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Thermal Behavior of Alkyl Monolayers on Silicon Surfaces Myung M. Sung,† G. Jonathan Kluth, Oranna W. Yauw, and Roya Maboudian* Department of Chemical Engineering, University of California, Berkeley, California 94720 Received June 5, 1997. In Final Form: September 3, 1997X Densely-packed alkyl monolayers similar to those previously reported by Linford et al.1,2 are formed by the reaction of 1-alkenes with hydrogen-terminated surfaces of both Si(111) and Si(100). The thermal behavior of these monolayers in vacuum has been studied using high-resolution electron energy loss spectroscopy. Both on Si(111) and on Si(100), the monolayers are found to be stable up to about 615 K. Desorption is signaled by a decrease in the intensity of C-H modes, accompanied by the appearance of Si-H modes, which suggests that desorption occurs through β-hydride elimination reactions. Upon further annealing to 785 K, C-H and Si-H modes essentially disappear, and a peak appears at 780 cm-1, which is attributed to a SiC vibrational mode. This behavior indicates that decomposition of the monolayers has taken place.
I. Introduction Self-assembled monolayers (SAMs) have been the subject of intense study because of their potential utility in such applications as wetting, adhesion, lubrication, chemical sensing, and high-resolution lithography.3,4 Several different varieties of SAMs have been investigated, including alkanethiols (CH3(CH2)n-1SH) on Au, Ag, and Cu and alkyltrichlorosilanes (CH3(CH2)n-1SiCl3) on SiO2, Al2O3, and mica. Octadecyltrichlorosilane (OTS) on oxidized silicon has been shown to be effective as a passivating layer5 as well as a means of reducing adhesion between the surfaces of micromachines.6 There are applications, however, where it is desirable to eliminate the need for the oxide layer in the self-assembly process.7 Recently it has been demonstrated that self-assembled monolayers can be formed which are directly bonded to silicon surfaces.1,2 These monolayers are formed by reacting an alkene (such as octadecene) with a hydrogenterminated surface. The resulting monolayer bonds to the surface through a Si-C bond and shows packing and wetting properties similar to those employing alkanethiol and alkyltrichlorosilane precursor molecules. The thermal stability of these films is an important consideration because they must be able to withstand the temperatures used in various processing and packaging steps.7 While monolayers with different head groups demonstrate similar properties upon their formation, their thermal behavior varies dramatically. Alkanethiols adsorbed on Au have been observed to desorb at about 450 K through cleavage of the Au-S bond.8 Alkylsiloxanes on oxidized silicon have been observed to decompose * E-mail:
[email protected]. Phone: (510) 6437957. Fax: (510) 642-4778. † Current address: Korean Institute of Chemical Technology, Taejon, Korea. X Abstract published in Advance ACS Abstracts, October 15, 1997. (1) Linford, M. R.; Fenter, P.; Eisenberger, P. M.; Chidsey, C. E. D. J. Am. Chem. Soc. 1995, 117, 3145. (2) Linford M. R.; Chidsey, C. E. D. J. Am. Chem. Soc. 1993, 115, 12631. (3) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: Boston, MA, 1991. (4) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932. (5) Wasserman, S. R.; Tao, Y. -T.; Whitesides, G. M. Langmuir 1989, 5, 1074. (6) Houston, M. R.; Maboudian, R.; Howe, R. T. Proceedings of SolidState Sensor and Actuator Workshop, Hilton Head, SC, June 2-6, 1996; p 42. (7) Maboudian, R.; Howe, R. T. J. Vac. Sci. Technol. B 1997, 15, 1. (8) Nishida, N.; Hara, M.; Sasabe, H.; Knoll, W. Jpn. J. Appl. Phys. 1996, 35, L799.
