This invention relates generally to field emission display devices, and in particular, to cathode structures for field emission devices.
Field emission displays (FEDs) are flat panel display devices that combine the size and portability advantages of liquid crystal displays (LCDs) with the performance of conventional cathode ray tubes (CRTs). FED devices typically include a field emission cathode positioned opposite a flat screen coated with phosphors. The phosphors emit light in response to bombardment by electrons from the cathode to produce an image. The field emission cathode emits electrons when subjected to an electric field of sufficient strength. The cathode typically includes thousands of microscopic emitter tips for each pixel of the screen. It is principally the emissive nature of the cathode that give FEDs the thin, flat screen features of an LCD with the viewing angle, brightness, and response speed of a CRT.
While FEDs are potentially very attractive devices, a limiting factor in the widespread adoption of the technology is the difficulty of manufacturing the devices, particularly the difficulty in manufacturing the FED cathodes. Field emission cathodes have been known for some time. See, for example, Spindt et al. J. of Appl. Phys. 47, 5248 (1976). The field emission cathodes described therein typically comprise sharp-tip metal electron emitters, such as molybdenum cones having a tip radius of the order of a few tens of nanometers. A method of manufacturing such cathodes with Mo cone emitters on a conductive substrate using semiconductor fabrication techniques is described, of example, in U.S. Pat. No. 5,332,627 to Watanabe et al. Another example of the use of semiconductor fabrication techniques, including patterning and etching, to manufacture emitter cone structures is provided in U.S. Pat. No. 5,755,944 to Haven et al.
Because of the difficulty of manufacturing metal cone emitters, planar emitters have been identified as alternative emitters for use in field emission cathodes. Planar emitters typically fall into two classes: edge emitters and surface emitters. Edge emitters emit electrons from the very edge of a layer of material regardless of the amount of material present, while surface emitters emit electrons from an entire surface area.
The performance of emitting materials in field emission cathodes is largely dependent on the design of the cathode structure. While cathode structures for sharp-tip metal cone emitters have been thoroughly investigated, cathode structures for planar emitters, both edge emitters and surface emitters, are not as well developed. Thus, there is a need for improved cathode structures for planar emitters for use in field emission displays.
A four-layer cathode structure for use in field emission display (FED) devices improves emission characteristics, such as current density and uniformity, for edge emitters and surface emitter. An image is displayed on an FED device in terms of pixels, each made up of multiple sub-pixels. A first layer of the four-layer cathode structure consists of conducting lines. The first layer is supported on an insulating substrate. The width of the conducting lines is smaller than the sub-pixel size. A second layer consists of thin non-conducting lines crossing the conducting lines. A third layer consists of a thick layer of non-conducting material with holes centered between the thin non-conducting lines of the second layer and extending over a portion of the thin-non-conducting lines. The fourth layer of the cathode structure consists of conducting lines containing holes of the same dimension as, and aligned with, the holes in the third layer. Because the holes in the third and fourth layers are aligned, portions of the conducting lines of the first layer and of the non-conducting lines of the second layer are exposed.
To complete the manufacture of a field emission cathode, emissive material is deposited onto the portions of the conducting lines of the first layer exposed through the aligned holes in the third and fourth layers of the cathode structure. For planar edge emitters, the emitting region is the edge of the emitter-covered conducting lines. The four-layer cathode structure insures that the emitting edges are completely exposed. The thin non-conducting lines isolate the emitting material and prevent it from wicking up the edges of the holes and creating short circuit with the fourth layer of conducting lines. Thus, the four-layer cathode structure improves FED device performance.