Langmuir 1993,9,2383-2388
2383
Structure and Tribological Properties of Ultrathin Alkylsilane Films Chemisorbed to Solid Surfaces J. Riihe,+ V. J. Novotny,* K. K. Kanazawa, T. Clarke,and G. B. Street IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, California 95120-6099 Received September 22,1992. I n Final Form: June 23,199P The tribologicalproperties of monomolecular layers of alkylsilanes were investigated. The monolayers were obtained by a self-assembly process of alkylchlorosilanes with different chain lengths and varying number of functional groups at the silane moiety. These ultrathin films were characterized using ellipsometry,Fourier transform infrared spectroscopy, and quartz microbalance measurements. Ordered, dense films oriented almost normal to the surfacewere obtained with monofunctional silaneswith chains up to 18 CH2 units, while films with longer chains or di- and trifunctional head groups exhibited disorder and poor orientation. The frictional behavior of the organic films attached to both pin and disk surfaces was studied in sliding experiments. The removal of organic films was followed in situ during sliding experimentswith scanningmicroellipsometry. It waa found that the lifetimeof the coated surfacesincreased strongly with increasing chain length of the alkyl substituent.
I. Introduction Ultrathin filmsas boundary lubricant systemshave been the subject of great interest recently,' particularly in magnetic storage devices, where thin film magnetic disks must be protected against damage caused by strong mechanical shearing. However,the thickness of the mobile lubricant must be kept a t a minimum to avoid problems with static friction (stiction) and to minimize loss of signal due to the separation between magnetic medium and the read/write head. At the same time, sufficient adhesion between the lubricant film and the surface of the overcoat is required to avoid depletion by spin-off2and head-disk interaction^.^ Several thin film systems for boundary lubrication have been studied. In particular, LangmuirBlodgett (LB) films of fatty acids4or their salta,Sultrathin films of end group functionalized polymers (e.g, perfluoropolyethers),6 and chemisorbed monolayers of substituted silanes' have been investigated for such purposes. However, it is not well understood how the mechanical properties of the surfaces modified with the boundary lubricant depend on the molecular structure of the film. In order to correlate the structure of the lubricant film with the tribological properties of the interface, it is important to know the density of the monolayer and the strength of ita attachment to the interface as well as the influence of the conformation of the alkyl chains on these properties.
In this study we have investigated the structure and tribological properties of monolayers of alkylsilanes with varying chain lengths chemisorbed to silicon surfaceswith a native silicon dioxide film. The influence of the number of reactive sites at the silane head group on the structure and properties of the film was investigated using mono-, di-, and trifunctional alkylsilanes. In order to obtain the best conditions for the formation of well-defined monolayers, the kinetics of the chemisorption process was studied with a quartz microbalance. Quartz microbalances have been used extensivelyto studyvapor-phase processes, deposition, or dissolution of films with thicknesses of around 1 pm* and also LB films? In the present work, this technique was employed for the study of the selfassembly of one monolayer onto a solid substrate in the flow system. The conformation of the alkyl substituents in the self-assembled films was studied with Fourier transform infrared spectroscopy (FTIR) in the grazing incidence mode. The frictional properties were determined in pin- and slider-on-disk experiments. In situ microellipsometry was used to get a quantitative measure of the wear caused by shearing of the films.
11. Experimental Section Materials and Reagents. Hexamethylenedisilazane (Cl), dimethyloctylchlorosilane $81, dimethyldodecylchlorodane (C12), dimethyloctadecylchlorosilane ((2181, dimethyltriacontylchlorosilane(C30),methyloctadecyldichlorosilane (ODs),and octadecyltrichlorosilane(OTS) were obtained from commercial sources (Petrach, Aldrich, Merck). They were purified by distillation under nitrogen or reduced pressure. C30 waa wed withoutany furtherpurification. Polished,p-dopedsiliconwafers with native oxide were employed aa substrates. They were extracted with chloroform for 3-12 h and dried in a stream of filtered nitrogen. Directly prior to we they were exposed to intense high-energy UV irradiation (185 nm) under nitrogen to remove any residual hydrocarbon impurities. The surfaces obtained were highly hydrophilic (wetting angle of water lo4 cycles) the surface was scanned without stopping the disk during the surface scan. In these cases, a circumferential average film thickness along the track was obtained. For each measurement a new pin or slider was used.
