Tokyo, July 13, 2001---Fujitsu Laboratories Ltd. today announced that it has developed fundamental process technology for making thin-film transistors (*1) (TFTs) that will make it possible to produce System LCDs (*2) and sheet computers. The new technology, which enables TFTs to be fabricated on a glass substrate at a process temperature of 450° or below, achieves mobility of above 500 cm2/Vs (*3), more than twice that of existing processes. The details of the technology were presented at the 2001 International Workshop on AM-LCD Displays, held July 11 - 13 in Tokyo.
Development Background
Liquid crystal displays (LCDs) using amorphous silicon (*4) are currently on the market in various product forms, including displays for personal computers. However, because the mobility of amorphous silicon is extremely small, at 0.8 cm2/Vs, it is impossible to integrate high-speed circuits with the pixel array onto the same glass substrate. Poly-silicon (*5) LCDs, on the other hand, are able to achieve 200-300 cm2/Vs mobility, making it possible to mount a low-frequency circuit onto the glass substrate. This technology already is being utilized for high-precision digital cameras and portable notebook computers.
Increasing mobility beyond 400 cm2/Vs would make it possible to integrate high-speed circuits and the display part on the same glass substrate, thus opening up the potential to develop System LCDs and sheet computers.
In order to increase mobility, it is necessary to reduce the grain boundaries in poly-silicon TFTs, requiring the development of crystallization technology that can create larger crystal grains. In addition, it is necessary to carry out this process at temperatures below 550° to prevent deformation of the glass.
Technology Details
With the goal of reducing the density of grain boundaries and enlarging crystal grains, Fujitsu developed a crystallization technology that utilizes a stable scanning diode-pumped solid-state CW laser (*6) instead of conventional excimer laser crystallization technology. Using this technology, poly-silicon film consisting of crystal grains one hundred times the size of typical grains was formed on a glass substrate. Using these crystals and a conventional process temperature of 450°, Fujitsu was successful in fabricating a poly-silicon TFT with 400-600 cm2/Vs mobility-double or triple that obtained by excimer crystallization technology-on a 30cm x 30cm glass substrate. This technology can easily be applied to 1m x1m or larger glass substrates.
Glossary of Terms:
- 1: System LCD:
- An LCD that provides high added value by offering not only a liquid crystal display and related driver circuits integrated onto a low-cost glass substrate using high-performance low-temperature poly-silicon technology, but also process, memory and other functions.
- 2: Thin-Film Transistor (TFT):
- A type of silicon-on-insulator (SOI) transistor.
- 3: Mobility:
- A basic parameter of transistor performance. The higher the mobility, the greater the ability of the transistor to drive large electric current, making high-speed and smaller circuitry possible. Originally used to indicate ease of carrier mobility, this parameter's value grows higher with smaller mass and lower scattering frequency.
- 4: Amorphous Silicon:
- Amorphous is used to describe substances that lack a long-distance correlation in their atomic configuration. Amorphous silicon film is one example of this substance and can be formed at low temperatures.
- 5: Poly-silicon, Crystal Grain Size, and Grain Boundary:
- Poly-silicon is formed from silicon grains of randomly oriented single crystals. The grain boundary refers to the space between each grain, while the crystal grain size simply refers to the circumference of each grain. When a carrier (i.e., an electron or hole) passes over a grain boundary, the carriers scatter, and the greater the frequency of this action, the lower the poly-silicon mobility.
- 6: CW Laser:
- A laser featuring continuous oscillations. Excimer lasers operate using pulse oscillations.
* All company/product names mentioned may be trademarks or registered trademarks of their respective holders and are used for identification purpose only.
About Fujitsu Laboratories Ltd.
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 Multimedia, Personal Systems, Networks, Peripherals, Advanced Materials and Electronic Devices.
About Fujitsu
Fujitsu is a leading provider of Internet-focused information technology solutions for the global marketplace. Its pace-setting technologies, best-in-class computing and telecommunications platforms, and worldwide corps of systems and services experts make it uniquely positioned to unleash the infinite possibilities of the Internet to help its customers succeed. Headquartered in Tokyo, Fujitsu Limited (TSE:6702) reported consolidated revenues of 5.48 trillion yen for the fiscal year ended March 31, 2001.
Internet: http://www.fujitsu.com/
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