Abstract:
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.

Description:
COPYRIGHT NOTICE 
     © 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. 
     Additional aspects and advantages will be apparent from the following detailed description of preferred embodiments, which proceeds with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective pictorial view showing in cross-section a thermionic emission assembly according to one embodiment. 
         FIG. 2  is an enlarged cross-sectional view of the thermionic emission assembly of  FIG. 1 . 
         FIG. 3  is an enlarged fragmentary view of a cathode, blocks, and filament posts of the thermionic emission assembly of  FIG. 1 . 
         FIG. 4  is a graph of the emission currents of different embodiments of the thermionic emission assembly. 
         FIG. 5  is a graph of the operating temperatures of different embodiments of the thermionic emission assembly. 
         FIG. 6  is a graph of the operating temperatures of an embodiment of the thermionic emission assembly calculated by a thermal modeling program. 
         FIG. 7  is a graph of the gap distance between a cathode and a Wehnelt cap of the thermionic emission assembly according to one embodiment. 
         FIG. 8  is a cross-sectional view of a test thermionic emission assembly according to one embodiment. 
         FIG. 9  is a graph of the operating temperatures of different embodiments of the test thermionic emission assembly. 
         FIG. 10  is a graph of the operating temperatures of an embodiment of the test thermionic emission assembly calculated by a thermal modeling program. 
         FIG. 11  is a graph of the gap distance between a cathode and Wehnelt cap of the test thermionic emission assembly according to one embodiment. 
         FIG. 12  is a graph of the gap distances between cathodes and Wehnelt caps of different embodiments of the test thermionic emission assembly. 
     
    
    
     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. 1  is a cross-sectional pictorial view of a thermionic emission assembly  100  according to one embodiment.  FIG. 2  is an enlarged cross sectional view of assembly  100  that is rotated relative to  FIG. 1  to show two filament posts. Assembly  100  may 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). Assembly  100  is typically operated in a vacuum (e.g., 1×10 −7  Torr).  FIG. 3  is an enlarged diagram of a portion of assembly  100  showing a cathode  102  held between a pair of blocks  104 . Cathode  102  may be a hexaboride cathode such as a cerium hexaboride (CeB 6 ) cathode or a lanthanum hexaboride (LaB 6 ) cathode. Cathode  102  may have a &lt;100&gt; 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. Cathode  102  includes an end portion  106  from which electrons are emitted. The shape and dimensions of cathode  102  may be different for various applications to achieve different beam currents, spot sizes, and brightness. For example, end portion  106  may 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 portion  106  may 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, cathode  102  may have a &lt;310&gt; crystal orientation and may be configured as a “tophat” cathode to form a beam having a large current and large spot size. 
     Blocks  104  may be made of pyrolytic graphite and may act as resistive heaters. Blocks  104  help to thermally isolate a pair of filament posts  108  from cathode  102  (only one filament post  108  shown in  FIG. 1 ). The thickness of blocks  104  is preferably from about 0.25 mm to about 0.39 mm to provide different thermal isolation characteristics. Filament posts  108  are electrically connected to cathode  102  and are bent toward each other in a “V” shape to compressively hold cathode  102  in place. The mount configuration of filament posts  108  and cathode  102  may be a shunted mini vogel mount (SMVM) configuration or a mini vogel mount (MVM) configuration. Filament posts  108  may be made of a material, such as molybdenum or molybdenum-rhenium alloy, that maintains a high modulus of elasticity even at high temperatures. 
     Filament posts  108  extend from blocks  104  through a disk-shaped ceramic base  110 . Filament posts  108  are rigidly fixed to ceramic base  110 , which serves as a base for cathode  102  and filament posts  108 . Thus, blocks  104  and filament posts  108  form a support structure for cathode  102 , which is mechanically coupled to ceramic base  110 . Ceramic base  110  is made of any ceramic material such as alumina (98.5%). Ceramic base  110  has 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 cap  112  is attached to ceramic base  110 . Wehnelt cap  112  may be made of titanium alloy (e.g., Ti6Al4V), titanium, or stainless steel. Wehnelt cap  112  is attached to ceramic base  110  by a field ring  114  and screws  116 . Field ring  114  is provided on a side of ceramic base  110  opposite from Wehnelt cap  112 , and screws  116  extend through ceramic base  110  between a flange  118  of Wehnelt cap  112  and field ring  114 . Ceramic base  110  provides electrical and thermal isolation between Wehnelt cap  112  and filament posts  108 . A negative voltage (e.g., −400 V) is applied to Wehnelt cap  112  relative to cathode  102  to suppress electron emission anywhere from cathode  102  other than its end portion  106 . Flange  118  may be mounted flush against ceramic base  110 , or a height adjustment ring (not shown) may be positioned between flange  118  and ceramic base  110 . Wehnelt cap  112  forms an interior region or a cavity that surrounds cathode  102 , blocks  104 , and portions of filament posts  108 . Wehnelt cap  112  has a top side  120  and a side wall  122  that extends between flange  118  and top side  120 . Side wall  122  may include one or more vent holes  124 , slots, or windows (slots and windows not shown). Side wall  122  has a preferred thickness from about 0.25 mm to about 1.02 mm. The thickness of side wall  122  and the configuration of vent holes  124 , slots, or windows affect thermal paths to provide more or less thermal isolation for top side  120  relative to flange  118  and other components of assembly  100 . For example, a relatively thin side wall  122  can provide for top side  120  a higher operating temperature than that provided by a thicker side wall  122 . A relatively thin side wall  122  can also provide between top side  120  and flange  118  a bigger thermal difference than that provided by a thicker side wall  122 . A high operating temperature for top side  120  may decrease the likelihood for oxide build up on it. 
