U.S. patent number 3,924,156 [Application Number 05/483,266] was granted by the patent office on 1975-12-02 for method and system for correcting an aberration of a beam of charged particles.
This patent grant is currently assigned to International Business Machines Corporation. Invention is credited to Samuel K. Doran, Merlyn H. Perkins.
United States Patent |
3,924,156 |
Doran , et al. |
December 2, 1975 |
Method and system for correcting an aberration of a beam of charged
particles
Abstract
A beam of charged particles is deflected in a closed path such
as a square, for example, over a cross wire grid at a constant
velocity by an X Y deflection system. A small high frequency jitter
is added at both axes of deflection to cause oscillation of the
beam at 45.degree. to the X and Y axes. From the time that the
leading edge of the oscillating beam passes over the wire until the
trailing edge of the beam passes over the wire, an envelope of the
oscillations produced by the jitter is obtained. A second envelope
is obtained when the leading edge of the beam exits from being over
the wire until the trailing edge of the beam ceases to be over the
wire. Thus, a pair of envelopes is produced as the beam passes over
each wire of the grid. The number of pulses exceeding ten per cent
of the peak voltage in the eight envelopes produced by the beam
completing a cycle in its closed path around the grid are counted
and compared with those counted during the previous cycle of the
beam moving in its closed path over the grid. As the number of
pulses decreases, the quality of the focus of the beam increases so
that correction signals are applied to the focus coil in accordance
with whether the number of pulses is increasing or decreasing.
Inventors: |
Doran; Samuel K. (Wappingers
Falls, NY), Perkins; Merlyn H. (Hopewell Junction, NY) |
Assignee: |
International Business Machines
Corporation (Armonk, NY)
|
Family
ID: |
23919393 |
Appl.
No.: |
05/483,266 |
Filed: |
June 26, 1974 |
Current U.S.
Class: |
315/382 |
Current CPC
Class: |
H01J
37/21 (20130101); H05B 3/56 (20130101); H01J
37/244 (20130101); H01J 37/153 (20130101); E01C
11/265 (20130101); H05B 3/36 (20130101); H05B
2203/003 (20130101); H05B 2203/026 (20130101); H01J
2237/24578 (20130101); H05B 2203/017 (20130101) |
Current International
Class: |
H01J
37/04 (20060101); H01J 37/02 (20060101); H01J
37/244 (20060101); H01J 37/21 (20060101); H01J
37/153 (20060101); H01J 029/70 () |
Field of
Search: |
;315/382,30,31,364 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Wilbur; Maynard R.
Assistant Examiner: Potenza; J. M.
Attorney, Agent or Firm: Leach, Jr.; Frank C. Galanthay;
Theodore E.
Claims
What is claimed is:
1. A method for correcting an aberration of a beam of charged
particles including:
directing the beam at a constant velocity in a predetermined path
across a target during each cycle;
ascertaining the lengths of time for the beam to enter and leave at
least one portion of the target;
and adjusting the aberration of the beam in accordance with the
lengths of time for the beam to enter and leave the portion of the
target in comparison with the lengths of time for the beam to enter
and leave the portion of the target during the prior cycle of
movement of the beam in its predetermined path.
2. The method according to claim 1 including:
oscillating the beam in a single predetermined direction during its
movement in the predetermined path with the single predetermined
direction being other than any direction of movement of the beam in
its predetermined path;
and ascertaining the length of time for the beam to enter the
portion of the target by voltage signals produced by the
oscillations when the beam enters the portion of the target and the
length of time for the beam to leave the portion of the target by
voltage signals produced by the oscillations when the beam leaves
the portion of the target.
3. The method according to claim 2 including:
directing the beam in a closed path to provide the predetermined
path;
and forming the target in the shape of a cross so that four
portions of the target are crossed during each movement of the beam
in the closed path.
4. The method according to claim 3 including:
counting the number of times that the voltages produced by the
oscillations pass a predetermined voltage limit during each
entrance or exit of the beam over the portion of the target to
ascertain the length of time for the beam to enter or leave the
portion of the target;
and adjusting the aberration of the beam in accordance with the
count during a cycle of movement of the beam in its predetermined
path in comparison with the count during the prior cycle of
movement of the beam in its predetermined path.
