NANO LETTERS
High-Yield Selective Placement of Carbon Nanotubes on Pre-Patterned Electrodes
2002 Vol. 2, No. 5 443-446
Justin C. Lewenstein,† Timothy P. Burgin,† Aline Ribayrol,‡ Larry A. Nagahara,† and Raymond K. Tsui*,† Physical Sciences Research Laboratories, Motorola Labs, 7700 South RiVer Parkway, Tempe, Arizona 85284, USA, and Espace Technologique, Saint Aubin, 91193 Gif-sur-YVette, France Received December 4, 2001; Revised Manuscript Received January 31, 2002
ABSTRACT Practical utilization of carbon nanotubes (CNTs) in nanoscale devices requires their directed placement on a substrate. Controlled placement has been achieved by a combination of the highly selective adsorption of single walled CNTs (SWNTs) onto open regions of amino-functionalized SiO2 in a polymeric resist, followed by liftoff. Careful selection of the surfactant used to suspend the SWNTs as well as other processing parameters allow controlled placement in high yield. This technique may provide a path for the large-scale fabrication of CNT-based devices and circuits.
Introduction. Research on carbon nanotubes (CNTs) is generating increasing interest in the field of nanotechnology. Their unique physical, electrical, and mechanical properties make them promising candidates for components in nanoscale devices.1 To create such devices in a practical manner, it is necessary to control the placement and orientation of CNTs on a surface. However, there have been relatively few literature reports of techniques for selectively placing isolated CNTs in a controlled manner on multiple sites over a large surface.2-5 Burghard et al. first reported that CNTs suspended in a solvent will adsorb preferentially to aminopropylsiloxane (APS)-derivitized SiO2 surfaces.2 More recently, Choi et al. reported selective placement of large numbers of SWNTs into nanolithographically defined regions of APS and subsequent deposition of electrodes to contact the SWNTs.5 An alternative methodology is to selectively place the SWNTs on a substrate with pre-patterned electrodes. This approach avoids the potential problem of introducing defects and contaminants that could arise in patterning electrodes after SWNT placement. It also allows for rapid screening of the placement effectiveness and electrical characterization of the SWNTs. This work reports the influence of various experimental factors on selective placement. The parameters investigated include the type of surfactant used to suspend the SWNTs and the configuration of the pre-patterned electrodes. * To whom correspondence should be addressed. E-mail: ray.tsui@ motorola.com. † Motorola Labs, USA. ‡ Motorola Labs, France. 10.1021/nl015690z CCC: $22.00 Published on Web 04/17/2002
© 2002 American Chemical Society
Experimental Details. Lightly n-doped Si (100) wafers with 200 nm of thermal SiO2 were used as substrates for electrode fabrication. Figure 1a shows a scanning electron microscope (SEM) image of one of the electrode structures in the array. Patterning via electron-beam lithography was carried out using a positive bi-layer resist (PMGI/UV3) that provided an undercut to facilitate metal liftoff. Reactive ion etching (CHF3) was used to create recessed regions in the exposed SiO2 surface. These depressions were filled with evaporated Ti/Au (nominally, 5 nm Ti and 45 nm Au) to make each electrode nearly coplanar with the SiO2 surface. These electrodes were nominally 150 nm wide. After metal liftoff, standard optical lithography and metal deposition were used to create large Au (nonplanar) contact pads to connect to the smaller electrodes. Another layer of resist (PMMA) was used as a mask for formation of the APS self-assembled monolayer (SAM) and subsequent SWNT placement. Small (0.4 × 2 µm2) openings or “trenches” were developed in the resist to expose the electrodes (Figure 1b). Residual contaminates were removed by a 60 s exposure to an O2 plasma, increasing the dimensions of the trenches by ∼0.2 µm. As an additional cleaning measure prior to APS formation, the substrates were exposed to an Ar plasma for 60 s. The samples were immediately immersed in a solution of aminopropyltriethoxysilane [0.5 vol % in ultrapure (UP, 18 MΩ‚cm) water] at room temperature.6 After 30 min they were rinsed with UP water and spun dry. The samples were then soaked in an aqueous suspension of SWNTs for 2 h, rinsed with
Figure 2. AFM images showing a trench in the PMMA in which the selective SWNT placement procedure has been successfully carried out, before (a) and after (b) the patterning resist was lifted off. The amino-functionalized region can be seen in (b) with a contrast different from the surrounding SiO2 surface. SWNTs so placed remain in position after liftoff. The SWNT at the bottom of the trench is the only one that bridges at least two electrodes. In fact, it electrically connects all but the leftmost of the four electrodes.
