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Electron beam lithography in passivated gold nanoclusters
Microelectronic Engineering 57–58 (2001) 837–841 www.elsevier.com / locate / mee
Electron beam lithography in passivated gold nanoclusters a a, b T.R. Bedson , R.E. Palmer *, J.P. Wilcoxon a
Nanoscale Physics Research Laboratory, School of Physics and Astronomy and The University of Birmingham, Edgbaston, Birmingham B15 2 TT, UK b Nanostructures and Advanced Materials Department, Sandia National Laboratories, Albuquerque, NM 87185, USA Abstract We have employed thin films of passivated gold nanoclusters, deposited from solution onto a range of surfaces (graphite, silicon, thermally grown silicon dioxide and sputtered silicon dioxide), as a negative tone electron beam resists. The best resolution achieved to date is 26 nm. Response curves obtained for monolayer films on the SiO 2 surfaces indicate that the sensitivity depends on the substrate, attributed to backscattering of the primary beam (and secondary electron generation), as confirmed by Monte Carlo simulations. 2001 Elsevier Science B.V. All rights reserved. Keywords: Electron beam lithography; Direct writing; Passivated metal clusters
The production of metallic nanostructures via electron beam writing is attractive because of the prospect of reducing the number of individual stages in the fabrication process. Passivated metal nanoclusters [1] are small (diameter 1–10 nm), suggesting high resolution performance and the metal loading is high compared with other organometallics [2], so that carbon contamination may be reduced. The technique of 40 kV electron beam writing in multilayers of passivated gold clusters has been used to create structures that demonstrate Coulomb blockade conduction characteristics at room temperature [3]. In this case the carbon contamination arising from the exposed ligands was exploited to create a series of tunneling gaps between metal islands in 100 nm wide structures. Structures with metallic conductivity have also been created via 100 kV e-beam writing and an additional annealing stage [4]. Increasing interest [5] in low energy e-beam lithography arises both because of the possible reduction of the proximity effect and the constraints imposed by miniaturisation of e-beam columns for parallel writing. We have recently reported electron beam lithography studies of passivated gold nanoclusters at 6 kV [6–8]. Here we discuss a systematic study of the response curves obtained at 6 kV, demonstrating average linewidths as small as 26 nm and deriving quantitative values for the sensitivity of the films. The passivated gold clusters were synthesized by an inverse micelle method described elsewhere [9]. The 4-nm gold cores are surrounded by a self-assembled monolayer of hexadecanethiol * Corresponding author. Fax: 1 44-121-414-7327. E-mail address: [email protected] (R.E. Palmer). 0167-9317 / 01 / $ – see front matter PII: S0167-9317( 01 )00562-7
2001 Elsevier Science B.V. All rights reserved.
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T.R. Bedson et al. / Microelectronic Engineering 57 – 58 (2001) 837 – 841
(C 16 H 33 S) ligands, giving a particle diameter of | 6 nm. The clusters were deposited from solution onto the clean surfaces of various substrates: highly oriented pyrolitic graphite (HOPG), silicon (with the native oxide layer intact), thermally grown silicon dioxide and sputtered silicon dioxide. Thin films of monolayer coverage (i.e. thickness | 6 nm) were calibrated by ellipsometry measurements to establish the film thickness [10]. The samples were introduced into a scanning electron microscope (SEM) equipped with an electron beam lithography system (Raith Elphy Quantum) for pattern generation. At 6 kV the SEM has a beam current of 6.5 pA and a spot size of less than 12 nm. The spot size obviously limits the ultimate resolution of the writing process. The pattern employed consisted of parallel (single pixel) lines 4 mm in length, spaced 200 nm apart. Each line of the pattern was assigned a different dose, ranging from 2.7 3 10 3 to 1.45 3 10 6 mC cm 22 . Following exposure and removal from the SEM the unexposed clusters were rinsed away by immersing in octane while in an ultrasonic bath. The lines produced by e-beam writing were then examined with an atomic force microscope (AFM). The width of the lines produced by e-beam writing in passivated nanocluster films of approximately monolayer coverage on thermally oxidised Si(100) substrate is found to increase with electron dose, as illustrated in Fig. 1. The average linewidth of the upper line in Fig. 1, which received the lowest dose, is 26 nm. Response curves describing the exposure for the monolayer films of passivated gold nanoclusters on sputtered and thermally grown SiO 2 is shown in Fig. 2. which plots the width of the exposed lines as a function of electron beam dose [7]. The threshold for electron beam writing (obtained from the onset of the response curves) is 2900 mC cm 22 in the case of sputtered SiO 2 , about half the value obtained for thermally grown SiO 2 , 5400 mCcm 22 . In addition, it is evident (from Fig. 2) that the linewidth increases at a much faster rate after the onset of lithography in the case of the sputtered SiO 2 , while saturation of the linewidth on sputtered SiO 2 starts at | 500 nm compared with
Fig. 1. AFM image showing a series of lines produced by e-beam writing at 6 kV after removal of the passivated gold clusters. The e-beam doses employed to create the four lines were (top to bottom) 11.7 3 10 3 , 22.3 3 10 3 , 45 3 10 3 and 90 3 10 3 mC cm 22 .
