nano-tribological characteristics of self-assembled monolayer and its application in nano-structure fabrication

Wear 255 (2003) 808–818

Micro/nano-tribological characteristics of self-assembled monolayer and its application in nano-structure fabrication In-Ha Sung a , Ji-Chul Yang a , Dae-Eun Kim a,∗ , Bo-Sung Shin b a b

Department of Mechanical Engineering, Yonsei University, 134 Shinchon-dong, Seodaemoon-gu, Seoul 120-749, South Korea Nanoprocess Group, Korea Institute of Machinery and Materials, 171 Jang-Dong, Yousung-gu, Daejeon 305-343, South Korea

Abstract The fundamental tribological characteristics of 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS), octadecyltrichlorosilane (OTS), and single chain alkanethiol self-assembled monolayers (SAMs) with various chain lengths were investigated in order to identify the mechanical scribing condition for micro-machining applications. The concept of the novel surface micro-machining explored in this work is to mechanical scribe away the SAM resist coated on the workpiece surface where, pattern formation by subsequent chemical etching is desired. From the experimental results, it was found that the FDTS surface was damaged about 20% more rapidly than the OTS surface due to higher friction, even though the surface energy of FDTS was lower than that of OTS. Also, it was found that thiol on a copper surface could be removed even under a few nN normal load. The nano-tribological characteristics of alkanethiol SAM on various metals were largely dependent on the native oxide layer of metals. Based on these findings, FDTS and 1-hexadecanethiol (HDT) were chosen as the resists for silicon and metal surfaces, respectively. By using the mechano-chemical process with a diamond-coated tip, nano-patterns with sub-micrometer width and depth on surfaces of Au, Ag, Cu and Si could be fabricated. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Mechano-chemical micro-machining process (MCMP); Scanning probe microscope (SPM); Self-assembled monolayers (SAMs); Tool–workpiece interaction

1. Introduction With the advancement of micro-systems and nanotechnology, the need for ultra-precision fabrication techniques has been steadily increasing. Currently, methods such as e-beam [1], electrodeposition [2], X-ray [3] and LIGA [4] as well as photolithography techniques are commonly used to fabricate micro-structures. These processes provide excellent dimensional accuracy and intricate structures. However, the shortcomings of these processes are that they require heavy capital investment and extensive processing time. Although, these techniques are usually adequate for mass production, they are not suitable for prototype fabrication of micro-parts and for processing of non-silicon materials. In order to overcome some of the demerits of these processes, so-called soft lithography techniques such as micro-transfer molding and micro-contact printing have been suggested. These techniques, however, have some problems such as low lateral resolution and distortion of the pattern due to elastomer deformation [5]. Driven by the need for a new fabrication technique that can get over the limitation of the minimum ∗ Corresponding author. Tel.: +82-2-2123-2822; fax: +82-2-312-2159. E-mail address: [email protected] (D.-E. Kim).

pattern width or the critical dimension by current lithography methods, numerous researches on scanning probe microscope (SPM)-based lithography have also been conducted. However, they face many technical problems such as low reproducibility and insufficient removal of material [6–9]. With the motivation to overcome these problems, a novel micro-fabrication technique that is rapid, flexible, and cost-effective has been previously developed [10,11]. The micro-fabrication technique, which is called mechano-chemical micro-machining process (MCMP), is based on fundamental understanding of tribological interaction between a sharp tip and the workpiece material. Fig. 1 illustrates the basic concept behind the mechano-chemical process. The resist that is used as a protective layer against the chemical etchant is selectively removed by the mechanical abrasion action. The desired depth of cut is achieved by controlling the thrust force applied to a single asperity tip that serves as the tool. Chemical etching is then performed to remove the workpiece material at locations where the resist has been removed. Since the main mechanism of material removal of the workpiece is chemical reaction, the problem of mechanical machining such as burr formation can be overcome. By adjusting the chemical etching time

0043-1648/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0043-1648(03)00058-9

