Vapor source assembly

Abstract

A vapor source assembly is described which incorporates a magnetic shield for preventing at least some of the lines of force in a magnetic field from passing through the crucible.

Parent Case Text

This application is a continuation-in-part of application Ser. No. 386,489 filed Aug. 8, l973, now abandoned.

Description

This invention relates to the vaporization of materials under high vacuum and, more particularly, to a vapor source assembly for use in a vacuum deposition system.

High vacuum deposition systems often employ electron beams for heating the material to be vaporized. Typically, the material is contained within a crucible or equivalent structure and is bombarded by one or more electron beams to heat the material in the crucible to a sufficiently high temperature to vaporize. Vaporization may occur through evaporation or sublimation, depending on the material. The vapor then moves from the crucible to a substrate positioned at an appropriate location, upon which the vapor condenses to form a desired coating.

Vapor source assemblies are often used in high vacuum evaporation and deposition systems as replaceable units. A vapor source assembly typically comprises a crucible for containing material to be vaporized, an electron beam gun for heating the material in the crucible, and a suitable supporting structure. The vapor source assembly may have means for producing one or more magnetic fields to direct the electron beam through a desired path onto the surface of the material in the crucible and to focus the beam to a desired concentration and thereby control the size of the impact area on the surface of the target.

Electron beam guns for use in vapor source assemblies generally comprise an electron emissive filament or other element for emitting electrons, and means for focusing the electrons into a beam. The beam of electrons is accelerated along an initial path by a suitable accelerating anode.

One type of vapor source assembly of particular advantage utilizes an electron beam gun positioned beneath the level of the crucible and such that the initial path of the electron beam is directed away from the material in the crucible (i.e. not directly at the target). A magnetic field or magnetic fields having lines of force extending transversely to the direction of travel of the electrons in the electron beam is used to deflect the beam of electrons through a curving path onto the target. Deflection of the beam by such so-called transverse fields enables the electron emissive filament to be positioned out of a line of sight of the target. Thus the filament is not directly exposed to materials vaporized from the target, and evolved condensible materials do not readily contact the surfaces of the filament. A substantial decrease in erosion of the electron emissive filament and a resulting longer life of the filament is achieved. Moreover, the tendency for negative ions and secondary electrons to be trapped in the electron beam is substantially reduced by the use of transverse fields. This reduces space charge build up which can detrimentally affect focusing and deflection. A successful electron beam assembly of this type is shown and described in U.S. Pat. No. 3,177,535.

As shown in the cited patent, vapor source assemblies often employ an upright crucible. Under some circumstances, spalling of condensed materials from cooled surfaces of the vacuum enclosure, and splashing and splattering of molten material from the upright crucible, constitute a potential impairment to satisfactory operation of an electron beam gun as described in the cited patent. Where these facts are a problem, they may be alleviated by positioning the electron emissive filament underneath the crucible and by deflecting the electron beam through a curving path of approximately 270.degree.. In such a case, the electron emissive filament is protected from splashing, splattering and spalling.

In operating a vapor source assembly of the foregoing described type, it may be desirable to provide a transverse field produced by pole members positioned on opposite sides of the crucible. Because the crucible is positioned between the pole members or pole pieces extending on opposite sides thereof, the transverse magnetic field for directing the beam through its arcuate path extends through the crucible. As a result, even within the crucible itself, the influence of the magnetic field will continue to bend the beam.

If the beam continues to curve within the crucible for the reason described above, the position of the impact region of the beam on the surface of the target material contained in the crucible will be dependent upon the level of that surface within the crucible. Thus, in the case of a molten pool contained within the crucible, variations in the level of the pool required constant monitoring and magnetic field changes in order to keep the same impact position on the surface of the target. In some cases, the level of the material within the crucible may lower to an extent where the beam may curve around too far and cause uneven heating in the crucible. With molten materials, this causes a stirring action which in some cases leads to severe crucible erosion. In extreme cases, the beam may completely curve around 360.degree. and emerge from the vapor area and cause damage to surrounding components. This not only is a dangerous condition, but it limits the amount of useful material that can be evaporated.

