Surface Chemistry of Mercaptan and Growth of Pyridine Short-Chain

Langmuir 2002, 18, 1587-1594

1587

Surface Chemistry of Mercaptan and Growth of Pyridine Short-Chain Alkoxy Silane Molecular Layers Jay J. Senkevich,* Christopher J. Mitchell, G.-R. Yang, and T.-M. Lu Rensselaer Polytechnic Institute, Department of Physics 1SC25, 110 8th Street, Troy, New York 12180 Received June 25, 2001. In Final Form: December 3, 2001 The use of molecular layers to modify the surface and interfaces of solid-state materials while retaining their bulk properties offers great potential. Despite the widespread interest, little work has been undertaken to characterize the growth and surface chemistry of the short-chain alkoxy silane molecular layers. Variable angle spectroscopic ellipsometry, contact angle goniometry, and X-ray photoelectron spectroscopy are used to undertake the work in the present study. Results indicate that 3-mercaptopropyltrimethoxysilane and 2-(trimethoxysilylethyl)pyridine are both unique among the short-chain alkoxy silanes and grow multilayer films in a toluene solution on hydroxylated SiO2 surfaces. In particular, the mercaptan molecular layers show evidence of a changing surface chemistry as a function of growth time. Further, added surface moisture on mercaptan molecular layers yields thicker films of a higher density with more reduced surface sulfur when subsequent growth is resumed as compared to a control sample. Further, the pyridine molecular layers possess negative optical birefringence much like the parylene polymers, polyimides, and phthalocyanine Langmuir-Blodgett films undertaken by previous researchers. In previous cases, the presence of a phenyl group with a large anisotropic molecular polarizability caused the large in-plane polarizability. Further, the pyridine molecular layers exhibited a high index of refraction of 1.567 ( 0.005 explaining its superior properties as a metallic diffusion barrier at dielectric/metal interfaces from previous research.

I. Introduction The ability to modify the chemistry of a surface yet retain the bulk properties of the substrate has profound importance for many and diverse applications. Some of these applications include tribology,1 interfacial bonding,2 corrosion inhibition,3 diffusion barriers for metallic diffusion into dielectrics,4 passivation layers for II-VI and III-V compound semiconductors,5 improvement of substrate biocompatibility,6 and promotion of the wetting of metals deposited on dielectrics. In particular, 3-mercaptopropyltrimethoxysilane has been used for passivation of Ga-As,7 for copper chemical vapor deposition,8,9 as a heavy metal ion adsorbent,10-14 and for protein im* To whom correspondence should be addressed. Tel: 518-2768369. Fax: 518-276-8761. E-mail: [email protected]. (1) Moser, A. E. Tribology of Self-Assembled Monolayers. Dissertation, University of Nebraska, Lincoln, NE, 1998. (2) Choi, G. Y.; Kang, J. F.; Ulman, A.; Zurawsky, W.; Fleischer, C. Langmuir 1999, 15, 8783. (3) Jennings, G. K.; Laibinis, P. E. Organic Coatings for Corrosion Control; ACS Symposium Series 689; American Chemical Society: Washington, DC, 1998; p 409. (4) Krishnamoorthy, A.; Chanda, K.; Murarka, S. P.; Ramanath, G. Appl. Phys. Lett. 2001, 78, 2467. (5) Janes, D. B.; Kolagunta, V. R.; Batistuta, M.; Walsh, B. L.; Andres, R. P.; Liu, J.; Dicke, J.; Lauterbach, J.; Pletcher, T.; Chen, E. H.; Melloch, M. R.; Peckham, E. L.; Ueng, H. J.; Woodall, J. M.; Lee, T.; Reifenberger, R.; Kubiak, C. P.; Kasibhatla, B. J. Vac. Sci. Technol., B 1999, 17, 1773. (6) Thom, V. H.; Altankov, G.; Groth, T.; Jankova, K.; Jonsson, G.; Ulbricht, M. Langmuir 2000, 16, 2756. (7) Hou, T.; Greenlief, C. M.; Keller, S. W.; Nelen, L.; Kauffman, J. F. Chem. Mater. 1997, 9, 3181. (8) Doppelt, P.; Stelzle, M. Microelectron. Eng. 1997, 33, 15. (9) Doppelt, P. Coord. Chem. Rev. 1998, 178-180, 1785. (10) Feng, X.; Fryxell, G. E.; Wang, L.-Q.; Kim, A. Y.; Liu, J.; Kemner, K. M. Science 1997, 276, 923. (11) Brown, J.; Mercier, L.; Pinnavaia, T. J. Chem. Commun. 1999, 70, 60. (12) Mercier, L.; Pinnavaia, T. J. Environ. Sci. Technol. 1998, 32, 2749. (13) Lim, M. H.; Blanford, C. F.; Stein, A. Chem. Mater. 1998, 10, 467.

