US3390222A

US3390222A - Electron beam apparatus with variable orientation of transverse deflecting field

Info

Publication number: US3390222A
Application number: US480287A
Authority: US
Inventors: Emmett R Anderson
Original assignee: Air Reduction Co Inc
Current assignee: Airco Inc
Priority date: 1965-08-17
Filing date: 1965-08-17
Publication date: 1968-06-25
Anticipated expiration: 1985-06-25
Also published as: DK114566B; CH448288A; GB1154512A; SE335582B; NL6611561A; LU51771A1; BE685528A; AT296446B; DE1565883B1

Description

June 25, 1968 E. R. ANDERSON 3,390,222

ELECTRON BEAM APPARATUS WITH VARIABLE ORIENTATION OF TRANSVERSE DEFLECTING FIELD Filed Aug. 17, 1965 j .Fl 1

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3,390,222 ELECTRON BEAM APPARATUS WITH VARIABLE gIIgENTATION F TRANSVERSE DEFLECTING LD' Emmett R. Anderson, Berkeley, Calif., assignor, by mesne assignments, to Air Reduction Company, Incorporated, a corporation of New York Filed Aug. 17, 1965, Ser. No. 480,287 6 Claims. (Cl. 1331) ABSTRACT OF THE DISCLOSURE Apparatus is described for heating a target material in a high vacuum electron beam furnace. A beam of electrons is deflected by a transverse magnetic field, the orientation of which is varied to provide a predetermined impact pattern on the surface of the target material.

The present invention relates generally to apparatus for heating material in an electron beam furnace and more particularly to apparatus for traversing an electron beam in a predetermined electron impact pattern on a target material in an electron beam furnace.

Normally in electron beam furnaces it is desirable to provide means for effecting accurate control over an electron beam so as to precisely regulate the point of impact of the beam on the target material. Such a controlled electron beam is highly advantageous in numerous applications. For example, in a reaction hearth it is desirable to maintain a substantially constant temperature throughout the reacting area. This was generally difficult to accomplish, since the edges of the reacting area tended to remain at a lower temperature than the more centrally located portions, since heat transfer is greatest at the edges. Operations involving the casting of generally rectangular shaped items, having relatively sharp corners, often entailed substantial difiiculties. Considerable difficulties also arose in vapor plating operations, where it was desired to provide generally uniform vapors for coating the substrate.

When a relatively narrow beam of electrons is employed, there is a tendency to gouge holes in the evaporant material, while a relatively wide beam of electrons tends to cause the formation of a rim of relatively slowly evaporating material around the periphery of the evaporant material.

However, an electron beam which periodically traverses a predetermined impact pattern on the surface of the target material minimizes the occurrence of uneven heating at the outer edges of the material and facilitates convenient casting of materials in desired shapes. In addition, such a periodic traversal effects thermal stirring of the material being heated which precludes resolidification along its outer edges. Moreover, in vapor plating operations such stirring promotes the maintenance of a relatively constant composition of the vapors, emanating from the surface of the evaporant material so that the substrate may be plated in a generally uniform manner.

It is a object of the present invention to provide apparatus for guiding an electron beam onto predetermined regions of a target material. It is another object of the present invention to provide apparatus for periodically traversing an electron beam across the surface of a target material, thereby heating and thermally stirring the material. It is another object to provide apparatus for periodically directing an electron beam across the surface nited States Patent 0 of the target material in a series of discrete steps to form a closed impact pattern. A still further object is theprovision of apparatus for controllably sweeping an electron beam across the surface of a target material in any desired pattern. Other objects and advantages of the present invention will become readily apparent from the following detailed description and accompanying drawings, wherein:

FIGURE 1 is a schematic view of a high vacuum electron beam apparatus incorporating features of the present invention.

FIGURE 2 is a sectional view taken along line 2-2 of FIGURE 1, and

FIGURE 3 is a schematic circuit diagram of another embodiment of an electrical circuit for traversing the electron beam in the apparatus shown in FIGURE 1,

Generally, in the illustrated apparatus the beam of electrons supplied by an electron gun 10 in an electron furnace 14 is directed into a magnetic field established by an electromagnetic means 18 disposed adjacent a crucible 22 containing a target material 26. Circuit means 30 are electrically coupled to the electromagnetic means 18 for varying the magnetic field such that a predetermined closed impact pattern is traversed by the beam of electrons on the surface of the target material 26.

More specifically, referring to the drawings and particularly to FIGURE 1, a high vacuum electron beam apparatus in accordance with the present invention includes an enclosure 32 which communicates with a vacuum pump 34 for continuously maintaining a relatively high vacuum therein. The crucible 22 is disposed within the enclosure for receiving the target material and is spaced from the electron gun 10.

