NANO LETTERS
Molecular Lithography for Wafer-Scale Fabrication of Molecular Junctions
2004 Vol. 4, No. 8 1391-1394
Gregory S. McCarty* Penn State Nanofabrication Facility/Materials Research Institute and Department of Engineering Science and Mechanics, The PennsylVania State UniVersity, 105 MRI Building, State College, PennsylVania 16802 Received April 28, 2004
ABSTRACT High-resolution patterning is critical for the development of functional devices and sensors with nanometer dimensions. To date, no patterning methodology exists with the capability to produce molecular-scale features reliably over an entire wafer. In this paper, a lithographic process is presented to fabricate pairs of electrodes reproducibly with nanometer-scale resolution quickly, efficiently, and economically. Additionally, this work demonstrates that molecules of interest can be added to, or removed from, patterned molecular junctions. This novel technology provides a relatively simple and economical means to produce devices capable of monitoring the addition of molecules to a structure.
The ability to transfer a desired pattern to a surface is one of the key technologies utilized in the production of electronic devices and is becoming increasingly more important to fundamental scientific research. In particular, the advancement of patterning technologies will facilitate the study of the electronic properties of individual molecules. Indeed, many research projects have focused on this goal, especially for the production of active electronic devices based on molecular properties.1-15 The results from these studies are promising but, to date, have not conclusively demonstrated the feasibility of the production of molecular electronic devices. In addition, a manufacturable method of producing structures with features on the molecular scale has yet to be developed. Many patterning methodologies have been explored, and each has its own advantages and disadvantages. For example, photolithographic-based patterning methods are typically utilized in semiconductor manufacturing applications because of their high throughput; however, these methodologies are limited in resolution to ∼100 nm. In contrast, direct write electron beam lithography has demonstrated the best resolution of the conventional patterning strategies, achieving wellspaced 8-10-nm-wide lines.16,17 However this methodology is both temporally and fiscally expensive, and the achievable resolution is greatly affected by the proximity of the features to one another and by the substrate material. More recently, novel techniques have been developed for patterning surfaces with extremely high resolution. These techniques include directly writing patterns onto a surface using the tip of a scanning probe microscope,18,19 selective etching processes for line formation,20 and the utilization of molecular layers * Corresponding author. E-mail:
[email protected]. 10.1021/nl049375z CCC: $27.50 Published on Web 07/09/2004
© 2004 American Chemical Society
as lift-off resists.21,22 The best resolutions reported with any of these techniques are on the order of 8 to 10 nm.20,22 Improvements are needed to increase the patterning resolution to only a few nanometers, the approximate size of a single molecule. In this letter, a patterning methodology is presented that enables the reproducible fabrication of stable, molecularscale separations between electrodes. All processes described are wafer scale, meaning that the operation or modification is performed on the entire wafer at the same time. This type of processing is efficient, thus providing a straightforward path to economical mass production. Photolithography is used to define the shape and size of each of the electrodes quickly and inexpensively, whereas molecular lithography defines the small separation between the electrodes. Figure 1 shows a series of optical microscopy images (AE) and corresponding cartoon depictions (F-J) outlining the steps involved in the fabrication of the electrode pairs. First, the silicon substrate is cleaned using standard semiconductor cleaning procedures, and two layers of metal are thermally evaporated onto the substratesa chrome layer is added first, and then a gold layer is added on top to create a total thickness of 120 nm. Then, photolithography is used to define an etch mask, and reactive ion etching is used to form the initial metallic feature, as depicted in Figure 1A and F. Next, the distance separating this electrode from the second electrode, which is created at a later step, is defined using molecular thin films based on alkanethiol self-assembled monolayers (SAMs). The wafer is immersed in a 1 mM solution of alkanethiol in 95% ethanol to form a dense, selflimiting single layer of molecules selectively on the gold surface. Because the SAM is self-limiting, wafers can be immersed in the solution for up to 96 h without adverse
Figure 1. Series of optical micrographs (A-E) and cartoon depictions (F-K) showing the steps used to fabricate the electrode structures. Each of the optical micrographs shows a 100 µm × 80 µm area. (A and F) First, a chrome adhesion layer and then a gold layer are thermally evaporated onto the substrate, and photolithography and reactive ion etching are used to generate the initial metallic feature. (B and G) This structure is modified with a molecular resist. No difference is observable using the optical microscope. To fine tune the distance between electrodes, multilayered resists can be employed rather than using standard SAMs. (C and H) A second photolithographic process is used to create the shape of the second electrode. The arrow points to the area exposed on the initial electrode. (D and I) A second layer of gold is evaporated evenly across the entire substrate. (E and J) The resist used in the photolithographic process is removed, effectively stripping the gold deposited on top of it and resulting in the completed electrode pair. A separation between the two electrodes is not clearly visible because of the resolution of optical microscopy. (K) Cartoon depiction of a side view showing the separation at the interface of a completed electrode pair.