mobilization.15,16 However, despite the potential widespread application of molecular layer chemistry little work has been undertaken to characterize the growth of shortchain alkoxy silanes and in particular mercaptan- and pyridine-terminated silanes. The short-chain alkoxy silane molecular layers have a significant advantage over the other types of silanes in that they are the most widely available with the most diversity in their functional groups. Such functional groups include pyridine (-NC5H5), mercaptan (-SH), tetrasulfide (-SSSS-), phenyl (-C6H5), phosphino (-PR2), methyl (-CH3), amino (-NH2), iodo (-I), bromo (-Br), cyano (-CN), and vinyl (-CHdCH2), among others. The possibility also exists not only to grow monolayers end-capped with these functional groups but to grow multilayers. Conceptually, multilayer growth can take place (1) due to the lack of self-limiting chemistry, that is, random growth (studied here), or (2) due to well-defined monolayer growth, subsequent chemical modification of the monolayer surface, and then further monolayer growth.17 Molecular layers may be of interest since longchain self-assembled monolayers (SAMs) have been shown to have excellent dielectric properties with high tunneling barriers.18 Self-assembled multilayer growth akin to (2) above is analogous to atomic layer epitaxy where the growth of thin films takes place layer-by-layer through the use of self-limiting chemical reactions.19 In particular, the mercaptan end-capped molecular layer’s surface chemistry is investigated as a function of thickness and (14) Im, H.-J.; Yang, Y.; Allain, L. R.; Barnes, C. E.; Dai, S.; Xue, Z. Environ. Sci. Technol. 2000, 34, 2209. (15) Hickman, J. J.; Bhatia, S. K.; Quong, J. N.; Shoen, P.; Stenger, D. A.; Pike, C. J.; Cotman, C. W. J. Vac. Sci. Technol., A 1994, 12, 607. (16) Yee, J. K.; Parry, D. B.; Caldwell, K. D.; Harris, J. M. Langmuir 1991, 7, 307. (17) Netzer, L.; Sagiv, J. J. Am. Chem. Soc. 1983, 105, 674. (18) Collet, J.; Tharaud, O.; Chapoton, A.; Vuillaume, D. Appl. Phys. Lett. 2000, 76, 1941. (19) Goodman, C. H. L.; Pessa, M. V. J. Appl. Phys. 1986, 60, R65.

10.1021/la010970f CCC: $22.00 © 2002 American Chemical Society Published on Web 02/01/2002

1588

Langmuir, Vol. 18, No. 5, 2002

Senkevich et al. The silanes grown on the native oxide were characterized by the same method as the native oxide itself. However, instead of just SiO2/Si(Jellison) another layer was added to the model to fit for the molecular layers. The isotropic Cauchy model was used for this layer assuming Cauchy parameters of An ) 1.45, Bn ) 0.01, and Cn ) 0 yielding an index of refraction of 1.458 at 634.1 nm.

Figure 1. 3-Mercaptopropyltrimethoxysilane (mercaptan SAM) and 2-(trimethoxysilylethyl)pyridine (pyridine SAM) used in this study.

