US3710072A

US3710072A - Vapor source assembly

Info

Publication number: US3710072A
Application number: US00141791A
Authority: US
Inventors: R Shrader; K Tsujimoto
Original assignee: Airco Inc
Current assignee: Airco Inc
Priority date: 1971-05-10
Filing date: 1971-05-10
Publication date: 1973-01-09
Anticipated expiration: 1990-01-09

Abstract

A vapor source assembly is described in which an electron beam is deflected in an arcuate path by a main magnetic field from an electron beam gun positioned below the level of the crucible to impinge upon the top surface of material in the crucible. A second magnetic field is produced within the first magnetic field and has lines of force of variable orientation to provide a controllable variation in beam deflection so that the beam may be swept upon the surface of the material contained in the crucible.

Description

Eli-W122], a llhlil r 39'i'll3a073 United States Patent 1 [111 3,7W,W2

Shrader et al. 1 Jan. 6, 1973 54 VAPOR SOURCE ASSEMBLY 3,390,222 6/1968 Anderson .Q ..13/31 3,235,647 2/1966 Hanks ..l3/3l [75] lnvemors' 223293, gz gm s g 12% 3,420,977 1 1969 Hanks em... .,...219 121 EB 9 1 both of Calif- 1,779,269 10/1930 ClOugh ..336/l84 X [73] Assignee: Airco, llnc., New York, NY. Primary Examiner-J. V. Truhe Assistant Examiner-Gale R. Peterson [22] Ffled' May 1971 Attorney-Fitch, Even, Tabin & Luedeka 21 Appl. 190.; 141,791

[57] ABSTRACT ,313/ A vapor source assembly is described in which an electron beam is deflected in an arcuate path by a Clmain magnetic from an electron beam gun pogi- [58] Field of Search ..2l9/12l EB, 121 EM; tioned below the [eve] f the crucible to impinge upon 313/495 I56; 13/31; 335/210 213; the top surface of material in the crucible. A second 336/184 magnetic field is produced within the first magnetic field and has lines of force of variable orientation to [56] References C'ted provide a controllable variation in beam deflection so UNITED STATES PATENTS that the beam may be swept upon the surface of the material contained in the crucible. 3,446,934 5/1969 Hanks ..2l9/l2l EB 3,483,417 12/1969 Hanks ..2l9/12l E18 X Claims, 2 Drawing Figures ay i l, it] n h f i l liiip H it, fi illilllll lhl 93 -iii ilir PATENTED JAN 9 I975 SHEET 1 OF 2 m 0% T WMO W NDM WMWM WW/J T RM mmw Y? ATTORNEYS PATENTEDJAN 9 I975 3.710.072

SHEET 2 UF 2 ROBERT L. SHRADER BY KAZUMI N. TSUJIMOTO r124, am, w

ATTOR NE Y5 INVENTORS I This invention relates to the evaporation of materials under high vacuum and, more particularly, to a vapor source assembly for use in a vacuum deposition system.

ll-lligh 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. 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 evaporant, an electron beam gun for heating the material in the crucible, and suitable supporting structure. One or more vapor source assemblies may be employed in a single high vacuum deposition system, depending upon the composition of the desired coating or coatings. For example, several vapor source assemblies may be employed to produce a coating of a composite material or such assemblies may be selectively operated to produce successive layers of different materials on the substrates.

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

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 socalled 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 longerlife 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 gun assembly of this type is shown and described in U. S. Pat. No. 3,177,535.

As shown in the cited patent, vapor source assem blies 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 factors 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. in such a case, the electron emissive filament is protected from splashing, splattering and spalling.

in order to provide uniform heating of the surface of the evaporant in the crucible while at the same time providing efficient heat transfer to the material in the crucible, it is typically desirable to provide a system for sweeping the beam across the surface of the crucible in a predetermined pattern. Moreover it is sometimes desirable that such sweeping be at a very high frequen cy to enhance the uniformity of the heating of the material in the crucible. Although known prior art systems have proved highly satisfactory in many instances, further improvements in efficiency of heating, while at the same time enhancing reliability of the equipment and minimizing its cost, are desirable.

it is an object of the present invention to provide an improved vapor source assembly for use in a vacuum deposition system.

Another object of the invention is to provide a vapor source assembly of improved heating cfficiency.

it is another object of the invention to provide a vapor source assembly of high heating efficiency which is low in cost and reliable of operation.

Another object of the invention is to provide a vapor source assembly which provides for very high frequency sweep of an electron beam over the surface of material in a crucible while maintaining a substantially uniform focus of the beam.

Other objects of the invention will become apparent to those skilled in the art from the following description, taken in connection with the accompanying drawings wherein:

lFlG. l is a full sectional view of a vapor source assembly constructed in accordance with the invention; and

HG. 2 is an exploded perspective view of the vapor source assembly of FIG. ll.