T.R. Bedson et al. / Microelectronic Engineering 57 – 58 (2001) 837 – 841
839
Fig. 2. Response curves for electron beam exposure of thin films ( | 6 nm) of passivated gold nanoclusters supported on both sputtered and thermally grown silicon dioxide.
| 100 nm in the case of thermal SiO 2 . Note however, complete saturation was not observed in either 22 case, even at the highest doses used (28.5 C cm ). We believe this behavior is due to the ability of the weakly bound nanoclusters to diffuse across the surface to the region where the line is being fabricated, thus providing a supply of material for further growth of the structure. The quantitative differences between the response curves for passivated nanoclusters on the sputtered and thermal SiO 2 substrates demonstrate that the substrate plays a key role in the mechanism of monolayer resist exposure. We attribute this effect to the scattering of the primary beam electrons in the substrate and the subsequent emergence of backscattered or secondary electrons in the vicinity of the initially irradiated region, with consequent exposure of the passivated nanocluster film. It seems probable that low energy secondary electrons generated both by the primary beam and the backscattered electrons which actually expose the passivated nanoclusters, via dissociation [11] or desorption of the ligands. Monte Carlo simulations [12] of the scattering process were undertaken for both types of SiO 2 substrate employed, with and without the passivated gold nanocluster film on the surface. The cluster film was modelled as a continuum with the calculated average atomic number, atomic weight and density values of the film. Fig. 3 illustrates two of the many simulations undertaken. The scattering width obtained, for thermal SiO 2 , 0.95 mm (Fig. 3a) compares with a value of 4 mm for the sputtered oxide (Fig. 3b). All the simulations were repeated for both thermal and sputtered SiO 2 using identical thicknesses of clusters and substrates. All maintain the difference in the scattering widths obtained i.e. thermal SiO 2 always demonstrates the narrower scattering width. The 23 increased scattering evident in sputtered SiO 2 reflects the reduced density ( | 0.43 g cm ) compared 23 with thermally grown SiO 2 (1.73 g cm ) which increases the mean free path for the electrons. This enhanced scattering explains both the lower threshold dose for e-beam writing on sputtered SiO 2 and the steeper slope of the response curve (Fig. 2).
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T.R. Bedson et al. / Microelectronic Engineering 57 – 58 (2001) 837 – 841
Fig. 3. Monte Carlo simulations of a 6-kV electron beam scattering in films of passivated gold nanoclusters on SiO 2 substrates. (a) Thin ( | 6 nm) cluster film on a thermal SiO 2 layer (65 nm thick) on bulk silicon and (b) a thin ( | 6 nm) cluster film on a sputtered SiO 2 substrate. Note the change of scale between (a) and (b). A total of 1000 trajectories are shown in each case.