with different surface chemistry cleans. The study here also investigates the role of water in the multilayer growth of mercaptan molecular layers via hydroylsis and condensation reactions and how water influences density and surface chemistry of the mercaptan multilayer SAM. The surface chemistry and thickness of the mercaptan molecular layers is of particular interest for promoting the wetting of metals, for example, Cu, Pd, Pt, Co, and Ni deposited on dielectric surfaces. Thick molecular layers of ∼50 Å, even though they possess the requisite surface chemistry, may be unacceptably thick for applications in the semiconductor industry where “atoms are being counted”. II. Experimental Section The short-chain alkoxy silanes were all grown on the native oxide of Si(100) 50 Ω cm test grade wafers (sample size, ∼15 mm × ∼25 mm). The surface was treated with a RCA-1 clean, 5:1:1 ratio of deionized water (DI-H2O)/conc NH4OH/conc H2O2 for 2 min at >70 °C. In one case, the oxide surface was cleaned at room temperature with chloroform four times, 0.1 M KOH for 2 min, and 0.1 M HNO3 for 5 min and rinsed after KOH and HNO3 with DI-H2O to check for the correlation between surface chemistry clean and the oxidation of the mercaptan molecular layer. After the RCA-1 clean, to remove the organics and hydroxylate the surface, the thickness of the oxide was 13.6 ( 1.1 Å for 25 data points assuming optical constants of bulk SiO220 and Si(Jellison).21 The uncleaned silicon substrate possessed a slightly thicker oxide of 20.0 ( 1.0 Å. The thickness was obtained by use of an M-44 variable angle spectroscopic ellipsometry (VASE) instrument (J.A. Woollam Co., Lincoln, NE) at three angles (75, 76, and 77°) to the sample normal. These angles are close to the Brewster angle of the silicon substrate. Assuming an index of refraction for silicon of 4.0 at 634.1 nm, its Brewster angle would be 76.0°. For analysis of relatively thick films, for example, >50 Å, the Brewster angle of the thin film is normally used; however, for angstrom thick films the Brewster angle of the substrate should be utilized. All the short-chain alkoxy silanes were purchased from Gelest Inc. (Tullytown, PA) with >97% purity. In all cases, 1 drop (∼7 mg) of the alkoxy silanes shown in Figure 1 {3-mercaptopropyltrimethoxysilane and 2-(trimethoxysilylethyl)pyridine} was added to 1.00 ( 0.01 g of toluene (with added anhydrous MgSO4 but not distilled; water solubility, 0.05% w/w at 20 °C) in a 15 mL poly(tetrafluoroethylene) (PTFE) beaker yielding a concentration of 0.7% w/w. The cleaned native oxide surface was heated to >135 °C for at least 2 min and then cooled to room temperature (RT) (>30 s) before dropping the wafer into the toluene/silane solution. When completed, the sample was washed ×four times with toluene and then blown dry with N2. The humidity in the laboratory during silane growth varied considerably as reflected during the time of the year (12-80%) at a temperature of 72.0 ( 0.4 °F. No glovebox was used for silane growth, and the toluene was not distilled prior to use to investigate the growth of the silanes under typical laboratory conditions to achieve a robust surface modification method. (20) Philipp, H. R. In Handbook of Optical Constants of Solids; Palik, E. D., Ed.; Academic Press: Orlando, 1985; pp 749-63. (21) Edwards, D. F. In Handbook of Optical Constants of Solids; Palik, E. D., Ed.; Academic Press: Orlando, 1985; pp 547-69.

n(λ) ) An + Bn/λ2 + Cn/λ4

(isotropic Cauchy model)

(1)

n is the index of refraction, and λ is the wavelength of the incident photon. Since the optical constants and the thickness of films 50 Å, optical constants could be measured and those optical constants might be similar to those for a single monolayer depending on the growth mechanism. However, to assume optical constants for films 50 Å); 0.7% v/v in toluene in a PTFE beaker with a humidity of 65-80%.

Figure 14. The equilibrium contact angles for the data presented in Figure 12 using the sessile drop technique with a drop volume of 5 µL and measured within 30 s.

By this simple example (no kinetic argument and not taking into consideration the type of sulfur, S0, S-, or S2-), sulfur would be oxidized to sulfonic acid under acidic conditions by the following net reaction:

S + H2O + O2 f H2SO3

Eθ(SHE) ) 0.780 (5)

The consequence of this simple example is that oxygen should be excluded if high-quality thin mercaptan molecular layers are to be deposited. This is best accomplished in a vacuum, in a glovebox, or in a purged reaction vessel. The oxidation of mercaptans (thiols) or sulfides is wellknown in biochemistry, and this is one reason free oxygen is excluded from the human body IV. Pyridine Molecular Layer Growth Like mercaptan, the pyridine silane grows multilayer films as shown in Figure 13 and shows an increase in its contact angle in the multilayer regime (Figure 14). The increase may be associated with a thickness increase. Pyridine (-C5H5N), unlike the mercaptan (-SH) moiety, is a hydrocarbon species and will not react with Si-OH or Si-OCH3 species; therefore, multilayer growth may take place by pathway a in Figure 2. However, this pathway may be justified due to pyridine’s optical birefringence. A pyridine molecular layer of >50 Å thick (grown at 65 °C for 1440 min) possessed a strong negative birefringence much like the chemical vapor deposited parylene polymer thin films,34 polyimide thin films,35-37 (34) Senkevich, J. J.; Desu, S. B.; Simkovic, V. Polymer 2000, 41, 2379.

