Laser processed hydrogenated amorphous silicon for field emission displays

Amorphous silicon is now considered to be a mature technology in the field of flat panel displays. The a-Si:H Thin Film Transistor has enabled the active matrix liquid crystal display that has become the standard. However, the technology is still not perfect and suffers from visual artefacts, especially when trying to render video rate images. Field emission displays (FED) have promised to resolve these issues, but lithographic limitations in the technology have so far failed to deliver a technology that is truly manufacturable. Carbon nanotube technology has shown promise but also shows a number of limitations. This paper describes a new approach to field emission technology and a route to manufacture. Conventional FEDs utilise the confining effects of electric fields at sharp fabricated microtips. It has been found that laser processed hydrogenated amorphous silicon shows remarkable field emission properties that can be readily exploited in display manufacture. Hydrogenated amorphous silicon after breakdown in an electric field forms filamentary regions that show stable field emission. The structure after breakdown, or conditioning, may have an inhomogeneous nano-scale granular structure. The filamentary channels can be determined by the distribution of the nano-particles and act as the source of emission. Gated filaments are a route to low voltage emission. This concept has been extended to form a two dimensional plane for field emission, but without the breakdown or conditioning step. The interaction of profiled excimer laser beams with thin films of hydrogenated amorphous silicon in contact with certain metals leads to surface roughening and the creation of nano particles. There is little confinement of the external field, yet large enhancement factors are found and emission is found at low thresholds and without hysteresis. The laser processing creates the conditions for a high internal electric field that is responsible for field emission. The uniform nature of the emission and the surface roughening effect makes the material a suitable candidate for simple fabrication of displays using materials and tools that are familiar to the industry. The laser processed material is coated with an insulator and metal and etched to form self aligned gated structures that extract electrons at low voltage. The electrons impinge on a phosphor plate through a vacuum space to form a display that is potentially superior to existing flat panel displays in terms of cost, speed and power consumption.