U.S. patent number 3,601,577 [Application Number 05/004,221] was granted by the patent office on 1971-08-24 for method and apparatus for viewing the impact spot of a charge carrier beam.
This patent grant is currently assigned to Steigerwald Strahltechnik GmbH. Invention is credited to Joachim Geissler, Edgar Meyer.
United States Patent |
3,601,577 |
Meyer , et al. |
August 24, 1971 |
METHOD AND APPARATUS FOR VIEWING THE IMPACT SPOT OF A CHARGE
CARRIER BEAM
Abstract
In equipment using a beam of charged particles for machining a
workpiece, the size and/or shape of the beam's impact spot is
checked by placing a sharply defined edge to intercept energy
radiated from the impact spot so that the radiation casts a shadow
on a detection device, which is provided by a fluorescent screen of
a photoelectric detector or by a combination of them.
Inventors: |
Meyer; Edgar (N/A),
Geissler; Joachim (N/A, DT) |
Assignee: |
GmbH; Steigerwald Strahltechnik
(DT)
|
Family
ID: |
25994259 |
Appl.
No.: |
05/004,221 |
Filed: |
January 20, 1970 |
Current U.S.
Class: |
219/121.23;
219/121.26; 250/473.1; 219/121.28 |
Current CPC
Class: |
H01J
37/147 (20130101); B23K 15/0013 (20130101); H01J
37/3005 (20130101); H01J 37/304 (20130101) |
Current International
Class: |
H01J
37/304 (20060101); H01J 37/147 (20060101); H01J
37/30 (20060101); B23K 15/00 (20060101); B23K
015/00 () |
Field of
Search: |
;219/121,121EB,121L,69
;250/49.5,41.9 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Truhe; J. V.
Assistant Examiner: Neill; Robert E. O'
Parent Case Text
This is a division of application Ser. No. 804,342 filed Jan. 24,
l969 as a continuation of application Ser. No. 473,355, filed July
20, 1965, now abandoned.
Claims
What is claimed is:
1. A device for monitoring the focused state of a beam of charged
particles at the impact spot on a workpiece, comprising means for
detecting energy radiated from the impact spot of the beam of
charged particles, and a diaphragm having a sharply defined edge
positioned between the impact spot and the detecting means to cast
a shadow on the detecting means with the sharpness of the shadow
representative of the focused state of the beam at the impact
spot.
2. The device as recited in claim 1 wherein the diaphragm includes
a knife edge to cast a linear shadow.
3. The device as recited in claim 1 wherein the diaphragm is formed
of a first apertured plate and a second disc spaced from the plate
along a path between the impact spot and the detecting means, with
the disc selectively sized with respect to the aperture in the
plate to prevent radiation from the impact spot from reaching the
detection means whenever the size of the impact spot is smaller
than a predetermined value.
4. The device as recited in claim 3 wherein the aperture and the
disc are circular.
5. The device as recited in claim 3 wherein distance between the
disc and the aperture is adjustable.
6. The device as recited in claim 3 wherein the detecting means
comprises a fluorescent screen and a photoelectric detector
arranged adjacent the screen to sense the light emission thereof in
the area where radiation is received from the impact spot whenever
the size of the impact spot is larger than said predetermined
value.
7. The device as recited in claim 3 and further including means for
periodically varying the focusing condition of the beam, means for
producing an output signal from the detection means in dependence
on the radiation received thereon, and means for stopping the
variation of the focusing condition upon occurrence of a minimum of
the output signal.
8. The device as recited in claim 1 wherein the detecting means
comprises a fluorescent screen.
9. The device as recited in claim 1 and further including means for
deflecting charged particle radiation from the impact spot onto the
detecting means.
10. The device as recited in claim 1 wherein the detection means
are responsive to charged particle radiation.