111. Results and Discussion Ellipsometric Results. Optical ellipsometry is a convenientway to measure the thickness of very thin layers and, therefore, it has been used extensively to study the self-assembly of OTS8 and other silaneslO on reflective surfaces. Figure 4 shows the thicknesses of the layers as a function of the chain length of the alkyl substituent. For calculations of the thickness from the ellipsometric parameters # and A, a refractive index of nf = 1.50 was assumed. Thicknesses were measured on three independent samples with five to ten readings each. The value of 2.5 nm obtained for OTS agreeswell with the literature.8 (10) (a) Gun, J.; Sagiv, J. J. Colloid Interface Sci. 1986,112,457. (b) Gun, J.; Iscovici, R.; Sagiv, J. J. Colloid Interface Sci. 1984,101,201. (c) Cohen, S. R.; Naamann, R.; Sagiv, J. J. Phys. Chem. 1986,90,3054. (d) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. F. D. J.Am. Chem. SOC.1987,109,3359. (e) Finklea, H. 0.; Robinson, L. R.; Blackburn, A.; Richter, B.; Allara, D.; Bright, T. Langmuir 1986,2, 239.
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Figure 3. Setup for tribological and scanning microellipsometry experiments.
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Figure 4. Ellipsometry measurements on silane monolayers; n repreaents the number of carbon atoms of the alkyl substituent (cf., ref 1).
The measured film thicknesses are in good agreementwith those of a densely packed monolayer having extendedalkyl chains. Only the layer thickness for a monolayer of C30 (3.3 nm) was below the value calculated for the contour length of an all trans chain (4.0 nm). The contour length was calculated using standard bond lengths (C-C = 0.154 nm) and bond angles (112’). It should be noted, however, that ellipsometry measurements alone do not give precise information about the absolute thickness and/or density of the monolayers because the refractive index of the surface attached monolayer is unknown. In principle, thicknesses and refractive indices could both be calculated from the parameters A and $ measured by ellipsometry. However, in ultrathin f i i only one parameter can be obtained with sufficient accuracy. Furthermore, the monolayer is assumed to be isotropic and homogeneous and especially the latter assumption is not always correct. AU these factors give rise to some uncertainty about the absolute thickness value. Therefore, additional techniques have to be employed to gain information about the chemisorbed monolayers. Also, contact angle measurementsdo not give information about the density of the monolayers. Indeed,
Figure 6. Adsorption and desorptionof the self-assembledlayer of dimethyloctadecylchlorosilane measured with a quartz microbalance: solvent toluene; 10-MHz crystal; 25 O C .
it was found that strongly hydrophobic surfaces can be already obtained when the surface coverage by the f h , as measured by other techniques, is still well below complete coverage. Quartz MicrobalanceResults. Both the kinetics and the quality of monolayer formation by a surface reaction can be studied by using the high sensitivity quartz microbalance. With this technique, the deposition of thin filmsfrom the vapor phase8 and liquid environment11J2 can be studied with resolution down to 0.02 monolayer. The balance uses shifts of the resonance of internally reflected transversal shear waves to measure the mass of the thin film.The results of quartz microbalance measurements on the self-assembly of C18 from toluene onto Si02 are shown in Figure 5. At the first stage of the experiments, pure toluene was run through the cell. The resonance frequency was recorded for about 3 h to measure the drift of the signal. Then the chlorosilane was added to the reservoir, so that a concentration of 1-2% was obtained. After the delay time of the system (which was reconfirmed using toluene with a dissolved dye), a rapid decrease of resonance frequency of the quartz crystal was observed. This frequency change can be attributed to chemisorption and physisorption of the chlorosilane onto the Si02 surface. The frequency decrease leveled off after approximately 20-30 min. Then, pure toluene was added and the material which was not chemisorbed was washed off. The resonance frequency of the crystal increased again, and after 1h no more changes could be observed. The frequency shift reflects not only the adsorption and desorption behavior but also the density and viscosity of the solvent. Therefore, care was taken to compare frequencies only under conditions where pure toluene was the solvent. The difference in resonance frequency of the crystal before addition and after desorption of physisorbed chlorosilane (Af,was 51 f 5 Hz. From this frequency shift, the mass increase per unit area of the quartz crystal Am can be calculated8 according to
where j~ represents the shear modulus of quartz (2.95 X loll dyn/cm2),PQ is the density of the quartz (2.65 g/cm3), fo the frequency of the unloaded quartz crystal (10 038 575 Hz in this experiment), and 4the frequency difference between before and after desorption of the thin film.This equation is valid for thin overlayers, which means that the thickness of the layer is negligible with respect to the wavelength of the shear wave in the overlayer. Thisis the case in the self-assembledmonolayers studied. From these (11) Kanazawa, K. K.; Gordon,J. G. Anal. Chem. 1966,57,1770. (12)Hin~berg,W.;Willson, C.; Kanazawa, K. K. J. Electrochem. SOC. 1985,133,1448.
