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Nanofriction of silicon oxide surfaces covered with thin water films
Wear 254 (2003) 924–929
Nanofriction of silicon oxide surfaces covered with thin water films A. Opitz a,∗ , S.I.-U. Ahmed b , J.A. Schaefer a , M. Scherge c b
a Institut für Physik, Technische Universität Ilmenau, PF 100565, D-98684 Ilmenau, Germany CSEM Centre Suisse d’Electronique et de Microtechnique SA, Rue Jaquet Droz 1, CH-2007 Neuchˆatel, Switzerland c IAVF Antriebstechnik AG, Im Schlehert 32, D-76187 Karlsruhe, Germany
Abstract A thin water film present on surfaces plays a central role in defining the micro- and nanotribological properties of a system. This paper presents a quantitative examination of the nanotribological effects of thin water films in ultra high vacuum (UHV) on OH-terminated (hydrophilic) and bare (no OH terminations, hydrophobic in vacuum) silicon oxide surfaces. Water film thickness was controlled by varying the water partial pressure in UHV. Friction was measured by scanning force microscopy (SFM) as a function of an external applied load. The surface energy and the shear stress of the nanotribological contact was then approximated by fitting the friction-load curves using the Derjaguin–Muller–Toporov (DMT) model. The surface energy as well as the adhesion force of the OH-terminated hydrophilic sample first decrease and later increase significantly at higher water partial pressures. No such dependence could be deduced from the friction-load curves at varying water pressures for the bare hydrophobic silicon oxide surface. However, at relatively high normal loads (pressures) and water partial pressures the bare hydrophobic silicon oxide is transformed to an OH-terminated surface. This transformation appears to occur only in the area of contact leading to the conclusion that it is friction-induced. This work shows that the chemical composition of the topmost surface layer defines the frictional behavior of the tribosystem. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Atomic force microscopy; Nanotribology; Silicon oxide; Water film; Hydrophilic; Hydrophobic; Nanowear
1. Introduction The fact that on the micro and nanoscales surface effects play a major role in determining the frictional response of systems is now well documented in various reports in [1–3]. This combined with the knowledge that, under ambient conditions, most surfaces are covered by a thin water film [1], indicates that a precise understanding of the tribological consequences of such thin water films is required at both, the micro and the nanoscales. This is particularly important for present day micro electromechanical systems (MEMS) [2] and the upcoming nano electromechanical systems (NEMS) [4]. In microtribological experiments with water confined in a slit formed by a smooth silicon ball versus a silicon flat with the same surface finish, the friction force curve showed several minima and maxima as function of the water film thickness [5]. At high water coverages of more than 10 monolayers friction is dominated by capillarity. With lower coverages, friction decreases due to reduction of this capillarity. Later several maxima (solid-like stick slips, increased ∗ Corresponding author. E-mail address: [email protected] (A. Opitz).
friction) and minima (liquid-like, decreased friction) were observed. Using the electrical double layer theory these effects were linked to the effects of confined monolayers [5]. Another ultra high vaccum (UHV) microtribology study showed that, as the water coverage on the surface is reduced by desorption, friction is reduced which arises from decreasing capillarity and is followed by increased friction due to a rise in cohesion [6]. Transitional friction from a high- to low-level has also been demonstrated due to the higher water adsorption potential when two surfaces are brought into contact [7]. Nanotribological investigations by Opitz et al. [8] on hydrophilic silicon oxide and hydrophobic silicon showed that confined water layers do not exist in a nanocontact. Results indicate that the tip apex of an scanning force microscopy (SFM) tip always makes direct solid–solid contact by penetrating the water film on the surface. Experiments also show that the water layers near the surface are ordered and that the magnitude of the friction force associated with these ordered water layers is small compared to capillary action resulting from thicker water films. Another nanotribological study has shown that the silicon oxide has a lower friction in dry nitrogen atmosphere than under ambient conditions [9].
0043-1648/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0043-1648(03)00248-5
A. Opitz et al. / Wear 254 (2003) 924–929
This paper investigates the nanotribological performance of two types of silicon oxide surfaces. One is OH-terminated (silanol) and hence hydrophilic while the other is bare, implying Si–O–Si (siloxane) bonding. The effect of varying water film thicknesses, on the two surfaces, upon the nanotribological characteristics is examined. The present study distinguishes itself from previous reports by our group [8,10] in that frictional behavior is linked to the surface energy and shear stress. For this purpose, friction-load curves were obtained and then fitted using the Derjaguin–Muller–Toporov (DMT) model [13]. From this fit both, surface energy as well as the shear stress were extracted. In a separate set of experiments, surface wear was also investigated and determined to be friction-induced.
