WO2009100877A1

WO2009100877A1 - An electron beam unit and method for adjusting electron beams generated by an electron beam unit

Info

Publication number: WO2009100877A1
Application number: PCT/EP2009/000919
Authority: WO
Inventors: Bernhard Jung; Klaus-Rainer Schulze
Original assignee: PTR Präzisionstechnik GmbH
Priority date: 2008-02-15
Filing date: 2009-02-10
Publication date: 2009-08-20
Also published as: DE102008009410A1; DE102008009410B4; EP2253006A1

Abstract

The electron beam unit comprises an electron beam generator (1) for generating an electron beam along an electron beam axis (5), an adjusting and focusing unit (2) for adjusting and focusing the electron beam in a focus plane (6), which adjusting and focusing unit (2) is arranged on the electron beam axis (5) downstream of the electron beam generator (1), and a control (3) for controlling the adjusting and focusing unit (2). The control (3) is adapted to control the adjusting and focusing unit (2) for shifting of the focus plane (6) along the electron beam axis (5) such that the focus plane is shifted back and forth at a predetermined frequency. The orientation of the electron beam axis (5) can be adjusted with the help of the adjusting and focusing unit in such a way that the impact points displayed by the electron beam impact point display unit (4), which is arranged on the electron beam axis (5), form a circular area.

Description

AN ELECTRON BEAM UNIT AND METHOD FOR ADJUSTING ELECTRON BEAMS GENERATED BY AN ELECTRON BEAM UNIT

The present disclosure is related to an electron beam unit and a method for adjusting electron beams generated by an electrode beam unit.

In electron beam welding, a beam generated in a Cathode- Wehnelt-Electrode-Anode system is formed and deflected, respectively, by magnetic fields, in which case the focusing of the beam in the center of a current conducting coil has an important influence.

It is necessary, especially for the focusing, that the beam passes centrally through the focusing coil, so that rotationally symmetric fields influence the beam and only the position of the focus plane is changed in the focusing. If this is not the case, the beam is deflected in addition to the change of the focus of the beam, so that the impact point of the beam on a work piece "wanders" by varying the focusing coil currents.

In order to secure that the electron beam takes the desired central path through the focusing coil, it usually is passed through so-called centering and adjusting coils after the generation of and before the focusing of the electron beam. These coils generate two intersecting magnetic fields (x and y direction). The electron beam path can be adjusted by making appropriate changes to the strength of the currents in the two coils so that the beam path passes the focusing coil in the center of the magnetic field.

A problem exists in finding the centering currents for the correct adjustment. There are several known solutions for this problem.

Traditionally, the operator of an EB machine (EB = electron beam) closely monitors the impact point of the electron beam (low power) on a planar surface arranged perpendicularly to the electron beam direction. This can be a workpiece plane, or can also be a separate test body; advantageously a special screen can be used, e.g., a luminescent screen having a surface with a luminescent coating. Such a luminescent screen is used as an illustrative example in the following. An illuminated point is formed on the screen by a beam focussed exactly on its surface. The point expands with increasing defocusing, and vice versa. By so-called through-focusing procedures, that is, by the changing of the focusing plane from a position above the screen in a cyclical manner to a position below the screen, the light point "breathes". That is, it becomes smaller up to a point and then becomes again larger, and so on, in a cyclical manner.

According to the above-described dependency of the power level distribution and of the beam location on the quality of the centered (or uncentered) passage through the focusing coil, a lateral "wandering" of the luminescent image on the screen is generated in addition to the above- described "breathing". The "wandering" can be eliminated by correctly aligning the centering currents (or, with a less effective system, can be minimized).

The operator of the machine works in iterative manner in order to achieve the same: he chooses a given setting of the centering currents and performs a cyclical change of the focusing plane through the luminescent screen in the direction of the beam axis (through-focussing). Dependent on his observation of the changing of the image on the luminescent screen, he recognizes that one or more of the centering currents need to be changed in a particular direction, and he repeats the through-focusing. These iterations can - depending on the experience of the operator - consume a remarkable amount of time, which is a disadvantage for the availability of the EB machine. Further, inexperienced operators possibly may not even obtain the optimal centering, which can be critical for the welding result.

