NANO LETTERS
Patterning and Visualizing Self-Assembled Monolayers with Low-Energy Electrons
2002 Vol. 2, No. 10 1161-1164
R. Krupke,* S. Malik, H. B. Weber, and O. Hampe Forschungszentrum Karlsruhe, Institut fu¨ r Nanotechnologie, D-76021 Karlsruhe
M. M. Kappes Forschungszentrum Karlsruhe, Institut fu¨ r Nanotechnologie, D-76021 Karlsruhe, Institut fu¨ r Physikalische Chemie II, UniVersita¨ t Karlsruhe,D-76128 Karlsruhe
H. v. Lo1 hneysen Physikalisches Institut, UniVersita¨ t Karlsruhe, D-76128 Karlsruhe, Forschungszentrum Karlsruhe, Institut fu¨ r Festko¨ rperphysik, D-76021 Karlsruhe Received July 3, 2002; Revised Manuscript Received August 19, 2002
ABSTRACT We show that a trimethylsilyl (TMS) self-assembled monolayer on a silicon surface is a self-developing positive resist, which can be patterned with low energy electrons. Contact angle measurements have been used to quantify the efficiency of the exposure as a function of exposure dose and acceleration voltage. Ash formation was negligible, as a 3-aminopropyltriethoxysilane (APTES) self-assembled monolayer could be formed on the patterned area without an intermediate development stage. APTES/TMS patterns have been visualized with scanning electron microscopy at low energy and atomic force microscopy. The functionality of the patterns has been tested by selective deposition of carbon nanotubes.
Self-assembled monolayers (SAM) are widely used to change the chemical properties of a surface.1 For instance, a silicon surface can be made hydrophobic or hydrophilic by functionalizing with self-assembled methyl- or amino-terminated monolayers, respectively.2,3 Hence the adhesion of specific substances can be inhibited or promoted at the molecular level. More recently SAMs have been patterned using atomic force microscopy or electron beam writing to create surface domains with different chemical properties that allow to selectively deposit nanosized objects such as carbon nanotubes or biomolecules.4,5 Furthermore, SAMs have become of interest to the semiconductor industry for high-resolution lithography due to the small size of the molecules. A quite extensive literature exists about e-beam patterning of SAMs.6-11 However, most work focuses on aliphatic silane SAMs with rather long hydrocarbon chains. In particular, despite a recent application using patterned TMS for selective carbon nanotube absorption,4 there is no detailed report about e-beam patterning of trimethylsilyl (TMS) SAM, the shortest of the group of methylsilane SAMs. It has been shown that for an effective exposure of a SAM with a thickness of the * Corresponding author. E-mail:
[email protected] 10.1021/nl025679e CCC: $22.00 Published on Web 09/18/2002
© 2002 American Chemical Society
order of 1-2 nm, the electron energy has to be much smaller than the 10 kV typically used in standard electron-beam lithography.13 The effect is related to the energy-dependent penetration depth of electrons. Because a TMS SAM has a thickness of only 0.5 nm, we expect even lower voltages to be required for an effective exposure, which would make TMS a favorable resist for parallel exposure with microfabricated electron-beam column arrays requiring acceleration voltages of the order of 1 kV or below.14 We have investigated the feasibility of electron-beam patterning of a TMS self-assembled monolayer by variation of exposure dose and electron acceleration voltage. The impact of exposure has been quantified by measurements of the contact angle Θ. For that purpose samples with large exposed areas were prepared. Additionally, using very low acceleration voltage, striped patterns were formed on different samples. As we will show below, these patterns can be visualized not only by atomic force microscopy but also by scanning electron microscopy using low-energy electrons. The issue of fragment formation has been addressed by backfilling the exposed area with a 3-aminopropyltriethoxysilane (APTES) self-assembled monolayer, and the func-
Figure 1. Schematic diagram of the sample preparation starting from a TMS SAM on silicon: (I) large area exposure with successive contact-angle measurements, (II) patterning of stripes, backfilled with APTES, and deposition of carbon nanotubes (see text).