Figure 1. SEM image of a typical electrode structure in the array is shown in (a). The dashed line indicates the region to be patterned and amino-functionalized for the selective placement of SWNTs, using the procedure shown in (b). The process involves patterning via electron-beam lithography to expose areas for chemical functionalization (i), formation of the APS SAM and then SWNT deposition (ii), and finally resist liftoff to remove SWNTs that are not attached to the APS (iii). For clarity, the pre-patterned electrodes are not shown in (b).
UP water and spun dry. All glassware was rigorously cleaned with piranha solution (3:1 H2SO4:H2O2) prior to use. The starting material for the SWNT suspensions was bucky paper7 filtered from toluene suspended SWNTs formed by laser ablation.8 The bucky paper was sonicated (Fisher Scientific FS15 ultrasonic cleaner) in UP water with 1% surfactant [Triton X-100 (TX-100) or sodium dodecyl sulfate (SDS)] until it went completely into suspension (2 d). The suspension was purified by three successive centrifugation steps (Eppendorf 5417C, 14 000 rpm, 99 min) to remove the amorphous carbon and other contaminants such as metallic catalyst particles present in the as-received SWNTs. For the results obtained in these experiments, SWNT suspensions were several months old. The starting concentration of SWNTs before centrifugation was 0.1 mg/mL. Removal of the patterned PMMA was carried out via Soxhlet extraction with acetone or methylene chloride for 24 h.9 Resist removal, however, was not a prerequisite for measuring the electrical properties of the SWNTs. Electrode structures in the arrays were probed (Micromanipulator Company 6000 probe station, HP 4145B semi444
conductor parameter analyzer) to determine locations where they were electrically connected. Locations of interest were imaged by atomic force microscopy (AFM, Digital Instruments DI 3100) to confirm SWNT placement.10 Results and Discussion. Figure 2a shows an AFM image of a trench in the PMMA, exposing a region over four electrodes in which SWNTs have been selectively placed by the method just described. The AFM image in Figure 2b shows the same area after liftoff of the PMMA. Typically, there were very few SWNTs deposited on the resist and most were removed during liftoff. The only exceptions were those partially attached to the APS (e.g., upper right corner in Figure 2b). The majority of the SWNTs had diameters of 1.6 nm or less, as determined by AFM height measurements. This suggests that the SWNTs were deposited as individual tubes or small bundles. Samples that underwent selective placement using TX100 suspended SWNTs produced an electrical yield that ranged from 20% to 55%. We defined the electrical yield as the probability that a SWNT would electrically bridge the two innermost electrodes (Figure 2). The yield, of course, would be higher if all combinations of electrode pairs were measured. In fact, random inspection of these trenches by AFM indicated the placement yield, (i.e., the chance of finding a SWNT with or without contact to the electrodes) was 100%. This is encouraging because it allows us flexibility in altering process parameters to improve the electrical yield and alignment of the SWNT. It also suggests that the technique could become a viable method for the large-scale fabrication of CNT-based devices and circuits. The choice of surfactant used to suspend the SWNTs is critical to the effectiveness of the selective placement procedure.11 TX-100 suspensions gave dramatically higher placement yields than those prepared with SDS despite a decade reduction in interaction time relative to published Nano Lett., Vol. 2, No. 5, 2002
placement techniques (Electrical yields: TX-100, 20-50%, 2 h; SDS, ∼5%, 10 h).5 The selectivity of the placement is also very high, evidenced by the fact that TX-100 suspended SWNTs did not adhere to the resist. In the absence of an APS monolayer, no adhesion of the SWNTs onto openings containing the electrodes was observed. The four-electrode structure was designed with an electrode spacing ranging from 150 to 300 nm. We found that the more closely spaced electrode configurations had a higher electrical yield. We believe this is because a SWNT of a given length will be more likely to contact two or more electrodes if these are more closely spaced. From control samples, we found by AFM that 70% of the deposited SWNTs had lengths of 500 nm or less. These short lengths are one reason there was a substantial difference between the electrical yield and the placement yield. The height to which the electrodes extend above the SiO2 surface is also important. We did not see significant improvements in yield until quasi-planar electrodes embedded in the SiO2 surface (height difference of ∼9 nm) were used. Nonplanar electrodes can act as a physical barrier to the approach of the SWNTs to the APS/SiO2 surface. An added benefit of quasi-planar electrodes is that they are less likely to cause bending in a SWNT and affect its electrical properties.12 Another factor that influenced selective placement yield was the purity of the SWNT suspension. The suspensions should be as clean as possible to prevent carbonaceous contaminants from competing with the SWNTs for amine binding sites on the surface of the APS SAM. Virtually all of the SWNTs examined by AFM remained after resist liftoff, indicating a strong binding force between the APS SAM and the SWNTs. The presence of multiple SWNTs in many trenches suggests that narrowing the trench should provide a reasonable yield of single, well-aligned SWNTs across the electrodes. Current versus bias voltage (I-V) measurements at room temperature were used to evaluate the electrical properties of the selectively placed SWNTs, with the bias being applied to the pre-patterned Au electrodes. A majority of the SWNTs showed no variation in conductance in response to changes in Vg, the voltage applied to the back gate (the substrate), and thus appeared to be metallic in nature. A smaller fraction of the SWNTs showed a significant increase in conductance for Vg < 0, and the inverse for Vg > 0, as shown in Figure 3. This behavior is generally accepted to be representative of the p-type characteristic of semiconducting SWNTs. Initial two-terminal resistances (Vg ) 0) measured for the SWNTs varied from a few MΩ to hundreds of MΩ. These values were somewhat higher than those reported in the literature.5,13 In addition, the I-V behavior for the SWNTs was often asymmetric. These observations are likely due to high contact resistances caused by residual contamination at the SWNT/electrode interface. Thermal annealing (405 °C in argon for approximately 45min) removed nearly all of the surface contamination from the substrate and dramatically reduced the contamination present on the CNTs (as evidenced by AFM). In addition, Nano Lett., Vol. 2, No. 5, 2002
Figure 3. I-V data for a selectively placed SWNT that shows strong current modulation by changing Vg, the voltage applied to the back gate (substrate). The maximum current level was limited to 10 nA to reduce the potential danger of damaging the SWNT. The inset shows an AFM image (550 × 200 nm2) of the SWNT (red) bridging two electrodes (brown). The image has been colorized to aid the viewer.
