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Micro/Nano Lithography

Assembling three-dimensional carbon nanotube-based cantilever arrays

When combined, nanoassembly and microlithography prove effective techniques for making very flexible cantilevers using carbon nanotube composites.
28 October 2007, SPIE Newsroom. DOI: 10.1117/2.1200710.0901

Carbon nanotubes (CNTs) offer many unique and outstanding properties. However, the extremely small dimensions of CNTs—with diameters on the nanometer scale—limit their practical application. One of the most serious bottlenecks to commercialization is how to precisely deposit the tubes on a surface, or substrate, either individually or as a bulk material.

Current approaches to the problem include chemical vapor deposition,1 soaking,2 and stamping.3 But these techniques require expensive instruments, long deposition times, and complicated procedures. In addition, CNT films formed using these techniques have the disadvantage that they are mostly ‘monolayers,’ that is, only a molecule thick. Recently, a new nanofabrication technique called layer-by-layer self-assembly has been used to grow CNT multilayers on surfaces.4 The assembled CNT films have uniform surfaces and show excellent mechanical properties.

In developing a simple and effective method of depositing CNT films, we chose to combine nano-self-assembly and traditional microlithography. We were able to fabricate 2D and 3D microstructures based on CNT multilayers using this combinative technique.5 In our design, the lateral dimensions of the microstructures are defined by lithography, and the vertical dimensions are controlled by self-assembly.

Figure 1. Layer-by-layer self-assembly of polyelectrolytes/CNTs and polyelectrolytes/nanoparticles.

Figure 2. Schematic diagram of the cantilever array fabricated on a silicon substrate. The enlarged area illustrates the multilayer, which consists of polycations (+), polyanions (–), Fe2O3 nanoparticles (–), and CNTs (–).

Figure 3. Fabrication process for the self-assembled cantilever.

Figure 4. Flexed cantilever in acetone with an external magnetic field. Four optical images illustrate the deflection of a cantilever at four angles: (a) 0°, (b) 45°, (c) 90°, and (d) 135°.

Among the range of nanofabrication techniques, self-assembly has attracted much attention due to its versatility and simplicity. It overcomes the size limitation of conventional ‘top-down’ lithography-based microfabrication methods. Ultrathin films can be built through alternate adsorption of oppositely charged species such as polyelectrolytes, nanoparticles, CNTs, and proteins. The species are held together by strong ionic bonds, and form uniform and stable thin films. With a proper (positive-negative-positive) alternation, the CNTs and nanoparticles can be put together in any order, and the thickness can be controlled with nanometer precision (see Figure 1).

One of the most important advantages of CNTs is their high mechanical strength. CNT/polymer composites show high Young's modulus (elasticity) of 17GPa.6 To investigate the potential applications of CNT films, we fabricated 3D magnetic cantilever arrays using polycations, CNTs, and Fe2O3 nanoparticles (see Figure 2). Both the roots and the beams are composed of CNT multilayers. Fe2O3 nanoparticles are integrated in the structure as a magnetically sensitive material, which makes actuation and measurement convenient.

Fabrication of the 3D cantilever arrays is based on the 2D patterning method and involves several UV lithography and photoresist development steps. A modified lift-off technique provides additional protection for the cantilevers.7 Figure 3 details the fabrication process. A layer of photoresist is spin-coated onto the substrate and patterned by lithography and development. A second lithography step is executed with part of the photoresist layer protected. The shielded material is used as a sacrificial layer underneath the cantilever beam. The polyelectrolyte/CNT/Fe2O3 multilayer is self-assembled on the substrate. Another photoresist layer is spin-coated onto the substrate to cover the assembled multilayer, followed by a third lithography step. The part of the photoresist that covers the cantilever root and beam is protected by the photomask. The other part is exposed under UV light and then dissolved by developer with the assistance of ultrasonic vibration. As a result, the unexposed layers and the intercalated cantilever remain on the substrate. The freestanding cantilever beam is released by stripping off the photoresist—both the bottom sacrificial layer and the upper protective layer—with acetone. The effective thickness of the cantilever is estimated to be 200nm.

An external magnetic field is applied by holding a permanent magnet 1cm above the device. Figure 4 shows the behavior of the cantilever beam during magnetic actuation. The beam is gradually deflected from 0° to about 135°. The device restores rapidly after the magnet is removed. There is no observable structural failure or damage following repeated deflections (more than 100 times). The CNT multilayer has proven to be very strong and flexible, properties that are attractive for many applications.

In summary, we have presented an effective, simple, low-cost, and low-temperature approach to making CNT-based 3D cantilever arrays by combining nano-self-assembly with microlithography. The strength and thickness of the cantilevers can be adjusted over a wide range. The devices are robust and easily deflected to over 90°. They can be used as magnetically driven microactuators. And because many biomolecules are easily adsorbed on the self-assembled multilayer, the devices also have potential for biosensing applications.

This work is partially supported by the Defense Advanced Research Projects Agency MEMS/NEMS Fundamental Research Program through the Micro/Nano Fluidic Fundamentals Focus (MF3) Center.

Tianhong Cui
Department of Mechanical Engineering
University of Minnesota
Minneapolis, MN

Tianhong Cui is the Nelson Associate Professor of Mechanical Engineering at the University of Minnesota. He received his BS degree from Nanjing University of Aeronautics and Astronautics in 1991, and his PhD degree from the Chinese Academy of Sciences in 1995. From 1999 to 2003 he was an assistant professor of electrical engineering at Louisiana Technical University. Prior to that, he was a Science and Technology Agency (STA) fellow at the National Laboratory of Metrology in Japan, and served as a postdoctoral research associate at the University of Minnesota and at Tsinghua University. His research awards include the Nelson Endowed Chair Professorship from the University of Minnesota, the Research Foundation Award from Louisiana Tech University, and the Alexander von Humboldt Award of Germany. He also received an STA Special Research Program fellowship and a New Energy and Industrial Technology Development Organization fellowship, both from Japan. His current research interests include micro- and nanoelectromechanical systems (MEMS and NEMS), nanotechnology, and polymer electronics.

Wei Xue
Department of Mechanical Engineering
Washington State University
Vancouver, WA

Wei Xue is an assistant professor of mechanical engineering at Washington State University Vancouver. He received his BS and MS degrees in electrical engineering from Shandong University, Jinan, China, in 1997 and 2000, respectively, and his PhD degree in mechanical engineering from the University of Minnesota, Minneapolis, in 2007. Before he joined Washington State University Vancouver, he worked as a postdoctoral research associate at the Department of Mechanical Engineering, University of Minnesota. His main research interests include microfabrication techniques, nanotechnology, polymer and silicon MEMS, micro- and nanoelectronics, chemical and biological sensors, modeling, and systems control.