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Electron-Beam-Induced Damage of Alkanethiolate Self-Assembled Monolayers (SAMs): Dependence on Monolayer Structure and Substrate Conductivity Chuanzhen Zhou,† Aaron Trionfi,‡ Julia W. P. Hsu,‡ and Amy V. Walker*,†,§ Department of Chemistry and Center for Materials InnoVation, Washington UniVersity in St. Louis, Campus Box 1134, One Brookings DriVe, St. Louis, Missouri 63130, and Center for Integrated Nanotechnologies, Sandia National Laboratories, P.O. Box 5800 MS-1415, Albuquerque, New Mexico 87185-1415 ReceiVed: December 1, 2009; ReVised Manuscript ReceiVed: April 20, 2010
We report the first studies of the relative importance of substrate conductivity and monolayer structure on the electron-beam-induced damage of alkanethiolate self-assembled monolayers (SAMs) adsorbed on Au and GaAs(001) using time-of-flight secondary-ion mass spectrometry. The results clearly show that the extent of damage observed is strongly dependent on the electrical conductivity of the substrate; at a given electron dose, the amount of degradation is greatest for SAMs adsorbed on the least conductive substrate, semiinsulating GaAs(001). This is because there is a buildup of static charge at the substrate/SAM interface, whereas for an electrically conductive substrate, electrons can be conducted away from the surface, leading to less electron-beam-induced damage. The monolayer structure also greatly affects the amount of electron beam damage. Disordered SAMs, such as nonanethiol adsorbed on Au, undergo more degradation at a given electron dose than ordered SAMs, such as octadecanethiol (ODT) adsorbed on Au. Comparison of the data for undecanethiol (UDT) on conducting GaAs, a disordered SAM, and ODT on semi-insulating GaAs, an ordered SAM, suggests that the detailed 2D monolayer structure plays a more important role than the electrical conductivity of the substrate in determining the extent of electron-beam-induced damage. In addition, differences in the detailed structure of SAMs on Au and GaAs affect the reaction pathways observed. These findings explain previously reported results that much higher electron beam doses are required to damage SAMs on metals compared with SAMs adsorbed on semiconductors and insulators. 1. Introduction In electron beam lithography, a beam of electrons, with a typical energy of 10-100 keV, is scanned across a resist, creating a patterned surface.1 Self-assembled monolayers (SAMs) are attractive resists for electron beam lithography because they are well-ordered, dense films on a wide range of substrates2,3 with a film thickness (1-2 nm) and a molecular diameter ( Iemitted).31 For the semiinsulating GaAs I, there is a higher buildup of electrons on the sample than for the doped GaAs substrates (GaAs II and GaAs III), leading to an increase in the electron-induced damage observed. Second, we also observe that the extent of electron-beaminduced damage is larger for disordered monolayers. On GaAs(001), at a given electron dose, for UDT, a disordered monolayer, we observe that the intensities of the unsaturated hydrocarbon and PAH ions (Figures 4 and 5) are much larger than those for ODT, an ordered monolayer. The percentage decrease in the molecular ion intensity, Ga2M+ (M ) -S(CH3)xCH3; x ) 10 (UDT), 17 (ODT)), is also much larger for disordered UDT (∼43%) than for ODT (∼13%) (Figure 8). Similarly, for disordered NT on Au, we observe that the extent of electron-beam-induced damage is larger than that for ordered ODT at a given electron dose (Figures 4, 6, and 7).