     Top side  120  includes a cap beam aperture  126  located at a center portion so as to be coaxially aligned with end portion  106  of cathode  102  when Wehnelt cap  112  and filament posts  108  are fixed to ceramic base  110 . The thickness of top side  120  tapers from about 0.5 mm to about 0.125 mm from side wall  122  to the center portion where aperture  126  is located. Aperture  126  functions to allow electrons emitted from cathode  102  and traveling along a beam axis or path  127  to pass out of Wehnelt cap  112 . Wehnelt cap  112  and cathode  102  are configured so that, prior to operation, end portion  106  of cathode  102  is set back from the center portion of top side  120  at 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 assembly  100  is first turned on and various parts of assembly  100  (e.g., cathode  102 , blocks  104 , filament posts  108 , ceramic base  110 , Wehnelt cap  112 ) 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 cap  112  allows 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 cathode  102  begins) and stable operation of assembly  100  is reduced compared to that of conventional electron emitters. Thus, assembly  100  makes more practical the turning off of cathode  102  when it is not in use. 
     Assembly  100  also includes an anode  128  attached to ceramic base  110 . Anode  128  functions to accelerate electrons that pass through aperture  126  of Wehnelt cap  112 . Anode  128  may be made of aluminum alloy (e.g., Ti6Al4V) or aluminum-copper alloy (CuAl) and functions as a thermal ground for assembly  100 . Anode  128  includes a base portion  130  having a first surface  130 S 1 , facing an outer surface of bottom side  120  of Wehnelt cap  112 . First surface  130 S 1 , of base portion  130  is relatively flat and substantially parallel to the outer surface of top side  120 . Anode  128  includes a spacer ring  132  that extends from base portion  130  to ceramic base  110  and that encircles Wehnelt cap  112 . A clamp ring  134  mounted on a side of ceramic base  110  opposite anode  128  attaches it to ceramic base  110 . Screws  136  extend from clamp ring  134  to spacer ring  132  through ceramic base  110 . Ceramic base  110  provides electrical and thermal isolation between anode  128  and Wehnelt cap  112  (and filament posts  108 ). A negative voltage (e.g., −5,300 V) is applied to cathode  102  relative to anode  128  to produce a beam of electrons traveling along a beam path. Spacer ring  132  is sized to provide a predetermined sized space between first surface  130 S 1 , of base portion  130  and the outer surface of top side  120  of Wehnelt cap  112 . For example, the space between base portion  130  and top side  120  may be in a range from about 1.0 mm to about 2.0 mm, and preferably about 1.5 mm. Spacer ring  132  includes holes  138  or slots in its sidewall for pumping out the source region (i.e., the interior of Wehnelt cap  112 ) and for aligning Wehnelt cap  112 . 
     Base portion  130  has an anode beam aperture  140  coaxially aligned with aperture  126  of Wehnelt cap  112 . The diameter of aperture  140  may be in a range from about 1.0 mm to about 2.0 mm, and preferably about 1.5 mm. Aperture  140  allows electrons traveling along the beam path and passing through aperture  126  to pass through anode  128 . Anode  128  includes a circumferential flange  142  that has openings  144  sized to receive support rods that attach anode  128  to thermal ground. 
     A differential pumping aperture (DPA) plate  146  is mounted on a second surface  130 S 2  of base portion  130  and is attached to anode  128  by screws  148 . DPA plate  146  includes an aperture  150  coaxially aligned with aperture  140  and aperture  126  so as to allow the electron beam passing through apertures  140  and  126  to pass through DPA plate  146  and prevent reverse air flow into Wehnelt cap  112 . (Cathode  102  operates at higher vacuum pressure than that at anode aperture  140 .) The electron beam passing through aperture  150  impinges a target specimen of an instrument in which assembly  100  is installed. 