Therefore, an alternative is known that depends on the measuring of the electron beam along the beam axis.

A disadvantage of this method is the necessity of a significant installation of apparatuses. Such a measurement sensor apparatus cannot be installed permanently, in particular, in short cycle time production machines which usually have chambers with small dimensions, and, therein, complicated work piece mounting devices. Moreover, it may be quickly contaminated by effects of the operation of the machine. In order to be able to insert the sensor when necessary, it is not only required to have the necessary (and often unavailable) free workspace, but appropriate mountings and an electrical connection to the outside are also required. US 4,160,150 describes an electron beam welding apparatus adapted to reduce the drift of the fused material flowing from the welding zone by constantly changing the status of the focusing of the electron beams.

US 3,748,467 describes a scanning electron microscope with means for periodically changing the focal length of the final condensing lens between two values and a means for at the same time displaying two images corresponding to the two focal lengths.

US 7,075,076 B2 describes an inspection unit adapted to capture images of an inspection area by various focusing adjustments.

The present invention provides an improved technique for adjusting an electron beam apparatus.

It is an object of the present invention to describe an improved technique for adjusting an electron beam apparatus.

This object is solved by an electron beam apparatus according to claim 1, and a method of adjusting an electron beam according to claim 7.

Further embodiments of the invention are found in the dependent claims.

By shifting the focusing plane, a constantly recurring image of the breathing and - in the case of an uncentered alignment - wandering electron beam can be generated on a screen or on a work piece.

Advantageously, the frequency is chosen to be so high that a type of stroboscope effect is produced, so that the eye of the operator can no longer perceive the breathing. Then, a resulting light effect can be seen that spans the entire surface excited by the through-focusing. This excited surface typically appears to be dumbbell-shaped (two-dimensional), wherein the spheres, more specifically circular surfaces, are formed at the turning points of the wandering movement of the beam. The "bar" of the dumbbell is shown in the middle and is comparatively thin. Therein lies the focus. The described dumbbell image is deformed when the centering currents are changed. The dumbbell is turned and lengthened or shortened in the image plane. This happens in such a way that the operator can immediately, effortlessly, and, thanks to the stroboscope effect, almost statically evaluate the effects of his changes.

An optimal centering is achieved when the length of the bar of the dumbbell becomes zero and when both spheres (circles) come to cover each other, i.e., form a single circle.

This optimization can be simply manually implemented. The operator can thus change the centering currents by means of a potentiometer or a digital input and visually observe the effects. It is also possible to achieve this optimization by means of an automatic image processor. The image, which may be obtained by, e.g., an appropriate camera, can be processed by means of appropriate software, and the centering current can be automatically changed until the image exhibits the above-described optimal situation (covering of the two dumbbell spheres), i.e., forming of a single circle in the luminescent image.

The present teachings are especially adapted for the exact adjustment of the centering of an electron beam for short cycle time type production machines.

Further important and useful features are disclosed in the description of exemplary embodiments and the figures. The figures show:

Fig. 1 an embodiment of an electron beam apparatus;

Fig. 2 a block diagram of an embodiment of an electron beam apparatus;

Fig. 3 a principal sketch of an embodiment of an adjusting unit;

Fig. 4 a first example of luminescent images on a screen, which are produced by employing an embodiment of the invention; and Fig. 5 a second example of luminescent images on a screen, which are produced by employing an embodiment of the invention.

Fig. 1 shows an electron beam apparatus that comprises an electron beam generator 1 and an adjusting and focusing unit 2. The electron beam generator 1 generates an electron beam that propagates along an electron beam axis 5. The adjusting and focusing unit 2 is arranged on the electron beam axis 5 in the beam direction downstream of the electron beam generator 1. The electron beam generator 1 comprises, in the present embodiment, a cathode 11, an electrode 12, e.g., a Wehnelt electrode, and an anode 13. The adjusting and focusing unit 2 comprises, in the present embodiment, an adjusting unit 21 and a focusing unit 22. In the present embodiment, the adjusting unit 21 is arranged between the electron beam generator 1 and the focusing unit 22. An electron beam impact point display unit 4 is arranged on the electron beam axis 5 in front of the electron beam unit. In the present embodiment, the electron beam impact point display unit 4 is arranged perpendicularly to the electron beam axis 5. The electron beam is focussed in the focus plane 6 by means of an adjusting and focusing unit 2, in the present embodiment, by means of the focusing unit 22. The focus plane 6 is therefore a plane that is perpendicular to the electron beam axis 5 and contains the focus of the electron beam.