11. A method of monitoring the focused state of a beam of charged
particles at its impact spot on a workpiece, comprising the steps
of positioning an impact spot radiation sensitive detection device
at a location to receive energy radiated from the impact spot,
selectively intercepting the flow of said energy to said detection
device with a sharply defined edge to cast a shadow image on the
radiation detection device with the sharpness of the shadow being
representative of the focusing condition of the beam at the impact
spot.
12. The method as recited in claim 11 and further including the
steps of adjusting the interception of the radiation to cast a
normally dark shadow image for a focused beam at the impact spot,
and sensing the brightening of the dark shadow image upon a
defocusing of the beam at the impact spot and producing a signal
representative thereof.
Description
This invention relates to an improved system for checking the focus
of a beam of charged particles upon an impact spot in equipment
using the beam for machining of a workpiece.
Charge carrier beam equipment, and above all, equipment using such
beams for the purpose of machining, require the beam's impact spot
on the subject be observed in order that the focus can be adjusted
to the desired, usually minute size. In addition, it is often
desirable to be able to observe the shape of the beam's impact
spot, so that a certain, usually round focus, can be obtained by an
appropriate adjustment of various focusing fields.
It is of prior art to check the dimension of the beam's focus in
charge carrier beam equipment by light-optical means. However, due
to the relatively small aperture of the optical light path which
normally passes through the pole shoes of the electron-optical
focusing lens, such a light-optical checking system has an
insufficient resolving power. Small foci can, therefore, no longer
be observed and checked with sufficient accuracy.
In addition, most of the known types of light-optical viewing
systems have a glass lens disposed in the corpuscular beam device
proper, and this lens is provided with a central bore to allow the
corpuscular beam to pass. The optically best suited center portion
of the lens is, thus, inevitably lost for observation, i.e., a
degraded image has to be tolerated. This is another reason why
small foci cannot be observed with sufficient accuracy.
Another drawback of the light-optical viewing system is the fact
that the optical lens incorporated in the corpuscular beam device
is quickly covered with evaporated material in the course of the
machining process, so that it becomes unsuitable for further use. A
protective glass plate is, therefore, arranged in front of the
lens, which collects the evaporated material and must, therefore,
be replaced from time to time.
If a high-power beam is used, the protective glass must be replaced
very often. This is, of course, accompanied by frequent,
undesirable interruptions of the work.
It is also prior art to check the focusing of a beam of charged
particles by arranging a collecting electrode above the workpiece
to be machined and to measure the current intercepted. When this
current reaches an extreme value, this is interpreted to be
equivalent to optimal focusing. This method has the disadvantage
that it works properly only with a few materials and that,
moreover, the adjustment is not made for minimum diameter of focus,
but for maximum temperature at any point of the impact area. In
addition, this method does not provide any information on the size
and shape of the impact spot of the beam.
The purpose of the present invention is to indicate a process for
checking the size and/or shape of the beam's impact spot, which is
free from the disadvantages of previously known processes and
which, moreover, offers a number of essential advantages. It is
also the purpose of this invention to provide a device for carrying
out the new process, which is of simple design in spite of its high
efficiency.
The invention, thus, refers to a process for checking the size
and/or shape of the beam's impact spot in equipment using a charged
particle beam for machining. According to the invention, the
particles and/or X-rays reflected from the impact spot of the beam
are used for such a check.
In this connection, it is particularly advantageous to image the
impact spot by charge carrier optical means, with the aid of the
particle radiation reflected from it, onto a radiation detector
which supplies information on size and shape of the image formed.
If, for example, a fluorescent screen is used as a radiation
detector, the corresponding sizes of the beam's impact spot can be
directly deduced from the size and shape of the image formed on
this screen. In this manner, even small and minute impact spots can
be observed without difficulty and can even be measured. If the
viewing beam is bent along the axis by a suitable deflection field,
observation is, in addition, no longer disturbed by the vapor
emanating from the object struck by the beam, i.e., perfect viewing
over a prolonged period is guaranteed even when a high-power beam
is used.
