electron beam induced carbonaceous deposition as ... - World Scientific

International Journal of Nanoscience Vol. 10, Nos. 4 & 5 (2011) 935
941 # .c World Scienti¯c Publishing Company DOI: 10.1142/S0219581X11008757

ELECTRON BEAM INDUCED CARBONACEOUS DEPOSITION AS A LOCAL DIELECTRIC FOR CNT CIRCUITS T. VIJAYKUMAR, NARENDRA KURRA and G. U. KULKARNI* Chemistry and Physics of Materials Unit and DST Unit on Nanoscience Jawaharlal Nehru Centre for Advanced Scienti¯c Research, Jakkur PO Bangalore 560 064, India *[email protected] Evaluating the electrical nature of carbon nanotubes (CNTs) from a collection requires establishing electrical contacts across individual CNTs lying on a dielectric layer. In this work, it is shown how a dielectric layer may be inserted underneath a chosen CNT. This has been accomplished by the electron beam induced carbonaceous deposition process in the presence of moisture and residual hydrocarbons present in the SEM chamber. When performed at a CNT location on a Si substrate, the CNT instead of getting buried underneath is found to be lifted on top of the carbonaceous platform, as if due to nonwetting nature of CNT surface. By ¯xing one end of the CNT on the Ag/Si substrate using a Pt deposit and lifting rest of the length to lie on a carbonaceous platform, the I
V data from nanotubes of varying resistances have been collected using conducting AFM. The chosen nanotubes have also been examined by Raman measurements. The method is particularly useful while working with a random collection of nanotubes resulting from a chemical process. Keywords: Electron beam lithography; electron beam induced deposition; carbonaceous deposition; CNT circuits; conducting AFM.

vertical to the substrate.7 In other methods, the CNTs are ¯rst spread on an insulating surface, usually SiO2 /Si, and then located using a microscopy tool to deposit electrodes on a chosen CNT. The latter step is usually achieved by performing electron (or ion) beam induced metal deposition (EBID or FIBID) facilitated by a scanning electron microscope (SEM) or by shadow masking prior to physical vapor deposition.8
11 This article describes a method by which a chosen CNT is placed over a lithographically laid dielectric layer, so as to enable electrical measurement along the length of the tube. The present work is a result of our exploration to study the nature of interaction between the CNT surface and the electron beam induced carbonaceous deposition. The

1. Introduction Carbon nanotubes (CNTs), because of their unique electrical, mechanical and other interesting properties, have been projected as prototype building blocks of nanoscale architectures.1,2 While fabricating CNT circuits, one of the important issues to be dealt with is addressing of a CNT lying on a substrate, produced out of a chemical process, either in pristine form or after functionalisation.2 Similar requirements come up while dealing with CNT collections enriched with metallic or semiconducting tubes.3,4 For nanotubes spread on a substrate, movable micro or AFM cantilevers have served as contacting electrodes.5,6 However, this only provides conduction across the CNT diameter except in such cases where the CNTs have been grown 935

936

T. Vijaykumar, N. Kurra & G. U. Kulkarni

carbonaceous deposition is usually performed by supplying the SEM chamber with a hydrocarbon source such as pump oil from di®usion system.12 It is believed that under the focused electron beam on a substrate, the hydrocarbon molecules undergo complex reactions leading to a carbonaceous deposition.12
15 Although the exact chemical nature is not known, its unique properties have been exploited. It has been used as etch resist for micromachining.12,13 It was also used as a local dielectric in MIM diode fabrication.14 The deposit has been applied to lower the electrical contact resistance and strengthen the mechanical characteristics16
22 of CNT
CNT and CNT
AFM tip interface.23
26 The growth dynamics of the deposit on CNT surface has been studied recently.27 In this article, we have carefully investigated the nature of the interaction between the CNT surface and the carbonaceous deposit while depositing the latter on Ag/Si substrates using e beam in moisture ¯lled chamber. There was no extra source of hydrocarbons in this case, except the residual hydrocarbons present in the vacuum chamber along with water vapor. The behavior of this deposit towards CNT is interesting in that the deposit instead of burying the CNT, caused its lifting. The carbonaceous deposit is quite insulating and its dielectric nature has been studied in detail using conducting AFM. These novel observations led us to construct circuits where electrical measurement along the length of a chosen CNT is possible using CAFM.