Fig. 2 shows a block diagram of an embodiment of the invention. The adjusting and focusing unit 2 comprises the adjusting unit 21 and the focusing unit 22, and is linked to a control 3. The adjusting and focusing unit 2 is, in the present embodiment, also linked with an image processing unit 7.

Fig. 3 shows an embodiment of the adjusting unit 21, which comprises a first adjusting coil 21 1 and a second adjusting coil 213. The first adjusting coil 211 comprises adjusting current contacts 212 and a second adjusting coil 213 comprises adjusting current contacts 214. The first adjusting coil 211 comprises, in the present embodiment, two coils that are arranged opposite to each other, 21 Ia (as shown on the left side of Fig. 3) and 21 Ib (as shown on the right on the side of Fig. 3). One of the adjusting contacts 212 is connected to the coil 211a, and the other adjusting current contact 212 is connected to the oppositely-arranged coil 211b. The two coils 21 1a and 21 1b are also connected to each other so that an electrical path between the adjusting current contacts 212 is formed through the two coils 21 1a and 211b. The second adjusting coil 213 also comprises, in the present embodiment, two coils 213a (shown on the bot- torn of Fig. 3) and 213b (shown on the top of Fig. 3) which are arranged opposite to each other and are connected to each other in a similar way so that an electrical path between the adjusting current contacts 214 is formed through the two coils 213a and 213b.

The adjusting unit 21 shown in Fig. 3 is, in the present embodiment of the electron beam unit, arranged in such a way that the electron beam axis 5 extends perpendicularly to the plane of Fig. 3 and through the area inside the coils 21 Ia, 21 Ib, 213a and 213b. Preferably, the intervals between the respective coils 211a, 21 Ib, 213a and 213b and the electron beam axis 5 are all the same.

Hereafter, the function of the described embodiment of the electron beam apparatus is described. In operation, the electron beam generator 1 generates an electron beam along the electron beam axis 5. The generated electron beam can be adjusted and focussed in a focus plane 6 by means of an adjusting and focusing unit 2. The adjusting of the electron beam is done in the present embodiment shown in the figures by adjusting the adjusting currents supplied to the adjusting current contacts 212 and 214. The adjusting currents generate two intersecting (crossing) magnetic fields between the coils 21 Ia and 211b of the first adjusting coil and the coils 213a and 213b of the second adjusting coil 213, which deflect the electron beam. In this way, the position in which the electron beam passes the focusing unit 22 can be determined by adjusting the adjusting currents. The focusing unit 22 is, by means of a control 3 (i.e., for example using a control program), controlled so that the focus plane 6 is displaced (shifted) back and forth along the electron beam axis 5 with a predetermined frequency. The shifting of the focus plane 6 is performed such that the focus plane 6 is shifted between a first plane, which is located between the focus unit 22 and the electron beam impact point display unit 4, and a second plane located on the other side of the electron beam impact point display unit 4 in the direction of the electron beam axis 5.

By this periodic through- focusing, the impact point (area) of the electron beam on the electron beam impact point display unit 4 displays a process of defocusing-focusing-defocusing- focusing, etc. If the predetermined frequency is high enough (e.g., > 15 Hz), the constantly changing impact points (areas) produce a "standing" (unchanging) image, e.g., as shown on Figs. 4 and 5, in the case of constant adjusting currents. The images, which can be seen in Figs. 4(a) and (b), are produced when the electron beam passes through the focusing unit 22 in a not absolutely centered manner. In this case, the focusing unit 22, in addition to the focus change, exerts a deflecting influence so that the impact point of the electron beam on the electron beam impact point display unit 4 wanders (laterally) caused by the variation of the focus coil current. In this way, the dumbbell-formed images are produced, as shown in Figs. 4(a) and 4(b). The length of the dumbbell is, in the case shown in Fig. 4(a), longer than the length of the dumbbell in the case shown in Fig. 4(b). The different dumbbell lengths are produced because the electron beam temporarily passes the focussing coil 22 at a greater distance from the center of the focusing unit 22 in the case shown in Fig. 4(a) than in the case shown in Fig. 4(b). Therefore, the deflecting influence exerted on the electron beam by the focusing unit 22 is larger in the case of Fig. 4(a). Therefore, the wandering of the impact points shown on the electron beam impact point display unit 4 in Fig. 4(a) is larger than in Fig. 4(b). Further, it can be seen that the direction of the wandering in Fig. 4(a) and (b) is different. This is because the deflecting influence of the adjusting unit 21 is different, for instance, due to different relationships between the adjusting currents supplied to the first adjusting coil 211 and the second adjusting coil 213.

