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Conducting polymers could simplify circuit fabrication.

Conducting Polymers Could Simplify Circuit Fabrication Since the discovery of conducting polymers about 10 years ago, the primary use for the materials was thought to be in battery systems.

New studies by researchers at the Univ. of California, Berkeley, however, suggest some of the polymers might have a greater impact in microelectronics by providing a simpler way to fabricate circuits.

Bruce Novak, leader of the Berkeley group, told attendees of the recent American Chemical Society meeting in Washington, DC, that the work could lead to a revolutionary advance "capable of radically simplifying the way electronic circuits are written."

The advance couldn't come at a better time. The emphasis on electronic miniaturization has fueled a search for ways of writing increasingly thinner electronic circuit lines onto substrates.

Researchers believe they can use lasers to draw the smaller circuit lines, but they still need to find a material the laser can write on, a material that would change its electrical characteristics upon exposure to light.

Current chip fabrication systems use photoresists and photolithography techniques to make circuit patterns. The approach works well but is time-consuming and involves many processing steps.

Novak approached the problem by trying to find polymeric materials that can be converted directly into conductors upon exposure to light. The Berkeley researchers focused on a preformed poly (p-phenylene sulfide), or PPS material, that can be phenylated at the sulfur center by allowing the polymer to react with diaryliodonium salts.

After photolysis, the materials would undergo a photochemical rearrangement that would change them from polar (ionic) to nonpolar and allow them to act as photoresist materials, the researchers thought.

While running tests on the materials, the researchers made a more dramatic discovery. They found that one of the intermediates involved in the photo-decomposition process may have the same electronic structure as the conducting polymer derived from doping PPS with oxidizing agents.

"If true, then the photolyzed regions of our polymer could be converted to a conductor if [we could find] ways to [isolate] this electronic structure,"

Novak told the ACS meeting.

The researchers found that this could be done by eliminating all sources of hydrogen atoms. Once this was achieved, the intermediate's normal decomposition cascade would be halted.

"True to this prediction, when dry thin films of these arylated poly (p-phenylene) [APPS materials] are masked and photolyzed, the photolyzed regions turn black and shiny and are persistently photoconductive, while the nonphotolyzed regions remain insulating," Novak said.

Using standard microlithography techniques, the Berkeley researchers showed that the exposed APPS regions became permanently conducting circuit elements, while the other regions remained insulators.

Novak stated that experiments with the photodoping process used in the APPS work routinely provided material conductivity values of moe than [ That value is consistent with conductivity values obtained from a known conductor.

UC-Berkeley has applied for a patent on the new procedure. But Novak says more work needs to be done, especially in the basic chemistry of the materials.

"We are working to imrpove conductive and photochemical efficiency," Novak says. "We are modifying the polymer, putting new groups on to see if we can get better properties. Eventually we plan to make devices, to prove feasibility."
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Author:Stambler, Irwin
Publication:R & D
Date:Apr 1, 1991
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