GB2164174A - Method and apparatus for controlling the focussing condition of a deflected electron beam - Google Patents

Abstract

Method and apparatus for controlling the focussing condition of an electron beam periodically deflected over a plurality of discrete deflection positions within a common target area. To an electron gun (1) with a beam source (2), a focussing unit (3) and a deflection unit (5), is added a control unit (6) for the deflection of the electron beam. To solve the problem of keeping a predetermined spot intensity over the whole target area, the electron beam is initially adjusted as to its focussing condition at each deflection position (F1....Fn) by means of variable deflection fields to a predetermined spot geometry. The focussing data corresponding to this spot geometry are stored with the corresponding position data in a memory unit (12). In use the appropriate focussing data are recalled from the memory unit (12) for each beam position and the focussing of the beam controlled accordingly. The control unit (16) has a microprocessor (7) for the automatic control of these processes. <IMAGE>

Description

SPECIFICATION Method and apparatus for controlling the focussing condition of a deflected electron beam The invention relates to a method for controlling the focussing condition of an electron beam which is periodically deflected over a plurality of discrete deflection positions within a common target area.

Such a method is used in welding and drilling by means of an electron beam, when the working locations are for example arranged in a raster in the target area. The method is moreover of use in surface hardening by electron beams as well as in electron beam melting, when it is required to direct the electron beam in a definite surface distribution to bring about melting (DE-OS 25 12 285). As indicated in the specification mentioned, the method according to the invention can however also be used advantageously in evaporating materials out of a crucible, when it is required to maintain a uniform deposition within a larger condensation zone of coatings condensed from the vapour.

In any event, the electron beam produces a definite surface pattern on its incident surface which here is indicated as a target surface, which pattern comprises a plurality of discrete deflection positions. The surface pattern comprises individual fields row-wise arranged in one or two dimensions, and through the size of these fields and the dwell time of the electron beam in the individual fields a spatially defined flux can be produced.

While it is desired in electron beam welding and drilling to have so far as possible identical beam data at each deflection position, it may be desirable in vacuum deposition to produce a vapour flux which is variable over the space by means of a current flux which is variable over the space in order to compensate for inhomogeneities in the area where the vapour condenses due to other causes.

With a mechanical relative movement between the electron gun and the target area, the beam deflection is unchanged so that the focussing condition is also maintained. A mechanical relative motion is however slow, and in many cases cannot be used on that account, or difficult, and thus only carried out at high cost. Movement of the crucible for example in melting or evaporation is thus excluded on similar grounds. Movement of the electron gun involves however extraordinary expense.

Thus, in the past use has mainly been made of the property of an electron beam being deflected more or less strongly by electromagnetic or electrostatic fields, so that thereby a plurality of discrete deflection positions can be produced within a relatively large target area, without any mechanical relative motion being effected. The principle of such a method is known from television picture tubes. Now such a measure has however the substantial disadvantage that the quality of the focus condition falls off with increasing deflection angle.

It is known from DE-AS 20 47 138 that an electron beam deflected from a fixed beam source into different discrete deflection positions can have very different beam cross sections at the incident positions, so that the flux density at the incident positions is correspondingly altered. This is an undesirable occurrence, and in the specification referred to it is therefore proposed to compensate for the different focussing condition at the separate incident locations by different dwell times. The effect of such a remedy is however extraordinarily limited and it cannot be used for numerous cases in which it is required to have an exact beam focussing or a definite beam diameter at the incident location of the electron beam.In particular, the known measure cannot be used when the relative dwell times of the electron beam in the different discrete deflection positions cannot be altered because for example the traverse time of the electron beam is predetermined for a very large number of deflection positions.

If for example material from an elongate vaporising crucible is to be deposited on a continuously moving strip, flux densities or dwell times which differ from place to place lead to an undesirable "stripe" pattern on the moving strip which cannot be tolerated for numerous end uses. In order to maintain definite deposition conditions over the whole width of the strip - this is an important requirement for a homogeneous coating thickness distribution transverse to the direction of travel of the strip-it is necessary to have a constant flux density over the entire deflection zone of the electron beam spot.

The invention is based therefore on the problem of providing a control method of the kind described above by which in the whole target area or over the whole deflection region an electron beam spot will maintain a predetermined flux density.

The solution of this problem is produced in the above described method, according to the invention, in that an initial adjustment is made of the focussing condition of the electron beam to a predetermined spot geometry by means of variable deflection fields, the focussing data corresponding to this spot geometry being stored with the relevant position data in a memory and, in use, the focussing data for each beam position is recalled from memory and the beam focussing controlled accordingly.

