When it comes to high-performance filtration, separation, sunlight collection, surface charge storage or catalysis, the effective surface area is what counts. Highly regular fractal structures seem to be the perfect candidates, but manufacturing can be quite cumbersome. Here it is shown-–for the first time—that complex 3D fractals can be engineered using a recursive operation in conventional micromachining of single crystalline silicon. The procedure uses the built-in capability of the crystal lattice to form self-similar octahedral structures with minimal interference of the constructor. The silicon fractal can be used directly or as a mold to transfer the shape into another material. Moreover, they can be dense, porous, or like a wireframe. We demonstrate, after four levels of processing, that the initial number of octahedral structures is increased by a factor of 625. Meanwhile the size decreases 16 times down to 300 nm. At any level, pores of less than 100 nm can be fabricated at the octahedral vertices of the fractal. The presented technique supports the design of fractals with Hausdorff dimension D free of choice and up to D = 2.322.
Although this paper does not concentrate on applications, but is merely a first demonstration of a new fabrication technology, it is instructive to illustrate a few potential applications. Due to the huge possible apparent porosity it is evident that the perforated fractal membrane will find applications in the field of filtration and (free molecular) separation. Likewise the extraordinary number of pores might be helpful as injector or spray nozzles and fast diffusion mixer devices. The property of a large effective surface area might be explored in capacitors, batteries, catalysis and dense membranes. Furthermore, hollow and smooth fractal surfaces might be useful in trapping light for solar cell and photonic applications. Fundamental studies of light behavior when entering such a fractal structure will be very interesting to explore. The presented fractal wireframes might be selective particle traps, advanced 3D fractal antennas for enhanced device integration in RF applications, and coolers or heaters, which remove or distribute heat very uniformly and as catalytic frames. This can be useful in sensor applications or chemical reactors. Furthermore, instead of creating an additional level of octahedral structures, the perforated LOCOS oxide might be used to locally dope silicon and create a variety of 3D photonic crystals or quantum dot and wire arrays. As a last example, it is worthwhile to think about metallic or semiconducting dots or wireframes as potentially allowed in the suggested fractal fabrication scheme. This might be useful in the emerging field of 3D electronic integration.
In summary, a new class of 3D structures fabricated by a repeated silicon nanomachining procedure is demonstrated: octahedral fractals. It makes use of the crystalline property of silicon to form octahedral pits and corner lithography to define the location of the next level of octahedra in the silicon mold. Another essential feature of this technique is its ability to accurately form multiscale structures based on a single conventional lithographic step. We demonstrated the fabrication of dense, perforated and wire-like fractal structures. Starting from a single micron-sized mask opening, we have shown the creation of 625 nanopores for each individual octahedral fractal resulting in an apparent porosity of 4% and a fractal with a freely accessible area increased by a factor of 6.5 with respect to the occupied wafer area.
Brian Wang is a Futurist Thought Leader and a popular Science blogger with 1 million readers per month. His blog Nextbigfuture.com is ranked #1 Science News Blog. It covers many disruptive technology and trends including Space, Robotics, Artificial Intelligence, Medicine, Anti-aging Biotechnology, and Nanotechnology.
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