News Release: The former Hitachi Displays

March 31, 2005

IPS low-temperature polysilicon TFT with high-definition (System-In-Display), developed using SELAX technology

Hitachi Displays, Ltd. (Executive President: Fumiaki Yonai), jointly with Hitachi's R & D division, has successfully developed a 2.4 inch VGA low-temperature polysilicon TFT display using SELAX* technology: "crystallization technology to form a high performance thin polysilicon film in any area specified".
Using SELAX technology, this product forms a part of a horizontal driver circuit on the glass so that VGA (480×640 dots; 339ppi) can provide the world's highest definition, verifying the possibility of deploying System-In-Display*1). Its development aims for use in TV phone on mobile devices such as mobile phones and high image quality displays such as TV broadcast receivers. Its display section adopts IPS technology, a high image quality LCD technology, used in the large size TV displays. A sample of products developed will be distributed, with the aim of making this a commercial product in the future.
Mass production of the product is planned for the second half of Fiscal 2005. We have prepared the production system in the low-temperature polysilicon line (large board; 730×920mm) in V3 ward, at Mobara-shi, Chiba Prefecture, Japan.
SELAX is an advanced technology that was presented by Hitachi's Central Research Laboratory in SID2002, the International Conference on Display held in May, 2002. It is well compatible with the traditional technology, low-temperature polysilicon TFT process, and has the advantage of being able to be processed at any position on a large glass board. Thus, it has been attracting attention as next generation technology. Hitachi's R&D Division and Hitachi Displays, Ltd. have jointly studied its mass production and succeeded in its commercialization.

[SELAX]Selectively Enlarging Laser X'tallization

(The following description is a summary of our news release issued from Hitachi in May 2002.) Polysilicon TFT provides an electron mobility*2) as high as 100-150cm2/Vs. In a lower operation frequency circuit, therefore, it can be mounted on the same glass board as the display screen. With an electron mobility of 300cm2/Vs or higher, a high function circuit also can be integrated with display. To increase the polysilicon TFT's electron mobility, it is necessary to reduce the crystal grain boundary which hampers the electron mobility. Generally, the crystallization method using excimer laser is adopted to enlarge crystal grains. This method, however, shortens the duration until the melted silicon congeals (about 100nano seconds), preventing the crystal grains becoming larger. Another problem is larger characteristic variation.
SELAX is a technology in which, by controlling a pulse width of solid-state laser and emitting it on the polysilicon, the silicon's thin film is optimally melted and congealed to form "pseudo-mono crystal silicon"*3). This technology produces a 20 times larger crystal grain size than the traditional one, and forms a flat surface polysilicon film. Controlling both crystal grain's surface direction and crystal growth direction successfully makes the crystal grain align with TFT's current direction in favor of an increase in mobility.
Compared with the traditional TFT, the new product successfully enhanced the characteristics: it increased two or more times the electron mobility and decreased the threshold voltage variation by 10% or less. Thanks to better TFT rising characteristics, even a lower voltage enables a high speed operation (delay in one stage of inverter: 8ns (five times shorter than in the traditional one) (Vdd =5V, gate length= 4 micron), reducing the circuit power consumption.

*1) System-In-Display: A high-value-added display, which uses a high performance low-temperature polysilicon TFT so that not only the display and its driving circuit but memory function, interface and processor function are on a low-price glass.

*2) Electron mobility: A basic parameter associated with electron mobility; a value representing basic transistor performance. The larger this value, the larger the current transistor that may flow through it so that lower power with a high speed circuit can be realized.

*3) Pseudo-mono crystal silicon semiconductor: A polysilicon semiconductor in which there are no grain boundaries that traverse the TFT's current direction. Also, one where no carriers are scattered and the electric current flows smoothly.

* IPS (In-Plane Switching) technology, based on an operation that differs from the normal TFT LCD, is a lateral electric field LCD technology developed by Hitachi. Hitachi disclosed this technology in 1995 and commercialized it in 1996, and it has evolved with its successors Super-IPS and Advanced-Super IPS. Due to the lateral electric field, LCD molecules rotate on a surface in parallel to the TFT board. The simple movement of such molecules results in the various excellent performances in terms of viewing angle, color reproducibility, and intermediate grade response time. This technology is adopted in 32-inch large-size LCD televisions, highly appreciated for their superior imagequality.

[Main Specifications] IPS low temperature polysilicon TFT LCD display

Display size 6.0cm(2.4") models
Pixels 480(H) x 640(V); 339ppi
Colors available 16M colors (Full color)
Viewing angle 170 degrees or more, horizontal/vertical directions
Color reproductivity 50% (NTSC)
8 bit digital (RGB series input)