In some cases, particularly for checking large impact spots, it may
be of advantage to image the impact spot by means of the particle
and/or X-rays reflected from it through a small pinhole diaphragm
onto a radiation detector providing information on the size and
shape of the image formed. While a device for applying this viewing
method is of extremely simple design, the check it makes possible
will, in many cases, be entirely sufficient.
In a particularly simple application of the new technique, the
particle and/or X-ray radiation reflected from the beam's impact
spot may also be used to project the shadow image of a knife edge
onto a radiation detector. In this case, the radiation detector
must above all provide information on the edge sharpness of the
image formed. When the separation between impact spot, knife edge
and radiation detector is known, the size of the impact spot can be
directly deduced from the edge sharpness obtained. If a closed
edge, i.e. a hole, is used, the course of the edge sharpness of the
shadow image will, in addition, provide information on the shape of
the beam's impact spot.
It is obvious that in the latter case the diameter of the hole may
be considerably larger than if the impact spot were imaged through
a pinhole diaphragm. Another advantage of the new method is the
fact that it also permits the surroundings of the beam's impact
spot to be observed in a simple manner. In this case, the
surroundings to be observed are covered with an additional,
preferably defocused beam. Together with the impact spot of the
beam employed for machining, the area covered is imaged on a
radiation detector, preferably a fluorescent screen, by the
particle and/or X-ray radiation reflected from the area covered by
the additional beam. The result is a bright image of the impact
spot which permits exact checking of the size and shape of this
spot, and a darker image of the surroundings which permits the beam
to be exactly positioned.
If a beam of sufficiently high aperture is used for machining, the
focusing arrangement can be chosen so that only part of the beam is
employed for the machining process, while another part of the beam
covers the surroundings to be imaged. In this case, a special
illuminating beam is thus no longer required. Taking into account
the spherical aberration of the imaging lens, the beam may, for
example, be focused so that its central portion is used for
machining, while an outer, annular zone of the beam illuminates the
surroundings to be observed.
If the machining process permits short-time interruptions (pulsed
beam, intermittent operation), the beam serving for machining may
also be used to form an image of the surrounding area without any
additional optical means. During the short interruptions in
machining, the beam is then either deflected rapidly, e.g. in the
form of scanning, to cover the area to be imaged, or its impact
zone is artificially enlarged by means of defocusing.
A preferred device for the application of the method covered by the
present invention consists of a viewing system which contains at
least one charge-carrier-optical imaging lens and one radiation
detector and which is arranged so that it intercepts at least part
of the corpuscular radiation reflected from the beam's impact
spot.
Of particular advantage is the use of a fluorescent screen as a
radiation detector, which will then directly display a
true-to-scale image of the impact spot.
Under certain conditions, it may also be advisable to use
appropriate deflection fields in order to project the image by a
scanning-type motion onto a photoelectric detector of lesser size
than the image, whose voltage controls the intensity of an
oscillograph ray synchronized with the scanning motion. In this
case, any desired image scale can be chosen.
If only the size of the beam's impact spot is to be checked, the
radiation detector may also be a photoelectric detector, which is
again smaller than the image of the impact spot formed on it at the
desired optimal focusing of the beam, and which is connected to an
indicator.
When this radiation detector is moved to the center of the
corresponding image, the indicator connected to it will show
maximum deflection when the beam is optimally focused. When the
detector is moved to the edge of the corresponding image, a minimum
deflection will be obtained when the beam is optimally focused.
If a beam of charged particles is used for machining, charged
particles are reflected from the impact spot of the beam, and these
charged particles moving away from the impact spot at high speed
are then intercepted by the viewing systems. At the same time,
X-ray radiation is also reflected from the impact spot of the beam,
which may be used as described above for checking the shape and/or
size of the beam's impact spot.