Very generally, the vapor source assembly of the invention comprises means it forming a crucible 12 for containing material 13 to be vaporized. An electron beam gun llll is positioned below the level of the crucible for producing an electron beam l5 directed initially in a direction away from the material in the crucible. Deflecting means to and H7 produce a main magnetic field having lines of force extending generally transversely of the initial electron beam direction to deflect the electron beam produced by the electron beam gun through an arcuate path from the electron beam gun to the crucible. The deflecting means are of a configura tion such that the main magnetic field is of generally uniform strength in the region of the electron beam gun and in the region of the crucible. Sweeping means l8 are provided for producing a sweeping magnetic field having lines of force of variable orientation within the first magnetic field to provide a controllable variation in beam deflection. The sweeping means are positioned closer to the electron beam gun than to the crucible with respect to the total amount of deflection of the electron beam produced by the electron beam gun.

Referring now more particularly to the drawings, 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. Two

recesses

23 and 25 are formed on the underside of the block and at the opposite end thereof from the recess 21 and are contiguous with each other to form a step. The crucible 12 is formed with frustoconical walls in the region of the block between the recess 21 and the recess 23. 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 aids in focusing the vapor flow, 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 pair of support blocks 49 and 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 suitable bolts 55 which extend through the magnet and into the block. The crucible forming means or block 11 rests upon the blocks 49 and 51 and upon the magnet 53, with the magnet 53 fitting within the recess 21 in the block 11.

The deflecting means 16 and 17 comprise a pair of parallel pole plates which are polarized by the magnet 53. The pole plates 16 and 17 are generally rectangular in shape except for

chamfers

57 and 59, respectively, 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 blocks 49 and 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 blocks 49 and 51 just above the base plate 47 and in the region between the pole plates 16 and 17. The electron beam gun 14 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 67 and 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 separated by a gap 73 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 67 and 69 by means of

high voltage straps

77 and 79, suitably bolted to the cathode blocks 67 and 69, respectively. Bolts 83 and secure the

high voltage straps

77 and 79 respectively 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 67 and 69. The plate 81 engages and is secured to the block 67 but the block 69 is made smaller than the block 67 so the plate 81 is in spaced relationship to the block 69. The bolts 85 and the bar 86 are insulated from the block 69 so 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 67 and 69, and forms a back for the opening. Thus, in effect, the filament is disposed in a recess in a beam forming block main tained 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 the bar 86 by cylindrical alumina insulators 97, and mechanically supported from the insulators by bolts 95. Thus, electrical separation is maintained between the bar 86 and the anode plate 91, the latter being at ground potential durof support, and may be removed simply by loosening the set screws 100 through clearance holes 103 in the pole pieces 16 and 17.

When properly energized, the electron beam gun 11 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 FIG. 1. Because of the transverse magnetic fields 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 to the top surface ofthe material in the crucible 12.

The sweeping means 18 comprise three solenoidally wound coils 101, 103 and 105. The

coils

101 and 105 have generally parallel axes and the coil 103 is supported extending transversely of the

coils

101 and 105 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 and comprises a pair of terminals 109 and a pair of terminals 111. The

coils

101 and 105 are connected in series with each other and to the terminals 109, and the coil 103 is connected to the terminals 111.

By applying currents of variable strength and direction to the terminals 109 and 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 con trol 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.

The housing 107 mates in the recess 37 in the block 11 to form a plugin arrangement and support the steering means at the appropriate position. This position is preferably between 70 and 130 of deflection of the beam by the main field, and not more than one halfthe total amount of deflection in any case. The opening 413 is provided such that the beam may pass up through the block and into the crucible 12 as shown in FIG. 1, due to the deflection force provided by the main magnetic field. The shelves 39 and 4] help to prevent vapor in the region above the crucible 12 from reaching the electron beam gun 14 and the filament 61 thereof such as to cause erosion of the filament.

ln operating the device, suitable attachment is made to the

straps

77 and 79 to provide a high voltage negative potential on the cathode blocks 67 and '79 to provide a high voltage negative potential on the cathode blocks 67 and 69 and on the beam former plate 01, 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.

The iealtage flux from the permanent magnet 53 extends into the region above the crucible 12 at the side thereof opposite the side from which the electron beam arrives. As is known to those skilled in the art, bombardment of metallic material by electron beams produces secondary electrons due to emission thereof by the molten material. Such secondary electrons can form space charges and deleteriously affect the operation of the device. By placing the permanent magnet 53 as is shown in the drawings, the leakage flux therefrom captures many of the secondary electrons thus emitted, substantially reducing the problems caused thereby and enhancing operation of the device.