The role of substrate electron scattering in the exposure of the thin ( | monolayer) passivated nanocluster films is confirmed by the experimental results for graphite and silicon (with native oxide) substrates. In particular, we were not able to write lines in monolayer cluster films on either of these substrates. This is consistent with the much reduced secondary electron coefficients, d, for these materials (1 for graphite and 1.1 for silicon [13]) compared with SiO 2 (where d lies between 2.1 and
T.R. Bedson et al. / Microelectronic Engineering 57 – 58 (2001) 837 – 841
841
4). Note that for most materials there is a maximum in the secondary electron emission for primary beam energies in the range 250–400 eV [14]. Thus, the sensitivity to e-beam exposure for the passivated gold nanocluster monolayers may possibly be highest in low energy electron beam lithography. In summary, we have investigated the exposure of thin ( | monolayer) films of passivated gold nanocluster (diameter | 6 nm) in direct low voltage electron beam writing experiments at 6 kV. A minimum linewidth of 26 nm resolution has been obtained so far. We conclude that electrons generated in the substrate dominate the exposure of monolayer cluster films. Thus both the sensitivity and the resolution obtained with such nanocluster films depends sensitively on the cluster / substrate combination. The fact that the response of the films to e-beam exposure must be determined in part by the electron scattering in both the metallic cluster core and the organic ligands (i.e. as well as the substrate), it may be possible to design nanoclusters for optimum performance in direct e-beam writing of metallic nanoclusters. Acknowledgements We thank Peter Nellist for useful discussions and Andrew Parker and Phil Buckle (Malvern) for supplying SiO 2 substrates. This work was supported by the EPSRC, ERDF and HEFC. TRB is grateful to the EPSRC and the University of Birmingham for financial support. References [1] P.J. Durston, J. Schmidt, R.E. Palmer, J.P. Wilcoxon, Appl. Phys. Lett. 71 (1997) 2940; P.J. Durston, J. Schmidt, R.E. Palmer, J.P. Wilcoxon, Appl. Phys. Lett. 72 (1998) 176; A.J. Parker, P.A. Childs, R.E. Palmer, M. Brust, Appl. Phys. Lett. 74 (1999) 2833; H. Osman, J. Schmidt, Y. Shigeta, K. Svensson, R.E. Palmer, Chem. Phys. Lett. (2000) in press. [2] H.G. Craighead, L.M. Schiavone, Appl. Phys. Lett. 48 (1986) 1748. [3] L. Clarke, M.N. Wybourne, M. Yan, S.X. Cai, J.F.W. Keana, Appl. Phys. Lett. 71 (1997) 617; L. Clarke, M.N. Wybourne, M. Yan, S.X. Cai, L.O. Brown, J. Hutchinson, J.F.W. Keana, J. Vac. Sci. Technol. B 15 (1997) 2925. [4] M.T. Reetz, M. Winner, J. Am. Chem. Soc. 119 (1997) 4539; J. Lohau, S. Friedrichowski, G. Dumpich, E.F. Wassermann, M. Winner, M.T. Reetz, J. Vac. Sci. Technol. B 16 (1998) 77. [5] A. Olkhovets, H.G. Craighead, J. Vac. Sci. Technol. B 17 (1999) 1366; C.K. Harnett, K.M. Satyalakshmi, H.G. Craighead, Appl. Phys. Lett 76 (2000) 2466. [6] T.R. Bedson, P.D. Nellist, R.E. Palmer, J.P. Wilcoxon, Microelectron. Eng. 53 (2000) 187. [7] T.R. Bedson, T.E. Jenkins, D.J. Hayton, J.P. Wilcoxon, R.E. Palmer, Appl. Phys. Lett. (2001) in press. [8] T.R. Bedson, R.E. Palmer, J.P. Wilcoxon, Appl. Phys. Lett. (2001) in press. [9] J.P. Wilcoxon, R.L. Williamson, R.J. Baughman, J. Chem. Phys 98 (1993) 9993. [10] T.E. Jenkins, D.J. Hayton, T.R. Bedson, R.E. Palmer, J.P. Wilcoxon, J. Phys. D (2001) submitted. [11] C. Olsen, P.A. Rowntree, J. Chem. Phys. 108 (1998) 3750. [12] Monte carlo simulation software by Kimio Kanda (Hitachi) using algorithms developed by D.C. Joy (Oak Ridge National Laboratory). Downloadable from http: / / www.nsctoronto.com [13] CRC, CRC Handbook of Chemistry and Physics, 81st Edition, CRC Press, Boca Raton, FL, 2000. [14] I.M. Watt, Chapter 1 The Principle and Practice of Electron Microscopy, 2nd Edition, Cambridge University Press, Cambridge, 1997, p. 6.
