LG Electronics Develops Flexible OLE Panel Driven by Oxide Semiconductor TFT

Aug 30, 2007 16:15 Naoki Tanaka, Nikkei Microdevices

LG Electronics Inc. of South Korea developed a flexible OLE panel that uses an oxide semiconductor In-Ga-Zn-O (IGZO) material for TFT.

The company presented the new panel at the 7th International Meeting on Information Display (IMID 2007). It is a 3.5-inch QCIF+ (176 x 220) panel that has IGZO TFT and OELD formed on a stainless-steel substrate with a thickness of 100 μm. The company unveiled the prototype panel during the author's interview.

"Organic semiconductor TFT has a higher reliability and mobility compared with organic TFT or amorphous Si-TFT," LG Electronics said. "It is superior to low temperature poly-Si TFT in uniformity. Thus, it is the front-runner drive element for the flexible OEL panel."

LG Electronics started the development of OEL panel driven by TFT made of a transparent oxide semiconductor IGZO in April 2006. The company presented a paper on a monochrome panel and demonstrated a color panel based on this development at the International Display Workshops (IDW) in December of the same year.

LG Electronics then improved the video display performance of the color panel from the initial level of approximately 5 fps to 60 fps and presented the prototype at the Society for Information Display (SID) in May 2007. The company used a glass substrate (manufactured by Corning Inc.) with a thickness of 0.63 mm for this prototype.

When a stainless-steel substrate is used, an organic planarizing film is first formed to ensure the surface flatness of the substrate. LG Electronics reportedly developed a new material and process so that the planarizing film will not be affected by the generation of cracks, etc., during the subsequent production steps, including gate insulating film formation, which require a high-temperature processing. The company used a plastic barrier film manufactured by LG Chem Ltd. of South Korea.

The flexible OEL panel presented during the author's interview has a radius of curvature of 6 cm. The company demonstrated in the video clip shown during the paper presentation that the characteristics of IGZO TFT would remain unchanged even when the radius of curvature was decreased to 4 cm.

It was also indicated that the display on the prototype flexible panel would not be affected when the panel was bent (radius of curvature was varied) to a certain degree by hand.

LG Electronics used a low-molecular material for the OLE material. The company employed the top-emission structure, in which the light is extracted from the side opposite to TFT, because the stainless-steel substrate is not transparent.

TFTs manufactured by using existing photolithography and wet etching techniques have a top-gate structure and uses SiOx for the gate insulating layer. The IGZO TFT on the stainless-steel substrate has a mobility of 6.7 cm2 V-1 s-1, on/off ratio of 3.5 x 10^5 and subthreshold switching (S value) of 0.61 V/digit.

The latest technology is part of the achievements by the development promoted as a national project titled "Smart Window Using Transparent Electronic Device." This project is supported by Electronics and Telecommunications Research Institute (ETRI), a national research institute in South Korea.

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