Sharp, SEL Revamp IGZO TFT

Jun 5, 2012
Shinya Saeki, Nikkei Electronics
The 6.1-inch LCD panel for mobile devices such as tablet computers
The 6.1-inch LCD panel for mobile devices such as tablet computers
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The 4.9-inch LCD panel for smartphones
The 4.9-inch LCD panel for smartphones
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The 13.5-inch OLED panel. There are a few missing longitudinal lines.
The 13.5-inch OLED panel. There are a few missing longitudinal lines.
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The 3.4-inch flexible OLED panel
The 3.4-inch flexible OLED panel
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Sharp Corp and Semiconductor Energy Laboratory (SEL) Co Ltd announced June 1, 2012, that they have developed an IGZO (In-Ga-Zn-O)-based TFT having properties improved by adding crystallinity.

The companies expect that the TFT will be used as driver elements of LCD and OLED panels for smartphones, tablet computers, etc, whose resolutions are becoming higher. Sharp plans to start volume production of an LCD panel using the new IGZO TFT within fiscal 2012 though it depends on customer demand, it said.

At a press conference that Sharp had June 1, 2012, the company showed LCD and OLED panels that use the newly-developed IGZO TFT. It showcased a 4.9-inch LCD panel whose pixel count and resolution are 1,280 x 720 and 302ppi and a 6.1-inch LCD panel with a pixel count of 2,560 x 1,600 and 498ppi.

"We have established a technology to mass-produce an LCD panel with a resolution of up to 500ppi with no problem," the company said.

Asked how Sharp will deal with its "CG silicon" TFT, which uses a low-temperature polysilicon TFT, in the future, it said, "Though it depends on users, it is sure that we will shift our focus to IGZO TFT."

As for the OLED panels, Sharp showed 13.5- and 3.4-inch panels whose pixel counts are 3,840 x 2,160 and 960 x 540, respectively. Their resolutions are both 326ppi. The 13.5-inch panel realizes color display by using red, green and blue (RGB) color filters for its white OLED devices. The 3.4-inch panel is flexible because it uses a resin substrate. The company did not disclose the material of the OLED device, light extraction method, etc because "the panels are just prototypes."

Sharp considers that OLED panel is important as "one field of mobile display." But, asked about its commercialization schedule, the company said, "Though it is technically possible, there are financial challenges such as costs as well as issues related to investment effect."

Crystal growth in c-axis direction

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Crystal growth in c-axis direction

IGZO is an oxide semiconductor. Sharp has been mass-producing an LCD panel that is for tablet computers and uses amorphous IGZO TFTs as driver elements at its Kameyama Plant No. 2 since March 2012. The carrier mobility of the amorphous IGZO TFT is 20 to 50 times higher than that of an amorphous silicon (Si) TFT, which is used for TVs, etc. And its leak current in the off-state is 1/100. As a result, it can realize a (1) higher resolution, (2) lower power consumption and (3) higher touch panel performance, compared with an LCD panel driven by amorphous Si TFTs, the company said (See related article).

In addition, the amorphous IGZO TFT enables volume production with a glass substrate as large as that used for amorphous Si TFTs.

Sharp said that it is possible to further improve the carrier mobility and the leak current in the off-state by increasing the crystallinity of the IGZO TFT. For example, the leak current at a temperature of 85°C can be reduced to 100yA/μm (y (yocto): 10-24) or less, said Shunpei Yamazaki, president of SEL. He also said that the variation of I-V characteristics in response to light irradiation can be reduced.

By taking advantage of those characteristics and combining the IGZO TFT with crystalline Si-based semiconductors, it becomes possible to realize the best performance ever in applications other than display such as memory, image sensor and CPU, Yamazaki said.

Sharp and SEL developed a technology for crystal growth of an IGZO layer in the c-axis direction. The IGZO has a hexagonal crystal structure when seen from the direction of the c-axis and a layered crystal structure when seen from the direction perpendicular to the c-axis. The two companies call the crystal structure "CAAC (C-Axis Aligned Crystal)."

Though they did not reveal the details of the manufacturing process, Yamazaki said, "The crystal grows by applying an annealing process after forming an amorphous IGZO layer. We have know-how related to the processing conditions." The basic manufacturing processes for the new IGZO TFT are the same as those for existing amorphous IGZO TFTs, Sharp said.

Sharp and SEL will announce the details of the new technology at the 2012 SID International Symposium, Seminar & Exhibition (SID 2012), the world's largest academic conference on display technologies, which runs from June 3 to 8, 2012, in Boston, the US (thesis number 9.2, 27.4, etc).