Surface Chemistry of Mercaptan

Langmuir, Vol. 18, No. 5, 2002 1593

Table 1. The Effect of Water on the Growth of the Mercaptan and Pyridine Alkoxysilane SAMs thickness (sol-gel chemistry) ∼9 days (Å)

contact angle (sol-gel chemistry) ∼9 days (deg)

pyridine (trimethoxy)

37.5 ( 17.7

43 ( 5

mercaptan (trimethoxy)

17.0 ( 1.0

43 ( 5

mercaptan (triethoxy)

61.1 ( 1.0

81 ( 11

SAM

a

thickness toluene w/H2O 60 min (Å)

contact angle toluene w/H2O 60 min (deg)

thickness toluene w/o H2O 60 min (Å)

contact angle toluene w/o H2O 60 min (deg)

110.6 ( 21.0a 14.8 ( 5.0b 53.4 ( 12.4a 14.3 ( 3.0b 33.7 ( 1.5a

47 ( 3b

14.0 ( 0.4

43 ( 3

52 ( 4b

9.6 ( 1.8

52 ( 3

43 ( 7a

12.2 ( 0.6

53 ( 3

b

With the H2O drop on the silicon substrate. With the H2O drop not on the silicon substrate.

and phthalocyanine Langmuir-Blodgett films.38 Negative birefringence is interpreted as the plane of the pyridine group being parallel to the plane of the substrate and therefore the pyridine silane lies on its side as shown in Figure 2a. The negative birefringence is a result of the large anisotropic molecular polarizability of the pyridine group. Adjacent pyridine groups are energetically stable due to this large molecular polarizability. Poly(p-xylylene) has a high melting temperature of 420 °C due to this strong bonding between its adjacent phenyl groups that are much like pyridine groups. The measured index of refraction for the pyridine molecular layer was 1.567 ( 0.005 at 634.1 nm for a 51.0 Å thin film using a simple isotropic Cauchy model. An index of 1.567 hints at a multilayer with a reasonable density since poly(p-xylylene) has an index of 1.650 in the as-deposited condition with only slight crystallinity. Such reasonable density for the pyridine molecular layers may be the reason for their barrier properties toward metallic diffusion of metals through dielectrics.4 In contrast, the mercaptan molecular layers (without added H2O) had an index of 1.386 ( 0.015 based on samples 61-90 Å thick. Since the mercaptan group has a lower molecular polarizability than the pyridine group, a lower index of refraction is expected. However, the mercaptan molecular layer’s index of refraction is significantly lower. This lower index may be a result of a random growth mechanism taking place producing lower quality multilayer film, especially when the physisorbed water at the surface is not well controlled, for example, in solution. Controlled availability of physisorbed water can possibly increase the quality of the mercaptan multilayers. For the 51 Å pyridine molecular layer film, the anisotropic Cauchy model would not converge to give definite values for nin-plane and nout-of-plane; however, an optical birefringence was found of ∆n ) (-0.070 to -0.095). Optical birefringence may be defined as

∆n ) nout-of-plane - nin-plane

(6)

β-Poly(p-xylylene) has been shown to have a birefringence of -0.1090 to -0.1304 depending on its film thickness and time/temperature of post-deposition annealing, and the amorphous phase of β-poly(p-xylylene) has an estimated birefringence of -0.0635.34 A birefringence near that of β-poly(p-xylylene) is reasonable considering the potential disorder in the monolayer growth of the pyridine molecular layer and since no post-deposition annealing was undertaken. More study should be undertaken to understand how post-deposition annealing influences the anisotropy and quality of the pyridine molecular layers. (35) Ree, M.; Kim, K.; Woo, S. H.; Chang, H. J. Appl. Phys. 1997, 81, 698. (36) Cho, T.; Morgen, M.; Lee, J. K.; Ryan, E. T.; Zhao, J.-H.; Malik, I.; Ho, P. Proc.sElectrochem. Soc. 1998, 98-3, 95. (37) Auman, B. C. Mater. Res. Soc. Symp. Proc. 1994, 337, 705. (38) Bourgoin, J.-P.; Doublet, F.; Palacin, S.; Vandevyver, M. Langmuir 1996, 12, 6473.

