Flexible Displays Advance

Efforts to develop a flexible display that could have applications in laptop computers and cell phones, store displays and even "electronic paper" were stepped up recently with the opening of two new facilities.

In September, E Ink Corp . opened a flexible microelectronics facility in Woburn, Mass. The 9500 ft² building will house E Ink's Microelectronics Technology Group, which will develop microelectronics — including flexible transistors — with the goal of creating paper-like display prototypes next year. In June, FlexICs opened a pilot facility in Milpitas, Calif., for producing semiconductors on plastic. The 18,000 ft² facility features a Class 100 cleanroom with special equipment designed for making semiconductors on plastic.

At the core of E Ink's display technology is a new material comprised of millions of tiny microcapsules. In one incarnation, each microcapsule contains positively charged white particles and negatively charged black particles suspended in a clear fluid (Fig. 1). When a negative electric field is applied, the white particles move to the top of the microcapsule, where they become visible to the user. This makes the surface appear white at that spot. At the same time, an opposite electric field pulls the black particles to the bottom of the microcapsules, where they are hidden. By reversing this process, the black particles appear at the top of the capsule, which now makes the surface appear dark at that spot. To form an E Ink electronic display, the ink is printed onto a sheet of plastic film that is laminated to a layer of circuitry.

In the first quarter of next year, the company will be offering products with a direct-drive backplane for signage. Such products do not require thin-film transistors (TFTs) on the backplane, said Mike McCreary, vice president of R&D at E Ink. This will enable the company to compete directly with Gyricon Media (Palo Alto, Calif.), a spin-off of Xerox that is already offering similar products (see www.gyriconmedia.com).

 
1. In an “electronic ink,” microcapsules are electronically manipulated to create an image. (Source: E Ink Corp.)

In 2003, E Ink "will be shipping either rolls or sheets" of this display material to Philips, its first TFT partner, McCreary said. "They will have custom but more or less traditional glass transistor backplanes, and they will take a flexible display material and that will be laminated to these backplanes. So, while that will not be flexible, it will cut the weight and thickness of the display almost by a factor of two. The generation after that is really where we start to go into 100% flexible displays, and that will replace the glass TFT with flexible support."

FlexICs is also hoping to be well-positioned in the new flexible transistor regime by acting as a display foundry, said Shyam Dujari, vice president of marketing. FlexICs has developed a way of fabricating polysilicon TFTs on plastic substrates using an excimer to convert amorphous silicon to polysilicon with a low-temperature process (<100°C). "Starting with gate oxidation, to dopant activation, all the way to the contact and anneal, we are able to do all these processes without ever exceeding the temperature at which the plastic will deform," Dujari said. Figure 2 shows an organic LED device structure on a plastic substrate. The company also has a strong focus on substrate engineering to mesh with the low-temperature process.

 
2. The goal of FlexICs is to use a low-temperature process to produce thin-film transistors and devices such as this organic LED (OLED) on flexible plastic substrates. (Source: FlexICs)

"What we intend to do is sell customers products that are based on their designs," he explained. "A display manufacturer would have us design a backplane which is an active matrix that goes on the back of the display. We would manufacture the display backplanes for them then to put on the display material and build the entire display. Because we are going to be doing a polysilicon TFT, we can also integrate the drivers into the display."