2. Experimental Section An n-type silicon wafer (
¼ 4
7
cm) was cleaned by sonicating in acetone followed by a rinse in double distilled water for 2 min. Silver was deposited by physical vapor deposition (Hind Hivac, Bangalore) on Si wafers using a TEM grid as proximity mask such that the resulting grid pattern of Ag served as an address system but with good electrical connectivity across the pattern. Multiwall carbon nanotubes, 10
200 nm in diameter and 5
10 m in length (Sigma Aldrich) were deposited on the Ag/Si substrate by spin coating a 15 L of the dispersant in dichlorobenzene (1 mg/5 mL) at 3000 rpm for 45 s. Electron beam induced carbonaceous deposition was performed using a ¯eld emission Scanning electron microscope (Nova NanoSEM 600, FEI Company) in low vacuum

mode. For working at low vacuum, the pole piece was mounted with low vacuum detector (LVD) and the chamber was ¯lled with water vapor. Typically, a working distance of 3
4 mm between the pole piece and the substrate was used while the water vapor pressure was 0.4 Torr. Selected regions on the substrate were exposed to e beam dosages of 0.7
2.5 C cm
2 at 10 kV in the TV mode. The set up for C-AFM consists of a Multimode scanning force microscope attached to a Nanoscope IV controller (Digital Instruments, USA) and an external multimeter (Keithley 236) as the source and measurement unit for current
voltage characteristics. Au coated tip served as one electrode and the substrate as a second electrode, which was isolated from the scanner by proper insulation. The electrical noise was minimized by proper earthing of the instrument. Raman spectroscopy measurements on CNTs in circuits were carried out using a microRaman spectrometer (Model 25-LHR-151-230, Melles-Griot, USA) with excitation wavelength at 632.8 nm with a laser spot size of 2 m.

3. Results and Discussion The process of carbonaceous deposition is depicted in Fig. 1. In Fig. 1(a) is shown a AFM image of the carbonaceous deposition on a Si substrate obtained by e beam (10 kV) lithographic writing with a dosage of 0.77 C cm
2 in presence of water moisture (0.4 torr), which is shown schematically on top. We have found that the deposition is facile in the environmental mode compared to high vacuum. The thickness of the square platform created (5:5  5:5 m) is  15 nm as revealed by the AFM z-pro¯le. Several such depositions were carried out by varying the e beam dose. Under the deposition conditions employed in this study, the thickness of the carbonaceous platform was found to vary linearly with the e beam dose as shown in the plot in Fig. 1(b). This way, the deposition itself can be ¯ne controlled to produce platforms of desired thicknesses. It appears that the water molecules at 0.4 torr pressure scrub o® residual hydrocarbon species present in vacuum and enable the carbonaceous deposition. The carbonaceous deposit is known to serve as a dielectric, as reported in some studies.14 This aspect has been examined in detail on carbonaceous platforms of di®erent thicknesses using the CAFM set up. On increasing the tip bias, we observed a sudden

Electron Beam Induced Carbonaceous Deposition as a Local Dielectric for CNT Circuits Water vapor Carbon deposit Si SEM chamber

25 20

5 nm

Height (nm)

e-beam

937

15 10 5 0 0.0

1 µm

0.4 0.8 1.2 Area dosage(C cm-2)

60 40

5 µm

20 0 -6 -3 0 V

3

Breakdown field (V.nm-1)

80

(b)

I (µA)

(a)

6

(c)

0.15 Ag/Si

0.10

0.05 5

10 15 Thickness (nm)

20

(d)

Fig. 1. (a) Carbonaceous deposition over a 5:5  5:5 m region. The height pro¯le obtained from AFM is also given. The top schematic illustrates the carbonaceous deposition process. (b) Variation in the height of the deposition with the e dose. (c) The current as measured by the conducting AFM tip in contact with the carbonaceous platform, against the applied bias. The current compliance was set to 80 A. Adjacent AFM image shows rupturing of the carbonaceous platform due to electrical breakdown. (d) Variation of the breakdown ¯eld with the thickness of the carbonaceous platform. The inset shows a schematic diagram of the AFM setup.

increase in current at a certain positive voltage in each case, attributable to circuit breakdown. In the example shown in Fig. 1(c) for a 25 nm thick platform, the breakdown occurred at a tip bias of  þ2:2 V. The breakdown indeed leads to rupturing of the dielectric platform as is apparent from the adjacent AFM image. This process was repeated for platforms of di®erent thicknesses and in each case, the breakdown ¯eld was calculated. The variation of the breakdown ¯eld with the dielectric layer thickness (see Fig. 1(d)) ¯ts an empirical power-law of the form given by, (see Ref. 28) EB ðdÞ ¼ const  d
n The curve ¯tting of data yielded a value of n  0:75  0:01. Typical values for n for dielectric ¯lms are in the range of 0.3
0.5.29 The observed higher value may arise due to nonideal electrode con¯guration (one electrode being AFM tip) as well as due to the possibility of high ¯eld emission from the tip.