5. The method according to claim 4 in which:
the aberration of the beam is the quality of the focus of the
beam;
and the focus is adjusted by changing the current flow through the
focus coil.
6. The method according to claim 5 including automatically
reversing the direction of change of the current flow through the
focus coil if the focus quality decreases.
7. The method according to claim 5 including:
adjusting the focus only every other cycle of movement of the beam
in its predetermined path;
and adjusting the focus only after completion of a cycle of
movement of the beam in its predetermined path following completion
of the cycle of movement of the beam in its predetermined path in
which comparison of the count with the count from the prior cycle
of movement of the beam in its predetermined path occurred.
8. The method according to claim 2 including:
counting the number of times that the voltages produced by the
oscillations pass a predetermined voltage limit during each
entrance or exit of the beam over the portion of the target to
ascertain the length of time for the beam to enter or leave the
portion of the target;
and adjusting the aberration of the beam in accordance with the
count during a cycle of movement of the beam in its predetermined
path in comparison with the count during the prior cycle of
movement of the beam in its predetermined path.
9. The method according to claim 8 in which:
the aberration of the beam is the quality of the focus of the
beam;
and the focus is adjusted by changing the current flow through the
focus coil.
10. The method according to claim 9 including automatically
reversing the direction of change of the current flow through the
focus coil if the focus quality decreases.
11. The method according to claim 9 including:
adjusting the focus only every other cycle of movement of the beam
in its predetermined path;
and adjusting the focus only after completion of a cycle of
movement of the beam in its predetermined path following completion
of the cycle of movement of the beam in its predetermined path in
which comparison of the count with the count from the prior cycle
of movement of the beam in its predetermined path occurred.
12. The method according to claim 1 in which:
the aberration of the beam is the quality of the focus of the
beam;
and the focus is adjusted by changing the current flow through the
focus coil.
13. The method according to claim 12 including automatically
reversing the direction of change of the current flow through the
focus coil if the focus quality decreases.
14. The method according to claim 13 including:
directing the beam in a closed path to provide the predetermined
path;
and forming the target in the shape of a cross so that four
portions of the target are crossed during each movement of the beam
in the closed path.
15. The method according to claim 12 including:
adjusting the focus only every other cycle of movement of the beam
in its predetermined path;
and adjusting the focus only after completion of a cycle of
movement of the beam in its predetermined path following completion
of the cycle of movement of the beam in its predetermined path in
which comparison of the count with the count from the prior cycle
of movement of the beam in its predetermined path occurred.
16. A system for correcting an aberration of a beam of charged
particles including:
means to move the beam at a constant velocity in a predetermined
path across a target during each cycle;
means to ascertain the lengths of time for the beam to enter and
leave at least one portion of the target during each cycle of
movement in the predetermined path;
and means to correct the aberration of the beam in accordance with
the lengths of time for the beam to enter and leave the portion of
the target in comparison with the lengths of time for the beam to
enter and leave the portion of the target during the prior cycle of
movement of the beam in its predetermined path.
17. The system according to claim 16 including:
means to oscillate the beam in a single predetermined direction
during its movement in the predetermined path by said moving means
with the single predetermined direction being other than any
direction of movement of the beam in its predetermined path;
said ascertaining means including:
means to obtain the voltages produced by said oscillating means
when the beam enters and leaves the portion of the target;
and means to count the number of times that the voltages produced
by the beam entering and leaving the portion of the target pass a
predetermined voltage limit during each cycle of movement of the
beam in its predetermined path;
and said correcting means includes means to correct the aberration
of the beam in accordance with the count obtained by said counting
means during a cycle of movement of the beam in its predetermined
path in comparison with the count obtained by said counting means
during the prior cycle of movement of the beam in its predetermined
path.
18. The system according to claim 17 in which said oscillating
means comprises means to apply a single high frequency jitter to
said moving means.
19. The system according to claim 18 in which:
said moving means includes first and second means to deflect the
beam in orthogonal directions;
and said first and second deflecting means moves the beam in a
closed path across portions of the target extending in the
orthogonal directions.
20. The system according to claim 19 in which:
the aberration of the beam is the quality of the focus of the
beam;
and said correcting means includes means to change the focus of the
beam in accordance with the count obtained by said counting means
during a cycle of movement of the beam in its predetermined path in
comparison with the count obtained by said counting means during
the prior cycle of movement of the beam in its predetermined
path.
