A process has been proposed for growing carbon nanotubes aligned substantially parallel with the nominal planar surfaces of substrates and further aligned with patterns on the substrates. Prior to growth, the patterns would be formed by micromachining the substrates, which could be silicon or siliconon- insulator (SOI) wafers. By making it possible to tailor the positions and orientations of individual carbon nanotubes grown on pre-patterned substrates, this process would enable advances in nanotube-based electronic and electromechanical devices.
The process would include chemical vapor deposition (CVD) of the carbon nanotubes on patterned catalysts on the substrates. In each case, the CVD gas would consist of a source of carbon (such as methane, ethylene, or carbon mono- xide) either by itself or in a mixture with other gases. Carbon nanotubes grow when a substrate with patterned catalyst is heated and exposed to this CVD gas mixture under appro- priate conditions [which can include enhancement by RF (radio frequency) plasmas and/or hot filaments].
The basic process admits of three main variants, each involving a different technique or combination of techniques to position and orient the growing carbon nanotubes. In the first variant (the basic process), the desired alignment would be enforced by use of in-plane pointed silicon cantilevers protruding from an undercut silicon layer on an SOI substrate (see Figure 1). Part of the upper surface of each cantilever would be coated with a thin film of a suitable catalyst (e.g., Ni, Co, or a suitable metal alloy or compound).
On the basis of prior experiments on the growth of nanotubes, it is expected that (1) the nanotubes will tend to nucleate at random times and locations, such that multiple tubes may grow out of each catalyst film, and (2) because of attractive van der Waals forces, the nanotubes will tend to grow along the cantilever surfaces and edges. It is also anticipated that if the tip of a growing nanotube reaches the tip of the cantilever, further growth would likely cause the nanotube to protrude from the tip because strainenergy cost of bending the nanotube to the small tip radius would exceed the energy of van der Waals attraction (see Figure 2).