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METHOD OF PREVENTING JUNCTION
LEAKAGE IN FIELD EMISSION DEVICES
RELATED APPLICATION INFORMATION
This is a continuation-in-part of U.S. patent application Ser. No. 08/907,256, filed Aug. 6, 1997, now abandoned, which is a continuation of Ser. No. 08/542,718, filed Oct. 13, 1995, now abandoned, which is a continuation-in-part of Ser. No. 08/307,365, filed Sep. 16, 1994, now abandoned.
This invention was made with Government support under Contract No. DABT63-93-C-0025 awarded to Advanced Research Projects Agency (ARPA). The Government has certain rights in this invention.
FIELD OF THE INVENTION
This invention relates generally to stabilizing the threshold voltage active elements in active matrix Field Emission Displays (FEDs).
BACKGROUND OF THE INVENTION
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 in active matrix FEDs typically contains the active semiconductor devices (e.g., field effect transistors) that control electron emissions 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 grids 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, releasing the photons that illuminate the screen. A single pixel of the display screen is typically illuminated by one or more emitter sites.
In a gated FED, the grid is separated from the base 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, granted 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", and incorporated herein by reference, describes a flat panel display that utilizes FEDs.
The quality and sharpness of an illuminated pixel site of the display screen is dependent upon 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 emissions 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 required to be isolated remain in an off condition. Thus,
shifts in the threshold voltage (Vr) (the voltage necessary to turn on the transistor for the pixel) are undesirable, and there is difficulty in maintaining the Vr at a level such that unwanted activation will not occur.
5 SUMMARY OF THE INVENTION
It is an object of the present invention to provide an improved method of constructing an FED with a lightblocking element that prevents photons generated in the
10 environment and by a display screen of the FED from affecting semiconductor junctions on a baseplate of the FED. It is a still further object of the present invention to provide an improved method of constructing FEDs using an opaque layer that protects semiconductor junctions on a baseplate
15 from light and which may also perform other circuit functions. It is a still further object of the present invention to provide an FED with improved junction leakage characteristics using techniques that are compatible with large-scale semiconductor manufacture. A further object of this inven
20 tion is to provide a means for protecting the cathode structure of an FED. A still further object of the present invention is to shield transistors and semiconductor junctions of an FED against X-rays and other electromagnetic radiation. Finally, it is still further an object of the present invention to
25 manufacture a high-quality FED display having a long life. In accordance with the present invention, an improved method of constructing FEDs for flat panel displays and other electronic equipment is provided. The method, generally stated, comprises the formation of radiation-blocking
30 elements between a cathodoluminescent display screen and baseplate of the FED. A light-blocking element protects semiconductor junctions on a substrate of the FED from photons generated in the environment and by the display screen. An X-ray-blocking element prevents damage to the
35 cathode structures from X-rays generated when electrons bombard the phosphor screen. The light-blocking element may be formed as an opaque layer adapted to absorb or reflect light. In addition to protecting the semiconductor junctions from the effects of photons, the opaque layer may
40 serve other circuit functions. The opaque layer, for example, may be patterned to form interlevel connecting lines for circuit components of the FED.
In an illustrative embodiment, the light-blocking element is formed as an opaque, light-absorbing material deposited
45 on a baseplate for the FED. As an example, a metal such as titanium that tends to absorb light can be deposited on the baseplate of an FED. Other suitable opaque materials include insulative light-absorbing materials such as carbon black, impregnated polyamide, manganese oxide and man
50 ganese dioxide. Moreover, such a light-absorbing layer may be patterned to cover only the areas of the baseplate that contain semiconductor junctions. The light-blocking element may also be formed of a layer of a material, such as aluminum, adapted to reflect rather than absorb light.
55 In another embodiment, an X-ray-blocking layer is formed, said layer comprising an X-ray-blocking material disposed between the picture elements and the cathodes. As an example, a metal such as Tungsten that has a high atomic number Z and tends to block X-rays may be used in order to
60 prevent, at least partially, X-ray radiation from damaging the cathode structures. Lead, titanium, and other metals, ceramics and compounds that have a high atomic number Z and tend to block X-rays may serve as suitable alternative materials. The X-ray-blocking layer can also be patterned to
65 cover only particular areas that house sensitive cathode structures and semiconductor junctions, and may be formed of layers of more than one type of X-ray-blocking material.