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Fig. 1. Schematic diagram of the mechano-chemical micro/nano-structure fabrication process.

and condition, three-dimensional structures with relatively high-aspect ratios on various metal surfaces as well as on silicon-based materials can be fabricated. Earlier studies demonstrated that the MCMP is a flexible, time-saving, cost effective, and reliable method which can be used for fabrication of micro-scale structures [10,11]. The motivation of this work is to extend the capabilities of MCMP to fabricate surface structures that are much smaller than what has been achieved previously. In order to fabricate nano-scale patterns and surface structures, it is essential for the resist used in MCMP to be as thin as possible. For this purpose, self-assembled monolayers (SAMs) are expected to be most adequate for the resist, since they have the merits of forming ultra-thin films by a simple chemical process at low cost. SAMs have been of great interest in recent years due to their potential in a wide range of technological applications such as corrosion inhibition and chemical sensing [12–16]. Particularly, good bonding strength, low surface energy and hydrophobic property of SAMs make them attractive candidates for use in friction and stiction reduction of micro-parts in micro-systems [16–18]. SAMs can be classified by their head group such as thiol (–SH) that binds to a metal surface and silane (–SiR3 ) that forms strong bonds with a silicon surface [19]. In this study, alkyltrichlorosilane (RSiCl3 ) and alkanethiol (RSH) precursor molecules were used as the resist for silicon and metal, respectively. Table 1 shows a summary of the mechano-chemical micro/nano-fabrication process with respect to the fabrication scale. Essentially, in order to fabricate nano-scale features, SAM has to be used as the resist material rather

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than SiO2 as in the case of MCMP. By doing so, patterns can be also be fabricated successfully on metal surfaces that was not possible with MCMP. To achieve good surface integrity by using the technique, it is important to understand the effects of the critical load for the resist removal, feed rate, and the surface damage characteristics. Also, the physical and chemical states of the workpiece surface such as surface roughness and the degree of contamination must be assessed. Most importantly, in order to fabricate nano-scale structures with good quality, the optimum scribing condition must be based on an understanding of the fundamental tribological characteristics of the SAM resist. In this paper, the frictional behavior and the surface damage characteristics of alkyltrichlorosilane and alkanethiol SAMs are investigated. The experimental results related to identifying the optimum machining condition for microfabrication are also described. Finally, some of the micro/ nano-structures fabricated on silicon and various metal surfaces using the mechano-chemical process are presented.

2. Experimental details 2.1. Sample preparation 2.1.1. Alkyltrichlorosilane SAMs Among various alkylsilane SAMs, alkyltrichlorosilane (RSiCl3 )-based 1H,1H,2H,2H-perfluorodecyltrichlorosilane (CF3 (CF2 )7 (CH2 )2 SiCl3 , FDTS) and octadecyltrichlorosilane (CH3 (CH2 )17 SiCl3 , OTS) were used for this work. As for the FDTS SAM, a Si(1 0 0) surface was first cleaned and oxidized by dipping into hydrogen peroxide (H2 O2 ) solution, and then immersed in a 1 mM iso-octane solution of FDTS under nitrogen flow in order to prevent bulk polymerization of the solution. On the other hand, the OTS SAM coating was prepared by immersing an oxidized Si(1 0 0) specimen in a 1 mM OTS–hexadecane solution. The Si(1 0 0) surface was oxidized by immersing in piranha solution (H2 SO4 :H2 O2 = 7:3, v:v) for 20 min. 2.1.2. Alkanethiol SAMs As substrates to be used for coatings, metal films of gold, silver and copper (>99.99%) were deposited to a thickness

Table 1 Summary of mechano-chemical micro/nano-fabrication process Features

Micro-structure (MCMP) [10,11]

Nano-structure [this work]

Machining principle

Abrasive interaction between a single asperity tool and workpiece Control of the depth of cut by thrust force Diamond (radius 2–5 ␮m) Si, metal (brass, Al, Cu)