The electron beam is directed onto the surface of the target material 26 by a transverse magnetic field. The magnetic field is established by the electromagnetic means 18. The electromagnetic means 18 includes a pair of parallelspaced

magnetic pole pieces

38 and 40 disposed on opposite sides of the crucible 22. The

pole pieces

38 and 40 are preferably in the form of generally flat, elongated plates which are of greater length than the diameter of the crucible 22. The

pole pieces

38 and 40 are fabricated of a relatively high permeability material such as soft iron. The

magnetic pole pieces

38 and 40 are coupled at one end by a bar 44 of high permeability material, which provides a low reluctance flux path therebetween. A pair of

coils

48 and 50 are successively disposed on the bar 44, and when energized establish a magnetic field between the pole pieces above the surface of the crucible 22. The beam of electrons is directed through the magnetic field and is thereby deflected into forming a predetermined impact pattern on the surface of the target material 26.

The electron gun 10 is preferably disposed above the target material 26 at a suitable angle thereto. Preferably the gun 10 is disposed at an angle of approximately 45 with respect to the normal to the surface of the target material 26. Thus, the beam of electron is directed into the magnetic field at an angle of approximately 45, and may be conveniently guided into a predetermined impact pattern on the surface of the target material 26.

When the

coils

48 and 50 disposed on the bar 44 are appropriately energized a magnetic field, having lines of flux 54, as shown in FIGURE 1, is established adjacent the surface of the target material 26. The

coils

48 and 50 preferably comprise suitably insulated multiturn coils, respectively coupled to appropriate sources 58 and 60 of current. The magnetic lines of flux 54 extending adjacent the surface of the target material 26 are generally transversely oriented with respect to the direction of travel of the beam of electrons passing therethrough and accordingly deflect the electron beam onto the surface of the target material 26.

If equal currents are supplied to both

coils

48 and 50, the lines of magnetic flux produced deflect the electron beam onto the surface of the target material 26 in an area lying along a generally elongated strip, passing diametrically across the surface of the target material 26 generally parallel to the

pole pieces

38 and 40. If the currents supplied to the

coils

48 and 50 are maintained generally equal in each of the coils but increased in amplitude the increased magnetic field intensity causes the beam to be defiected along the elongated strip toward the position designated by the letter e on the surface of the crucible.

It is readily apparent that any variance between the currents respectively supplied to the

coils

48 and 50 varies the orientation of the lines of flux constituting the magnetic field and correspondingly changes the deflection of the electron beam passing through the field, thereby changing its'area of impingement on the surface of the target material 26. Since electrons are generally deflected in a direction, perpendicular to the magnetic lines of flux, the electron beam is deflected onto the surface of the target material 26 in the direction of the pole piece coupled to the coil receiving the lesser amount of energization.

From the foregoing it can be seen that the electron beam may be directed onto predetermined areas on the surface of the crucible by appropriately adjusting the currents respectively supplied to the

coils

48 and 50. Since these currents vary the orientation of the lines of magnetic flux, which determine the deflection of the beam of electrons passing therethrough, the electron beam may be directed onto the surface of the target material 26 forming a predetermined impact pattern. For example, the

coils

48 and 50 may be suitably periodically energized such that the beam of electrons is deflected onto the target material 26 so as to define a periodically repetitive circular impact pattern or circular sweep about the perimeter of the material.

If the characteristics of the magnetic field are sufficiently linear, operations such as the perimeter sweeping described above, may be conveniently achieved by coupling the

coils

48 and 50 to the electrical circuit means 30, as is illustrated in FIGURE 1. The circuit means 30 comprises a pair of sine wave generators 58 and 60, which are connected in series respectively with coil 48 and coil 50. The serially connected

coils

48, 50 and generator 58, 60 are coupled in parallel, and the parallel combination is connected in series with a D-C bias source of a greater voltage than the peak voltage of the sine wave generators. The sine wave generators are connected 90 out of phase with respect to each other, so that when the current produced by one of the sources is at a maximum, the current produced by the other source is at a minimum.

It is quite advantageous to employ current for establishing the magnetic field which does not cause reversal of the field. A reversing magnetic field would introduce unwanted complexities. For example, if a reversing magnetic field is established the current supplied to one coil is negative while the current supplied to the other coil is positive resulting in the establishment of two adjacent magnetic fields which may lead to a lack of control over the beam of electrons.

As previously mentioned, the electron gun is preferably positioned at approximately a 45 angle with respect to the target material 26. Thus, the beam of electrons generated enters the magnetic field at a 45 angle. This is advantageous in that it permits convenient establishment and adjustment of the magnetic field for direct ing the beam of electrons onto desired regions on the target material 26. If the electron gun 10 is positioned directly above the target material it is necessary to establish a magnetic field having a relatively complex orientation in order to form a desired impact pattern. The establishment of such a field may require a reversing current rather than a pulsating D-C current, which as previously indicated results in loss of control of the beam. This problem is avoided by directing the beam of electrons into a magnetic field at an angle to the target material, and by employing a magnetic field established by pulsating D-C currents supplied to the

coils

48 and 50.

