FR2568720A1 - METHOD AND DEVICE FOR CONTROLLING THE FOCUSING STATE OF A DEVIETED ELECTRON BEAM - Google Patents

Abstract

THIS IS TO CONTROL THE FOCUSING STATE OF AN ELECTRON BEAM PERIODICALLY DEVIATES ON DIFFERENT SCAN POSITIONS ON A TARGET SURFACE IN ORDER TO OBTAIN A PREDETERMINED POWER DENSITY IN ALL THESE POSITIONS. THE ELECTRON CANNON 1 CONSISTS OF A BEAM GENERATOR 2, A FOCUSING UNIT 3 AND A DEVIATION UNIT 5, TO WHICH IS ASSOCIATED A CONTROL UNIT 6. THE FOCUS STATE OF THE BEAM IN EACH POSITION (F. ..F), BY MEANS OF VARIABLE DEVIATION FIELDS, TO OBTAIN A GIVEN GEOMETRY OF THE FOCUS TASK. FOR EACH TASK, THE FOCUS DATA IS STORED WITH THE POSITIONS DATA IN A MEMORY 12. IN OPERATION, THE CONTROL UNIT QUESTS THE MEMORY AND CORRECTS THE FOCUS ACCORDINGLY.

Description

i

  The invention relates to a method for controlling

  of the focusing state of an electron beam deviated periodically over a large number of positions of

  discrete scanning contained in a target surface com-

mune.

  Such a process can be used for welding

  ge or drilling with an electron beam when

  that the work points are arranged, for example,

  We have a screened distribution in the target surface. The pro-

  yielded can also be used for super quenching

  by means of electron beams, as well as in

  the case of fusion by electronic bombardment during

  that it is a question of injecting the power of the beam of elect

  sections in the molten bath with a defined distribution on the surface (R.F.A. patent application published under number 28 19 285). As shown in the cited document, the method according to the invention can also be used

  with a particular advantage in the evapora-

  tion of materials contained in a crucible, when it is important to ensure that the layers condensed from the vapor a composition which remains uniform in the

  limits of a large condensation zone.

  In all cases, the electron beam pro-

  duit on its impact surface, which is designated here by

  target surface expression, a defined surface pattern

  nor which consists of a large number of positions of ba

  discreet layering. The pattern is made up of distinct areas

  your juxtaposed in one or two dimension (s), and we

  can obtain a defined power contribution in its location

  reading by the size of these areas and by time

  stopping the electron beam in each of the zones.

  Whereas, in the case of welding or drilling

  executed by means of an electron beam, it is often

  haitable to obtain beam data as identi-

  as possible in all scan positions.

  It may be desirable in the case of evaporation 2 L o2568720

  under vacuum to determine an evaporation power

  reliable with the location by a contribution of power which varies with the location, to compensate for faults

  of homogeneity, attributable to other causes, which

  festering in the vapor condensation area.

  If there is a relative movement

  mechanical tif between the electron gun and the surface

  target, the beam deflection remains unchanged, so

  that the focusing state also remains maintained.

  However, the relative mechanical displacement is long and, therefore, it is unusable in many cases, or even it can be of a difficult execution and

  therefore linked to a great complication. It is thus

  if, for example, in the case of fusion or evaporation, it is excluded, for obvious reasons,

  to move the crucible. However, moving the ca-

  not electron is extremely complicated.

  It is for these reasons that, in the past, we mainly took advantage of the property of the electron beam to be deflected more or less strongly

  using electromagnetic fields or electrostati-

  ques, so it is possible to achieve a large number of discrete scanning positions within a relatively large target area without

  have to execute a mechanical relative movement. The main-

  cipe of such a process is known by cathode-ray tubes

  TV ques. However, this technique has a serious drawback in that the quality of the focusing state decreases with the increase in

the angle of deviation.

