Electric lamp and discharge devices – Electrode and shield structures
Reexamination Certificate
2000-08-15
2004-01-20
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
Electrode and shield structures
C313S336000
Reexamination Certificate
active
06680562
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to the field of electron sources for use in electron beam applications, and in particular to Schottky emitters.
BACKGROUND OF THE INVENTION
Electron emission cathodes, typically referred to as electron sources, are used in devices such as scanning electron microscopes, transmission electron microscopes, semiconductor inspection systems, and electron beam lithography systems. In such devices, an electron source provides electrons, which are then guided into an intense, finely focused beam of electrons having energies within a narrow range. To facilitate formation of such a beam, the electron source should emit a large number of electrons within a narrow energy band. The electrons should be emitted from a small surface area on the source into a narrow cone of emission. Electron sources can be characterized by a brightness, which is defined as the electron current divided by the real or virtual product oft he emission area and the solid angle through which the electrons are emitted. A practical source should be bright and should operate for an extended period of time with little or no maintenance and minimal noise, that is, variations in the amount and energy of the emitted electrons.
Electrons are normally prevented from leaving the atoms at the surface of an object by an energy barrier. The amount of energy required to overcome the energy barrier is known as the “work function” of the surface. One type of electron source, a thermionic emission source, replies primarily on heat to provide the energy to overcome the energy barrier and emit electrons. Thermionic emission sources are not sufficiently bright for use in many applications.
Another type of electron source, a cold field emission source, operates at room temperature and relies on a strong electric field to facilitate the emission of electrons by tunneling through the energy barrier. A field electron source typically includes a narrow tip at which electrons leave the surface and are ejected into the surrounding vacuum. While cold field emission sources are much smaller and brighter than thermionic emission sources, cold field emission sources exhibit instabilities that cause problems in many applications.
Yet another type of electron source is referred to as a Schottky emission cathode or Schottky emitter. Although the term “Schottky emission” refers to a specific operating mode of an emitter, the term “Schottky emitter” is used more broadly to describe a type of electron emitter that may be capable of operating in a variety of modes, including Schottky emission mode. Schottky emitters use a coating on a heated emitter tip to reduce its work function. The coating typically comprises a very thin layer, such as a fraction of a monolayer, of an active metal. In Schottky emission mode, a Schottky emitter uses a combination of heat and electric field to emit electrons, which appear to radiate from a virtual point source within the tip. With changes to the emitter temperature and electric field, the Schottky emitter will emit in other emission modes or combinations of emission mode, including extended Schottky emission mode and thermal field mode. Schottky emitters are very bright and are more stable and easier to handle than cold field emitters. Because of their performance and reliability benefits, Schottky emitters have become a common electron source for modem focused electron beam systems.
FIG. 1
shows part of a typical prior art Schottky emitter
12
, such as the one described in U.S. Pat. No. 3,814,975 to Wolfe et al. for “Electron Emission System.” Schottky emitter
12
includes a filament
14
that supports and heats an emitter
16
having an apex
22
from which the electrons are emitted. Applicants herein use the term “emitter” alone to refer to that portion of the electron source from which electrons are emitted (e.g., emitter
16
of
FIG. 1
) and the term “Schottky emitter” refers to the entire electron source assembly (e.g., Schottky emitter
12
), often including a suppressor cap described below. Heating current is supplied to filament
14
through electrodes
24
that penetrate a base
26
. Schottky emitter
12
typically operates with apex
22
at a temperature of approximately 1,800 K. Emitter
16
is typically made from a single crystal of tungsten oriented in the <100>, <110>, <111>, or <310> direction. Emitter
16
could also be made of other materials, such as molybdenum, iridium, or rhenium. Emitter
16
is coated with a coating material to lower its work function. Such coating materials could include, for example, compounds, such as oxide, nitrides and carbon compounds, of zirconium, titanium, hafnium, yttrium, niobium, vanadium, thorium, scandium, beryllium or lanthanum. For example, coating a (100) surface of tungsten with zirconium and oxygen lowers the work function of the surface from 4.5 eV to 2.8 eV. By reducing the energy required to emit electrons, the coating on the emitter makes it a brighter electron source.
At the high temperatures at which Schottky emitter
12
operates, the coating material tends to evaporate from emitter
16
and must be continually replenished to maintain the low work function at apex
22
. A reservoir
28
of the coating material is typically provided to replenish the coating on emitter
16
. The material from reservoir
28
diffuses along the surface and through the bulk of emitter
16
toward apex
22
, thereby continually replenishing the coating there. Schottky emitter
12
includes a reservoir
28
of coating material positioned at the junction of emitter
16
and filament
14
. Methods for coating emitters and fabricating reservoirs of coating materials are known. For example, reservoir
28
may be formed by adding a powder of a precursor material, such as zirconium hydride, to a solvent, such as water or isoamyl acetate, to make a slurry and then adhering the slurry to the emitter
16
. When the emitter is heated, the zirconium hydride decomposes into zirconium and hydrogen, which evolves off. The emitter
16
is then heated in an atmosphere of oxygen to form a zirconium oxide coating and reservoir. It will be understood that the term zirconium oxide is used to indicate any combination of zirconium and oxygen atoms and is not limited to any particular atomic ratio.
At the high operating temperatures of the Schottky emitter
12
, not only does the coating material on emitter
16
and apex
22
evaporate, the coating material also evaporates directly from the reservoir, depleting it. The evaporation rate of the coating material in the reservoir increases exponentially with the temperature. Thus, the useful life of the reservoir depends upon the amount of material in the reservoir and its temperature. At a constant temperature, increasing the mass of the reservoir increases its life. Large increases in reservoir mass are not practical, however, because the coating material in a large reservoir tends to separate from the emitter, reducing the reservoir mass and causing problems in the vacuum system.
When reservoir
28
is depleted, Schottky emitter
12
no longer functions properly, and it is necessary to shut down the electron beam system in which Schottky emitter
12
is installed to replace the emitter. Because such electron beam systems are often critical links in the manufacturing of complex integrated circuits, shutting down a system can delay production and is therefore costly. It is desirable, therefore, to extend the life of the reservoir as much as possible, thereby extending the life of the emitter.
FIG. 2
shows a part of another prior art Schottky emitter
34
, similar to the one described in J. E. Wolfe, “Operational Experience with Zirconiated T-F Emitters,”
J. Vac. Sci. Tech.
16(6) (1979) and U.S. Pat. No. 5,449,968 to Terui for “Thermal Field Emission Cathode.”
FIG. 2
shows an emitter
36
connected to a filament
38
at a junction
44
and terminating in an apex
46
. (Emitter
12
of
FIG. 1
also included a junction, but it was hidden by reservoir
28
.)
den Hartog Sander G.
Jun David S.
Magera Gerald G.
McGinn James B.
Schwind Gregory A.
Berck Ken A
FEI Company
Patel Nimeshkumar D.
Scheinberg Michael O.
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