Chemical Modification of Self-Assembled Monolayers by Exposure to

XPS data showed that hydroxyl (C−OH) and aldehyde (CH O) groups were incorporated onto the surface of the SAMs upon irradiation at 0.5, 1, and 2 Tor...
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J. Phys. Chem. B 2000, 104, 7403-7410

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Chemical Modification of Self-Assembled Monolayers by Exposure to Soft X-rays in Air Tae K. Kim, Xiao M. Yang, Richard D. Peters, B. H. Sohn, and Paul F. Nealey* Department of Chemical Engineering and Center for Nanotechnology, UniVersity of Wisconsin, Madison, Wisconsin 53706 ReceiVed: January 11, 2000; In Final Form: April 24, 2000

Methyl-, vinyl-, and trifluoroacetoxy-terminated self-assembled monolayers (SAMs) of alkylsiloxanes on SiOx/ Si substrates were exposed to soft X-rays (0-4000 mJ/cm2) at air pressures from 2 × 10-2 to 2 Torr. The exposed and unexposed monolayers were characterized by using advancing-contact-angle measurements of water, ellipsometry, and X-ray photoelectron spectroscopy (XPS). No significant differences in the thicknesses of the monolayers were observed under any exposure conditions. Advancing-contact angles of water (θa) on all of the monolayers did not change with increasing dose up to 2000 mJ/cm2 for exposures performed at 2 × 10-2 Torr. A 15% loss of fluorine was observed from the CF3COO-terminated SAMs at this pressure at a dose of 4000 mJ/cm2. The θa decreased monotonically with dose for all monolayers exposed at 0.5, 1, and 2 Torr of air pressure. The rate of decrease of θa increased with increasing air pressure. A simple kinetic model based on competing oxidation and cross-linking reactions of reactive surface species fit the data well. The model adequately described the asymptotic value of the contact angle at high doses for the three exposure pressures and was insightful for the analysis of the role of oxygen in surface-modification reactions. Loss of fluorine from the CF3COO-terminated SAMs followed the same trends as the contact-angle data. XPS data showed that hydroxyl (C-OH) and aldehyde (CHdO) groups were incorporated onto the surface of the SAMs upon irradiation at 0.5, 1, and 2 Torr of air pressure, irrespective of the initial terminal groups of the SAMs. The hydroxyl groups were shown to be reactive for the formation of bilayer structures. These results are relevant for the optimization of chemical contrast and sensitivity in imaging layers based on SAMs for nanolithographic techniques using ionizing radiation.

Introduction Chemical modification of self-assembled monolayers (SAMs) by irradiation with soft X-rays may provide the basis for patterning surfaces at sub-100-nm-length scales with regions of different functionality using X-ray lithography. In this paper we describe the exposure conditions under which SAMs of alkylsiloxanes on SiOx/Si exhibit significant changes in wetting behavior and chemical reactivity as a function of dose. The experiments were carried out using synchrotron radiation and exposure systems designed for processing photoresists. Exposure conditions were chosen to be similar to those used in prototypical equipment for the development of X-ray lithography for the microelectronics industry. The results are (1) pertinent to understanding damage to organic materials upon exposure to X-rays at lithographically relevant doses, intensities, and exposure conditions and (2) directly applicable to patterning SAMs using tools already in development. Disadvantages of performing experiments under these conditions are that the exposure environment cannot be controlled as well as in experiments using, for example, ultrahigh vacuum and a monochromatic source and that the irradiated samples must be analyzed ex situ. The objectives of this study were (1) to design high-contrast and sensitive imaging layers based on SAMs that may be patterned with lithographically relevant doses of ionizing radiation, (2) to investigate the effects of the exposure environ* To whom correspondence should be addressed. E-mail: nealey@ engr.wisc.edu.

ment (vacuum, humidity, and gas composition) on the chemical modification of exposed SAMs, and (3) to compare and contrast the damage to monolayers exposed under lithographically relevant conditions to previous reports of damage to SAMs caused by exposure to X-rays in photoelectron spectrometers. The primary result of this study is that when SAMs of alkylsiloxanes on SiOx/Si are exposed to X-rays in the presence of oxygen, an increasing number of hydroxyl and aldehyde groups are incorporated onto the surface of the monolayer when the dose is increased. Initially hydrophobic surfaces can be rendered hydrophilic, or initially inert surfaces can be rendered reactive. These results suggest that, under proper exposure conditions, SAMs may exhibit adequate contrast and sensitivity for industrially relevant fabrication techniques. The use of highresolution ionizing radiation (X-rays, extreme-UV radiation, electron beams) to effect the chemical contrast provides immediate opportunities for patterning SAMs at sub-100-nm-length scales. X-ray damage to SAMs has previously been studied by a number of research groups. In the course of investigating the structure of SAMs of alkylsiloxanes on SiOx with X-ray reflectivity, Wasserman et al.1 observed that methyl- and vinylterminated monolayers irradiated in air appeared to be oxidized. Samples that had been irradiated by X-rays from a rotating anode source in an XPS did not appear to be oxidized, and they concluded the oxidation of the SAMs required the high flux of the synchrotron source.1 They also reported that exposure of monolayers containing C-Br bonds resulted in the loss of bromine in X-ray reflectivity experiments and during XPS