Diamond (radius 150 nm) Si, metal (Au, Ag, Cu)

SiO2 Not possible (plowing → deburring) ∼1.5 ␮m

FDTS (C8 F17 C2 H4 SiCl3 ) HDT (C16 H33 SH), DDT (C12 H25 SH) ∼0.25 ␮m

Tool (tip) Workpiece materials Resist Si Metal Minimum pattern width

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of 200–250 nm on Si(1 0 0) by using either e-beam evaporation or RF-sputtering technique. A thin layer of either chromium (30 nm) or titanium (50 nm) was deposited on the silicon surface prior to the metal film deposition process in order to improve the adhesion. The metal coated specimens were first cleaned and then immersed in a 0.01–0.1 M ethanolic solution of alkanethiols for 0.5–24 h at room temperature [20,21]. Alkanethiols used in this work were the following: single chain n-alkanethiol (CH3 (CH2 )n SH) with various chain lengths, n: 1-heptanethiol (HT, n = 6), 1-decanethiol (DT, n = 9), 1-dodecanethiol (DDT, n = 11), and 1-hexadecanethiol (HDT, n = 15). A study of the effect of immersion time on thiol structure showed that no significant changes were observed in the chemical composition of the monolayers [22]. It was also reported that the electrochemical properties of thiol layers that formed within a few seconds in a solution with concentrations above 0.02 M were equivalent to those formed in 24 h [23]. 2.2. Characterization of SAMs X-ray photoelectron spectroscopy (Al K␣ X-ray source, hν = 1486.6 eV) and field emission scanning auger spectroscopy (Ar ion sputter) were used for the analyses of chemical compositions and structures of SAM-coated surfaces. Also, in order to identify the wetting properties of SAM surfaces, contact angle of water droplets were measured. A few micro-liters of deionized water were dropped on the SAM surface for the contact angle measurements. The measurements were carried out at room temperature and relative humidity level of 20%. Spectroscopic ellipsometry was used for the measurements of the thickness of SAMs (angle of incidence 70◦ ). 2.3. Measurement of micro/nano-tribological characteristics The sliding frictional force was measured using a custom built pin-on-disk type micro-tribotester. A Si3 N4 ball (3 mm diameter) was mounted on the silicon load beam that was instrumented with a set of semi-conductor strain gages. The resolution of the frictional force that could be detected by the setup was about 100 ␮N. The load was applied by the elastic deflection of the load beam. For all the friction measurements the normal load of 10 mN was applied and the sliding speed was kept at 600 ␮m/s, unless otherwise noted. From the preliminary tests, the sliding distance was set to 5 m, since no surface damage was detected and the frictional behavior showed steady-state values during the total sliding distance. In order to investigate the nano-tribological characteristics of SAM-coated surfaces, an atomic force microscope (AFM) and a lateral force microscope (LFM) were used for scratch tests and friction measurements, respectively. For scratch tests, diamond-coated tips (radius ∼150 nm, cantilever normal stiffness ∼0.4, 17 N/m) were used. Friction tests were