The vapor source assembly of the invention is particularly well suited to the evaporation of aluminum, and provides very high evaporation rates with virtually no crucible erosion. Steering or sweep of the beam is accomplished without serious variation in the focusing of the beam because of the high uniformity of the main magnetic field. By using a permanent magnet, rather than an electromagnet, less heat is generated by the system and a more simple electrical system may be utilized. Moreover, there is no possibility for the main magnetic field to suddenly fail, causing the electron beam to be projected elsewhere within the vacuum enclosure to possibly damage other components. The plug-in type steering mechanism greatly facilitates assembly of the device and the general arrangement protects the cathode or emitter of the electron beam gun from impingement of ions thereon which would cause erosion.

The invention also provides a substantial improvement with respect to electron beam focusing. More particularly, the precise amount of focus on the electron beam may be closely controlled by controlling the relative amounts of deflection produced by the main magnetic field and the deflection provided by the steering means. The beam may be swept over a relatively wide variation without deterioration of focus. By placing the steering means relatively close to the emitter, that is, where less than 120 of total beam deflection has 1181(61'1 place, a favorable lever arm relationship is established. This reduces the total energy necessary for sweeping the beam. As a result, a very high frequency sweep may be achieved by using laminated cores in the

solenoidal coils

101, 103 and 105.

Some other advantages exist in the mechanical construction of the invention. The large funnel-shaped upper cone 27 above the crucible 12 can be used as a hopper for loosely compacted charge material so that a nearly full crucible of molten metal can be made from a charge of relatively low bulk density. Replacement of the emitter assembly or electron beam gun 14 may be accomplished relatively quickly because of the plug-in type arrangement with the pins '99 passing into the blocks 10 and 51. The fact that the housing 107 of the sweeping means 10 is in contact on three sides with the block 11 shields the housing 107 from radiant heat and provides heat conduction to the cooled block 11 for cooling the

coils

101, 103 and 105.

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 and 115 may be attached to the edges of the pole plates 16 and 17 by bolts 117. The plates 113 and 115 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 (as seen in HQ. I).

It may therefore be seen that the invention provides an improved vapor source assembly for use in a high vacuum evaporation system. The vapor source assembly of the invention is of simple construction, is low in cost, and provides superior operation than prior art devices.

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 drawings. Such modifications are intended to fall within the scope of the appended 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 positioned below the level of said crucible for producing an electron beam directed, initially, in a direction away from the material to be vaporized, deflecting means for producing a main magnetic field having lines of force extending generally transversely of the initial electron beam direction to deflect the electron beam produced by said electron beam gun through an arcuate path from said electron beam gun to said crucible, said deflecting means including a pair of pole plates arranged substantially parallel with each other on opposite sides of said cruciblc, said pole plates being of a configuration such that said main magnetic field is of generally uniform strength in the region of said electron beam gun and in the region of said crucible, and sweeping means including a plurality of coils positioned on coplanar axes between said pole plates and oriented for producing a sweeping magnetic field when said coils are energized with variable currents, said sweeping field being substantially smaller than said main magnetic field and having lines of force of variable orientation within said main magnetic field to provide a controllable variation in beam deflection, said sweeping means being positioned closer to said electron beam gun than to said crucible with respect to the total amount of deflection of the electron beam produced by said electron beam gun.

2. A vapor source assembly according to claim 1 wherein said deflecting means include a permanent magnet.

3. A vapor source assembly according to claim 1 wherein said deflecting means include a permanent magnet extending between said pole plates and positioned adjacent the side of said crucible opposite the electron beam path such that leakage flux from said permanent magnet intercepts secondary electrons emitted from the surface of the material in said crucible.

4. A vapor source assembly comprising, means forming a crucible for containing material to be vaporized, an electron beam gun positioned below the level of said crucible for producing an electron beam directed, ini tially, in a direction away from the material in said crucible, agair of pole plates positioned on opposite sides of sai crucible and said electron beam gun in substantially parallel relationship with each other, means for polarizing said pole plates to produce a main magnetic field therebetween, said pole plates extending a sufficient distance in a direction away from said crucible such that the main magnetic field produces deflection of the electron beam produced by said electron beam gun through an arcuate path from said electron beam gun to said crucible, sweeping means for producing a sweeping magnetic field in the path of the electron beam in a region wherein said electron beam has undergone a deflection from the initial path of between and said sweeping means comprising at least three solenoidal coils arranged with coplanar axes and means for energizing said coils with variable currents to produce variation in the orientation of the lines of force of said sweeping magnetic field.

