Nanometer Scale Gap Made by Conventional Microscale Fabrication

Using this shadow, we demonstrate that a nanometer scale gap, denoted as a step junction, can be built between the two metal layers. The current−vol...
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NANO LETTERS

Nanometer Scale Gap Made by Conventional Microscale Fabrication: Step Junction

2004 Vol. 4, No. 9 1699-1703

Jaewon Choi, Kangho Lee, and David B. Janes* School of Electrical and Computer Engineering, Purdue UniVersity, 465 Northwestern AVe., West Lafayette, Indiana 47907-2035 Received June 9, 2004; Revised Manuscript Received July 23, 2004

ABSTRACT This paper describes a simple way to build a nanometer scale gap, using conventional microscale fabrication techniques such as optical lithography, electron-beam evaporation, and lift-off. Using electron-beam evaporation and lift-off of a metal line, it is possible to transfer the positive slope at the developed pattern edge in positive photoresist to a corresponding negative (undercut) slope in the resulting edge of the metal line. This negative slope on the metal pattern edge is subsequently used as a shadow for a second, thinner metal layer. Using this shadow, we demonstrate that a nanometer scale gap, denoted as a step junction, can be built between the two metal layers. The current− voltage characteristics of the resulting gaps have been characterized and the gaps have been imaged using scanning electron microscopy. To illustrate the application of a step junction structure as a nanoscale gap electrode, conductive bridges consisting of organic monolayers and nanoscale gold clusters were formed between the two electrodes. The observation of the clusters in scanning electron microscope (SEM) images and enhanced conductivity in the devices indicates that the step junction structure provides a means to realize efficient electrical contacts to nanoscale conductive elements.

Introduction. One of the key challenges in realizing molecular scale electronic components is the requirement for well-defined contact structures at the nanometer scale. Several approaches have been reported for realizing appropriate contacts, using “pre-formed” contacts, which generally eliminate the need for direct evaporation of metal layers on top of the molecular layers and associated potential problems.1 Various techniques to realize nanometer scale gaps include mechanically controlled break junctions (MCB),2 break junctions using electromigration (electrical break junction; EBJ),3 electrodeposition4 and, recently, carbon nanotube-extracted lithography (CEL).5 A direct e-beam lithography technique, which can realize features at the sub10 nm scale, has also been proposed.6 While these techniques have supplied useful tools for research, they generally are not simple parallel processes compatible with current silicon technology.7 None of them can easily be scaled to mass production in order to make a large number of junctions for integrated circuits (IC) in a short time and in a controllable manner. MCB is a decent scientific tool, like the scanning probe microscope (SPM), but cannot be developed into a technology for ICs. With EBJ, it might be possible to build small-scale circuits, but it is not clear that large-scale integration can be achieved by breaking individual junctions electrically. Furthermore, it is * Corresponding author. E-mail: [email protected], Tel: 765-4949263, Fax: 765-494-2706. 10.1021/nl049113x CCC: $27.50 Published on Web 08/10/2004

© 2004 American Chemical Society

commonly necessary to cool wires to liquid helium temperatures in order to realize nanoscale gaps between electrodes using EBJ.8 It is also questionable whether a large number of devices can be fabricated in a rapid and controllable manner with electrodeposition because the rate and quality of electrodeposition strongly depends on the shape and number of objects under deposition. Approaches based on direct-write e-beam lithography generally are too slow to compete with approaches based on optical lithography. CEL is also difficult to develop to a process for ICs because the fabrication of CEL is still limited by the alignment of carbon nanotubes. With MCB, EBJ, and CEL, it is typically not possible to have different materials on the two sides of the junction because the junction is initially one material or it is evaporated all at once. The capability to utilize different materials on the two sides of the gap could enable a variety of other applications of nanoscale devices in a controllable manner, including rectifiers or spin devices.9 This paper describes a new method to fabricate nanometer scale gaps with conventional microfabrication techniques. This method provides a relatively simple way to build a large number of nanometer scale gaps in a short time without using e-beam lithography or other complicated processes. The resulting structure is denoted as a step junction. Experimental Procedure. The initiation of the step junction structure is the positive and slow slope of positive