performed using a V-shaped Si cantilever (radius ∼10 nm, normal stiffness ∼0.4 N/m). All the experiments were performed at a relative humidity level of 20–30% under a class 100 clean hood at room temperature. 2.4. Chemical etching In order to characterize the surface damage of the specimens after the scratch test, the specimens were wet etched. Iron(III) chloride etchant (FeCl3 ) was used for copper etching, and oxygenated cyanide solution (K2 S2 O3 /K3 Fe(CN)6 /K4 Fe(CN)6 ) was used to etch silver and gold [24,25]. Using these etchants, the thiol-covered regions could be etched 106 times slower than the areas where the thiol has been removed [13]. Therefore, by using this etching method, it was possible to verify whether or not the thiol film was removed from the surface during the scratch test. Also, the scratched region of the FDTS coated silicon surface was etched anisotropically with KOH solution. These etching methods were also used for nano-pattern fabrication. 3. Micro/nano-tribological characteristics of SAMs 3.1. Effect of load and surface condition Contact angle measurements showed that the contact angles of water on FDTS and OTS SAMs were about 104◦ and 100◦ , respectively. The higher contact angle of the FDTS surface could be attributed to the lower surface energy (6 mJ/m2 ) of FDTS than that of the OTS surface (20 mJ/m2 ) [26]. Also, from the ellipsometry measurements of FDTS and OTS SAMs, it was determined that the thickness was about 3.0 and 2.8 nm, respectively. Fig. 2 shows the XPS wide scan spectra of FDTS and OTS SAM surfaces. No peaks related to the chlorine species appeared in either spectrum, which indicated that the trichlorosilane group of the SAMs reacted completely with the hydroxyl groups on the silicon substrate, and therefore, FDTS and OTS SAMs were synthesized successfully on the silicon surface [27]. Fig. 3 shows the influence of normal load on the friction of FDTS and OTS SAMs. Frictional (lateral) force signals were obtained by scanning a 5 ␮m × 5 ␮m area at speed of 10 ␮m/s using a LFM. From the results, it was found that the OTS SAM showed lower friction than the FDTS SAM in the testing range of the loads, despite the fact that FDTS has a lower surface energy. This result is in good agreement with the previous results reported on the size effect of the terminal group on frictional behaviors of alkylthiol surfaces [28]. The difference in the frictional behavior of these SAMs may be due to the difference in the size of the CH3 and CF3 terminal groups. Namely, the CF3 terminal molecules of FDTS provide a more densely packed arrangement than the CH3 molecules of OTS since both SAMs have almost the

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Fig. 4. Friction coefficients of FDTS and OTS SAMs with respect to sliding distance.

Fig. 2. XPS survey spectra of (a) OTS and (b) FDTS SAM surface.

same lattice spacing even though the CF3 molecule is larger than the CH3 molecule. Hence, the lateral motion within the FDTS SAM is likely to cause deformations of more SAM molecules and consequently may lead to more energy dissi-

Fig. 3. Frictional (lateral) force signal from LFM with respect to the normal load.

pation. Another possible reason of the frictional difference may be due to higher surface roughness and less uniformity of the FDTS coated surface compared with the OTS coated surface [29,30]. These factors seem to be responsible for the relatively large fluctuation in the lateral force signals of the FDTS SAM surface. Figs. 4 and 5 show the frictional behaviors and wear characteristics of both SAMs with respect to the sliding distance. The friction coefficient of the OTS surface was maintained at a relatively low value of about 0.09 up to 9 m sliding distance, whereas the friction coefficient of the FDTS surface started to increase after 7 m of sliding. From the optical images of the wear tracks of both SAMs shown in Fig. 5, it was found that the surface damage of the FDTS SAM was more severe than that of the OTS surface. The poor wear resistance of the FDTS SAM probably resulted in the early increase in the friction coefficient. Also, it is plausible that the surface damage of the FDTS SAM was accelerated by the high frictional interaction. FDTS SAM is better as a resist material than OTS SAM in micro-machining since it shows higher friction and surface damage. In order to identify the load at which FDTS can be removed cleanly, scratch tests were performed with a 150 nm radius diamond tip using an AFM. Fig. 6 shows the AFM image and cross-sectional profile of the micro-pocket that was fabricated by scratching once under a 1 ␮N load followed by etching in 1 M KOH solution for 30 min. As shown in the figure, wear particles were piled up on both sides of the scan area and wear scars were generated after the mechanical scribing and chemical etching processes. However, no significant change in the depth occurred. On the contrary, the cross-sectional profile shown in Fig. 7, which was obtained by a single scan under a 5 ␮N load, revealed that a micro-pocket with 200 nm depth could be fabricated on the Si surface after chemical etching. From the experimental results presented so far, it can be concluded that the FDTS SAM is more suitable as the resist material for micro/nano-structure fabrication on silicon surface than the OTS SAM. Also, at least a few micro-newton load

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Fig. 6. (a) AFM image and (b) cross-sectional profile of a micro-pocket fabricated on FDTS SAM-coated Si surface (1 scratch under 1 ␮N load/30 min etching in KOH).

