In Place Fabrication Solves Organic Polymer Shortcoming

"We want to build electronic devices like transistors and flexible circuits," says Dr. Seong Kim, assistant professor of chemical engineering.

Kim and Sudarshan Natarajan, graduate student in chemical engineering, looked at fabricating circuits from polythiophene. This conjugate conducting organic polymer is easily made in a beaker, but once the polymer is created from chaining together a series of identical smaller molecules – monomers – it is a powder that cannot be molded or used for film coating.

"Conjugate conducting polymers are not soluble nor are they meltable," Kim told attendees at the annual meeting of the American Chemical Society today (Sept. 8) in New York. "Some researchers have made them soluble by adding elements to the polymer backbone, but making circuit boards with these is difficult and requires high energy."

Kim and Natarajan solve the fabrication problem by combining the synthesis and processing steps, which are done separately in conventional methods, into a single step.

"We bypass the problem," says Kim. "We make the material at the site of application."

The researchers use a prepared substrate and deposit the monomer – the small molecule that chains to make the polymer – using standard physical vapor deposition. Once they have a thin film of the monomer on the substrate, they apply a mask, similar to those used in standard silicon electronics manufacture, to the surface. The masked monomer film is then exposed to ultraviolet light.

The light causes two monomers to join forming a dimer, then a third molecule to form a trimer and so on. Dimers and trimers then join to begin forming the much longer polymer chains until all the monomer exposed to light is polymerized.

This reaction differs from normal polymerization where the chain begins at one point and grows by adding individual monomers. In this new process, monomers are joining each other wherever they are struck by the photons in the ultraviolet light. The process takes about seven minutes to complete. The researchers then wash off the soluble, uncoupled monomers, leaving only the pattern of conducting polymers indicated by the mask.

Aiming to incorporate conjugate conducting organic materials into the current silicon-based microtechnology, the researchers tried a variety of inorganic substrates including copper, gold and silicon.

However, neither copper nor silicon will make a flexible circuit, so the researchers are also investigating using other flexible substrates such as plastics. A circuit made on plastic materials would find applications where the flexibility is critical. For example, flexible circuits would be ideal for lightweight flexible-screen displays creating electronic paper.

A variety of conducting polymers are also light emitting. The proper combination of red, yellow and green can produce full color images. Another example would be in biomedical applications.

The researchers are looking at a variety of other organic conducting polymers for use in in-place fabrication of circuits and electronic devices.

A seed grant from Penn State's National Science Foundation-supported Materials Research Science and Engineering Center seed grant supported this work.