A thermionic emission assembly includes a Wehnelt cap that has a cap beam aperture and a cavity within which a cathode is supported. Electrical energy applied to the cathode causes it to reach a sufficiently high temperature to emit a beam of electrons that propagate through the cap beam aperture. An anode having an anode beam aperture is positioned in spatial alignment with the cap beam aperture to receive the electrons. The anode accelerates the electrons and directs them through the anode beam aperture for incidence on a target specimen. A ceramic base forms a combined interface that electrically and thermally separates the Wehnelt cap and the anode. The interface thermally isolates the Wehnelt cap from the anode to allow the cathode to rapidly reach the sufficiently high temperature to emit the beam of electrons.

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© 2011 Applied Physics Technologies. A portion of the disclosure of this patent document contains material to which a claim for copyright is made. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the file or records of the U.S. Patent and Trademark Office, but reserves all other copyright rights whatsoever.

TECHNICAL FIELD

This disclosure relates generally to electron emitters and, in particular, to a thermionic emission assembly for generating an electron beam.

BACKGROUND INFORMATION

Various electron emitters or “sources” are used in equipment such as electron beam microscopes. Source alignment conventionally is performed by manual alignment of several individual piece part assemblies during operation.

At the expense of lifetime, the cathode is never turned off to avoid long emission stability delays. In other words, the cathode remains operating to avoid a time-consuming wait for stable operation to be reached before source alignment can be performed.

SUMMARY OF THE DISCLOSURE

A preferred embodiment of a thermionic emission assembly includes a Wehnelt cap that has a cap beam aperture and an interior region within which a cathode is supported. Electrical energy applied to the cathode causes it to reach a sufficiently high temperature to emit a beam of electrons that propagate through the cap beam aperture. An anode having an anode beam aperture is positioned in spatial alignment with the cap beam aperture to receive the electrons propagating through the cap beam aperture. The anode accelerates the electrons and directs them through the anode beam aperture for incidence on a target specimen. A ceramic base is positioned in the assembly to form a combined interface that electrically and thermally separates the Wehnelt cap and the anode. The interface thermally isolates, in absence of substantial heat dissipation, the Wehnelt cap from the anode to allow the cathode to rapidly reach the sufficiently high temperature to emit the beam of electrons.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

With reference to the above-listed drawings, this section describes particular embodiments and their detailed construction and operation. The embodiments described herein are set forth by way of illustration only and not limitation. Skilled persons will recognize in light of the teachings herein that there is a range of equivalents to the example embodiments described herein. Most notably, other embodiments are possible, variations can be made to the embodiments described herein, and there may be equivalents to the components, parts, or steps that make up the described embodiments.

For the sake of clarity and conciseness, certain aspects of components or steps of certain embodiments are presented without undue detail where such detail would be apparent to skilled persons in light of the teachings herein and/or where such detail would obfuscate an understanding of more pertinent aspects of the embodiments.

As skilled persons will appreciate in light of this disclosure, certain embodiments are capable of achieving certain advantages over the known prior art, including some or all of the following: (1) pre-alignment of a cathode, Wehnelt cap, and anode assembly; (2) relatively low mass and thermally isolated Wehnelt cap for shorter duration to reach stable operation; (3) reduced component part count for an electron gun module; (4) use of a one-piece ceramic disk for electrical and thermal isolation of the Wehnelt cap and anode; (5) reduced manufacturing costs for a thermionic emission assembly; (6) relative ease of manufacturing a thermionic emission assembly; and (7) a drop-in, field-replaceable thermionic emission assembly. These and other advantages of various embodiments will be apparent throughout the following detailed description.