2. Experimental setup Rectangular pieces of p-doped (1–10 cm, B-doped) Si(1 0 0) with an SFM determined root mean square (RMS) roughness of about 0.1 nm measured over a 0.6 m×0.6 m area were used as the sample material. Due to the natural oxide present on Si, samples are OH-terminated and, therefore, hydrophilic [2]. The hydrophilicity was also separately confirmed by contact angle measurements. Prior to mounting into the UHV chamber, the samples were sequentially cleaned using ultrasonic assistance in isopropanol and methanol for 5 min each. Afterwards, the samples were thoroughly rinsed with bi-distilled water. The oxide film measured on such samples had a thickness of 1.3 nm determined by X-ray photoelectron spectroscopy (XPS). The hydrophobic silicon oxide samples were prepared by heating at 600 K for 30 min. During this heating process the OH groups desorbs and the surface is terminated with Si–O–Si bonds [14]. However, such a surface is stable only in vacuum. The friction force microscope (FFM) used for this work operates in an ultra high vacuum system with a base pressure of 5 × 10−10 mbar. In this microscope, cantilever movement is detected by beam deflection. The cantilevers
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made from silicon were calibrated by the geometry of cantilever and the detector sensitivity [15]. The spring constant of the cantilever was determined to be about 0.11 N/m, which allows for high resolution in friction force measurements. The Si tips used in all experiments were covered by a native oxide and are hydrophilic. The manufacture quoted tip radius is smaller than 15 nm. During friction measurements, however, the tip radius changes and is proposed in the paper to be about 30 nm. The water partial pressures were varied using a special water dosing system based on the work by Bazock et al. [16]. The main feature of the system is that water vapor from a liquid source (cleaned using freeze-thaw cycles) is introduced to the UHV system through a micometer-sized diameter hole. Here a leak valve is used. With this method, the water pressure near the sample is one to two orders of magnitude higher then in other parts of the chamber. Friction-load measurements using the SFM were performed in lateral mode under a varying applied external load. For each friction measurement the tip was scanned 300 nm in the forward and backward direction at a velocity of 300 nm/s and the friction was determined from the friction hysteresis curve. For the time dependent study, the applied external load was 60 nN. The chemical composition of the hydrophilic surface at different pressures was determined with X-ray photoelectron spectroscopy in a different UHV system.
3. Results The friction-load curve measurements using FFM were made as a function of the residual water pressure in the UHV system and are plotted in Fig. 1. The friction force depends on the normal force by a nonlinear law. The different black curves show the dependence of friction-load curves on the residual water pressure for the hydrophilic silicon oxide samples. The grey curve shows one example for hydrophobic silicon oxide; curves obtained at other pressures were almost identical and are, therefore, not shown. The DMT
Fig. 1. Friction force as function of normal force at different water pressures. The black curves are for silanol terminated hydrophilic silicon oxide, the grey curve is for siloxane terminated hydrophobic silicon oxide. Strong dependence of the friction-load curves of hydrophilic silicon on the partial water pressure is visible. Only one representative curve for hydrophobic silicon oxide is shown since curves at other pressures are almost identical. The solid lines are fitted by the Eq. (1).
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Fig. 2. The surface energy as a function of water pressure determined by fitting the friction-load curves in Fig. 1 with Eq. (1). The black curve is for silanol terminated hydrophilic silicon oxide, the grey curve is for siloxane terminated hydrophobic silicon oxide. The dashed lines are drawn to guide the eye.
model [13] was used for fitting the friction-load curves. The model is based on the Bowden–Tabor-approach [17] where the friction force is proportional to the real contact area and is an extended Hertz-model [18] with respect to the adhesion force. The function of friction force FF is given by, 2/3 R (FL + 4πγR)2/3 (1) FF = πτ K here is τ the shear stress (MPa) in friction contact, R the tip radius (∼30 nm), K the reduced E-modulus (50.1 GPa for SiO2 ) and FL the external load. The normal force is calculated according to FN = FL + FAd , were the adhesion force is equal to FAd = 4πγR, with γ the surface energy (mJ m−2 ) for the tribological contact. The shear stress has no dependence on water pressure and is determined to have the values: τhydrophilic = 375 ± 30 MPa and τhydrophobic = 40 ± 3 MPa. Both results have a relative error of about 8%; this is the measurement error. The dependence of surface energy upon the residual water pressure is shown in Fig. 2. No dependence of the friction force for hydrophobic silicon oxide is could be determined (γhydrohobic = 99 ± 3 mJ m−2 ). On the other hand, the hydrophilic silicon oxide surface shows a strong dependence on the surface energy. The surface energy first decreases from 10−10 to 10−8 mbar residual water pressure and upon
further increase in the residual water pressure the surface energy increases. Fig. 3 shows XPS spectra of the O 1s region on the hydrophilic surface at different pressures. At 1 × 10−10 mbar only a single peak at 533 eV due to Si–O bonding was observed. No peak attributable to Si–OH bonding was detected (Fig. 3a). However, such a component at 535 eV is clearly discernable at 3 × 10−8 mbar (Fig. 3b) indicating the presence of OH groups at this pressure. The nanofriction was also measured as a function of scanning time for the hydrophilic and hydrophobic silicon oxide. In these experiments, the tip and sample were placed in contact and when the desired partial water pressure was reached, the tip was moved repeatedly relative to the surface under an applied normal load of 60 nN. This corresponds to a DMT model calculated pressure of 1.9 GPa. Under such conditions, changes of the frictional behavior during exposure was clearly evident (Fig. 4). Two effects can be seen: First, starting from UHV, the friction force measured increases with rise in the water partial pressure. Secondly, at a particular pressure the friction force measured immediately after exposure and after 10 min of repeated sliding is different. For a particular pressure, the friction immediately after exposure is higher and after 10 min of sliding is lower. After this exposure the friction force on an adjacent area
Fig. 3. O 1s XPS lines at (a) 1 × 10−10 mbar and (b) 3 × 10−8 mbar for oxidized silicon samples. The O 1s has in a silicon environment (Si–O–Si) a binding energy of 533 eV, and for the hydroxyl environment (Si–OH) of 535 eV [23].