tionality of APTES/TMS patterns has been tested by selective deposition of carbon nanotubes (Figure 1). TMS SAMs were formed by chemical vapor deposition on Si samples cut from p-type Si (100) wafers with native oxide coverage. The samples were first exposed to an oxygen plasma. This treatment serves three purposes: (a) cleaning the surface from organic residues, (b) oxidizing the Si surface, and (c) terminating the surface with OH groups. The hydrophilicity manifests itself as a reduction of the contact angle from originally Θ ≈ 48° to Θ < 10°. The hydrophilic samples were heated with 1 mL hexamethyldisilazane (Sigma-Aldrich, 99.9% purity) to 150 °C for 5 h in a sealed aluminum cylinder (200 mL). Then the samples were rinsed with chloroform and 2-propanol. After the treatment the samples became hydrophobic with Θ ) 90 ° - 94 ° due to coverage with a trimethylsilyl monolayer. Patterning of the samples was done with a high-resolution LEO 1530 field emission scanning electron microscope (SEM) in combination with a Raith ELPHY writing system. The SEM can operate down to voltages as low as 100 V. To image the surface, the in-lens detector was used. Samples with large exposed areas (5 × 5 mm2) were subjected to contact angle measurements of a sessile drop of water. Backfilling experiments were carried out by soaking patterned samples in a 1 mM solution of APTES (Sigma-Aldrich, 99% purity) in chloroform for 30 min. The samples were rinsed in chloroform and 2-propanol and subjected to scanning electron microscopy and atomic force microscopy. Several TMS-terminated silicon samples were exposed over a large area (5 × 5 mm2) with varying the exposure dose from d ) 50 µC/cm2 to d ) 2 mC/cm2 at an acceleration voltage of V ) 10 kV. The contact angle as a function of the exposure dose is shown in Figure 2. The contact angle, after an initial drop to Θ ≈ 90 °, remains constant up to d ) 600 µC/cm2. At higher dose, the contact angle becomes smaller, reaching Θ ≈ 65 ° at d ) 2 mC/cm2. The inset of Figure 2 shows the contact angle as a function of the acceleration voltage. The voltage has been varied from V ) 10 kV to V ) 1 kV at a constant dose of d ) 200 µC/cm2. 1162
Figure 2. Contact angle Θ as a function of the exposure dose d at a fixed acceleration voltage V ) 10 kV (squares). Inset shows Θ as a function of acceleration voltage V at a fixed dose d ) 200 µC/cm2 (circles). The full lines are guides to the eye.
The contact angle is independent of the voltage for V > 2 kV. At lower voltage a significant reduction of Θ is observed. Cos Θ is related to the free energies of the liquid-air interface, γLA (the ordinary surface tension of the liquid), and the difference of the free energy between the solid-air interface, γSA, and the solid-liquid interface, γSL. The latter two quantities cannot usually be measured separately, but their difference can. The relation is cos Θ ) (γSA - γSL)/γLA For water at room temperature, γLA ) 7.2 µJ/cm2. As cos Θ varies from 1 to 0 during TMS SAM formation, the quantity γSA - γSL changes by about 45 meV per surface molecule of SiO215, which corresponds to the expectation for suppressed hydrogen bonding of water to the surface resulting from the formation of the methyl SAM. Conversely, cos Θ increases during electron irradiation, which we attribute to the removal of methyl groups and eventually the recovery of a hydrophilic surface. In trimethylsilyl, the weakest bond is the C-Si bond, which makes it likely that complete methyl groups detach from the Si atom under electron irradiation. After exposure to air, the remaining dangling bond may rapidly react with water and form an OH-terminated, hydrophilic surface. Returning to Figure 2, it is apparent that for high acceleration voltage Θ shows only a weak dose dependence for 0 < d e 600 µC/cm2. Note, however, that there is a strong dependence on acceleration voltage (below 2 kV) for small doses. We argue that this is due to methyl groups being more effectively removed at very low acceleration voltage. For an understanding, it is important to consider the energy dependent scattering of electrons in the solid.16 Electrons penetrating the solid undergo elastic and inelastic scattering. Whereas elastic scattering merely changes their trajectory, inelastic scattering also leads to energy transfer to the solid, which can break bonds. Both scattering mechanisms depend on energy which leads to a smaller penetration depth and shrinking interaction volume with decreasing energy. PenNano Lett., Vol. 2, No. 10, 2002
Table 1: Electron Ranges RKO in Silicon for Various Incident Electron Energies E0a E0
10 keV
5 keV
2 keV
1 keV
0.5 keV
0.2 keV
RKO
1.5 µm
0.5 µm
100 nm
30 nm
10 nm
2 nm
a Values below E ) 5 keV are extrapolated from ref 17 as a crude 0 estimate.
etration depth and interaction volume can be determined by Monte Carlo simulation using the Rutherford and BetheBloch formula for the elastic and inelastic scattering, respectively. An expression for the maximum electron range RKO, which closely approximates the depth dimension of the interaction volume, has been derived by Kanaya and Okayama:17
Figure 3. SEM picture of a pattern on silicon with 1 µm long and 50 nm wide stripes of APTES SAMs (bright) surrounded by TMS SAMs (dark), taken at V ) 500 V.
