Electric lamp and discharge devices – With luminescent solid or liquid material – Vacuum-type tube
Reexamination Certificate
2000-05-26
2003-08-05
Patel, Vip (Department: 2879)
Electric lamp and discharge devices
With luminescent solid or liquid material
Vacuum-type tube
C313S495000, C313S493000, C313S509000
Reexamination Certificate
active
06603257
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to field effect electron emitters and an improved method for forming field-effect electron emitters, a form of cold cathode electron emitters for a cathodo/electro-luminescent flat panel display.
DESCRIPTION OF THE RELATED ART
A cathodoluminescent field-effect flat panel display consists of a two-dimensional cold cathode with a matrix addressing scheme constructed above the cathode whereby electrons may be allowed to selectively flow with an intensity determined by the matrix addressing scheme to a distally disposed, phosphor-bearing, anodically-biased plate. The display is termed “field effect” because the source of electrons arises from an array of needle-like emitters (from one to several hundred per picture element, or pixel). A positive potential is applied between the first electrode in the addressing scheme and the needle-like emitters or cold cathode structures. The mechanical shape of the needle-like structure produces a compression of equipotential lines and, therefore, an enhancement of the electric field at the tip of the structure by many orders of magnitude. This enhancement is sufficient for the emission of electrons. The material itself, especially its work function, is important to predict the effect. Electrons experience an acceleration through an electric field that is maintained between the matrix addressing scheme and the anodic plate of the device. The anode consists of an array of cathodoluminescent phosphors. Generally, these phosphors are deposited within a black matrix and the entire layer is covered with an aluminum film, very similar to the construction of the face plate of a color cathode ray tube (CRT) however no shadow mask is involved.
As is usual in flat panel displays, the drive electronics present a register of signals, one signal for each picture element in a scan line, to vertically disposed matrix lines. Each signal is modulated either in amplitude or duration or both to effect the luminance level desired. For full color, triplexes of phosphors emitting red, green and blue are addressed. Once the scan line is serviced, the data is replaced by the data appropriate to the second scan line. This line is selected and the data applied. This operation continues until all scan lines have been serviced. To avoid the perception of flickering light, the time allowed to service all scan lines is in the order of 10 to 15-milliseconds. If the phosphorescence is short, each pixel must be restimulated in a shorter time. An alternative is to fabricate a storage means at every pixel. Generally, this is done by fabricating a thin film field effect transistor at every pixel. This electronic storage thereby compensates for the lack of phosphor or other electro-optical persistence. As yet, this alternative is employed commercially only in liquid crystal displays called “TFT-LCD”s. Cathodoluminescent phosphors generally demonstrate both direct luminescence when light is emitted while the phosphor is being driven, and phosphorescence when the metastable phosphor returns to ground state after stimulation. Photons are emitted as electrons fall back into lower energy orbitals. The timing of these effects and human eye physiology determines the maximum time allowed to service all scan lines. The reciprocal of this time is the refresh rate, the number usually reported in display literature. At some point, long phosphorescence times become undesirable because images, intended to change dynamically, smear.
Additional important considerations arise from considering the useful service life of the device. Phosphors exhibit coulomb-aging. Each phosphor material is characterized by the total number or coulombs that the material can accept to lose half its emission efficiency. From this point of view, using higher voltage phosphors at low current for a given energy is better than using lower voltage, higher current phosphors. Ideally, a field effect device (FED) uses CRT phosphors that benefit from 60 years of development. These phosphors have useful lives of up to 20,000 hours of operation. Unfortunately, CRT phosphors operate at high voltage (greater than 6,000-volts). This creates an engineering problem. The anodic plate must be distally disposed farther from the cathode/grid structure. Holding the anodic plate in appropriate position requires intervention to focus electron bundles and spacer members with challenging aspect ratios. The spacer members must be tens of millimeters high but roughly 0.025 millimeters in cross section.
Seminal work in cold cathodes was reported by Spindt et al., in “Physical Properties of Thin-film Field Emission Cathodes with Molybdenum Cones,” Journal of Applied Physics, Volume 47, No. 12, December 1976, pages 5248-5263. Spindt et al., discuss field emission cathode structures in U.S. Pat. Nos. 3,665,241, 3,755,704, and 3,812,559. The flat panel display industry refers to the process that produces arrays of molybdenum cones as described by Spindt et al., as the Spindt process. This process demonstrates the availability of currents in the range of 50-150 microamperes per cone. The service life of emitters using the Spindt process can be limited by ion polishing of the cones. The electric field at a sharp tip is inversely proportional to the radius of the tip. Extremely sharp tips with a radius of tens of nanometers may be dulled to a radius in the hundreds of nanometers by suffering ion impact, or sputtering. It is important to maintain a high quality vacuum in an FED to minimize this effect. Molybdenum has a work function on the order of 4.5 to 5eV, and therefore, offers no enhancement of the emission effect due to work function.
An example of a method of enhancing the efficiency of a needle-like cold cathode array is given in U.S. Pat. No. 5,908,699 ('699) entitled “Cold cathode electron emitter and display structure”. The '699 reference discloses the use of nano-crystalline carbon to create a robust needle-like tip. However, carbon has a relatively high work function (about 5eV) and therefore, such displays require relatively high potential differences between the cathode needles and the addressing matrix electrode. With the introduction of cesium as a cathode material in '699, the effective work function is reduced to the order of 1.05 to 1.3eV, dramatically reducing the operating voltages (e.g., on the order of 4 or 5 to 1). However, cesium is a difficult material. Cesium tends to act as a scavenger in vacuum devices. In fact, cesium is often introduced into vacuum devices as a getter. Cesium is difficult to handle in manufacturing since it is unstable in air. While a low work function material seems highly desirable, cesium may not maintain its properties over the required lifetime of the cathode.
Certain phosphors, for example, ZnS: Mn, emit light when stimulated by an electric field.
This phenomenon is called electroluminescence and the flat panel displays predicated on this phenomenon are called electroluminescent displays (EL). Commercial devices are made in two basic ways: (1) powder EL generally uses a thick phosphor layer in a direct current grid work and (2) thin film EL generally uses a layered structure that includes a thin film of EL phosphor and at least one transparent insulator, for example yttrium oxide (Y
2
O
3
), in a conductive cross grid. Both structures, powder and thin film EL, are simple, and the display is fabricated on a single insulating plate. Since the luminance arises only in the driven phosphor, one of the conductive electrodes must be substantially transparent. Usually, indium tin oxide (ITO) is used for the transparent electrode. A typical vertical structure is ITO, Y
2
O
3
, ZnS:Mn, Y
2
O
3
, and Al. Usually, ITO forms vertical lines and aluminum forms horizontal lines. If Al is deposited first, a transparent cover plate is needed, and the initial insulated plate need not be transparent. If ITO is deposited first, the substrate must be transparent. This type of display is commonly referred to as “acTFEL”, meaning a thin film electrolumine
Elqaq Deirdre Heyde
Hasan Mohamed Ali
Calkins Charles W.
Kilpatrick & Stockton LLP
Patel Vip
Quarterman Kevin
University of North Carolina at Charlotte
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