S0743-7463(97)00592-1 CCC: $14.00
beginning at about 740 K through the cleavage of C-C bonds, resulting in a gradual decrease in chain length.9 The thermal behavior of alkyl monolayers on silicon has not been examined previously. In the present study we have employed high-resolution electron energy loss spectroscopy (HREELS) to examine the thermal behavior of the alkyl monolayers formed on silicon surfaces. It is found that the monolayers are stable up to 615 K. Hydrogen is observed on the surface following desorption, likely the result of β-hydride elimination reactions. The desorption temperature is about 100 K lower than that found for alkylsiloxane monolayers on oxidized silicon, lending further support to the β-hydride elimination pathway. At higher temperatures, SiC vibrational modes become evident, indicating that the chains have decomposed. II. Experiment Samples were cut from B-doped Si(100) wafers with resistivity in the range of 1-50 Ω cm. The native oxide was etched away by placing the sample in concentrated hydrofluoric acid (40% in water) for 1 min at room temperature, followed by rinsing in deionized water (resistivity ) 18 MΩ) and reagent grade 2-propanol. The sample was oxidized using a piranha solution (3:1 mixture of H2SO4/H2O2) for 30 min at about 100 °C, followed by a water rinse. The chemical oxide formed by this step was etched away by placing the sample in a saturated NH4F solution (40% in water) for 4 min, followed by a short (1-2 s) water rinse and a 20 s rinse in 2-propanol. The NH4F treatment has been shown to produce a hydrogen-terminated surface nearly free of carbon and oxygen.10 To form the monolayer, the alkene precursor was first purified by bubbling nitrogen through it at room temperature for 30 min. The hydrogen-terminated surface was placed in the alkene, and the nitrogen bubbling continued for 30 min. The alkene (Aldrich, 92%) was then heated with continuous nitrogen bubbling. Solution temperature and time in the alkene were found to have a significant influence on the resulting monolayers. With the octadecene (C18) at room temperature, an immersion time of 90 min resulted in a water contact angle of only 88°. With an increase of the temperature to 150 °C for 90 min, the water contact angle increased to 103°. At 200 °C, the same water contact angle could be achieved with only 30 min of immersion time. Extended time at 200 °C only slightly improved the contact angle. Results for the hexadecane contact angle showed similar trends; these results are summarized in Table 1 for monolayers prepared from the reaction of octadecene with NH4F-treated Si(100). After the desired time at high temperature, the samples were cooled to room temper(9) Kluth, G. J.; Sung, M. M.; Maboudian, R. Langmuir 1997, 13, 3775. (10) Dumas, P.; Chabal, Y. J. Chem. Phys. Lett. 1991, 181, 537.
© 1997 American Chemical Society
Thermal Behavior of Alkyl Monolayers on Silicon Surfaces
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Table 1. Water and Hexadecane Contact Angles for Monolayers Prepared from Octadecene on Si(100) as a Function of Solution Temperature and Time in Solution temp ( °C)
time (min)
θ(H2O)
θ(HD)
25 150 200 200
90 90 30 75
88 103 104 106
40 40 41
ature, removed from the alkene, and then rinsed with chloroform. Contact angles for the alkyl monolayers approach those of the alkylsiloxane monolayers, indicating that these monolayers are well-ordered and methyl-terminated. This procedure is slightly different from the one previously reported for formation of these monolayers.1,2 Instead of continuous nitrogen bubbling, the reaction vessel in the previous report was evacuated, flooded with argon, vented to a bubbler, and then heated to up to 200 °C for 60 min. As will be shown below, however, the two procedures result in similar films. Furthermore, similar quality films are obtained on both Si(111) and -(100). The measurements reported in this paper were performed on octadecene monolayers prepared at 200 °C for 60 min, resulting in a water contact angle of 106°. Octene (C8) monolayers were prepared at 100 °C for 90 min; the water contact angle was 100°. Samples are introduced into ultrahigh vacuum (UHV) by means of a load lock system. Both the UHV chamber and the load lock have been described previously.11 The chamber contains low-energy electron diffraction, Auger electron spectroscopy (AES), HREELS, and a differentially-pumped quadrupole mass spectrometer coupled to a temperature controller for temperatureprogrammed desorption (TPD). The base pressure is 1 × 10-10 Torr. After the sample was cooled to 120 K, HREEL spectra were obtained in the specular mode using an incident electron energy of 6 eV. The resolution of the elastic peak was typically between 40 and 50 cm-1 at about 104 counts/s. The spectra were unchanged even after several hours of exposure to the incident electron beam. In order to have simple sample transfer capabilities, the sample temperature cannot be measured directly. The temperature was measured by a type K thermocouple attached to a Ta plate behind the sample. The thermocouple reading was calibrated for sample temperature by preparing a clean Si(100) surface in vacuum through sputtering and annealing to 1200 K. The clean surface was then exposed to atomic hydrogen, which was subsequently desorbed in a TPD experiment. The desorption temperature of the monohydride species was then compared with the value of 795 K reported in the literature12 to calibrate the sample temperature. HREELS measurements were complemented by contact angle analysis and atomic force microscopy (AFM). A model A-100 Rame´-Hart NRL goniometer was used to measure water and hexadecane contact angles in room air using the sessile drop method.13 A Digital Instruments Nanoscope III was used in tapping mode to obtain AFM images. Scanning was performed in air at room temperature.