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Because the unsaturated hydrocarbon and PAH ion yields for UDT on degenerately doped GaAs (Figures 4c,d and 5a) are larger than those for ODT on semi-insulating GaAs (Figures 1 and 2), we conclude that the detailed monolayer structure is more important than the substrate electrical conductivity in determining the extent of electron-beam-induced damage of SAMs. The picture is more complex when comparing wellordered SAMs on Au (resistivity ) 3.8 × 10-8 Ω · m) and degenerately doped GaAs ((1.7-2.7) × 10-5 Ω · m). Although ODT forms well-ordered SAMs on both GaAs(001) and Au, the SAM structures on these two substrates are very different. Alkanethiolate SAMs adsorbed on Au with more than nine methylene (-CH2-) units in the backbone are well-ordered with large domain sizes (∼250 nm2).2,3,43 In contrast, alkanethiolate SAMs adsorbed on GaAs(001) are well-ordered if there are 15 or 16 methylene units in the backbone, but they have small domain sizes.29 McGuinness et al.29 reported for ODT adsorbed on GaAs(001) that the domain correlation length was ∼70 Å (implying a domain size of ∼49 nm2). The changes in the intensities of the unsaturated hydrocarbon and small PAH ions, characteristic of electron-beam-induced damage, are approximately the same for ODT on these substrates (Figures 4, 5b, and 6b and the Supporting Information). However, on GaAs, the percentage decrease in the molecular ion, Ga2M+, (∼13%, Figure 3) is less than the percentage decrease in the molecular ion, AuM2-, on Au (∼22%), suggesting that the extent of damage is larger on Au. Additionally, on conductive GaAs, the formation of the large PAH ions, C12H8+ and C13H9+, albeit at an electron dose of g600 µC cm-2 (Figure 5b), but these ions are not observed on Au even after an electron dose of 4000 µC cm-2 (Figure 6), which suggests that the extent of electron beam damage is larger on GaAs. Taken together, these observations suggest that, when the substrate is conducting enough, the monolayer structure quantitative determines the reaction pathways of electron-beam-induced damage. 5. Conclusions We observe that both the SAM structure and the electrical conductivity of the substrate affect the extent of electron-beaminduced damage. When semi-insulating GaAs is used as the substrate, more damage is created in the monolayer at a given electron dose compared with SAMs adsorbed on conductive GaAs and gold. At the electron beam energies employed (40 keV), there is a buildup of electrons on the sample, leading to more damage created in the monolayer at a given electron dose. Studies using ordered and disordered SAMs adsorbed on conductive GaAs(001) and Au indicate that the monolayer structure is an important factor in the extent of electron-induced damage. Disordered SAMs (UDT adsorbed on GaAs(001) and NT adsorbed on Au) are observed to undergo more degradation than ordered ODT SAMs adsorbed on these substrates. Comparison of the data for UDT on conducting GaAs, a disordered SAM, and ODT on semi-insulating GaAs, an ordered SAM, suggests that the detailed 2D monolayer structure is more important than the electrical conductivity of the substrate in determining the extent of electron-beam-induced damage. However, the results of ODT adsorbed on Au and conducting GaAs suggest that the different monolayer structures result in quantitatively different reaction pathways. These findings help explain previously reported results that much higher electron beam doses are required to degrade SAMs on metals compared with SAMs adsorbed on semiconductors and insulators. Acknowledgment. We thank J. Reno for providing the semiinsulating GaAs wafer (GaAs I) and E. Vogel for measuring