10.1021/jp000145s CCC: $19.00 © 2000 American Chemical Society Published on Web 07/18/2000

7404 J. Phys. Chem. B, Vol. 104, No. 31, 2000 analysis.1 Laibinis et al.2 and Graham et al.3 investigated damage to CF3COO- and CF3CONH-terminated monolayers supported on substrates that produced different photoelectron yields. The X-ray source of a photoelectron spectrometer (XPS) was used to irradiate the samples and to analyze the damage. On the basis of an in situ analysis of the loss of fluorine from the monolayers, they showed that electrons, not X-rays, are the principal cause of damage. In a combined XPS and FTIR study, Sagiv et al.4 analyzed the damage to nine different monolayer/substrate systems (including CH3- and COOH-terminated SAMs) upon exposure to X-rays under XPS-measurement conditions. They determined that organic monolayers are susceptible to significant chemical and structural damage without major loss of material from the surface even during relatively short exposure times and that the presence of radiation-sensitive functional groups, such as COOH-, affects the extent of damage and the degradation pathways. They concluded that XPS data alone are insufficient to quantitatively characterize radiation-induced chemical and structural transformations in the monolayers. Park et al.5 demonstrated the selective cleavage of the nitro group from a nitrobenzaldimine monolayer upon irradiation with X-rays (300-800 eV), and SAMs of (p-chloromethyl)phenyltrichlorosilane monolayers have been patterned by exposure to soft X-rays6 and proximity X-rays,7 taking advantage of the loss of chlorine from the exposed regions. Damage to SAMs has also been investigated for surfaces irradiated with high-energy and low-energy electrons. Irradiation of SAMs with high-energy electrons (such as in electron-beam lithography) is similar to irradiation with X-rays in that the molecular mechanisms of chemical and structural transformations involve primary and secondary electrons (with a distribution of energies) generated in the substrate. Craighead et al.8 investigated the use of SAMs of alkylsiloxanes as ultrathin resists in electron-beam lithography. Their comprehensive analysis of electron-beam-induced damage using a number of analytical techniques revealed that (1) the major effect of radiation was the loss of hydrogen via cleavage of C-H bonds, (2) eventually a carbonaceous residue was left at high doses, and (3) oxygenated functional groups were formed on the surface when the irradiated samples were removed from the exposure chamber and were exposed to the atmosphere. In a first approximation, low-energy-electron irradiation is akin to exposing SAMs to photoelectrons of discrete energies. Rowntree et al.9 exposed SAMs of alkanethiols on Au to low-energy electron beams in the 0-20-eV range. The principal disassociation process was dehydrogenation, and the disassociation probability exhibited a maximum at 10 eV. Dehydrogenation was not uniform throughout the film thickness but was strongly localized at the film/vacuum interface. Grunze et al.10,11 investigated the damage to SAMs of alkanethiols on Au upon exposure to low-energy electrons in the 10-100-eV range. The monolayers were very sensitive to irradiation, and they observed dehydrogenation, desorption of the film, and the appearance of new sulfur species. Lithographic applications of low-energy irradiation of SAMs were also demonstrated.12,13 In this study, we analyzed the surface properties of SAMs of alkylsiloxanes that were irradiated using soft X-rays from a synchrotron source in the presence of air. Methyl-, vinyl-, and CF3COO-terminated SAMs of alkylsiloxanes were investigated so as to compare our results to the literature. Significant changes in wetting behavior and chemical reactivity were achieved for doses of order 100-1000 mJ/cm2 and for exposure pressures of 1-2 Torr, whereas little change in the surface properties was observed for exposure at a pressure of 2 × 10-2 Torr,