Figure 1. Cross sectional view of photoresist edge, following evaporation of metal for step layer (10 nm Ti/400 nm Au).

photoresist. Figure 1 illustrates the photoresist profile (following metallization), which includes a positive sidewall slope that is exposed to a normal-incidence metallization as long as the overhang at the top of the photoresist, which is formed by chlorobenzene treatment,10 is minimized, e.g., through mild over-development. The sample illustrated in Figure 1 was fabricated using the following sequence. An oxidized silicon piece was cleaned by toluene, acetone, and methanol, and then desiccated with hexamethyldisilazane for stable adhesion of the photoresist. The sample was spincoated with AZ 1518 photoresist at a spin speed of 4000 rpm for 40 s. After soft baking (85 °C for 15 min), the sample was soaked in chlorobenzene for 10 min and dried using N2. An appropriate pattern mask was used to expose the sample in a Karl-Suss contact aligner at 23 mW/cm2 for 7 s. After exposure, the pattern was developed for around 25 s using a 1:1 mixture of AZ developer with distilled (DI) water. The positive slope of the photoresist edge will result in a negative slope on the sidewall of a metal layer which is evaporated onto the sample. To illustrate the reverse slope formation, 10 nm titanium (Ti) and 400 nm gold (Au) was deposited using an e-beam evaporator. Figure 1 is an SEM image of the sample following evaporation, illustrating the metal “wing” on the sidewall of the photoresist. Following lift-off of the metal on the top of the photoresist region (and associated removal of the photoresist), the patterned metal region shown on the right side of Figure 1 remains; this structure will be denoted as the step layer. Figure 2 illustrates the subsequent deposition of a second, thinner, metal layer (denoted as the “finger” layer). Because of the negative slope at the edge of the step layer, the metal deposited in this second evaporation is blocked from reaching the substrate surface in the region shadowed by the overhang of the step layer. By using this shadowing effect, it is possible to build a nanometer scale gap between the two metal regions, as illustrated in Figure 2b. Note that a photolithography step is used to define the finger layer; this pattern overlaps the step electrode so a critical alignment is not required in order to form a nanoscale gap. If the metal wing on the step layer is 1700

present when the finger layer evaporation is performed, the gap will be relatively large. Alternatively, the wing on the step layer can be removed before finger layer deposition using mechanical agitation via ultrasonication. This approach results in smaller gap dimensions. The standard step junction procedure results in a mechanically stable structure, although the nonplanarity of the two contacts may restrict the ability to deposit nanoscale elements, such as metal nanoparticles, within the gap. The overhang at the edge of the step layer also makes inspection of a gap using SEM or atomic force microscopy (AFM) more difficult. To get around the problem of nonplanar contacts, we devised a way to make a modified structure, denoted as a sacrificial step junction, in which the two layers have the same thickness. As shown in Figure 3, multiple layers of metal, Ti, Au, and chromium (Cr), were evaporated for the step layer after lithography on a silicon dioxide surface. After lift-off of the step layer, it was found that wings of approximately 1 µm width remained on the edges of the step layer. Ultrasonication in acetone for 20 min was used to remove the majority of the wing region, reducing the overall dimension of the wings to the range of 20∼50 nm. The finger layer is formed by a subsequent sequence of optical lithography, e-beam evaporation and lift-off, using the same nominal thicknesses of Ti and Au employed in the step layer. After the formation of the finger layer, the Cr on the step layer is stripped with commercial, nitric-acid-based Cr etchant.11 Immediately after the Cr strip, a sidewall of Au/ Ti remains at the edge of the step layer, as shown in Figure 3b, because the Cr etching is performed without mechanical agitation. To get rid of this sidewall, ultrasonication was used either during the Cr etch or in methanol after the Cr etch. Since this etch procedure also lifts off the Ti/Au layer that was deposited on top of the step layer during the finger layer evaporation, the resulting structure consists of two electrodes with nominally equal thicknesses of Ti/Au metallization. The final device structure also includes contact pad regions using thicker metal layers deposited on the step and finger layers (outside of the region of the gap), to allow stable electrical measurement on the devices. A cross-sectional view of a sacrificial step junction structure, with the sacrificial layer still present, is shown in Figure 4. To make this cross-sectional SEM view, the sample was cut along a crystal plane of the substrate silicon wafer, resulting in the striations observed in the figure. However, the gap formed under the shadow of step layer is clearly visible, with a gap width below 30 nm. Figure 4 represents an interim stage in the processing; the remainder of the sacrificial step-junction procedure is as follows. On these samples, the step layer was defined using AZ 1518 photoresist and e-beam evaporation of a 5 nm thick Ti adhesion layer, 20 nm of Au, and a 50 nm thick sacrificial layer of Cr. The pattern was lifted off in acetone with ultrasonic agitation. The finger layer was formed by photolithography and subsequent evaporation of 5 nm of Ti and 20 nm of Au. Finally, a thick metal pad layer, consisting of Au on Ti, was deposited in order to allow stable probing on each junction. Nano Lett., Vol. 4, No. 9, 2004

