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Formation of Dense Self-assembled Monolayers of (n-Decyl)trichlorosilanes on Ta/Ta2O5 Randy De Palma,*,‡ Wim Laureyn,† Filip Frederix,†,‡ Kristien Bonroy,† Jean-Jaques Pireaux,§ Gustaaf Borghs,† and Guido Maes‡ InteruniVersity Microelectronics Center (IMEC), MCP-ART, Kapeldreef 75, B ˜ -3001 LeuVen, Belgium, Physical and Quantum Chemistry, Catholic UniVersity LeuVen, Celestijnenlaan 200F, B-3001 LeuVen, Belgium, and Laboratoire Interdisciplinaire de Spectroscopie Electronique (LISE), Faculte´ s UniVersitaires Notre-Dame de la Paix (FUNDP), 61 Rue de Bruxelles, B-5000 Namur, Belgium ReceiVed July 6, 2006. In Final Form: September 20, 2006 Tantalum pentoxide (Ta2O5) is a promising material for the realization of biological interfaces because of its high dielectric constant, its high chemical stability, and its excellent passivating properties. Nevertheless, the deposition of highly organized silane SAMs to realize well-defined and tailored Ta2O5-based (bio)interfaces, has not been studied in great detail as of yet. In this work, we have investigated the formation of a highly ordered, dense monolayer of trichlorosilanes on Ta2O5 surfaces. Specifically, two different cleaning procedures for Ta2O5 were compared and (n-decyl)trichlorosilane (DTS) was used to study the effect of both cleaning methods on the silanization of Ta2O5. Both types of cleaning allowed the formation of complete and crystalline DTS monolayers on Ta2O5, in contrast with the incomplete, disordered silane layer assembled on uncleaned Ta2O5. The deposited self-assembled monolayers were studied by means of contact angle goniometry, Brewster angle FTIR, X-ray photoelectron spectroscopy, cyclic voltammetry, and ellipsometry. Infrared analysis exhibited a highly ordered DTS silane film on Ta2O5 and indicated a larger tilt angle of the alkyl chains on this substrate by comparison to DTS on SiO2. Furthermore, with use of ellipsometry and XPS, the silane film thickness on Ta2O5 was determined to be substantially smaller than that reported in the literature for DTS on SiO2, supporting the observations of an increased tilt angle (∼45°) on Ta2O5 than on SiO2 (∼10°). By means of cyclic voltammetry, the formation of a dense, essentially pinhole-free, silane film was observed on the cleaned samples. In conclusion, the fully characterized and optimized procedure for the silanization of Ta2O5 surfaces with trichlorosilanes will allow the formation of well-defined, reproducible, and controllable chemical interfaces on Ta2O5.
Introduction Self-assembled monolayers (SAMs) are highly ordered twodimensional structures that form spontaneously on a variety of substrates.1 The most common adsorbate/substrate combinations are organosilanes on oxide surfaces2 and sulfur-containing molecules on gold.3 Although the latter combination has received the most attention, probably because of its ease of preparation, organosilanes possess some advantageous features. The covalent nature of the assembly process results in a superior stability, which allows extensive handling and further modification without deterioration of the SAM.4 Moreover these SAMs are compatible with complementary metal oxide semiconductor (CMOS) technology and permit the use of optical techniques such as fluorescence, often used as read-out methods (e.g., microarray).5 Tantalum pentoxide (Ta2O5) is an emerging promising material for the realization of biosensor devices because of its high dielectric constant,6 its excellent chemical stability,7 and its interesting optical properties. Gebbert et al. have already shown * To whom correspondence should be addressed. Randy De Palma, IMEC, MCP-ART, Kapeldreef 75, B-3001 Leuven, Belgium. Phone: +32-16281083. Fax: +32-16-281097. E-mail:
[email protected]. † IMEC. ‡ Catholic University Leuven. § Faculte ´ s Universitaires Notre-Dame de la Paix. (1) Schreiber, F. J. Phys.: Condens. Matter 2004, 16, R881. (2) Onclin, S.; Ravoo, B. J.; Reinhoudt, D. N. Angew. Chem., Int. Ed. 2005, 44, 6282. (3) Nuzzo, R. G.; Allara, D. J. Am. Chem. Soc. 1983, 105, 4481. (4) Maoz, R.; Cohen, H.; Sagiv, J. Langmuir 1998, 14, 5988. (5) Glo¨kler, J.; Angenendt, P. J. Chromatogr. B 2003, 797, 229. (6) Zaima, S.; Furuta, T.; Yasuda, Y. J. Electrochem. Soc. 1990, 137, 1297. (7) Christensen, C.; de Reus, R.; Bouwstra, S. J. Micromech. Microeng. 1999, 9, 113.