Kim et al. irrespective of the initial terminal group. The changes in surface properties were related to the incorporation of oxygenated functional groups. Experimental Section Materials. Octadecyltrichlorosilane, 7-octenyltrichlorosilane, and 11-bromoundecyltrichlorosilane were purchased from Aldrich and Gelest and were used as received. Trifluoroacetic anhydride, anhydrous toluene, anhydrous hexane, anhydrous tetrahydrofuran, and anhydrous borane-tetrahydrofuran complex (1 M solution in tetrahydrofuran) were obtained from Aldrich and were used without further purification. Silicon 〈100〉 wafers were obtained from III TYGH and were cleaved into approximately 2.5- × 2.5-cm squares. Deposition of SAMs. Silicon substrates were placed in a glass dish and covered with piranha solution [H2SO4:H2O2 ) 70:30 (v/v)] at room temperature for 30 min. Caution! Piranha solution reacts Violently with organic compounds and should not be stored in closed containers. The mixture was heated for an additional 30 min at 90 °C and then cooled to room temperature. The silicon substrates were immediately rinsed with electronicgrade deionized (DI) water (resistivity g18 MΩ/cm) several times and were blown dry with nitrogen. To deposit SAMs from octadecyltrichlorosilane [CH3(CH2)17SiO-/SiO2, CH3-terminated monolayers], the cleaned substrates were immersed in 0.5% (v/v) octadecyltrichlorosilane (50 µL) in anhydrous toluene (10 mL) in a glovebox (under N2 atmosphere). After 24-25 h, the substrates were removed from the alkyltrichlorosilane solutions and rinsed with toluene and/ or chloroform. Optimal immersion times for formation of complete monolayers were determined from kinetic studies of contact angle and thickness. The substrates were baked at 120 °C for 5 min. Then they were removed from the glovebox, were rinsed with ethanol (200 proof) several times, and were dried under a stream of nitrogen. The same procedure was used to deposit SAMs from 7-octenyltrichlorosilane [CH2dCH(CH2)6SiO-/SiO2, CH2dCHterminated monolayers] and 11-bromoundecyltrichlorosilane [Br(CH2)10SiO-/SiO2, Br-terminated monolayers] except that the substrates were not baked at 120 °C. On some samples, Br-terminated monolayers were deposited on the surfaces of previously deposited and irradiated monolayers. Hydroboration of the CH2)CH-Terminated Monolayers. Hydroboration of the CH2dCH-terminated monolayers to form hydroxyl-terminated surfaces was performed according to the procedure described by Wasserman et al.14 The CH2dCHterminated monolayers were immersed in 1 M BH3-THF complex solution (ca. 15 mL) under an N2 atmosphere. After 2 h, the substrates were washed with anhydrous tetrahydrofuran, and approximately 15 mL of 30% H2O2/0.1 M NaOH was injected into the jars containing the borane-terminated monolayers. After 5 min, the OH-terminated monolayers were rinsed with DI water several times. Trifluoroacetylation of OH-Terminated Monolayers. The CF3COO-terminated monolayers (CF3COO(CH2)8SiO-/SiO2) were prepared by immersion of the OH-terminated monolayers in 2% (v/v) trifluoroacetic anhydride (200 µL) in anhydrous hexane (10 mL) for 5 min.14 They were then rinsed with ethanol several times and dried under a stream of nitrogen. Exposure of SAMs to X-rays. Monolayers were irradiated on the ES-1 beamline at the Center for Nanotechnology located at the Synchrotron Radiation Center at the University of Wisconsin. The soft X-rays are nonmonochromatic, and the wavelength is centered at 1.1 nm (E ≈ 1127 eV). Air pressures

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TABLE 1. Characterization of Monolayers Used in This Study by Ellipsometry and Contact-Angle Analysis monolayers CH3-terminated CH2dCH-terminated HO-terminated CF3COO-terminated Br-terminated

CH3(CH2)17SiO-/SiO2 CH2dCH(CH2)6SiO-/SiO2 HO(CH2)8SiO-/SiO2 CF3COO(CH2)8SiO-/SiO2 Br(CH2)10SiO-/SiO2

θa (deg)

θr (deg)

θs (deg)

thickness (Å)

110 ( 1.8 98 ( 1.4 60 ( 1.8 90 ( 2.0 95 ( 1.0

97 ( 1.5 87 ( 1.6 50 ( 2.6 73 ( 2.7 75 ( 1.9

101 ( 1.7 92 ( 1.6 56 ( 1.5 81 ( 3.6 80 ( 1.5

25.6 ( 1.8 10.4 ( 1.5 11.5 ( 1.2 12.5 ( 1.3 13.4 ( 1.1

of 0.5, 1, and 2 Torr were achieved by refilling the exposure chamber with air (20% relative humidity) after the chamber had been pumped down to 2 × 10-2 Torr. Doses were delivered from 0 to 4000 mJ/cm2. During exposure, the beamline remained stationary while the samples moved on an XY stage. Depending upon the dose and the current density of the beam, the time required for exposure was in the range of 5-60 s. X-ray radiation is absorbed in the gaseous-exposure environment such that the intensity of the beam reaching the surface of the monolayers, I, can be calculated from the equation

I ) Io exp(-µx)

