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Liquid silicon promises ‘green’ devices
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 500°C 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/Vs-almost 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 15µm patterning. The company is now studying nanolevel
droplets to enable a 0.5µm 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.”
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