When working with materials that sublimate, if the beam continues to bend back after striking the surface of the target, the beam has a tendency to tunnel into the material at an angle and give poor coating distribution patterns. Ideally, the beam should strike the target material perpendicular to the surface and continue in a straight line to the bottom of the crucible.

It is an object of the present invention to provide an improved vapor source assembly.

Another object of the invention is to provide a vapor source assembly in which variations in the level of the target surface do not result in variations in beam impact position.

A further object of the invention is to provide a vapor source assembly wherein the electron beam strikes the target material perpendicular to the surface and continues in a straight line to the bottom of the crucible regardless of target surface level.

Other objects of the invention will become apparent to those skilled in the art from the following description, taken in connection with the accompanying drawing wherein the sole FIGURE is a side view, partially in section, of a vapor source assembly constructed in accordance with the invention.

Very generally, the vapor source assembly of the invention comprises means 11 forming a crucible 12 for containing material 13 to be vaporized. An electron beam gun 14 produces an electron beam 15, and the beam is deflected through an arcuate path to the crucible by a transverse magnetic field having lines of force extending transversely of the path of the electron beam. The transverse field is produced between a pair of pole members 16 and 17 positioned on opposite sides of the crucible. Magnetic shield means 19 are positioned between the pole members and the crucible for preventing at least some of the lines of force extending between the pole members from passing through the crucible.

Referring now more particularly to the drawing, the crucible forming means 11 comprise a copper block. A recess 21 is formed in the block on the underside at one end thereof extending across the entire width of the block. An annular recess 23 is machined on the underside of the block coaxially with the crucible 12. A recess 25 is formed in the block at the end opposite the recess 21. The crucible 12 is formed with frustoconical walls. A second frustoconical section 27 extends from the upper rim 29 of the crucible 12 to the upper surface of the block 11 and forms an upper cone or hopper for the crucible. This protects the pole pieces 16 and 17 from condensation of vapor thereon, and provides a hopper for melting loosely compacted material. The block 11 is cooled by coolant passages 31 bored therein and suitable coolant conduits 33 communicate with the passages 31 for conducting flowing coolant, such as water, to and from the passages. Suitable coolant connections 35 are provided for the conduits 33.

A generally horizontal U-shaped recess 37 is formed in the block 11 just below the upper surface thereof to leave two generally horizontal shelves 39 and 41 extending therefrom above the recess 25. An opening 43 is formed in the block 11 extending from the upper surface thereof to the lower surface of the lower shelf 41 in the recess 25. The opening 43 also partially intercepts the frustoconical surface 27, extending downwardly about half way from the upper surface of the block 11 to the upper rim 29 of the crucible 12. The opening 43 and the recess 37 thus leave a vertical wall 45 separating the opening 43 from the recess 37 and extending between the shelves 39 and 41.

Support for the vapor source assembly includes a generally rectangular base plate 47 of non-magnetic material having a support block 51 supported thereon and secured thereto by any suitable means, such as by welding. A permanent magnet 53 in rectangular block form is supported between the base plate 47 and the block 11 by bolts, not shown, which extend through the magnet and into the block. The crucible forming means or block 11 rests upon the block 51 and upon the magnet 53, with the magnet 53 fitting within the recess 21 in the block 11.

The deflecting means comprise a pair of parallel pole plates 16 and 17 which are polarized by the magnet 53. The pole plates 16 and 17 are generally rectangular in shape except for chamfers 57 formed in the lower edges toward one end. This provides the plates with a slight taper. The lower edge of each of the plates 16 and 17 rests upon the upper surface of the base plate 47 and the facing sides of the plates 16 and 17 abut the adjacent sides of the block 51. The same faces of the plates 16 and 17 also abut opposite sides of the crucible forming block 11. Suitable screws, not shown, are provided for attaching the plates 16 and 17 to the base plate 47 and the block 11 to form a rigid assembly.