This means in practice that to set up the equipment the electron beam is brought into successive, defined deflection positions by means of definite deflection currents in the deflection unit and the focussing condition or spot geometry is manually adjusted, under visual inspection, by a potentiometer. Reference will be made below to the apparatus needed to do this. At each deflection position, defined by X-Y-coordinates, the electrical data defining the focussing condition are determined and placed into the memory locations for the focussing data belonging to the relevant deflection position by actuating an input sensor. This process is time-consuming, depending upon the degree of precision required; after a single adjustment the apparatus can however hold the unchanging stored data over a very long period of time.

In automatic operation of the apparatus the focussing data belonging to each beam position are then recalled from memory according to the deflection position in question and the beam focussing controlled accordingly. This process is effected cyclically or periodically with a corresponding frequency, which can readily go up to 1000 Hz.

In vacuum deposition for example a suitably high deflection frequency has the effect that the deposition process, despite the discontinuous impingement of the electron beam on the surface of the material to be vaporised, proceeds practically continuously because of the heat capacity of the melt. With the solution according to the invention the problem posed is overcome to the fullest extent, that is to say, over the whole deflection region of the electron beam, the spot is maintained at a predetermined flux density and the negative influence of different beam deflections is automatically and rapidly corrected in each deflection position. This makes possible the maintenance of a precisely predetermined interaction between the electron beam and the target material at each deflection position.So, for example, in welding, weld locations (spotwelds) always have the same propertjes.

In vacuum deposition from an elongate crucible of which the long axis lies transverse to the direction of movement of a travelling strip, coatings will have an extraordinarily good homogeneity with regard to coating thickness and coating composition. This is especially noteworthy if the same strip is passed over a plurality of vaporising crucibles from which different vaporised materials are released. In particular however the control method according to the invention is characterised by a high degree of reproducibility of the processing parameters concerned.

The control method according to the invention also makes possible a more compact construction and thus more economical apparatus. While it has up to now been necessary to keep the distance between the electron gun and the target area as large as possible, in order to keep the deflection angle correspondingly small, it is possible, with the control method according to the invention, to move up the target area closer to the electron gun and use a larger maximum deflection angle.

This leads to equipment dimensions of a small constructional volume which can also be more quickly and inexpensively evacuated.

Obviously with the invention the relative dwell times in each deflection position can be selected also to be different. Since the relative dwell duration is no longer a correction factor for different focussing conditions, the relative dwell times no longer need to be altered so that once set, the scanning frequency remains constant. Thus it is especial!y useful in vacuum deposition to select the relative dwell times in the deflecting positions at the ends of the crucible to be longer because at these positions there is an increased heat requirement as a result of the additional water cooling provided here on the end walls of the crucible. The relative dwell times needed for each deflection position are predetermined through a program control which follows exactly the focussing control according to the invention.The adjustment of definite dwell times by a program control is already part of the prior art (DE-OS 28 12 285) and will not be further described here.

Aside from the end walls of a deposition crucible it is however desirable as a rule to maintain an electron beam spot with a constant or identical flux density. Such a measure is in fact indispensible for the production of identical spot welds or drilled bores.

The invention relates also to an apparatus for carrying out the method described above.

Such an apparatus has as usual an electron gun with a beam source, a focussing unit and a deflection unit for deflecting the beam on X Y-coordinates. The apparatus has furthermore a control unit for deflecting the electron beam into the individual deflection positions defined by the X-Y-coordinates while maintaining definite dwell times.

For the solution of the same problem according to the further invention (a) the electron beam gun is provided in the path of the.electron beam with a multi-pole arrangement by which the electron beam is focussable with a frequency up to 1000 Hz, and further comprises (b) the control unit memory locations for the X-Y-coordinates of the deflection positions and memory locations for the focussing data associated with each deflection position as well as a microprocessor, by which the movement of the beam into the individual deflection positions and the dwell times as well as the focussing conditions in the individual deflection positions are controllable according to the information recalled from the memory locations.

The use of multi-pole arrangements, socalled four pole and eight pole arrangements, is known in electron microscopes, but these multi-pole arrangements are in those cases static that is to say used with unmodulated current.

Further advantageous features of the invention are to be found in the remaining subclaims.

One embodiment of the subject of the invention will now be further described with reference to Figures 1 to 7.

They show: Figure 1 a schematic representation of an electron beam gun in combination with a block diagram of the associated control unit for heating a vaporising crucible, Figure 2 is a plan view of a target area with deflection positions arranged in two rows, Figure 3 is a plan view of a target area with deflection positions arranged in a cirle, Figure 4 is a part sectional view of an electron gun in the region of the beam path, Figure 5 is an axial section through the lower part of Figure 4 in which the section planes stand at right angles to one another according to line V-V of Figure 7, Figure 6 is a radial section through the subject of Figure 5, along the line VI-VI, and Figure 7 is a radial section along the line VII-VII of Figure 5.