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2386 Langmuir, Vol. 9, No. 9, 1993 values, an areal density of the self-assembled film of 224 ng/cm2is obtained. This result can be compared with a calculated value based on the ellipsometry measurements and the density of the chlorosilane. The thickness of a silane layer prepared under identical conditions using a silicon wafer instead of a quartz crystal as a substrate was measured to be 2.5 nm. If the density of the attached material is assumed to be 0.9-0.95 (which is close to the density of the chlorosilane or alkylsiloxane polymers d = 0.9-0.96 g/cm3) a calculated mass density of 225-238 ng/ cm2 is obtained and is in excellent agreement with the quartz microbalance results. It can be concluded from these measurements that the initial rate of the surface reaction is high, but that the reaction slows down more and more as the surface coverage approaches100% . To obtain dense monolayers under the reaction conditions used, reaction times of >1 h have to be employed. Otherwise the surface reaction and the surface coverage remain incomplete. The kinetics of the chemisorption of C30 from hexadecane was also measured using the same technique. The results were in good qualitative agreement with measurements on the corresponding octadecylsilane. The frequency shift upon addition of the chlorosilane was also initially very rapid. However, contrary to the behavior of C18, the frequency increase leveled off rapidly and became very slow at surface coverages well below 50 % Under the conditions employed, some changes in the resonance frequencycould still be observed even after several hours. Generally it was found that the monolayer coverage was not complete. This would explain why the thickness as measured by optical ellipsometry always produced values smaller than expected for a dense and extended monolayer. However, quantitative measurements of the surface coverage by chemisorption reactions with slow kinetics are difficult to obtain with the quartz microbalance, as the long time drift of the signal becomes comparable with the measurement signal. For measurements on systems with slow kinetics, a setup with better thermal and electrical stability would be required. When the chemisorption of di- and trifunctional alkylsilanes (ODs and OTS) was studied using the quartz microbalance, it was very difficult to obtain reproducible results. Frequently the attached layers had areal densities larger and sometimes smaller than those expected for a monolayer. Apparently, the idealized structure in Figure 1 does not represent the structure of the OTS and ODS very well. These results are in good agreement with ellipsometricmeasurements and with FTIR measurements which frequently showed thicknesses larger than a monolayer. Evidently, the thickness of the attached films is very strongly dependent on moisture traces in the solvent and on the amount of moisture adsorbed on the surface of the substrate, because water will lead to a condensation of siloxaneoligomers prior to attachmentof the f i i ,which would favor a multilayer build-up. FTIR Results. The ordering of alkyl chains in the monolayer was studied by FTIR in the grazing incidence mode. This technique utilizes the fact that a p-polarized light beam reflected from a surface under grazing angle of incidence can only excite vibrational modes with transition dipole moments perpendicular to the surface. Modes with dipole momentsparallel to the surface cannot be excited and cannot be detected in the spectra. Therefore, the relative intensities of bands in the vibration spectra give information about the orientation of the corresponding groups.
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Figure 6. Infrared spectra of the C-H stretching region of (a, top) bulk C18 in CCL and a self-assembledmonolayerof C18and (b, bottom) C18, OTS,and C30; all monolayer spectra were measured in the grazing incidence mode at 25 OC;the spectrum of OTS is displaced by 5E-4 and that of C30 by + 93-4 for clarity.
Figure 6a shows the FTIR spectrum of a self-assembled monolayer of C18. In the region of 2800-3000 cm-l, four peaks are easily discernible and can be attributed to the following m o d e ~ : ~ 2967 ~ J cm-1 ~ va asymmetric C& stretching vibration; 2921 cm-l = va asymmetric CH2 stretching vibration; 2876 cm-l = us symmetric CHs stretching vibration; 2852 cm-' = us symmetric CH2 stretching vibration. The peaks due to the CH2 vibrational modes are greatly reduced in intensity in a monolayer compared with the bulk and are similar to the intensity of the CH3 stretching vibration. By comparison, in a bulk spectrum (Figure 6a), the peaks due to the CH3vibrations are almost completely obscured by the intensity of peaks due to the CH2 vibrations. Comparable intensity ratios of the two stretching vibrations have also been reported in LB films of fatty acid salts such as cadmium arachidate.l3 Order in the monolayer can be more directly examined by the study of temperature dependenceof the IR spectra. When the sample with the self-assembled monolayer WBB heated, the spectra in Figure 7 were obtained. When curves a to c are compared, it is seen that raising the temperature leads to an increase in intenaity of the CH2 bands relative to those of the CH3 vibrations. The thermal energy increases the disorder in the packing, leading eventually to a melting of the alkyl chains. This loss of order leads to an increase of the number of CH2 groups which do not (13)(a) Rabolt, J. F.;Bums, F.C.; Schlotter, N. E.;Swalen, J. D.J. Chem. Phys. 1983, 78, 946. (b) Naeelli, C.;Rabolt, J. F.;Swalen, J. D . J. Chem. Phys. 1986,82, 2136.
