British Technology Aims at Large Economical FEDs

Field emitter display (FED) technology can produce very bright displays viewable over wide angles but has not seriously challenged established LCD techniques. This is because an FED array of many millions of fragile microchip emitters is very difficult to manufacture with the screen size required for applications such as television. Practical screen sizes are currently limited to ~5 in. Printable Field Emitters Ltd. (PFE) was formed to exploit a technique that avoids use of microtip emitters. PFE uses this technique to construct FED displays with emitters that can be deposited and patterned using thick film-type inks and simple lithography processes. PFE claims its technology is suitable for scaling up to display sizes of 1 m or more. Production lines can be employed. The technique offers low-voltage drive electronics with CRT display characteristics.

The material was developed from an epoxy composite used by Professor Rod Latham of the University of Aston (Birmingham, England). His material gave densities of 100 mA/cm² with low fields, but its vapor pressure was too high for use in a sealed vacuum environment. PFE, with Latham, developed a new class of materials suitable for use as field emission cold cathodes in sealed vacuum displays. They can be formulated as inks for use with standard patterning techniques such as screen printing and optical lithography.

Materials with 'switch-on' fields down to 3.5 V/mm have been produced, this value equaling the best examples of diamond and diamond-like carbon emitters. However, the emitter site density still needs to be raised further by two orders of magnitude to ~100,000 emitters per cm² to increase display brilliance. Micro-engineered buffer resistors can be built into emitting sites at no extra cost.

Fig. 1. A MIMIV structure in the composite cathode layer forms the emitter.

PFE fabricated a demountable turbo-pumped test device comprising a simple diode structure with a 0.5 mm gap and three 2 x 2 mm pixels. It used a glass-based emitter with built-in buffering and the industry standard phosphors: red (yttrium oxysulfide:Eu), green (zinc oxide:Zn) and blue (zinc sulfide:Ag) on indium tin oxide conductors. Potentials up to 7.5 kV were applied to the three anodes in turn to activate the red, green and blue phosphors. An external circuit could gate the power supply to modulate the pixels as digits in a three bit counter. Green and blue pixels have been switched for 449 hours, but an electrical discharge eliminated the red pixel in this device after 150 hours. Light emission at a brightness of 1800 C/m^-2 was demonstrated.

PFE has raised funds to build demonstration addressable 16 x 16 devices this year and has gathered a team to improve characteristics of the emitter material. It has expanded into its own laboratory facilities at the Rutherford Appleton Laboratory (Chilton, Oxon, UK). PFE plans to license its technology to large display producers rather than manufacture displays itself and supply emitter material formulations.