The permanent magnet 53 thus polarizes the pole plates 16 and 17 so that the main magnetic field is established between the pole plates. The main magnetic field will, of course, have some variation in its strength with distance from the magnet. The taper prevents the strength from falling off excessively, and for all practical purposes, the main magnetic field is of substantially uniform strength, the variation in strength being negligible.

The electron beam gun 14 is mounted to the block 51 just above the base plate 47 and in the region between the pole plates 16 and 17. The electron beam gun 20 may be of any suitable construction. In the illustrated embodiment, however, the electron beam gun includes an electron emitting filament 61 formed generally in the shape of an inverted U. The filament may have the transverse section of the U formed in a coil, as is known in the art, to provide a large emissive area. The filament is formed of tungsten or other suitable emissive material.

The filament is clamped in an upright position at each of its parallel legs by clamping blocks 63. The clamping blocks 63 are held by bolts 65 and clamp the filament against one of two cathode blocks 69. The clamping blocks are formed with a suitable notch, not shown, for accommodating the filament legs. The cathode blocks are made out of molybdenum and are mounted adjacent each other and formed with a central beam forming opening 71 therein which the transverse portion of the filament 61 spans. The blocks are electrically separated so that a suitable heating current can be passed through the filament superimposed on the high voltage, described below, for heating the filament to an emissive temperature.

Each of the cathode blocks is provided with a recess 75 in the back side in which the clamping blocks 63 seat. Filament current and high negative voltage are supplied to the cathode blocks 69 by means of high voltage straps 79, suitably bolted to the cathode blocks. Bolts 85 secure the high voltage straps to the cathode blocks, passing into a non-magnetic steel bar 86 on the opposite side of the cathode blocks. A beam former plate 81 extends up the back of and over the top of the cathode blocks 69. The plate 81 is electrically connected in such a way as to prevent electrical shorting of the filament current. The beam former plate 81 closes the top of the opening 71 defined by the cathode blocks 69, and forms a back for the opening. Thus, in effect, the filament is disposed in a recess in a beam forming block maintained at a very high negative potential.

An anode plate 91 is provided folded over the top of the beam former plate 81 and spaced a distance therefrom. The anode plate is supported on a suitably shaped support bracket 93 spaced from and insulated from the bar 86. The anode plate 91 is held at ground potential during the operation of the electron beam gun 14.

When properly energized, the electron beam gun 14 produces a stream of electrons in the form of the electron beam 15 which initially issues from the opening 71 in a direction generally away from the target. This initial direction is substantially horizontal and to the right in the drawing. Because of the transverse magnetic field set up between the pole plates 16 and 17, and because the direction of the lines of force in such fields is appropriately selected in accordance with the right hand rule, the electrons of the electron beam are deflected upwardly and then around through an arcuate path for a change in direction of approximately 270.degree. to the top surface of the material 13 in the crucible 12.

Sweeping means 18 are provided for moving the beam over the target surface. These comprise three solenoidally wound coils, one of which 103 is shown. The unshown coils have generally parallel axes and the coil is supported extending transversely of the unshown coils at one end thereof to form a generally U-shaped structure. The coils are suitably supported within a U-shaped housing 107 of generally square cross sectional configuration. Electrical connection is provided on the housing for the coils by means of terminals 111.

By applying currents of variable strength and direction to the various coils through the terminals 111, a magnetic field may be established within the region bounded by the coils. This magnetic field has lines of force of variable or controllable orientation. As the beam passes through this region, it may be deflected according to the orientation of the lines of force therein. By appropriate control of the direction and strength of the lines of force, the beam may be moved or swept across the surface of the target material within the crucible 12.

In operating the device, suitable attachment is made to provide a high voltage negative potential on the cathode blocks 69 and on the beam former plate 81, and also to provide a heating current for the filament 61. The beam thus formed is deflected around through the main magnetic field, passing through the region defined by the sweep means 18. By suitably controlling the sweep means, a repetitive impact pattern may be produced on the surface of the melt at a very high frequency. Vapor thus produced moves to the substrate, not shown, to thereby coat the substrate.

