Electric lamp or space discharge component or device manufacturi – Process – With assembly or disassembly
Reexamination Certificate
2002-02-14
2004-01-13
Ramsey, Kenneth J. (Department: 2879)
Electric lamp or space discharge component or device manufacturi
Process
With assembly or disassembly
Reexamination Certificate
active
06676471
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to field emission displays (FEDs) and, more particularly, to a method for preventing junction leakage in FEDs.
2. State of the Art
Flat panel displays have recently been developed for visually displaying information generated by computers and other electronic devices. Typically, these displays are lighter and utilize less power than conventional cathode ray tube displays. One type of flat panel display is known as a cold cathode field emission display (FED).
A cold cathode FED uses electron emissions to illuminate a cathodoluminescent screen and generate a visual image. An individual field emission cell typically includes one or more emitter sites formed on a baseplate. The baseplate typically contains the active semiconductor devices that control electron emission from the emitter sites. The emitter sites may be formed directly on a baseplate formed of a material such as silicon or on an interlevel conductive layer (e.g., polysilicon) or interlevel insulating layer (e.g., silicon dioxide, silicon nitride) formed on the baseplate. A gate electrode structure, or grid, is typically associated with the emitter sites. The emitter sites and grid are connected to an electrical source for establishing a voltage differential to cause a Fowler-Nordheim electron emission from the emitter sites. These electrons strike a display screen having a phosphor coating. This releases the photons that illuminate the screen. A single pixel of the display screen is typically illuminated by one or several emitter sites.
In a gated FED, the grid is separated from the baseplate by an insulating layer. This insulating layer provides support for the grid and prevents the breakdown of the voltage differential between the grid and the baseplate. Individual field emission cells are sometimes referred to as vacuum microelectronic triodes. The triode elements include the cathode (field emitter site), the anode (cathodoluminescent element) and the gate (grid). U.S. Pat. No. 5,210,472 to Stephen L. Casper and Tyler A. Lowrey, entitled “Flat Panel Display In Which Low-Voltage Row and Column Address Signals Control A Much Higher Pixel Activation Voltage”, describes a flat panel display that utilizes FEDs.
In flat panel displays that utilize FEDs, the quality and sharpness of an illuminated pixel site of the display screen is dependent on the precise control of the electron emission from the emitter sites that illuminate a particular pixel site. In forming a visual image, such as a number or letter, different groups of emitter sites must be cycled on or off to illuminate the appropriate pixel sites on the display screen. To form a desired image, electron emission may be initiated in the emitter sites for certain pixel sites while the adjacent pixel sites are held in an off condition. For a sharp image, it is important that those pixel sites that are required to be isolated remain in an off condition.
One factor that may cause an emitter site to emit electrons unexpectedly is the response of semiconductor junctions in the FED to photons generated by the luminescent display screen and photons present in the environment (e.g., lights, sunshine). In an FED, P/N junctions can be used to electrically isolate each pixel site and to construct row-column drive circuitry and current regulation circuitry for the pixel operation. During operation of the FED, some of the photons generated at a display screen, as well as photons from the environment, may strike the semiconductor junctions on the substrate. This may affect the junctions by changing their electrical characteristics. In some cases, this may cause an unwanted current to pass across the junction. This is one type of junction leakage in an FED that may adversely affect the address or activation of pixel sites and cause stray emission and a degraded image quality.
One possible situation is shown in FIG.
1
.
FIG. 1
illustrates a pixel site
10
of a field emission display (FED)
13
and portions of adjacent pixel sites
10
′ on either side. The FED
13
includes a baseplate
11
having a substrate
12
formed of a material such as single crystal P-type silicon. A plurality of emitter sites
14
is formed on an N-type conductivity region
30
of the substrate
12
. The P-type substrate
12
and N-type conductivity region
30
form a P/N junction. This type of junction can be combined with other circuit elements to form electrical devices, such as FETs, for activating and regulating current flow to the pixel sites
10
and
10
′.
The emitter sites
14
are adapted to emit electrons
28
that are directed at a cathodoluminescent display screen
18
coated with a phosphor material
19
. A gate electrode or grid
20
, separated from the substrate
12
by an insulating layer
22
, surrounds each emitter site
14
. Support structures
24
, also referred to as spacers, are located between the baseplate
11
and the display screen
18
.
An electrical source
26
establishes a voltage differential between the emitter sites
14
and the grid
20
and display screen
18
. The electrons
28
from activated emitter sites
14
generate the emission of photons from the phosphor material contained in a corresponding pixel site
10
of the display screen
18
. To form a particular image, it may be necessary to illuminate pixel site
10
while adjacent pixel sites
10
′ on either side remain dark.
A problem may occur, however, when photons
32
(i.e., light) generated by a light source
33
, sunlight or other environmental factors strike the semiconductor junctions formed in the substrate
12
. In addition, photons
32
from an illuminated pixel site
10
may strike the junctions formed at the N-type conductivity regions
30
on the adjacent pixel sites
10
′. The photons
32
are capable of passing through the spacers
24
, grid
20
and insulating layer
22
of the FED
13
, because often these layers are formed of materials that are translucent to most wavelengths of light. As an example, the spacers
24
may be formed of a translucent polyimide, such as kapton or silicon nitride. The insulative layer
22
may be formed of translucent silicon dioxide, silicon nitride or silicon oxynitride. The grid
20
may be formed of translucent polysilicon.
The exposure to photons from the display screen
18
and the environment may change the properties of some junctions on the substrate
12
associated with the emitter sites
14
. This in turn may cause current flow and initiate electron emission from the emitter sites
14
on the adjacent pixel sites
10
′. The electron emission may cause the adjacent pixel sites
10
′ to illuminate when a dark background may be required. This will cause a degraded or blurry image. Besides isolation and activation problems, light from the environment and display screen
18
striking junctions on the substrate
12
may cause other problems in addressing and regulating current flow to the emitter sites
14
of the FED
13
.
In experiments conducted by the inventors, junction leakage currents have been measured in the laboratory as a function of different lighting conditions at the junction. At a voltage of about 50 volts and depending on the intensity of light directed at a junction, junction leakage may be on the order of picoamps (i.e., 10
−12
amps) for dark conditions to microamps (i.e., 10
−6
amps) for well-lit conditions. For an FED, even relatively small leakage currents (i.e., picoamps) will adversely affect the image quality. The treatise entitled “Physics of Semiconducting Devices” by S. M. Sze, copyright 1981 by John Wiley and Sons, Inc., at paragraphs 1.6.1 to 1.6.3, briefly describes the effect of photon energy on semiconductor junctions.
In the construction of screens for cathode ray tubes, screen aluminizing processes are used to form a mirror-like finish on the inside surface of the screen. This layer of aluminum reflects light towards the viewer and away from the rear of the tube. In U.S. Pat. No. 3,814,968 t
Cathey, Jr. David A.
Lee John
Micro)n Technology, Inc.
Ramsey Kenneth J.
TraskBritt
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