FIG. 1is a cross-sectional pictorial view of a thermionic emission assembly100according to one embodiment.FIG. 2is an enlarged cross sectional view of assembly100that is rotated relative toFIG. 1to show two filament posts. Assembly100may be used in a number of different instruments or applications that are operationally enabled by use of an electron beam. Such applications include, but are not limited to, a scanning electron microscope (SEM), a transmission electron microscope (TEM), surface analysis and metrology, and high current applications (microwave tubes, lithography, electron-beam welders, X-ray sources, and free electron lasers). Assembly100is typically operated in a vacuum (e.g., 1×10−7Torr).FIG. 3is an enlarged diagram of a portion of assembly100showing a cathode102held between a pair of blocks104. Cathode102may be a hexaboride cathode such as a cerium hexaboride (CeB6) cathode or a lanthanum hexaboride (LaB6) cathode. Cathode102may have a <100> crystal orientation and a diameter from about 0.254 mm to about 1.27 mm, preferably from about 0.38 mm to about 0.77 mm. Cathode102includes an end portion106from which electrons are emitted. The shape and dimensions of cathode102may be different for various applications to achieve different beam currents, spot sizes, and brightness. For example, end portion106may be formed to have a cone angle θ of approximately 90° and a tip diameter from about 16 μm to about 25 μm to provide a relatively high beam current. Alternatively, end portion106may have a smaller cone angle (e.g., 60°) and tip diameter (e.g., 5 μm) to form a beam having high brightness and a small spot size. Moreover, cathode102may have a <310> crystal orientation and may be configured as a “tophat” cathode to form a beam having a large current and large spot size.

Blocks104may be made of pyrolytic graphite and may act as resistive heaters. Blocks104help to thermally isolate a pair of filament posts108from cathode102(only one filament post108shown inFIG. 1). The thickness of blocks104is preferably from about 0.25 mm to about 0.39 mm to provide different thermal isolation characteristics. Filament posts108are electrically connected to cathode102and are bent toward each other in a “V” shape to compressively hold cathode102in place. The mount configuration of filament posts108and cathode102may be a shunted mini vogel mount (SMVM) configuration or a mini vogel mount (MVM) configuration. Filament posts108may be made of a material, such as molybdenum or molybdenum-rhenium alloy, that maintains a high modulus of elasticity even at high temperatures.

Filament posts108extend from blocks104through a disk-shaped ceramic base110. Filament posts108are rigidly fixed to ceramic base110, which serves as a base for cathode102and filament posts108. Thus, blocks104and filament posts108form a support structure for cathode102, which is mechanically coupled to ceramic base110. Ceramic base110is made of any ceramic material such as alumina (98.5%). Ceramic base110has a thickness from about 1.5 mm to about 2.5 mm. The ceramic material is chosen because of its electrical insulating, high strength, and high temperature survivability properties.

A Wehnelt cap112is attached to ceramic base110. Wehnelt cap112may be made of titanium alloy (e.g., Ti6Al4V), titanium, or stainless steel. Wehnelt cap112is attached to ceramic base110by a field ring114and screws116. Field ring114is provided on a side of ceramic base110opposite from Wehnelt cap112, and screws116extend through ceramic base110between a flange118of Wehnelt cap112and field ring114. Ceramic base110provides electrical and thermal isolation between Wehnelt cap112and filament posts108. A negative voltage (e.g., −400 V) is applied to Wehnelt cap112relative to cathode102to suppress electron emission anywhere from cathode102other than its end portion106. Flange118may be mounted flush against ceramic base110, or a height adjustment ring (not shown) may be positioned between flange118and ceramic base110. Wehnelt cap112forms an interior region or a cavity that surrounds cathode102, blocks104, and portions of filament posts108. Wehnelt cap112has a top side120and a side wall122that extends between flange118and top side120. Side wall122may include one or more vent holes124, slots, or windows (slots and windows not shown). Side wall122has a preferred thickness from about 0.25 mm to about 1.02 mm. The thickness of side wall122and the configuration of vent holes124, slots, or windows affect thermal paths to provide more or less thermal isolation for top side120relative to flange118and other components of assembly100. For example, a relatively thin side wall122can provide for top side120a higher operating temperature than that provided by a thicker side wall122. A relatively thin side wall122can also provide between top side120and flange118a bigger thermal difference than that provided by a thicker side wall122. A high operating temperature for top side120may decrease the likelihood for oxide build up on it.

