Langmuir 2006, 22, 5617-5624
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Substrate Changes Associated with the Chemistry of Self-Assembled Monolayers on Silicon Theresa M. McIntire,† S. Rachelle Smalley,† John T. Newberg,† A. Scott Lea,‡ John C. Hemminger,† and Barbara J. Finlayson-Pitts*,† Department of Chemistry, UniVersity of California IrVine, IrVine, California 92697-2025, and Pacific Northwest National Laboratory, P.O. Box 999, MSIN K8-93, Richland, Washington 99352 ReceiVed January 16, 2006. In Final Form: March 31, 2006 Alkylsiloxane self-assembled monolayers (SAMs) are used in the semiconductor industry and, more recently, as proxies for organics adsorbed on airborne mineral dust and on buildings and construction materials. A number of methods have been used for removing the SAM from the substrate after reaction or use, particularly plasmas or piranha (H2SO4/H2O2) solution. However, when the substrates are reused to make new SAMs, the impact of the cleaning methods on the chemistry of subsequently formed SAMs on the surface is not known. Here we report atomic force microscopy, X-ray photoelectron spectroscopy, Auger electron spectroscopy, and Fourier transform infrared studies of changes in a silicon substrate upon repetitive deposition and removal of SAMs by these two methods. It is shown that a thicker layer of silicon oxide is formed, and the surface becomes irregular and roughened, particularly after the piranha treatment. This layer of silica impacts the structure of the SAMs attached to it and can serve as a reservoir for trace gases that adsorb on it, potentially contributing to the subsequent reactions of the SAM. The implications for the use of such surfaces as a proxy for reactions of organics on airborne dust particles and on structures in the boundary layer are discussed.
I. Introduction Reactions of trace gases in the atmosphere with surfaces and with compounds adsorbed on surfaces are increasingly recognized as being important in urban and regional air pollution as well as for global climate change. Historically, such processes have been studied primarily in the laboratory by monitoring the loss of reactant gases and the formation of gaseous products. Following changes in the surface itself in real time under atmospheric conditions of pressure, temperature, oxygen, and water vapor has been problematic because of the need for high vacuum in many surface analytical techniques. Despite the challenges, such surface data are critical to understanding the molecular-level chemistry so that the results can be extrapolated reliably to the atmosphere. A promising approach for real-time in situ studies under conditions relevant to the atmosphere is the use of attenuated total reflectance (ATR) spectroscopy combined with Fourier transform infrared (FTIR). For example, ATR-FTIR has been used to study the chemistry of oxides of nitrogen and sulfur1,2 in thin water/ice films. In addition, it has been used to study the kinetics and products of the oxidation of organics in the presence of air and water vapor.3,4 In the studies of Dubowski et al.,4 for example, alkene self-assembled monolayers (SAMs) were attached to the surface of a silicon ATR crystal, and the loss of CdC and formation of CdO were followed with time using FTIR. Silicon is a particularly convenient ATR crystal because it transmits light in the mid-IR and is relatively chemically resistant. In the absence of special treatment, such crystals have a thin * To whom correspondence should be addressed. Tel: (949) 824-7670. Fax: (949) 824-3168. E-mail:
[email protected]. † University of California Irvine. ‡ Pacific Northwest National Laboratory. (1) Sayer, R. M.; Horn, A. B. Phys. Chem. Chem. Phys. 2003, 5, 5229. (2) Ramazan, K. A.; Wingen, L. M.; Miller, Y.; Chaban, G. M.; Gerber, R. B.; Xantheas, S. S.; Finlayson-Pitts, B. J. J. Phys Chem. A 2006, doi: 10.1021/ jp056426n. (3) Thomas, E. R.; Frost, G. J.; Rudich, Y. J. Geophys. Res. 2001, 106, 3045.