Having realized the potential use of carbonaceous deposition as a dielectric, we attempted to build dielectric layers for CNT circuits. While trying to build carbonaceous material around CNTs, we came across an interesting phenomenon, which is detailed in Fig. 2. An individual CNT on Si substrate was chosen (Fig. 2(a)). A square region of carbonaceous deposition was produced as shown in Fig. 2(b) using an e dose of 0.7 C cm
2 . It is rather not di±cult to make out from the image that the CNT is located on top of the deposit, instead of getting buried underneath. Under the deposition conditions employed by us, the carbonaceous deposition seems to have e®ectively lifted the CNT on top. This is somewhat similar to the observation made in the case of 100 nm Fe
Co
Ni nanoparticles, which were welded ¯rmly to the carbonaceous layer beneath.30 In order to have a better understanding, we performed the deposition in two stages sequentially with e dosage of 1.5 C cm
2 (Fig. 2(c)), such that the two deposits formed steps of heights  25 and

T. Vijaykumar, N. Kurra & G. U. Kulkarni

Before CNT

CNT

Si

after

Carbonaceou

Si

s deposit

938

(a)

(b)

100

100 nm

Height(nm)

80 60 40 20 0 0 (c)

5 10 15 20 25 30 35 Location on Nanotube (d)

Fig. 2. (a) SEM image of the CNT on Si substrate. The lifting of the entire CNT during the carbonaceous deposition is shown in (b). (c) AFM image of a CNT lifted by two successive depositions ( 16 nm and 20 nm, respectively). The AFM height pro¯les were collected at the locations marked on the CNT. Three such pro¯les (at locations 4, 9 and 21) are shown as examples. (d) Variation in the height of the CNT from the Ag/Si surface along the CNT length at di®erent locations. The height was calculated by subtracting the diameter of the NT from the height measured using AFM. Scale bar, 2 m.

16 nm, respectively. The height of the CNT from the substrate surface was estimated from the z-pro¯le analysis along its length at di®erent locations (see Fig. 2(c)). Far away from the carbonaceous platforms (at locations marked 1
5), the height measured using AFM pro¯le was around  180 nm corresponding to the CNT diameter itself. This value has been subtracted from z-pro¯les, and the resulting height values are shown in Fig. 2(d) for the di®erent locations. It is interesting to see that, at locations closer to the platform, the height of the CNT is as much as  80 nm well beyond the platform thickness (25 nm). The carbonaceous deposit has not only lifted the CNT above itself, but seems to have brought some deformation in it. The CNT height at further locations drops to  30 nm implying that it eventually landed on the carbonaceous platform but only to be lifted again by the second platform (see Fig. 2(d)). This observation is indeed surprising and to our knowledge, it is for the ¯rst time. As the e beam deposition

takes place focused on the substrate where there is abundance of secondary electrons, the CNT gives way to the process, lifting itself up the deposition. The hydrophobic nature of the pristine CNT surface31 may also play a role here. Unlike the case of nanoparticles,30 the CNTs in our case were found to roll away easily from their original positions even with minimal force of few nanonewtons applied during AFM scans, revealing that the interaction of CNT with the carbonaceous layer is rather weak (see Fig. 3). The above observations prompted us to produce circuits by selectively lifting individual CNTs using the dielectric carbonaceous layer. As the CNT is only loosely bound as noted earlier (see Fig. 3(b)), we followed a method where a desired CNT was ¯rst located and one end of the CNT was ¯xed to Ag/Si substrate by depositing Pt on top from a metalorganic precursor preloaded in the SEM chamber using the EBID process. This formed one electrode

Electron Beam Induced Carbonaceous Deposition as a Local Dielectric for CNT Circuits

939

Carbonaceous platform CNT foot print

4 nm

CNT

Carbonaceous Platform (a)

(b)

Fig. 3. (a) AFM scan leading to rolling of NT away from the carbonaceous platform. The trench width is 180 nm while the depth is 0.6 nm. (b) Schematic showing only the bottom part of CNT contacting the carbonaceous platform, before being rolled away by the AFM tip. Scale bar, 2 m.

