Patent Number: 
Section: description

FIG. 5 illustrates a general cathode 110 of the present invention in one embodiment. In particular, cathode 110 is a single crystal, in contrast to the conventional micro-polycrystalline tantalum foils. The single crystal cathode of the present invention need not be an absolute xe2x80x9csinglexe2x80x9d crystal, but is defined as a structure that has xc2x11xc2x0 variations. Because of its single crystalline structure, the single crystal cathode does not have misoriented grains. As a result, the cathode 110 of the present invention has fewer structural non-uniformities than the conventional micro-polycrystalline foils and therefore, a more uniform emission characteristic. As a result, the single crystal cathode 110 of the present invention solves the patchiness problems with conventional cathodes. Further, one or more of the crystalline planes of the single crystal cathode 110 has a work function variation substantially less than 2%, which is sufficiently uniform to be used as SCALPEL(trademark) (and similar projection electron lithography) process thermoionic cathode. In a more preferred embodiment, the cathode 110 is made of a refractory crystalline material, such as tantalum, tungsten, rhenium or molybdenum. In another preferred embodiment, the cathode 110 is made of an alloy including at least one of tantalum, tungsten, rhenium or molybdenum. In an even more preferred embodiment, the single crystal cathode 110 is made of tantalum. Tantalum has a body centered cubic (bcc) structure. As illustrated in FIG. 5, both emissive planes A (100) and B (111) of the tantalum single crystal have relatively low work functions, 4.0 eV and 4.15 eV, respectively, in contrast to conventional polycrystalline foil cathodes, which have work functions of approximately 4.25 eV. Emissive plane C (110) has a relatively high work function of 4.8 eV. As is known to one of ordinary skill in the art, work function and current are inversely related; the lower the work function, the higher the current that can be generated. Therefore, both emissive planes A (100) and B (111) permit the SCALPEL(trademark) system in which they are used, to produce a higher current. FIG. 6 illustrates a method of bonding the general cathode 110 to a conventional heat supplying/structural member, such as filament 46 in FIG. 3, to produce a bonded article, in one embodiment of the present invention. In steps 200 and 210, the single crystal cathode 110 and filament 46, respectively, are obtained. In step 220, the single crystal cathode 110 and filament 46 are bonded using a local bonding technique. A global bonding technique, such as spot welding cannot be used to bond the cathode 110 of the present invention to a filament 46, because spot welding modifies the structure of the single crystal cathode 110 in an area adjacent to the weld spot. The filament 46 must be pressed heavily against a surface of the tantalum single crystal and a high current (up to 1,000 A) must be run through to bring the point of interaction to a temperature of at least 3,000xc2x0 C. During traditional spot welding, the high current runs through the entire cathode 110 causing global thermal radiance and related stress/tension over the entire cathode 110. The present invention solves the problems with bonding a single crystal cathode of the present invention with conventional filaments by utilizing a local bonding technique to bond the single crystal cathode 110 and the filament 46 together. A local bonding technique does not recrystallize (by causing the grain misorientation to increase to 5-20xc2x0) a center of the single crystal cathode, and therefore produces a bonded article including single crystal cathode 110 and filament 46 which is usable in a projection electron lithography system, such as the SCALPEL(trademark) system. In a more preferred embodiment, the local bonding technique is laser welding. Laser welding is significantly different from traditional spot welding, in that laser welding does not depend on sample surface roughness, surface and volumetric electrical resistance, supporting jig resistance, sample and jig thermal conductivity. Laser welding does depend on the sample melting point and to some extent, on the sample surface optical reflection and cleanness. As a result, laser welding is effective in bonding two different materials, for example molybdenum and stainless steel or iridium and tungsten. Another attractive feature of laser welding is its inherent xe2x80x9cpointnessxe2x80x9d; the laser spot is tightly focused down to a 150-200 xcexcm Dia. and the weld depth does not even approach 50 xcexcm. As a result of its limited heating area and laser pulse short duration, typically a few microseconds, laser welding can be thermally characterized as a pure adiabatic process, namely, when a cathode""s microscopic area is melted, the surrounding area remains cold. This is one major advantage of laser welding over spot welding, namely, laser welding works on microscopic part of an entire cathode, whereas spot welding affects a large cathode area. In an even more preferred embodiment, the single crystal cathode is made of tantalum. In an even more preferred embodiment, the single crystal cathode is disk-shaped and has a diameter of 600-1500 xcexcm and a height of 200-300 xcexcm. In an even more preferred embodiment, the conventional member is both a heat supplying and structural support member. In an even more preferred embodiment, the conventional member is a ribbon shaped filament. In an even more preferred embodiment, the filament is made of one of tungsten, a tungsten-rhenium alloy, and a tungsten-tantalum alloy. In an even more preferred embodiment, the single crystal cathode 110 is utilized in a source which includes Kovar(trademark) or molybdenum/rhenium (Mo/Re) posts, and a tungsten (W) filament. A typical tantalum single crystal cathode disc dimensions are 1 mm Dia and 0.2 mm thickness, and its mass is 2.6xc3x97103 gms. The specific heat of tantalum is 0.15 J/gm. If a laser weld spot is 200 xcexcDia and 50xcexc deep, the tantalum mass to be melted is 2.7xc3x9710xe2x88x925 g, and assuming the necessary spot temperature elevation from room temperature to a melting point of 3,000xc2x0 C., the amount of absorbed heat is 12 mJ. If this heat is dissipated entirely in the tantalum cathode, then the entire tantalum cathode disc would be warmed only by 62xc2x0 C. In contrast, a spot welding current pulse would elevate the entire tantalum cathode disc temperature by approximately 500xc2x0 C., because the lowest pulse energy producing a reliable weld is approximately 4 J. FIG. 7 illustrates a laser-welded tantalum single crystalline cathode emission image. As illustrated in FIG. 7, neither grains nor boundaries are detected. As desired, the laser weld did not adversely affect the cathode""s sensitive crystalline structure. The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.