the advantage of a capacitance sensor for the detection of biological recognition events.8 More recently, several groups have used Ta2O5 as a base material for optical waveguides9 and surface plasmon resonance sensors.10 Due to its excellent passivating properties and its chemical inertness, Ta2O5 is also often used as a protective passivation layer in semiconductor manufacturing.7 Deposition of tantalum and its oxides can be done by a variety of techniques, such as physical vapor deposition, chemical vapor deposition, and thermal oxidation. This makes the use of these materials very flexible. At the same time Ta2O5 is known to be a biocompatible material used in for example implants11 and other biomedical devices. Silane SAMs would offer an ideal platform for engineering the surface of these tantalum-based devices, provided that procedures are developed to assemble these silanes with high quality and fidelity. However, to our knowledge, the controlled deposition of high-quality silane SAMs on Ta2O5 has never been studied before in great detail. In general, the preparation of SAMs on Ta2O5 is largely unexplored in comparison to SAMs on gold and SiO2. There are some extended investigations on surface functionalization of Ta2O5 but these make use of alkyl phosphate monolayers instead of alkylsilanes.12,13 Therefore, silane SAMs on Ta2O5 are believed to merit further investigation since they are likely to have (8) Gebbert, A.; Alvarez-Icaza, M.; Sto¨cklein, W.; Schmid, R. D. Anal. Chem. 1992, 64, 997. (9) Huang, N. P.; Michel, R.; Voros, J.; Textor, M.; Hofer, R.; Rossi, A.; Elbert, D. L.; Hubbell, J. A.; Spencer, N. D. Langmuir 2001, 17, 489. (10) Lu, H. B.; Homola, J.; Campbell, C. T.; Nenninger, G. G.; Yee, S. S.; Ratner, B. D. Sens. Actuators, B 2001, 74, 91. (11) Zhang, Y.; Ahn, P. B.; Fitzpatrick, D. C.; Heiner, A. D.; Poggie, R. A.; Brown, T. D. J. Musoskeletal Res. 1999, 3, 245. (12) Hofer, R.; Textor, M.; Spencer, N. D. Langmuir 2001, 17, 4014.