Figure 2. Cross-sectional views illustration gap formation of step junction (a) after evaporating the first electrode layer (step layer) and (b) after evaporating the second electrode layer (finger layer). The step layer is defined by lift-off after evaporation.

Figure 3. Cross-sectional views of fabrication procedure of sacrificial step junction showing structure (a) following evaporation of the finger layer, (b) after stripping of sacrificial layer (Cr), and (c) removal of remaining wing using ultrasonication.

To determine the distribution of gap sizes, cross-sectional images were taken in a field emission SEM on a sample that had been exposed to 10 min of ultrasonication. In this case, more than 50% of junctions showed gap sizes in the range of 10∼30 nm. Gap sizes below 10 nm were observed in approximately 20% of the devices; in this size range, it is not possible to accurately determine the gap size due to the resolution of the SEM and the difficulty in imaging along a sidewall. The remaining fraction of the junctions showed gaps larger than 30 nm associated with relatively large, residual wings. The detailed statistics of the local gap dimension within a junction cannot be completely determined from a cross sectional view, which only allows a detailed view of the region near the exposed end of the gap. Due to the roughness of the step edge, there is a variation in the gap distance along the width of the gap, which results in local regions within the gap that are smaller than the stated gap dimension. Electrical Measurement of Nanoscale Species Using Step Junction. To illustrate the application of the step junction structure as an electrical contact to nanoscale Nano Lett., Vol. 4, No. 9, 2004

components, a series of conduction experiments was performed using sacrificial step junction devices. Currentvoltage characteristics were measured for each device following the gap fabrication procedure; at this point, the conduction reflects the tunneling current through the nanoscale gap and conduction through any leakage paths in the substrate. The initial currents were typically in the pA range for good devices. After gathering the initial I-V data, the sample was cleaned with argon/oxygen plasma. Subsequently, the sample was soaked in a 5 mM solution of 1,4benzenedimethanethiol (BDMT) solution in ethyl alcohol for 1 day. After removing the sample from the solution, the sample was cleaned with ethyl alcohol to remove any physisorbed molecules. The sample was then immersed in a citrate-stabilized gold sol with 20 nm gold clusters. After 1 day, it was taken out from the solution and rinsed with DI water to get rid of residual citrate from surface. In a previous study, AC bias was used to bridge a molecular junction with gold clusters.12 In the current study, the aim was to have the Au clusters bind to the pre-formed molecular monolayer of BDMT. After drying the sample with nitrogen, the sample 1701

