Portland, Ore. Carbon nanotubes are nearly ideal one-dimensional semiconductors, but current fabrication techniques, such as arc discharge, laser ablation and chemical-vapor deposition, leave their ends capped. To attain good wetting with solders, low-contact resistance with the substrate and unimpeded emission from the nanotubes' tips requires opening their ends with an extra etching step. That process invariably damages them, says Georgia Institute of Technology professor Ching-Ping Wong. Sometimes the ones with poor adhesion from the substrate are entirely dislodged, fraying their ends, causing them to age prematurely and overall degrading their electrical performance in circuits. Current carbon nanotube growth methods also can't be combined with inexpensive low-temperature substrates.
Wong claims to have a new carbon nanotube growth and fabrication technique that solves all those problems by separating the growth from the assembly of nanotube-based devices. His "carbon nanotube transfer" technology is a two-step process whereby sheets of open-ended carbon nanotubes are separately grown on silicon substrates, then transferred to epoxy substrates in a manner similar to that used for flip-chips. Wong, a professor of materials science and engineering, performed the work with fellow Georgia Institute of Technology professor Dennis Hess and doctoral candidates Lingbo Zhu and Yangyang Sun.
"What we are offering is a new paradigm for transferring and integrating carbon nanotubes onto integrated circuits and microelectronic packages," said Wong. "We have circumvented both the high-growth temperature and poor adhesion that currently plague other implementation methods."
For the demonstration, Wong's group used standard chemical-vapor deposition (CVD) equipment to make nanotube sheets on silicon, and flip-chip fabrication equipment to assemble a field emitter for a display on an inexpensive epoxy substrate.
The first step was fabricating the silicon chip with an array of open-ended carbon nanotubes. Then the team transferred the chip to a temperature-sensitive substrate, here an epoxy (FR-4) board coated with copper foil. Under-bump metallization layers were sputtered onto the copper substrate, and stencil printing was used to pattern eutectic tin-lead solder that was polished into 30-micron bumps. The silicon substrates on which the carbon nanotubes were grown were then flipped and aligned to the correspond- ing copper substrates, and soldered to simultaneously form both electrical and mechanical connections. "We have demonstrated for the first time that we can grow open-ended carbon nanotubes in situ--that is, we can fabricate carbon nanotubes that are open-ended where they are needed, then transfer them to a temperature-sensitive substrate," Wong said.