In Figure 1 is set out a very diagrammatic representation of an electron beam gun 1, with which are associated a beam source 2, an electromagnetic focussing unit 3, the multipole arrangement 4, according to the invention, and a suitable deflection unit 5 for deflecting the beam on X-Y-coordinates. Details of such an electron gun will be explained below with reference to Figures 4 to 7.

The electron gun 1 has an associated control unit 6 of which the main component is a microprocessor 7. With this microprocessor is associated on an input unit 8 with a keyboard 9 for inputting control instructions etc. The input unit has furthermore adjustment devices 10 and 11 for influencing the focussing data by means of the multi-pole arrangement 4.

The adjustment devices 10 and 11 are symbolised by potentiometer knobs.

To the microprocessor 7 is, moreover, connected a memory unit 12 having memory locations 13 for the X-Y-coordinates of the deflection positions and memory locations 14 for the focussing data associated with each deflection position.

Connected to the microprocessor 7 is a control circuit 15 for controlling the multi-pole arrangement 4 as well as a control circuit 16 for the control of the deflection unit 5.

The deflection program including the relative dwell times in the individual deflection positions cari also be loaded by means of the input unit 8 and the microprocessor 7.

Beneath the electron gun 1 is shown an elongate vaporising crucible 17 which is filled with a material 18 to be vaporised. The electron beam emerging from the deflection unit 5 is now deflected periodically over the surface of the material 18, and takes up discrete deflection positions F1 to Fn according to the above described program, here symbolised by black lines. The two extreme deflection positions of the electron beam are shown by broken lines 19 and 20. It is apparent that the extreme conditions are at the deflection positions F, and Fn. An electron beam, which in the middle of the deposition crucible 17 has a circular spot has, at the positions F1 and Fnt a strongly distorted or ellipticall spot geometry.

This is where the invention comes in and for each of the deflection positions by means of the adjusting devices 10 and 11, the multipole arrangement is adjusted in such manner that the spot geometry at all impingement points corresponds to a predetermined, preferably identical, focussing condition. The focussing data worked out for each deflection position are input into the memory locations 14 by operating the keyboard 9 and held ready for recall. The surface in which all the deflection positions lie is shown as a target area 21; it is bounded in Figure 2 by the inner rectangule 22 which represents the inner containing wall of the vaporising crucible 17.

In Figure 2 can be seen a deflection pattern made up from a total of twenty four deflection positions, in which the discrete deflection positions are shown as shaded circles. The diameters of the shaded circles correspond to the diameter of the electron beam momentarily stopped in the relevant deflection position. As illustrated, the twenty four deflection positions are distributed in two rows, each with twelve deflection positions, but the length of a row or column and the number of deflection positions therein is practically selectable at will, and also the number of rows or columns is essentially freely selectable. The locations of the individual deflection positions, more accurately the mid-points of the individual deflection positions, are determined by X-Y-coordinates which proceed from an origin P.The advance of the electron beam is effected by stepwise adjustment of the deflection current of the deflection coils involved at any one time. The deflection current in the X-direction is represented by a staircase function with twelve steps. Details of such beam deflection are however prior art and will not be further explained therefore. The required X-Y-coordinates are input into the memory location 13.

As soon as the defined beam position is reached the relevant focussing data are recalled from the memory locations 14 and the beam correspondingly corrected, whereby the extremely uniform surface pattern shown in Figure 2 is achieved.

In Figure 3 is shown that individual deflection positions F, to F6 can be arranged on a circle, which is provided for example in a rotationally symmetrical vaporising crucible 1 7a as a target. In this case the target area 17 is a circular surface which is bounded by the inner wall of the vaporising crucible. Also here it happens that in each deflection position F1 to F6 in any location the same focussing condition, that is to say the same beam diameter with absolutely circular form of the beam cross-section, is to be produced. Assuming a uniform beam flux there will be here a constant flux density in all cases.

Figure 4 shows the important parts of a practical embodiment of an electron beam gun 1. In the beam source 2, of which only the Wehnelt-electrode 22 is visible, there is a cathode, not shown here, which emits an electron beam along the axis A-A of the gun.

The acceleration of the electron beam is produced in the usual way by an accelerating anode 23 which projects beyond a funnel shaped connector 24 in the direction towards a beam guide tube 25, of which here only the outlines are indicated in broken line. Beneath the accelerating anode 23 is a shut-off valve 26 which is pivotable out of the beam path by a control shaft 27. The beam source 2 is surrounded by a vacuum tight double wall outer tube 28 so that necessary operating vacuum can be maintained in the region of the beam source 2.

A locating flange 29 is associated furthermore with the electron gun 1 to which a casing tube 30 is secured below in which all the electro-optic parts are assembled. Here is found also the deflector unit, further explained below with reference to Figures 5 and 7, of which in Figure 4 only the pole shoes 31 and 32 are to be seen projecting from below the casing tube.