If a viewing system is used for checking the size and shape of the
impact spot, which images the particle radiation reflected from the
impact spot by charge-carrier-optical means on a radiation
detector, preferably a fluorescent screen, this viewing system may
be arranged in a tilted position with respect to the axis of the
corpuscular beam so that it intercepts the corpuscles reflected to
one side. In this connection, it will be found particularly
advantageous to bend the axis of the viewing system, for example by
a magnetic deflection field, in order to protect the fluorescent
screen from evaporation. The viewing beam may be bent at any
desired point, i.e. both between the impact spot and the
image-forming lens and between the lens and the radiation
detector.
The viewing system can also be arranged at right angles to the axis
of the beam. In this case, it is indispensable that a deflection
field be available between the impact spot and the viewing system
for deflecting the particles reflected sideways towards the viewing
system. No additional means are therefore required to protect the
fluorescent screen from evaporation.
The viewing system may contain one or two image-forming lenses. It
is particularly advantageous to provide two image-forming lenses
and to use the one serving to focus the beam as a first
image-forming lens. In this case, appropriate means, preferably
magnetic deflection fields, have to be provided for separating the
beam used for machining from the reflected particle radiation.
This arrangement offers another special advantage if it is required
for the particular machining application to guide the machining
beam, e.g. with the aid of a deflection system below the lens, in
the desired manner over the workpiece. If in this case above all
the elastically reflected charge carriers are used for imaging the
impact spot, and if an electrical deflection field is employed for
deflection, the image of the impact spot will remain stationary on
the fluorescent screen even when the beam moves over the surface of
the workpiece.
If a low-power beam is used for machining or if the viewing system
images the impact spot of the beam at a particularly high
magnification, the image formed on the fluorescent screen may be
too weak. In this case, the viewing system should preferably be
equipped with an image converter tube. Another possibility is to
increase the energy of the reflected particle radiation by
post-acceleration, in order to obtain a brighter screen image.
The device covered by the present invention can also be used to
automatically adjust a focus of minimum size. In this case, an
arrangement may, for example, be provided which periodically
changes the current flowing through the lens serving to focus the
beam. A photoelectric detector is then arranged in front of the
fluorescent screen of the viewing system, which will, for example,
cover only the central portion of the screen image. This detector
is connected to a system which maintains the current flowing
through the focusing lens at a constant level when the detector
current has reached a peak. With the aid of this device, the
current flowing through the focusing lens is thus automatically
adjusted so that a focus of minimum size will be produced. This
automatic adjustment of a minimum-size focus can also be obtained
if only a photoelectric detector is used as a radiation detector
instead of a fluorescent screen.
Having briefly described the invention, it will be described in
greater detail, along with other objects and advantages thereof, in
the following detailed description which may be more easily
understood by reference to the accompanying drawings, of which:
FIG. 1 is a partly sectioned side elevation of a device in
accordance with the present invention;
FIG. 2 is an enlarged view of a portion of the device shown in FIG.
1;
FIG. 3 is a cross-sectional view of another embodiment of a viewing
system;
FIG. 4 is a perspective view of a viewing system using an
oscillograph tube for image formation;
FIG. 5 is a elevation view of a viewing system equipped with a
post-acceleration section;
FIG. 6 is a partially sectioned elevation view of embodiment of the
present invention in which the lens serving to focus the beam used
for machining is employed as the first image-forming lens of the
viewing system;
FIG. 7 is a partially sectioned elevation view of another
embodiment of a viewing system according to the present invention
in which the focusing lens is likewise used as the first
image-forming lens;
FIG. 8 is a partially sectioned elevation view of another
embodiment of the present invention in which the X-ray radiation
reflected from the impact spot of the beam is imaged with the aid
of a pinhole diaphragm;
FIG. 9 is a partially sectioned elevation view of another
embodiment of the present invention in which the radiation
reflected from the impact spot projects an image of a knife edge
onto a fluorescent screen;
FIG. 10 is a perspective view of another embodiment of the present
invention in which the radiation reflected from the impact spot
images a knife edge on a fluorescent screen;
FIG. 11 is an elevation view of an embodiment of the present
invention in which the radiation reflected from the impact spot
projects the image of a pinhole diaphragm onto a fluorescent
screen;
FIG. 12 is a partially sectioned view of an embodiment of the
present invention which is suitable for the automatic adjustment of
a focus of minimum size.
