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Fujitsu Develops Efficient Silicon-Crystal Production Technology for Thin-Film Transistors
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Fujitsu Laboratories Ltd.

Fujitsu Develops Efficient Silicon-Crystal Production
Technology for Thin-Film Transistors

Will Help Make "Sheet Computers" A Reality

Tokyo, May 20, 2002-Fujitsu Laboratories Ltd. today announced that it has developed technology enabling the efficient and practical production of thin-film transistor (TFT)*1 silicon crystals, an essential step towards the realization of system LCD panels, or so-called "sheet computers," *2 and has confirmed the effectiveness of this new technology through test operation of SRAM and shift-register peripheral circuits.

The new technology represents a major advance in lowering the cost of producing TFTs for sheet computers, bringing their development one step closer to realization.

Details of the new technology are to be presented at the Society for Information Display's conference (SID 2002) opening in Boston on May 19.

Using TFTs, it is possible to embed a multitude of circuit functions onto the glass substrate of an LCD or organic electro luminescent (EL) display panel to create a thin, lightweight sheet computer. Such TFTs, however, would need to operate at very high speeds, requiring higher electron mobility than can be achieved with the low-temperature polysilicon TFTs currently available. In July of last year, Fujitsu Laboratories developed a technology called CW-laser Lateral Crystallization (CLC) to fabricate TFTs on a glass substrate with performance equivalent to that of a typical MOSFET LSI on monocrystalline silicon. This CLC technology resulted in a dramatic improvement in the performance level of low-temperature polysilicon TFTs and represented an important milestone towards the development of sheet computers. However, because a CW laser was used to produce crystals with high electron mobility, it was necessary to reduce the size of the spot cast by the laser to achieve large power densities, making the production process very time-consuming.

About the Technology
The new technology resolves this problem by adjusting the level of crystallization*3 to the performance level of the TFTs required in different areas of the panel, generating the high-mobility crystallization needed for high-performance TFTs in the peripheral circuits, and normal crystallization levels for other areas that do not require high-performance TFTs. A sheet computer would require high-performance TFTs only in the peripheral circuit regions, while the display pixels (which occupy the vast majority of the surface area) do not need high-performance TFTs. Furthermore, because TFTs in the pixels are laid out in a regular, sparse pattern, optimum crystallization can maintain the required level of TFT performance while increasing the speed of CLC crystallization by roughly 50 times.

The new technology is actually a combination of three technologies:
  1. Selective irradiation of TFT areas - A technology for selectively crystallizing the areas where there are TFTs. Silicon is eliminated from areas where there are no TFTs, so crystallization is not necessary.

  2. Beam splitting - A technology for splitting a laser beam to increase the number of beams. Since TFT area can be as small as 5 µm2, it only takes a small amount of energy to achieve crystallization. The excess energy can be directed into beams reaching other TFTs.

  3. Simultaneous irradiation with multiple beams - Irradiating simultaneously with multiple laser beams accelerates the crystallization process.

These techniques, taken together, yield roughly 50 times the throughput of previous CLC technologies. Compared to excimer lasers, which can only produce relatively low-mobility TFTs, this crystallization process is potentially two to three times as fast.

Fujitsu will continue its progress in developing TFT integration technology for LCDs and organic EL displays, moving forward toward the development of sheet computers.

*1: Thin-film transistor
A transistor operated by a thin film of silicon over an amorphous insulating substrate. More specifically, TFTs produced using a low-temperature process (below 550°C) are both inexpensive and can be used with large glass surfaces, and are known as low-temperature polysilicon TFTs. The electron mobility of low-temperature polysilicon TFTs is usually in the range of 400 cm2/Vs, but Fujitsu's new CLC technology breaks new ground by achieving electron mobility levels in excess of 500 cm2/Vs. There are also high-temperature polysilicon TFTs, which are made using a high-temperature process on a quartz substrate.
*2: Sheet computer
A slim, light computer that integrates high-performance, low-temperature polysilicon TFT circuits onto a display panel. Transforming the display panel, which has hitherto been peripheral to the CPU, into a computer could well position it as the most common form of human-machine interface.
*3. Crystallization
In producing a polysilicon TFT, amorphous silicon is deposited on an insulating substrate and exposed to a laser beam, which fuses and then crystallizes it. Through this crystallization process, amorphous silicon is transformed into polysilicon. CLC technology can produce polysilicon with very favorable crystalline properties.

About Fujitsu Laboratories
Founded in 1968 as a wholly owned subsidiary of Fujitsu Limited, Fujitsu Laboratories Limited is one of the premier research centers in the world. With a global network of laboratories in Japan, China, the United States and Europe, the organization conducts a wide range of basic and applied research in the areas of IT Core Systems, IT Media, Networks, Peripherals, Advanced Materials and Electronic Devices.

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Minoru Sekiguchi, Robert Pomeroy
Fujitsu Limited, Public & Investor Relations
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Fujitsu Laboratories Ltd.
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