Fig. 4(c) shows the case in which the electron beam travels exactly through the center of the focusing unit 22. In this case, the impact point of the electron beam does not wander across the electron beam impact point display unit 4 when the focus coil current is varied.

The cyclical through-focusing induces a constant changing of the size of the impact point. However, when the frequency of the through-focusing is > 15 Hz, the change in size is no longer recognizable by the human eye, i.e., the human eye sees only a circular surface as shown in Fig. 4(c).

Fig. 5 shows an example which is different than the example shown in Fig. 4 in that the amplitude of the through-focusing, that is the maximum defocusing, is smaller. Therefore, the two dumbbell units shown in Fig. 5(a) and (b), and also the circular surface shown in Fig. 5(c), are respectively smaller than the corresponding images from Fig. 4.

The adjustments of the control of the adjusting and focusing unit 2 necessary to obtain a circular shape as shown in Figs. 4(c) and 5(c) can be manually made by observing the impact points shown by the electron beam impact point display unit 4. Preferably the adjustment is carried out by means of an image processing unit 7 adapted to scan and evaluate an image of the electron beam impact point display unit 4 so that the corresponding adjustments of the control signals, for instance, of the adjusting currents that are to be supplied to the adjusting current contacts 212 and 214, can be automatically generated. The image processing unit 7 is adapted to evaluate the image of the impact points displayed by the electron beam impact point display ^• unit 4 and to control the adjusting and focusing unit 2 so that a circular surface, like those shown in Figs. 4(c) and 5(c) is produced. The evaluation of the image and the control are advantageously carried out in an iterative manner. The evaluation can, for example, comprise that the image processing unit determines the orientation of the (axis of the) dumbbell in the image. This orientation corresponds to the direction in which the electron beam is shifted in comparison to a central path through the focusing unit 22. With the knowledge of this direction, the control 3 can determine iteratively corrections of the control signals for the adjusting focusing unit 2. Dependent on the changing size of the dumbbell, recognized by the image processing unit, the iteration can be so controlled that the electron beam is constantly brought closer to the center of the focusing unit 22. Dependent on the performance level and the precision of the image processing unit (7), the predetermined frequency can be advantageously selected to be > 20 Hz, > 25 Hz, > 30Hz, >50 Hz or > 100 Hz or even larger. It is advantageous if the image to be processed by the image processing unit is so stable that a simple evaluation of the images is possible. Here, it is advantageous to select the predetermined frequency such that it is larger than the imaging frequency (of the imaging device used for the image processing). Alternatively, the electron beam impact point display unit 4 can be luminescently designed so that a stable image is produced even with smaller predetermined frequencies. Similarly, changing frames (individual pictures) from a video camera can also be evaluated.

In an embodiment, the electron beam unit is part of a machine which is adapted for the processing of a workpiece by means of an electron beam, e.g., electron beam welding, annealing, or other electron beam processings. Then, the electron beam impact point display unit 4, e.g., a luminescent screen, can also be inserted into the processing chamber of the machine, in addition to a mounting device for a workpiece. For instance, the electron beam impact point display unit 4 can be arranged on the mounting device and can be secured against any slipping. The electron beam axis 5 and the luminescent screen 4 are arranged such that the electron beam impacts the luminescent screen. In case of a manual adjustment, the operator turns on a beam having an appropriate (low) beam intensity and operates the control 3 such that the focus plane 6 is shifted along the electron beam axis 5 in the cyclical manner, and undertakes the optimization of the adjusting current with the available adjustment elements by visual observation, preferably with the help of a video camera and a connected monitor.