     The configuration of assembly  100  shown in  FIGS. 1 and 2  affords a number of improvements over known electron sources. For example, because filament posts  108 , Wehnelt cap  112 , and anode  128  are attached to a common base—ceramic base  110 —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 assembly  100  may replace the defective source without the need to align the various parts of assembly  100  after it is installed in the electron microscope. Moreover, ceramic base  110  functions as a thermal insulator and thereby provides mutual thermal isolation of anode  128 , Wehnelt cap  112 , and filament posts  108 . The placement of ceramic base  110  as the only component between Wehnelt cap  112  and anode  128  allows assembly  100  to achieve stable operation more quickly than that which is achieved by conventional systems. Additionally, the design of Wehnelt cap  112  facilitates a substantial reduction in the amount of time between start up and stable operation of assembly  100  compared to that of conventional systems. The thickness of flange  142  contributes to a limited extent the diminution of warm-up time attributable to the design of Wehnelt cap  112 . 
     Emission Current and Temperature Testing 
       FIG. 4  is a graph of the emission current measured for three different embodiments of assembly  100  used in a SEM. For each embodiment, assembly  100  was first conditioned by placing 7.0 kilovolts (kV) between the cathode/Wehnelt cap assembly and anode  128 . For each embodiment, cathode  102  was heated to approximately 1750 kelvin (K). 
     In a first embodiment, CuAl was used for Wehnelt cap  112  and anode  128 . The thickness of side wall  122  of Wehnelt cap  112  was about 1.0 mm, and side wall  122  included eight equally spaced vent holes  124 . The thickness of blocks  104  was set to about 0.25 mm. Curve  402  represents 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 cap  112  and anode  128 . The thickness of side wall  122  of Wehnelt cap  112  was about 0.25 mm, and side wall  122  included eight equally spaced vent holes  124 . The thickness of blocks  104  was set to about 0.38 mm. Curve  404  represents the emission current measured for the second embodiment. (The discontinuity in curve  404  stems 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 cap  112  and anode  128 . The thickness of side wall  122  of Wehnelt cap  112  was about 1.0 mm, and side wall  122  included eight equally spaced vent holes  124 . The thickness of blocks  104  was set to about 0.25 mm. Curve  406  represents 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. 5  is a graph of operating temperatures of Wehnelt cap  112  and anode  128  of the first and third embodiments. The temperatures of Wehnelt cap  112  and anode  128  were measured by thermocouples attached to field ring  114  and clamp ring  134 . Curves  502  and  504  represent the temperatures of, respectively, Wehnelt cap  112  and anode  128  of the first embodiment. Curves  506  and  508  represent the temperatures of, respectively, Wehnelt cap  112  and anode  128  of the third embodiment. As shown in  FIG. 5 , Wehnelt cap  112  and anode  128  of the third embodiment stabilized at significantly higher temperatures than those of Wehnelt cap  112  and anode  128  of 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 assembly  100 . In the fourth embodiment, Ti6Al4V was used for Wehnelt cap  112  and anode  128 . The thickness of side wall  122  of Wehnelt cap  112  was about 0.5 mm, and side wall  122  included eight equally spaced vent holes  124 . Results from the model are depicted in  FIGS. 6 and 7 .  FIG. 6  is a graph of the temperatures of ceramic base  110  and different portions of Wehnelt cap  112 , and  FIG. 7  is a graph of the gap distance between end portion  106  of cathode  102  and first side  120  of Wehnelt cap  112 . Curve  602  represents the temperature of ceramic base  110 , curve  604  represents the temperature of an outer portion of flange  118 , curve  606  represents the temperature of a middle portion of flange  118 , curve  608  represents the temperature of an outer portion of first side  120 , and curve  610  represents the temperature of a middle portion of first side  120 . Curves  602 ,  604 ,  606 ,  608 , and  610  show that the temperatures of ceramic base  110  and Wehnelt cap  112  become relatively stable in approximately 30 minutes after startup.  FIG. 7  shows that the gap distance varies relative to the changes in temperature of ceramic base  110  and Wehnelt cap  112 . The initial precipitous decrease in the gap distance results from expansion of cathode  102  and blocks  104 . The subsequent increase in gap distance results from expansion of Wehnelt cap  112  and ceramic base  110 . Filament posts  108  may also expand and thereby cause the gap distance to decrease.  FIG. 7  demonstrates, however, that Wehnelt cap  112  and ceramic base  110  as compared to filament posts  108  expand 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 posts  108  and radiation losses. The model demonstrates that gap distance becomes relatively stable in approximately 20 minutes after startup. 