  It is already known from the R.F.A. patent application published under No. 20 47 138 that an electron beam emitted by a stationary electron source and which

  is diverted to different discrete scanning positions

  tes, can have a very variable section on zo-

  nes of impact, so the power density on

3 2568720

  the impact areas vary accordingly. This constitutes

  an undesirable phenomenon and that is why it is propo-

  in the above document to compensate for variations

  of the state of focusing on the different areas of im-

  pact by variable downtime. However, the effects obtained are extremely limited and this measure is unusable for many cases in which it

  is important to get an exact focus of the beam

  hoop, or a defined diameter of the beam at the point

  impact of the electron beam. In particular, the me-

  above is unusable when it is not possible

  ble to vary the relative downtime of the

  electron beam in the different positions of ba

  discreet layering because, for example, the time of ba-

  layering of the electron beam is predetermined for a

  very large number of scanning positions.

  When, for example, using a crucible

  of evaporation in an elongated form, substances are evaporated

  these on a strip that we scroll in moving con-

  continuous above the evaporation crucible, local differences in power density or electron beam stopping time result in an undesirable pattern of "scratches" on the moving band, pattern

  which is unacceptable in many use cases.

  To obtain evaporation conditions defined on

  the entire width of the strip - which represents a con-

  essential edition to ensure a homogeneous distribution

  of the thickness of the layer in the transverse direction

  when the tape runs - we need a focal spot of electron beam with a density of

  constant power over the entire scanning range.

  The invention therefore aims to provide

  a control method of the kind described at the beginning which allows

  set to get a focal spot of electron beam with a predetermined power density over the entire

  target area, i.e. over the entire scanning range

4 2568720

ge.

  According to the invention, the resolution of the problem po-

  se is obtained, in the process described at the start, by the fact that at the start, the focusing state of the electron beam is adjusted to a focal spot geometry

  predetermined in each of the available scanning positions

  cretes, by means of variable deflection fields-, we

  morise the focusing data corresponding to this geometry of the focal spot in a memory, with the

  corresponding position data and during operation

  operation, we call in memory for each position

  tion of the beam, the corresponding focusing data

  dantes and we correct the focus control of the

beam accordingly.

  In practice, we proceed as follows: when adjusting the device, we bring the beam

  of electrons successively in scanning positions

  ge defined by deflection currents defined in the deflection unit and manually adjusted

  the focusing state or the geometry of the focal spot

  manually by means of a potentiometer, under observation

  visual vation. We will come back with more precision

  below on the conditions of construction of the devices

  positive to achieve to achieve this result. For each

  that scanning position, which is defined by its co-

  X-Y data, we calculate the defined electrical data -

  sant focus state and, by pressing an input key, we introduce in the locations of

  memory focusing data belonging to the po-

  scanning position considered. This procedure is

  gue, all the longer as the required precision is

  larger, but after the initial setting of the device

  tif, the stored data can be kept

constant for a very long time.

  In the automatic operation of the device

  tif, the focusing data belonging to each position of the beam are called in the memory, in

  depending on the scanning position considered, and the fo-

  beam calibration is corrected accordingly. This-

  The procedure takes place cyclically or periodically with an appropriate frequency, which can reach without

disadvantages up to 1000 Hz.

  In vacuum evaporation, a frequency of

  such a high sweep results, for example, in that

  this at the thermal inertia of the molten bath, the evaporation process takes place in a practically continuous manner,

  despite the discontinuity of the surface bombardment

  that of the material to be evaporated by the electron beam.

  With the solution according to the invention, the problem posed is entirely solved, that is to say that over the entire range

  scanning of the electron beam, we get a ta-

  focal length of electron beam with density

  of predetermined power and that the unfavorable influence

  ble variations in the deflection of the electric beam

  in the different scanning positions is

  automatically canceled and with great speed. This

  helps maintain interactions exactly predeter-

  mined between the electron beam and the matter at-

  tint in each scan position. Thus, for example, in the case of welding, the welding zones (the welding points) always have the same properties. In the case of evaporation under vacuum of substances contained in shaped crucibles

  elongated, whose longitudinal axis extends transverse-

  lying to the direction of travel of a moving strip, we

  obtains layers with extreme homogeneity

  great thickness with regard to the layer thickness

  and the composition of the layer. This is an advantage by-

  particularly noticeable when the same strip scrolls

  successively above several evaporating crucibles

  tion where it is a question of evaporating several different materials

  annuities. However, the control method according to the invention

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  tion is distinguished in particular by a high reproduc-

  the parameters influenced.