silicon oxide surface layer, with some of the Si atoms terminated in -OH groups. As a result, their surface is similar to silicate and borosilicate materials that are used in many laboratory systems, such as those made of quartz or glass. In addition, this surface is a reasonable model for airborne dust particles and building materials, which contain silicates5,6 and which act as substrates for heterogeneous chemistry in the troposphere. The focus of studies to model heterogeneous atmospheric processes has been the chemistry occurring in the thin surface layer. Relatively little attention has been given to the role of the substrate, including potential effects of the pretreatment of the surface such as cleaning. For example, in the case of ATR studies, where the cost of the infrared transmitting crystals is such that the crystals are cleaned and reused multiple times, changes in the substrate have the potential to impact subsequent chemistry. Numerous methods exist for cleaning substrates, such as mechanical cleaning,7 chemical cleaning,8-13 ultrasonics,14 and supercritical fluids.15-18 Alkylsiloxane SAMs are used extensively (4) Dubowski, Y.; Vieceli, J.; Tobias, D. J.; Gomez, A.; Lin, A.; Nizkorodov, S.; McIntire, T.; Finlayson-Pitts, B. J. J. Phys Chem. A 2004, 108, 10473. (5) Finlayson-Pitts, B. J.; Pitts, J. N., Jr. Chemistry of the Upper and Lower Atmosphere - Theory, Experiments, and Applications; Academic Press: San Diego, CA, 2000. (6) Diamant, R. M. E. The Chemistry of Building Materials; Business Books Limited: London, 1970. (7) Qin, K. D.; Li, Y. C. J. Colloid Interface Sci. 2003, 261, 569. (8) Henderson, R. C. J. Electrochem. Soc. 1971, 118, C223. (9) Henderson, R. C. J. Electrochem. Soc. 1972, 119, 772. (10) Kern, W. J. Electrochem. Soc. 1990, 137, 1887. (11) Pan, T. M.; Lei, T. F.; Chao, T. S.; Liaw, M. C.; Ko, F. H.; Lu, C. P. J. Electrochem. Soc. 2001, 148, G315. (12) Ermolieff, A.; Marthon, S.; Rochet, X.; Rouchon, D.; Renault, O.; Michallet, A.; Tardif, F. Surf. Interfacial Anal. 2002, 33, 433. (13) Bowling, A.; Kirkpatrick, B.; Hurd, T.; Losey, L.; Matz, P. Ultra Clean Processing of Silicon Surfaces V 2003, 92, 1. (14) Tomozawa, A.; Ohnishi, A.; Kinoshita, H.; Nakano, T. Ultra Clean Process. Silicon Surf. 2000 2001, 76-77, 235. (15) Sherman, R.; Whitlock, W. J. Vac. Sci. Technol. B 1990, 8, 563. (16) Sherman, R.; Grob, J.; Whitlock, W. J. Vac. Sci. Technol. B 1991, 9, 1970. (17) Sherman, R.; Hirt, D.; Vane, R. J. Vac. Sci. Technol. A 1994, 12, 1876. (18) Ryan, K. M.; Erts, D.; Olin, H.; Morris, M. A.; Holmes, J. D. J. Am. Chem. Soc. 2003, 125, 6284.
10.1021/la060153l CCC: $33.50 © 2006 American Chemical Society Published on Web 05/18/2006
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in the semiconductor industry, where two of the most common approaches to cleaning silicon surfaces are the use of “piranha” solution or plasma cleaners. Piranha cleaning consists of immersing the sample in a mixture of H2SO4 and H2O2 to oxidize surface contaminants, while plasma cleaning uses inert gas ions (e.g., argon) to sputter off surface contaminants. However, whether these treatments alter the surface in a manner that impacts the subsequent chemistry occurring in thin films on the substrate is not known. Here we report studies of changes in the chemical and physical characteristics of silicon surfaces after deposition and then removal of SAMs either by plasma cleaning or by piranha solution. To mimic the processing of ATR crystals used to follow SAM chemistry in real time,4 repetitive cycles of SAM attachment and removal were carried out. Surface topography and physical roughness were examined using atomic force microscopy (AFM). The elemental surface composition was monitored using X-ray photoelectron spectroscopy (XPS). The oxide layer depth was monitored using Auger electron spectroscopy (AES). A further probe of the nature of the surface comprised studies of the interaction of water vapor with the surface using ATR-FTIR. It is shown that both piranha cleaning and plasma cleaning for the removal of SAMs from silica lead to the formation of