affects on the resulting molecular film. Figure 1B and G depicts an initial structure that has been modified by such a molecular resist. At this point in the processing, a second electrode is defined using a lift-off photolithographic process. Specifically, a photoresist is spun onto the entire wafer and photolithographically defined such that the area exposed overlaps the initial electrode by several micrometers (Figure 1C and H). Another gold layer, approximately 45 nm thick, is then evaporated onto the substrate to coat all exposed surfaces uniformly, as shown in Figure 1D and I. Finally, the photoresist for the second pattering process is removed. This serves to remove the gold layer evaporated on top of the resist, resulting in the final structure shown in Figure 1E and J. Because of the extremely small size of the separation between the two electrodes, it is not visible by optical microscopy. A cartoon depiction of the side view of a separation between the electrodes is presented in a completed electrode pair (Figure 1K). To fine tune the distance between electrodes, multilayered resists can be employed rather than using single-layer SAMs. In this case, an alkanethiol with a reactive end group, such as mercaptoundecanoic acid or mercaptohexadecanoic acid, is grown on the surface of the first gold electrode (Figure 1B and G). Next, the wafer is immersed in a 2 mM solution of copper perchlorate in ethanol so that Cu2+ ions form an atomically thin layer on top of the carboxylic acid terminal 1392
Figure 2. Optical and scanning electron micrographs showing an electrode structure fabricated on a silicon substrate. (A) Optical micrograph showing an individual electrode structure. The initial electrode is labeled Au#1, and the electrode that was deposited second is labeled as Au#2. The scale bar is 50 µm. (B) Highresolution optical micrograph showing the interface between the two metallic electrodes, labeled Au#1 and Au#2. The scale bar is 20 µm. (C) Scanning electron micrograph of the interface between the two metallic layers, labeled Au#1 and Au#2. The scale bar is 100 nm. The separation between the electrodes is observable but is difficult to resolve. (D) A similar scanning electron micrograph taken of an electrode pair on a second substrate fabricated using a multilayered resist to achieve a larger separation distance between two electrodes. The scale bar is 100 nm.
group of the first SAM, which serves as a linker upon which a second layer will grow. Another alkanethiol layer is grown, and the process repeats until the desired thickness is obtained and the fabrication of the second electrode ensues. The growth of such multilayer molecular resists has been reported previously.22,23 The completed substrates are 2-in. silicon wafers containing thousands of electrode pairs. These electrode pairs were fabricated with a molecular resist thickness of 2.4 nm. Figure 2 presents a series of images depicting one set of electrodes on a completed silicon substrate. The optical micrograph in Figure 2A illustrates two square metallic pads, labeled Au#1 and Au#2, which allow electrical contact to be made to the device. The junction between the Au#1 electrode and the Au#2 electrode is circled. A higher-resolution optical micrograph of the interface is presented in Figure 2B. No separation is visible with optical microscopy; however, a separation between the electrodes is clearly delineated in the scanning electron micrograph presented in Figure 2C. The surface of the electrodes is rough, thus the separation distance between the electrodes is somewhat variable. A similar scanning electron micrograph taken of a second substrate fabricated to achieve a larger separation distance between two electrodes is presented in Figure 2D. This silicon substrate was processed using a multilayered molecular resist with a thickness of 5.6 nm. In this case, the observed separation between the electrodes is 6 ( 2 nm. To probe the electronic properties of the device, this methodology has been used to fabricate electrode pairs on silicon substrates that are uniformly coated with a 90-nmthick thermally grown layer of silicon dioxide. This silicon Nano Lett., Vol. 4, No. 8, 2004
Figure 3. Current-voltage characteristics collected for a number of completed electrode pairs. (A) Nine current voltage curves for a single device with a design separation of 2.5 nm. Triplicate traces were collected at three separate times: at 0, 4 and 20 h. (B) Six current-voltage curves for six different electrode pairs from the same substrate with a design separation of 2.5 nm. These electrodes conduct an average current of 1.7 nA at 1 V of applied bias. (C) Current-voltage curves for eight different electrode structures from the same substrate with a design separation of 3.6 nm. These electrodes show an average of 400 pA of current at 1 V of applied bias.