From Figure 5 which is to a larger scale, it is seen that the beam guide tube 25 and the casing tube 30 are connected together by an annulus 33. At the upper end of the casing tube 30 there is moreover a flange 34 with which the casing tube 30 is tightened against the fixing flange 29 which extends radially inwardly up to the beam guide tube 25. In this way a hermetically sealed annular space 35 is formed between the beam guide tube 25 and the casing tube 30, in which an annular case member is arranged, having annular spaces 36 and 37, having an inner wall 38, an outer wall 39 and an annular part 40. The respective parts are connected by the securing flange 29. In this case member are arranged the focussing unit 3, the multi-pole arrangement 4 and the deflection unit 5.A cooling medium is flowed through the annular spaces in order to hold the working temperatures of the electrooptical apparatus at the lowest possible level.

The multi-pole arrangement 4, in the present case a four pole arrangement, is arranged directly beneath the focussing unit 3 and opens up the possiblity to force the electron beam in the plane given by section line VI-VI into a beam cross-section which produces at the target area a spot with predetermined geometry.

The influence of four pole arrangements on the electron beam is known per se and will not be further described therefore.

As can be seen from Figure 6, the multipole arrangement 5 comprises an annular closed magnet yoke 41 with recesses 42 in which coil cores 43 with pole shoes 44 and magent windings 45 are arranged. The cores 43 are connected to the magnet yoke 41 by screwed spigots 46. It will be understood that the beam guide tube 25 and the inner wall 38 are made from non-magnetic material.

In Figure 7 is shown in detail the deflection unit 5. Diametrically opposed spool cores 48 and 49 with pole shoes 50 and 51 as well as magnet windings 52 and 53 are arranged on a magnet yoke 47. Displaced by 90 from them are coil cores 54 and 55, on which are magnet windings 56 and 57. The cores 54 and 55 are extended through the annulus 33 and each carry opposite this annulus pole shoes 31 and 32 (Figures 4 and 5). Details of the focussing unit 3 and the deflection unit 5 are however known in the prior art so that further description thereof is unnecessary.

The coil core 43 of the multi-pole arrangement 4, depending on the frequency with which the variable focussing must be effected, are laminated or made out of transformer stampings. This allows focussing at a correspondingly high frequency which would not be possible with the focussing device 3. With an eight pole arrangement the number of poles would be doubled as compared to Figure 6, the pole separation being in each case 45". In the multiple arrangements like poles are always opposed, so that the magnet fields do not run diametrically through the axis A-A of the gun. On the other hand, each pair of adjacent poles is energised so that they form opposite poles. In use of a four pole arrangement, the control circuit 15 must have two independent current outputs, while an eight pole arrangement must have four independent current outputs.

Claims (7)

1. Method for controlling the focussing condition of an electron beam deflected periodically over a plurality of discrete deflection positions within a common target area, characterised in that an initial adjustment is made of the focussing condition of the electron beam to give a predetermined spot geometry by means of variable deflection fields, the focussing data corresponding to this spot geometry being stored with the relevant position data in a memory and, in use, the focussing data for each beam position is recalled from memory and the beam focussing controlled accordingly.

2. Apparatus for carrying out the method of claim 1, using an electron gun having a beam source, a focussing unit and a deflection unit for deflecting the beam on X-Y-coordinates within a target area and a control unit for the deflection of the beam in the individual deflection positions defined by the X-Y-coordinates while maintaining a definite dwell period, characterised in that (a) the electron gun has a multi-pole arrangement in the beam path by which the beam can be focussed with a frequency up to 1000 Hz, and that (b) the control unit contains memory locations for the X-Y-coordinates for the deflection positions and memory locations for the focussing data associated with each deflection position, as well as a microprocessor, by which the movement of the beam to the individual deflection positions and the dwell times as well as the focussing conditions in the individual deflection positions are controllable according to data recalled from the memory locations.

3. Apparatus according to claim 2, characterised in that the multi-pole arrangement is a four pole arrangement.

4. Apparatus according to claim 2, characterised in that the poles of the multi-pole arrangement have laminated coil cores with pole shoes.

5. Apparatus according to claim 2, in which the electron gun has a cylindrical cavity bounded by a beam guide tube and a casing in which the focussing unit and the deflection unit are arranged, characterised in that the multi-pole arrangement is arranged in the same cavity between the focussing unit and the deflection unit.

6. A method for controlling the focussing condition of an electron beam substantially as hereinbefore described with reference to the accompanying drawings.

7. Apparatus for controlling the focussing condition of an electron beam substantially as hereinbefore described with reference to the accompanying drawings.