Electric lamp and discharge devices – With luminescent solid or liquid material – Vacuum-type tube
Reexamination Certificate
2003-03-04
2004-05-11
Williams, Joseph (Department: 2879)
Electric lamp and discharge devices
With luminescent solid or liquid material
Vacuum-type tube
C313S292000
Reexamination Certificate
active
06734619
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to evacuated flat-panel displays such as those of the field emission cathode and plasma types and, more particularly, to a process for forming load-bearing spacer structures for such a display, the spacer structures being used to prevent implosion of a transparent face plate toward a parallel spaced-apart back plate when the space between the face plate and the back plate is hermetically sealed at the edges of the display to form a chamber, and the pressure within the chamber is less than that of the ambient atmospheric pressure. The invention also applies to products made by such process.
2. State of the Art
For more than half a century, the cathode ray tube (CRT) has been the principal device for electronically displaying visual information. Although CRTs have been endowed during that period with remarkable display characteristics in the areas of color, brightness, contrast and resolution, they have remained relatively bulky and power hungry. The advent of portable computers has created intense demand for displays which are lightweight, compact, and power efficient. Although liquid crystal displays (LCDs) are now used almost universally for laptop computers, contrast is poor in comparison to CRTs, only a limited range of viewing angles is possible, and battery life is still measured in hours rather than days. Power consumption for computers having a color LCD is even greater and, thus, operational times are shorter still, unless a heavier battery pack is incorporated into those machines. In addition, color screens tend to be far more costly than CRTs of equal screen size.
As a result of the drawbacks of liquid crystal display technology, field emission display technology has been receiving increasing attention by industry. Flat-panel displays utilizing such technology employ a matrix-addressable array of cold, pointed, field emission cathodes in combination with a luminescent phosphor screen.
Somewhat analogous to a cathode ray tube, individual field emission structures are sometimes referred to as vacuum microelectronic triodes. Each triode has the following elements: a cathode (emitter tip), a grid (also referred to as the gate), and an anode (typically, the phosphor-coated element to which emitted electrons are directed).
Although the phenomenon of field emission was discovered in the 1950's, it has been within only the last ten years that extensive research and development have been directed at commercializing the technology. As of this date, low-power, high-resolution, high-contrast, monochrome flat-panel displays with a diagonal measurement of about 15 centimeters have been manufactured using field emission cathode array technology. Although useful for such applications as viewfinder displays in video cameras, their small size makes them unsuited for use as computer display screens.
In order for proper display operation, which requires field emission of electrons from the cathodes and acceleration of those electrons to the phosphor-coated screen, an operational voltage differential between the cathode array and the screen of at least 1,000 volts is required. As the voltage differential increases, so does the life of the phosphor coating on the screen. Phosphor coatings on screens degrade as they are bombarded by electrons. The rate of degradation is proportional to the rate of impact. As fewer electron impacts are required to achieve a given intensity level at higher voltage differentials, phosphor life may be extended by increasing the operational voltage differential. In order to prevent shorting between the cathode array and screen, as well as to achieve distortion-free image resolution and uniform brightness over the entire expanse of the screen, highly uniform spacing between the cathode array and the screen must be maintained. During tests performed at Micron Display Technology, Inc. in Boise, Id., it was determined that, for a particular evacuated, flat-panel field emission display utilizing glass spacer columns to maintain a separation of 250 microns (about 0.010 inches), electrical breakdown occurred within a range of 1100-1400 volts. All other parameters remaining constant, breakdown voltage will rise as the separation between screen and cathode array is increased. However, maintaining uniform separation between the screen and the cathode array is complicated by the need to evacuate the cavity between the screen and the cathode array to a pressure of less than 10
−6
torr, so that the field emission cathodes will not experience rapid deterioration.
Small area displays (e.g., those which have a diagonal measurement of less than 3.0 cm) may be cantilevered from edge to edge, relying on the strength of a glass screen having a thickness of about 1.25 mm to maintain separation between the screen and the cathode array. Because the displays are small, there is no significant screen deflection in spite of the atmospheric load. However, as display size is increased, the thickness of a cantilevered flat glass screen must increase exponentially. For example, a large, rectangular television screen measuring 45.72 cm (18 in.) by 60.96 cm (24 in.) and having a diagonal measurement of 76.2 cm (30 in.) must support an atmospheric load of at least 28,149 newtons (6,350 lbs.) without significant deflection. A glass screen, or face plate (as it is also called), having a thickness of at least 7.5 cm (about 3 inches) might well be required for such an application. But that is only half the problem. The cathode array structure must also withstand a like force without significant deflection. Although it is conceivable that a lighter screen could be manufactured so that it would have a slight curvature when not under stress and be completely flat when subjected to a pressure differential, with the fact that atmospheric pressure varies with altitude and as atmospheric conditions change, such a solution becomes impractical.
A more satisfactory solution to cantilevered screens and cantilevered cathode array structures is the use of closely spaced, load-bearing, dielectric spacer structures, each of which bears against both the screen and the cathode array plate, thus maintaining the two plates at a uniform distance between one another, in spite of the pressure differential between the evacuated chamber between the plates and the outside atmosphere. By using load-bearing spacers, large area displays might be manufactured with little or no increase in the thickness of the cathode array plate and the screen plate.
Load-bearing spacer structures for field-emission cathode array displays must conform to certain parameters. The spacer structures must be sufficiently nonconductive to prevent catastrophic electrical breakdown between the cathode array and the anode (i.e., the screen). In addition to having sufficient mechanical strength to prevent the flat-panel display from imploding under atmospheric pressure, they must also exhibit a high degree of dimensional stability under pressure. Furthermore, they must exhibit stability under electron bombardment, as electrons will be generated at each pixel location within the array. In addition, they must be capable of withstanding “bakeout” temperatures of about 400° C. that are likely to be used to create the high vacuum between the screen and the cathode array back plate of the display. Also, the material from which the spacers are made must not have volatile components which will sublimate or otherwise outgas under low pressure conditions.
For optimum screen resolution, the spacer structures must be nearly perfectly aligned to array topography, and must be of sufficiently small cross-sectional area so as not to be visible. Cylindrical spacers must have diameters no greater than about 50 microns (about 0.002 inch) if they are not to be readily visible. For a single cylindrical lead oxide silicate glass column having a diameter of 25 microns (0.001 in.) and a height of 200 microns (0.008 in.), a buckle load of about 2.67×10
−2
newtons (0.006 lb.)
Cathey David A.
Elledge Jason B.
Hofmann James J.
Xia Zhong-Yi
Micro)n Technology, Inc.
TraskBritt
Williams Joseph
LandOfFree
Anodically bonded elements for flat-panel displays does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Anodically bonded elements for flat-panel displays, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Anodically bonded elements for flat-panel displays will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3253213