In FIG. 1, there is shown a device 1 for machining material by
means of an electron beam. The beam-generating system of this
device consists of the cathode 2, the control electrode 3 and the
grounded anode 4. The units 5 and 6 serve to generate the heating
voltage for the cathode 2, the bias voltage for the control
electrode 3, and the high voltage. In the direction of the beam,
below the anode 4, is an electromagnetic deflection system 7
serving for beam adjustment. The unit 8 is the power supply unit of
the deflection system 7. Below the system 7 is a diaphragm 9 which
can be displaced in the diaphragm plane in a manner not illustrated
in the drawing.
The electromagnetic lens 10 serves to focus the electron beam 12 on
the workpiece 13. The lens 10 is supplied with power by the unit
11. The workpiece 13 rests on the schematically represented
universal stage 14 which can be shifted from left to right by means
of the crank 15 and from front to rear by another crank not shown
in the drawing.
A portion 16 of the electrons reflected from the impact spot of the
electron beam 12 on the workpiece 13 reaches the viewing system 20.
This consists of the two image-forming lenses 17 and 18 which
project an image of the impact spot of the beam onto the
fluorescent screen 19. The size and shape of the beam's impact spot
can be directly determined from the magnified image of the impact
spot on the fluorescent screen 19. A thin diaphragm 50' is arranged
in the plane of the first aerial image, which ensures that only
electrons of a certain predetermined speed reach the fluorescent
screen 19. The viewing system 20 is bent behind the lens 18, and a
deflection field 21 (schematically illustrated) deflects the
electrons 16 towards the fluorescent screen 19. The deflected path
of rays in the viewing system protects the fluorescent screen 19
from damage and coating by material evaporated from the workpiece
during working.
It is also possible to replace the fluorescent screen 19 by a
photoelectric detector connected to an indicator. This detector
must be slightly smaller than the image of the beam's impact spot
formed on the detector when the desired optimal focusing of the
electron beam 12 has been achieved. With this arrangement the
focusing state of the electron beam 12 can be directly indicated by
the deflection of the indicator's pointer.
FIG. 2 is an enlarged view of a portion of the device shown in FIG.
1. A high-aperture beam is illustrated. Utilizing the spherical
aberration of the focusing lens 10, the focusing of the beam 22 is
chosen so that only the shaded central portion of the beam 22 is
focused on the workpiece 13. The annular zone of the beam
surrounding this central portion is here used to illuminate the
surroundings of the beam's impact spot. With the aid of the viewing
system 20, a bright image of the impact spot and a darker image of
the surroundings of this spot can then be observed.
It is also possible to use the device shown in FIG. 1 in order at
the same time to observe the surroundings of the beam's impact
spot. For this purpose, the power supply unit 11 of the lens need
only be controlled so that the lens 10 alternates between focusing
and defocusing the beam 12. Furthermore, a deflection system can be
arranged below the lens 10 as is shown, for example, in FIG. 6, and
a voltage can be fed to this deflection system, which will cause
the beam 12 to stop in the respective machining position during
successive intervals and to be deflected during the intermediate
intervals so that it will cover the surroundings of the working
point in a scanning-type pattern.
In the embodiment illustrated in FIG. 3, the viewing system
consists of an image-forming lens 23 and the fluorescent screen 24.
The viewing system is here arranged at right angles to the axis of
the electron beam 12. An electromagnetic deflection field 25 serves
to deflect the electrons reflected from the impact point of the
beam into the axis of the viewing system 23, 24. This arrangement
offers the advantage that even without a thin diaphragm it is
essentially only electrons of a predetermined speed which reach the
fluorescent screen 24. In addition, this arrangement may be made
somewhat more compactly than that of FIG. 1, and the fluorescent
screen is automatically protected from evaporation.