preliminary annealing results showed a reduction in the measured two-terminal resistance by as much as 3 to 4 orders of magnitude, giving final resistance values as low as 100 KΩ14, in agreement with those reported in the literature.5,13 In summary, process parameters have been identified that allow the selective placement of SWNTs over pre-patterned electrodes with very high yields. Compared to the approach where electrodes are fabricated after the SWNTs have been deposited on a surface, the present procedure offers greater flexibility in the electrode fabrication process and allows rapid placement and screening of SWNT-based devices. With further refinement, this process may provide a path for the large-scale fabrication of CNT-based devices and circuits. Acknowledgment. We thank H. Goronkin for his support of this work, and the Advanced Processing and Characterization Laboratories in Motorola Labs for technical assistance. The work is supported in part by the European Community project SATURN (IST-1999-10593). References (1) Carbon Nanotubes: Synthesis, Structure Properties and Applications; Dresselhaus, M., Dresselhaus, G., Avouris, P., Eds.; SpringerVerlag: Berlin, 2001. (2) Burghard, M.; Duesberg, G. S.; Philipp, G.; Muster, J.; Roth, S. AdV. Materials 1998, 10, 584. (3) 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. (4) Ahlskog, M.; Seynaeve, E.; Vullers, R. J. M.; Van Haesendonck, C. J. Appl. Phys. 1999, 85, 8432. (5) Choi, K. H.; Bourgoin, J. P.; Auvray, S.; Esteve, D.; Duesberg, G. S.; Roth, S.; Burghard, M. Surface Science 2000, 462, 195. (6) Utilizing PMMA required the use of an aqueous solution of (aminopropyl)triethoxysilane to form the APS SAM because it is incompatible with the organic solvents typically used. (7) Liu, J.; Rinzler, A. G.; Dai, H.; Hafner, J. H.; Bradley, R. K.; Boul, P. J.; Lu, A.; Shelimov, K.; Huffman, C. B.; Rodriguez-Macias, F.; Shon, Y.; Lee, T. R.; Colbert, D. T.; Smalley, R. E. Science 1998, 280, 1253. 445
(8) The SWNTs were purchased from Tubes@Rice, now known as Carbon Nanotechnologies, Inc., Houston, TX. (9) Postdeposition annealing was typically required to completely remove the residue present after liftoff with hot acetone. (10) Due to the very high resolution needed to image SWNTs by AFM, we saw a large increase in the frequency at which an AFM tip needs to be changed, with some tips lasting only for a few scans before loosing the necessary resolution. CNT-AFM tips, such as those described by Choi, N.; Uchihashi, T.; Nishijima, H.; Ishida, T.; Mizutani, W.; Akita, S.; Nakayama, Y.; Ishikawa, M.; Tokumoto, H. Jpn. J. Appl. Phys. 2000, 39, 3707, might be useful for imaging these types of samples.
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(11) Burgin, T.; Lewenstein, J.; Tsui, R.; et al., future publication. (12) Yao, Z.; Postma, H.; Balents, L.; Dekker, C. Nature 1999, 402, 273. (13) Bezryadin, A.; Verschueren, A. R. M.; Tans, S. J.; Dekker, C. Phys. ReV. Lett. 1998, 80, 4036. (14) With the current window dimensions, multiple SWNTs (2-3) were typically present between a given pair electrically connected electrodes. The 100 KΩ value was determined form a measurement of two SWNTs (or small bundles) in parallel having a total resistance of