a much more roughened surface consisting of an irregular layer of silicon oxide. The nature of these “cleaned” surfaces as well as the impacts on their subsequent chemistry and use in SAM studies are discussed. II. Experimental Methods Two types of silicon substrates were used in this study: (1) p-type silicon (111) wafers (Wacker Siltronic Corporation) and (2) silicon ATR crystals (Harrick Scientific Corporation, 8 cm × 1 cm × 4 mm), having 10 reflections along the length of the crystal. The substrates were cleaned in boiling ethanol and then in boiling chloroform to remove surface adsorbed organics. They were then further cleaned either using piranha solution or by plasma cleaning. The piranha cleaning consisted of immersing the sample for 30 min. in a solution of 70:30 (v/v) H2SO4 (Fisher Certified ACS Plus, 95.7%) and H2O2 (30%, Electron Microscopy Sciences, ACS reagent grade), followed by thorough rinsing and boiling in Nanopure water (Barnstead, 18.1 MΩ cm). All samples were dried with nitrogen. [Caution: A piranha solution reacts violently with organic materials and should be handled with extreme caution.] Plasma cleaning was carried out by placing the samples in an argon plasma discharge (Harrick Plasma Cleaner/Sterilizer PDC-32G, low power) at 6.8 W for ∼10 min. Upon removal from the plasma cleaner, the substrates were placed in Nanopure water and dried in nitrogen (UHP, Oxygen Services, 99.999%). SAMs of 7-octenyltrichlorosilane (C8), Pfaltz & Bauer, 97%) were deposited on cleaned silicon pieces according to well-established techniques.19 This particular SAM was chosen because the extensive morphology, kinetics, and product studies on its reaction with ozone were available for comparison.4,20 After drying the cleaned substrates with nitrogen, the surfaces were placed in an ∼60 mM solution of 7-octenyltrichlorosilane in hexadecane for 30 min. The C8) SAMcoated wafers were then placed in boiling chloroform to remove any physisorbed material.4 The boiling was repeated an additional time to ensure a smooth, well-ordered coating. Samples were removed from the boiling chloroform and wiped with solvent-soaked laboratory lens paper to remove any unreacted starting material. To determine the effect of multiple cycles of SAM deposition followed by cleaning on the substrate, one set of samples was subjected to repetitive clean-SAM deposition-clean cycles using the piranha to remove the SAM from the surface. A second set was (19) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92. (20) McIntire, T. M.; Lea, A. S.; Gaspar, D. J.; Jaitly, N.; Dubowski, Y.; Li, Q.; Finlayson-Pitts, B. J. Phys. Chem. Chem. Phys. 2005, 7, 3605.
McIntire et al. subjected to a similar set of cycles but using plasma cleaning. Four different techniques, described in the following section, were used to probe the surface morphology and chemical composition: (1) AFM, (2) XPS, (3) AES, and (4) water uptake detected using FTIR. (1) AFM. Intermittent contact-mode AFM images were obtained in air at ambient pressure and humidity using an AutoProbe CPResearch (ThermoMicroscopes, Sunnyvale, CA; now Veeco Instruments, Santa Barbara, CA) scanning probe microscope. The piezoelectric scanner was calibrated using a 5.0 µm grating in the xy and z directions using an AFM reference (Pacific Nanotechnology, Santa Clara, CA; Model No. P-000-0004-0). The tips were silicon (ultrasharp cantilevers, model no. NSC11, MikroMasch). Topographs were obtained as 256 × 256 pixels and were flattened line by line and analyzed using the AutoProbe image processing software supplied by the manufacturer of the AFM. The root-mean-square (RMS) surface roughness over selected scanned areas was calculated from RRMS ) [ΣN (zn - z)2/(N - 1)]1/2, where z is the average z height, zn is the height at each point on the sample, and N is the number of points sampled. (2) XPS. XPS spectra of the silicon substrates before and after the clean-SAM deposition-clean cycles were obtained in an ESCALAB MKII ultrahigh vacuum (UHV) instrument (VG Scientific) equipped with three individually pumped chambers, allowing for rapid transfer (