1 (thiol-covered region) with respect to the scratching load are plotted in Fig. 9. The results indicate that the amount of carbon and sulfur in the scratched regions, decrease abruptly due to removal of thiol, as the scratching load increases.

Fig. 5. Optical images of wear tracks and particles of (a) FDTS and (b) OTS SAMs.

is needed to remove the FDTS SAM coating in a single scan. Fig. 8 shows the optical image of a 30 ␮m ×30 ␮m region on an HDT coated copper surface after one scratch by a diamond tip under a 1 nN load and the AES spectra of the surface. In Fig. 8(b), Area 1 is a thiol-covered region and Area 2 is the scratched region and Area 3 is a wear particle that was generated due to scratching. Wear scars and wear particles could be found in the scratched region. Compared with the C/Cu ratio of the AES atomic concentration obtained at the thiol-covered area, the decrease in the value obtained at the scratched region indicated that the thiol layer was removed. Moreover, it could be found that the major chemical composition of the wear particles is carbon, which suggests that it originated from CH3 and CH2 of HDT. The ratios of C/Cu and S/Cu in Area 2 (scratched region) to those in Area

Fig. 7. (a) AFM image and (b) cross-sectional profile of a micro-pocket fabricated on FDTS SAM-coated Si surface (1 scratch under 5 ␮N load/30 min etching in KOH).

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Fig. 9. The ratios of C/Cu and S/Cu in Area 2 (scratched region) to those in Area 1 (thiol-covered region) with respect to the scratching load.

could not be easily damaged as compared with the HDT coated copper surface. It is known that copper is more chemically reactive with oxygen than silver or gold. In the case of a copper evaporated film the native oxide layer was formed to a thickness of 1–1.5 nm after exposure to air for about 2 min [31], and in the case of silver an oxide layer of 1–2 monolayer(s) was also formed in air [32]. Therefore, the assembly of thiol will be affected by the formation of a metal oxide layer and consequently, the tribological characteristics of a thiol surface will be also affected by it.

Fig. 8. (a) Optical image and (b) AES spectra of HDT–Cu surface after mechanical scribing (1 scan, under 1 nN normal load).

From these results, it may be concluded that the thiols on the metal surface can be damaged by an abrasive interaction even under an ultra-low load of 1 nN. This observation can be confirmed by the AFM image of the HDT–copper surface after chemical etching, as shown in Fig. 10. The surface was etched only in the scratched region to a depth of about 200 nm. In the case of an HDT coated gold surface, no evidence of surface damage was found even when the surface was scratched under a few nN load for tens of cycles. Fig. 11 shows the optical image of a 40 ␮m × 40 ␮m surface region obtained after five scans under a 1 ␮N load using the diamond tip. This implies that the HDT coated gold surface

Fig. 10. (a) AFM image and (b) cross-sectional profile of HDT–Cu surface after chemical etching (30 s in iron chloride etchant).

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Fig. 11. Optical image of HDT–Au surface after five scans under 1 ␮N load.

supported this viewpoint [32,34,35]. The analysis revealed that the oxygen in the metal oxide surface was removed and metal thiolate was formed during the thiol adsorption process. From these considerations, it is suggested that in the case of thiol-coated metal oxide surfaces, the contact between the tip and the metal oxide molecules that were mixed in the thiol layer as well as thiol molecules can be possible, and therefore, the surface shows high friction and wear characteristics. It can be also concluded that high friction and surface damage characteristics of thiol-coated oxide surfaces are due to the heterogeneity and high concentration of defects of the thiol films. In summary, the nano-tribological characteristics of alkanethiolate SAMs on metal surfaces were found to be

