Potential Microchip Insulator Shows Surprising Behavior When Tested In Ultra-Small Scale

Scientists analyzing polymers as possible insulators for advanced microchips were surprised to learn that the materials behave much differently when they are in ultra-thin layers than they do when they are in larger pieces.

That finding may be especially relevant because developing smaller, more powerful computer chips depends in part on finding new insulating materials that work in extremely thin layers.

Details about the research, which was conducted at the Commerce Department's National Institute of Standards and Technology, are being presented today during the ACS meeting. The accompanying scientific paper was written by NIST scientists Darrin Pochan, Eric Lin, Wen-Li Wu and Sushil Satija.

Plastics that might make superior insulators have a drawback: they quickly turn rubbery at certain temperatures. That is a bad trait for materials in computer circuits because computers heat up while in operation. The disproportionate expansion or contraction of layered chip materials causes stresses that result in circuit failures.

Scientists have known that large, so-called bulk-size samples of plastics turn abruptly from hard and glassy to soft and rubbery at a point called the glass-transition temperature. Above the glass-transition temperature the plastic material expands at a far greater rate than it does at temperatures below the glass transition.

But now, the NIST research has shown that extremely thin films--layers only about five molecules thick--of a polymer called polystyrene do not show the same rapid changes with temperature. Instead, they expand and turn rubbery gradually, at a constant rate. Therefore, models for predicting how the polymer will behave in ultra-thin layers do not work properly, since those models are based on the characteristics of bulk samples.

The ramifications of the new findings are unclear; researchers do not yet know whether the different characteristics of ultra-thin films will make them better or worse insulators in the microscale. Either the polystyrene does not become rubbery at all in its thin-film state, or perhaps it makes the transition at a temperature higher than the range in the experiment, which reached a maximum of 140 degrees Celsius (284 degrees Fahrenheit), said Pochan, a physical chemist in the Polymers Division of the NIST Materials Science and Engineering Laboratory.

"Only recently, have people thought about the physical consequences of placing polymers in spaces similar to, or smaller than, the molecular dimensions of the polymer molecules themselves," said Pochan, who helped perform the research. "Polymer molecules behave in a somewhat predictable way in the bulk. But when you take those same long-chain molecules and put them in a thin film, they behave in very different ways than they do when they are in the bulk. People only recently have begun modeling thin-film effects in polymers with contradictory property predictions. We are just starting to measure it."

For microchips to work, their tiny circuits must be electrically insulated from one another. Consequently, the required thickness of an insulator is one factor dictating how small chips can be made.

Scientists used a technique called X-ray reflectivity to study how the polystyrene changed with increasing temperature. They also studied the polymer when it was covered with another material to see how other insulating polymers might behave in the multilayered environment of a microchip. For that experiment, they used neutrons generated at the NIST Center for Neutron Research to see through the material covering the polystyrene.

The research is ongoing. Further experiments will include work to investigate how the polymer behaves when covered with different materials to better model real-life microchip applications.

For more information, interested parties may contact Pochan at (301) 975-3657.

As a non-regulatory agency of the Commerce Department's Technology Administration, NIST promotes U.S. economic growth by working with industry to develop and apply technology, measurements and standards.