In case of an automatic adjustment, the adjusting currents are adjusted by using the image processing and the resulting automatic adjustments, as described above.

After the conclusion of the adjustment - which lasts a maximum of 1 minute with an experienced operator or automatic adjustment - then the next (e.g., loaded in the changing cycle) workpiece can be directly subjected to the processing. The electron beam is then optimally centered. The luminescent screen is normally removed beforehand from the machine, because the volume of the working chamber is often so small that the necessary space for storing it, for example, for pivoting it out of the way, is not available. The removal of the luminescent screen from the machine also prevents the luminescent screen from being contaminated during the use of the electron beam unit.

It is explicitly stated that all features disclosed in the description and/or the claims are intended to be disclosed separately and independently from each other for the purpose of original disclosure as well as for the purpose of restricting the claimed invention independent of the composition of the features in the embodiments and/or the claims. It is explicitly stated that all value ranges or indications of groups of entities disclose every possible intermediate value or intermediate group of entities for the purpose of original disclosure as well as for the purpose of restricting the claimed invention, in particular also as a limit of a value range.

Claims

1. An electron beam apparatus comprising an electron beam generator (1) for generating an electron beam along an electron beam axis (5); an adjusting and focusing unit (2) for adjusting the electron beam axis and focusing the electron beam in a focus plane (6), which adjusting and focusing unit (2) is arranged on the electron beam axis (5) downstream of the electron beam generator (1), and a control (3) for controlling the adjusting and focusing unit (2), wherein the control is adapted to control the adjusting and focusing unit (2) for shifting of the focus plane (6) along the electron beam axis (5) such that the focus plane is shifted back and forth with a predetermined frequency while the position in which the electron beam axis (5) passes through the adjusting and focusing unit (2) is adjusted, the electron beam unit being adapted such that an electron beam impact point display unit (4) is arrangeable on the electron beam axis (5) in the area of the back and forth movement of the focus plane (6).

2. An electron beam unit according to claim 1, comprising an image processing unit (7) adapted to evaluate impact points displayed on the electron beam impact display unit (4).

3. An electron beam unit according to claim 1 or 2, wherein the predetermined frequency is > 15 Hz, preferably > 20 Hz and even more preferably > 50 Hz.

4. An electron beam unit according to one of claims 1 to 3, wherein the adjusting and focusing unit (2) comprises an adjusting unit (21) and a focusing unit (22).

5. An electron beam unit according to one of claims 1 to 4, wherein the adjusting and focusing unit (2) comprises two adjusting coils (211, 213) adapted to produce two intersecting magnetic fields.

6. An electron beam unit according to claims 4 or 5, wherein the adjusting and focusing unit (2) comprises a focusing coil arranged around the electron beam axis.

7. A method of adjusting of an electron beam produced by an electron beam apparatus comprising the steps: a) generating an electron beam, wherein an electron beam impact point display unit (4) is arranged on the electron beam axis (5) such that the electron beam intersects the same; b) focusing the electron beam in a focus plane (6) by means of an adjusting and focusing unit of the electron beam unit, wherein the focus plane (6) is located in the area of the electron beam impact point display unit (4); c) shifting the focus plane (6) back and forth along the electron beam axis (5) with a predetermined frequency over a range of the electron beam axis (5) that contains the intersection of the electron beam impact point display unit (4) and the electron beam; and d) adjusting the exact orientation of the electron beam axis (5) using the adjusting and focusing unit such that the impact points displayed by the electron beam impact point display unit (4) in step c) form a circular area.

8. A method according to claim 7, wherein the predetermined frequency is chosen to be so high that the impact points shown by the electron beam impact point display unit (4) in step c) give the visual impression of a stationary image.

9. A method according to one of claims 7 or 8, wherein the predetermined frequency is > 15 Hz, preferably > 20 Hz, and even more preferably > 50 Hz.

10. A method according to one of claims 7 to 9, wherein the impact points on the electron beam impact point display unit (4) are evaluated with an image processing unit (7), and wherein the results of the evaluation are used for automatically adjusting the orientation in step d).