Direct-Writing of Three-Dimensional Structures Using Laser-Based Processes
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Introduction Creating freestanding three-dimensional (3D) structures at small scales is difficult using traditional fabrication approaches. Common bottom-up techniques employing multilayered lithography enable a building up and etching away of layers to obtain the desired structures, but these are limited by the difficulty of creating 3D structures of arbitrary shape and local material properties. Similarly, top-down approaches for micro- and nanoassembly are inadequate to the task of handling, precisely positioning, and joining together the even smaller core units of the preferred structures. Laser direct-write addition (LDW) using chemical vapor deposition (CVD) overcomes these difficulties by creating materials of interest at the precise location required. Gaseous or liquid precursors can be used for material deposition induced by properly chosen, placed, and controlled energetic beams such as laser, electron, or ion beams, which can be adjusted to freely target any location in three dimensions.
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Growth rate and precision strongly depend on beam properties. The major advantage of beam-induced growth, compared with standard lithography techniques, is that arbitrary 3D structures can be written directly onto and away from the substrate exposed to the energetic beam. Figure 1 shows a schematic illustration of the general CVD process. The substrate is surrounded by the appropriate liquid or gas. The laser beam is focused on the intended growth location, providing the energy necessary to induce a chemical reaction to change the liquid or gas reactants into the solid material of interest. Motion of the substrate or beam enables direct-writing of arbitrary 3D structures. Laser direct-write CVD (LDW-CVD) is a powerful technique for 3D fabrication on a small scale. To demonstrate this point, we use the example of perhaps the most recognized metal-framed structure, the Eiffel Tower in Paris. As shown in Figures 2a and 2b, construction of the world’s tallest structure at the time (300 m)
Figure 1. (a)–(c) Schematic illustration of the general beam-induced chemical vapor deposition process. The substrate is surrounded by the appropriate liquid or gas (pink background). The laser beam (green cone) is focused on the intended growth location, providing the energy necessary to induce a chemical reaction, thus changing the liquid or gas reactants into the solid material of interest (yellow structure). Motion of the substrate or beam enables the direct-writing of arbitrary 3D structures, in this case, a microelectrode cage for trapping particles.
required support elements that were later removed after the structure was completed. Similarly, LDW-CVD of aluminum combined with a pre-shaped polycarbonate support substrate was used to fabricate a copy of the Eiffel Tower 3 mm high—one hundred thousand times smaller than the original (Figures 2c and 2d). In this case, the support substrate was later removed by dissolving it in a suitable solvent to provide the completed freestanding structure.
MRS BULLETIN • VOLUME 32 • JANUARY
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