Electron beam lithography in passivated gold nanoclusters a a, b T.R. Bedson , R.E. Palmer *, J.P. Wilcoxon a
Nanoscale Physics Research Laboratory, School of Physics and Astronomy and The University of Birmingham, Edgbaston, Birmingham B15 2 TT, UK b Nanostructures and Advanced Materials Department, Sandia National Laboratories, Albuquerque, NM 87185, USA Abstract We have employed thin films of passivated gold nanoclusters, deposited from solution onto a range of surfaces (graphite, silicon, thermally grown silicon dioxide and sputtered silicon dioxide), as a negative tone electron beam resists. The best resolution achieved to date is 26 nm. Response curves obtained for monolayer films on the SiO 2 surfaces indicate that the sensitivity depends on the substrate, attributed to backscattering of the primary beam (and secondary electron generation), as confirmed by Monte Carlo simulations. 2001 Elsevier Science B.V. All rights reserved. Keywords: Electron beam lithography; Direct writing; Passivated metal clusters
The production of metallic nanostructures via electron beam writing is attractive because of the prospect of reducing the number of individual stages in the fabrication process. Passivated metal nanoclusters [1] are small (diameter 1–10 nm), suggesting high resolution performance and the metal loading is high compared with other organometallics [2], so that carbon contamination may be reduced. The technique of 40 kV electron beam writing in multilayers of passivated gold clusters has been used to create structures that demonstrate Coulomb blockade conduction characteristics at room temperature [3]. In this case the carbon contamination arising from the exposed ligands was exploited to create a series of tunneling gaps between metal islands in 100 nm wide structures. Structures with metallic conductivity have also been created via 100 kV e-beam writing and an additional annealing stage [4]. Increasing interest [5] in low energy e-beam lithography arises both because of the possible reduction of the proximity effect and the constraints imposed by miniaturisation of e-beam columns for parallel writing. We have recently reported electron beam lithography studies of passivated gold nanoclusters at 6 kV [6–8]. Here we discuss a systematic study of the response curves obtained at 6 kV, demonstrating average linewidths as small as 26 nm and deriving quantitative values for the sensitivity of the films. The passivated gold clusters were synthesized by an inverse micelle method described elsewhere [9]. The 4-nm gold cores are surrounded by a self-assembled monolayer of hexadecanethiol * Corresponding author. Fax: 1 44-121-414-7327. E-mail address: [email protected] (R.E. Palmer). 0167-9317 / 01 / $ – see front matter PII: S0167-9317( 01 )00562-7
2001 Elsevier Science B.V. All rights reserved.
838
T.R. Bedson et al. / Microelectronic Engineering 57 – 58 (2001) 837 – 841
(C 16 H 33 S) ligands, giving a particle diameter of | 6 nm. The clusters were deposited from solution onto the clean surfaces of various substrates: highly oriented pyrolitic graphite (HOPG), silicon (with the native oxide layer intact), thermally grown silicon dioxide and sputtered silicon dioxide. Thin films of monolayer coverage (i.e. thickness | 6 nm) were calibrated by ellipsometry measurements to establish the film thickness [10]. The samples were introduced into a scanning electron microscope (SEM) equipped with an electron beam lithography system (Raith Elphy Quantum) for pattern generation. At 6 kV the SEM has a beam current of 6.5 pA and a spot size of less than 12 nm. The spot size obviously limits the ultimate resolution of the writing process. The pattern employed consisted of parallel (single pixel) lines 4 mm in length, spaced 200 nm apart. Each line of the pattern was assigned a different dose, ranging from 2.7 3 10 3 to 1.45 3 10 6 mC cm 22 . Following exposure and removal from the SEM the unexposed clusters were rinsed away by immersing in octane while in an ultrasonic bath. The lines produced by e-beam writing were then examined with an atomic force microscope (AFM). The width of the lines produced by e-beam writing in passivated nanocluster films of approximately monolayer coverage on thermally oxidised Si(100) substrate is found to increase with electron dose, as illustrated in Fig. 1. The average linewidth of the upper line in Fig. 1, which received the lowest dose, is 26 nm. Response curves describing the exposure for the monolayer films of passivated gold nanoclusters on sputtered and thermally grown SiO 2 is shown in Fig. 2. which plots the width of the exposed lines as a function of electron beam dose [7]. The threshold for electron beam writing (obtained from the onset of the response curves) is 2900 mC cm 22 in the case of sputtered SiO 2 , about half the value obtained for thermally grown SiO 2 , 5400 mCcm 22 . In addition, it is evident (from Fig. 2) that the linewidth increases at a much faster rate after the onset of lithography in the case of the sputtered SiO 2 , while saturation of the linewidth on sputtered SiO 2 starts at | 500 nm compared with
Fig. 1. AFM image showing a series of lines produced by e-beam writing at 6 kV after removal of the passivated gold clusters. The e-beam doses employed to create the four lines were (top to bottom) 11.7 3 10 3 , 22.3 3 10 3 , 45 3 10 3 and 90 3 10 3 mC cm 22 .