(1)

where Io is the intensity of the incident beam entering the chamber, µ is the absorptivity (a function of pressure and gas composition), and x is the distance between the entrance to the chamber and the sample.15 In our experiments, the length of the pathway was 21 cm. Doses reported in this paper were calculated from the intensity of the beam as it entered the chamber and were not corrected for gas-phase absorbance in the exposure environment. For example, for a dose reported as 2000 mJ/cm2 under 1 Torr of air pressure, the actual dose delivered to the sample was 1870.8 mJ/cm2. Contact-Angle Measurements. Contact-angle measurements of deionized water were obtained using a Rame´-Hart goniometer under ambient atmosphere conditions. A syringe was used to dispense 10-20-µL drops of DI water (resistivity g18 MΩ/ cm). Advancing-contact angles (θa) were measured by forcing a small droplet of DI water from the end of a blunt-tip microliter syringe. Receding-contact angles (θr) were measured by retracting the plunger to remove a portion of the DI water. Staticcontact angles (θs) were measured by dropping a small drop of DI water on the surface. Contact angles were measured on the opposite edges of 3 drops and averaged. Ellipsometry measurements. Ellipsometry measurements were made on a Rudolf Research/Auto EL II ellipsometer using a He-Ne laser (λ ) 632.8 nm) at an incident angle of 70° relative to the surface normal of the substrates. The thicknesses of the SAM and the oxide layer cannot be measured simultaneously, and at least three separate spots were measured on each substrate before and after deposition of the SAM to determine the thickness of the oxide layer and the thickness of the SAM plus the oxide layer. The thickness of the oxide layer of silicon wafers was typically 12-18 Å. A refractive index of 1.45 was used for calculation of the thicknesses of the monolayers and the oxide. X-ray Photoelectron Spectroscopy (XPS) Measurements. The XPS spectra were obtained using a Perkin-Elmer 5400 ESCA photoelectron spectrometer with a Mg KR source (E ) 1253.6 eV, 300 W). The scan area was 1 × 3 mm, in which the gold-foil surface normal was at 45° to the entrance of the concentric hemispherical analyzer. The XPS measurements were made in a vacuum in the range 10-8-10-9 Torr. Survey spectra were collected for each sample using a pass energy of 89.45 eV. High-resolution scans of the carbon and bromine region were also performed using pass energies of 35.75 eV and an acquisition time of 20 min. The peaks were interactively fit using

Figure 1. Survey spectra (left) and high-resolution spectra of the C(1s) regions (right) of (a) CH3-terminated monolayer, (b) CH2dCHterminated monolayer, (c) OH-terminated monolayers formed from the CH2dCH-terminated monolayer after hydroboration and oxidation, and (d) CF3COO-terminated monolayers formed from the OH-terminated monolayer treated with trifluoroacetic anhydride.

Gaussian and Lorentzian profiles. Curve-fitting of the C(1s) spectra was achieved on the basis of a nonlinear least-squares method. The energy reference for the XPS spectra was the C(1s) peak at 285 eV. Results and Discussion Characterization of SAMs Prior to Exposure. Advancing-, receding-, and static-contact angles of DI water on all the monolayers used in this study and the thicknesses of all monolayers as determined by ellipsometry are presented in Table 1. Our values for contact angles and thicknesses for the monolayers agreed well with those reported in the literature.14,16-19 Figure 1 shows XPS-survey spectra (left) and high-resolution spectra (right) of the C(1s) peak for the organotrichlorosilane monolayers on silicon oxide. The high-resolution C(1s) spectra for CH3- and CH2dCH-terminated monolayers exhibited sharp, symmetric peaks centered at 285 eV. The high-resolution C(1s) spectrum of the CH2dCH-terminated monolayer after hydroboration and oxidation exhibited an asymmetric, broad peak indicative of the formation of the hydroxyl terminal group. A good curve fit was obtained by modeling the asymmetric C(1s) peak with contributions from methylene (CH2) at 285 eV and hydroxyl (C-OH) at 286.4-286.5 eV. XPS data for the OHterminated monolayers after reaction with trifluoroacetic anhydride confirmed the presence of fluorine (CF3, 293.6 eV) in both the survey and the high-resolution spectra and the presence of the ester linkage (CdO, 290.0 eV) in the high-resolution spectra. The shifts in binding energy from the methylene peak agree with literature values.20 SAMs Remain Intact After X-ray Exposure. The characteristic escape depth for electrons from substrates induced by

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Figure 2. Thickness of monolayers as a function of X-ray dose as determined by ellipsometry. Lines are drawn through the points to guide the eye.