  The control method according to the invention also makes it possible to build more compact devices and, consequently, of a more advantageous price. So

  that so far there has been a tendency to maintain as large a

  as far as possible the distance between the electron gun and the target surface, in order to be able to limit the scanning angle to even smaller values, it is now possible, with the control method according to the invention, to approximate the electron gun at a smaller distance from the target surface by accepting a larger maximum deflection angle. This leads to dimensioning the device with a lower volume,

  in which we can also make the vacuum faster-

  ment and with a lower expense.

  Naturally, in the subject of the invention, it is also possible to choose different values for the

  relative downtime in each sweep position.

  However, since the relative downtime does not

  is more of a corrective quantity for differences

  these states of focus, the relative down times no longer have to be modified, so that, in the case of a number calculated once and for all, the interrogation frequency remains constant. Thus, for example, in the case of vacuum evaporation, it is advantageous to choose for the relative stopping times in the scanning positions situated at the ends of the

  crucible longer values because at these

  rights, the heat demand is greater, due

  of the additional presence in these places of the pa-

  kings of ends of the crucible, which are cooled by

  the water. The relative downtime required for each

  that scanning position are predetermined by a com-

  program command that follows exactly the command of

  calibration according to the invention. The time setting

  rêt defined by a program command is already part of-

  state of the art (R.F.A. patent application published under number 28 12 285) and will therefore not

  described in detail in this memo.

  However, outside the ends of a hollow

  evaporation set, it is generally desirable to obtain

  ning a focal spot of an electron beam having

  a constant or identical power density in all

  your scan positions. This measure is even in-

  dispensable when welding points have to be produced

or identical holes.

  The invention also relates to a device

  tif for the implementation of the method described above.

  This device includes, as in the classic construction-

  sique, an electron gun with a generator

  beam, a focusing unit and a focusing unit

  viation used to deflect the beam according to the co-

  ordinates X-Y. The device further includes a control unit for deflecting the electron beam into the different individual scanning positions defined by X-Y coordinates, while maintaining

  defined downtime. Consequently, according to other ca-

  characteristics of the invention and to solve the same problem,

  a) the electron gun present on the path

  electron beam roof, a multi-pole device

  re by means of which the electron beam can be cor-

  rigged in focus with a frequency of up to

dre 1000 Hz and, moreover ,.

  b) the control unit includes places

  memory for X-Y coordinates of positions

  and memory locations for data

  focal points corresponding to each scanning position, as well as a microprocessor by means of which the

  beam movement which brings it to different positions

  scan times, and downtime and

8 2568720

  focus states in the different scanning positions can be controlled according to the query

memory locations.

  The use of multipolar devices, called quadrupoles or octopoles in electron microscopes, is certainly known but, in these cases, these multipolar devices are used

  statically, i.e. with high intensity.

  According to another characteristic of the invention, the poles of the multipolar device have

  coil cores made of sheets with parts for

  laires. According to another characteristic of the invention,

  in a device in which the electron gun pre-

  feels a cylindrical cavity delimited by a tube of

  beam guide and an envelope tube, and in which

  the focusing unit and the deflection unit are arranged, the multipolar device is placed in the same cavity, between the focusing unit and the unit

of deviation.