Top side120includes a cap beam aperture126located at a center portion so as to be coaxially aligned with end portion106of cathode102when Wehnelt cap112and filament posts108are fixed to ceramic base110. The thickness of top side120tapers from about 0.5 mm to about 0.125 mm from side wall122to the center portion where aperture126is located. Aperture126functions to allow electrons emitted from cathode102and traveling along a beam axis or path127to pass out of Wehnelt cap112. Wehnelt cap112and cathode102are configured so that, prior to operation, end portion106of cathode102is set back from the center portion of top side120at a predetermined gap distance. For example, the gap distance may be in a range from about 200 μm to about 300 μm, and preferably about 250 μm. When assembly100is first turned on and various parts of assembly100(e.g., cathode102, blocks104, filament posts108, ceramic base110, Wehnelt cap112) heat up, the gap distance changes as a result of expansion of the various parts until the temperatures of the various parts become relatively stable. The gap distance affects operating characteristics such as emission current, crossover, and crossover location. The design of Wehnelt cap112allows it to increase in heat to a stable temperature relatively quickly. The amount of time between start-up (i.e., the time when electron emission from cathode102begins) and stable operation of assembly100is reduced compared to that of conventional electron emitters. Thus, assembly100makes more practical the turning off of cathode102when it is not in use.

Assembly100also includes an anode128attached to ceramic base110. Anode128functions to accelerate electrons that pass through aperture126of Wehnelt cap112. Anode128may be made of aluminum alloy (e.g., Ti6Al4V) or aluminum-copper alloy (CuAl) and functions as a thermal ground for assembly100. Anode128includes a base portion130having a first surface130S1, facing an outer surface of bottom side120of Wehnelt cap112. First surface130S1, of base portion130is relatively flat and substantially parallel to the outer surface of top side120. Anode128includes a spacer ring132that extends from base portion130to ceramic base110and that encircles Wehnelt cap112. A clamp ring134mounted on a side of ceramic base110opposite anode128attaches it to ceramic base110. Screws136extend from clamp ring134to spacer ring132through ceramic base110. Ceramic base110provides electrical and thermal isolation between anode128and Wehnelt cap112(and filament posts108). A negative voltage (e.g., −5,300 V) is applied to cathode102relative to anode128to produce a beam of electrons traveling along a beam path. Spacer ring132is sized to provide a predetermined sized space between first surface130S1, of base portion130and the outer surface of top side120of Wehnelt cap112. For example, the space between base portion130and top side120may be in a range from about 1.0 mm to about 2.0 mm, and preferably about 1.5 mm. Spacer ring132includes holes138or slots in its sidewall for pumping out the source region (i.e., the interior of Wehnelt cap112) and for aligning Wehnelt cap112.

Base portion130has an anode beam aperture140coaxially aligned with aperture126of Wehnelt cap112. The diameter of aperture140may be in a range from about 1.0 mm to about 2.0 mm, and preferably about 1.5 mm. Aperture140allows electrons traveling along the beam path and passing through aperture126to pass through anode128. Anode128includes a circumferential flange142that has openings144sized to receive support rods that attach anode128to thermal ground.

A differential pumping aperture (DPA) plate146is mounted on a second surface130S2of base portion130and is attached to anode128by screws148. DPA plate146includes an aperture150coaxially aligned with aperture140and aperture126so as to allow the electron beam passing through apertures140and126to pass through DPA plate146and prevent reverse air flow into Wehnelt cap112. (Cathode102operates at higher vacuum pressure than that at anode aperture140.) The electron beam passing through aperture150impinges a target specimen of an instrument in which assembly100is installed.

The configuration of assembly100shown inFIGS. 1 and 2affords a number of improvements over known electron sources. For example, because filament posts108, Wehnelt cap112, and anode128are attached to a common base—ceramic base110—the components of assembly can be packaged together as an integrated structure and aligned prior to deployment in an instrument such as an electron microscope. Thus, for example, if an electron source of an electron microscope becomes defective, the defective source may be removed and assembly100may replace the defective source without the need to align the various parts of assembly100after it is installed in the electron microscope. Moreover, ceramic base110functions as a thermal insulator and thereby provides mutual thermal isolation of anode128, Wehnelt cap112, and filament posts108. The placement of ceramic base110as the only component between Wehnelt cap112and anode128allows assembly100to achieve stable operation more quickly than that which is achieved by conventional systems. Additionally, the design of Wehnelt cap112facilitates a substantial reduction in the amount of time between start up and stable operation of assembly100compared to that of conventional systems. The thickness of flange142contributes to a limited extent the diminution of warm-up time attributable to the design of Wehnelt cap112.

