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Flat panels go nanotech: Taiwan’s ITRI pioneers CNT-FED, part two
Chris Hall, DigiTimes.com, Taipei [Monday 24 January 2005]

TFT LCD looks firmly entrenched as the dominant flat-panel technology, but in fact the displays industry is keen to find a technology that delivers better performance – and, crucially, at lower cost. Alternative technologies exist, but often suffer from their own well known drawbacks. In the case of OLED technology, for example, there is the infamously short lifetime of the display.

One promising technology is the carbon-nanotube field-emission display (CNT-FED). In Taiwan, key development work on CNT-FEDs is ongoing at the Electronics Research and Service Organization of the Industrial Technology Research Institute (ERSO/ITRI). Recently, following ITRI’s announcement of the development of a 20-inch CNT backlight unit (CNT-BLU), DigiTimes.com had the opportunity to talk with Dr. CC Lee, deputy director of flat-panel development at ERSO/ITRI.

This is Part II of a three-part interview. Part III will follow on January 25. Part I appeared on January 21.

Q: How do these kinds of advantages – replacing the backlight unit of a TV, and screening-printing and so on – how do these compare these with OLED technology? With OLED, you have no backlight unit for example. OLEDs are printable, maybe. OLEDs also have very fast response times, no viewing angle problems and very good color quality. How does CNT-FED technology compare?

A: CNT-FED technology is similar. It has a wide viewing angle. There is no limitation to the viewing angle because both OLED and CNT-FED are self-emissive technologies, so that means there is no limitation on the viewing angle. The response time is the same because it is an emissive technology.

The crucial distinction is that CNT-FED uses inorganic material, so the overall stability of CNT-FED technology is greater than that of OLED. One company claims that they could make a white OLED backlight and apply it to some consumer devices where lifetime is not a concern. But if you tried to use the same backlight in a TFT-LCD TV, then the long-term reliability of the material would become an important issue.

Generally, CNT-FED technology offers all the advantages of OLED – there is no need for a BLU; there are fast response times; there is a wide viewing angle and excellent color quality – but the lifetime of the CNT-FED would be much longer.

Q: Can you quantify the lifetime issue at all?

A: The lifetime is relative to the brightness. If we are comparing OLED and CNT-FED displays, we need to keep the brightness at exactly the same level; then we could compare the lifetime and reliability of the displays.

Basically, we don’t see OLED as a competitor for CNT-FED in brightness. OLED often claims a lifetime of over 20,000 hours, but that is only possible at a brightness of 100 nits. For a viewable TV display, you normally need a brightness of 500 nits. But if you increase the brightness of an OLED display to that level, you will increase the strain on the organic material, and the lifetime will drop.

Q: So your advice to the OLED industry would be to develop CNT-FED technology instead?

A: Not entirely. They are very different industries. CNT-FED is a screen-printing technology, whereas OLED uses ink-jet printing. It’s a totally different printing technology. So if an OLED company wants to invest in CNT-FED technology it would not be easy. On the other hand, companies from the PCB, CRT and PDP industries would find it easier to enter CNT-FED production because the core technology is the same – screen-printing plus vacuum sealing.

If all the display makers suddenly decided they would like to get into CNT-FED production, the big losers would be OLED and color-STN (CSTN), for that reason. For the TFT-LCD, PDP and CRT makers, it would be relatively easy produce CNT-FEDs in their facilities.

Q: CNT-FEDs have a reputation for low power consumption. Can you quantify this?

A: The emissive characteristics of CNT-FEDs are pretty good. They simply require a small voltage applied to the cathode, and an emission will be generated. We can reduce the voltage to less than 40 volts, if we can reduce the diameter of the cathode structure. The voltage is low compared to PDP displays, where the working voltage is around 60V-100V.

Q: Can you also quantify color quality? What is the color gamut?

A: The color gamut is the same as that of CRT, at around 95% because the phosphor is similar to that of TFT. If we wanted to change that we would have to change the material of the phosphor.

Q: Do CNT-FEDs have any disadvantages?

A: Vacuum sealing for mass production remains a critical issue, as we mentioned. The ability to control the uniformity of the display is a second critical issue because with the larger size cathode we need to be able to control the emitting current. However, we have figured out some processes and devices for solving the uniformity issue.

Q: Which technology works best for CNT-FED production, screen-printing or vapor deposition?

A: In terms of equipment, the printer is cheaper than vapor deposition, and in Taiwan there are some companies that can make the printer. Printing is a mature technology.

But screen-printing is also a complicated technology, combining both equipment and materials. Plus, you need an engineer, and only then can you print. But with vapor deposition you just need to buy the equipment and do some fine-tuning of the recipe, so it’s totally equipment dependent.

Generally the cost of printing is lower because the equipment cost is lower, but the cost of the materials is higher. There is some trade-off between the equipment and the materials.

Q: How does OLED production compare, in terms of materials used?

A: Materials for OLED production are organic, so you need to control the environment when you do the printing process – in a vacuum or a nitrogen atmosphere – otherwise they will react with oxygen and water. For CNT-FEDs, however, the material is inorganic, so you can print with the material exposed to air, and you don’t need to control the environment.

With CNT-FED, the curing process will decompose any organic material, until finally the inorganic material for the CNT-FED is stable.

Q: Can you outline the general formation of a CNT-FED display?

A: Generally for a CNT-FED there are two plates for the display, the anode and cathode. We need to screen-print the phosphor and black matrix on the anode plate. And we also need to print the cathode – it’s silver. Then we print the glass for the insulator, and also print the carbon nanotubes (CNTs) to form the emitter on the cathode. Finally we do the vacuum sealing process to combine anode and cathode.

Q: You say that vacuum sealing is a critical part of the process?

A: Yes it is critical, but the sealing process is similar to the vacuum sealing process for CRT and PDP production. That means we can easily obtain the equipment and technology from the existing infrastructure for CRT and PDP. But we need to be able to control the degree of vacuum inside the CNT-FED panel – it needs to be about 10-7. In other words, the process is standard, and the material is standard because the process has already been established for CRT and PDP, but controlling the process is harder.

For a CRT, the volume of the device is higher, so the vacuum is easier to produce and control. But for the CNT-FED, the volume of the device is relatively small, so if you want to pump out residual gas, you need to think carefully about how to pump it out completely, so that no residual gases remain. CRT and PDP have given us the foundations of how to do this, but the capability to maintain a high degree of vacuum and control it successfully is difficult. There’s no doubt this is a critical aspect of CNT-FED mass production.

Q: At this point in the project, what success are you having with this critical point, the vacuum process?

A: Right now we can control the curing temperature of the sealing process. We have studied the curing process as well as types of getter technology, and we have assessed how to reduce the residual gas, so we can obtain a degree of vacuum better than 10-6. If we want to increase the degree of vacuum to 10-7, we have to go through an aging process, following vacuum sealing. This involves applying an electron beam to bombard the buffer, to clean the residual gas. Then we can increase the degree of vacuum to 10-7.

http://www.digitimes.com/news/a20050124PR201.html