Microfabrication of three-dimensional boron structures by laser chemical processing

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Abstract
A method for microfabrication of three-dimensional structures in free space is presented. Laser-assisted chemical vapor deposition is used to grow a material at a point where a laser beam locally heats the substrate. By moving the substrate relative to the laser beam with a micropositioning system, three-dimensional shapes can be created. Helical shapes are generated utilizing three linear translational axes as well as an additional rotational axis. Tilting the substrate to align the growth direction with the laser beam direction facilitates improved process control. The smallest structures that can be grown with this technique are about 1 μm. Amorphous boron fibers and crystalline boron springs have been manufactured as two examples of micromechanical elements. The amorphous boron fibers show excellent mechanical properties: A modulus of elasticity of 420–450 GPa, a fracture strain of 2.7%–3.7%, and a fracture stress of 12–17 GPa. The crystalline boron springs produced so far display only moderate mechanical characteristics. This is because of the irregular fiber surface with protruding grains and amorphous nodules. Nevertheless, a maximum relative resilience of about 9% for a tightly wound spring has been obtained. The crystal structure of the grains examined show a fair agreement with the β-boron phase, even though distinct deviations are observed. Metastable arrangement of the B12 icosahedral units is suggested as a possible explanation. A high density of (100) twins and stacking faults are identified by transmission electron microscopy. By further process and system development it is believed that micromechanical details can be tailored more or less arbitrarily for different applications.