Material Properties of Carbon-Infiltrated Carbon Nanotube-Templated Structures for Microfabrication of Compliant Mechanisms

Abstract

Carbon nanotubes can be grown vertically from a substrate to form dense forests hundreds of microns tall. The space between the nanotubes can then be filled with carbon using chemical vapor deposition to create solid structures. These infiltrated structures can be detached from the substrate and operated as single-piece MEMS. To facilitate the design of compliant microdevices using this process, we explored the influence of two fabrication parameters—iron layer thickness and infiltration time—on the material’s mechanical properties, using the fracture strain to judge suitability for compliance. We prepared samples of a simple meso-scale cantilever beam pattern at various levels of these parameters, applied vertical loads to the tips of the beams, and recorded the forces and deflections at brittle failure. These data were then used in conjunction with a nonlinear FEA model of the beams to determine Young’s modulus and fracture stress for each experimental setting. From these data the fracture strains were obtained. The highest fracture strain observed was 2.48%, which is approximately 3.5 times that of polycrystalline silicon. This was obtained using an iron layer thickness of 10 nm and an infiltration time of 30 minutes. We used a test device—a compliant gripper mechanism for holding mammalian egg cells—to demonstrate the use of this material in compliant MEMS design.

Mechanical Property Measurement of Carbon Infiltrated Carbon Nanotube Structures for Compliant Micromechanisms

“…A statistical analysis of this data shows that the infiltration time has a significant effect on both the ultimate strength and the elastic modulus, but not on maximum strain. There is also an interaction of infiltration time with iron thickness that significantly affects the elastic modulus, although iron thickness on its own was not found to directly affect the three properties studied here [14].…”

“…Comparing the finite element results to the test results, we used an iterative process to calculate corrected values of the Young's modulus and failure stress for each sample. This finite element model and correction procedure is described in greater detail in [14].…”

“…A statistical analysis of these corrected values was then conducted to determine the relationships between these properties and the two process parameters mentioned above. The finite element modeling and statistical analysis are covered in greater detail in [14].…”

“…A recent study that used carbon as the deposition material in the CNT-M process indicates that carbon-infiltrated CNT (CI-CNT) structures could far surpass the compliant capabilities of existing MEMS materials [14], most specifically with regard to maximum tolerable strains. Although this was the first study exploring CVD infiltration with carbon, other uses of carbon in the MEMS microfabrication process have been investigated.…”

Fabrication and Testing of Planar Stent Mesh Designs Using Carbon-Infiltrated Carbon Nanotubes

“…As shown in Ref. [18], initial experiments with carbon deposition have shown that carbon-infiltrated structures exhibit a remarkable degree of compliance and strain in bending, where tension is the typical failure mode. The high strain and superior biocompatibility marks CI-CNT as a potentially appropriate and even superior material to typical steels and alloys for stent fabrication.…”