Under some circumstances, evaporation of certain materials (e.g. aluminum) is carried out more efficiently with a diffuse beam rather than a narrow, highly focused beam. To this end, small plates 113 may be attached to the edges of the pole plates 16 and 17 by bolts 117. The plates 113 extend inwardly from the pole plates 16 and 17. The wider the plates, the greater the beam diffusion which results. The plates are of sufficient length to exceed the width of the central section of the housing 107.

In the illustrated embodiment, the crucible is frustoconical in shape, therefore having a circular cross section. The illustrated magnetic shield means 19 comprise a mild steel annulus or cylindrical wall which fits in the recess 23 surrounding the base of the crucible and which is held therein by a set screw 121. The mild steel cylinder operates to shunt the magnetic field in the region of the crucible. This prevents at least some of the magnetic lines of force from passing through the crucible and therefore eliminates the previously mentioned undesirable characteristics which are sometimes associated with many existing transverse field electron beam vapor source assemblies. Any suitable shape and material may be used for the magnetic shield.

As may be seen in the illustrated embodiment, it may not be necessary that the shield 19 extend the full height of the crucible 12. In the embodiment shown, the shield not only blocks lines of force at its level, but sufficiently inhibits field strength directly above the shield level so as to produce the desired results. In fact, it may be preferable to use a shield of a size as small as possible consistent with the desired results, since the larger the shield, the stronger the magnets producing the field need to be to maintain a given field strength.

The vapor source assembly of the invention provides more uniform heating because of the avoidance of undesired variation of beam spot position on the surface of the target. This minimizes the chance of erosion by preferential heating adjacent one side of the crucible. In addition, longer runs per crucible charge are possible because of the minimized effect of variation in the level of material within the crucible. In a vapor source assembly of the invention, the crucible can typically be depleted to less than one-third full. In the case of evaporating an aluminum charge of 12 kilowatts, such result provides over one hour of continuous running, which is a substantial improvement over that previously possible. Since the beam impact position is not affected by the level of the evaporant material, constant adjusting of the beam position with variations in material height is unnecessary. Moreover, the danger of the beam completely flipping out of the crucible as a result of extremely low pool level is eliminated. In the case of sublimating materials, erosion takes place vertically rather than tunneling back at an angle, providing a better and more predictable coating distribution. Because of its simplicity, the magnetic shield is relatively low in cost.

The precise height of the magnetic shield may be selected in accordance with the desired characteristics of the system. It has been found that with a crucible depth of one inch, an axial height of the magnetic shield 19 of 1/2 inch provides satisfactory operation. Its thickness should be enough to provide sufficient blockage of the magnetic lines of force.

It may therefore be seen that the invention provides an improved electron beam vapor source assembly. Crucible erosion is reduced, and beam position remains constant with changes in pool or target level. Better coating uniformity is attainable in the case of sublimating materials, and the ability to evaporate more material per run is provided.

Various modifications of the invention in addition to those shown and described herein will become apparent to those skilled in the art from the foregoing description and accompanying drawing. Such modifications are intended to fall within the scope of the appended claims.

Claims

What is claimed is:

1. A vapor source assembly comprising means forming a crucible for containing material to be vaporized, an electron beam gun for producing an electron beam, deflecting means for producing at least one magnetic field having lines of force extending transversely of the path of the electron beam to deflect the electron beam through an arcuate path from said electron beam gun to said crucible, said deflecting means including a pair of pole members positioned on opposite sides of said crucible and between which the lines of force of the magnetic field extend, and magnetic shield means positioned between said pole members and said crucible for preventing at least some of the lines of force extending between said pole members from passing through said crucible.

2. A vapor source assembly according to claim 1 wherein said magnetic shield means comprise a wall of magnetic material coaxially of said crucible.

3. A vapor source assembly according to claim 2 wherein said crucible is of circular cross section and wherein said wall is cylindrical.

4. A vapor source assembly according to claim 1 wherein said deflecting means include means for steering the electron beam to vary the position of impact thereof on material contained in said crucible.