IBM Research announced an engineering breakthrough that could accelerate the replacement of silicon transistors by carbon nanotubes to power future computing technologies. IBM scientists demonstrated a new way to shrink transistor contacts without reducing performance of carbon nanotube devices, opening a pathway to faster, smaller and more powerful computer chips beyond the capabilities of traditional semiconductors. The results are reported in the journal Science.
IBM’s breakthrough overcomes a major hurdle that silicon and any semiconductor transistor technologies face when scaling down. In any transistor, two things scale: the channel and its two contacts. As devices become smaller, increased contact resistance for carbon nanotubes has hindered performance gains. These new results could overcome contact resistance challenges all the way to the 1.8 nanometer node—four technology generations away.
Carbon nanotube chips could greatly improve the capabilities of high performance computers, enabling Big Data to be analyzed faster, increasing the power and battery life of mobile devices and the Internet of Things, and allowing cloud data centers to deliver services more efficiently and economically.
Silicon transistors, tiny switches that carry information on a chip, have been made smaller year after year, but they are approaching a point of physical limitation. With Moore’s Law running out of steam, shrinking the size of the transistor—including the channels and contacts—without compromising performance has been a vexing challenge troubling researchers for decades.
IBM has previously shown that carbon nanotube transistors can operate as excellent switches at channel dimensions of less than ten nanometers—less than half the size of today’s leading silicon technology. IBM’s new contact approach overcomes the other major hurdle in incorporating carbon nanotubes into semiconductor devices, which could result in smaller chips with greater performance and lower power consumption.
Carbon nanotubes represent a new class of semiconductor materials that consist of single atomic sheets of carbon rolled up into a tube. The carbon nanotubes form the core of a transistor device the superior electrical properties of which promise several generations of technology scaling beyond the physical limits of silicon.
Electrons in carbon transistors can move more easily than in silicon-based devices, and the ultra-thin body of carbon nanotubes provide additional advantages at the atomic scale. Inside a chip, contacts are the valves that control the flow of electrons from metal into the channels of a semiconductor. As transistors shrink in size, electrical resistance increases within the contacts, which impedes performance. Until now, decreasing the size of the contacts on a device caused a commensurate drop in performance—a challenge facing both silicon and carbon nanotube transistor technologies.
In the Science paper, the researchers describe the development of the single-walled carbon nanotube (SWNT) transistor technology with an end-bonded contact scheme that leads to size-independent contact resistance to overcome the scaling limits of conventional side-bonded or planar contact schemes.
IBM researchers had to forego traditional contact schemes and invented a metallurgical process akin to microscopic welding that chemically binds the metal atoms to the carbon atoms at the ends of nanotubes. This end-bonded contact scheme allows the contacts to be shrunken down to below 10 nanometers without deteriorating performance of the carbon nanotube devices.
For any advanced transistor technology, the increase in contact resistance due to the decrease in the size of transistors becomes a major performance bottleneck. Our novel approach is to make the contact from the end of the carbon nanotube, which we show does not degrade device performance. This brings us a step closer to the goal of a carbon nanotube technology within the decade.— Dario Gil, vice president of Science & Technology at IBM Research
Qing Cao, Shu-Jen Han, Jerry Tersoff, Aaron D. Franklin, Yu Zhu, Zhen Zhang, George S. Tulevski, Jianshi Tang, and Wilfried Haensch (2015) “End-bonded contacts for carbon nanotube transistors with low, size-independent resistance” Science doi: 10.1126/science.aac8006