta(4p) features for the adsorption of Ta[N(CH3)2]5 on: (a) unmodified SiO2 (chemical oxide), (b) a 30 Å, and (c) a 84 Å thick polyglycidol thin film. The spectra have been fit to two peaks using Gaussian-Lorentzian product function with the lower binding energy peak corresponding to N(1s), and the higher binding energy one corresponding to Ta(4p).
84 Å film suggests a change in the next-nearest neighbor environment for N bound to Ta in these thicker organic films. This suggestion is further bolstered by other features apparent in the spectra in Figure 7, namely, there is a continuous decrease in the area of the N(1s) peak with the thickness of the polyglycidol thin film. Of more importance is the behavior of the N/Ta atomic ratio, which is 5 for the parent Ta complex, and 0 if it is completely stripped of ligands. This ratio can be calculated by accounting for the respective photoelectron cross-sections, as described in the Supporting Information. From this calculation we find N/Ta ratios of 4.1 ( 0.4 for chemical oxide, 2.0 ( 0.4 for 30 Å and 0.8 ( 0.2 for 84 Å thick polyglycidol film The observed trend clearly indicates that as the density of terminal -OH groups increases, Ta[N(CH3)2]5 adsorbs on the surface by losing an increasingly higher number of dimethylamido ligands. While the N/Ta ratio of ∼4 hints toward a single ligand exchange reaction
Gas-Surface Reactions
Langmuir, Vol. 24, No. 16, 2008 8617 Scheme 2
Figure 8. N(1s) XP spectrum for 3-aminopropyldimethylethoxysilane adsorbed on a 84 Å thick polyglycidol thin film. The spectrum has been fit to a single peak using Gaussian-Lorentzian product function after background subtraction.
and formation of one Ta-O bond for chemical oxide, the ratios of ∼2 and ∼1 suggest 3 and 4 Ta-O bonds are formed on average when reacting with the polyglycidol thin films. The density of the number of hydroxyl carbons present in the layers would seem to justify a significant amount of ligand loss. Using the corrected (2D) values we find that -OH/Ta increases from 2.2 to 4.5 to 5.3 for polyglycidol thicknesses of 30, 66 and 84 Å. Thus, there seems to be a slight deficiency in terms of the number of -OH available to produce the observed amount of ligand loss in the thinnest 30 Å film (we expect 3 -OH per Ta). For the thicker 84 Å film, however, there seems to be ample density of -OH to produce the observed amount of ligand loss. Indeed, it seems unavoidable to conclude that based on the Ta ARXPS results that the great majority of the Ta complexes are beneath the surface, where they can react with surface, shortchain terminal and backbone -OH. A final comment as to the reaction of Ta[N(CH3)2]5 with the polyglycidol layers concerns stereochemistry issues. We address these issues in cartoon form in Scheme 2. As we have just discussed, the reaction of the Ta complex with chemical oxide appears to be rather clean, and involves only a single ligand exchange reaction. This is despite the fact that the chemical oxide surface possesses a high density of -OH. One reason for this could be stericsthe -OH on chemical oxide are largely confined to an atomically flat plane, and the Ta complex may have difficulty accessing multiple -OH sites for ligand exchange. In the case of the polyglycidol layers, the conformation and configuration of the -OH groups is considerably more relaxed. If, indeed, the Ta complex is able to penetrate the organic layer (cf. Scheme 2B and C), then it is likely to become virtually surrounded by -OH groups, and considerable loss of ligand becomes possible. This seems to be the case for the thicker (84 Å) polyglycidol thin film examined here. Additional ligand loss may also occur through reactions not involving polyglycidol, such as reactions involving two of the N(CH3)2 ligands, leading to the liberation of dimethylamine and the formation of an azametalacyclopropane.40,64 C. 3-Aminopropyldimethylethoxysilane on Polyglycidol. In some cases it may be desirable to modify the -OH termination intrinsic to polyglycidol to another functional group, perhaps as a way to attenuate the amount of ligand loss. For example, complete stripping of all ligands by the organic thin film could render the Ta complex completely inactive concerning additional (64) Beaudoin, M.; Scott, S. L. Organometallics 2001, 20, 237–239.