In order to verify this consideration, the scratch tests of HDT on a fresh copper surface were performed. To minimize the oxide layer formation on copper surfaces, HDT SAM was coated on the copper surface that was transferred immediately, from the vacuum chamber to the HDT solution as soon as the copper films were deposited. The results of the scratch tests showed that in contrast to the results from a copper oxide surface, no evidence of surface damage was found even after tens of scratch cycles under a few nN load. Fig. 12 shows the thickness of thiol coated on metal surfaces measured by ellipsometry. From this figure, it can be observed that in the case of a copper oxide substrate, the thickness of the thiols was between 15 and 55 nm, which is an indication of multilayer coating. On the contrary, thiol films of 1–2 monolayer thickness were formed on the fresh substrates. According to a previous study [33], the copper oxide layer is transformed to a multilayer copper thiolate complex when it comes in contact with thiol. This causes copper ions to co-exist in the thiol layers. The structure characterization of alkanethiolate SAMs on metal oxide surfaces by XPS

Fig. 12. HDT SAM thickness on various conditions of metal surface after 24 h immersion measured by ellipsometry.

Fig. 13. Effect of scan speed on the frictional force of various thiols on (a) Au and (b) Cu surfaces (diamond tip, 10 nN).

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heavily dependent on the metal oxide layer that was formed before the thiol adsorption process. 3.2. Effect of scan speed The effect of scan speed on the frictional force of various SAMs was observed using a diamond tip in order to apply the results to optimize the mechanical scribing process. The scan speed corresponds to the feed rate in the mechano-chemical process. Scan length was set to 10 ␮m and normal load was set to 10 nN. From Fig. 13, it can be seen that frictional force increased with the scan speed. It has been reported that the carbon chain of thiols can be compressed viscoelastically with slow recovery time of the order of 0.08 s in the case of an HDT film on a gold surface [36]. From this viewpoint, in the case of a 5 ␮m/s scan speed, the tip slides 0.4 ␮m during the recovery time. Consequently, when the scan length of 10 ␮m and the reciprocating motion of the tip are considered, thiol molecules that experienced the contact with the tip can have enough time to be recovered in most situations except when the tip scans the region near the edges of both sides. However, at the scan speed of 200 ␮m/s, the tip slides 16 ␮m during the recovery time of 0.08 s. In this case, most of the thiol molecules in the scanned region cannot be recovered, and therefore, they cannot play a role as a boundary lubricant. Also, it is reasonable that short-chain thiols tend to show lower friction than long-chain thiols as the scan speed increases, since they may have faster recovery time than the long-chain thiols. For successful nano-pattern fabrication, it is important to find the appropriate feed rates that can generate good pattern shapes as well as high material removal rate. In the

Fig. 14. Cross-sectional profiles of the nano-patterns fabricated on HDT–Cu surface with respect to various feed rates: (a) 3 ␮m/s; (b) 10 ␮m/s.

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experiment, it was found that the quality of the fabricated pattern could be often degraded at relatively high feed rates. Fig. 14 shows the examples of the cross-sectional profiles of the patterns on HDT coated copper surfaces that were fabricated at feed rates of 3 and 10 ␮m/s. It is interesting to note that at relatively high feed rate of 10 ␮m/s, ridges formed on the sides of the pattern and they could not be removed effectively by chemical etching. This may be due to the fact that plowing could occur at a high feed rate. In other words, it is likely that the thiol resist would be simply displaced to the sides of the pattern rather than be removed at relatively high feed rates. Based on the experimental results, it was recommended that the scan speed in the range of 2–3 ␮m/s was appropriate for the machining condition.

Fig. 15. Nano-patterns on FDTS coated Si surface with 2 ␮m spacing (pattern width ∼250 nm, depth ∼100 nm): (a) SEM image; (b) AFM image and profile.