X-ray irradiation (E e 15 keV) is 20-40 Å.2 This inelastic scattering length is large enough to affect any of the bonds of the hydrocarbons in our monolayers, including the possible cleavage of the Si-C bond or any C-C bonds. Ellipsometric measurements, however, revealed that the thicknesses of monolayers exposed to doses of X-rays as high as 4000 mJ/cm2 were no different (within experimental error) from monolayers as deposited, irrespective of the terminal group of the monolayers (see Figure 2). These results indicate the photochemical cleavage and/or surface reaction is confined to the monolayer-air or -vacuum interface during X-ray irradiation. These results are consistent with the data of Graham et al.3 They observed that the thickness of monolayers decreased ∼1 Å after 5 h of exposure to X-rays (Al KR E ) 1486.6 eV) under pressure of ∼10-9 Torr in a Surface Science X-100 XPS spectrometer. Tidswell et al.21 and Fenter et al.22,23 also reported that components of monolayers supported on silicon substrates that have been irradiated using a synchrotron X-ray source remain largely intact. Chemical Modification of SAMs with Exposure to X-rays Depends on Air Pressure and Dose. Changes in the surface chemistry of the monolayers after exposure to X-rays were probed by measuring advancing-contact angles of DI water as a function of dose and air pressure in the exposure chamber (see Figure 3). The advancing-contact angles on the CH3-, CH2dCH-, and CF3COO-terminated monolayers prior to exposure were 110°, 98°, and 90°, respectively. For exposures performed at a pressure of 2 × 10-2 Torr, the contact angles of the monolayers remained constant for doses up to 2000 mJ/ cm2. XPS spectra (not shown) of unexposed monolayers and monolayers exposed at 2 × 10-2 Torr are indistinguishable. (These XPS data provide additional evidence that the monolayers are not cleaved from the surface upon exposure.) For exposures performed at pressures of 0.5, 1, and 2 Torr, the contact angles of DI water decreased monotonically with increasing dose. As the pressure increased, the rate of decrease of the contact angles increased. The initially hydrophobic surfaces were converted to hydrophilic surfaces (θa ∼ 45°) for all terminal groups if the exposures were performed at 2 Torr and using high doses. These results indicate that one or more of the chemical species present in the exposure environment must participate in the

Kim et al. surface reactions responsible for the decrease in contact angles of water on the monolayers. The major species whose concentrations increase with increasing air pressure are nitrogen, oxygen, and water. Incorporation of oxygen-containing functional groups on the surface of all of the monolayers upon exposure to X-rays at pressures greater than 0.5 Torr provides strong evidence for the participation of oxygen in the surface reactions. A discussion of the role of oxygen is presented in detail in the next section. Nitrogen is not believed to play a significant role because (1) only small and irreproducible peaks were observed in the spectra near 399 eV, the photoelectron energy corresponding to nitrogen-containing compounds, for both exposed and unexposed monolayers and (2) the area under these small peaks did not increase for monolayers exposed to X-rays with 95% N2 at atmospheric pressure. Water is not believed to play a significant role because monolayers exposed to X-rays at 0.5 and 1 Torr of air with 20% and 100% relative humidity exhibited the same degree of chemical modification as determined by XPS and contact angles. Damage to the CF3COO-terminated monolayers could be determined as a function of pressure and dose by monitoring the amount of fluorine on the surface with XPS. Figure 4 shows survey spectra for CF3COO-terminated monolayers irradiated with increasing doses of X-rays at 1 Torr of air pressure. The height of the F(1s) peak decreased as the dose increased from 0 to 650 mJ/cm2, and the peak disappeared for doses above 1000 mJ/cm2. A more quantitative measure of the loss of fluorine with doses at 2 × 10-2, 0.5, and 1 Torr was obtained by calculating the ratio of the area of the F(1s) peak to the area of the Si(2p) peak (F/Si, see Figure 5). [The area of the Si(2p) peak is used as an internal standard in the spectra.] In agreement with the contact-angle measurements presented above, chemical modification of the surface of the CF3COO-terminated monolayers does not occur to a significant extent at a pressure of 2 × 10-2 Torr for doses up to 2500 mJ/cm2 (F/Si remained constant). At a dose of 4000 mJ/cm2, F/Si decreased by approximately 15%. Graham et al. investigated the damage to CF3CONH-terminated monolayers supported on Al, Ti, Cu, and Au substrates exposed to monochromatized Al KR X-rays (E ) 1486.6 eV) over time periods up to 5 h in a photoelectron spectrometer operated at 10-8-10-9 Torr.3 They estimated the flux of photons in their experiments to be 2 × 1012 photons/ (cm2 s). On the basis of these parameters, a 25-min exposure results in a dose of 1000 mJ/cm2. They report a 10% loss of fluorine for monolayers on gold at a dose of approximately 2400 mJ/cm2 and a 30% loss of fluorine at a dose of approximately 12 000 mJ/cm2. The agreement between our results and those of Graham et al. is satisfactory, considering the differences in the experiments: (1) the flux of photons from our synchrotron source was 2-3 orders of magnitude higher than the flux of photons from the photoelectron spectrometer; (2) the time periods for exposures were seconds using the synchrotron source, as compared to hours for the Al KR source; (3) the maximum dose in our experiments was 4000 mJ/cm2, and the maximum dose in the experiments of Graham et al. was 12 000 mJ/cm2; and (4) silica substrates produce fewer photoelectrons/ photon than heavy-metal substrates such as Cu and Au. Exposures performed at 0.5 and 1 Torr resulted in a monotonic decrease in F/Si with dose, and the rate of disappearance of fluorine from the surface of the monolayers increased with increasing pressure. At 0.5 Torr, fluorine was not detected on the surface for doses higher than 2500 mJ/cm2, and at 1 Torr, fluorine was not detected on the surface for doses higher than 1000 mJ/cm2. Graham et al. pointed out that some

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Figure 3. Advancing-contact angles of DI water as a function of air pressure and with increasing X-ray dose for (a) CH3-terminated monolayers, (b) CH2dCH-terminated monolayers, and (c) CF3COO-terminated monolayers.

