[FPDI/GD] Chimei Innolux Prototypes CNT-based Touch Panel

Nov 12, 2010
Tetsuo Nozawa, Nikkei Electronics
A CNT-based touch panel
A CNT-based touch panel
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A mobile device equipped with the panel
A mobile device equipped with the panel
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The three manufacturing methods for the CNT-based touch panel and a problem of the CNT film
The three manufacturing methods for the CNT-based touch panel and a problem of the CNT film
[ If it clicks, the expanded picture will open ]

Chimei Innolux Corp showcased a touch panel that uses carbon nanotubes (CNTs) as a conductive material at FPD International 2010/Green Device 2010.

The company also embedded the touch panel in a smartphone, etc and demonstrated them.

"It eliminates the need for an etching process for patterning, making it possible to make a touch panel in a simple process," Chimei Innolux said.

There are three manufacturing methods for the CNT-based touch panel. First, two films applied with CNTs are attached to each other by using a spacer (CNT resistive film method). Second, a patterned ITO (indium tin oxide) film and a CNT film are attached to each other (hybrid capacitance method with CNT and ITO). Third, a CNT film is simply attached to a transparent substrate, etc (CNT super-thin film capacitance method).

Chimei Innolux is ready to start volume production of the touch panel by using any of those methods. The CNT film is made by applying CNTs to a PET (polyethylene terephthalate) film.

With the first method, the company prototyped a 4.1-inch touch panel with a resolution of 480 x 800 pixels. Its frame frequency is 125Hz. And its light transmission rate is 74-78% though it differs depending on the substrate.

A touch panel prototyped with the second method has a size of 3.2 inches, a resolution of 320 x 480 pixels, a frame frequency of 85Hz and a light transmission rate of 87%. And a touch panel made with the third method has a size of 3.2 inches, a resolution of 320 x 480 pixels, a frame frequency of 70Hz and a light transmission rate of 87%.

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All of the three panels support multi-touch input. The first and third panels can detect up to two points at the same time while the second panel can detect up to five points at the same time. The first and third methods are suited to realize a simple manufacturing process, which is the advantage of CNTs. But the third method, especially, has a problem.

Specifically, with the third method, there is a large anisotropic nature in electric resistance due to the orientation of CNTs on the CNT film. For example, it might occur that the x-coordinate of a finger can be detected with a high accuracy while the detection accuracy for its y-coordinate is low.

"When two points having the same x-coordinate and different y-coordinates are touched at the same time, it is possible that they cannot be distinguished," Chimei Innolux said.

To address this problem, the company developed the second method, which combines a simply patterned ITO film and a CNT film. The electric resistance of the ITO film is isotropic and, thus, mitigates the problem of the CNT film.

It takes time to replace ITO film

This time, Chimei Innolux applied the CNT film to a touch panel, not to the transparent electrodes of a display.

"As a replacement for an ITO film, the CNT film has a high electric resistance," the company said. "A high electric resistance is not a serious problem for a touch panel. So, it is much easier to use the CNT film for a touch panel than to use it for replacing an ITO film."