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All The Chips That Are Fit to Print

In a development that could open new markets for wall-sized televisions and unbreakable cellular phone displays, researchers at the Palo Alto Research Center (PARC) Tuesday said they have created the first plastic semiconductor transistor array patterned entirely using jet printing.

Jet printing? You read it right. Central to the system is a polymer that can be dissolved into a liquid, creating a semiconducting ink that scientists apply directly onto flexible substrates for easier microchip production. The ink is the brainchild of Canadian researchers affiliated with imaging giant Xerox Corp. , of which the Palo-Alto, Calif.-based PARC is a subsidiary.

As the flat-panel display business continues to grow, researchers expect this new fabrication method to lower the cost of active-matrix displays by replacing vacuum deposition and photolithography in current manufacturing techniques. "PARC contributed greatly to the amorphous silicon transistor that is at the heart of all active-matrix liquid crystal displays," said Mark Bernstein, the Center's director. "With this [latest] breakthrough, PARC is well-positioned to revolutionize display technology yet again."

As the researchers explained, the new arrays are printed using additive and subtractive techniques on either flexible or rigid substrates, in a process analogous to color registration. During the additive production process, polymer inks are jetted directly onto substrates just where they are needed, and other materials are deposited everywhere else. A mask is then jet-printed on top, and those materials not protected by the mask are dissolved away through a subtractive process that lasts less than one second.

Both phases of this production cycle require precise layer-to-layer registration, and PARC printers are controlled by a patent-pending computer vision system that ensures proper alignment of the layers. According to PARC Research Scientist Dr. Raj Apte, even if one of the transistors warps or deforms during processing, a common complication with flexible substrates, this vision system can correct alignment and continue the printing process.

"The printer correctly positions each layer of color with respect to the other layers, even if the paper or substrate has shrunk or warped," Apte noted. "The layers to align [always]...create the transistor arrays."

There is, however, much more involved in the fabrication of a low cost transistor array than just printing the polymer semiconductor. As with any integrated electronic device, metals and insulators must also be deposited and patterned into a multi-layer structure that has the right electronic circuit and an appropriate physical size. Under a grant from the National Institute of Standards and Technology, PARC scientists such as Apte also have integrated the jet-printed polymer into a prototype display circuit, in which printing techniques define all the patterns.

These circuits, developed in collaboration with scientists at Motorola and The Dow Chemical Co. , could eventually be used to create roll-to-roll displays on flexible, thin plastic. The reusable paper displays would be much less fragile than current materials, and could be rolled to fit in a compact space or expanded to provide a larger display area when necessary.