Nanostructures such as Nanowires (NWs) and Carbon Nanotubes (CNTs) have been identified as some of the most promising candidates to extend and even replace materials currently used in microelectronic manufacturing processes. For example, metallic CNTs have been proposed as nano-electronic interconnects due to their high current carrying capacity, whereas semiconducting CNTs have been indicated as nanoscale transistor elements due to their large range band gap. Both metallic and semiconducting CNTs are also, due to their excellent mechanical properties, promising structural materials for Micro-Electro-Mechanical Systems (MEMS) and Nano-Electro-Mechanical Systems (NEMS). These and similar applications cannot be fully accomplished yet since the fabrication of nanostructures still faces a variety of unsolved issues, which vary from one application to another but may, however, be similar in some aspects.
A first issue is related to the growth of nanostructures in a predefined direction e.g. a growth which is substantially parallel to a main surface of a substrate i.e. in a direction, when the main surface of the substrate is lying in a plane, substantially parallel to the plane of the main substrate.
Thermal and/or plasma enhanced chemical vapor deposition (CVD) have been used extensively to grow carbon nanotubes (CNTs) and pattern devices around them. However, the methods reported in the state of the art to grow these CNTs lack either one or at least the combination of following parameters: predictability and control in terms of CNT density, on-chip location, and orientation. For transistor applications and for use in MEMS and NEMS, it would be extremely advantageous to achieve horizontal single CNT growth between electrode pairs.
In prior art methods, controlling on-chip CNT density and position has been achieved by dielectrophoresis, see R. Krupke et al., Nano Lett., 7 (6), 1556, 2007. This is a technique that makes use of a non-uniform electric field to align pre-grown CNTs dispersed in a liquid medium on to a surface. This is a non-standard processing technique and does not yield very good contact resistance (CNTs just stuck by Van der Waals forces). There have also been attempts to grow aligned CNTs by using an in-situ electric field between 2 large electrodes during CVD, for example H. Dai et al., App. Phys. Lett., 81 (5), 913, 2002. Also localized growth by CVD has been attempted by defining Si-oxide islands on TiN electrodes (on both electrodes of the electrode pair). See Yaakobowitch et al, Proc. MEMS 2010, 432. The catalyst particles are here still spread out over the whole wafer, but growth happens preferentially on the oxide islands. The pictures shown in this reference clearly indicate that CNTs also grown on other parts of the wafer and also many catalyst particles are present on Si-oxide islands of both electrodes, making the growth of a single CNT not realistic. Furthermore, electrical contact is only achieved when the CNT also touches the TiN electrode as Si-oxide is non-conducting.