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Fig. 4. Friction force as a function of time at different behaviors on siloxane terminated hydrophobic silicon oxide. The friction force is shown in black immediately after exposure and after a sliding time of 10 min. The friction force on an adjacent part of the sample not initially in contact with the tip is shown in grey. The dashed lines are drawn to guide the eye.
Fig. 5. Topography (a) and friction force (b) are shown on hydrophilic silicon oxide after exposure. Changes of topography and friction force are visible in the center of image. The measurement was performed after the friction tests for Fig. 3 at a partial water pressure 5 × 10−6 mbar. The central part relates to the friction after sliding and the outer part to the friction of the partially siloxane terminated hydrophobic silicon oxide surface, which is formed due to UHV exposure. See text for details.
not in contact, was also measured. The friction results on this adjacent area (grey line in Fig. 4) indicate that the friction behavior is that of the siloxane terminated hydrophobic silicon oxide surface. The effect of friction on the initially hydrophobic silicon oxide are shown in topography and friction force images in Fig. 5. After 10 min of sliding, a narrow wear track is visible. This is distinguished from the undamaged area by a lower topography and higher friction. The outer part, not in contact during friction measurements relates to hydrophobic friction on silicon oxide. This clearly shows that friction-induced changes occur on the surface properties during testing.
4. Discussion The surface of hydrophilic silicon oxide undergoes changes during water exposure. An initially hydrophilic
surface introduced and stored for a long time in a UHV environment steadily looses OH groups due to desorption. At a pressure of about 10−10 mbar, the surface is at best only partially OH-terminated. This is supported by the XPS measurements performed at 1 × 10−10 mbar (Fig. 3b), which shows no evidence of Si–OH bonding indicating that the concentration of OH groups at this pressure is below the detection limits of XPS. This implies that the surface is largely terminated with Si–O–Si bonds, which according to Iler [12] renders the surface hydrophobic. This change also affects the surface energy, which now approaches that of the hydrophobic silicon oxide surface (Fig. 2). Note the high value of the surface energy in this region. We propose that this may be due to the greater bonding interaction between the Si–O–Si terminations of the tip and sample. These interactions decrease gradually with increase in the water partial pressure. At this stage water acts as a nanolubricant by shielding both surfaces from bonding interactions with
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one another. However, above a certain threshold pressure (Fig. 2), water begins to attack the siloxane (Si–O–Si) bonds forming silanol (Si–OH) groups [14]. Consequently, the surface energy starts increasing. Once the surface is fully hydroxylated, as has been reported to occur at a pressure of about 10−8 mbar [19], and at higher partial pressures, water adsorbs on these OH-groups by hydrogen bonding and forms water layers. Measurements made at this stage and at subsequently higher pressures are largely dominated by capillary effects and comparable to friction changes which occur during increasing relative humidity [1,5,20]. Fig. 1 also shows that the friction of siloxane terminated hydrophobic silicon oxide does not change by varying the water partial pressures. However, after prolonged exposure, with sample and tip in contact under high externally applied loads, the surface is affected and siloxane groups convert to silanol groups. An important aspect is that the hydrophobic silicon oxide surface changes only in the contact area between tip and sample. This hypothesis was tested in the experiment plotted in Fig. 4 and the scanned area depicted in topography and force images in Fig. 5. The friction increase at each pressure (black curves) is greater because the siloxane groups are converted to silanol groups due to the tip-sample contact under an applied load. In a microtribological study, Nevchupa et al. [7] have proposed in a microtribological experiment that the adsorption potential of water increases when two surfaces come into contact with one another. Applying this hypothesis here, since tip and sample were in contact before the experiment, water accumulation occurs in the contacting area which, together with the applied load, induces a tribochemical reaction transforming the siloxane bonds to silanol bonds, leading to higher friction. Areas not in contact (grey curve in Fig. 4) are not affected, this has already been discussed above. It should also be noted that in Fig. 4 the friction force decreases after 10 min of repeated sliding. This is attributed to the chemical composition of the scratched (worn) surface during sliding. The fact that this surface is scratched is evident from the topography images which clearly shows a trench indicating material removal and, hence, wear. Moon et al. [21,22], in ambient environment AFM experiments of silicon oxide layers, reported that the presence of OH bases promotes the nano-scratching of silicon oxide layers. An interesting observation is that the decreased friction measured after 10 min of sliding is not measured again in subsequent measurements. This is confirmed from the friction image (Fig. 5) after completion of the experiments. The worn area has acquired the same friction force measured on the initial surface immediately after exposure. This could be due to a meta stable oxide created upon scratching which changes once the tip is no longer in contact or to the effect of increased water exposure and tip contact upon subsequent scanning, or a combination of the two and remains the subject for further investigations.
