Researchers at Stanford University have made stacked carbon nanotube ICs with an average density of 100 carbon nanotubes per micrometer with a current density of up to 122 microamperes per micrometer.
“Now that we have this nanotube that’s on par with conventional silicon,” says researcher H.-S. Philip Wong, “we can think about building high-performance systems.”
The Stanford team grew CNTs on quartz begins with quartz. A layer of gold was deposited on top and then peeled away with thermal tape, taking the CNTs with them.
The nanotubes can then be transferred to the target surface, where the thermal tape is eased off and the gold chemically removed, leaving an array of parallel carbon nanotubes on the surface.
One transfer yields around eight nanotubes per micrometer, as measured perpendicular to the direction that current would flow across the devices.
The deposition process can be repeated more than a dozen times by laying down a gluelike polymer before each successive deposition of carbon nanotubes which prevents the CNTs from sticking to one another.
The Stanford team used the multiple-transfer strategy to create a monolithic 3-D IC.
The team built a crossbar switch—a circuit that can be used to connect different inputs and outputs—out of a layer of silicon, two layers of resistive RAM, and then a layer of carbon nanotube transistors. They were able to build the stack of circuits without raising temperatures above 400 °C, which could damage the transistors.