III. Results and Discussion Figure 1 shows a typical HREEL spectrum for octadecene reacted with a hydrogen-terminated Si(100) surface. The peak assignments are summarized in Table 2. The spectrum is dominated by the C-H stretch at 2920 cm-1 and the C-H bends between 1280 and 1450 cm-1.14,15 C-H modes are also present at 730 and 880 cm-1. The peak at 1060 cm-1 is assigned to the Si-O-Si asymmetric stretch;16 the presence of oxygen is confirmed by AES. (11) Kluth, G. J.; Maboudian, R. J. Appl. Phys. 1996, 80, 5408. (12) Sinniah, K.; Sherman, M. G.; Lewis, L. B.; Weinberg, W. H.; Yates, J. T.; Janda, K. C. J. Chem. Phys. 1990, 92, 5700. (13) Neumann A. W.; Good, R. J. Surface and Colloid Science Vol. II: Experimental Methods; Plenum Press: New York, 1979. (14) Bellamy, L. J. The Infrared Spectra of Complex Molecules; Wiley: New York, 1958. (15) Snyder, R. G.; Schachtschneider, J. H. Spectrochim. Acta 1963, 19, 85. (16) Ibach, H.; Bruchmann, H. D.; Wagner, H. Appl. Phys. A 1982, 29, 113.
Figure 1. HREEL spectrum of a self-assembled monolayer formed from the reaction of octadecene with NH4F-treated Si(100).
Figure 2. HREEL spectra of alkyl monolayer on (a) Si(111) prepared in our laboratory and (b) Si(111) prepared by Chidsey’s group. Table 2. Peak Assignments for Alkyl Monolayers Prepared from Reaction of Octadecene with NH4F-Treated Si(100) peak position (cm-1)
assignment
ref
2920 2280 2100 1450 1360 1280 1060 880 780 730 620
C-H stretch Si-O-Si overtone Si-H stretch CH2 scissor CH3 symmetric bend CH2 twist-rock Si-O-Si stretch CH3 rock SiC CH2 rock-twist Si-H bend
14 20 19 14, 15 14, 15 14, 15 16 15 17 14, 15 19
There may also be a contribution to this peak from the C-C stretch at 1050 cm-1, as has been observed for ethyl groups adsorbed on Si(100).17 The positions of the C-H modes are consistent with those of normal alkanes such as butane and hexane, and the spectrum is similar to that of alkylsiloxane monolayers on oxidized Si(100),9 except for the significantly smaller intensity of the Si-O-Si stretch. Similar quality monolayers were also formed on Si(111) as judged by the HREEL spectrum shown in Figure 2a. Monolayers on Si(111) resulted in a spectrum with a greater elastic peak intensity but poorer resolution compared to monolayers on Si(100). Because the procedure for preparing samples differed slightly from that of the monolayers examined in previous reports,1,2 steps were taken to ensure that the two procedures produced essentially the same monolayers. Professor Chidsey’s group (17) Widdra, W.; Huang, C.; Yi, S. I.; Weinberg, W. H. J. Chem. Phys. 1996, 105, 5605.
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Figure 5. HREEL spectra of the C-H region for (a) OTScoated oxidized Si(100) before annealing, (b) octadecene reacted with NH4F-treated Si(100) before annealing, (c) OTS-coated oxidized Si(100) upon annealing to 815 K, and (d) octadecene reacted with NH4F-treated Si(100) upon annealing to 785 K. The peaks have been scaled such that the maximum intensity is the same for all peaks.
Figure 3. Tapping mode AFM image of octadecene reacted with NH4F-treated Si(111) from a 500 nm2 area. The range of the z axis is 5 nm.
Figure 4. HREEL spectra as a function of annealing temperature for an octadecene monolayer on Si(100).
at Stanford graciously provided a sample of octadecene adsorbed on hydrogen-terminated Si(111) which we examined with HREELS. The spectrum for that sample is shown in Figure 2b. The samples are found to be very similar; the major difference is the amount of oxygen found, a difference that is confirmed by AES. As will be shown below, the thermal behavior of both samples is also very similar; thus, it is concluded that the different preparation procedures result in essentially the same monolayer. A representative AFM image for monolayers formed on the Si(111) surface is shown in Figure 3. The (111) surface is quite smooth, with a root mean square roughness of 1.4 Å. Monolayers formed on Si(100) showed a rougher surface (root mean square roughness of 3.6 Å), reflecting the topography of the underlying silicon following NH4F treatment. The Si(111) surface is found to be atomically flat over thousands of angstro¨ms,10 whereas the (100) surface is rough due to the evolution of (111) facets as etching proceeds.18 Figure 4 shows HREEL spectra for octadecene reacted with Si(100) as a function of annealing temperature. The sample was annealed to the indicated temperature for 1 min, then cooled to and held at 580 K until the pressure (18) Neuwald, U.; Hessel, H. E.; Feltz, A.; Memmert, U.; Behm, R. J. Surf. Sci. 1993, 296, L8-L14.
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