for the CNT. The carbonaceous deposition was then carried out (e dose, 0.7 C cm
2 Þ to cover the rest of the length of the CNT so that it got almost entirely lifted on top the carbonaceous platform of 10 nm thick (see Fig. 4), without causing extra deformation. For the two CNTs under consideration, the carbonaceous deposition was carried out such that a 2 m region in the middle was left free for microRaman measurement. The I
V measurement in the case of CNT-i was nearly linear in contrast to the non-linear behavior found in CNT-ii (see Fig. 4(a), indicating that the former was more metallic. In the corresponding Raman spectrum from CNT-i (see Fig. 4(b)), the intensity of the G band around 1560 cm
1 is relatively high compared to that of the D band at 1360 cm
1 . For CNT-ii, the spectrum is broad with D band intensity comparable to the G band. Clearly, the CNT-i is more crystalline32,33 which goes well its electrical nature. Thus, the present

i

technique allows a unique way of determining the electric nature of randomly spread over CNTs on a surface. Although the method employs CAFM to contact the CNT, it is unconventional in that the electrical measurement here refers to the length of the tube lying on the dielectric carbonaceous platform. A note on the nature of interaction between carbonaceous deposit and CNT surface is worthwhile. When a carbon source is deliberately introduced, the deposit produced can wet a carbon nano¯bre surface and clamp it.34 On the other hand, there are several studies which relied only on residual hydrocarbons. The deposit in these cases may come up below as well as on top27 which has been used to stick two CNTs,19 to glue CNTs to electrodes18 and AFM tips,17,25,26 or to clamp and freeze deformation in a CNT, while bending it by a nanomanipilator.20,21 It has been used to clamp CNTs and form junctions.22 In such studies, the e

G-band

ii

I(µ A)

D-band

50

i

0 -2

-1

1

V (V )

2

-50

-100

(a)

ii 1200

1400 1600 Raman Shift (cm-1)

1800

(b)

Fig. 4. (a) I
V characteristics of CNTs i and ii wired into a circuit using CAFM and corresponding AFM images have shown. (b) Raman spectra of the CNTs recorded at the black spots. Scale bar, 3 m.

940

T. Vijaykumar, N. Kurra & G. U. Kulkarni

beam energy is typically above 20 keV. In our study, we have relied on low beam energy (10 keV) in presence of residual hydrocarbons and water vapor, to arrive at optimal conditions for lifting of CNTs. The role of water vapor as reactive environment provides more ionic current and enhances the crosslinking in the deposit. The adsorbed water molecules may not be supporting the growth of carbonaceous deposition on the CNT surface at lower e beam energies. Hence, using this method one can de¯ne local dielectric for CNTs and measure longitudinal electrical transport properties.

4. Conclusion Electron beam induced carbonaceous deposit platforms of controlled thicknesses, up to 30 nm have been produced on Ag coated Si substrates, by varying the e dosages at 10 keV beam energy in the presence of 0.4 torr moisture. Conducting AFM measurements have shown that thus produced carbonaceous platforms could withstand a few volts of tip bias before exhibiting dielectric breakdown. The dewetting nature of the CNT from the carbonaceous surface was exploited in constructing circuits, where the CNT end glued to the Ag/Si substrate using PtEBID formed one contact while the other end lying on the dielectric carbonaceous platform was contacted by the CAFM probe. This way we could evaluate the electrical properties of the CNTs randomly spread over the substrate surface. Importantly, the measurement is performed along the nanotube length, which should be the true representation of its 1D behavior. The method is particularly useful while dealing with semiconducting/ metallic CNTs or in the case of functionalized CNTs. This method may be used to bring controlled deformation in CNTs while causing minimal damage to the surface itself. Preliminary measurements have shown that it is extendable to graphene as well.

Acknowledgments The authors thank Professor C. N. R. Rao, FRS for his encouragement. Support from the Department of Science and Technology, Government of India is gratefully acknowledged. NK acknowledges CSIR for funding. The authors thank Veeco India Nanotechnology Laboratory at JNCASR for providing the AFM facility. NK thanks Selvi for helping in operating FESEM.

