[info] advancednano: eetimes: printed silicon transistor with polysilicon electron mobility and 20 micron features

[Alejandro Dubrovsky]

(
http://www.eetimes.com/news/latest/showArticle.jhtml?articleID=202805929&pgno=2&printable=true
)

Silicon circuits made ink-jet printable

R. Colin Johnson
(11/13/2007 3:07 PM EST)
URL: http://www.eetimes.com/showArticle.jhtml?articleID=202805929
 
SAN FRANCISCO, Calif. -- Silicon ink for printing electronic circuitry
atop flexible foil substrates was unveiled today at the Printed
Electronics conference (Nov. 12-15, 2007, San Francisco). Kovio, Inc.
(Sunnyvale, Calif.) described its "green" silicon ink for thin-film
transistors (TFTs) that achieve the performance of polysilicon
transistors, but at a third their price and consuming only 5 percent of
the chemicals and 25 percent of the energy of single-crystal silicon.
Kovio claimed that radio-frequency identification tags using its silicon
ink will drop Kovio's price from 15 cents today to 5 cents by 2008, when
Kovio begins volume production of its inkjet-printed RFID tags.

"We have the world's first all-printed silicon transistor," said Amir
Mashkoori, CEO and chairman of Kovio. "Our thin-film silicon transistors
have very high mobilities for a printed device and we can make both
p-type and n-type devices for CMOS circuits. Right now our design rules
are 20 micron, but we have 10 micron working in the lab, which is where
Intel started in 1971. Intel's first microprocessor used just over two
thousand transistors: similarly, our first devices for RFID tags will
use less than about a thousand transistors when we go into mass
production by the end of next year [2008]."

Kovio is building its own fab, which uses temperatures too high for
plastic substrates (which is why Kovio uses a stainless steel foil
substrate), but which does not require the expensive processing
equipment and clean-room environment of single-crystal silicon fabs.
Silicon ink devices can be fabricated on roll-to-roll printing
equipment, which is how Kovio plans to dramatically drop the price of
RFID tags and similar applications using all types of flexible
electronics.

"From a capital viewpoint, we can build a printable silicon fab for
about $10 million, compared with $1 billion for a traditional silicon
fab," said Mashkoori. "Of course, we will need more of them as volume
ramps up, but the point is that it is a much smaller incremental cost.
Plus we need only about five percent of the materials (one percent of
substrate cost and three percent of the cycle time) to create new
devices."

By way of comparison, single crystal silicon transistors today can
achieve mobilities as high as 600 centimeters squared per volt second
(sq cm/Vs), and polysilicon transistors, like those that drive LCD
displays, have mobilities of about 100 sq cm/Vs. Unfortunately, there is
a big gap between single-crystal silicon and the printable organic
transistors that are being demonstrated at dozens of labs worldwide.
Organic transistors have dismal electron mobilities of less than 1 sq
cm/Vs in contrast with Kovio's silicon ink, which rivals polysilicon
with its 80 sq cm/Vs electron mobilities. Most important, silicon ink
can produce transistors that are fast enough for RFID and most other
electronic interface protocols.

Kovio's only reported rival for silicon ink today is a research project
reported by Seiko Epson Corp. last year that used a silane compound of
hydrogen and silicon, called polysilane, which was inkjet-printed in a
nitrogenous atmosphere, followed by baking at 500 degrees Celsius and
excimer-laser annealing. Unfortunately, the Seiko Epson formulation only
achieved electron mobilities 6.8 sq cm/Vs when inkjet-printing
transistors: too slow for RFID applications and almost 12 times slower
than Kovio's 80 sq cm/Vs process.

"Single-crystal silicon is faster than us, but we are faster than all
the organics and printable silicon circuits reported today," said Vik
Pavate, vice president of business development at Kovio. "Most
importantly, our printable silicon is fast enough for RFID applications;
in fact, the speed of our RFID tags exceeds the specifications for both
HF [high-frequency, or 13.56 MHz] and UHF [ultra-high frequency, or 900
MHz] bands."

Silicon ink was the brainchild of Professor Joe Jacobson and his student
Colin Bulthaup at the Massachusetts Institute of Technology, who
co-founded Kovio when it spun off from MIT in 2001. Besides being speedy
enough for easy integration into the existing RFID infrastructure,
Kovio's silicon ink is greener than single-crystal silicon chips.
Silicon ink uses an additive approach, whereby the only materials
consumed go into the makeup of the circuitry. Traditional silicon
fabrication uses the opposite, or subtractive, approach, which grows
wafer-wide layers of materials, then etches away what is unwanted the
way a sculptor chips away at a block of marble: leaving most of the
material as waste.

"We are taking an additive approach to making silicon circuits, which is
more economical in its both its price and its conservation of
resources," Pavate said.

Since with Kovio's process the circuitry is already on a flexible
substrate, it can be attached to an RFID tag's antenna by means of
roll-to-roll printing equipment instead of with the more expensive
pick-and-place semiconductor-chip-handling equipment used to make
single-crystal silicon RFID tags.

Kovio has filed more than 86 patents and has had about a dozen granted
so far, protecting the processes by which it achieves polysilicon
transistor performance from its silicon-ink-printed transistors. Kovio
is also reserving as trade secrets certain parts of its process, which
it believes give it a proprietary advantage and make reverse engineering
very difficult for other companies.

So far Kovio has signed as customers Toppan Forms Co. Ltd. (Tokyo), a
Japanese business-form printer, and Cubic Transportation Systems, Inc.
(San Diego, Calif.), producer of automated fare-collection systems for
public transport, both of which have joint development and supply
agreements with Kovio.

Kovio employs 31 people, 22 of whom are engineers, and has a dozen
investors, ranging from major venture capitalists, such as Kleiner
Perkins Caufield & Byers, to industrial giants, such as Panasonic.