In order to produce a substantially circular impact pattern the amplitudes of the sine wave generators 58 and 60 are maintained generally equal to each other. The diameter of the sweep circle is systematically related to the amplitude of the current supplied by the sine wave generators 58 and 60. Thus, if it is desired to sweep the outer periphery of the surface of the target material 26 the amplitude of the sine wave generators 58 and 60 may be empirically adjusted for accomplishing such an operation. Alternatively, if it is desired to sweep a circular pattern having a smaller diameter the amplitude of the currents supplied by the sine Wave generators 58 and 60 is decreased until the desired pattern is achieved. An additional control over the sweep is achieved by virtue of the D-C bias source 64. A variance in the amplitude of the output of the bias source 64 effects a corresponding change in the location of the center of the sweep circle. Thus, it is possible to conveniently produce a circular impact pattern having a suitable diameter on a desired region of the surface of the material 22 by appropriately adjusting the amplitude of the sine wave generators 58 and 60 and of the output of the D-C bias source 64.

In certain instances there may be minor non-uniformities when attempting to magnetically guide an electron beam at a predetermined repetitive impact pattern in the manner previously described. Such non-uniformities are to a large extent caused by the non-linear characteristics of a magnetic circuit. Minor compensation for such nonuniformities may be achieved in certain instances by varying the amplitudes of the sine wave generators 58 and 60 with respect to each other. Additional compensation may be achieved by appropriately shaping the output wave forms of the sine wave generators 58 and 60 together with changing their amplitude relationship.

However, in. some instances compensating measures may be insufficient to overcome the non-uniformities, particularly when even relatively small deviations are undesirable. In such instances, an electrical circuit, such as that illustrated in FIGURE 3, is coupled to the

coils

48 and 50 replacing the sine wave generators 58 and 60 and the DC bias source 64. Basically, such a circuit provides for the approximation of a circular impact pattern by periodically directing the electron beam about the outer periphery of the target material 26 in a series of discrete steps. It has generally been found that such a stepping control circuit provides an adequate approximation, when a series of eight discrete positions are employed, although a smaller or larger number of steps may be readily provided if such is deemed desirable. Accordingly, the circuit illustrated in FIGURE 3 and the subsequent description provides a series of eight discrete steps of energization to each coil to cause the beam of electrons to traverse the periphery of the target material 26.

The circuit shown generally includes an eight position, two pole electro-mechanical stepping switch 68, which is stepped by a suitable pulse generator 72 coupled to a stepping relay 76 provided in the stepping switch 68. The switch 68 includes two

banks

80 and 84 of contacts, each bank being provided with a wiper 88 and 92. The Wipers 88 and 92 are mechanically linked such that they each advance simultaneously through successive steps. The contacts in the first bank 80 are designated by the letters a through h. Correspondingly, the contacts in the second bank 84 are designated by the letters a" through h" as shown.

The wipers 88 and 92 are respectively coupled to corresponding ends of the

coils

48 and 50, and the other ends of the coils are coupled to a negative terminal of a D-C energy source 96. The positive end of the D-C energy source 96 is coupled to corresponding ends of a bank 100 of sixteen adjustable resistors, one resistor being provided for each position on the

banks

80 and 84 of the stepping switch 68. The other ends of the resistors are connected to the associated fixed contacts of the

banks

80 and 84.

To obtain the desired impact pattern on the surface of the target material 26, which is designated by the points of impact a through k, the points of impact of the beams are each initially statically adjusted by utilizing the stepping relay 76 to advance the Wipers 88 and 92 into communication with the; associated resistors in the resistor bank 100. The resistance of the associated resistors are adjusted so as to direct the beam onto the desired area of impact. For example, the stepping relay 76 may be operated to advance the wiper 88 such that it is in electrical contact with the relay contact in the bank 80 designated by the letter a'. Since the wiper 92 necessarily advances simultaneously with the wiper 88, the wiper 92 is at this instant in contact with the relay contact in the bank 84 designated by a". Thus, the coil 48 is coupled to the D-C energy source 96 through the associated resistor 104. correspondingly, the coil 50 is coupled to the DC energy source 96 through a resistor 108. Since the adjustable resistor 104 and 108 vary the amplitude of the current supplied to their

respective coils

48 and 50, the resistors 104 and 108 may be readily adjusted empirically to cause the beam of electrons to be deflected onto the position designated by the letter a on the surface of the target material 26. Similarly, the wiper 88 may be advanced to the position designated by the letter b while the wiper 92 advances to the position designated by the letter b". The coil 48 then communicates with the energy source 96 through an adjustable resistor 112 while the coil beam onto the area designated by the letter b. In a similar fashion the remaining positions or steps about the periphery'of the target material 26 designated by.the letters 0 through h are empirically adjusted.

