On-chip interconnections using carbon nanotube technology have been demonstrated at above 1GHz by researchers at California's Stanford University working with semiconductor companies Toshiba and TSMC.
"This is the first time anyone has been able to show digital signals going through nanotubes at 1GHz," claimed Stanford's Professor Philip Wong. "There had been a lot of expectations that nanotubes could do this, but no experimental proof so far."
Carbon nanotubes come in two types, semiconducting and metallic, and it is metallic-type nanotubes which have the characteristics required for high-speed on-chip interconnect.However, no way has been found to grow them predictable from point to point, and there is no effective way to accurately position carbon nanotube interconnects (CNTs) grown elsewhere.
In the Stanford experiment, researcher Gael Close with Toshiba scientists designed and array of 256 ring oscillators using 11,000 transistors on a 6.5mm[super2] chip.
In each oscillator one connection was missing, brought out as a pair of electrodes.
"After the semiconductor foundry TSMC made the chip, Close then engaged in a few more fabrication steps at our nanofabrication facility to complete the missing connections with the nanotubes," said the research team.
The CNTs were 50-100nm in diameter and around 5[micro]m long - purchased from a commercial vendor where they were synthesised for maximum electrical conductivity.
"The quality of the nanotubes and their connections varied widely, but in the end 19 of the ring oscillators were successfully connected," said the researchers.
The nanotubes rested directly above the transistors they were connecting, minimising electrical capacitance and allowing for the transmission of zeroes and ones at 1.02GHz in the best case. In 16 of the 19 good connections, the oscillators ran at speeds better than 800MHz.
In a separate experiment, Stanford demonstrates its ability to connect electrodes using a multi-walled carbon nanotube.