BACKGROUND OF THE INVENTION Electron beam sources arewell known fortheiruse in vapor deposition, where a thin film of any of a variety of materials is evaporated onto a substrate. The evaporation occurs when a high energy beam of electrons is directed onto molten material residing in a crucible in close proximity to the substrate. Vapor deposition is frequently used for optical coatings, although numerous other applications are well known in the industry.

While e-beam sources, sometimes referred to as"e-beam", have proven a relatively reliable means for accomplishing vapor deposition, there is still room for improvement. Conventional e-beam sources, despite their many good points, have still been relatively complicated devices, in which the alignment and coherence of the electron beam has been the result of careful design and assembly of a multitude of components.

For example, in a conventional e-beam source, a filament provides a source of electrons which are repelled from the cathode through a beam former toward an anode. The beam is deflected into the crucible by the relative positioning of the elements, including particularly the filament and beam former relative to the cathode and anode, and the associated magnetic fields. A characteristic arrangement is shown in U. S. Patent No. 5,418,348, assigned to the assignee of the present invention. The beam former also acts in part to prevent impingement of stray electrons onto the anode, in part by limiting the exit orifice for the filament so that only a minority of the filament is exposed. This naturally represents a trade-off between reliability and efficiency of the e-beam source, such that efficiency of the filament output is limited in favor of increased reliability.

In addition, the thermo-mechanical environment in which e-beam sources typically operate is quite harsh. Vibration, internal stress and thermal influences can cause conventional e-beam sources to go out of alignment from time to time, requiring maintenance by skilled personnel. In addition, the plating of e-beam source components by incidental evaporative deposition can require cleaning of the e-beam source components including possible disassembly.

The filament of the e-beam source also requires replacement from time to time. The proper alignment of the filament represents a key aspect of any assembly or disassembly of the e-beam source. In conventional e-beam sources designs, this has involved careful installation of the filament, generally by skilled personnel.

As a result, there has been a long term need for an e-beam source design which includes greater efficiency, fewer components, greater durability and improved maintenance requirements leading to improved long term performance.

SUMMARY OF THE INVENTION The present invention provides an electron beam source which has fewer components, simpler design, greater efficiency, improved reliability and easier

installation of replacement filaments, all leading to the desired improvements in long term performance.

In particular, the present design includes a redesigned emitter assembly which permits elimination of the beam former while at the same time providing improved filament efficiency. The emitter arrangement of the present invention includes a generally cylindrical chamber or channel for receiving the filament. The shape of the chamber assists in causing the beam of electrons emitted from the filament to be more coherent, resulting in greater efficiency while at the same time permitting the elimination of the beam former. In addition, the electron tails associated with conventional designs are substantially reduced if not eliminated.

In addition, the e-beam source of the present invention uses a monolithic insulator which provides not only greater rigidity to the assembled components, but also greater thermal stability. The resulting increased rigidity and thermal stability provides better long term alignment of the e-beam source assembly. The improved long term alignment helps to reduce maintenance issues.

Further, the present invention includes an elegantly simple self-aligning filament design. The filament receiver within the cathode is constructed to be precisely aligned. The filament is designed with equal length filament legs that are configured to mate to the filament receiver. The result is that the filament can be installed within the cylindrical receiver formed in the cathode without special tools or skills, while still achieving precise alignment.

These and other features of the present invention may be better appreciated form the following Figures taken together with the accompanying Detailed Description of the Invention.

THE FIGURES Figures 1A-1C show exploded, partly assembled and assembled perspective views, respectively, of an exemplary embodiment of the electron beam source according to the present invention.

Figure 2 shows in front elevational view the electron beam source of Fig. 1 C.

Figures 3A and 3B show a monolithic insulator in accordance with the present invention in front elevational and side elevational views, respectively.

Figures 4A-4C show the left hand cathode structure of the electron beam source of Figure 1, including the generally cylindrical channel in which the filament is received, in perspective, side elevational and bottom view, respectively.

Figures 4D-4F show the right hand cathode structure of the electron beam source of Figure 1, including the generally cylindrical channel in which the filament is received, in perspective, side elevational and bottom view, respectively.