Zhou et al. the resistivity of the Au substrates employed in this study. A.V.W. acknowledges the financial support of the National Science Foundation. This work was performed, in part, at the U.S. Department of Energy, Center for Integrated Nanotechnologies, at Los Alamos and Sandia National Laboratories. Sandia National Laboratories is a multiprogram laboratory operated by Sandia Corporation, a Lockheed-Martin Company, for the U.S. Department of Energy’s National Nuclear Security Administration under Contract No. DE-AC04-94AL85000. Supporting Information Available: A short description of the reaction pathways involved in the electron-beam-induced damage of HDT, ODT, and MHA SAMs adsorbed on Au using 40 keV electrons; the change in the relative ion intensities of the hydrocarbon fragment ions C2H3+, C4H5+, C4H7+, C6H9+, and C6H11+ with an electron dose for ODT adsorbed on GaAs(001) substrates with different doping levels; the variation of the relative ion intensities of the PAH ions C8H9+, C9H8+, C10H8+, and C13H9+ with an electron dose for ODT adsorbed on GaAs(001) substrates with different doping levels; the change in the relative ion intensities of the hydrocarbon fragment ions C2H3+, C4H5+, C4H7+,C5H7+, C5H9+, C6H9+, and C6H11+ with an electron dose for NT and ODT adsorbed on Au; the change in the relative ion intensities of the hydrocarbon fragment ions C2H3+, C4H5+, C4H7+, C6H9+, and C6H11+ with an electron dose for UDT and ODT adsorbed on GaAs(001). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) McCord, M. A.; Rooks, M. J. Microlithography. In SPIE Handbook of Microlithography, Micromachining and Microfabrication; Rai-Choudhury, P., Ed.; SPIE Publications: Bellingham, WA, 1997; Vol. 1, pp 139-250. (2) Ulman, A. Chem. ReV. 1996, 96, 1533–1554. (3) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151–256. (4) Zharnikov, M.; Grunze, M. J. Vac. Sci. Technol., B 2002, 20, 1793– 1807. (5) 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, 2823–2828. (6) Lercel, M. J.; Redinbo, G. F.; Pardo, F. D.; Rooks, M.; Tiberio, R. C.; Simpson, P.; Craighead, H. G.; Sheen, C. W.; Parikh, A. N.; Allara, D. L. J. Vac. Sci. Technol., B 1994, 12, 3663–3667. (7) Lercel, M. J.; Rooks, M.; Tiberio, R. C.; Craighead, H. G.; Sheen, C. W.; Parikh, A. N.; Allara, D. L. J. Vac. Sci. Technol., B 1995, 13, 1139– 1143. (8) Tiberio, R. C.; Craighead, H. G.; Lercel, M.; Lau, T.; Sheen, C. W.; Allara, D. L. Appl. Phys. Lett. 1993, 62, 476–478. (9) Geyer, W.; Stadler, V.; Eck, W.; Zharnikov, M.; Go¨lzha¨user, A.; Grunze, M. Appl. Phys. Lett. 1999, 75, 2401–2403. (10) Seshadri, K.; Froyd, K.; Parikh, A. N.; Allara, D. L.; Lercel, M. J.; Craighead, H. G. J. Phys. Chem. 1996, 100, 15900–15909. (11) Zhou, C.; Jones, J. C.; Trionfi, A.; Hsu, J. W. P.; Walker, A. V. J. Phys. Chem. C 2009, 114, 5400–5409. (12) Zharnikov, M.; Frey, S.; Heister, K.; Grunze, M. Langmuir 2000, 16, 2697–2705. (13) Zharnikov, M.; Geyer, W.; Go¨lzha¨user, A.; Frey, S.; Grunze, M. Phys. Chem. Chem. Phys. 1999, 1, 3163–3171. (14) Frey, S.; Heister, K.; Zharnikov, M.; Grunze, M. Phys. Chem. Chem. Phys. 2000, 2, 1979–1987. (15) Mu¨ller, H. U.; Zharnikov, M.; Vo¨lkel, B.; Schertel, A.; Harder, P.; Grunze, M. J. Phys. Chem. B 1998, 102, 7949–7959. (16) Olsen, C.; Rowntree, P. A. J. Chem. Phys. 1998, 108, 3750–3764. (17) Rowntree, P.; Dugal, P.-C.; Hunting, D.; Sanche, L. J. Phys. Chem. 1996, 100, 4546–4550. (18) Huels, M. A.; Dufal, P.-C.; Sanche, L. J. Chem. Phys. 2003, 118, 11168–11178. (19) Cyganik, P.; Vandeweert, E.; Postawa, Z.; Bastiaansen, J.; Vervaecke, F.; Lievens, P.; Silverans, R. E.; Winograd, N. J. Phys. Chem. B 2005, 109, 5085–5094. (20) Harnett, C. K.; Satyalakshmi, K. M.; Craighead, H. G. Appl. Phys. Lett. 2000, 76, 2466–2468. (21) Ballav, N.; Schilp, S.; Zharnikov, M. Angew. Chem., Int. Ed. 2008, 120, 1421–1424.
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