RKO ) 0.0276AE01.67/Z0.889 F [µm] where the incident energy E0 is given in keV, A is the molar mass in g/mol, F is the density in g/cm3, and Z is the atomic number of the target. Values of RKO are listed in Table 1. As can be seen from the numbers in Table 1, most of the energy is dissipated deep into the silicon substrate for high acceleration voltages. At smaller voltages, energy is dissipated closer to the surface, and hence the SAM is exposed more effectively with more bonds presumably being broken. There are two additional features in the curve which we would like to comment on. First, there is a significant reduction of Θ for d > 600 µC/cm2. We believe that contamination due to irradiation in high vacuum is the origin. Pyrolysis of adsorbed hydrocarbons is generally observed in this regime.18 Second, the weakly irradiated samples with d e 300 µC/cm2 have a systematically smaller contact angle than the nonirradiated samples with d ) 0, independent from the irradiation dose. We assume that cycling the irradiated samples through the high-vacuum environment of the SEM modifies the SAM in addition to the change resulting from irradiation with electrons. Although our SEM is also suitable for exposure at voltages V < 1 kV, we could not prepare large-area samples at this voltage suitable for contact-angle measurements due to a reduced and deformed maximum exposure area. We have therefore prepared patterned samples to explore the lowvoltage regime. The strategy was to first remove locally the methyl-SAM with very low energy electrons. If the TMS SAM was removable without the formation of ashes, then an intermediate development step would not be needed and the recovered silicon areas could immediately be backfilled with another silane SAM. We have chosen an amino-SAM (APTES) because its formation can be verified by adsorption of various organic substances from solution or dispersion (Figure 1, II). Samples with a TMS top layer were patterned with an electron beam at V ) 500 V and d ) 500 µC/cm2. Subsequently the samples were soaked in APTES/chloroform in order to obtain the amino-SAM formation. Then the samples were rinsed with chloroform and 2-propanol. Nano Lett., Vol. 2, No. 10, 2002
Figure 4. (upper) Lateral-force AFM image demonstrating the enhanced friction of the cantilever on APTES SAM stripes (bright) compared to the surrounding TMS SAM. The pattern is different from Figure 3 because the exposure dose and line width have been varied. (lower left) Topography AFM image of a 50 nm wide stripe of APTES SAMs (bright) surrounded by TMS SAMs. The height profile along the white horizontal line is shown on the right.
Examination of the samples with SEM reveals that the APTES stripes can be visualized under the conditions of low acceleration voltage plus the use of the in-lens detector (Figure 3). With regard to the previous discussion on the penetration depth of electrons, it is obvious that not only is the patterning more effective at low acceleration voltage but also is the imaging. We have also observed that the patterns are visible with the in-lens detector only but not with the Everhart-Thornlay detector. The use of the in-lens detector allows for a smaller working distance and hence the collection of more secondary electrons. The contrast between APTES and TMS SAM can appear inverted, depending on the beam current density. We attribute the contrast to differences in the surface potential. A line of APTES SAMs has been scanned with an AFM tip in contact mode (Figure 4). The measured height difference ∆d ≈ 0.8 nm is in agreement with the difference in height of an APTES and TMS SAM with ∆d ≈ 1 nm - 0.4 nm.19,20 However, because 1163
in the SWNT strands in order to accommodate the largest adhesion length possible. In summary, we have demonstrated that a TMS SAM is a self-developing positive resist that can be patterned with low-energy electrons (E0 < 1 keV). Fragment formation is insignificant, as an APTES SAM can be formed on the recovered silicon surface without an intermediate development step. Patterns of APTES/TMS have been visualized by SEM, using a similar energy as for the patterning. The functionality of the pattern has been shown by selective deposition of carbon nanotubes on stripes of APTES. Acknowledgment. The authors thank A. Berlinger and T. Koch for the AFM characterization and D. Gerthsen for helpful discussions. References (1) (2) (3) (4)
Figure 5. SEM images demonstrating the selective deposition of carbon nanotube bundles (arrows) on stripes of APTES SAMs.
