Three-dimensional microstructuring of carbon by thermoplastic spacer evaporation during pyrolysis

The three-dimensional microstructuring of carbon is useful for microelectromechanical devices and electrode arrays. A microstructure in the form of a microscale bridge (consisting of a girder and two adherent substructures) on an alumina substrate with a surface roughness of 1–2 μm (which allowed bonding by mechanical interlocking) was attained by using a novel low-cost process that involved thermoplastic spacer (paraffin wax) evaporation during pyrolysis of an epoxy-based film that coated the spacer and parts of the substrate. Fillers were chosen to reduce the shrinkage during pyrolysis and to increase the electrical conductivity. Multiwalled carbon nanotube as a filler was particularly effective for reducing the cracking tendency. Carbon black and silver nanoparticles as sole fillers were ineffective, producing cracked bridges. The total filler content (nanotube, optionally along with silver nanoparticles) had to exceed 3 vol.% in order to attain good control of the shape of the bridge. The method used a novolac epoxy resin in combination with an amine curing agent (without ultra-violet curing). The epoxy was chosen for low viscosity and strong bonding to the substrate. A bridge with a girder of length 90–300 μm, separated from the substrate by a height of 5–15 μm, was attained.