After completion of the positioning procedure the pulse generator 72 is activated and coupled to the stepping relay 76. The stepping relay 76 then advances the wipers 88 and 92 to successively engage the fixed contacts in the

respective banks

80 and 84 in response to the pulses of current provided by the pulse generator 72. Since the frequency of the pulse generator 72 controls the energization of the stepping relay 76, it is generally desirable to employ a pulse generator of the variable frequency variety to provide convenient adjustment of the sweep speed around the periphery of the target material 26.

If desired, in certain applications, where substantially increased sweep speed is advantageous, such that the response time of the relay 66 is insufficient, a solid state switching apparatus may be conveniently provided. Such an apparatus generally may include a semi-conductor ring counter which drives a pair of parallel banks of gate circuits to selectively couple the coils to the energy source.

Thus, a novel apparatus has been provided for sweeping an electron beam in predetermined paths on a target material. In addition, apparatus has been set forth for carrying out such sweeping operations by producing an approximate impact pattern comprising a series of discrete positions, and sweeping the beam through such positions at a predetermined periodic rate.

Various other changes and modifications may be made in the above-described apparatus without deviating from the spirit and scope of the present invention.

Various features of the present invention are set forth in the following claims.

What is claimed is:

1. Apparatus for heating a target material in a high vacuum electron beam furnace comprising means for generating a beam of electrons, electromagnetic means including a pair of pole pieces for establishing a magnetic field having lines of flux extending generally transversely through said beam of electrons, a multiturn coil connected to each of said pole pieces, and circuit means connected to said coils for varying the orientation of said magnetic field in a repetitious pattern, said circuit means including means for supplying a periodically varying D-C current to each of said coils, said currents being out of phase with respect to each other, whereby a predetermined impact pattern is formed on the surface of said target material.

2. An apparatus in accordance with claim 1 wherein the D-C currents supplied to the coils are approximately out of phase with respect to each other.

3. Apparatus in accordance with claim 1 wherein the circuit means comprises a sine wave generator coupled to each of said coils, said sine Wave generators being approximately 90 out of phase with respect to each other, and a D-C bias source coupled in series with said sine wave generators and with said coils, said DC bias source having a higher voltage than the peak voltage of the sine wave generators.

4. Apparatus for heating a target material in a high vacuum electron beam furnace comprising electromagnetic means disposed adjacent said target material for establishing a magnetic field having lines of flux extending adjacent the surface of said target material means for generating a beam of electrons, said beam of electrons being directed at an angle of approximately 45 with respect to said surface of the target material, means coupled to said electromagnetic means for supplying pulsating D-C currents thereto for varying the orientation of said magnetic field, and circuit means coupled to said orientation varying means for repetitively varying the orientation of said magnetic field, whereby a predetermined closed impact pattern is formed on the surface of said target material.

5. Apparatus for heating a target material in a high vacuum electron beam furnace comprising means for generating a beam of electrons, a pair of magnetic pole pieces for establishing a magnetic field having lines of flux extending generally transversely through'said beam of electrons, and circuit means coupled to said pole pieces for periodically varying the orientation of said magnetic field so as to direct said beam of electrons'to traverse the surface of said target material in a series of discrete steps to form a predetermined closed impact pattern, said circuit means including a pair of switch means each having a common connection, which in response to activating pulses is sequentially coupled to a plurality of individual connections, means for applying a different voltage to each of said individual connections, and means coupled to said switch means for providing activating pulses thereto.

6. Apparatus for heating a target material in a high vacuum electron beam furnace comprising means for generating a beam of electrons, a pair of magnetic pole pieces for establishing a magnetic field having lines of fiux extending generally transversely through said beam of electrons, and circuit means coupled to said pole pieces for periodically varying the orientation of said magnetic field so as to cause said beam of electrons to transverse the surface of said target material in a series of discrete steps, thereby forming a predetermined closed impact pattern on said target material, said circuit means comprising a multi-turn coil coupled to each of said pole pieces, a D-C energy source, a bank of adjustable resistors, a stepping switch including two banks of fixed contacts, each of said contacts in said banks being coupled to said energy source through an associated resistor, a pair 7 8 of wipers for providing selective communication between References Cited said contacts in each of said banks and one of said multi- UNITED STATES PATENTS turn coils, said Wipers being mechanically linked to ad- 3,270,118 8/1966 Gaydou 13 31XR vance slmultaneously, a stepping relay for advancing said 3,312,858 4/1967 Dietrich 219 121 XR mechanically linked wipers, and a pulse generator of predetermined frequency coupled to said stepping relay ROBERT K. SCHAEFER, Primary Examiner.

for energizing Said Stepping y- M. GINSBURG, Assistant Examiner.