5. A vapor source assembly comprising, means forming a crucible for containing material to bc vaporized, an electron beam gun positioned below the level of said crucible for producing an electron beam directed, initially, in a direction away from the material to be vaporized, deflecting means for producing a main mag netic field having lines of force extending generally transversely of the initial electron beam direction to deflect the electron beam produced by said electron beam gun through an arcuate path from said electron beam gun to said crucible, said deflecting means being of a configuration such that said main magnetic field is of generally uniform strength in the region of said electron beam gun and in the region of said crucible, and sweeping means producing a sweeping magnetic field substantially smaller than said main magnetic field and having lines of force of variable orientation within said main magnetic field to provide a controllable variation in beam deflection, said sweeping means being positioned closer to said electron beam gun than to said crucible with respect to the total amount of deflection of the electron beam produced by said electron beam gun, said sweeping means including at least three solenoidal coils arranged with coplanar axes, and means for energizing said coils with variable currents to produce variation in the orientation of the lines of force of said sweeping magnetic field.

6. A vapor source assembly according to claim 5 wherein two of said solenoidal coils are arranged with their axes substantially parallel with each other in spaced relationship on opposite sides of the electron beam path, and wherein the third of said solenoidal coils is arranged with its axis substantially perpcndicular to the axes of the other of said solenoidal coils, forming a generally U-shaped assembly therewith.

7. A vapor source assembly according to claim 6 wherein said two parallel solenoidal coils are connected in series.

8. A vapor source assembly according to claim 6 wherein said crucible forming means include socket means for receiving said U-shaped coil assembly and supporting same in an operative position.

Claims

1. A vapor source assembly comprising, means forming a crucible for containing material to be vaporized, an electron beam gun positioned below the level of said crucible for producing an electron beam directed, initially, in a direction away from the material to be vaporized, deflecting means for producing a main magnetic field having lines of force extending generally transversely of the initial electron beam direction to deflect the electron beam produced by said electron beam gun through an arcuate path from said electron beam gun to said crucible, said deflecting means including a pair of pole plates arranged substantially parallel with each other on opposite sides of said crucible, said pole plates being of a configuration such that said main magnetic field is of generally uniform strength in the region of said electron beam gun and in the region of said crucible, and sweeping means including a plurality of coils positioned on coplanar axes between said pole plates and oriented for producing a sweeping magnetic field when said coils are energized with variable currents, said sweeping field being substantially smaller than said main magnetic field and having lines of force of variable orientation within said main magnetic field to provide a controllable variation in beam deflection, said sweeping means being positioned closer to said electron beam gun than to said crucible with respect to the total amount of deflection of the electron beam produced by said electron beam gun.

4. A vapor source assembly comprising, means forming a crucible for containing material to be vaporized, an electron beam gun positioned below the level of said crucible for producing an electron beam directed, initially, in a direction away from the material in said crucible, a pair of pole plates positioned on opposite sides of said crucible and said electron beam gun in substantially parallel relationship with each other, means for polarizing said pole plates to produce a main magnetic field therebetween, said pole plates extending a sufficient distance in a direction away from said crucible such that the main magnetic field produces deflection of the electron beam produced by said electron beam gun through an arcuate path from said electron beam gun to said crucible, sweeping means for producing a sweeping magnetic field in the path of the electron beam in a region wherein said electron beam has undergone a deflection from the initial path of between 70* and 130*, said sweeping means comprising at least three solenoidal coils arranged with coplanar axes and means for energizing said coils with variable currents to produce variation in the orientation of the lines of force of said sweeping magnetic field.

5. A vapor source assembly comprising, means forming a crucible for containing material to be vaporized, an electron beam gun positioned below the level of said crucible for producing an electron beam directed, initially, in a direction away from the material to be vaporized, deflecting means for producing a main magnetic field having lines of force extending generally transversely of the initial electron beam direction to deflect the electron beam produced by said electron beam gun through an arcuate path from said electron beam gun to said crucible, said deflecting means being of a configuration such that said main magnetic field is of generally uniform strength in the region of said electron beam gun and in the region of said crucible, and sweeping means producing a sweeping magnetic field substantially smaller than said main magnetic field and having lines of force of variable orientation within said main magnetic field to provide a controllable variation in beam deflection, said sweeping means being positioned closer to said electron beam gun than to said crucible with respect to the total amount of deflection of the electron beam produced by said electron beam gun, said sweeping means including at least three solenoidal coils arranged with coplanar axes, and means for energizing said coils with variable currents to produce variation in the orientation of the lines of force of said sweeping magnetic field.

6. A vapor source assembly according to claim 5 wherein two of said solenoidal coils are arranged with their axes substantially parallel with each other in spaced relationship on opposite sides of the electron beam path, and wherein the third of said solenoidal coils is arranged with its axis substantially perpendicular to the axes of the other of said solenoidal coils, forming a generally U-shaped assembly therewith.