Figures 5A and 5B show a filament in accordance with the present invention in top plan and side view, respectively.

Figure 6A shows in perspective view the left cathode block and the left filament clamp in accordance with the present invention to provide simple but accurate alignment of the filament within the cathode channel.

Figure 6B shows in perspective view the left and right cathode blocks with the filament in place, but the right filament clamp removed, to show how the filament legs mate with the cathode block.

DETAILED DESCRIPTION OF THE INVENTION The present invention provides an electron beam source which does not require a beam former, is built around a single monolithic insulator, and provides simple but reliable filament alignment. In addition, the electron beam source provides greater efficiency, a more coherent electron beam, fewer maintenance issues, and improved protection against debris shorting out the filament among other improvements. E-beam sources such as the present invention are used for depositing optical coatings on a wide variety of devices including wavelength divided multipliers, camera and other lenses, and laser mirrors.

Referring first to Figures 1A, 1B and 1C, Figure 1A shows an exploded perspective view of an exemplary e-beam source in accordance with the present invention, while Figure 1B shows a partly assembled perspective view of such an e-beam source and Figure 1C shows an assembled perspective view thereof. A monolithic insulator 100 can be seen at the center of the assembly in Figure 1 A. The monolithic insulator 100, which is shown in greater detail in Figures 3A-3B and may be seen to be essentially an inverted"T"shape, includes a plurality of mounting holes 105 through the upright portion of the inverted"T", with other mounting holes 110 at either end of the base of the inverted"T". The monolithic insulator 100 may typically be formed from high purity alumina oxide by any convenient technique such as sintering. The purity of the alumina oxide should be sufficient to provide acceptable voltage standoff for the high voltage environment in which the e-beam source of the present invention typically operates. By appropriate selection of materials and thicknesses, the insulator 100 provides a rigid, thermally stable

foundation upon which to mount the remaining components of the e-beam source of the present invention. In an exemplary embodiment, the insulator 100 may be on the order of slightly less than two inches long, with an overall height of the inverted"T" on the order of one-and-one-quarter inches and a depth of about two-thirds of an inch. The thickness of the cross-bar, or bottom, portion 100A of the inverted"T"may be slightly more than one-fourth of an inch, while the thickness of the upright portion 100B of the inverted"T"may be on the order of one-third of an inch. In general, the dimensions of the insulator must be of sufficient thickness to provide high voltage separation at 10kV or more.

Connected to either side of the insulator are, respectively, left cathode block 115 and right cathode block 120, typically fabricated of molybdenum through brazing or other suitable techniques. The left and right cathode blocks 115 and 120, which are shown in greater detail in Figures 4A-4C and 4D-4F (discussed hereinafter), include high voltage leg 125 on the left cathode block and high voltage leg 130 on the right cathode block. The legs provide a connection to a voltage source (not shown) for heating the filament appropriately.

The left and right cathode blocks are maintained at different electrical potentials, and to help achieve this the blocks 115 and 120 are mounted to different mounting holes 105. It can be seen from Figure 1 A that the left cathode block 115 is provided with through-holes 135 and mounts to the monolithic insulator 100 via holes 105 on one diagonal and mating screws 117 while the right cathode block 120 includes through-holes 140 and mounts to the monolithic insulator via holes 105 on the opposite diagonal and mating screws 117.

The cathode blocks 115 and 120 each include a portion of a substantially cylindrical chamber or channel 145, cut off at the front face, in which a filament 150 is retained. The filament 150 is fixedly positioned at each end to the left and right cathode blocks, respectively, by means of left filament clamp 155 and right filament clamp 160 which clamp filament legs 165 therebetween. In the exemplary embodiment shown, the clamps 155 and 160 are held in position by screws 117, although the particular means by which the clamps are affixed to the cathode blocks is not important. The clamps 155 and 160 provide a shield at the ends of the filament to prevent electron tails from escaping the emitter assembly. The filament is typically fabricated from pure straight grain tungsten. In the exemplary embodiment discussed herein, the filament is arranged such that approximately seventy-five percent of the front surface (front view) of the filament is exposed, in contrast with the fifty percent or less typically exposed in prior art devices. This

increased exposure leads to greater beam strength and increased efficiency, and is made possible, in part, by the arrangement of the channel 145 and the resulting focusing action.