21. The system according to claim 20 in which said focus change
means includes means to change the focus only every other cycle of
movement of the beam in its predetermined path with the change
occurring only after completion of a cycle of movement of the beam
in its predetermined path following completion of the cycle of
movement of the beam in its predetermined path in which comparison
of the count with the count from the prior cycle of movement of the
beam in its predetermined path occurred.
22. The system according to claim 17 in which said counting means
counts the number of times that the voltages exceed a predetermined
minimum voltage.
23. The system according to claim 22 in which the predetermined
minimum voltage is ten percent of the peak voltage.
24. The system according to claim 23 in which:
the aberration of the beam is the quality of the focus of the
beam;
and said correcting means includes means to change the focus of the
beam in accordance with the count obtained by said counting means
during a cycle of movement of the beam in its predetermined path in
comparison with the count obtained by said counting means during
the prior cycle of movement of the beam in its predetermined
path.
25. The system according to claim 24 in which said focus change
means includes means to change the focus only every other cycle of
movement of the beam in its predetermined path with the change
occurring only after completion of a cycle of movement of the beam
in its predetermined path following completion of the cycle of
movement of the beam in its predetermined path in which comparison
of the count with the count from the prior cycle of movement of the
beam in its predetermined path occurred.
26. The system according to claim 17 in which:
the aberration of the beam is the quality of the focus of the
beam;
and said correcting means includes means to change the focus of the
beam in accordance with the count obtained by said counting means
during a cycle of movement of the beam in its predetermined path in
comparison with the count obtained by said counting means during
the prior cycle of movement of the beam in its predetermined
path.
27. The system according to claim 26 in which said focus change
means includes means to change the focus only every other cycle of
movement of the beam in its predetermined path with the change
occurring only after completion of a cycle of movement of the beam
in its predetermined path following completion of the cycle of
movement of the beam in its predetermined path in which comparison
of the count with the count from the prior cycle of movement of the
beam in its predetermined path occurred.
28. The system according to claim 16 in which said oscillating
means comprises means to apply a single high frequency jitter to
said moving means.
Description
In U.S. Pat. No. 3,644,700 to Kruppa et al, there is shown an
apparatus for stepping a square-shaped beam of charged particles in
a substantially raster fashion over a predetermined area. An
improved apparatus for stepping a square-shaped beam of charged
particles over a predetermined area is shown and described in the
copending patent application of Michel S. Michail et al for "Method
And Apparatus For Positioning A Beam Of Charged Particles," Ser.
No. 437,585, filed Jan. 28, 1974, now U.S. Pat. No. 3,900,736 and
assigned to the same assignee as the assignee of this
application.
In the aforesaid Kruppa et al patent and the aforesaid Michail et
al application, it is necessary for the beam to be properly focused
to obtain the desired precise application of the beam in various
portions of the predetermined area. As described in the aforesaid
Kruppa et al patent and the aforesaid Michail et al application,
the beam is moved through A, B, and C cycles.
The present invention provides a method and system for measuring
the quality of the focus of the beam of charged particles at
various intervals and automatically correcting the focus. These
measurements and the corrections occur during certain of the C
cycles.
The present invention also is capable of ascertaining the
illumination of the beam of charged particles, the quality of the
focus of the beam of charged particles, and the astigmatism of the
beam of charged particles so that manual correction of these
aberrations can occur. These corrections would occur when the
electron gun is set up to produce the beam.
The present invention measures the quality of the focus of the beam
by directing the beam in a predetermined closed path across a
target which will interrupt the beam of charged particles whenever
the beam is completely engaging the target. Since the beam is
deflected in orthogonal directions, the quality of the focus of the
beam in both of its deflection directions is ascertained.
The present invention preferably accomplishes this by utilizing a
cross wire grid as the target and moving the beam in a square over
the grid at a constant velocity. Thus, the beam will pass over a
portion of the wire grid extending in the X direction, for example,
next over a portion of the wire grid extending in the Y direction,
then over another portion of the wire grid extending in the X
direction, and finally over another portion of the wire grid
extending in the Y direction during each cycle of movement of the
beam over the grid.
