Electric lamp and discharge devices – With luminescent solid or liquid material – Vacuum-type tube
Reexamination Certificate
2000-08-21
2003-02-18
Patel, Ashok (Department: 2879)
Electric lamp and discharge devices
With luminescent solid or liquid material
Vacuum-type tube
C313S1030CM, C313S1050CM, C313S306000, C313S308000, C313S309000, C313S311000, C313S336000, C313S351000, C313S497000
Reexamination Certificate
active
06522061
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to field emission devices and more particularly a field emission device utilizing a microchannel gain element to multiply electron emission.
BACKGROUND OF THE INVENTION
It has become increasing common in the construction of image-generating devices to utilize a field emission device or “cold” cathode as a source of electrons for exciting a surface that generates a visible light. The cathode is placed in a vacuum enclosure and electrons are emitted from the cathode by action of a strong electric field adjacent the cathode. The electric field results from the cathode's geometry and from the use of a collector or anode adjacent the cathode. The anode is biased with a strong positive electric potential relative to the cathode. The emitted electrons generate a beam that passes between the cathode and anode. This beam can be modified by a third electrode known as a gate or grid. Further electrodes can also be added within the vacuum space between the cathode and the gate to further modify the electron beam. The resulting assembly can be termed a triode, tetrode, pentode, or the like, depending upon the number of electrodes present, in addition to the anode and cathode.
A generalized field emission device according to the prior art is detailed schematically in FIG.
1
. The cathode
20
includes an emitter section
22
having a sharp emitter point
23
. The emitter
22
is located behind a gate
24
having an opening
26
through which electrons pass to strike an anode
28
. The cathode
20
, gate
24
and anode
28
are separated from each other spatially and are enclosed in an evacuated envelope
30
. Field emission or “cold” cathodes such as cathode
20
are known generally to those who are skilled in the art. Current methods of fabricating and characterizing these devices is described in a special issue of the Journal of Vacuum Science and Technology B. Microelectronics and Nanometer Structures, Second Series, vol. 12, no. 2. March/April 1994and is hereby expressly incorporated herein by reference.
When the anode
28
of
FIG. 1
is employed as a display device (such as in a flat panel display) a phosphor is provided integrally to the anode. The phosphor is sensitive to electron excitation and emits visible light. Likewise, the cathodes
20
are organized in arrays that are addressable to generate an image. Where the structure of
FIG. 1
is to be employed in electron microscope, the object to be analyzed is part of the anode
28
.
In a display device, the brightness of light emitted by the phosphor screen depends upon the density of electrons that strike a given area of the screen and the energy derived from the voltage difference between the anode and the cathode. The brightness also depends upon the efficiency of the phosphor in converting the energy of electrons to photons of visible light. Typically, to achieve a high current density, and therefore a high-brightness in a display device the electric field that extracts electrons at the emitter
22
must be in the range of 10
7
Volts/centimeter. This high field strength invariably leads to cathode tip damage by erosion. Such erosion makes the emitted beam unstable in the short term and impairs long-term reliability. Thus, erosion makes construction of flat panel displays using the structure
FIG. 1
limited in brightness. Cathode damage and beam instability are also encountered in developing electron sources for integrated circuit lithography and electron microscopy.
IBM Research Report RC19596 (86076), Jun. 3, 1994, entitled “Emission Characteristics Of Ultra-Sharp Cold Field Emitters” (now published in the Journal of Vacuum Science & Technology. B12(6), p.3431. Nov./Dec. 1994) by Ming L. Yu, et al describes cathode deterioration at high current densities due to ohmic power dissipation, electron static stress and ion etching from residual gases in the evacuated space. The report also relates to long term and short term current instabilities during electron emission at high current densities.
It is desirable that electrons emitted by each cathode be as concentrated as possible when used in a display device. Transverse spreading of electrons generates larger pixels and, thus, lowers the density of pixels on the screen. Spreading of the electron beam also results in noise and reduced contrast, since electrons strike the phosphor in an area other than the intended pixel. To combat spreading, displays according to the prior art have located the anode/phosphor screen as close to the cathode array as necessary to attain desired resolution, contrast and signal-to-noise ratio. However, the placement of the anode and cathode in close proximity makes construction difficult and imposes design constraints that increase construction cost and/or degrade performance.
While the cathode structure
FIG. 1
can also be utilized in a color display device, by providing phosphors of three different colors (red, green and blue, for example) in a cluster with three independent emitters, the above-described problems remain. Additionally, accurate control of color generated by the three, clustered, subpixels must also be maintained. Electron beam spread and instability complicate the maintenance of good color fidelity.
In order to overcome the problems inherent in a field emission cathode operating at high current, it is contemplated that the cathode can be operated at a substantially lower current. However, a lower current, while reducing erosion and increasing electron beam stability, does not generate a beam of sufficient density. The resulting weak beam is typically insufficient to cause currently available phosphors to emit a bright visible light. The emitted electron beam is also insufficient to perform detailed electron microscopy or electron beam lithography with improved performance and results.
In the field of lithography for producing integrated circuits, in particular, the goal is to create complex, microscopic circuit patterns on a semiconductor wafer substrate composed, for example, of silicon or gallium arsenide. To accomplish the patterning process, the wafer is first covered with a protective layer of polymeric, light-sensitive material, known as a photoresist. The photoresist is dried by a baking or “curing” process, and then is exposed selectively to light in a pattern defined by a “mask” having transparent and opaque areas analogous to the desired circuit pattern. Where the mask is transparent, light passes through, onto the substrate, exposing each portion of the circuit pattern. The exposed circuit pattern on the substrate undergoes a chemical change in response to the light, while the remaining unexposed photoresist is unchanged chemically. When the photoresist is “developed,” in a manner similar to photographic film, a defined circuit pattern on the substrate at a microscopic level results. The developed photoresist enables selective processing of the semiconductor wafer to produce a layered structure with a variety of conducting, semiconducting and insulating media that define the finished circuit.
A mask can be placed directly on the surface of the substrate to produce a 1:1 scale image on the substrate. This is termed “contact lithograplhy.” since the mask essentially contacts the substrate. Alignment of the mask is important in a contact arrangement, and a special contact aligner is used for this purpose, since the mask must maintain alignment with the substrate within a close tolerance range as the substrate undergoes several different layers of lithography.
Other techniques for producing circuit patterns on a substrate entail the use of “proximity alignment,” “projection alignment,” or “step-and-repeat alignment.” The most widely employed in the semiconductor industry is currently the step-and-repeat technique. A “stepper” mechanism transfers the image from a relatively larger 5× or 10× scale (for example) mask to the substrate using, a reduction process that employs a sophisticated optical system. The substrate is moved in increments, or “stepped,” under the
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