|Xiang and colleagues demonstrate a versatile approach to fabricate in-plane multi-contact nanowire structures on a single silicon chip. This approach may offer a versatile means to fabricate complex networks of nanowire materials, with the possibility of integration of field effect devices and sensors.|
Reviewed by Jeff Morse, PhD, National Nanomanufacturing Network
- Xiang Y, Keilbach A, Codinachs LM, Nielsch K, Abstreiter G, Fontcuberta i Morral A, and Bein T. 2010. Multiple Nanowire Species Synthesized on a Single Chip by Selectively Addressable Horizontal Nanochannels. Nano Letters 10(4):1341–1346. DOI: 10.1021/nl904207n.
Advanced fabrication processes to create nanowire structures from a range of metallic and semiconductor materials have been explored for many years now. More recently, these structures have gained interest for device specific applications including sensors, nanofluidics, and three-dimensional (3D) interconnects for advanced nanoelectronics. Typical approaches to achieve metal and semiconducting nanowire structures include directional crystal growth, sol-gel techniques, and template-based methods. Template-based techniques represent the more versatile approach, where a nanoporous template structure is formed first and then filled with nanowires synthesized by electrochemical or chemical vapor deposition. In this manner, a range of nanowire materials, diameters, and densities are possible, being limited only by the initial properties of the nanopore structure. Various techniques are available to create ordered or random arrays of nanopores, including mesoporous metal oxide, track-etched polycorbonate, porous silicon, and porous anodic alumina (PAA). While these advances have proven invaluable for focused studies on the properties of dimensionally confined materials structures, high value applications still require viable strategies for integration and scale-up, as well as the ability to synthesize multiple species of nanowires on a common platform.
Recently Xiang et. al. reported on a versatile approach to fabricate in-plane multi-contact nanowire structures on a silicon chip. Utilizing the PAA method, the authors’ investigation first fabricated selectively addressable nanochannel structures. To accomplish this, a 1-µm aluminum thin film was deposited on an oxidized silicon substrate by thermal evaporation and annealed in a forming gas atmosphere. The multi-contact architecture was formed by patterning finger stripes of the aluminum using standard photolithography and wet chemical etching processes, with finger width in the range of 1-5 µm, and a 25 µm spacing between all fingers. The finger width can be further reduced to control the number of nanopores and ultimately nanowires for a given application. Various contact configurations were explored from single up to five contacts in a sample. Nanochannels were grown during the anodization step in the aluminum fingers, and various electrolytes and conditions were investigated in order to control the density, diameter and length of the horizontal nanopores. Depending on the conditions, nanopores diameters in the 20-45 nm range were routinely achieved, with lengths determined by the anodic etch time. Furthermore, adjacent aluminum stripes that were electrically isolated from the anodized contact were unaffected by the anodization process.
Nanowires of gold, copper, nickel, cobalt and tellurium were electrodeposited within the multicontact nanopore structures using both direct current and pulsed techniques. Additionally, chemical vapor deposition was used to selectively form silicon nanowires by first electrodepositing a gold catalyst layer on the nanopore wall. Analysis of the nanowire arrays indicated some incomplete filling of the nanopores, likely due to the presence of barrier layers resulting from the anodization process. Subsequent optimization of the pore etch process significantly improved the uniformity of the resulting nanowire structures. Thus, an effective, reproducible method to fabricate multiple species of nanowires on a common substrate has been reported. By further extending the materials to other semiconductors and metals, this approach may offer a versatile means to fabricate complex networks of nanowire materials, with the possibility of integration of field effect devices and sensors.
Images reproduced with permission from Xiang Y, et. al., Nano Letters 10(4):1341–1346. DOI: 10.1021/nl904207n. Copyright 2010 American Chemical Society.