  Other features and advantages of the

  vention will be better understood on reading the description

  tion which will follow from an exemplary embodiment of the in-

  vention and its mode of operation, with reference to the accompanying drawings in which, Figure 1 is a schematic representation of an electron beam, as well as a block diagram of the corresponding control unit for heating a evaporator crucible; Figure 2 is a top view of a target surface having scanning positions arranged in two rows;

  Figure 3 is a top view of a surface above

  ble having scanning positions arranged in a

circle; -

  FIG. 4 represents an electron gun in

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  part in section, in the region of the beam path; Figure 5 is an axial section of the part

  lower of Figure 4, the section planes being per-

  pendulars between them, along the line V-V of Figure 7; Figure 6 is a radial section of the object of Figure 5, taken along the line VIVI; and Figure 7 is a radial section taken along

line VII-VII in Figure 5.

  In FIG. 1, a schematic representation has been given of an electron gun 1 of which a

  beam generator 2, an electric focusing unit

  tromagnetic 3, the quadrupole device 4 according to the invention, and a traditional deflection unit 5

  for the deflection of the beam in the X-Y coordinates.

  The details of such an electron gun are exposed from

  more precisely with reference to FIGS. 4 to 7.

  The electron gun 1 is associated with a unit

  control 6 whose central part is a microprocessor

  sister 7. With this microprocessor is associated an input unit 8 equipped with a keyboard 9 for the introduction of control commands, etc. The input unit comprises

  in addition to adjustment devices 10 and 11 intended for

  fluence focusing data by means of the device

  multipole tif 4. Adjustment devices 10 and 11

  are symbolized by potentiometer buttons.

  To the microprocessor 7 is further connected a memory unit 12 provided with memory locations 13 for the X-Y coordinates of the scanning positions and

  of memory locations 14 for focal data

  sation belonging to each scan position.

  Downstream of the microprocessor 7, a control circuit 15 for controlling the multipolar device 4 is connected, as well as a control circuit 16 for

  piloting the deflection unit 5.

2568720_

  The scanning program, including times

  relative stops in the various scanning positions

  ge, is also imposed by means of the input unit 8

and microprocessor 7.

  Below the electron gun 1, an elongated evaporator crucible 17 is shown which is filled with a substance to be evaporated 18. The electron beam leaving the deflection unit 7 is then deflected periodically on the surface of the substance to

  evaporate 18, this beam taking, depending on the program-

* described to me above, discrete scanning positions

  F1 to Fn which are symbolized here by black lines.

  The two extreme scanning positions of the electron beam are indicated by dashed lines 19 and 20. It is observed that, in the scanning positions F1 and Fn, there are extreme conditions. A beam

  of electrons which still has a focal focal spot

  area in the middle of the evaporator crucible 17, would have

  highly distorted or elliptical focal spot geometries

  ticks in positions F1 and Fn. This is where

  comes the invention, which works in such a way that for

  each of the scanning positions, the multi-device

  area 4 is adjusted by means of the adjusting devices and 11 in such a way that the geometry of the focal spot corresponds in all the impact zones to

  determined focusing states, preferably identi-

  ques. The focus data obtained in each of the various scanning positions is entered into the memory locations 14 by actuation of the keyboard 9 and they are kept ready to be called up in these locations. The area where all the scanning positions are located is called the area

  target 21; in figure 2, it is surrounded by the rectan-

  inner rail 22, which represents the inner boundary wall

  of the evaporator crucible 17.

  In FIG. 2, one can recognize a scanning pattern composed of a total of twenty-four scanning positions, and in which the scanning positions

  discrete are represented by the hatched circles.