Figure 4. XPS survey spectra of the CF3COO-terminated monolayers on silicon oxide irradiated at 1 Torr of air pressure and for doses of (a) 0 mJ/cm2, (b) 150 mJ/cm2, (c) 650 mJ/cm2, (d) 1000 mJ/cm2, and (e) 2000 mJ/cm2.

Figure 5. Plot of the ratio of the F(1s) peak intensity to the Si(2p) peak intensity for CF3COO-terminated monolayers exposed at 2 × 10-2, 0.5, and 1 Torr of air pressure for the doses indicated.

of the damage to the SAMs even in ultrahigh vacuum may result from reactions with species in the vapor phase, such as background amounts of oxygen, water, and nitrogen. Our results indicate that the presence of oxygen may be required for or

greatly accelerates surface reactions that result in the loss of fluorine from the monolayers upon irradiation. Irradiation of SAMs in Air Results in Terminal Groups That Contain Oxygen. Detailed information about the nature of the chemical modification of monolayers upon exposure to X-rays in the presence of air was obtained using XPS. Figure 6 (left, middle) shows the high-resolution C(1s) spectra for the irradiated CH3- and CH2dCH-terminated monolayers, respectively, as a function of X-ray dose. As the dose was increased for both monolayers, the C(1s) peak became increasingly asymmetric toward higher binding energies. The binding energies, relative to methylene, CH2, for oxidized carbon atoms are as follows: CH2OH, 1.5 eV; CdO, 3.0 eV; and CO2H, 4.5 eV. A good curve fit to the XPS data was obtained by modeling the asymmetric C(1s) peaks with contributions from methylene (CH2), hydroxyl (C-OH), and aldehyde (CdO) groups. This qualitative analysis of the XPS spectra, along with the decrease in the advancing-contact angle of water, leads to the conclusion that polar, oxygen-containing functional groups are incorporated onto the surface of CH3- and CH2dCH-terminated monolayers upon exposure to X-rays in the presence of air. The concentration of hydroxyl and aldehyde groups on the exposed CH3- and CH2dCH-terminated monolayers cannot be quantitatively calculated from the XPS data because the curve fitting is sensitive to the elementary distribution perpendicular to the surface.16 It also was not possible to obtain quantitative information from the oxygen peaks (not shown), due to the large background signal from the oxygen atoms of the silica substrate. A reaction mechanism that explains the incorporation of hydroxyl and aldehyde groups on the surface of the CH3terminated monolayers consists of the following steps: (1) free radicals are produced at the surface of the monolayer by removal of a hydrogen in a number of processes involving primary and secondary electrons emitted from the substrate upon irradiation, (2) the free radicals recombine with radical intermediates, such as hydrogen, react with neighboring groups to form cross-links, or react with oxygen (hence, the dependence on oxygen concentration) to form hydroperoxy radicals, and (3) the hydroperoxy radicals decompose along well-known pathways to generate a mix of products including alcohols, aldehydes and carboxylic acids. Although production of radicals may be most probable at the surface,8 free radicals are produced throughout the thickness of the monolayer. It is likely that free radicals in the interior of the chains react to form cross-links or undergo recombination processes with radical intermediates. Many of these processes result in loss of hydrogen.8 A simple kinetic model incorporating these ideas is presented in Scheme 1. Consider the surface of the SAM to be populated

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Kim et al.

SCHEME 1. Simple Kinetic Model of Reaction Mechanism for Chemical Modification of SAMs by Exposure to X-rays in Air

with sites, S, that can be converted to free radicals, S*, upon irradiation. The radicals either react with oxygen to form oxidized species on the surface, S-O, or recombine with radical intermediates or form cross-links, S-X. If we assume that the contact angle of water on surfaces terminated in S or S-X equals 110° and that the contact angle of water on surfaces terminated with S-O equals 0°, then the fraction of sites converted from S to S-O as a function of delivered dose, f(dose), is given by the expression24

f(dose) )

[1 + cos(110°)]2 - [1 + cosθ(dose)]2 [1 + cos(110°)]2 - [1 + cos(0°)]2

(2)

where θ(dose) ) the contact angle of water on CH3-terminated monolayers as a function of delivered dose (Figure 3a). An expression for the fraction of oxidized surface sites as a function of dose was found using rate laws for Scheme 1. Assuming a pseudo-steady-state concentration of the highly reactive intermediate species, S*, is reached quickly,

f(dose) )

