Many known vacuum fluorescent displays use a filament and grid combination to source electrons for the vacuum fluorescent display. To improve the efficiency and brightness of displays, it is desirable to move the electron source closer to the phosphors that are the light source. However, because the filaments and grids are typically suspended, mechanical shock to the display can give rise to vibrations in the grid and the filament. This potential for vibrations limits the practical minimum distance between the electron source comprising the filament and grid and the phosphors because it is undesirable to allow the filament and/or grid to short circuit to the substrate carrying the phosphors.
A known type of display that eliminates the filament and grid includes an array of field emitters as the electron source. The field emitter array is fabricated directly to a substrate, such as silicon, and therefore does not have the suspension and vibration characteristics of the filament and grid electron sources. In known vacuum fluorescent displays using field emitter arrays, electrons travel from the field emitters across a short distance to phosphors mounted on transparent anodes, which are mounted on a transparent substrate. For reconfigurable displays, the substrates may include transparent thin film transistors to control the voltage levels of the anodes. The electrons impinging on the phosphors excite the phosphors so that the phosphors emit light. The emitted light travels through the transparent conductor, transparent thin film devices, if any, and the transparent substrate on which the phosphors and thin film devices are mounted to be viewed by a viewer of the display.
Descriptions of field emitters and their construction are included in the articles, C. A. Spindt, I. Brodie, L. Humphrey and E. R. Westerberg, "Physical properties of thin-film field emission cathodes with molybdenum cones," Journal of Applied Physics, Vol. 47 , No. 12, December 1996, Pages 5248-5263, and C. A. Spindt, C. E. Holland, A. Rosengreen, I. Brodie, "Field-Emitter Arrays for Vacuum Microelectronics," IEEE, Transactions on Electronic Devices, Vol. 38, No. 10, October 1991, Pages 2355-2363. The disclosures of the above two articles are incorporated herein by reference.
These known field emitter displays are unsuitable for high brightness display applications because they have a limited brightness achievable for a given amount of power supplied to the display. As much as 40% of the light emitted by the phosphors is reabsorbed into the phosphor and substrate before it reaches the eye of the viewer of the display. Thus for high brightness display applications, the art has been typically limited to filament and grid vacuum fluorescent displays.
Use of filament and grid vacuum fluorescent displays places limitations on the size and structure of the display. Typically, a display with a filament and a grid requires the grid to be approximately one millimeter from the emissive phosphors. This has a variety of impacts on display performance, one is display efficiency, which is reduced because the electrons must travel over a fairly large distance, i.e., over 1 mm, to reach the phosphors.
A second impact is that the spacing of display elements is limited, which has an extremely noticeable impact on reconfigurable vacuum fluorescent displays. For example, when pixels are spaced as close as 500 .mu.m, the operation of one pixel may have an undesirable coupling effect on the operation of a neighboring pixel. Pending U.S. patent application, Ser. No. 08/205,462, assigned to the assignee of this invention, recognizes this coupling effect and sets forth a structure including an isolation grid on the substrate surrounding each pixel that eliminates the coupling effect and also the necessity for a suspended acceleration grid. This structure allows movement of the pixels closer together without any resulting undesirable coupling between the pixels. For example, displays using the isolation grid have achieved a pixel pitch of about 370 .mu.m while retaining a 50% fill factor, where the fill factor is defined as the ratio of the phosphor covered area of the pixel (the light emitting area) to the total pixel area. Further reducing the pixel pitch while maintaining the fill factor is a challenge, though, because the isolation grid limits the spacing between neighboring pixels to no less than about 100 .mu.m and more typically in the range of 120-150 .mu.m. This thus limits the pixel density of the reconfigurable display.
One prior field emitter display proposal places the field emitters on the same substrate with the phosphors. This configuration eliminates the necessity to mount the phosphors on a transparent substrate but the pixel fill factor is seriously reduced due to the space required for the large multiplicity of field emission structures fabricated on the common substrate with the phosphor. Thus such displays will have inadequate brightness, fill factor and/or pixel density for many applications.