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'Green' silicon ink prints cheaper RFID tags

Posted: 15 Nov 2007 ?? ?Print Version ?Bookmark and Share

Keywords:silicon ink? green technology? RFID?

Silicon ink for printing electronic circuitry atop flexible foil substrates was unveiled this week at the Printed Electronics conference in the United States. Kovio Inc. described its "green" silicon ink for 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 RFID 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.

'World's first'
"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?m, but we have 10?m working in the lab, which is where Intel started in 1971. Intel's first microprocessor used just over 2,000 transistors: similarly, our first devices for RFID tags will use about less than a thousand transistors when we go into mass production by the end of next year [2008]."

Kovio is building its own fab that 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 600sq.cm/Vs, and polysilicon transistors, like those that drive LCD displays, have mobilities of about 100sq.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 1sq.cm/Vs in contrast with Kovio's silicon ink, which rivals polysilicon with its 80sq.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 500C and excimer-laser annealing. Unfortunately, the Seiko Epson formulation only achieved electron mobilities 6.8sq.cm/Vs when inkjet-printing transistors: too slow for RFID applications and almost 12 times slower than Kovio's 80sq.cm/Vs process.

'Fast enough for RFID'
"Single-crystal silicon is faster than us, but we are faster than all the organics and printable silicon circuits reported today," said Vik Pavate, VP 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 [ultrahigh 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 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.

Patent count
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, a Japanese business-form printer, and Cubic Transportation Systems Inc., 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.

- R. Colin Johnson
EE Times




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