5. Conclusions The friction force was examined as a function of the water pressure in UHV using friction force microscopy. Different friction behaviors and changes depend on the surface termination and on the partial pressure of water in vacuum. The surface energy of initially silanol terminated silicon oxide undergoes variations depending on the partial water pressure. At very low pressures, silanol groups partially desorb from the surface and tip leading to a rise in the surface energy and, subsequently, in the friction. When the water partial pressures is increased friction is reduced due to shielding effect of water. At higher pressures water converts the siloxane to silanol groups leading to increased surface energies and friction. In contrast, the siloxane terminated silicon oxide surface shows negligible changes in the surface energy and friction at different water partial pressures. However, for the siloxane terminated surfaces in contact with the tip and with repeated sliding at high loads converts the siloxane to silanol groups. Repeated sliding also scratch the surface and induces wear. This study highlights the significant differences in nanotribological behavior that exist between siloxane and silanol terminated silicon oxide surfaces.
Acknowledgements This work was supported by a grant from the Deutsche Forschungsgemeinschaft (Projekt Sche 425/2-4). References [1] M. Scherge, S.N. Gorb, Biological Micro- and Nanotribology, NanoScience and Technology, Springer-Verleg, Berlin, 2001. [2] R. Maboudian, Surface processes in MEMS technology, Surf. Sci. Rep. 30 (1998) 207. [3] B. Bhushan, Tribology Issues and Opportunities in MEMS, Kluwer Academic Publishers, Dordrecht, 1998. [4] S.E. Lyshevski, MEMS and NEMS: Systems, Devices and Structures, CRC Press, Boca Raton, 2002. [5] M. Scherge, X. Li, J.A. Schaefer, The effect of water on friction in MEMS, Tribol. Lett. 6 (1999) 215. [6] E.A. Deulin, A.A. Gatsenko, B.A. Loginov, Friction force of smooth surfaces of SiO2 –SiO2 as a function of residual pressure, Surf. Sci. 433–435 (1999) 288. [7] R.A. Nevchupa, M. Scherge, S.I.-U. Ahmed, Transitional microfriction behavior of silicon induced by spontanous water adsorption, Surf. Sci. 517 (2002) 17. [8] A. Opitz, S.I.-U. Ahmed, J.A. Schaefer, M. Scherge, Friction of thin water films: a nanotribological study, Surf. Sci. 504 (2002) 199. [9] L. Scandella, E. Meyer, L. Howald, R. Lüthi, M. Guggisberg, J. Gobrecht, H.-J. Güntherodt, Friction forces on hydrogen passivated (1 1 0) silicon and silicon oxide studied by scanning force microscopy, J. Vac. Sci. Technol. B 14 (1996) 1255. [10] (a) A. Opitz, S.I.-U. Ahmed, M. Scherge, J.A. Schaefer, Friction mechanisms from velocity and load dependent measurements on hydrophilic silicon from air to UHV, in preparation; (b) A. Opitz, Nanotribologische Untersuchungen von ultradünnen Wasserfilm auf hydrophoben und hydrophilen Siliziumoberfächen, Ph.D. thesis, TU Ilmenau (2003).
A. Opitz et al. / Wear 254 (2003) 924–929 [11] B.V. Derjaguin, V.M. Muller, Y.P. Toporov, Effect of contact deformations on the adhesion of particle, J. Colloid Interface Sci. 53 (1975) 314. [12] R.K. Iler, The Chemistry of Silica, Wiley/Interscience, Chichester, 1979. [13] U.D. Schwarz, P. Köster, R. Wiesendanger, Quantitative analysis of lateral force microscopy experiments, Rev. Sci. Instrum. 67 (1996) 2560. [14] M.J. Bazock, L. Muehlhoff, J.N. Rossell Jr., W.J. Choyke, J.T. Yates Jr., Methods in semiconductor surface chemistry, J. Vac. Sci. Technol. A 5 (1987) 1. [15] F.P. Bowden, D. Tabor, The Friction and Lubrication of Solids, Clarendon Press, Oxford, 1950. [16] H. Hertz, Ueber die Berührung fester elastischer Körper, J. Reine Angew. Math. 92 (1881) 156.
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[17] O. Sneh, M.A. Cameron, S.M. George, Adsorption and desorption kinetics of H2 O on a fully hydroxylated SiO2 surface, Surf. Sci. 364 (1996) 61. [18] M. Fujihira, D. Aoki, Y. Okabe, H. Takano, H. Hokari, Effect of capillary force on friction force microscopy: a scanning hydrophilicity microscope, Chem. Lett. 7 (1996) 499. [19] W.C. Moon, S.A. Contera, T. Yoshinobu, H. Iwasaki, Nanotribology of clean and oxide-covered silicon surfaces using atomic force microscopy, Jpn. J. Appl. Phys. 39 (2000) 272. [20] W.C. Moon, T. Yoshinobu, H. Iwasaki, Nanotribology of Si oxide layers on Si by atomic force microscopy, Ultramicroscopy 86 (2001) 49. [21] B. Hornetz, H.-J. Michel, J. Halbritter, ARXPS studies of SiO2 –SiC interface and oxidation of 6H SiC single crystal Si-(0 0 1) and C-(0 0 1) surface, J. Mater. Res. 9 (1994) 3088.