Instead of a viewing system provided with a fluorescent screen, a
device of the type illustrated in FIG. 4 may be used. Here, the
particle radiation passing through the image-forming lens 23 is
moved by a deflection field applied between the deflector plates
26, 27 and 28, 29 in a scanning-type pattern over the photoelectric
detector 30 which is smaller than the image of the impact spot. The
voltage generated by the detector is amplified by the amplifier 31
and controls the intensity of the oscillograph ray via the control
electrode 33 of the oscillograph tube. The tube 32 contains a
deflection system consisting of the plate pairs 26', 27' and 28',
29', to which deflecting voltages are applied in synchronism with
those applied to the plates 26, 27 and 28, 29. Since the deflecting
voltages fed to the systems 26, 27 and 28, 29 as well as 26', 27',
28', 29' are, moreover, proportional to each other, an image of the
impact spot of the beam 12 is thus formed on the fluorescent screen
34 of the tube 32.
FIG. 5 shows a viewing system which is particularly suited to those
applications in which the energy of the particle radiation
reflected from the workpiece is not sufficient for forming a
desirably bright screen image. Here, a post-acceleration section is
inserted between the image-forming lens 23 and the fluorescent
screen 38, where the two accelerating electrodes 35 and 36 are
provided. The fluorescent screen 37 is in the conventional manner
provided with a thin metal foil 37 which is connected to the
accelerating electrode 36. The generator 39 serves to generate the
post-acceleration voltage.
In the example shown in FIG. 6, the beam-generating system 2, 3, 4
is disposed at an angle to the axis of the focusing lens 10. An
electromagnetic deflection field 40 serves to deflect the electron
beam 12 into the axis of the lens 10. The latter focuses the
electron beam 12 in the conventional manner on the workpiece 13.
The electrons 16 reflected from the impact spot of the beam on the
workpiece 13 pass through the focusing lens 10 and are deflected
into the axis of the image-forming lens 41 by the deflection field
40. In this case, the viewing system consists of the first
image-forming lens, which is here the focusing lens 10, the second
image-forming lens 41 and the fluorescent screen 42. The focusing
lens 10 forms an image of the beam's impact spot at 43, looking
directly down on the spot, a desired arrangement in many
applications. The deflection field 40 may also be disposed between
the lens 10 and the workpiece 13.
Since in the aforementioned example the focusing of the electron
beam 12 by means of the lens 10 influences also the focusing in the
image plane 43, the image plane shift must be compensated for by
the lens 41 in order to ensure that the image of the focus is
always sharply defined on the fluorescent screen 42. For this
purpose the power supply units 44 for the imaging lens 41 and 45
for the focusing lens 10 are interconnected.
If below the focusing lens 10 an electrostatic deflection system 46
is used to displace the electron beam 12 on the workpiece 13, the
image of the focus formed on the fluorescent screen 42 will remain
stationary even when the electron beam moves along the workpiece
13.
The arrangement shown in FIG. 6 serves at the same time as an ion
trap, thus increasing the life of the cathode. In addition, the
fluorescent screen 42 is automatically protected from evaporation
of material from the workpiece.
In the example illustrated in FIG. 7, reflected particles 74
passing laterally of the main beam 12 through the lens 10 are
separated by a deflection field 47 and projected onto the
fluorescent screen 49 by means of an image-forming lens 48. In this
case also, the focusing lens 10 is used as the first imaging lens
of the viewing system, and the fluorescent screen is automatically
protected from evaporation.
In the example shown in FIG. 8, the particle and/or X-ray radiation
50 reflected from the impact spot of the electron beam 12 on the
workpiece 13 is imaged on a fluorescent screen 52 via a small
pinhole diaphragm 51. This embodiment is particularly useful when
large foci have to be imaged. It is advisable to provide a
deflection field in the path of the viewing beam in order to
protect the fluorescent screen.