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interesting applications, induced by the wide range of properties of Ta/Ta2O5-based materials. Since the pioneering work of Sagiv and co-workers in the 1980s,14 the field of SAMs on SiO2 has exponentially grown and several reviews have covered this subject.15,16 However, in contrast to the increasing sophistication of the molecular architectures of silanes, the actual mechanism of their monolayer formation on SiO2 still remains a subject of debate. Many studies have been carried out to elucidate this process, but the results are often conflicting. In the past, fundamental investigations were also performed to formulate procedures that would guarantee reproducibility and high quality of silane deposition on SiO2.17 These procedures were however never successfully applied for the functionalization of Ta2O5 and the mechanism of silane SAM formation on Ta2O5 was never extensively investigated before. For SiO2 it is for example commonly known that several parameters play an important role in the formation of silane SAMs. Primarily, the surface needs to be activated prior to assembly to clean the substrate of contaminants and to maximize the number of surface hydroxyl functions.18 Especially water was recognized as one of the key parameters in the self-assembly process since it was shown to be necessary for the hydrolysis of trichlorosilanes, their subsequent condensation on the surface hydroxyl groups, and the cross-polymerization with nearby silanol functions. Consequently, there is a general consensus that trace amounts of physisorbed water on the surface are essential for the formation of well-packed monolayers.19,20 However, in a revealing modeling study performed by Stevens, it was shown that crosspolymerization, which is considered as an essential element in the formation of stable SAMs, cannot occur on SiO2 in case of highly packed monolayers.21 In this paper we discuss the effect of a number of parameters, such as the type of cleaning, the substrate dehydration, and the deposition time on the formation of silane SAMs on Ta2O5. A CH3-terminated trichlorosilane with a 9-long alkane chain (ndecyltrichlorosilane) is chosen as a model system to study and optimize the formation of silane SAMs on Ta/Ta2O5. Although longer alkane chains are known to create more ordered SAMs, this chain length was preferred because most commercially available silanes with different end groups typically contain a C10-11 alkane chain (e.g., cyanoundecyltrichlorosilane or bromoundecyltrichlorosilane). Toluene is chosen as a solvent since it is known to dissolve an optimal quantity of water (i.e., ∼0.15 mg/mL) necessary for the formation of well-packed silane SAMs.22 Finally, the silane SAM structure/formation on SiO2 versus Ta2O5 is compared. Experimental Section Materials. For contact angle, XPS, ellipsometry, and cyclic voltammetry analysis, 50 nm of Ta was deposited on a 6-in. silicon wafer using an electron-beam evaporation system (Pfeiffer 580 PLS) operated at a speed of 5 Å/s. The samples for Brewster angle FTIR were prepared by depositing 3 nm of Ta, via chemical vapor deposition, on both sides of a double-sided polished silicon wafer. The tantalum target (99.69%) was purchased from Praxair. (n-Decyl)(13) Brovelli, D.; Ha¨hner, G.; Ruiz, L.; Hofer, R.; Kraus, G.; Waldner, A.; Schlo¨sser, J.; Oroszlan, P.; Ehrat, M.; Spencer, N. D. Langmuir 1999, 15, 4324. (14) Netzer, L.; Iscovici, R.; Sagiv, J. Thin Solid Films 1983, 99, 235. (15) Ulman, A. Chem. ReV. 1996, 96, 1533. (16) Schwartz, D. K. Annu. ReV. Phys. Chem. 2001, 52, 107. (17) Brzoska, J. B.; Azouz, I. B.; Rondelez, F. Langmuir 1994, 10, 4367. (18) Zhuravlev, L. T. Langmuir 1987, 3, 316. (19) Fairbank, R. W. P.; Wirth, M. J. J. Chromatogr. A 1999, 830, 285. (20) Wang, R. W.; Wunder, S. E. Langmuir 2000, 16, 5008. (21) Stevens, M. Langmuir 1999, 15, 2773. (22) Mc Govern, M. E.; Kallury, K. M. R.; Thompson, M. Langmuir 1994, 10, 3607.
