Displays are metamorphosing into "system-level displays" by integrating once-external functions onto the glass together with higher-performance thin-film transistors (TFTs). Indeed, by fabricating all the necessary circuits on the glass or plastic substrate of the display and adding a power supply, developers theoretically could achieve a complete PC or PDA.
But higher-performance TFTs must be developed if displays are to achieve true system-display status.
Low-temperature polysilicon (LTPS) TFT technology exhibits roughly 100-times better performance than amorphous-silicon TFTs. Some LTPS LCDs on the market already integrate drivers; others have added SRAM and D/A converters on the glass substrate. But compared with single-crystal silicon transistors, their performance is still far below what is required.
The performance of a TFT is linked to its mobility, defined in centimeters squared per volt-second. The mobility of amorphous-silicon TFTs is less than 1 cm2/Vs, compared with 100 to 150 cm2/Vs for LTPS TFTs. But a single-crystal silicon transistor has mobility of about 1,000 cm2/Vs.
"LCD manufacturers dream of fabricating circuits that have the same performance as single-crystal silicon transistors," said Nobuo Sasaki, senior research fellow of Fujitsu's Silicon Technologies Laboratories. The evolution of LTPS TFTs and continuous-grain silicon TFTs holds the key to system-level displays that integrate not only drivers but also high-speed circuits on glass.
Next-generation LTPS TFT LCDs, with mobility of about 300 cm2/Vs, are expected to integrate most of the peripheral circuitry now located around the panel. When mobility reaches 500 cm2/Vs in the following generation, sources said, display panels should be able to integrate other higher-speed circuits, such as processors.
But mobility is not the only obstacle standing in the way of the system-level display. Even as the mobility of LTPS TFTs pushes higher, "there are various other factors, such as uniformity, that we have to find solutions for," said Masanori Sakamoto, chief specialist at Toshiba's LCD Research and Development Center. Toshiba currently pegs the mobility of its LTPS LCDs at about 100 cm2/Vs. The company's road map for LCDs envisions mobility of 300 cm2/Vs in 2003 and 500 cm2/Vs in 2005.
Currently, excimer laser annealing is used to generate polysilicon film from amorphous silicon. The annealing process yields a relatively small silicon grain, measuring about 0.3 micron in diameter. Since larger grains are a factor in achieving higher mobility and therefore higher performance, the crystallization process is considered crucial to development of systems-on-displays. Researchers have recognized that they must move beyond excimer laser annealing if they are to achieve mobility of 500 cm2/Vs.
Lateral growth is a potential solution. Researchers at Columbia University are working on sequential lateral solidification to grow a polysilicon film in a lateral direction via the continuous application of a pulsed-laser system.
At Fujitsu, the research team headed by Sasaki has developed a lateral-scanning system using a continuous-wave laser. The team reported in 2001 that it had cleared the goal of 500-cm2/Vs mobility. "Scanning with a continuous-wave laser grows a silicon film of equivalent performance to single-crystal silicon," said Sakamoto. Grain boundaries appear along the direction of the scan. The boundary-to-boundary length is said to exceed 5 microns more than 10 times larger than for grains made by excimer laser.
TFT performance further depends on such factors as the presence of defects in the Si film, the surface-texture overlap ratio and the scanning speed. Fujitsu reported that optimum control of those factors enables TFTs formed along the scanning direction to achieve mobility of 566 cm2/Vs.
Fujitsu is developing production equipment now and intends to introduce system-level LCDs based on laterally grown Si film transistors in 2003.
Sharp Corp., meanwhile, plans an autumn introduction for continuous-grain silicon TFT LCDs with mobility of about 300 cm2/Vs.
Sharp and Semiconductor Energy Laboratory Co. Ltd. jointly developed continuous-grain (CG) silicon in 1998. In CG-silicon thin film, atoms continuously combine with each other even at the grain boundaries, which enables electrons to move much faster than in LTPS TFTs, said Sharp.
The company believes that CG-silicon TFTs can operate at 150 MHz when fabricated on a 2-micron process. Sharp is remodeling an amorphous-silicon TFT line at its Tenri fab to launch volume production of low-temperature CG-silicon TFT LCDs by October. The fab will be the base for Sharp's CG-silicon business, for which the company targets sales of about $2.8 billion in 2005. That would equate to 80 percent of Sharp's current LCD business.
"We'll introduce system LCD products that will change the image of current products completely," said Hirohide Nakagawa, deputy general manager of Sharp's System LCD Division.