July 23, 2009

Carbon Nanotubes integrated with MEMS

Researchers in Isreal have integrated suspended carbon nanotubes into micro-fabricated (MEMS) devices. They grow the carbon nanotubes onto the MEMS. Chirality is not controlled in their method. Other researchers have used DNA to sort carbon nanotubes by chirality but these methods have not been integrated.

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The integration of suspended carbon nanotubes into micron-scale silicon-based devices offers many exciting advantages in the realm of nano-scale sensing and micro- and nano-electromechanical systems (MEMS and NEMS). To realize such devices, simple fabrication schemes are needed. Here we present a new method to integrate carbon nanotubes into silicon-based devices by applying conventional micro-fabrication methods combined with a guided chemical vapor deposition growth of single-wall carbon nanotubes. The described procedure yields clean, long, taut and well-positioned tubes in electrical contact to conducting electrodes. The positioning, alignment and tautness of the tubes are all controlled by the structural and chemical features of the micro-fabricated substrate. As the approach described consists of common micro-fabrication and chemical vapor deposition growth procedures, it offers a viable route toward MEMS–NEMS integration and commercial utilization of carbon nanotubes as nano-electromechanical transducers.

In separate work, european researchers have used carbon nanotubes to weigh a single atom. Eventually arrays of carbon nanotubes could be used to determine the composition of atoms in any gas. The two methods could work together to make very sensitive sensors.

Weighing atoms - separate european work

A noted impediment in the present technique is the inability to control the CNT chirality. Thus, the electrical properties of the tethered tubes vary greatly between different devices. It should be noted that the mechanical properties of CNTs, on the other hand, do not depend much on the chirality. Although undesired, this large variability in device performances is not a major impediment considering the fact that MEMS devices suffer from exactly the same problem.

In fact, commercial devices are often individually tested and calibrated. Thus the variability between CNT devices can readily be overcome during this testing stage. In fact, Raman spectroscopy can readily be implemented to characterize the tubes and to identify their metallic or semi-conducting properties. A detailed study of the Raman maps of our devices is currently underway and will be discussed elsewhere. With the fabrication process described here at hand, the manner by which CNTs can be effectively integrated into MEMS devices strongly depends on theMEMS device design so the CNT response to mechanical deformation is fully and efficiently exploited. Additionally, in these devices the tubes have to be properly anchored, and the range of deformation limited to avoid plastic effects.

To summarize, we used a simple, CVD CNT growth method to achieve the integration of carbon nanotubes into micro-fabricated devices. Material selectivity of the CNT growth enabled us to tailor novel electronic devices as a step toward building CNT-based NEMS devices. The procedure described above is suitable for straightforward utilization of CNTs as electromechanical elements in otherwise silicon based fabrication, thus opens up the prospects of commercial utilization of CNT technology in micro-electronic-based applications.

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