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dynamic profiles were reduced when compared with initial film thicknesses but were not approaching zero. Ellipsometric profiles average over areas of 20 X 50 pm2 and do not exclude a possibility that the organic film is completely removed in localized areas where microscopic asperitieswere initiallypresent. At failure, the wear tracks extended through the monolayer, the silicon oxide layer, and into the silicon substrate, and consequently,the optical and mechanical evaluation of the wear track was not very meaningful. For this reason, lifetimes were defined as number of cycles to optically detectable failure. Few experiments had very short lifetimes compared with the average lifetimes due to particulate contamination on the wafer surfaces and these data were excluded from further analysis. The initial dynamic friction coefficient was 0.10 f 0.03. In some experiments, the friction coefficient suddenly increased up to 0.5 when failure was detected optically; however in other experiments the increase of friction coefficient was small. The friction coefficientalone does not provide a reliable determination of the lifetime of the interface. It was found essential to control not only the surface of the disk but also the surface of the sliders or pins. The Al20dTiC pins were treated with the same alkylsilanes as used for the wafers under identical conditions. Model experiments by FTIR on C18 attached to an aluminum oxide surface (natural oxide of evaporated aluminum) show that the packing of the alkylsilanes on the surface of the aluminum oxide is comparableto that on silicon dioxide. However, it was observed that there are serious aging effects in such films attached to aluminum oxide. This can be attributed to the fact that the Al-0-Si bonds are hydrolytidy less stable than the Si-oSi bonds. Therefore, in the experimentsdescribed here, onlyfreshly coated sliders and pins were used for tribological experiments. The removal of the organic films occurred on both wafer and pin surfaces, but it was systematicallymonitored only on wafer surfaces. It should be noted that the tribological properties were dependent on the preparation conditions, especially the reaction time for the self-assembly process. Samples obtained a t reaction times of a few minutes showed significantly inferior tribological properties. This is not surprising, as the kinetic measurements with the quartz microbalance show that in such samples monolayer formation is not complete. These results demonstratethat monolayer characterization is needed to complement the tribological data. Generally, it was found that increasing the length of the alkyl chain of the chemisorbed silane greatly enhanced the tribologicallifetimes as shown in Figures 8 and 9. While the initial dynamic friction coefficient was independent of the length of the alkyl chain within the experimental error, films consisting of monolayers with long alkyl chains had a lifetime several orders of magnitude longer than monolayers of the analogous short chain compounds. Whereas the short chain compounds C1 and C8 did not give much protection for the sheared surface, the long chain compounds acted as reasonably good boundary lubricants. The reason for this improved performance could be that the longer alkyl chains are muchmore flexible and can dissipate the mechanical energy from the shearing process more easily than the short chain compounds or that longer chains have stronger chain-chain interactions. Essentially no differences could be seen when monolayers with a different degree of cross-linking in the film were compared. Within the experimental error, monolayers obtained from chemisorption of a monochloro-, dichloro- and trichlorosilane ((218, ODs, OTS)showed no
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Figure7. Infraredreflection spectra of the C-H stretching region of a self-assembled monolayer of C18 at various temperatures: (a) 25 OC; Jb)68 O C ; (c) 100 OC;(d) cooled back to 25 OC. Curves b-d are dmplaced by + 8 0 4 each for clarity.