     Test Assembly 
       FIG. 8  is a cross-sectional view of a test assembly  800  used to perform thermal analysis on a cathode  802 , blocks  804 , filament posts  808 , a ceramic base  810 , and a Wehnelt cap  812 . (The last two digits of the three-digit reference numerals identifying corresponding component parts of test assembly  800  and thermionic emission assembly  100  are the same.) In a first embodiment of test assembly  800 , Wehnelt cap  812  is made of Ti6Al4V and a side wall  822  has a thickness of about 0.5 mm. Side wall  822  of the first embodiment includes eight equally spaced holes  824  having diameters of approximately 1.6 mm. In a second embodiment of test assembly  800 , Wehnelt cap  812  is made of Ti6Al4V and side wall  822  has a thickness of about 0.25 mm. Side wall  822  of the second embodiment includes four equally spaced windows (not shown) having dimensions of approximately 5.7 mm by 2.9 mm. 
       FIG. 9  is a graph of the thermal results of tests performed on the first and second embodiments of test assembly  800 . During the tests, an input power of 5.5 watts (W) was supplied to cathode  802  to heat it to approximately 1750 K. The final temperature of filament posts  808  was approximately 478.15 K. Curve  902  represents the temperature of a first side  820  of Wehnelt cap  812  of the first embodiment, and curve  904  represents the temperature of a flange  818  of Wehnelt cap  812  of the first embodiment. Curve  906  represents the temperature of first side  820  of Wehnelt cap  812  of the second embodiment, and curve  908  represents the temperature of flange  818  of Wehnelt cap  812  of the second embodiment.  FIG. 9  demonstrates that Wehnelt caps  812  of the first and second embodiments heat up quickly and the temperatures become relatively stable in approximately 20 minutes after startup.  FIG. 9  also demonstrates that first side  820  of the second embodiment as compared to first side  820  of the first embodiment is more thermally isolated from flange  818 . 
     A thermal modeling program was used to construct a thermal model for the first embodiment of test assembly  800 . In the model, an input power of 5.5 W was supplied to cathode  802 .  FIG. 10  is a graph of the temperatures of cathode  802 , filament posts  808 , and Wehnelt cap  812  calculated for the model. In  FIG. 10 , the left side y-axis represents temperatures of Wehnelt cap  812  and filament posts  808 , and the right side y-axis represents temperatures of cathode  802 . Curves  1002 ,  1004 ,  1006 , and  1008  represent the temperatures of various portions of Wehnelt cap  812 . Specifically, curves  1002 ,  1004 ,  1006 , and  1008  represent the temperatures of, respectively, an outer edge of flange  818 , a middle portion of flange  818 , an outer edge of first side  820 , and a middle portion of first side  820 . Curve  1010  represents the temperature of posts  808 ; and curves  1012  and  1014  represent the temperatures of, respectively, a middle portion of cathode  802  and an end portion of cathode  802 .  FIG. 10  demonstrates that the temperatures of cathode  802 , filament posts  808 , and Wehnelt cap  812  of the first embodiment of test assembly  800  stabilize relatively quickly compared to known electron sources. 
     The thermal modeling program also calculated the change in the gap distance between the end portion of cathode  802  and first side  820  of Wehnelt cap  812  of the first embodiment of test assembly  800 . The change in gap distance is represented by curve  1102  of  FIG. 11 . The gap distance variation corresponds to the temperature variation of cathode  802  and Wehnelt cap  812 . The precipitous initial decrease in the gap distance results from expansion of cathode  802  and blocks  804 . The subsequent increase in the gap distance results from expansion of Wehnelt cap  812  and ceramic base  810 . Filament posts  808  may also expand and thereby cause the gap distance to decrease. Curve  1102  demonstrates, however, that Wehnelt cap  812  and ceramic base  810  as compared to filament posts  808  expand more and at a faster rate. Other factors may also play a role in the gap distance response, such as conduction losses through filament posts  808  and radiation losses. The model demonstrates that gap distance becomes relatively stable in 20 or fewer minutes after startup. 
       FIG. 12  is a graph of the gap distances calculated by the thermal modeling program for alternative designs of Wehnelt cap  812  and ceramic base  810 . Curve  1202  represents the gap distance of a design in which the thickness of ceramic base  810  has been reduced from that of the first embodiment of test assembly  800 . Curve  1204  represents the gap of a design in which four windows (similar to the windows described in the second embodiment of test assembly  800 ) replace the eight holes  824  of the first embodiment. The alternative designs show more of a mismatch between the expansion of Wehnelt cap  812  and filament posts  808 . 
     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.