Other objects, advantages and capabilities of the present invention will become more apparent as the description proceeds.
BRIEF DESCRIPTION OF THE DRAWINGS 5
FIG. 1 is a cross-sectional schematic view of a prior art FED showing a pixel site and portions of adjacent pixel sites;
FIG. 2 is a cross-sectional schematic view of an emitter site for an FED having a light-blocking element formed in 10 accordance with the invention;
FIG. 3 is a perspective view of a cathode structure for an FED having an X-ray-blocking element formed in accordance with the invention;
FIGS. 4A and 4B are elevational views of a pixel/ emission site of an FED; and
FIG. 5 is another elevational view of a pixel/emission site of an FED according to the present invention.
DETAILED DESCRIPTION OF THE 20
It has been found that photons, generated by the luminescent display screen, as well as photons present in the environment (e.g. sunshine), cause an emitter site to emit 2J electrons unexpectedly. In some FEDs, 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 3Q 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 35 may adversely affect the address or activation of pixel sites and cause stray emissions and consequently a degraded image quality.
In experiments conducted by the inventors, junction leakage currents have been measured in the laboratory as a 40 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 range from picoamps (i.e., 10~12 amps) for dark conditions to microamps (i.e., 10~6 amps) for well-lit conditions. In FEDs, even 45 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 50 semiconductor junctions.
Moreover, it has been found by the inventors that unblocked electromagnetic radiation may damage the semiconductor junctions or the cathode structure. Exposure to photons from the display screen and external environment 55 may change the properties of some junctions on the substrate associated with the emitter sites, causing current flow and the initiation of electron emissions from the emitter sites on the adjacent pixel sites. The electron emissions may cause the adjacent pixel sites to illuminate when a dark back- 60 ground is desired, again causing a degraded or blurry image. In addition to isolation and activation problems, light from the environment and display screen striking junctions on the substrate may cause other problems in addressing and regulating current flow to the emitter sites of the FED cell. 65
For example, a problem may occur when photons (i.e., light) generated by a light source strike the semiconductor
junctions formed in the substrate. Further, photons from an illuminated pixel site may strike the junctions formed at the N-type conductivity regions on the adjacent pixel sites. The photons are capable of passing through the spacers, grid and insulating layer of the FED, because these layers are often formed of materials that are translucent to most wavelengths of light, such as spacers formed of a translucent polyamide (e.g., kapton or silicon nitride), or an insulative layer may be formed of translucent silicon dioxide, silicon nitride or silicon oxynitride. The grid may also be formed of translucent polysilicon.
U.S. Pat. No. 3,814,968, granted to Nathanson et al, addresses the problem with aluminization deposited on the screen member. However, such an approach does not work for high resolution active matrix FEDs, because cathode voltages are relatively low (e.g., 200 volts), and an aluminum layer formed on the inside surface of the display screen cannot be penetrated by enough electrons emitted at these low voltages. Therefore this approach is not suitable in an active matrix FED.
It is also known in the art to construct FEDs with circuit traces formed of an opaque material, such as chromium, that overlie the semiconductor junctions contained in the FED baseplate. As an example, U.S. Pat. No. 3,970,887, granted to Smith et al., describes such a structure (see FIG. 8). However, these circuit traces are constructed to conduct signals, and are not specifically adapted for isolating the semiconductor junctions from photon bombardment. Accordingly, most of the junction areas are left exposed to photon emission and the resultant junction leakage.
Another problem which may arise is caused by the presence of X-rays or radiation, often generated when electrons impinge upon the phosphor screen. The term "X-ray" means an electromagnetic radiation which has wavelengths in the range of 0.06 nm to 12.5 nm; visible light has wavelengths in the range of 400 nm to 800 nm. In FEDs, generated X-rays are emitted in virtually all directions. Because of the close proximity of the cathode to the X-ray emitting anode in an FED, it has been found that the cathode structure may be damaged by such exposure. In particular, if a silicon chip is used as a substrate on which the cathode structure is built up, the transistors or semiconductor junctions on the baseplate are susceptible to damage from these X-rays.
Referring now to drawing FIG. 1, an example embodiment is shown with 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 comprising, for example, 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 FEDs, 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