References 1. R. H. Baughman, A. A. Zakhidov and W. A. de Heer, Science 297, 787 (2002). 2. C. N. R. Rao and A. Govindaraj, Nanotubes and Nanowires, RSC Nanoscience and Nanotechnology series (Royal Society of Chemistry, Cambridge, 2005). 3. M. S. Arnold, S. I. Stupp and M. C. Hersam, Nano Lett. 5, 713 (2005). 4. R. Voggu, C. S. Rout, A. D. Franklin, T. S. Fisher and C. N. R. Rao, J. Phys. Chem. C 112, 13053 (2008). 5. S. Dohn, K. Mølhave and P. Bøggild, Sensor Lett. 3, 300 (2005). 6. P. J. De Pablo, M. T. Martínez, J. Colchero, J. G. Herrero, W. K. Maser, A. M. de Benito, E. Muñoz and A. M. Baró, Mater. Sci. Eng. C 15, 149 (2001). 7. M. R. Maschmann, A. D. Franklin, A. Scott, D. B. Janes, T. D. Sands and T. S. Fisher, Nano Lett. 6, 2712 (2006). 8. T. W. Ebbesen, H. J. Lezec, H. Hiura, J. W. Bennett, H. F. Ghaemi and T. Thio, Nature 382, 54 (1996). 9. C. Sch€onenberger, A. Bachtold, C. Strunk, J. P. Salvetat and L. Forró, Appl. Phys. A: Mater. Sci. Processing 69, 283 (1999). 10. J. C. Lewenstein, T. P. Burgin, A. Ribayrol, L. A. Nagahara and R. K. Tsui, Nano Lett. 2, 443 (2002). 11. S. Dohn, K. Mølhave, D. N. Madsen, R. Mateiu, P. Bøggild, A. M. Rasmussen, M. Brorson and C. J. H. Jacobsen, Mater. Res. Soc. Symp. Proc. 772, 153 (2003). 12. T. Djenizian, B. Salhi, R. Boukherroub and P. Schmuki, Nanotechnology 17, 5363 (2006). 13. N. Morita, N. Kawasegi and K. Ooi, Nanotechnology 19, 155302 (2008). 14. N. Miura, H. Ishii, J. I. Shirakashi, A. Yamada and M. Konagai, Appl. Surf. Sci. 113{114, 269 (1997). 15. I. C. Chen, L. H. Chen, C. Orme, A. Quist, R. Lal and S. Jin, Nanotechnology 17, 4322 (2006). 16. A. Bachtold, M. Henny, C. Terrier, C. Strunk, C. Sch€onenberger, J. P. Salvetat, J. M. Bonard and L. Forró, Appl. Phys. Lett. 73, 274 (1998). 17. Q. Chen, S. Wang and L.-M. Peng, Nanotechnology 17, 1087 (2006). 18. P. Rice, T. M. Wallis, S. E. Russek and P. Kabos, Nano Lett. 7, 1086 (2007). 19. Y. G. Wang, T. H. Wang, X. W. Lin and V. P. Dravid, Nanotechnology 17, 6011 (2006). 20. M. Wang, J. Wang, Q. Chen and L.-M. Peng, Adv. Funct. Mater. 15, 1825 (2005). 21. M. Wang, L.-M. Peng, J. Wang and Q. Chen, Adv. Funct. Mater. 16, 1462 (2006). 22. F. Banhart, Nano Lett. 6, 329 (2001).

Electron Beam Induced Carbonaceous Deposition as a Local Dielectric for CNT Circuits

23. T. Fujii et al., J. Vac. Sci. Technol. B 9, 666 (1991). 24. H. W. P. Koops, J. Kretz, M. Rudolph, M. Weber, G. Dahm and K. L. Lee, Jpn. J. Appl. Phys. Part 1 33, 7099 (1994). 25. M.-F. Yu, O. Lourie, M. J. Dyer, K. Moloni, T. F. Kelly and R. S. Ruo®, Science 287, 637 (2000). 26. S. Kim, J. Kim, M. Berg and A. de Lozanne, Rev. Sci. Instrum. 79, 103702 (2008). 27. K. Rykaczewski, M. R. Henry and A. G. Fedorov, Appl. Phys. Lett. 95, 113112 (2009). 28. J. J. O'Dwyer, The Theory of Electrical Conduction and Breakdown in Solid Dielectrics (Clarendon: Oxford, 1973).

941

29. A. D. Milliken, A. J. Bell and J. F. Scott, Appl. Phys. Lett. 90, 112910 (2007). 30. A. V. Moskalenko, D. J. Burbridge, G. Viau and S. N. Gordeev, Nanotechnology 18, 025304 (2007). 31. B. A. Kakade and V. K. Pillai, J. Phys. Chem. C 112, 3183 (2008). 32. R. A. DiLeo, B. J. Landi and R. P. Ra®aelle, J. Appl. Phys. 101, 064307 (2007). 33. S. R. C. Vivekchand, A. Govindaraj, M. M. Seikh and C. N. R. Rao, J. Phys. Chem. B 108, 6935 (2004). 34. W. Ding, D. A. Dikin, X. Chen, R. D. Piner, R. S. Ruo®, E. Zussman, X. Wang and X. J. Li, Appl. Phys. 98, 014905 (2005).