[S-O](dose) ) C[1 - exp(-k1‚dose)] [S]o

(3)

with

C)

k2[O2]R/k3 k2[O2]R/k3 + 1

(4)

where C describes the selectivity of the oxidation pathway, or the fraction of S* that reacts with O2, and the subscript o refers to the initial concentration of surface sites. Experimentally determined values of f(dose) calculated from the contact-angle data on CH3-terminated SAMs and eq 2 were fit to eq 3. The best fits to the data using a constant value of k1 and optimized values of C for each pressure are shown in Figure 7. The simple kinetic model: (1) captures the change in surface properties of CH3-terminated SAMs as a function of dose and exposure pressure, as indicated by the good agreement between fits of eq 3 and the experimentally determined values of f(dose), (2) adequately predicts an asymptotic limit (selectivity) in conversion of S to S-O as a function of pressure, and (3) allows the calculation of the pressure, Pchar, above which the formation of S-O is favored over the formation of S-X and below which the formation of S-X is favored over the formation of S-O. This pressure is approximately 1.1 Torr and falls within the range of pressures studied. The order of the reaction S* f S-O with respect to oxygen, R, was determined from eq 4 by plotting and finding the slope of ln[C/(1-C)] versus ln[O2], yielding a value of 1.5 ( 0.3. On the basis of our proposed reaction mechanism, we anticipated the reaction to be first-order in oxygen. If we fit the f(dose) data in Figure 7, allowing both k1 and C to vary for each pressure, then the best fits were obtained for k1 ) 0.0006, 0.0007, and 0.0016 and for C ) 0.45, 0.64, and 0.73 for pressures of 0.5, 1, and 2 Torr, respectively. Using these values of C, R was found to be 0.9 ( 0.1, the expected first-order

dependence, but k1 varies with the oxygen concentration. In both approaches, oxygen appears to play a greater role in the chemical modification of the surfaces of the SAM than that which is depicted in Scheme 1. Possible mechanisms by which oxygen may directly participate in the oxidation of surface groups include (1) X-ray absorption and ionization of oxygen in the vapor phase and subsequent reaction with the surface and (2) photoelectron capture and ionization of oxygen adsorbed onto the surface or in the vapor phase and its subsequent reaction with the surface. It is reasonable to expect that the ionized oxygen must be within its mean free path of the surface in order to react with the SAM. If we assume that all photons absorbed within this length scale result in reactive, ionized species (80% nitrogen species, 20% oxygen species) and that all ionized oxygen species react to form oxidized surface species, then we calculated that, at most, 0.1% of the surface sites are converted from S to S-O by this mechanism.25 If ionized oxygen species are produced within the mean free path by direct capture of photoelectrons emitted from the surface, then the photoelectron and the oxygen molecule must be at the same place at the same time in order to react. The probability of such an event is extremely low, based on the ratio of residence times of oxygen molecules and photoelectrons within the mean free path of oxygen molecules from the surface. (Residence time ) mean free path of oxygen/particle velocity.) We do not take into account, however, the possibility that oxygen may be adsorbed onto the surface for appreciable lengths of time. The role of oxygen in the chemical modification of SAMs upon exposure to soft X-rays is complicated and warrants further study with more sophisticated spectroscopic techniques than those which have been employed in this study. The simple reaction scheme proposed here represents the primary processes taking place and adequately describes the measured contact-angle variation. Additional mechanistic detail requires additional information about the changing surface state. The contact-angle data for CH2dCH- and CF3COO-terminated SAMs (Figure 3b,c) also were fit to the same model, recognizing that the species represented by S, S*, and S-X, as well as the specific rate constants, may be different for each type of SAM. We obtained very similar values for k1 (0.0018 and 0.0012 for CH2dCH- and CF3COO-terminated SAMs, respectively) and R (1.6 for both SAMs). The quality of the fits to eq 3 was, in fact, better than for the data shown in Figure 7 for CH3-terminated SAMs. Chemical modification of the CF3COO-terminated monolayers upon exposure to X-rays in the presence of air involves the cleavage of the outermost atoms of the monolayer and, possibly, reaction with oxygen in the exposure environment. Most of the damage occurs in the vicinity of the -CF3 group. The high-resolution C(1s) spectra for the CF3COO-terminated monolayers (Figure 6, right) show a similar rate of loss for both CF3 and O-CdO with increasing dose. This suggests that fragments consisting of CF3CO may be lost as a unit. In a manner similar to the exposed CH3- and CH2dCH-terminated monolayers, a new peak at approximately 288 eV appears in the C(1s) spectra for the CF3COO-terminated monolayers that were exposed at doses above approximately 1000 mJ/cm2. Again, a good curve fit to the XPS data was obtained by modeling the asymmetric C(1s) peaks with contributions from methylene (CH2), hydroxyl (C-OH), and aldehyde (CdO) groups. Reactivity of Chemically Modified SAMs after Exposure to X-rays. Most of the applications of this research will involve patterning monolayers with regions of different functionality

Modification of Self-Assembled Monolayers

J. Phys. Chem. B, Vol. 104, No. 31, 2000 7409

Figure 6. XPS high-resolution spectra of the C(1s) region for CH3- (left), CH2dCH- (middle), and CF3COO- (right) terminated monolayers exposed at 1 Torr of air pressure. The doses were as follows. CH3-terminated monolayers (a) 0, (b) 650, (c) 1000, and (d) 2000 mJ/cm2; CH2d CH-terminated monolayers: (a) 0, (b) 100, (c) 1200, and (d) 2000 mJ/cm2; and CF3COO-terminated monolayers (a) 0, (b) 150, (c) 650, (d) 1000, and (e) 2000 mJ/cm2.