By applying a high frequency jitter to oscillate the beam in a
single direction at an angle to each of X and Y axes, the
oscillations produced by the jitter form two envelopes as the beam
passes over a wire. A first envelope of the oscillations occurs
from the time that the leading edge of the beam engages the wire
until the trailing edge of the beam is disposed over the wire. A
second envelope occurs when the leading edge of the beam exits from
the wire until the trailing edge of the beam exits from the
wire.
Each of the envelopes is indicative of the current density of the
beam in the direction in which the beam is being deflected. The
envelopes represents a voltage proportional to the jitter component
of the current of the current collector. Since the jitter is
sweeping a constant two percent slice of beam area onto and off of
the wire, the peak amplitude of the envelope voltage is
proportional to the average current density of the two percent
slice. Thus, each of the two envelopes produced by the beam
crossing a wire extending in the Y direction when the beam is
deflected in the X direction is a profile of the current density
distribution of the beam in the X direction.
If the beam were free of aberrations, each of these envelopes would
be square shaped. However, when the beam is out of focus, the edge
definition of the beam is smeared so that the current density
profile also is smeared. Additionally, if the beam is astigmatic,
the envelopes or profiles of the current density in one direction
may be sharp while the profiles in the other direction may be
smeared or vice versa. Furthermore, if the beam is not properly
aligned so that it does not have uniform illumination, the top of
the current density profile will not be flat but will be
slanted.
Therefore, with the profile of the current density not being square
shaped when the beam has an aberration with the beam being square
shaped, the quality of the current density of the beam can be
ascertained by the use of the envelopes. The envelopes indicate
whether the beam is properly focused, has no astigmatism, and is
illuminated properly.
Accordingly, as the focus becomes worse, the sides of the profile
of the envelope tend to become more inclined or curved. Since the
area underneath the envelope must always be the same irrespective
of whether the beam has aberrations or not, an indication of the
quality of the focus of the beam in comparison with the focus of
the beam during the prior scan can be ascertained through
determining whether the envelope sides are becoming more straight
to indicate increasing quality of the focus of the beam or more
inclined to indicate decreasing quality of the focus of the beam.
If the beam has astigmatism, this will affect the focus
quality.
Similarly, the illumination of the beam also can be ascertained
through determining whether the envelope has a flat top or not. If
the top ceases to be flat, then the area near the top of the
envelope tends to become smaller to indicate poor illumination.
Accordingly, the quality of the focus of a beam can be compared to
that during the previous pass of the beam over the wire by counting
the number of times that the voltage within the envelope exceeds a
certain limit, which is preferably ten percent of the peak voltage
produced by the jitter. At ten percent of the peak voltage produced
by the jitter, the inclination of the sides of the profile of the
envelope is readily comparable with the inclination of the sides of
the profiles of the envelope produced during the prior pass of the
beam over the wire. Of course, other suitable percentages, except
fifty percent, of the peak voltage could be employed.
When initially starting to measure the quality of the focus, the
comparison is with the information from the satisfactory focus of
the beam from the previous C cycle in which focusing occurred. It
should be understood that other operations concerning the quality
of the beam occur during other C cycles so that a number of A, B,
and C cycles normally occur between focus measurements and noise
could have eliminated the prior information. Thus, the signal
produced by the first cycle of movement of the beam over the target
could produce a correction signal in the wrong direction.
If the number of times the voltage exceeds the selected limit
increases beyond the number from the prior cycle, than the
direction of change of the magnitude of current to the focus coil
is reversed. If the count of the pulses is less than that produced
by the prior pass of the beam over the grid, the direction of
change of the magnitude of current to the focus coil is
continued.
There are a total of sixteen complete cycles of the beam along its
closed path over the cross wire grid during each C cycle in which
automatic focus correction is occurring. This is sufficient to
satisfactorily focus the beam. Whenever the direction of change of
the magnitude of the current is reversed three consecutive times
when the correction signal is supplied to the focus coil, the best
focus value has been obtained.
To ascertain whether the illumination of the beam is satisfactory,
the counting of the number of times that the voltages within the
envelope exceeds another voltage limit, which is preferably ninety
percent of the peak voltage produced by the jitter, enables
ascertainment of the quality of the illumination of the beam. Thus,
when the number of times that the voltage within the envelope
exceeds the selected limit increases from the prior pass of the
beam over the grid, this is an indication that the illumination of
the beam is being improved. This is because an increase in the
number of times that the voltage exceeds ninety percent of the
maximum voltage produced by the jitter is an indication of an
increase in area in the top portion of the profile so that the top
of the profile of the envelope is becoming more flat.