Nanofriction of silicon oxide surfaces covered with thin water films A. Opitz a,∗ , S.I.-U. Ahmed b , J.A. Schaefer a , M. Scherge c b
a Institut für Physik, Technische Universität Ilmenau, PF 100565, D-98684 Ilmenau, Germany CSEM Centre Suisse d’Electronique et de Microtechnique SA, Rue Jaquet Droz 1, CH-2007 Neuchˆatel, Switzerland c IAVF Antriebstechnik AG, Im Schlehert 32, D-76187 Karlsruhe, Germany
Abstract A thin water film present on surfaces plays a central role in defining the micro- and nanotribological properties of a system. This paper presents a quantitative examination of the nanotribological effects of thin water films in ultra high vacuum (UHV) on OH-terminated (hydrophilic) and bare (no OH terminations, hydrophobic in vacuum) silicon oxide surfaces. Water film thickness was controlled by varying the water partial pressure in UHV. Friction was measured by scanning force microscopy (SFM) as a function of an external applied load. The surface energy and the shear stress of the nanotribological contact was then approximated by fitting the friction-load curves using the Derjaguin–Muller–Toporov (DMT) model. The surface energy as well as the adhesion force of the OH-terminated hydrophilic sample first decrease and later increase significantly at higher water partial pressures. No such dependence could be deduced from the friction-load curves at varying water pressures for the bare hydrophobic silicon oxide surface. However, at relatively high normal loads (pressures) and water partial pressures the bare hydrophobic silicon oxide is transformed to an OH-terminated surface. This transformation appears to occur only in the area of contact leading to the conclusion that it is friction-induced. This work shows that the chemical composition of the topmost surface layer defines the frictional behavior of the tribosystem. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Atomic force microscopy; Nanotribology; Silicon oxide; Water film; Hydrophilic; Hydrophobic; Nanowear
1. Introduction The fact that on the micro and nanoscales surface effects play a major role in determining the frictional response of systems is now well documented in various reports in [1–3]. This combined with the knowledge that, under ambient conditions, most surfaces are covered by a thin water film [1], indicates that a precise understanding of the tribological consequences of such thin water films is required at both, the micro and the nanoscales. This is particularly important for present day micro electromechanical systems (MEMS) [2] and the upcoming nano electromechanical systems (NEMS) [4]. In microtribological experiments with water confined in a slit formed by a smooth silicon ball versus a silicon flat with the same surface finish, the friction force curve showed several minima and maxima as function of the water film thickness [5]. At high water coverages of more than 10 monolayers friction is dominated by capillarity. With lower coverages, friction decreases due to reduction of this capillarity. Later several maxima (solid-like stick slips, increased ∗ Corresponding author. E-mail address: [email protected] (A. Opitz).
friction) and minima (liquid-like, decreased friction) were observed. Using the electrical double layer theory these effects were linked to the effects of confined monolayers [5]. Another ultra high vaccum (UHV) microtribology study showed that, as the water coverage on the surface is reduced by desorption, friction is reduced which arises from decreasing capillarity and is followed by increased friction due to a rise in cohesion [6]. Transitional friction from a high- to low-level has also been demonstrated due to the higher water adsorption potential when two surfaces are brought into contact [7]. Nanotribological investigations by Opitz et al. [8] on hydrophilic silicon oxide and hydrophobic silicon showed that confined water layers do not exist in a nanocontact. Results indicate that the tip apex of an scanning force microscopy (SFM) tip always makes direct solid–solid contact by penetrating the water film on the surface. Experiments also show that the water layers near the surface are ordered and that the magnitude of the friction force associated with these ordered water layers is small compared to capillary action resulting from thicker water films. Another nanotribological study has shown that the silicon oxide has a lower friction in dry nitrogen atmosphere than under ambient conditions [9].
0043-1648/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0043-1648(03)00248-5
A. Opitz et al. / Wear 254 (2003) 924–929
This paper investigates the nanotribological performance of two types of silicon oxide surfaces. One is OH-terminated (silanol) and hence hydrophilic while the other is bare, implying Si–O–Si (siloxane) bonding. The effect of varying water film thicknesses, on the two surfaces, upon the nanotribological characteristics is examined. The present study distinguishes itself from previous reports by our group [8,10] in that frictional behavior is linked to the surface energy and shear stress. For this purpose, friction-load curves were obtained and then fitted using the Derjaguin–Muller–Toporov (DMT) model [13]. From this fit both, surface energy as well as the shear stress were extracted. In a separate set of experiments, surface wear was also investigated and determined to be friction-induced.