have their dipole moments parallel to the surface and can therefore be excited by the p-polarized IR irradiation. Upon cooling back to room temperature, the spectrum returns nearly to the original form and the basic features are almost completely retained. From the fact that the intensity of the CH2 absorption band relative to the intensity of the CH3 band is slightly larger than before the heating of the sample, it can be concluded that the crystallization back from the melt is not complete and that some “disordered” portion remains. It can be concluded that the C18 molecules in the monolayer are well oriented and that the alkyl chains are standing almost perpendicular to the surface. The orientation of the alkyl chains in monolayer films of the long alkyl chain compound ((730)and of OTS showed a somewhat different behavior as shown in Figure 6b. The self-assembled monolayer with the triacontyl alkyl chain had a much larger relative intensity of the CH2 stretching vibration compared to the CH3 band, indicating a much more disordered packing of the alkyl chains. Indeed, the intensity of the CH2 bands in these films is so large, that a disorder has to be assumed. This result agrees well with ellipsometric and quartz microbalance measurements on such films, which show that the areal density of the film is lower than that of a layer of closely packed all-trans alkyl chains. The spectrum of OTS agreed well with spectra reported in the literature.1° However, it should be noted that the relative intensity ratio of CH2 to CH3 absorption bands, which is a sensitive indicator of the ordering in the film, varied from sample to sample (which is also the case when different literature datalo are compared). This could be explained either by differences in the tilt angle or, much more likely, by increased disorder in the film. Increased disorder would be consistent with the quartz microbalance measurements giving lower than expected areal densities and indicate that the structure depicted in Figure IC is too idealized. The patchy multilayer buildup would result in larger apparent densities than expected. The actual structure might include empty sites and/or patches of multilayer buildup. Tribological Results. Tribological properties of the self-assembled films were determined in pin-on-disk (wafer) on slider-on-disk (wafer) wear experiments. Thickness of organic films in the sliding track monitored microellipsometrically decreased gradually until sudden catastrophic failure took place. Before failure, the remaining thicknesses of the organic layers determined in multiple static profiles or single circumferentially averaged
Riihe et al.
2388 Langmuir, Vol. 9,No. 9, 1993
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Figure 8. Lifetimes to failure of interfaces modified with selfassembled f i i s of silanes from Figure 1 as a function of the length of the alkyl chain; pin on disk experiments, silicon substrate; load 0.16 N; velocity 1 m/s; relative humidity 30%; error bars me determined from variation of results on several samples.
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layers" obtained by dip coating of a trifunctionalalkylsilane onto a silicon surface and exposure to moisture show, despite their ill-defined structure, excellent tribological properties. As seen in Figures 8 and 9, the lifetime of such 'multilayers" (thickness 10nm) exceeds those of the selfassembled monolayers by up to a factor of about 10 if both samples are sheared under identical conditions. One explanation for this behavior could be that the material is firmly anchoredto the surface by reaction with the surface hydroxyl groups and the cohesion within the f i b is strong, due to the cross-linkingwhich occurs during the film preparation. On the other hand, the crude methud of preparation will surelyleave alarge number of functional groups which do not participate in the cross-linking process. Therefore, the film cannot be expected to consist of arigid three-dimensionalnetwork,but it should be much more flexible and therefore contain mobile groups which could absorb the mechanical energy from the shearing. Despite the improvementof the tribological properties of the interfaces with the self-assembled monolayers, the lifetime of surfaces protected with such boundary lubricants is still below that of surfaces with attached perfluoropolyether films. Ultrathin perfluoropolyether (PFPE) filmsare known to have excellent tribologicalproperties.sg In order to make a comparison of the tribological behavior of the silanes and PFPE,silicon disks and sliders (same as above) were coated with a 10.0 nm thick film of an end group functionalized perfluoropolyether (ZDOL; source, Aussimont). After thermal treatment (1h, 150 "C),the disk and slider were rinsed with 1,1,2-trifluorotrichloroethane in order to remove nonattached polymer. The thickness of the attached films was about 1.5 nm. Such slider/disk assemblies tested under identical conditions as the silanes had lifetime to failure of 1@-106 c~c1es.l~
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Figure 9. Lifetimes to failure of interfaces modified with selfassembled films of silanes from Figure 1 as a function of the length of the alkyl chain; slider on disk experiments; silicon substrate; for experimental conditions see Figure 8.
differences in the tribological behavior. This seems to indicate that once the anchoringto the surface is achieved by the chemisorption, additional two-dimensional crosslinking (as shown in the idealized structures of parts b and c of Figure 1)gives no additional improvementof the tribological properties. However,the picture is somewhatdifferent if multilayers are also taken into account. Highly disordered 'multi-
Self-assembled monolayers of mono-, di-, and trifunctional silaneswere formed on silicon oxide surfaces. While under conditions employed in this work, the monofunctional silanes formed ordered monolayers, the multifunctional silanes formed disordered multilayers. The silane monolayers chemisorbed to silicon dioxide surfaces act as goodboundary lubricate. The friction of coated interfaces is lower compared with the friction of bare interfaces and tribologicallifetimes of lubricated interfaces increase with increasing chain length of the alkyl substituent. Additional cross-linking of the chemisorbed material in the film does not affect the wear behavior.
Acknowledgment. The authors thank J. Rabolt for the kind use of his FTIR spectrometer. (14) R a e , J.; Kuan, S.;Novotny, V. J.; Clarke,T.;Street, G. B. To be submitted for publication.