T.R. Bedson et al. / Microelectronic Engineering 57 – 58 (2001) 837 – 841
839
Fig. 2. Response curves for electron beam exposure of thin films ( | 6 nm) of passivated gold nanoclusters supported on both sputtered and thermally grown silicon dioxide.
| 100 nm in the case of thermal SiO 2 . Note however, complete saturation was not observed in either 22 case, even at the highest doses used (28.5 C cm ). We believe this behavior is due to the ability of the weakly bound nanoclusters to diffuse across the surface to the region where the line is being fabricated, thus providing a supply of material for further growth of the structure. The quantitative differences between the response curves for passivated nanoclusters on the sputtered and thermal SiO 2 substrates demonstrate that the substrate plays a key role in the mechanism of monolayer resist exposure. We attribute this effect to the scattering of the primary beam electrons in the substrate and the subsequent emergence of backscattered or secondary electrons in the vicinity of the initially irradiated region, with consequent exposure of the passivated nanocluster film. It seems probable that low energy secondary electrons generated both by the primary beam and the backscattered electrons which actually expose the passivated nanoclusters, via dissociation [11] or desorption of the ligands. Monte Carlo simulations [12] of the scattering process were undertaken for both types of SiO 2 substrate employed, with and without the passivated gold nanocluster film on the surface. The cluster film was modelled as a continuum with the calculated average atomic number, atomic weight and density values of the film. Fig. 3 illustrates two of the many simulations undertaken. The scattering width obtained, for thermal SiO 2 , 0.95 mm (Fig. 3a) compares with a value of 4 mm for the sputtered oxide (Fig. 3b). All the simulations were repeated for both thermal and sputtered SiO 2 using identical thicknesses of clusters and substrates. All maintain the difference in the scattering widths obtained i.e. thermal SiO 2 always demonstrates the narrower scattering width. The 23 increased scattering evident in sputtered SiO 2 reflects the reduced density ( | 0.43 g cm ) compared 23 with thermally grown SiO 2 (1.73 g cm ) which increases the mean free path for the electrons. This enhanced scattering explains both the lower threshold dose for e-beam writing on sputtered SiO 2 and the steeper slope of the response curve (Fig. 2).
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T.R. Bedson et al. / Microelectronic Engineering 57 – 58 (2001) 837 – 841
Fig. 3. Monte Carlo simulations of a 6-kV electron beam scattering in films of passivated gold nanoclusters on SiO 2 substrates. (a) Thin ( | 6 nm) cluster film on a thermal SiO 2 layer (65 nm thick) on bulk silicon and (b) a thin ( | 6 nm) cluster film on a sputtered SiO 2 substrate. Note the change of scale between (a) and (b). A total of 1000 trajectories are shown in each case.