  The diameters of the hatched circles correspond to the diameter

  meter of the electron beam immobilized briefly in

  the scanning positions considered. In the case shown

  felt, the twenty-four scan positions are repaired

  ties in two rows of twelve scanning positions each

  cune but the length of a row or line and the

  number of scan positions found there

  can be chosen practically at will, and the number

  rows or lines can also be chosen free-

  is lying. The position of the different scanning positions, more precisely the centers of the different scanning positions, is determined by the X-Y coordinates which

  originate from an origin or reference point P. The de-

  placement of the electron beam is effected by modifi-

  stepwise cation of the deflection current of the corresponding deflection coil. The variation of the deflection current in direction X can be represented by a stepped curve with twelve steps. The details of such a beam deflection are part of the state of the art and will therefore not be explained in more detail. The corresponding X-Y coordinates are entered in the memory locations 13. As soon as the beam has reached the defined position, the data of

  corresponding focal points are called in em-

  memory placements 14 and the beam is corrected accordingly, so that the surface pattern is obtained

  extremely regular shown in Figure 2.

  With reference to FIG. 3, it has been shown that the different scanning positions F1 to F6 can also be arranged on a circle, when we have to

  bombard an evaporator crucible 17a having a sy-

  metrics. of rotation. In this case, the target surface 17 is

  a circular surface which is limited by the internal edge

12 2568720

  laughing of the evaporator crucible. Here, too, it is im-

  bearing in obtaining in each scanning position F1 and F6, the same focusing state, that is to say the same

  beam diameter, with an absolutely circular shape

  re of the beam section. If we assume a constant beam power, we get this way

  in all cases a unit of constant power.

  Figure 4 shows the essentials

  of a practical embodiment of an electric cannon

  sections 1. In the beam generator 2, of which

  sees that the Wehnelt electrode 22, is a catho-

  de, not shown in the drawing, which emits an electron beam along the axis A-A of the gun. The acceleration of the electron beam is carried out, in the conventional way, by an accelerating anode 23 which is extended by an intermediate piece 24 in the form of a funnel in the direction

  a beam guide tube 25, which has not been shown

  felt that the outlines, in broken lines. Above-

  under the accelerating anode 23, there is also a stop valve 26 which can be moved away from the trajectory

  of the beam by means of an operating shaft 27.-The general

  beam rector 24 is surrounded, with seal tight to the

  empty, for an outer tube 28 with double wall, for

  see maintain the necessary operating vacuum

  in the region of the beam generator 2.

  The electron gun 1 also has a bri-

  fixing 29 which is followed downwards by a

  envelope tube 30 in which all the elements are housed

  electronic optics. The barrel includes in-

  be the deflection unit, which will be described in more detail with reference to FIGS. 5 and 7 and of which only pole pieces 31 and 32 are seen in FIG. 4

  which emerge from the envelope tube downwards. -

  It can be seen in FIG. 5, drawn on a larger scale, that the beam guide tube 25 and the envelope tube 30 are connected to each other by a

13 2568720

  annular part 33. At the upper end of the tube

  veloppe 30 is further located a flange 34 by means of which the envelope tube 30 is clamped against the fixing flange 29 which extends radially inward to the tube 25 for guiding the beam. In this way, it is formed between the beam guide tube 25 and

  the tube envelops 30 a hollow cylindrical space 35 her-

  metrically closed, in which, a cylindrical sleeve

  hollow having an inner wall 38, an outer wall

  39 and an annular part 40, is arranged leaving

  free from the annular slots 36. The elements

  strips are connected to the fixing flange 29. It is in this sleeve that the focusing unit 3, the multipolar device 4 and the deflection unit 5 are housed. The cavities are traversed by a cooling fluid in order to maintain operating temperatures of electronic optical devices at such a low level

as possible.

  The multipolar device 4, which is a quadri-

  pole in the case considered, is arranged immediately below the focusing unit 3 and it provides the possibility of imposing on the electron beam, in the plane indicated by the section line VI-VI, a section of

  beam which produces a spot in the target surface

  wedge having the predetermined geometry. The influence of the quadrupole on the electron beam is known in

  itself and therefore will not be explained in more detail

if we.

  As can be seen from FIG. 6, the arrangement

  multipolar tif 4 is composed of a magnetic yoke 41 closed on itself in a ring, provided with recesses 42

  in which are arranged cores of coils 43 mu

  nis of pole pieces 44 and of electric windings

  magnets 45. The coil cores 43 are assembled to the cylinder head 41 by studs 46. It goes without saying that the bundle guide tube and the interior wall 38 are

14 2568720

14 ooc

made of non-magnetic material.