reactions. Thus, in the following set of experiments we have used 3-aminopropyldimethylethoxysilane, H2N(CH2)3Si(CH3)2(OCH2CH3) to transform the -OH termination of polyglycidol to one with -NH2 termination. This molecule, for example can be used to modify SiO2 (e.g., chemical oxide) to produce a surface with a high density of -NH2.41 Also, due to the single alkoxy group, it will only react with a single -OH group. We first consider the reaction of polyglycidol with the aminoalkoxysilane. After carrying out the solution phase reaction we find that the total ellipsometric thickness of the organic layer was found to increase by 4.4 Å. This is close to the molecular length of the aminoalkoxysilane, namely 5.4 Å, predicted using a molecular model (ACD/ChemSketchTM, Toronto, ON, Canada). For a more detailed characterization, we conducted XPS on this sample. In addition to the peaks associated with C, O and Si, also observed for bare polyglycidol, peaks corresponding to N are also observed. This suggests that we have successfully bound the aminoalkoxysilane to polyglycidol. In Figure 8 we display the N(1s) spectrum for modification of a 84 Å thick polyglycidol with the aminoalkoxysilane. The peak position of 398.6 eV that we obtained in the present case matches well with what we have reported earlier for the organic layer formed by binding this same molecule directly to chemical oxide,41 namely 399.2 eV. In earlier work we have reported that the density of this aminoalkoxysilane adsorbed on chemical oxide is 3.15 × 1014 molec-cm-2.41 Using the area under the N(1s) peak for that case as a reference, we estimate the density of -NH2 on this 84 Å polyglycidol thin film to be 4.9 × 1014 molec-cm-2, a value larger than that on chemical oxide. This value does not take into account possible photoelectron attenuation effects. Unfortunately, due to the relatively weak signal observed for the N(1s), and the fact that a smaller aperture must be used for ARXPS, conducting angle resolved measurements was not practical. Nevertheless, comparison to the (attenuation corrected) value for the -OH density is of some usesfor this film it is 3.8 × 1015 cm-2. If these values are representative it suggests that the procedure has modified approximately 13% of the -OH. This is a lower limit as we have not accounted for attenuation effects on the aminoalkoxysilane. In any event, this small value suggests much of the modification may occur on or near the surface of the organic thin film. Next we exposed the -NH2 modified-polyglycidol sample to Ta[N(CH3)2]5, and in Figure 9a we plot the Ta(4f) spectrum measured after a saturation dose. The Ta complex clearly still chemisorbs to the surface, but the intensity is reduced from that
8618 Langmuir, Vol. 24, No. 16, 2008
Figure 9. (a) XP spectrum of the Ta(4f) feature following saturation exposures to Ta[N(CH3)2]5 on a 84 Å thick polyglycidol thin film modified with 3-aminopropyldimethylethoxysilane. Ts ) 25 °C. The inset shows the peak area of the Ta(4f) region as a function of takeoff angle. (b) XP spectrum in the binding energy region corresponding to the N(1s) and Ta(4p) features for the adsorption of Ta[N(CH3)2]5 on this same -NH2 modified polyglycidol thin film.
seen from an unmodified 84 Å polyglycidol thin film [cf. Figure 6a]. Using the procedure described above, we calculate that the density of Ta adsorbed on this modified polyglycidol is 2.5 ( 0.2 × 1014 atoms · cm-2. This value represents one uncorrected for photoelectron attenuation effects. As such, the first point of comparison should be to the value found for Ta on unmodified 84 Å polyglycidol, 4.9 ( 0.4 × 1014 atoms · cm-2. Thus, these values suggest the modification has produced a reduction in chemisorptive capacity by 50%. In the absence of additional information the meaning of this is unclear at this point, as the Ta complex may react with one or both functional groups, although reaction with -OH would be favorable energetically. We can note at this time that the uncorrected Ta density is essentially 1/2 of that of the uncorrected density of -NH2. In order to estimate the spatial extent of the reaction of Ta[N(CH3)2]5 with the -NH2 modified polyglycidol thin film, we acquired angle resolved XP spectra for the Ta(4f) peak, and these results are shown in the inset of Figure 9a. From this data, and applying the 2D model, we find that dTa/λTa ) 0.20 ( 0.12, a value somewhat smaller than that obtained for the unmodified polyglycidol thin film (0.31 ( 0.09). Using λTa ) 50.1 Å implicates dTa ) 10 ( 5 Å. Additional information as to the nature of the chemisorbed species can be obtained as before by examining the reaction stoichiometry, i.e., acquiring the N(1s)/Ta(4p) XP spectrum after exposure to Ta[N(CH3)2]5. This is shown in Figure 9b. An analysis of this feature reveals a N/Ta atomic ratio of 5.7. Obviously, in this case, the interpretation is complicated by the presence of N in the organic thin film.
Sharma et al.