Once the cathode blocks 115 and 120 are affixed to the insulator 100, it will be appreciated that the insulator bottom cross-bar portion 100A extends slightly beyond the cathode blocks 115 and 120. Left hand emitter support block 170 is then fastened to the end of the cross-bar portion 1 OOA, typically by means of screws 107 mating with holes 110. Right hand emitter support block 175 is similarly fastened to the right end of the cross-bar portion 1 OOA. The emitter support blocks may typically be fabricated from OFHC copper, and are electrically separated from the cathode blocks as shown in the front elevational view of Figure 2.

Beam deflection shield 180 fastens to the front face of cathode block 115 by means of screws 117 through holes 185 and into mating holes 190 in the front face of the left cathode block 115, although the shield extends across the full width of the filament. It will be appreciated that the shield fastens on only one side to avoid shorting the electrical potential across the cathode blocks. The deflection shield 180 may typically be fabricated from molybdenum, and can include a beveled rear upper surface to assist in guiding the emitted beam. A typical beveling angle will be on the order of 45 degrees, although the specific angle may vary over a relatively wide range, with the primary objective being that the bevel is from back to front, so that the thin edge is at the front top of the shield. The beam deflection shield serves to keep the emitted beam from becoming too large.

An emitter shield 195 then fastens to the left and right emitter support blocks 170 and 175 in front of the beam deflection shield 180 by means of screws 117 passing through holes 200 and into mating holes 205. The emitter shield 195 is typically fabricated of OFHC copper. An anode 210 (Figure 1A only ; removed in Figure 1 B for purposes of clarity), typically fabricated from tantalum, is fastened across the top of the emitter support blocks 170 and 175 by means of screws 117 through holes 215 in the anode 210 into mating holes 220 in the tops of the support blocks 170 and 175. An emitter cover 225, typically fabricated from OFHC copper sheet, fastens across the back of the emitter assembly to the left and right emitter support blocks by means of screws 117 through holes 230 into mating holes 235 (not shown in Figure 1) in the back of the emitter support blocks.

Still further, and shown particularly in Figure 1 B, a filament insertion tool 250 may be seen in alignment with the filament 150. The insertion tool fits within the coil of the filament 150, and helps maintain proper alignment of the filament relative to

the channel 145. As shown in Figure 1B, but shown in greater detail in connection with Figure 6A discussed hereinafter, the ends of the filament insertion tool 250 fit within notches in the clamps 155 and 160 to ensure alignment of the filament during clamping.

Referring particularly to Figure 2, the e-beam source of the present invention may be seen in front elevational view. In particular, the relative arrangement of the monolithic insulator, cathode blocks and emitter support blocks--and the electrical separation therebetween--can be better appreciated. In particular, the emitter support blocks 170 and 175 and cathode blocks 115 and 120 can all be seen to be mounted on the insulator 100 but spaced apart from one another. In addition, the separation between the cathode blocks can also be seen. In addition, the extent to which the beam deflection shield 180 exposes the filament 150 in the exemplary embodiment depicted can be better appreciated. The gap between the filament and the channel, typically be on the order of. 025" although a wide range is acceptable, may also be appreciated. It has been discovered that increased efficiency of the filament results when the diameter of the cavity or channel is matched to the diameter of the filament to provide a substantially uniform gap.

Referring next to Figures 3A-3B, the monolithic insulator 100 can be better appreciated. As previously discussed, the insulator 100 provides a rigid, single support for the remaining components which is thermally stable and mechanically rigid. The result is an e-beam source that maintains alignment of the remaining components throughout an extended period of use, thus reducing maintenance issues. It will be appreciated that the shape and relative dimensions of the insulator 100 can be varied over a wide range while still providing the substantial benefits described herein. In addition, while it is presently preferred that the insulator be fabricated in a unitary manner, in at least some embodiments adequate performance will result if the insulator is integrated from multiple components. Further, while the specific inverted"T"shape is particularly suited to the exemplary embodiment discussed herein, the particular shape may be varied in some embodiments while still yielding acceptable performance or achieving other objectives of the present invention.