The role of substrate electron scattering in the exposure of the thin ( | monolayer) passivated nanocluster films is confirmed by the experimental results for graphite and silicon (with native oxide) substrates. In particular, we were not able to write lines in monolayer cluster films on either of these substrates. This is consistent with the much reduced secondary electron coefficients, d, for these materials (1 for graphite and 1.1 for silicon [13]) compared with SiO 2 (where d lies between 2.1 and
T.R. Bedson et al. / Microelectronic Engineering 57 – 58 (2001) 837 – 841
841
4). Note that for most materials there is a maximum in the secondary electron emission for primary beam energies in the range 250–400 eV [14]. Thus, the sensitivity to e-beam exposure for the passivated gold nanocluster monolayers may possibly be highest in low energy electron beam lithography. In summary, we have investigated the exposure of thin ( | monolayer) films of passivated gold nanocluster (diameter | 6 nm) in direct low voltage electron beam writing experiments at 6 kV. A minimum linewidth of 26 nm resolution has been obtained so far. We conclude that electrons generated in the substrate dominate the exposure of monolayer cluster films. Thus both the sensitivity and the resolution obtained with such nanocluster films depends sensitively on the cluster / substrate combination. The fact that the response of the films to e-beam exposure must be determined in part by the electron scattering in both the metallic cluster core and the organic ligands (i.e. as well as the substrate), it may be possible to design nanoclusters for optimum performance in direct e-beam writing of metallic nanoclusters. Acknowledgements We thank Peter Nellist for useful discussions and Andrew Parker and Phil Buckle (Malvern) for supplying SiO 2 substrates. This work was supported by the EPSRC, ERDF and HEFC. TRB is grateful to the EPSRC and the University of Birmingham for financial support. References [1] P.J. Durston, J. Schmidt, R.E. Palmer, J.P. Wilcoxon, Appl. Phys. Lett. 71 (1997) 2940; P.J. Durston, J. Schmidt, R.E. Palmer, J.P. Wilcoxon, Appl. Phys. Lett. 72 (1998) 176; A.J. Parker, P.A. Childs, R.E. Palmer, M. Brust, Appl. Phys. Lett. 74 (1999) 2833; H. Osman, J. Schmidt, Y. Shigeta, K. Svensson, R.E. Palmer, Chem. Phys. Lett. (2000) in press. [2] H.G. Craighead, L.M. Schiavone, Appl. Phys. Lett. 48 (1986) 1748. [3] L. Clarke, M.N. Wybourne, M. Yan, S.X. Cai, J.F.W. Keana, Appl. Phys. Lett. 71 (1997) 617; L. Clarke, M.N. Wybourne, M. Yan, S.X. Cai, L.O. Brown, J. Hutchinson, J.F.W. Keana, J. Vac. Sci. Technol. B 15 (1997) 2925. [4] M.T. Reetz, M. Winner, J. Am. Chem. Soc. 119 (1997) 4539; J. Lohau, S. Friedrichowski, G. Dumpich, E.F. Wassermann, M. Winner, M.T. Reetz, J. Vac. Sci. Technol. B 16 (1998) 77. [5] A. Olkhovets, H.G. Craighead, J. Vac. Sci. Technol. B 17 (1999) 1366; C.K. Harnett, K.M. Satyalakshmi, H.G. Craighead, Appl. Phys. Lett 76 (2000) 2466. [6] T.R. Bedson, P.D. Nellist, R.E. Palmer, J.P. Wilcoxon, Microelectron. Eng. 53 (2000) 187. [7] T.R. Bedson, T.E. Jenkins, D.J. Hayton, J.P. Wilcoxon, R.E. Palmer, Appl. Phys. Lett. (2001) in press. [8] T.R. Bedson, R.E. Palmer, J.P. Wilcoxon, Appl. Phys. Lett. (2001) in press. [9] J.P. Wilcoxon, R.L. Williamson, R.J. Baughman, J. Chem. Phys 98 (1993) 9993. [10] T.E. Jenkins, D.J. Hayton, T.R. Bedson, R.E. Palmer, J.P. Wilcoxon, J. Phys. D (2001) submitted. [11] C. Olsen, P.A. Rowntree, J. Chem. Phys. 108 (1998) 3750. [12] Monte carlo simulation software by Kimio Kanda (Hitachi) using algorithms developed by D.C. Joy (Oak Ridge National Laboratory). Downloadable from http: / / www.nsctoronto.com [13] CRC, CRC Handbook of Chemistry and Physics, 81st Edition, CRC Press, Boca Raton, FL, 2000. [14] I.M. Watt, Chapter 1 The Principle and Practice of Electron Microscopy, 2nd Edition, Cambridge University Press, Cambridge, 1997, p. 6.