ITHACA, N.Y. A multidisciplinary group at Cornell University is chasing a disposable chip technology that would layer plastic circuitry on top of thin, flexible silicon sheets to create throwaway information displays. This cheap technology could be built into consumer items such as milk cartons that scroll through pictures of missing children, or one-sheet newspapers with a button to toggle through the pages.
The group aims to layer a thin film of polymer-based transistors and interconnections on top of amorphous-silicon sheets, the easy-to-produce, flexible alternative to the thick, crystalline-pure silicon that is used in today's advanced chip designs.
The sheets of amorphous silicon resemble ordinary overhead projector transparencies and could be built as large as meters on a side. The substrates have already been bonded to polymer-based electronics in previous work, although no one knows exactly why the process works. Now the Cornell team is looking to define the molecular bond between the polymer and silicon.
"The technology for marrying these dissimilar materials is still poorly understood," said Cornell professor and team leader Paulette Clancy. "Our funding is supposed to just model and simulate the polymer-to-silicon interface, but we will also seek experimental verification."
To assist in the verification job, Clancy and Fernando Escobedo, both specialists in chemical engineering, have enlisted fellow Cornell professors Michael Thompson and George Malliaras in materials science and engineering; Michael Teter, a professor of physics; and Edwin Kan, a professor of electrical engineering.
The major roadblock to disposable electronics is the basic incompatibility of silicon, which is inorganic, with organic polymers. Organic and inorganic chemistries are so dissimilar that college courses almost always separate them into two tracks.
In this study, the professors teaching those separate courses will put their heads together with Clancy in an effort to figure out how the dissimilar materials can work together.
"To understand the sandwiched materials, we will need to accumulate knowledge that is hard to come by amorphous silicon is not very well-understood and polymer-based electronics even less [so]," said Clancy.
Amorphous silicon is used in monolithic applications such as solar cells and LED replacements. "Amorphous silicon is not rougher; it just doesn't have a perfect crystal pattern," said Thompson, the group's silicon materials expert. "If you look closely, it looks like platelets stacked on little pedestals, but with no logic to how they are stacked."
Polymer-based electronic materials are even more of a cipher. They have proven essential for blue and green light emission, which is difficult to achieve with semiconductors. But their incompatibilities with inorganic amorphous silicon are manifold.
Silicon deposition and other common processes are routinely run at hundreds or thousands of degrees, whereas low-temperature organics for polymer construction are typically kept below 100C. Researchers have overcome those problems by trial and error. For engineers it is enough that applications work, but Clancy's team plans to explain why.
"Electrically active organic polymers can act as displays by emitting light like an LED," said Thompson. "You can make sound with organic piezoelectric elements, and you can make sensitive sensors. We'll use the silicon layer for computation and the polymer layer for what it's best at. By using both technologies you can fine-tune the process to get anything you want." The silicon-polymer bond was made to work with a laser that superheats the boundary between the two materials. Theoretically, the factor of 10-to-1 difference in temperature expansion coefficients should doom the material to self-destruction as soon as its temperature starts changing, but for some reason it holds up.
"The theories say it should not work, so we are going to take a close look to find out what is really happening," said Thompson.
Both sides now
Clancy is directing attacks from each side of the interface Thompson from the inorganic silicon side and Malliaras working on the organic polymer.
For more complex architectures, Teter will work directly with Clancy and Escobedo to perfect his novel approach to polymer circuit design that harnesses quantum mechanics. The goal is to combine high computational speeds with higher accuracy than conventional circuitry.
Kan's job, then, will be to use the molecular structures modeled by Teter, Clancy and Escobedo to build real devices.