V. The Effect of Water on Molecular Layer Growth Mercaptan and Pyridine Silanes. Whether a monolayer or multilayer is desired, it is important to have a robust process. It is often easier to grow a monolayer as compared to multilayers since the chemistry of the first monolayer is often different from that of subsequent monolayers; hence, the often observed “self-limiting” chemistry. In certain cases, multilayer growth is possible and facilitated by the presence of physisorbed water. To make certain that water is a major factor in the growth of the mercaptan and pyridine molecular layers, two additional studies were undertaken. Table 1 shows the thicknesses and contact angles for mercaptan and pyridine molecular layers grown by solgel chemistry, water added to toluene solution, and anhydrous toluene. In all cases, the effect of adding water to the toluene solution was to increase the thickness of the molecular layers. When the substrate is placed in the beaker containing toluene, water, and the alkoxysilane, the water droplet has a tendency to stick to the native oxide surface since it is extremely hydrophilic and water is immiscible with toluene (0.05% w/w). At the points where the water droplet(s) sticks to the native oxide surface, the mercaptan and pyridine silanes can grow very thick films of >50 Å. In the case of the mercaptan silane, it is miscible in water and therefore may oligomerize before the substrate is placed in the beaker (resulting in a cloudy water droplet).39 Once oligomerization of the mercaptan silane occurs away from the substrate, it is not necessarily available to react at the surface. As the mercaptan molecular layers grow thicker, more reduced sulfur resides at its surface. The large contact angle for the mercaptan silane (triethoxy) (81 ( 11°) in Table 1 is close to that of pure sulfur (78°); therefore, the value is not unreasonable.40 Additionally, the sol-gel method of molecular layer growth is less controlled since water is added and the growth rate is rather slow at room temperature. A more successful approach was with nearly anhydrous conditions where the chemical reactions at the hydroxylated SiO2 surface are well controlled and dominate the molecular layer growth. The pyridine multilayer growth is much simpler than the mercaptan case and is likely dependent on the hydrolysis of the methoxy groups, which takes place more easily with additional water present. The pyridine silane is not miscible with water; however, the presence of water at the substrate surface can still hydrolyze the methoxy groups to the reactive hydroxyl groups. VI. Conclusions Silane molecular layers enable control of the dielectric surface chemistry while retaining its bulk properties. (39) Kurth, D. G.; Bein, T. Langmuir 1993, 9, 2965. (40) Janczk, B.; Chibowski, E.; Wojcik, W. Powder Technol. 1985, 45, 1.

1594

Langmuir, Vol. 18, No. 5, 2002

However, much of the emphasis on understanding selfassembled chemistry has been with the alkylthiolates on metallic surfaces and chlorosilyl SAMs on hydroxylated surfaces. Much less work has been undertaken with the alkoxysilanes, especially those with short hydrocarbon chains, commonly available, but which form disordered monolayers. The work here has investigated the growth and surface chemistry of pyridine- and mercaptanterminated alkoxy silanes. The presence of water appears to be a significant factor for multilayer growth of both the mercaptan and pyridine silanes. As the mercaptan molecular layer grows, its surface chemistry possesses more reduced sulfur (and therefore less oxidized sulfur). This is evident with short immersion times, for example, 0.25 min, and also longer immersion times, for example, 48 h. The starting molecule, 3-mercaptopropyltrimethoxysilane, is shown to be unoxidized, and therefore a surface reaction is responsible for the oxidation of the mercaptan group. The surface oxidation state of the sulfur is independent of the oxide surface clean but more likely due to the presence of both moisture and oxygen to oxidize the mercaptan molecular layer surface. Further, nearly independent of the molecular layer thickness, the integrated areas of the S2p XPS spectra are the same whereas the C1s spectra increase with increasing molecular layer thickness. From this observation and concurrent metallization studies, it is hypothesized that the sulfur resides only at the mercaptan molecular layer surface. As a consequence, a sulfurcontaining molecule should be eliminated during multilayer growth. Whether this is an oxidized (e.g., CH3SO3H) or reduced (e.g., CH3SH) sulfur-containing molecule was not determined.

Senkevich et al.

The addition of moisture at the mercaptan molecular layer surface after 24 h of growth influenced subsequent growth (24 h additional) by increasing the film thickness and showing more evidence of reduced sulfur at the film surface. A 4 h H2O dip caused an increase in thickness (146%) and more reduced sulfur (71%) than a 4 h exposure to a 100% RH environment (96% and 65%, respectively). The multilayer films exposed to water vapor possess a higher index of refraction, 1.434 ( 0.005, versus films not exposed to water vapor, 1.386 ( 0.015. However, all the mercaptan films exhibited lower indices of refraction as compared to thin films such as polyethylene and therefore may possess a lower density. In contrast, the pyridine multilayer films showed consistently much higher indices of refraction, for example, 1.567 ( 0.005 at 634.1 nm for a 51.0 Å film. They are also largely negatively birefringent ∆n ) (-0.070 to -0.095) similar to other films containing flexible phenyl groups. The negative birefringence along with the knowledge that pyridine is unreactive toward alkoxy silane groups leads to the hypothesis that these molecular layers grow by a different pathway as compared to the mercaptan molecular layers. The higher index of refraction, with the observation by other researchers that pyridine molecular layers are good metallic diffusion barriers for dielectrics, gives evidence that these multilayers are of high quality. Acknowledgment. Jay J. Senkevich is the recipient of the Rensselaer Roy Palmer Baker Graduate Fellowship. LA010970F