In FIG. 9, a knife edge 53 is arranged on one side of the workpiece
13. This knife edge is imaged on the fluorescent screen 55 by the
particle radiation reflected from the impact spot 54 of the beam in
the form of a shadow. Between the knife edge 53 and the fluorescent
screen 55 a deflection field 56 is indicated schematically, which
serves to deflect the particle radiation. This deflection field and
the protective shields 57 and 58 arranged in front of the
fluorescent screen 55 prevent the accumulation of evaporated
material on the fluorescent screen.
The deflection field may also be arranged between the workpiece 13
and the knife edge 53 to protect the knife edge 53 from evaporation
as well as the screen.
The size of the beam's impact spot 54 can without difficulty be
deduced from the sharpness range of the shadow image formed on the
fluorescent screen 55. Thus, an optimally focused beam will produce
a sharp transition from bright to dark on the fluorescent screen,
while this transition is gradually weakened as the focus changes
from the optimum.
If the knife edge 53 is replaced by a circular diaphragm, a shadow
image of this diaphragm will be formed on the fluorescent screen
55. The transition from bright to dark will again be sharply
defined when the beam 12 is optimally focused. When the impact
point 54 is enlarged, the transition from bright to dark will be
uniformly weakened along the entire periphery of the shadow image
only if the impact spot is circular in shape. Any deviation from
circular shape is revealed by different width of the transition
zone from bright to dark, so that in this case it is also possible
to obtain information on the shape of the impact spot.
The deflection field 56 shown in FIG. 9 may, under certain
circumstances, disturb the projection of the shadow image. FIG. 10
therefore shows a device in which such a disturbance is largely
avoided. The particle radiation 59 reflected from the beam's impact
spot 54 is deflected by the magnetic field formed between the pole
shoes 60 and 61 and projects a shadow image of the knife edge 62
onto the fluorescent screen 63. In this instance, the magnetic
field acts in the direction of the knife edge 62, so that the
radiation is deflected around a horizontal axis. The deflected
radiation follows an oblique path to the rear, while the direction
of the magnetic field is obliquely forward.
In the device presented schematically in FIG. 11, the radiation
reflected from the workpiece 13 passes through an annular diaphragm
64 which can be displaced in the direction of the arrow. A stop 65
serves to cap the center of the diaphragm 64. When the beam 12 has
been optimally focused--as is indicated by the solid lines--the
diaphragms 64 and 65 can be so adjusted one with respect to the
other that the fluorescent screen 66 remains dark. When the beam 12
is then defocused--as is indicated by the dashed lines--a bright
annulus will appear on the fluorescent screen 66, the total
brightness of which depends on the focusing state.
Behind the fluorescent screen 66 is a photoelectric detector 67. If
this detector is connected to an indicator, it is possible to
deduce optimal focusing of the beam 12 from minimum deflection of
the indicator. The detector 67 can also be used in a simple manner
to achieve automatic optimal focusing of the beam.
In FIG. 12, the viewing system 68 which includes the elements of
any of FIGS. 9, 10 or 11 serves to project a shadow image of the
impact spot of the beam 12 on the workpiece 13 onto a fluorescent
screen 69. In front of this screen is a photoelectric detector 70
which covers a portion of the screen image where brightness will be
smallest at optimum focusing of the beam. This detector is
connected to an amplifier 71 which in turn is connected to a unit
72. This unit generates a switching pulse when the voltage supplied
by the detector 70 has reached a minimum value. The unit 73
influences the power supply unit 11 of the lens in such manner that
the current flowing through the focusing lens 10 is periodically
varied. This causes also the size of the beam's impact spot on the
workpiece 13 to be varied periodically. During this variation, the
focus passes through the state of optimal focusing, and at this
instant a switching pulse is generated by the unit 72 which cuts
off the unit 73 and thus keeps the current, which at this moment
flows through the focusing lens, at a uniform level.
A focus of minimum size is thus adjusted automatically.
Instead of the viewing system 68 with the fluorescent screen 69 it
is also possible to use a viewing system which is directly based on
a photoelectric detector instead of the fluorescent screen 69.
* * * * *