The objective of the monolithic structure is to provide rigidity while maintaining electrical separation between components and thermal stability. Likewise, while alumina oxide is presently preferred forthe exemplary embodiment described herein, the insulator may be fabricated from any material having suitable electrical standoff characteristics combined with a low coefficient of thermal expansion and mechanical

rigidity. An additional advantage of the structure shown is that the insulator can be more massive than conventional insulator arrangements, which results in better cooling.

Referring next to Figures 4A-4F, the left and right cathode blocks may be better appreciated. In particular, the channel 145 can be seen to be substantially cylindrical in the embodiment shown. Referring particularly to Figures 4A-4C, a cathode bar 405 can be seen affixed to the left hand cathode block 115 behind the channel 145. The cathode bar 405, which extends outward from the left hand cathode block into a notch 410 in right hand cathode block 120 (best seen in Figure 4F) without actually contacting right cathode block 120. For both the right and left cathode blocks, the emitter legs 125 and 130, respectively, are affixed to the associated cathode blocks 115 and 120 by means of dowels 420 and screws 425.

The cathode bar 405 may be affixed to the cathode block by brazing or other means, and prevents electron beam leakage backward behind the filament.

The channel 145, which is formed by both the left and right channel block in the exemplary embodiment, but could be formed substantially by either block with only the necessary electrical contact on the other block, can be best appreciated from Figures 4A-4C and 4D-4F. The channel can be seen to be substantially cylindrical in shape in the exemplary embodiment shown in the Figures. The channel 145 serves to enclose the top, bottom and back portion of the filament, while still exposing substantially all of the outward-facing portion of the filament 150 so that the electron beam is directed outward in the desired direction. While the channel 145 is substantially cylindrical in shape, more complicated shapes such as square or v-groove, parabolic or ellipses are also acceptable and may in some instances provide desirable focusing of the beam. The general objective of the channel is, in essence, to enhance collation of the beam by preventing emissions from other than the desired direction. At the same time the channel may be configured to extend slightly beyond the filament to protect the filament from debris which may fall onto the emitter assembly during normal operation of the e-beam source.

The cathode blocks 115 and 120 may both be seen to include a notch on the side thereof closest to the insulator. The notches allow the cathode blocks to extend somewhat over the upright portion of the insulator 100, which assists in preventing plating of the insulator and helps to extend the period before the e-beam source requires cleaning.

Next, with reference to Figures 5A-5B, the filament can be seen to include a conventional filament portion 500, together with symmetrical legs 165 on either end

of the filament portion 500. By careful trimming of the legs 165 to be equal length, the filament can be mounted in the channel 145 in ready alignment. These elements will be better appreciated from the discussion of Figures 6A-6B, below.

With reference next to Figures 6A and 6B, the relationship between the filament 145, the mating portion of the left cathode block 115 and filament clamp (Figure 6A) and right cathode block and filament leg 165 (Figure 6B) can be better appreciated. From these Figures, the manner in which the filament legs 165 provide an automatic alignment of the filament within the channel 145 can be appreciated.

In particular, from Figure 6B, the filament can be seen to extend exactly to the vertical face 605 of the associated cathode blocks. From Figure 6A, the filament clamp 155 and associated screw can be seen in relationship to the left cathode block 115. The filament clamp 155 thus fixes the filament in position between the clamp and block, with the alignment determined by placing the ends of the filament legs against the vertical face 605. A notch 615 is positioned in the front surface of the clamps 155 and 160 to receive the ends of the alignment tool 250 on which the filament is supported during insertion. Once the filament is properly clamped in place, the insertion tool is removed. As noted previously, the filament clamps also cap the ends of the channel, so that electron tails off the ends of the filament are substantially prevented.

Having fully described a preferred embodiment of the invention and various alternatives, those skilled in the art will recognize, given the teachings herein, that numerous alternatives and equivalents exist which do not depart from the invention.

It is therefore intended that the invention not be limited by the foregoing description, but only by the appended claims.