In order to make some progress we recall the uncorrected values for the density of the terminal -R′NH2 groups, and the adsorbed Ta species on the modified polyglycidol surface: 4.9 and 2.5 × 1014 cm-2. Thus, if these numbers are correct, they indicate that there are two potential attacking -R′NH2 groups per Ta species. If a single ligand exchange reaction occurs between the Ta complex and one of these -R′NH2 groups, releasing HN(CH3)2, we would be left with a total N/Ta ratio of 6, very close to the value measured. On the other hand if two ligand exchange reactions occurred per Ta species we would be left with a N/Ta ratio of 5. Thus, loss of approximately a single, or perhaps two ligands per Ta complex seems a likely occurrence. We note that loss of two ligands, forming a “bipod” of sorts, where the legs are the attacking -R′NH2, species is exactly the scenario that we have found for the reaction of Ti[N(CH3)2]4 with -R′NH(i-C3H7) terminated selfassembled monolayers.40 There are obviously many alternatives, including reactions involving residual -OH, and also situations where a single attacking -R′NH2 could displace two -N(CH3)2 ligands forming an imido linkage (-R′N ) Ta). However, loss of 4-5 ligands (indicated for unmodified polyglycidol of this thickness) seems unlikely, as a N/Ta ratio of 2-3 would be the result, quite different from the measured value. Regardless, modification of the polyglycidol by the aminoalkoxysilane has caused the Ta species to retain on average 3 more ligands. This is a remarkable change. What may have caused such a drastic change? Perhaps the best explanation could be that the terminal groups present at or near the surface have changed (to -NH2), and that these species are less likely, perhaps due to a reduced thermodynamic driving force, to more completely exchange ligands.
IV. Conclusions We have investigated the growth of hyperbranched polyglycidol films on the surface of silicon dioxide, and their subsequent reaction with Ta[N(CH3)2]5. Consistent with previous work we found that the film thickness varied approximately linearly with reaction time up to thicknesses ∼150 Å. Initiation using an organic superbase produced essentially the same results as sodium methoxide. Contact angle measurements confirmed that the films grown over the range 30-145 Å were hydrophilic, and atomic force microscopy indicated formation of smooth, uniform thin films. Using in situ XPS we demonstrated that the adsorption of Ta[N(CH3)2]5 on polyglycidol is self-limiting, and exhibits first order Langmuirian kinetics. The saturation density of Ta adsorbed on polyglycidol was found to increase with increasing thickness of the organic film. From angle resolved XPS we found that Ta penetrates the organic film, on average to a depth of ∼15-25 Å irrespective of the thickness of polyglycidol, for thicknesses g30 Å. The composition of the adsorbed Ta species depends strongly on the thickness of the polyglycidol thin filmsfor a 30 Å polyglycidol thin film, approximately 3 ligands are lost upon chemisorption via ligand exchange reactions with -ROH groups, while for a thicker 84 Å film about 4 ligands are lost. Under the same reaction conditions, on the unmodified chemical oxide substrate, only a single ligand is lost. Polygycidol can be modified by 3-aminopropyldimethylethoxysilane, and this results in conversion of >10% of the -OH groups of a 84 Å thick polyglycidol thin film to -NH2 termination. Ta[N(CH3)2]5 reacts with the modified layer, and the saturation coverage is about 1/2 of that found on the unmodified surface, Ta penetrates
Gas-Surface Reactions
to a depth of about 10 Å, and only about one ligand is lost upon chemisorption, quite different from the unmodified surface. Acknowledgment. This research was supported by the NanoscaleInterdisciplinaryResearchTeamonInorganic-Organic Interfaces (NSF-ECS-0210693). Additional support was provided by the Semiconductor Research Corporation via the Center for Advanced Interconnect Science and Technology (SRC task 1292.003), and additional grants from NSF (ECE0304483, CTS-0529042). M.S. thanks Intel Corporation for a Ph.D. Fellowship. The authors also thank Krista Cosert for technical assistance.
Langmuir, Vol. 24, No. 16, 2008 8619
Supporting Information Available: Details of the experimental procedures including materials used, substrate preparation, synthesis conditions for polyglycidol, as well as details of the characterization of the organic films using contact angle, AFM, ellipsometry and XPS. Details regarding the adsorption of 3-aminopropyldimethylethoxysilane on polyglycidol. System and process details for chemisorption of Ta[N(CH3)2]5 on organic films. Details of XPS data analysis including Ta(4f)/O(2s) peak fits, C(1s) peaks and models for polyglycidol growth. Details of the model for the estimation of absolute density of Ta adsorbed on the organic layers. This material is available free of charge via the Internet at http://pubs.acs.org. LA800790U
