Micro-springs and tilted micro-cantilevers less than ten microns wide are a couple of structures made from 3D-structured carbon nanotubes (CNTs) in a methodology developed by Imec and MichiganUniversity.
The 3D structuring method for making vertically-aligned corrugated nanostructures is based on an iterative sequence of CVD growth of CNTs and capillary self-assembly. A wide variety of robust 3D CNT microstructures can be fabricated.
The process starts by lithographic patterning of a catalyst film on a Si wafer, followed by growth of vertically-aligned CNT microstructures by thermal CVD at atmospheric pressure.
Next, the straight CNT microstructures are transformed into sloped shapes by condensation and subsequent evaporation of liquid on the surface, the so-called capillary forming step.
These two process steps can then be iterated and finally result in 3D corrugated structures. Arrays of such structures with varying lateral dimensions can be fabricated as illustrated below. This study further focuses on the fabrication of (arrays of) CNT ‘bellows’ and CNT ‘cantilevers’.
First, the utility of cylindrical CNT ‘bellows’ for use as vertical microsprings was investigated. It was found that the properties of the corrugated CNT microsprings can be tuned on the basis of their geometry alone e.g. the compliance (which is the reciprocal of the stiffness) of the springs is inversely related to their wall thickness.
Microspring arrays are used commercially in probecards for the testing of electronic devices. For this particular application, the availability of a method for making arrays of compliant springs with small footprint is an important step forward.
Secondly, the method was used to create cantilevers. Iteration of growth and forming of semicircular shapes creates intricate overhanging cantilevers with multiple folds.
The structures can be packed in dense arrays and oriented in any direction.
The process enables cost effective fabrication of vertical CNT walls with a thickness below 300 nm, and aspect ratios exceeding 1:100.