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Liquid silicon promises 'green' devices

Posted: 16 May 2006 ?? ?Print Version ?Bookmark and Share

Keywords:Yoshiko Hara? silicon? Seiko Epson? liquid-silicon? transistor?

In pursuit of printable silicon devices, Seiko Epson Corp. has developed a liquid-silicon material and formed a silicon film and a transistor through a spin-coating process. The material, jointly developed with JSR Corp., is cyclopentasilane (CPS), a silane compound of hydrogen and silicon.

"If we can improve the process, we would be able to deliver brand-new electronic devices that are environmentally friendly," said Tatsuya Shimoda, deputy managing director of corporate R&D at Seiko Epson.

Epson believes liquid materials could lower the cost of semiconductor fabrication, in part because they would eliminate large vacuum equipment from the production line. The introduction of inkjet printing technology would further lower the cost. Leveraging its printer technology, Epson had already applied a microliquid process to fabricate devices such as color filters and organic-LED displays.

In the work with JSR, polysilane is polymerized from highly purified CPS by exposing ultraviolet light from the outside and avoiding contamination. Since polysilane is insoluble, the JSR and Seiko Epson researchers developed a version of the material that contains only Si and H. They found that this form of polysilane dissolves into CPS, some of which are left unpolymerized. The highly pure polysilane liquid becomes a polysilicon film by spin coating or inkjet printing in a nitrogenous atmosphere, followed by baking at over 500C and excimer-laser annealing.

For their experimental fabrication of transistors, the engineers used spin coating or inkjet printing in place of a conventional chemical-vapor-deposition (CVD) process to form the polysilicon layer. They used a conventional process to form the gate, source and drain. "We've cleared the most difficult process in fabricating a transistor with our liquid-silicon material," said Masahiro Furusawa, manager of the Flex Group at Epson's Technology Platform Research Center.

A transistor made from a spin-coated film, with patterning done by conventional photolithography, had a mobility of 108cm2/Vsalmost the same as a polysilicon TFT made by the CVD method. This shows that the material has high potential, said Furusawa.

The transistor made by inkjet patterning, meanwhile, had a mobility of 6.8cm2/Vs. "The mobility is not high yet, but this is mainly because we have not optimized the process, material and substrate. We believe that the printed silicon TFTs show sufficient potential," said Furusawa.

For the prototype transistors, the team used a glass substrate. But the polysilane is repellent against glass, causing the film to become as thick as 300nm. "Some breakthrough is necessary for the substrate, especially when used with inkjet printing," Furusawa said. "Conventional polysilicon film is about 100nm thick. If the film becomes thin, laser annealing can make a better crystal."

Epson is also attempting to make the ink droplet smaller for minuter patterning. In inkjet printers in volume production, a droplet of 2pL enables 15?m patterning. The company is now studying nanolevel droplets to enable a 0.5?m pattern.

"It was a dream to make a display with inkjet printing 10 years ago, but we've realized it," Shimoda said. "So it is not a dream now to make a Pentium with the microliquid process."

- Yoshiko Hara
EE Times

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