Monolithic Integration

by Michael Mullaney on November 18, 2011

As a loyal reader of The Approach and our steady stream of news stories, you’ve likely heard quite a bit about graphene. The material increasingly is at the forefront of nano and materials research. And for a good reason—this stuff has some seriously cool properties and potential applications.

Graphene is a single layer of carbon atoms. Linger on this fact for a moment: graphene is only one atom tall. For all intents and purposes, the only thing that flatter or less tall than graphene (unless you get theoretical and bring quarks, neutrinos, or other exotic elementary particles to the party) is nothing at all.

It’s difficult to conceptualize this kind of uber-flatness, and nanoscale graphene is far too small for us to get a good look at. However, if you were able to shrink yourself down to the nanoscale and pay a visit to a sheet of graphene, it would look the like above image: endless carbon atoms arranged like a chicken-wire fence, stretching far beyond the horizons.

It’s not a coincidence that “graphene” sounds a lot like “graphite.” Graphite is bulk carbon and made up of countless layers of graphene all crammed together. The charcoal we use in our barbeques, and diamonds we use in dentists’ drills and wear as jewelry are also mostly carbon, but with their atoms arranged in a slightly different way.

Researchers theorized for many years about the existence of graphene, but it wasn’t until 2004 that someone was able to isolate it. The tool they used for this major feat? Store-bought scotch tape. They gently dabbed the sticky side of the tape on bulk graphite. Their low-tech approach did the trick. The adhesiveness of the tape was strong yet gentle enough to strip away layers of graphene from the graphite. Using a powerful electron microscope, the researchers were then able to find and identify individual layers of graphene on the tape. True story.

Graphene has all sorts of interesting properties. It’s arguable that the most intriguing use for graphene, however, is for nanoelectronics. The microprocessors and chips at the heart of modern electronics lean heavily on two materials: silicon and copper. Silicon is a stellar semiconductor, which means it can be made to be conducting or insulating–can be switched “on” or “off.” There are billions of these on/off switches, called transistors, on the chips in your cell phone, laptop, digital camera, TV, or basically any other electronic device. We use very thin layers of nanoscale copper called interconnects to carry electrons around the silicon chips.

As our phones and computers get smaller and smaller, they demand smaller chips. Similarly, there’s a chip industry mantra called Moore’s Law, which states the number of transistors on a computer chip—and thus the chip’s speed—should double every 18 to 24 months. Moore’s Law is a tough customer. Looking ahead several years and several generations of chips, the industry has come to terms with the harsh reality that silicon and copper’s days are numbered. The smaller we shrink our silicon transistors and copper interconnects, the more we see how their effectiveness is impeded by weird quantum phenomena. The rules of physics take a turn for the strange and unexpected when stuff gets so tiny.

Researchers at Rensselaer are investigating ways to create graphene that would be a suitable replacement for both interconnects and transistors. Engineering prof Nikhil Koratkar is looking at graphene for transistors, while physics prof Sarok Nayak is studying graphene interconnects.

The blue ocean, pie-in-the-sky goal is to bring these ideas together and one day make chips from a single material. This idea, called monolithic integration, means the transistor and interconnects would be made of essentially the same stuff. It would likely shave off many dozens of steps from the chip manufacturing process, saving an abundance of time, money, and effort.

Is graphene be the material that finally makes all of this a possibility? Could be. Nikhil, Saroj, their students, and others are Rensselaer are working hard to make it so.

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