Improvement of the elastic modulus of micromachined structures using carbon nanotubes

Microelectromechanical flexural structures have been fabricated using sandwiched multilayers consisting of bundled singled walled carbon nanotubes(SWNTs) incorporated into silicon nitride (Si₃N ₄) films. The Si₃N₄-SWNT composite layer was patterned by reactive ion etching followed by release in XeF₂ to create freestanding bridge structures. The mechanical stiffness of the micromechanical bridges was monitored via force-displacement (F-D) curves obtained using an Atomic Force Microscope (AFM). Inclusion of SWNTs resulted in an increase in the spring constant of the bridge by as much as 64%, with an average increase of 25%. In a second experiment, micromachined bridges fabricated using dissolved wafer process were coated with debundled SWNTs. The SWNTs suspended in N-methyl-2-pyrrolidinone (NMP) solvent were sprayed locally on each bridge using a piezoelectric print head. Resonance frequency measurements were done in vacuum (∼10^-4 Torr) on the bridges after successive SWNT depositions. A 20% increase in the resonance frequency of the bridges was observed. The observed increase in stiffness in the first set of experiments as well as the observed increase in the frequency in the second set of experiments can be attributed to the high axial modulus of elasticity (∼1 TPa) of the carbon nanotubes.