IBM Cooks Up Super-Thin Graphene Chips
Jun 10, 2011 12:01 PM PT
IBM researchers have demonstrated a graphene circuit which integrates all circuit components onto a single wafer made of silicon carbide.
Graphene is a mesh of carbon atoms that's one atom thick. Integrating it monolithically -- meaning in one unit -- with other materials is a problem researchers have been grappling with since 2004, when the material's properties were first demonstrated.
IBM announced the demonstration of its monolithically integrated graphene circuit in a paper published Friday in the journal Science.
The paper's main point is that monolithic integration of graphene is feasible, IBM Fellow Phaedon Avouris, who led the research team, told TechNewsWorld.
"This is the first demonstration of such monolithic integration and is a great accomplishment," Jeanie Lau, an associate professor at the University of California, Riverside's physics department, remarked.
The research was conducted at IBM's T.J. Watson Research Center in Yorktown Heights, N.Y.
What Is Graphene?
Graphene is essentially carbon that comes in sheets one atom thick densely bonded together in a honeycomb crystal lattice.
You can think of graphene as a plane of graphite -- that stuff making up your pencil lead -- which is one atom thick. Visualize graphene's structure as chicken wire.
In 2004, physicists at the University of Manchester and the Institute for Microelectronics Technology in Chernogolovka, Russia, first isolated individual graphene flakes and measured their electronic properties.
About IBM's Announcement
The integrated graphene circuit announced by the IBM researchers operates as a broadband radio frequency mixer at frequencies up to 10 GHz.
Frequency mixers are used in all communications devices -- radios, TVs, cellphones and radio satellites, for example, IBM's Avouris said.
The circuit announced by IBM is an analog circuit. It's designed to work -- meaning switch on and off -- at 10GHz, which is 10 billion times a second, Avouris said. IBM is working to improve on that.
"We have graphene transistors working at over 200 GHz," Avouris said. "It is possible that eventually graphene can operate in the terahertz range."
How the Graphene Circuit Was Made
Creating graphene circuits is difficult because combining graphene with other materials has slowed down the circuits.
"Wafer-scale fabrication of graphene has been one of the biggest challenges in realizing the promise of graphene electronics," UC Riverside's Lau told TechNewsWorld.
To make the integrated graphene circuit, the IBM team first heated a silicon carbide wafer to desorb, or release, silicon and form a layer of graphene on its surface.
The graphene film was allowed to grow to two or three layers in thickness, and then the silicon surface was heated to 1,400 degrees Celsius.
Top-gated dual-fingered field-effect transistors were then integrated with aluminum inductors. Following this, the wafer was spin-coated with a thin polymer, and a layer of hydrogen silsesquioxane was applied to create active channels.
The active channels were then carved by e-beam lithography, the excess graphene was removed with an oxygen plasma laser, and the circuit was cleaned with acetone.
Hydrogen silsesquioxane is a negative tone electron-beam resist. Photoresists are used in the fabrication of printed circuit boards.
IBM's circuit measures one square millimeter, Avouris said.
The Future for Graphene Circuits
CMOS (complementary metal oxide semiconductor) technology, which is currently used in integrated circuits, is rapidly approaching the limits of what it can do, and graphene is one of the alternatives being eyed as a replacement.
However, the technology to produce graphene circuits "is still in its infancy, and more work needs to be done to iron out the wrinkles," Lau pointed out.
Among other things, the device yield rate needs to be improved, Lau suggested.
Using graphene in electrodes is the application that's probably closest to commercialization, Lau said.
However, there are many other potential uses of graphene because of its uniqueness, she added.
Graphene is "transparent like plastic but conducts heat and electricity better than metal, it's an elastic thin film, it's an impermeable membrane, and it's chemically inert and stable," Lau said.