News Release: The former Hitachi Displays

February 16, 2007

Development of IPS Low-temperature Polysilicon TFT Manufacturing Method Using SELAX Technology

Hitachi Displays, Ltd., in conjunction with the R & D division of Hitachi, Ltd. has successfully developed a manufacturing technology suitable for producing low-temperature polysilicon on existing amorphous TFT production lines.

Using SELAX (*1) technology, a crystallisation technology used for the partial formation of high performance thin polysilicon film, the new manufacturing technology allows the function of low-temperature polysilicon to be added to a section of the amorphous TFT, enabling the production capacity of low-temperature polysilicon to be improved without major investment in the existing amorphous TFT production line.

IPS LCDs that can produce fine still and moving images are being used increasingly to meet demand for better definition and higher quality displays for today's mobile equipment, including mobile phones. Demand for IPS LCDs with high definition VGA is expected to increase particularly with the growing diversity in mobile phone applications, such as mobile TV broadcasting. VGA with pixels four times greater than the currently mainstream QVGA are generally manufactured in low-temperature polysilicon production lines on which large electron movement (*2) and circuit embedding are possible. We have investigated increasing the low-temperature polysilicon production capacity with a view to expanding VGA production.

Low-temperature polysilicon cannot be produced on existing amorphous TFT production lines where crystallisation of thin film silicon is not possible. This new manufacturing technology allows silicon crystallisation and transverse growth of crystal grains at the same time by applying a solid laser only to the parts requiring large electron movement, such as the surrounding embedded circuit in the amorphous TFT substrate. Crystal regions can be formed in place on a large glass substrate with better crystal quality in a shorter period of time than those in the low-temperature polysilicon process using a typical excimer laser.

This technology will be applied to commercial production by the end of this year. A production system is planned at the V3 line (for 730 x 920mm large substrates) in Mobara City, Chiba Prefecture. Once the system is in operation, one million units will be added to the production capacity of the existing low-temperature polysilicon production line (two million units per month for 2.2-in type). Additional SELAX facilities will be built to increase capacity with future growth in demand.

SELAX, used for the development of the manufacturing technology, was presented at the SID2002 (Society for Information Display 2002 International Symposium) international display conference by the Central Research Laboratory of Hitachi, Ltd. in May 2002, and attracted considerable interest as a next generation technology due to its ability to enable processing at any place on a large glass substrate with a high degree of compatibility with the existing low-temperature polysilicon TFT process. The technology has now been improved further in conjunction with Hitachi Displays, Ltd. and the R & D division of Hitachi, Ltd., resulting in this manufacturing technology.

Terminology

(*1) SELAX technology: Short for Selectively Enlarging Laser X'tallization
A controlled solid laser is eradiated to polysilicon then applied to the TFT LCD panel to melt and solidify thin silicon film in optimum condition to produce "pseudo-monocrystal silicon*." Larger electron movements are needed to produce finer TFT LCDs. This requires larger crystal grains and a smaller crystal grain boundary that prevents electrons from moving. SELAX technology can be used to produce flat polysilicon films with crystal grains about 20 times the size of those produced by existing technology.
* Pseudo-monocrystal silicon: Polysilicon semiconductor with no grain boundaries that cross the TFT current direction. On this semiconductor, carriers are not diffused and the current flows with ease.
(*2) Electron movement: A value representing the ease of electron movement, and one of the basic parameters indicating transistor performance. As this value increases, larger current can be produced in the transistor, allowing faster circuits with low power consumption.