Emission Current and Temperature Testing

FIG. 4is a graph of the emission current measured for three different embodiments of assembly100used in a SEM. For each embodiment, assembly100was first conditioned by placing 7.0 kilovolts (kV) between the cathode/Wehnelt cap assembly and anode128. For each embodiment, cathode102was heated to approximately 1750 kelvin (K).

In a first embodiment, CuAl was used for Wehnelt cap112and anode128. The thickness of side wall122of Wehnelt cap112was about 1.0 mm, and side wall122included eight equally spaced vent holes124. The thickness of blocks104was set to about 0.25 mm. Curve402represents the emission current measured for the first embodiment. The emission current reached a maximum value of about 43 μA in 33 minutes after startup. The emission current reached 99% of its maximum value in approximately 12 minutes after startup.

In a second embodiment, CuAl was used for Wehnelt cap112and anode128. The thickness of side wall122of Wehnelt cap112was about 0.25 mm, and side wall122included eight equally spaced vent holes124. The thickness of blocks104was set to about 0.38 mm. Curve404represents the emission current measured for the second embodiment. (The discontinuity in curve404stems from a gap in the data collection process.) The emission current reached a maximum value of approximately 65.4 μA in 3.59 hours after startup. The emission current reached 99% of its maximum value in approximately 24 minutes after startup.

In a third embodiment, Ti6Al4V was used for Wehnelt cap112and anode128. The thickness of side wall122of Wehnelt cap112was about 1.0 mm, and side wall122included eight equally spaced vent holes124. The thickness of blocks104was set to about 0.25 mm. Curve406represents the emission current measured for the third embodiment. The emission current reached a maximum value of about 43 μA in 1.29 hours after startup. The emission current reached 99% of its maximum value in approximately 30 minutes after startup.

FIG. 5is a graph of operating temperatures of Wehnelt cap112and anode128of the first and third embodiments. The temperatures of Wehnelt cap112and anode128were measured by thermocouples attached to field ring114and clamp ring134. Curves502and504represent the temperatures of, respectively, Wehnelt cap112and anode128of the first embodiment. Curves506and508represent the temperatures of, respectively, Wehnelt cap112and anode128of the third embodiment. As shown inFIG. 5, Wehnelt cap112and anode128of the third embodiment stabilized at significantly higher temperatures than those of Wehnelt cap112and anode128of the first embodiment. Thus, the third embodiment may be less prone to oxide build up.

Thermal modeling software was also used to construct a thermal model for a fourth embodiment of assembly100. In the fourth embodiment, Ti6Al4V was used for Wehnelt cap112and anode128. The thickness of side wall122of Wehnelt cap112was about 0.5 mm, and side wall122included eight equally spaced vent holes124. Results from the model are depicted inFIGS. 6 and 7.FIG. 6is a graph of the temperatures of ceramic base110and different portions of Wehnelt cap112, andFIG. 7is a graph of the gap distance between end portion106of cathode102and first side120of Wehnelt cap112. Curve602represents the temperature of ceramic base110, curve604represents the temperature of an outer portion of flange118, curve606represents the temperature of a middle portion of flange118, curve608represents the temperature of an outer portion of first side120, and curve610represents the temperature of a middle portion of first side120. Curves602,604,606,608, and610show that the temperatures of ceramic base110and Wehnelt cap112become relatively stable in approximately 30 minutes after startup.FIG. 7shows that the gap distance varies relative to the changes in temperature of ceramic base110and Wehnelt cap112. The initial precipitous decrease in the gap distance results from expansion of cathode102and blocks104. The subsequent increase in gap distance results from expansion of Wehnelt cap112and ceramic base110. Filament posts108may also expand and thereby cause the gap distance to decrease.FIG. 7demonstrates, however, that Wehnelt cap112and ceramic base110as compared to filament posts108expand more and at a faster rate. Other factors may also play a role in the gap distance response, such as electrical conduction losses through filament posts108and radiation losses. The model demonstrates that gap distance becomes relatively stable in approximately 20 minutes after startup.

