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 ."
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.