FEDs: Back to the Future
I've always been enamored by field-emission technology. I first became aware of it in the early '90s while working at Raytheon, and it appealed to me because of the apparent similarity to cathode-ray tubes (CRTs). Back then, LCD technology was advancing at a rapid pace, but it had numerous limitations, and the allure of field-emission displays (FEDs) was very seductive. At the time, I may have even been overly optimistic about the relative simplicity of the design – build an array of nanometer-scale emitter tips (often referred to as "Spindt" tips) surrounded by relatively simple semiconductor structures and space them very close to an anode phosphor surface charged at a few thousand volts. Electrons will emit from the tips of the structures and strike the phosphor and cause light to emit from the face of the display – just like the identical mechanism in a CRT. Everything I understood about CRTs and how they formed pleasing images would still apply to FEDs, but in a very thin package. FEDs even looked like they would suit laptop applications in the most enchanted views of some.
Of course, the next 15 years did not yield the results we hoped for, while LCD technology surely lived up to its promise. Major problems – such as keeping the geometry of the tips intact for any significant period of time and finding methods for actually maintaining the space between the top and bottom layers of the display – dissuaded investors and derailed plans for commercial success, and to date, no one has successfully commercialized an FED.
Fortunately, not everyone gave up, and since that time, there have been numerous advances in the field. The most newsworthy have been the efforts by Canon on surface-conduction electron-emitter displays (SEDs) where instead of tips, the field emission occurs across a very narrow gap between electrodes of just a few nano-meters. A relatively small number of electrons scatter out of the gap and jump to the front phosphor glass through a 10-kV field to produce light. This appears to address the problem of emitter integrity with a similarly simple construction. Meanwhile, a number of organizations are having similar success using carbon nanotubes (CNTs) to produce an electron-emitting surface with demonstrable lifetime potential. CNTs seem to be on the forefront of a number of new innovations in display technology, and it is a popular research area for academic institutions as well as commercial companies. There are well-reported working demonstrations of these approaches, which is very encouraging.
However, at Display Week 2007, I was excited to see that there has also been dramatic success developing the original Spindt-type FED. Significant encouraging progress could be seen. Futaba presented a paper on its progress with FED technology and reported achieving 10,000 hours of lifetime and good evidence of being able to get to 25,000 or more soon. They also reported on a novel spacer approach using 5 μm x 0.6 mm glass fibers as the spacers, which would certainly make them invisible under normal conditions. One issue with spacers in FEDs is not just their innate invisibility, but whether they cause any disturbance in the paths of the electron beams around them, which also produces visible errors in the image. I did not get to examine a Futaba FED so I can't comment on this aspect, but a new company called Field Emission Technology (FET) did exhibit a 19-in. Spindt-tip field-emission panel with an impressively bright, uniform, full-color image that appeared to overcome both the spacer and tip-uniformity problems of the past. FET is based in Japan and is minority owned by Sony Corp., who has invested significantly in FED technology in the past. While they would not disclose many details of their demonstration panel to me, they did tell me that they have been testing their Spindt-tip design for more than a year now and had good lifetime data as well as the ability to repeat the recipe in different sizes with relatively good yields for a small-scale process.
Field emission is a vacuum-packaged technology such as CRTs and plasma panels, with all the baggage and inherent cost that comes along with that. Some of my colleagues believe the window of opportunity for FEDs is already closing in light of the parallel progress of organic light-emitting-diode (OLED) displays, but I'm not ready to agree. While it may never be as economically produceable as true solid-state technologies such as OLED or LCD technology, I believe FED technology in its several forms may become a compelling alternative for high- to very-high-resolution displays, especially under hostile environments such as extreme temperature ranges. FED technology is a very strong technology for video because it produces a short-duration pulse-type image just like CRTs with almost any imaginable combination of hold and refresh-rate possible. Television images on good FEDs with the right color gamut look as good as anything on the marketplace today. A lot of things can still go wrong on the way to mass producing OLED TVs, and in that case, FEDs may yet earn their 15 minutes of fame.
Full coverage of all the highlights and technology at Display Week is coming in the August issue of ID, and whether you were in Long Beach or not, I'm sure you will find it a very useful update on the industry we all serve.
I'm very pleased this month to welcome Dr. Louis Silverstein as our guest editor this month. A great deal of what I have learned about color and vision has come from Lou's frequent SID seminars and numerous published papers. Lou has assembled a terrific portfolio of articles that explore the latest issues in high-dynamic-range displays, extending the range and realism of color gamuts, and issues with multiple (more than three) primaries. I'm sure you will enjoy them, and I am very grateful to Dr. Silverstein and his authors for their great work.
– Stephen Atwood