TABLE 2. Thickness Differences between Irradiated Monolayers and Irradiated Monolayers after Reaction with 11-Bromoundecyltrichlorosilane thickness (Å)a dose (mJ/cm2)

CH3terminated

CH2dCHterminated

CF3COOterminated

0 547 1065 1557 2070

0 5.3 6.2 7.1 8.6

0 5.4 6.7 7.3 8.5

0 5.9 6.8 7.9 8.8

a Thickness of irradiated monolayer after reaction - thickness of irradiated monolayer.

Figure 7. Fit of values of f(dose) calculated from contact-angle data in Figure 3a and eq 2. The values were fit to eq 3, and the best-fit parameters are given in the figure for the different pressures.

with X-ray or extreme UV lithography and using the patterned monolayers in additive nanofabrication processes. Some of these processes may take advantage of differences in wetting properties between exposed and unexposed regions, for example, to control the orientation and macroscopic ordering of microdomains in thin films of diblock copolymers. Other processes, such as the selective deposition of metals, require a difference in reactivity between the exposed and unexposed regions. To investigate the chemical reactivity of the irradiated monolayers, the surfaces were exposed to 11-bromoundecyltrichlorosilane. Hydroxyl groups that are present on the irradiated monolayer surface should serve as sites for the chemisorption of the second organotrichlorosilane layer. The thickness

of the second layer is expected to be proportional to the concentration of hydroxyl groups on the surface of the irradiated monolayers. A similar strategy has been used to produce patterned coplanar molecular assemblies of different silanes.26 Table 2 presents the change in thickness of the irradiated monolayers after treatment with 11-bromoundecyltrichlorosilane. Irrespective of the terminal group of the exposed monolayers, the thickness of the 11-bromoundecyltrichlorosilane layer increased with increasing dose. The thickness of the second layer in all cases was significantly less than that of a 11-bromoundecyltrichlorosilane monolayer deposited on silica (13.4 Å, see Table 1), implying the density of hydroxyl groups on the exposed monolayers was not sufficient to form a complete bilayer structure, even at the highest doses. Confirmation of the deposition of a Br-terminated second layer was obtained with XPS. Figure 8 shows survey and high-resolution spectra from a Br-terminated monolayer deposited on silica, a CF3COOterminated monolayer after exposure to 2010 mJ/cm2 at 2 Torr,

7410 J. Phys. Chem. B, Vol. 104, No. 31, 2000

Kim et al. Acknowledgment. Funding for this work was provided by the Semiconductor Research Corp. (Grant No. 98-LP-452), NSF Small Grant for Exploratory Research (Grant No. CTS9708944), and NSF Career Award (Grant No. CTS-9703207). Facilities were supported by DARPA/ONR (Grant No. N0001497-1-0460) and the NSF (Grant No. DMR-95-31009). We also thank Franco Cerrina and Thatcher Root for helpful discussions regarding the role of oxygen and the reaction mechanism for the chemical modification of SAMs. References and Notes

Figure 8. Survey spectra and high-resolution spectra of the Br(3d) region (inset) for (a) a Br-terminated monolayer as deposited on silica, (b) a CF3COO-terminated monolayer irradiated at 1 Torr of air pressure with a dose of 2010 mJ/cm2, and (c) the exposed CF3COO-terminated monolayer after reaction with 11-bromoundecyltrichlorosilane.

and the same exposed CF3COO-terminated monolayer after reaction with 11-bromoundecyltrichlorosilane. Bromine peaks in the survey spectra were assigned to Br(3p) (183.5 eV) and Br(3d) (70.6 eV). The high-resolution spectra show that bromine was not present on the CF3COO-terminated monolayer after irradiation but was present after treatment with 11-bromoundecyltrichlorosilane. These results indicate that the hydroxyl groups generated during the exposure of monolayers to X-rays in the presence of air are reactive and that the concentration of reactive hydroxyl groups increases with increasing dose. Conclusions The chemical functionality of SAMs can be modified and controlled by exposure to synchrotron radiation in the presence of oxygen. Hydroxyl and aldehyde groups are incorporated onto the surface of the monolayers, and their concentration is strongly dependent on the dose and the air pressure in the exposure environment. The chemical transformation is not strongly dependent on the initial terminal group of the SAM. Unexposed monolayers and monolayers exposed under appropriate conditions exhibit strong contrasts in wetting behavior and chemical reactivity. These results suggest that SAMs may be patterned with regions of different chemical functionality at the nanoscale level using lithographic techniques based on ionizing radiation and that these patterns may be useful in additive processes for nanofabrication.

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