LAKE WALES, Fla. — IBM has surmounted one of the last hurdles to a future of carbon nanotube electronics — a method of connecting them to traditional electronics with nanoscale nickel end-contacts.
IBM foresees a carbon nanotube future where wearables, Internet of Things (IoTs), implantable medical monitors as well as More-than-Moore semiconductors will all use carbon nanotubes as their base, because they are cheap and easy to fabricate at low temperatures.
One stumbling block to high-end usage however, is making reliable contacts to their tiny one-nanometer ends. To solve that problem, IBM will present a nickel annealing method at the IEEE International Electron Devices Meeting 2016 (IEDM, December 3-7, San Francisco).
Schematic of a carbon-nanotube transistor based CMOS inverter with entirely end-bonded contacts to the nanotube channels.
Carbon nanotubes are tiny (one nanometer) rolled up sheets of graphene. Today they are being used in slurries to create wearables and Internet of Things devices (IoTs) that do not require high-performance, but which need to be flexible and use inexpensive substrates.
However, for the next-generation of More-than-Moore semiconductors, carbon nanotubes are the ideal materials for transistor channels because they are just one-nanometer in diameter, much smaller than even the most experimental traditional lithographic complementary-metal-oxide semiconductors (CMOS) today.
One of the remaining obstacles to nanotube transistors, however, was how to make a transitional end-cap contact to such a tiny hollow one-nanometer tube. Today instead of making end caps, labs merely deposit a metal across the top of the nanotube, forming a poor resistance-ridden contact. Now IBM, however, thinks it has solved this problem with a genuine metallic end-cap that melds with the carbon.
"We have solved the Achilles heel of nanotube transistors today — forming a reliable, low-resistance contacts to their ends using an inexpensive metal and processing method," researcher Shu-Jen Han at IBM Research (Yorktown Heights, N.Y.) told EE Times.
IBM already created a method of self-assembling nanotubes on-chip as the channels of transistors, but until now did not have a low-resistance processes for attaching to their end-caps. Now it does.
The novel technique works at temperatures as low as 752 Fahrenheit (400 Celsius) and melts the carbon nanotube end-cap into a nickel contact to the rest of the semiconductors.
"We consider this a brilliant solution to the contact issue," Han's told EE Times.
The result is a single-point end contact made at low temperatures, but which can withstand temperatures as high as 1,112 (600 Celsius) while processing the rest of the CMOS chip. Its magic, according to Han, is that it scales with the size of the carbon nanotube, since the carbon flows into the nickel forming a rock-solid covalent bond. In addition, IBM has already demonstrated that the process works equally well with both p-type and n-type CMOS transistors (see figure).
The graph shows the both the p-type field effect transistors (FET) and the n-type FET in one carbon nanotube inverter have matching characteristics required for complementary metal oxide transistor (CMOS) semiconductors.
— R. Colin Johnson, Advanced Technology Editor, EE Times