Figure 4. Field emission SEM photograph (cross-sectional view) of sacrificial step junction structure, before removal of sacrificial layer. Note that the nanoscale gap at the interface between the two regions is defined by the shadow of the step layer (left contact).

was immersed in a 5 mM BDMT solution in ethyl alcohol in order to promote the formation of a molecular path between the cluster and the contacts. The ideal formation of a molecule/cluster/molecule bridge across the gap is shown in the inset of Figure 5a. The current-voltage (I-V) characteristic from a device after assembling a bridge with clusters and BDMT molecules is shown in Figure 5a. As illustrated in Figure 5b, an SEM photograph of a portion of the gap region for this device shows a gold cluster bridging the gap. Currents of approximately 30 µA at 1 V are observed following deposition of the molecule/cluster/molecule bridge across the gap. This device exhibited less than 10 pA of current at 1 V when characterized before the deposition of molecules and clusters. The single-electron charging energy for an isolated 20 nm Au cluster in a vacuum is around 70 meV; the local environment in the step junction structure (higher dielectric constant, capacitive coupling to the electrodes) will decrease this energy to a value comparable to the thermal energy at room temperature. Therefore, a well-defined Coulomb blockade region in the I-V curve is not expected.13 Note that the observed increase in conductance correlates to the observation of cluster bridges across the gaps. In companion samples that were soaked in ethanol without the BDMT molecules and DI water, the conductivity of the gaps was not observed to increase. Although it is difficult to say exactly how the cluster is coupled to the electrodes, it is thought to be through the monolayers of BDMT that were grown on the two contacts before cluster deposition. Based on an estimate of the number of BDMT molecules which contact a metal nanocluster on each side, the observed current levels correspond to a resistance in the range of 10 MΩ per molecule of BDMT. This figure is within a factor of 10 of the predicted resistance of BDMT in a metal/molecule/metal structure.14 As mentioned above, field emission SEM inspection indicated sub-10 nm size gap sizes in a portion of the devices. The results of electrical measurements also indicate gaps in 1702

Figure 5. (a) Measured I-V characteristics of a sacrificial step junction device bridged with a gold cluster and BDMT molecules. The inset is showing an ideal formation of bridge over sacrificial step junction gap with gold cluster and BDMT molecules. Figure (b) shows a SEM photograph of an Au nanocluster bridging the gap on the measured device; the scale bar in the picture is 30 nm long.

the nanometer size range. Specifically, a portion of the devices showed reasonably high tunneling conduction before self-assembly, with the magnitude of tunneling conduction indicating a 1∼2 nm local gap size between the two metal electrodes.15 As evidence that local gap sizes as small as molecular dimension can be realized, approximately 5% of the step junction devices on other batches showed significant increases in conductivity (10 KΩ ∼ 10 MΩ) after deposition of short organic molecules, such as BDMT or 1,4-benzenedithiol. Since this conductivity is observed without the help of a cluster bridge, the observation of conductivity indicates Nano Lett., Vol. 4, No. 9, 2004

that regions within these gaps are small enough to allow bridging by these small organic molecules. Discussion and Conclusion. The step junction described in this paper represents a simple way to achieve batch fabrication of nanometer scale gaps. The technique provides the potential to build molecular scale gaps with current lithography facilities and technology. There are several engineering challenges that need to be addressed, including the irregularity. This problem can be eliminated by using plasma descumming16 after step layer lithography. The removal of the residual wings would also be more precise if we could use mild mechanical scrub or even chemicalmechanical polishing. The sacrificial step junction provides several advantages over the standard step junction. Using the sacrificial stepjunction, it is possible to inspect the gap with SEM or AFM, and also to merge the approach with other nanofabrication techniques, such as dip-pen nanolithography17 and cluster printing.18 Using the step junction procedure, it is also easy to build asymmetric pairs of electrodes for contacts to nanometer scale species, which might have significant potential applications.9 In experiments on molecule-only devices using these asymmetric junctions, the measured I-V curves show asymmetries with consistent polarities. The use of a junction with asymmetric contacts would allow engineering of the contacts, as well as the molecular-scale species, to achieve desired device characteristics. Although beyond the scope of this paper, temperature-dependent I-V measurements have been performed on devices consisting of standard step junctions that were exposed to organic molecules. The devices show stable behavior over temperatures between 80 K and room temperature. This illustrates that the step junction structure is a useful tool for exploring conductivity of nanoscale systems with the capacity of measuring temperature-dependent conductivity, unlike other tools such as SPM or MCB. Several applications using step junction structure are now under consideration, including three-terminal molecular devices and chemical sensors.