De Palma et al. trichlorosilane (97%) was purchased from ABCR GmbH. Toluene (Spectranal) was purchased from Riedel-de hae¨n. Ru(NH3)63+, CCl4 (p.a.), and molecular sieves (3 Å, 8-12 mesh) were purchased from Acros. H2O2 (30%) and methanol were purchased from Air Products. H2SO4 (95-97%), NH4OH (28-30%), and acetone (Puranal) were purchased from Honeywell. KCl (p.a.) was purchased from Merck. Pretreatment (Cleaning, Activation, and Dehydration) of Ta/ Ta2O5 Substrates. Two types of pretreatments are assessed in this paper to study the influence of cleaning, activation, and sample dehydration on the silane deposition on Ta/Ta2O5 surfaces. This pretreatment step, prior to the self-assembly process, is known to strongly influence the formation of silane SAMs. Commonly the surface is activated by removing the organic contaminations and maximizing the number of available hydroxyl groups. In some procedures dehydration of the substrate is used to avoid the presence of a water film on the surface. The first procedure used in this study (further denoted as “wet cleaning”) solely consists of a two-step treatment with a combination of acids. The first step is a piranha (H2O2/H2SO4; 1/3) cleaning for 15 min, which ensures the removal of all organic contaminations and promotes further oxidation of the native Ta2O5 film. (CAUTION: piranha solution reacts Violently with organic material.) This step is followed by an ammonia cleaning (H2O/H2O2/NH4OH; 4/1/1) at 70 °C for 15 min, which is known to promote the formation of a stoichiometric oxide (i.e., Ta2O5) by sequential etching of the “old” oxide (NH4OH) and oxidation to form a “new” oxide (H2O2). These samples are dried using a N2 flow prior to use. The second procedure (further denoted as “dry cleaning”) consists of a combination of the wet cleaning with a surface dehydration and a UV/O3 treatment. Following the wet cleaning, the samples are dried using a N2 flow and dehydrated on a hotplate at 110 °C for 30 min under a N2 atmosphere to remove the water on the surface and limit it to approximately a monolayer. Finally, this is followed by a dry UV/O3 treatment for 15 min at ∼50 °C to remove leftover contaminants. Silanization of Ta/Ta2O5 Substrates Using Trichlorosilanes. Silanization was performed in a closed glass container for 3 h at room temperature by inserting the samples, immediately after cleaning, in a solution of dried toluene containing 0.5% (v/v) (ndecyl)trichlorosilane. Toluene was dried over hot molecular sieves (∼110 °C) for at least 2 h prior to use. The glass container was preconditioned by silanizing it with the same silane (thorough rinsing is obligatory) to avoid loss of silane towards the glass surface and to favor its reaction with the Ta/Ta2O5 surface. To avoid the intrusion of water in the solution, the glass container was dehydrated in the oven at 110 °C and continuously flushed with N2 during the course of the silanization process. After silanization the samples were rinsed extensively with consecutively toluene, CCl4, acetone, H2O, and methanol to rinse away the noncovalently bound silanes; the samples were then blown dry using a N2 flow. To promote the potential cross-linking (siloxane formation) between the silane chains, the samples were heated on a hotplate at 110 °C (under N2 atmosphere) for 15 min. Characterization of Silane Self-assembled Monolayers. The instrumentation used for X-ray photoelectron spectroscopy (XPS) analysis is a HP5950 A spectrometer with an Al KR monochromatized source at a collection angle of 51.5°. Curve fitting for XPS was done using software developed at the FUNDP-LISE, which is comprised of a simplex routine with peaks of a mixed Gaussian (∼75%) and Lorentzian form, a Shirley background, and a peak width of 1.4-1.6 eV. Singlet peaks were used for both the C1s and O1s subregions and doublets for the Ta4f subregion. Contact angle measurements were performed using an OCA-20 video-based device from Dataphysics and the supplied SCA20 software. Static contact angles were determined on sessile drops (1 µL droplets) of ultrapure water and contact angles were calculated using the ellipse fitting. Advancing and receding contact angles were determined by depositing a 2 µL drop of ultrapure water on the surface and lowering the syringe until it gently touched the drop. Next the volume of the drop was increased and subsequently decreased with 2 µL at a continuous rate of 0.2 µL/s. The highest
Dense SAMs of (n-Decyl)trichlorosilanes on Ta/Ta2O5
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Table 1. Surface Elemental Concentrations Determined by XPS (Total Set to 100%) before and after Silanization of Uncleaned, Wet-Cleaned, and Dry-Cleaned Ta/Ta2O5 with DTSa surface elemental conc (%)
C1s subregion conc (%)
Ta4f subregion conc (%)
ratio Ta modifications no clean wet clean dry clean
Ta
O
C
Si
C/Si O/Ta CHxb C-Oc (OdC)-Od Ta2O5e TaxOyf
16.7 45.1 34.7