dioxide layer serves to insulate the electrode pairs from the semiconducting silicon substrate. When two conductors, such as those fabricated in this work, are in close proximity to each other a tunneling current can pass between them. This current is directly dependent on the material comprising the electrodes and on the distance and material between them. To probe the current passed by the fabricated electrode pairs, tungsten probes are used to make connections to the gold electrode pads, and current-voltage characteristics for fabricated devices are collected using a semiconductor parameter analyzer and microprobe station. Whereas the current-voltage measurements provide some information about the size of the separations, at this time no definitive separation values for the electrodes have been determined. Thus, the separation distances are currently labeled by the designed separation, which refers to the thickness of the molecular resist used in the fabrication of the device. Figure 3 displays current-voltage curves for several electrode pairs. In Figure 3A, nine current-voltage traces are presented for a single electrode pair fabricated with a 2.5-nm separation distance. Triplicate traces were collected at three separate times: at 0, 4, and 20 h. These traces coincide nicely, demonstrating the stability of an electrode pair over time. In Figure 3B, current-voltage curves are presented for six different sets of electrodes on the same wafer, designed using a 2.5-nm-thick molecular resist. Although some variability is evident, the traces vary by less than 1 order of magnitude, demonstrating a relatively uniform separation distance between each of the electrode pairs. These electrodes pass an average of 1.7 nA of current at an applied bias of 1 V. Similarly, Figure 3C depicts eight currentvoltage traces collected from electrode pairs fabricated with a molecular resist designed to be 3.6 nm thick. For this larger separation, the traces are less variable, signifying less variation in the separation distance among the electrode pairs probed. As expected, these electrode pairs, which are designed using a thicker molecular resist to achieve a greater separation distance, pass less current than the electrode pairs fabricated using a 2.5-nm-thick resist. The electrode pairs fabricated with a 3.6-nm-thick resist pass an average of only 400 pA of current at 1 V of applied bias. No tunneling current Nano Lett., Vol. 4, No. 8, 2004
Figure 4. (A) Schematic representation of NOPE, the highly conductive molecular wire candidate molecule that is added to the molecular junction in this work. (B) Triplicate current-voltage characteristics for one set of electrodes before (blue) and after (red) a 12-h immersion in a NOPE solution. After immersion in the NOPE solution, the current that this electrode structure can pass increases by about 20-fold because of the addition of the highly conductive molecule to the electrode junction. The three traces depicted in green were recorded after this device was immersed for 24 h in a dodecanethiol (C12) solution. This treatment serves to displace the conductive NOPE molecules from the junction, replacing them with less conductive dodecanethiol molecules. The current passed returns to a value close to the as-fabricated value, suggesting that most of the NOPE molecules have been removed from the molecular junction.
was observed for electrode structures with designed separations of 6.2 nm or greater (data not shown). These data clearly suggest that separations of only a few nanometers have been achieved. This technology was utilized to investigate the electronic properties of a molecular species; the substrate designed with a 2.5-nm separation distance was immersed in a 2 mM solution of the molecular wire candidate 4,4′-di(ethynylphenyl)-2′-nitro-1-benzenethiolate (NOPE) in ethanol (Figure 4A). The device was immersed in the solution for 12 h, subsequently rinsed with acetone and ethanol, and finally dried under nitrogen gas. Current-voltage traces were collected in triplicate from one electrode pair both before (blue series) and after (red series) this treatment and are presented in Figure 4B. Immersion in the NOPE solution serves to increase the tunneling current from 1.5 to 44 nA at 1 V of applied bias. This has been attributed to the insertion of the conjugated NOPE molecules into the junction between the electrodes.24,25 To demonstrate that the increased tunneling current was not due to a structural alteration of the electrode configuration, the inserted NOPE molecules were displaced through immersion in a solution of conventional alkanethiol molecules. After 24 h in a 1 mM dodecanethiol solution (C12) in ethanol, the sample was rinsed in acetone and ethanol and finally dried under a stream of nitrogen. Again the currentvoltage characteristics for the device were recorded. As shown in Figure 4B, the traces (depicted in green) are comparable to those recorded prior to the NOPE immersion. These data suggest that the less conductive alkanethiol molecules substitute into the separation between the electrodes, effectively replacing the NOPE molecules. This simple experiment demonstrates the ability of these devices to monitor the addition and removal of molecules from the molecular junction. 1393
In conclusion, this letter presents a method of fabricating metallic electrodes separated by distances on the molecular scale using a combination of conventional photolithographic and molecular lithographic techniques. The electronic properties of these devices are presented “as fabricated”, after the addition of NOPE molecules to the junction and after these molecules are displaced through an alkanethiol exchange. The addition of conductive NOPE molecules increases the current through the structure, effectively demonstrating the ability of the device to detect the electronic properties of an incorporated molecular species. This ability is further elucidated when these molecules are exchanged for less conductive dodecanethiol molecules, and the current-voltage characteristics are strikingly similar to those obtained prior to immersion in the NOPE solution. To the best of my knowledge, this is the first study to measure the electronic properties of a device before and after the addition of a molecule of interest. The scientific significance of this technology is profound because it provides a feasible means for systematic analyses of molecular conduction. Acknowledgment. I thank the Penn State Materials Research Institute and the Penn State site of the NNIN for financial support, the Penn State site of the NNIN, Professor David Allara, and Professor Andrew Ewing for the use of equipment and laboratory space. I acknowledge D. Allara and S. Uppili and J. Tour and Y. Yao for the generous gift of the NOPE molecules used in this study and Professor David Allara, Dr. Leslie Sombers, Ms. Christine McGuiness, and the staff of the Penn State site of the NNIN for insightful ideas and suggestions. References (1) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L., II.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705-1707. (2) Bumm, L. A.; Arnold, J. J.; Dunbar, T. D.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. B 1999, 103, 8122-8127. (3) Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Science 1999, 286, 1550-1552.
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NL049375Z
Nano Lett., Vol. 4, No. 8, 2004