2. Experimental setup Rectangular pieces of p-doped (1–10 cm, B-doped) Si(1 0 0) with an SFM determined root mean square (RMS) roughness of about 0.1 nm measured over a 0.6 m×0.6 m area were used as the sample material. Due to the natural oxide present on Si, samples are OH-terminated and, therefore, hydrophilic [2]. The hydrophilicity was also separately confirmed by contact angle measurements. Prior to mounting into the UHV chamber, the samples were sequentially cleaned using ultrasonic assistance in isopropanol and methanol for 5 min each. Afterwards, the samples were thoroughly rinsed with bi-distilled water. The oxide film measured on such samples had a thickness of 1.3 nm determined by X-ray photoelectron spectroscopy (XPS). The hydrophobic silicon oxide samples were prepared by heating at 600 K for 30 min. During this heating process the OH groups desorbs and the surface is terminated with Si–O–Si bonds [14]. However, such a surface is stable only in vacuum. The friction force microscope (FFM) used for this work operates in an ultra high vacuum system with a base pressure of 5 × 10−10 mbar. In this microscope, cantilever movement is detected by beam deflection. The cantilevers
925
made from silicon were calibrated by the geometry of cantilever and the detector sensitivity [15]. The spring constant of the cantilever was determined to be about 0.11 N/m, which allows for high resolution in friction force measurements. The Si tips used in all experiments were covered by a native oxide and are hydrophilic. The manufacture quoted tip radius is smaller than 15 nm. During friction measurements, however, the tip radius changes and is proposed in the paper to be about 30 nm. The water partial pressures were varied using a special water dosing system based on the work by Bazock et al. [16]. The main feature of the system is that water vapor from a liquid source (cleaned using freeze-thaw cycles) is introduced to the UHV system through a micometer-sized diameter hole. Here a leak valve is used. With this method, the water pressure near the sample is one to two orders of magnitude higher then in other parts of the chamber. Friction-load measurements using the SFM were performed in lateral mode under a varying applied external load. For each friction measurement the tip was scanned 300 nm in the forward and backward direction at a velocity of 300 nm/s and the friction was determined from the friction hysteresis curve. For the time dependent study, the applied external load was 60 nN. The chemical composition of the hydrophilic surface at different pressures was determined with X-ray photoelectron spectroscopy in a different UHV system.
3. Results The friction-load curve measurements using FFM were made as a function of the residual water pressure in the UHV system and are plotted in Fig. 1. The friction force depends on the normal force by a nonlinear law. The different black curves show the dependence of friction-load curves on the residual water pressure for the hydrophilic silicon oxide samples. The grey curve shows one example for hydrophobic silicon oxide; curves obtained at other pressures were almost identical and are, therefore, not shown. The DMT
Fig. 1. Friction force as function of normal force at different water pressures. The black curves are for silanol terminated hydrophilic silicon oxide, the grey curve is for siloxane terminated hydrophobic silicon oxide. Strong dependence of the friction-load curves of hydrophilic silicon on the partial water pressure is visible. Only one representative curve for hydrophobic silicon oxide is shown since curves at other pressures are almost identical. The solid lines are fitted by the Eq. (1).
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Fig. 2. The surface energy as a function of water pressure determined by fitting the friction-load curves in Fig. 1 with Eq. (1). The black curve is for silanol terminated hydrophilic silicon oxide, the grey curve is for siloxane terminated hydrophobic silicon oxide. The dashed lines are drawn to guide the eye.
model [13] was used for fitting the friction-load curves. The model is based on the Bowden–Tabor-approach [17] where the friction force is proportional to the real contact area and is an extended Hertz-model [18] with respect to the adhesion force. The function of friction force FF is given by, 2/3 R (FL + 4πγR)2/3 (1) FF = πτ K here is τ the shear stress (MPa) in friction contact, R the tip radius (∼30 nm), K the reduced E-modulus (50.1 GPa for SiO2 ) and FL the external load. The normal force is calculated according to FN = FL + FAd , were the adhesion force is equal to FAd = 4πγR, with γ the surface energy (mJ m−2 ) for the tribological contact. The shear stress has no dependence on water pressure and is determined to have the values: τhydrophilic = 375 ± 30 MPa and τhydrophobic = 40 ± 3 MPa. Both results have a relative error of about 8%; this is the measurement error. The dependence of surface energy upon the residual water pressure is shown in Fig. 2. No dependence of the friction force for hydrophobic silicon oxide is could be determined (γhydrohobic = 99 ± 3 mJ m−2 ). On the other hand, the hydrophilic silicon oxide surface shows a strong dependence on the surface energy. The surface energy first decreases from 10−10 to 10−8 mbar residual water pressure and upon
further increase in the residual water pressure the surface energy increases. Fig. 3 shows XPS spectra of the O 1s region on the hydrophilic surface at different pressures. At 1 × 10−10 mbar only a single peak at 533 eV due to Si–O bonding was observed. No peak attributable to Si–OH bonding was detected (Fig. 3a). However, such a component at 535 eV is clearly discernable at 3 × 10−8 mbar (Fig. 3b) indicating the presence of OH groups at this pressure. The nanofriction was also measured as a function of scanning time for the hydrophilic and hydrophobic silicon oxide. In these experiments, the tip and sample were placed in contact and when the desired partial water pressure was reached, the tip was moved repeatedly relative to the surface under an applied normal load of 60 nN. This corresponds to a DMT model calculated pressure of 1.9 GPa. Under such conditions, changes of the frictional behavior during exposure was clearly evident (Fig. 4). Two effects can be seen: First, starting from UHV, the friction force measured increases with rise in the water partial pressure. Secondly, at a particular pressure the friction force measured immediately after exposure and after 10 min of repeated sliding is different. For a particular pressure, the friction immediately after exposure is higher and after 10 min of sliding is lower. After this exposure the friction force on an adjacent area
Fig. 3. O 1s XPS lines at (a) 1 × 10−10 mbar and (b) 3 × 10−8 mbar for oxidized silicon samples. The O 1s has in a silicon environment (Si–O–Si) a binding energy of 533 eV, and for the hydroxyl environment (Si–OH) of 535 eV [23].