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4. Micro/nano-structure fabrication using mechano-chemical process The micro/nano-tribological characteristics of alkylsilane and alkanethiol SAMs described so far can be used for micro-fabrication process. In this section, some of the micro/nano-structures fabricated on silicon and metal surfaces using the mechano-chemical process are presented. 4.1. Nano-patterns on silicon surface FDTS SAM was used as the resist for silicon workpiece since it has the proper characteristics for micro-fabrication such as high friction and surface damage behaviors as mentioned previously. It is also known that FDTS SAM has high thermal stability and low adhesion and stiction behavior due to low surface energy [17,26,37]. These characteristics of FDTS SAM may help in reducing the contamination and wear of the tool tip used in the mechano-chemical

Fig. 16. (a) SEM and (b) AFM image of nano-patterns fabricated on HDT–Cu surface (500 nm spacing).

process. Fig. 15 shows the FDTS SAM-coated Si(1 0 0) surface that has been processed to form fine patterns with 2 ␮m spacing. The pattern has a depth of about 100 and 250 nm width. 4.2. Micro/nano-structures on metal surfaces According to a study on the structure of thiol-coated surfaces, long-chain thiols have a more densely packed structure than the short-chain thiols [38,39]. Researches on soft lithography also suggest that only long chain alkanethiol SAMs (n > 11) can effectively protect the substrates from the chemical etchant [40]. Therefore, HDT and DDT with long hydrocarbon chains were chosen as the resist for fabricating patterns on metal surfaces. In the experiments, the short-chain molecules such as HT could be used as the resist, but the lateral resolution of the fabricated pattern and the surface quality were not satisfactory. Also, the dimensional tolerance of the patterns showed less uniformity and reproducibility. Fig. 16 shows patterns fabricated on an HDT–copper surface with about 350 nm width and 120 nm depth. DDT coated copper and HDT coated silver surfaces that have the nano-patterns of 1.5 and 1.0 ␮m spacing are shown in Figs. 17 and 18, respectively.

Fig. 17. (a) SEM and (b) AFM images of nano-patterns fabricated on DDT–Cu surface (1.5 ␮m spacing).

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Fig. 19. (a) AFM image and (b) cross-sectional profile of nano-gratings fabricated on HDT–Cu surface (grating spacing ∼750 nm). Fig. 18. (a) SEM and (b) AFM images of nano-patterns fabricated on HDT–Ag surface (1 ␮m spacing).

As an extension of application of the micro-machining process presented in this work, fabrication of a micro-structured surface with sub-micro gratings was demonstrated. Fig. 19 shows the HDT–SAM-coated Cu/Si specimen that has fine gratings with 750 nm spacing and 200 nm height. All these patterns could be fabricated with relative ease and short time in comparison to some of the other methods by which similar patterns can be made. It is also expected that finer patterns on a defect-free surface can be fabricated if a sharper tip is used and the process is performed under more controlled environment.

on silicon and metal surfaces. In this work, FDTS and HDT were used as resists for silicon and metal surfaces, respectively. By using the mechano-chemical process with SAMs, high resolution patterns with 250–350 nm width and 100–200 nm depth could be fabricated on both silicon and various metal surfaces such as gold, silver and copper.

Acknowledgements This research has been supported by a grant (no. M102KN010001-02K1401-00723) from Center for Nanoscale Mechatronics & Manufacturing of the 21st Century Frontier Research Program.

5. Conclusions References In this work, the micro/nano-tribological characteristics of alkylsilane and alkylthiol SAMs were investigated. The tribological characteristics of the SAMs were exploited to develop the so-called mechano-chemical micro-fabrication technique that is based on mechanical abrasion and chemical etching. In order to fabricate the patterns with nano-scale dimensions, the mechanical abrasion process must be carefully controlled to remove the resist material where the chemical etchant is expected to remove the workpiece material. Experimental results showed that SAMs can be successfully used as a resist for nano-scale structure fabrications

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