Fabrication of Anodic‐Alumina Films with Custom‐Designed Arrays of Nanochannels

Abstract

Among the strategies for growing one-dimensional nanostructures such as nanotubes and nanowires, a very viable approach is deposition of the desired material into a template with arrays of well-aligned nanochannels.[1±7] A competitive template with such characteristics is the porous anodic aluminum oxide (AAO) film, whose nanochannels can even laterally self-organize into hexagonally close-packed (hcp) domains exhibiting short-range order provided it is grown under specific anodization conditions. [8,9] It has been demonstrated that the range of the order can be further extended by several orders of magnitude using lithographic-guiding techniques, [4,10±12] and that the pore size distributions of such guided arrays are much narrower than those of the self-organized ones. The successful fabrication of such long-range-ordered nanochannel arrays, hereafter referred to simply as ordered arrays, has not only broadened the potential applications of AAO films but also opened possibilities for the fabrication of arrays of nanostructures arranged according to a custom-designed geometry. For example, one can envision an array with only part of it covered by nanodots or nanowires while the rest of the surface area remains empty. Depending on its geometry, such an array, with designed optical and electronic properties, could be used as a photonic crystal and/or a waveguide.[13±16] One of the viable approaches for fabricating such a custom-designed array is to grow the desired material into a template with a partially closed nanochannel array. Herein, we demonstrate a focused ion beam (FIB) directwrite lithographic method for selectively closing part of the channels of an ordered array on an AAO film in order to create a custom-designed nanochannel array. The initial ordered arrays were fabricated by FIB lithographic guiding techniques where the closure of the nanochannels within a certain area was achieved by raster scanning the FIB over the area, thus directly bombarding the AAO film. The successful fabrication of such a template with custom-designed nanochannel arrays opens up numerous possibilities for the creation of nanowire or nanodot arrays with desired geometric patterns. The fabrication process always starts from growing an ordered array by anodizing a finely polished aluminum sample that has been patterned with a guiding lattice on its surface. The lattice is a two-dimensional array of hcp concave pits created by FIB direct-write lithography. An ordered array is achieved when the lattice constant of the guiding lattice is carefully matched with the electrolyte and anodization voltage.[17] For the present work, we set the lattice constant, and therefore the spacing, of the ordered array to be 100 nm, and grew the nanochannels to a typical aspect ratio of larger than » 50. Figure 1a shows a micrograph of a typical ordered array on an AAO film taken by scanning with a 50 keV gallium FIB with a beam current of 1.1 pA and a diameter of » 10 nm over the sample while collecting the secondary electron signal to provide th...

Microcantilevers with Nanochannels

“…[16][17][18][19][20] Moreover, replicas of these nanochannels can also be applied in many important fields such as nanoimprint lithography, superhydrophobic surface, and photonic crystals. [21][22][23] Recently, AAO technology has experienced a major technological leap by adopting photolithography. [24][25][26] This combination will result in various MEMS/NEMS structures with nanochannels in the years to come.…”

Focused‐Ion‐Beam‐Based Selective Closing and Opening of Anodic Alumina Nanochannels for the Growth of Nanowire Arrays Comprising Multiple Elements

“…The small beam diameter (<10 nm) of the FIB suggests that it should be possible to selectively close an entire macroscopic array of nanochannels, leaving only a single nanochannel or a small part of the array open. [15] Such unique AAO templates may be useful as platforms for the construction of devices on the nanometer scale that can be readily connected to the macroscopic world or studied by optical probes with micrometer-scale resolution.…”

“…In previous work, a method for fabricating custom-designed arrays of nanochannels by selectively closing specific regions of the nanochannels using FIB bombardment has been reported. [15] This method facilitates the selective growth of materials within open nanochannels, enabling the creation of a nanocomposite comprising arrays of filled and empty nanochannels. In order to take full advantage of the potential of AAO templates, the ideal situation would involve filling different regions of an array with different materials.…”

Integrated Direct DNA/Protein Patterning and Microfabrication by Focused Ion Beam Milling

“…FIB is well known as a powerful microfabrication tool combining high resolution with great precision; it has been widely used in maskless milling and deposition, microsystem technology inspection, metrology, and failure analysis. [25][26][27][28][29][30] For biomolecule-patterning applications, FIB has been used to fabricate structured substrate materials, such as ordered arrays of pits, [31] nanostructured surfaces [32] and localized single nanopores; [33] but not, to our knowledge, for direct patterning of biomolecules. In contrast to the existing applications described above, in our study FIB was employed successfully for the first time for direct, top-down patterning of DNA and proteins.…”