Test Assembly

FIG. 8is a cross-sectional view of a test assembly800used to perform thermal analysis on a cathode802, blocks804, filament posts808, a ceramic base810, and a Wehnelt cap812. (The last two digits of the three-digit reference numerals identifying corresponding component parts of test assembly800and thermionic emission assembly100are the same.) In a first embodiment of test assembly800, Wehnelt cap812is made of Ti6Al4V and a side wall822has a thickness of about 0.5 mm. Side wall822of the first embodiment includes eight equally spaced holes824having diameters of approximately 1.6 mm. In a second embodiment of test assembly800, Wehnelt cap812is made of Ti6Al4V and side wall822has a thickness of about 0.25 mm. Side wall822of the second embodiment includes four equally spaced windows (not shown) having dimensions of approximately 5.7 mm by 2.9 mm.

FIG. 9is a graph of the thermal results of tests performed on the first and second embodiments of test assembly800. During the tests, an input power of 5.5 watts (W) was supplied to cathode802to heat it to approximately 1750 K. The final temperature of filament posts808was approximately 478.15 K. Curve902represents the temperature of a first side820of Wehnelt cap812of the first embodiment, and curve904represents the temperature of a flange818of Wehnelt cap812of the first embodiment. Curve906represents the temperature of first side820of Wehnelt cap812of the second embodiment, and curve908represents the temperature of flange818of Wehnelt cap812of the second embodiment.FIG. 9demonstrates that Wehnelt caps812of the first and second embodiments heat up quickly and the temperatures become relatively stable in approximately 20 minutes after startup.FIG. 9also demonstrates that first side820of the second embodiment as compared to first side820of the first embodiment is more thermally isolated from flange818.

A thermal modeling program was used to construct a thermal model for the first embodiment of test assembly800. In the model, an input power of 5.5 W was supplied to cathode802.FIG. 10is a graph of the temperatures of cathode802, filament posts808, and Wehnelt cap812calculated for the model. InFIG. 10, the left side y-axis represents temperatures of Wehnelt cap812and filament posts808, and the right side y-axis represents temperatures of cathode802. Curves1002,1004,1006, and1008represent the temperatures of various portions of Wehnelt cap812. Specifically, curves1002,1004,1006, and1008represent the temperatures of, respectively, an outer edge of flange818, a middle portion of flange818, an outer edge of first side820, and a middle portion of first side820. Curve1010represents the temperature of posts808; and curves1012and1014represent the temperatures of, respectively, a middle portion of cathode802and an end portion of cathode802.FIG. 10demonstrates that the temperatures of cathode802, filament posts808, and Wehnelt cap812of the first embodiment of test assembly800stabilize relatively quickly compared to known electron sources.

The thermal modeling program also calculated the change in the gap distance between the end portion of cathode802and first side820of Wehnelt cap812of the first embodiment of test assembly800. The change in gap distance is represented by curve1102ofFIG. 11. The gap distance variation corresponds to the temperature variation of cathode802and Wehnelt cap812. The precipitous initial decrease in the gap distance results from expansion of cathode802and blocks804. The subsequent increase in the gap distance results from expansion of Wehnelt cap812and ceramic base810. Filament posts808may also expand and thereby cause the gap distance to decrease. Curve1102demonstrates, however, that Wehnelt cap812and ceramic base810as compared to filament posts808expand more and at a faster rate. Other factors may also play a role in the gap distance response, such as conduction losses through filament posts808and radiation losses. The model demonstrates that gap distance becomes relatively stable in 20 or fewer minutes after startup.

FIG. 12is a graph of the gap distances calculated by the thermal modeling program for alternative designs of Wehnelt cap812and ceramic base810. Curve1202represents the gap distance of a design in which the thickness of ceramic base810has been reduced from that of the first embodiment of test assembly800. Curve1204represents the gap of a design in which four windows (similar to the windows described in the second embodiment of test assembly800) replace the eight holes824of the first embodiment. The alternative designs show more of a mismatch between the expansion of Wehnelt cap812and filament posts808.

Combinations of the above embodiments, and other embodiments not specifically described herein will be apparent to skilled persons upon reviewing the above description. Though the present invention has been set forth in the form of the embodiments described above, it is nevertheless intended that modifications to the disclosed systems and methods may be made without departing from inventive concepts set forth herein.