  In FIG. 7, the deflection unit 5 is shown

  presented in detail. On a magnetic yoke 47, are diametrically opposed coil cores 48 and 49, provided with pole pieces 50 and 51 as well as

  of electromagnet windings 52 and 53. In posi-

  tions offset by 90 relative to these cores, are dis-

  laid coil cores 54 and 55 on which are wound electromagnet windings 56 and 57. The coil cores 54 and 55 are extended through the annular part 33 and carry the pole pieces 31 and 32 beyond this annular part (see Figures 4 and). However, the features of the focusing unit 3 and the deflection unit 5 are part of the state of the art so that one can dispense with

  to expand further on their subject.

  The coil cores 43 of the multipo- device

  4 are made in a laminated form, or in

  transformer sheets, depending on the frequency with which

  the variable focus correction must be exe

  cut. They allow in this way to achieve a cor-

  focusing rection with the desired high frequency

  that it would not be possible to achieve with the device

  focus 3. In the case of an octopole, the name-

  bre of magnetic poles would be doubled compared to that shown in Figure 6 and the pole spacing would be 45. In multipolar devices, the poles of the same name are placed face to face, so that the magnetic fields do not cross the axis A-A of the barrel diametrically. On the other hand, two coils

  of neighboring electromagnets are traversed by the cou-

  rant so as to form opposite poles. In the use-

  sation of a quadrupole, the control circuit 15 must have two independent current outputs and, in

  if using an octopale, it must have

  from four independent current outputs.

  z o2568720 Of course, various modifications may be made by those skilled in the art to the device which

  has just been described only by way of non-binding example

  mitative without departing from the scope of the invention.

16 2568720

Claims (4)

CLAIMS R E V E N D I C A T I O N S   1 - Method for controlling the state of focus   tion of an electron beam deflected periodically on   a large number of discrete scanning positions   naked in a common target surface, characterized in that at the beginning, the focusing state of the beam is adjusted   electron beam on a predefined focal spot geometry   completed in each of the discrete scan positions   tes, by means of variable deflection fields, we memorize   focus data corresponding to this   focal spot geometry in a memory, with the data   born from corresponding positions and, during operation   in the memory, for each position,   tion of the beam, the corresponding focusing data   and correct the focusing of the beam in con- sequence.   2 - Device for implementing the process   die according to claim 1, using an electro-cannon   which includes a beam generator, a focusing unit and a deflection unit for deflecting the beam in the X-Y coordinates within the limits of the target area, and a control unit   used to deflect the electron beam in the different   scan positions defined by the coordinates   born X-Y, retaining defined downtime, characteristic terrified in that:   a) the electron gun (1) present on the tra-   jectory of the electron beam, a multipo-   area (4) by means of which the electron beam can be corrected in focus with a frequency of up to 1000 fz, and in that   b) the control unit (6) comprises places   memory elements (13) for the X-Y coordinates of the positions   scanning and memory locations (14)   for focusing data belonging to the posi- 17 2568720   scanning position, as well as a microprocessor (7) by means of which the movement of the beam, which brings it to the different scanning positions, and the stopping times as well as the focusing states in the different scanning positions can be ordered from   interrogating the memory locations (13, 14).   3 - Device according to claim 2, charac-   terized in that the multipolar device (4) is a quadrupole device.   4 - Device according to claim 2, charac-   terized in that the poles of the multipolar device have coil cores (43) composed of sheets and pole pieces (44).   - Device according to claim 2, in   which the electron gun has a cylindrical cavity   only limited by a beam guide tube and a tu-   be envelope and in which are placed the focusing unit and the deflection unit, characterized in that the multipolar device (4) is arranged in the same cavity (35), between the focusing unit (3) and the deflection unit (5).