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Fig. 4. Friction force as a function of time at different behaviors on siloxane terminated hydrophobic silicon oxide. The friction force is shown in black immediately after exposure and after a sliding time of 10 min. The friction force on an adjacent part of the sample not initially in contact with the tip is shown in grey. The dashed lines are drawn to guide the eye.
Fig. 5. Topography (a) and friction force (b) are shown on hydrophilic silicon oxide after exposure. Changes of topography and friction force are visible in the center of image. The measurement was performed after the friction tests for Fig. 3 at a partial water pressure 5 × 10−6 mbar. The central part relates to the friction after sliding and the outer part to the friction of the partially siloxane terminated hydrophobic silicon oxide surface, which is formed due to UHV exposure. See text for details.
not in contact, was also measured. The friction results on this adjacent area (grey line in Fig. 4) indicate that the friction behavior is that of the siloxane terminated hydrophobic silicon oxide surface. The effect of friction on the initially hydrophobic silicon oxide are shown in topography and friction force images in Fig. 5. After 10 min of sliding, a narrow wear track is visible. This is distinguished from the undamaged area by a lower topography and higher friction. The outer part, not in contact during friction measurements relates to hydrophobic friction on silicon oxide. This clearly shows that friction-induced changes occur on the surface properties during testing.
4. Discussion The surface of hydrophilic silicon oxide undergoes changes during water exposure. An initially hydrophilic
surface introduced and stored for a long time in a UHV environment steadily looses OH groups due to desorption. At a pressure of about 10−10 mbar, the surface is at best only partially OH-terminated. This is supported by the XPS measurements performed at 1 × 10−10 mbar (Fig. 3b), which shows no evidence of Si–OH bonding indicating that the concentration of OH groups at this pressure is below the detection limits of XPS. This implies that the surface is largely terminated with Si–O–Si bonds, which according to Iler [12] renders the surface hydrophobic. This change also affects the surface energy, which now approaches that of the hydrophobic silicon oxide surface (Fig. 2). Note the high value of the surface energy in this region. We propose that this may be due to the greater bonding interaction between the Si–O–Si terminations of the tip and sample. These interactions decrease gradually with increase in the water partial pressure. At this stage water acts as a nanolubricant by shielding both surfaces from bonding interactions with
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one another. However, above a certain threshold pressure (Fig. 2), water begins to attack the siloxane (Si–O–Si) bonds forming silanol (Si–OH) groups [14]. Consequently, the surface energy starts increasing. Once the surface is fully hydroxylated, as has been reported to occur at a pressure of about 10−8 mbar [19], and at higher partial pressures, water adsorbs on these OH-groups by hydrogen bonding and forms water layers. Measurements made at this stage and at subsequently higher pressures are largely dominated by capillary effects and comparable to friction changes which occur during increasing relative humidity [1,5,20]. Fig. 1 also shows that the friction of siloxane terminated hydrophobic silicon oxide does not change by varying the water partial pressures. However, after prolonged exposure, with sample and tip in contact under high externally applied loads, the surface is affected and siloxane groups convert to silanol groups. An important aspect is that the hydrophobic silicon oxide surface changes only in the contact area between tip and sample. This hypothesis was tested in the experiment plotted in Fig. 4 and the scanned area depicted in topography and force images in Fig. 5. The friction increase at each pressure (black curves) is greater because the siloxane groups are converted to silanol groups due to the tip-sample contact under an applied load. In a microtribological study, Nevchupa et al. [7] have proposed in a microtribological experiment that the adsorption potential of water increases when two surfaces come into contact with one another. Applying this hypothesis here, since tip and sample were in contact before the experiment, water accumulation occurs in the contacting area which, together with the applied load, induces a tribochemical reaction transforming the siloxane bonds to silanol bonds, leading to higher friction. Areas not in contact (grey curve in Fig. 4) are not affected, this has already been discussed above. It should also be noted that in Fig. 4 the friction force decreases after 10 min of repeated sliding. This is attributed to the chemical composition of the scratched (worn) surface during sliding. The fact that this surface is scratched is evident from the topography images which clearly shows a trench indicating material removal and, hence, wear. Moon et al. [21,22], in ambient environment AFM experiments of silicon oxide layers, reported that the presence of OH bases promotes the nano-scratching of silicon oxide layers. An interesting observation is that the decreased friction measured after 10 min of sliding is not measured again in subsequent measurements. This is confirmed from the friction image (Fig. 5) after completion of the experiments. The worn area has acquired the same friction force measured on the initial surface immediately after exposure. This could be due to a meta stable oxide created upon scratching which changes once the tip is no longer in contact or to the effect of increased water exposure and tip contact upon subsequent scanning, or a combination of the two and remains the subject for further investigations.