the interaction potentials between the AFM tip and an APTES and a TMS SAM are very different, one has to be careful with an interpretation of height values. We also have observed strong frictional forces while scanning on APTES (Figure 4). Strong frictional forces have been reported for contact mode scanning on APTES SAMs.13 Our observation of strong frictional forces on APTES stripes indicates that the APTES formation did occur as proposed. Ash formation, as known for long hydrocarbon chains, apparently does not play a role here. As further proof we have tested the functionality of the pattern. APTES is widely used to make a glass surface sticky to organic material, where to the contrary TMS inhibits adhesion. Our APTES/TMS pattern on silicon should promote selective deposition of organic material on the APTES stripes. We used as a test material bundles of single walled carbon nanotubes (SWNT) suspended in dimethylformamide (DMF). The samples with the APTES/TMS pattern have been immersed into the suspension for 10 min and successively rinsed with chloroform and 2-propanol. Figure 5 demonstrates indeed that bundles of SWNTs have been deposited on stripes of APTES SAMs with high selectivity. The strong adhesion of SWNT to APTES even leads to kinks
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(5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20)
Ulman, A. An introduction to ultrahin organic films: from Langmuir-Blodgett to self-assembly; Academic Press: San Diego, 1991. Sugimora, H.; Nakagiri, N. Langmuir 1995, 11, 3623. Haller, I. J. Am. Chem. Soc. 1978, 100, 8050. Liu, J.; Casavant, M. J.; Cox, M.; Walters, D. A.; Boul, P.; Lu, W.; Rimberg, A. J.; Smith, K. A.; Colbert, D. T.; Smalley, R. E. Chem. Phys. Lett. 1999, 303, 125. Harnett, C. K.; Satyalakshmi, K. M.; Craighead, H. G.; Langmuir 2001, 17, 178. Lercel, M. J.; Tiberio, R. C.; Chapman, P. F.; Craighead, H. G.; Sheen, C. W.; Parikh, A. N.; Allara, D. L. J. Vac. Sci. Technol. B 1993, 11, 2823. Seshadri, K.; Froyd, K.; Parikh, A. N.; Allara, D. L.; Lercel, M. J.; Craighead, H. G. J. Phys. Chem. 1996, 100, 15900. Mu¨ller, H. U.; Zharnikov, M.; Vo¨lkel, B.; Schertel, A.; Harder, P.; Grunze, M. J. Phys. Chem. B 1998, 102, 7949. Weimann, T.; Geyer, W.; Hinze, P.; Stadler, V.; Eck, W.; Go¨lzha¨user, A.; Microelectron. Eng. 2001, 57, 903. Hild, R.; David, C.; Mu¨ller, H. U.; Vo¨lkel, B.; Kayser, D. R.; Grunze, M.; Langmuir 1998, 14, 342. Carr, D. W.; Lercel, M. J.; Whelan, C. S.; Craighead, H. G.; Seshadri, K.; Allara, D. L. J. Vac. Sci. Technol. A 1997, 15, 1446. Eck, W.; Stadler, V.; Geyer, W.; Zharnikov, M.; Go¨lzha¨user, A.; Grunze, M. AdV. Mater. 2000, 12, 805. Harnett, C. K.; Satyalakshmi, K. M.; Craighead, H. G. Appl. Phys. Lett. 2000, 76, 2466. Chang, T. H. P.; Thomson, M. G. R.; Kratschmer, E.; Kim, H. S.; Yu, M. L.; Lee, K. Y.; Rishton, S. A.; Hussey, B. W.; Zolgharnain, S. J. Vac. Sci. Technol. B 1996, 14, 3774. Williams, R.; Goodman, A. M. Appl. Phys. Lett. 1974, 25, 531. Goldstein, J. I. Scanning electron microscopy and X-ray microanalysis; Plenum Press: New York, 1981. Kanaya, K.; Okayama, S. J. Phys. D: Appl. Phys. 1972, 5, 43. Lercel, M. J.; Craighead, H. G.; Parikh, A. N.; Seshadri, K.; Allara, D. L. Appl. Phys. Lett. 1996, 68, 1504. Petri, D. F. S.; Wenz, G.; Schunk, P.; Schimmel, T. Langmuir 1999, 15, 4520. Sindorf, D. W.; Maciel, G. E. J. Phys. Chem. 1982, 86, 5208.
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