INTRODUCTION
Today's display industry demonstrates trends toward polarity, with manufacturers at one end cultivating large screens and better definition than ever before. On the other hand, development efforts are continuously enhancing thin, lightweight, low power consumption displays for use in mobile equipment.

Furthermore, although CRTs remain the main stream display format and the standard by which all other display are measured, other electronic display technologies are attracting strong attention. These electronic displays encompass LCDs, PDPs, VFDs, EL and LEDs.(1),(2) Each serves in a different market sector or niche, depending on the screen size, the pixel count and the panel configuration.

Figure 1 shows the target applications for the different technologies as function of the number of pixels and diagonal size. As we can see, a large technological gap exists between 80" diagonal displays and over 150" diagonal displays. Interesting to note, that due to the growing markets for outdoor digital large displays, presently a number of technologies coexist and compete for these applications.

Electronic sign boards, that must be very bright, generally use VFDs, LEDs or gas discharge devices (see comparison in figure 2). For the purpose of obtaining large area displays one can start from the existing monolithic type displays with the diagonal of less than 30" and expect that eventually these technologies will expand in order to obtain SWTVs. Alternatively one can start from the existing outdoor large area display technologies and improve the resolution and the picture quality in order to obtain the same result.

LARGE AREA OUTDOOR DISPLAYS, COMPARISON OF BENEFITS
Figure 2 presents a comparison of extremely bright display technologies for large electronic outdoor screens.(3) VFDs and gas discharge devices possess large areas of illumination and therefore usually are superior to LEDs in picture

quality. LEDs offer a limited area of illumination. This becomes a drawback in panels that use large pixel pitches. Under these conditions an LED needs to offer greater brightness per area than other devices. Panels that use such LEDs illustrate this disadvantage clearly. The display seems to glitter or twinkle at the illumination point resulting in poor picture quality. Furthermore, most LEDs use frontal lenses to improve the brightness. However, these lenses decrease the viewing angle. Meanwhile, gas discharge devices offer poor contrast because the discharge phenomenon prevents the devices from turning off completely. Also, the discharge phenomenon is unstable at temperatures below freezing.

In contrast to LEDs, VFDs offer better luminous efficiency using 30% less power than other devices. For these reasons in the past companies such as Sony chose the picture element tubes based on VFD technology (PET VFD) which offers an extremely bright color picture element plus stable operation and excellent picture quality (see figure 3).

Large Area Electronic Billboards using PET VFDs
LARGE AREA ELECTRONIC BILLBOARDS USING PET VFDs
The elementary picture tube of the outdoor electronic billboard shown in figure 3 consists of VFD elements organized in a 4x4 matrix (figure 4). Each pixel possesses 256 gray scale (see specifications in figure 5). Due to the low resolution (20mm pitch) the applications of the PET VFD electronic billboards are limited to

relatively large distance viewing. In the PET, electron emission from a cathode illuminates the 3-color RGB phosphors. A shield electrode between the cathode and the anode protects the low voltage electrodes from the effects of the high voltage. The X and Y matrix grids behind the cathode turn the emission flow off or on controlling the cathode emissions corresponding to each phosphor dot. The pulse width of the Y matrix grid changes the brightness of the corresponding phosphor dot. (3) Figure 4. Structure of a PET VFD. (3)

The cathode consists of a thin wire filament with barium oxide coating to emit thermal electrons. The phosphor is aluminized and anode voltages as high as 11 kV are applied.

As we explained above, the question is if the current PET VFD technologies can be improved in resolution and in such a way to extend their application to SWTVs where the observer is located as close as 10 feet from the surface of the display.

Limitations of the PET VFD technology
LIMITATIONS OF THE PET VFD TECHNOLOGY
There are a number of major factors that inhibit the PET VFD technology to be utilized for SWTVs:(4)

1. Power consumption and dissipation. Assuming the resolution needed for SWTVs is at least 4 times larger than the current elements, each new PET of the same size will consists of 64 pixels that will result in an excessive power consumption.
2. The use of hot cathodes. Hot cathodes have a wide emission spread. As a result an increase in resolution requires an additional masking electrode. Current PET VFDs already absorb 50% of all anode current at the masking electrode!
3. Cathode uniform heating. Each cathode filament requires 650°C in order to obtain sufficient electron emission. At least 2/3 of the cathode length should be at this temperature in order to obtain uniform illumination. Unfortunately, if the temperature of the cathode is increased to over 750°C, the oxide cathode material will evaporate. As a result and due to the metallic supports for the cathode, the current PET VFDs cannot achieve a pitch smaller than 15mm.

Based on these limitations, we concluded that in order to obtain a suitable PET for SWTVs one needs to transit from PET VFDs that utilizes hot cathodes to PET FEDs utilizing cold cathodes.