5. Conclusions The friction force was examined as a function of the water pressure in UHV using friction force microscopy. Different friction behaviors and changes depend on the surface termination and on the partial pressure of water in vacuum. The surface energy of initially silanol terminated silicon oxide undergoes variations depending on the partial water pressure. At very low pressures, silanol groups partially desorb from the surface and tip leading to a rise in the surface energy and, subsequently, in the friction. When the water partial pressures is increased friction is reduced due to shielding effect of water. At higher pressures water converts the siloxane to silanol groups leading to increased surface energies and friction. In contrast, the siloxane terminated silicon oxide surface shows negligible changes in the surface energy and friction at different water partial pressures. However, for the siloxane terminated surfaces in contact with the tip and with repeated sliding at high loads converts the siloxane to silanol groups. Repeated sliding also scratch the surface and induces wear. This study highlights the significant differences in nanotribological behavior that exist between siloxane and silanol terminated silicon oxide surfaces.
Acknowledgements This work was supported by a grant from the Deutsche Forschungsgemeinschaft (Projekt Sche 425/2-4). References [1] M. Scherge, S.N. Gorb, Biological Micro- and Nanotribology, NanoScience and Technology, Springer-Verleg, Berlin, 2001. [2] R. Maboudian, Surface processes in MEMS technology, Surf. Sci. Rep. 30 (1998) 207. [3] B. Bhushan, Tribology Issues and Opportunities in MEMS, Kluwer Academic Publishers, Dordrecht, 1998. [4] S.E. Lyshevski, MEMS and NEMS: Systems, Devices and Structures, CRC Press, Boca Raton, 2002. [5] M. Scherge, X. Li, J.A. Schaefer, The effect of water on friction in MEMS, Tribol. Lett. 6 (1999) 215. [6] E.A. Deulin, A.A. Gatsenko, B.A. Loginov, Friction force of smooth surfaces of SiO2 –SiO2 as a function of residual pressure, Surf. Sci. 433–435 (1999) 288. [7] R.A. Nevchupa, M. Scherge, S.I.-U. Ahmed, Transitional microfriction behavior of silicon induced by spontanous water adsorption, Surf. Sci. 517 (2002) 17. [8] A. Opitz, S.I.-U. Ahmed, J.A. Schaefer, M. Scherge, Friction of thin water films: a nanotribological study, Surf. Sci. 504 (2002) 199. [9] L. Scandella, E. Meyer, L. Howald, R. Lüthi, M. Guggisberg, J. Gobrecht, H.-J. Güntherodt, Friction forces on hydrogen passivated (1 1 0) silicon and silicon oxide studied by scanning force microscopy, J. Vac. Sci. Technol. B 14 (1996) 1255. [10] (a) A. Opitz, S.I.-U. Ahmed, M. Scherge, J.A. Schaefer, Friction mechanisms from velocity and load dependent measurements on hydrophilic silicon from air to UHV, in preparation; (b) A. Opitz, Nanotribologische Untersuchungen von ultradünnen Wasserfilm auf hydrophoben und hydrophilen Siliziumoberfächen, Ph.D. thesis, TU Ilmenau (2003).
A. Opitz et al. / Wear 254 (2003) 924–929 [11] B.V. Derjaguin, V.M. Muller, Y.P. Toporov, Effect of contact deformations on the adhesion of particle, J. Colloid Interface Sci. 53 (1975) 314. [12] R.K. Iler, The Chemistry of Silica, Wiley/Interscience, Chichester, 1979. [13] U.D. Schwarz, P. Köster, R. Wiesendanger, Quantitative analysis of lateral force microscopy experiments, Rev. Sci. Instrum. 67 (1996) 2560. [14] M.J. Bazock, L. Muehlhoff, J.N. Rossell Jr., W.J. Choyke, J.T. Yates Jr., Methods in semiconductor surface chemistry, J. Vac. Sci. Technol. A 5 (1987) 1. [15] F.P. Bowden, D. Tabor, The Friction and Lubrication of Solids, Clarendon Press, Oxford, 1950. [16] H. Hertz, Ueber die Berührung fester elastischer Körper, J. Reine Angew. Math. 92 (1881) 156.
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[17] O. Sneh, M.A. Cameron, S.M. George, Adsorption and desorption kinetics of H2 O on a fully hydroxylated SiO2 surface, Surf. Sci. 364 (1996) 61. [18] M. Fujihira, D. Aoki, Y. Okabe, H. Takano, H. Hokari, Effect of capillary force on friction force microscopy: a scanning hydrophilicity microscope, Chem. Lett. 7 (1996) 499. [19] W.C. Moon, S.A. Contera, T. Yoshinobu, H. Iwasaki, Nanotribology of clean and oxide-covered silicon surfaces using atomic force microscopy, Jpn. J. Appl. Phys. 39 (2000) 272. [20] W.C. Moon, T. Yoshinobu, H. Iwasaki, Nanotribology of Si oxide layers on Si by atomic force microscopy, Ultramicroscopy 86 (2001) 49. [21] B. Hornetz, H.-J. Michel, J. Halbritter, ARXPS studies of SiO2 –SiC interface and oxidation of 6H SiC single crystal Si-(0 0 1) and C-(0 0 1) surface, J. Mater. Res. 9 (1994) 3088.