Nano Lett., Vol. 4, No. 9, 2004

Acknowledgment. This research is supported in part by the National Science Foundation, the Department of Energy, and Indiana 21st Century Fund. We thank Henny Halimun and Sunkook Kim for mask design, and also thank Vania Petrova in the University of Illinois for field emission SEM inspection. Field emission SEM inspection was carried out in the Center for Microanalysis of Materials, University of Illinois, which is partially supported by the U.S. Department of Energy under grant DEFG02-91-ER45439. References (1) Walker, A. V.; Tighe, T. B.; Stapleton, J.; Haynie, B. C.; Upilli, S.; Allara, D. L.; Winograd, N. Appl. Phys. Lett. 2004, 84, 20. (2) Reed, M. A.; Zhou, C.; Mueller, C. J.; Burgin, T. P.; Tour, J. M. Science 1997, 278, 252. (3) Park, H.; Lim, A. K. L.; Park, J.; Alivisatos, A. P.; McEuen, P. L. Appl. Phys. Lett. 1999, 75, 301. (4) Morpurgo, A. F.; Marcus, C. M.; Robinson, D. B. Appl. Phys. Lett. 1999, 74(14), 2084. (5) Chung, J.; Lee, K.-H.; Lee, J. Nano Lett. 2003, 3(8), 1029. (6) Hu, W.; Bernstein, G. H.; Sarveswaran, K.; Lieberman, M. Proc. IEEE-Nano 2003. (7) Austin, M. D.; Chou, S. Y. Nano Lett. 2003, 3(12), 1687-1690. (8) Park, J.; Pasupathy, A. N.; Goldsmith, J. I.; Soldatov, A. V.; Chang, C.; Yaish, Y.; Sethna, J. P.; Abruna, H. D.; Ralph, D. C.; McEuen, P. L. Thin Solid Films 2003, 438, 457. (9) Deshmukh, M. M.; Prieto, A. L.; Gu, Q.; Park, H. Nano Lett. 2003, 3(10), 1383. (10) Campbell, S. A. The Science and Engineering of Microelectronic Fabrication; Oxford University Press: Oxford, 1996; p 275. (11) Chromium Etchant 1020 of Transene, Inc., www.transene.com. (12) Amlani, I.; Rawlett, A. M.; Nagahara, L. A.; Tsui, R. K. Appl. Phys. Lett. 2002, 80(15) 2761. (13) Bezryadin, A.; Dekker, C.; Schmid, G. Appl. Phys. Lett. 1997, 71(9), 1273-1275. (14) Dorogi, M.; Gomez, J.; Osifchin, R.; Andres, R. P.; Reifenberger, R. Phys. ReV. B 1995, 52(12), 9071-9077. (15) Kergueris, C.; Bourgoin, J. P.; Palacin, S.; Esteve, D.; Urbina, C.; Magoga, M.; Joachim, C. Phys. ReV. B 1999, 59, 19, 12505-12513. (16) Wolf, S.; Tauber, R. N. Silicon Processing for the VLSI Era, Vol. 1 - Process Technology; Lattice Press: Sunset Beach, CA, 2000. (17) Piner, D.; Zhu, J.; Xu, F.; Hong, S.; Mirkin, C. A. Science 1999, 283, 661-663. (18) Santhanam, V.; Andres, R. P. Nano Lett. 2004, 4, 1, 41.

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