Carbon Thin Film Cathodes for Large Area Displays
CARBON THIN FILM CATHODES FOR LARGE AREA DISPLAYS
Carbon films are known for their low electron affinity, strong mechanical strength and high thermal conductivity. Carbon is one of the most chemically stable elements in general. These superlative properties, combined with the ease of growing thin films by a variety of deposition techniques and a relatively simple process involved in making a thin film flat cathode, make carbon films one of the most desirable material for a field emission cathode. Many researchers recognize the potential of carbon films and are vigorously studying and improving carbon film emission properties, such as threshold field, emission current density and emission site density. (5)-(10)

We recently reported that an emission site density of 2 x 10 5/cm2 and a current density of 100 mA/cm2 could be achieved routinely over a relatively large area at an extraction field of about 10 V/¼m. Films with such emission properties are already good enough to be used for low to medium resolution displays. To make any useful device, however, the cathode has to be addressable. Two problems proved to be particularly challenging in pursuing an addressable carbon cathode: 1) patterning the emission film and 2) overcoming the edge emission from the cathode feed lines. In fact these two problems are universal for any thin film cathode regardless of the emitting material. The carbon film is deposited by a hot-filament assisted chemical vapor deposition (HFCVD) process using hydrogen and methane as feed gases. Process parameters such as filament and substrate temperature, gas flow rate, and chamber pressure were carefully controlled to obtained films of optimum emission properties.

The emission properties and a detailed explanation of the patenting processes of our carbon cathodes were reported elsewhere.(11)-(14) Figure 6 shows a magnified image of a matrix addressable carbon film emissive cathode. The overall cathode fabrication process is simple and low cost. This particular cathode was developed to be used in a picture element tube (PET) for large area display applications. The image shows four pixels, each with red-green-blue subpixels. The pixel pitch is 10mm.

A color PET device was designed(4) (figure 7) using this addressable carbon cathode, an addressable grid structure and an RGB anode.

PET FEDs Using Carbon Film
PET FEDs USING CARBON FILM ELECTRON EMISSIVE CATHODES FOR SWTVs
Applied Nanotech of USA implemented the design shown in figure 7 in an operational 8x8 pixels PET FED with the pitch of 10mm.(15) The 8x8 matrix of color pixels was enclosed in a similar way like the PET VFD using conventional vacuum tube sealing. The device showed high brightness and high reliability. As compared with the PET VFDs the number of auxiliary elements inside the tube is smaller, the manufacturing process is simplified and the thickness of the PET FED is reduced as shown in figure 8.

Similar to the construction of the electronic billboard comprised of PET VFDs, a mosaic structure of PET FEDs can be built relatively easy in order to obtain a very large slim wall TV. In figure 9 we simulated a SWTV built of this kind of PET FEDs. The picture simulates approximately an 8ft x 10ft display in a configuration of 320 x 240 color pixels at 10mm pitch. In order to have a sensation of the quality of the image on the display, looking at a 8"x6" similar picture at arms length will simulate the quality of the viewing from 30 feet. If we look at the same picture from approximately 10", it would simulate the quality of the image on SWTV as seen from 10 feet. Contrary to the PET VFD technology, the PET FED technology can be extended to pitches smaller than 10mm. The direction of the development will depend on the balance between picture quality and system cost.

Figure 9. A computer simulated image of a 10ft x 8ft, 320 x 240 pixels SWTV built of 1cm pitch PET FEDs. Holding a 8"x6" similar picture at arm's length will impress on you the quality of the image as seen from 30ft from the surface of the actual SWTV.

Conclusions
CONCLUSIONS
The SWTB can become a reality by improving technologies utilized in the growing industry of large area outdoor electronic signs. As the pitch required for the SWTVs is smaller than 10mm, technologies such as LEDs and VFDs cannot be considered presently. Our latest achievements in the quality of the electron field emission from carbon films leads us to believe that our PET FED approach is a suitable candidate for both, high resolution outdoor electronic billboard, as well as "slim wall televisions" (SWTV).

Acknowledgments
The technical results presented in this paper couldn't haven been realized without the extensive contribution of K. Kasano and K. Tatsuda from Ise Electronic Corporation in Japan and the participation of my close collaborators Z. Li Tolt, R.L. Fink and L.H. Thuesen. Also I would like to thank the Advanced Technology Program of US Department of Commerce for financial support for this research (award No. 70NANB5H1076).

References

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About the author
Dr. Zvi Yaniv is the President and Chief Operating Officer of Nano-Proprietary, Inc. (NPI. He has published over 200 articles, holds more than 150 patents, and has extensive contacts in the U.S., Europe, Israel and the Far East. Dr. Zvi Yaniv was a founder of Kent Display Systems in Kent, Ohio, the "no-power" reflective LCD Company and of OIS Optical Imaging Systems, Inc. Earlier, Dr. Yaniv held ranking positions with the Practical Engineering College, Beer-Sheva; National Institute for Technical Training, Tel-Aviv; and Ben-Gurion University of the Negev. Dr. Yaniv holds a B.Sc. in physics/mathematics and a M.Sc. in electro-optics with distinction from the Hebrew University of Jerusalem, and earned a M.Sc. and a Ph.D. in physics at Kent State University. In March 2000, Dr. Yaniv was nominated and he accepted the honorific title of Senior Research Fellow of the IC2 Institute of the University of Texas. In January 2001 Dr. Yaniv founded the Nanoparticles Applications Center, now affiliated with Texas State University. In December 2003 Dr. Yaniv was nominated and accepted to become a strategic advisor to Governor Nobuyoshi Sumita of Shimane Prefecture in Japan in the field of job creation utilizing the advances in nanotechnology. He can be reached at zyaniv@appliednanotech.net

This article is reprinted courtesy of the Society for Information Display.