An object of this invention is to provide a method and system for
correcting an aberration of a beam of charged particles.
Another object of this invention is to provide a method and system
for focusing a beam of charged particles.
A further object of this invention is to measure the quality of the
focus of the beam of charged particles and adjust the focus in
accordance with the quality.
Still another object of this invention is to automatically correct
the focus of a beam of charged particles at selected intervals of
time.
The foregoing and other objects, features, and advantages of the
invention will be more apparent from the following more particular
description of the preferred embodiment of the invention as
illustrated in the accompanying drawings.
In the drawings:
FIG. 1 is a schematic view showing a beam of charged particles and
the apparatus for controlling the beam.
FIG. 2 is a schematic top plan view of a portion of the wire grid
over which the beam of charged particles moves during focusing.
FIG. 3 is a schematic plan view of the beam showing the beam in its
position when not moved by the jitter frequency and the position to
which it is moved by the jitter frequency.
FIG. 4 is a schematic view showing a pulse produced by the beam
passing over a portion of the wire grid with the oscillations from
the jitter frequency appearing on the pulse as the beam enters and
leaves the portion of the wire grid.
FIG. 5 is a view showing two different shaped envelopes produced by
the oscillations from the high frequency jitter resulting from the
beam entering or leaving a portion of the wire grid with one of the
envelopes representing a good focus condition of the beam and the
other of the envelopes representing a poor focus condition of the
beam.
FIG. 6 is a view of the shape of an envelope produced by the
oscillations from the high frequency jitter resulting from the beam
entering or leaving a portion of the wire grid when illumination of
the beam is poor.
FIG. 7 is a block diagram showing the relationship of circuitry
used for correcting aberrations of the beam.
FIG. 8 is a schematic wiring diagram of the bandpass filter and
amplifier of the circuitry of FIG. 7.
FIG. 9 is a schematic wiring diagram of the automatic gain control
of the circuitry of FIG. 7.
FIG. 10 is a block diagram of the detecting and counting means of
the circuitry of FIG. 7.
FIG. 11 is a block diagram of an up down counting means of the
circuitry of FIG. 7.
FIG. 12 is a block diagram of the focus control unit of the
circuitry of FIG. 7.
FIG. 13 is a block diagram of an illumination detecting means of
the circuitry of FIG. 7.
FIG. 14 is a block diagram of the apparatus for producing the
jitter frequency.
FIG. 15 is a timing diagram showing the relationship of various
signals during focusing of the beam.
FIG. 16 is a schematic diagram showing the relationship of the
focus coil current and the number of pulses counted during cycles
of the beam passing over the wire grid.
Referring to the drawings and particularly FIG. 1, there is shown
an electron gun 10 for producing a beam 11 of charged particles in
the well-known manner. As shown and described in the aforesaid
Michail et al application, the electron beam 11 is passed through
an aperture 12 in a plate 14 to shape the beam 11. The beam 11 is
preferably square shaped and has a size equal to the minimum line
width of the pattern that is to be formed.
The beam 11 passes between a pair of blanking plates 15, which
determine when the beam 11 is applied to the material and when the
beam 11 is blanked. The blanking plates 15 are controlled by
circuits of an analog unit 17, which has a column control unit 16
connected thereto.
The analog unit 17 is controlled by a digital control unit 18 in
the manner more particularly shown and described in the copending
patent application of Phillip M. Ryan for "Method And Apparatus For
Controlling Movable Means Such As An Electron Beam," Ser. No.
398,734, filed Sept. 19, 1973, now U.S. Pat. No. 3,866,013, and
assigned to the same assignee as the assignee of this application.
The digital control unit 18 is connected to a computer 19, which is
preferably an IBM 370 computer.
The beam 11 then passes through a circular aperture 20 in a plate
21. This controls the beam 11 so that only the charged particles
passing through the centers of the lenses (not shown) are used so
that a square-shaped spot without any distortion is produced.
The beam 11 is next directed through stigmator coils 21A and 21B
and then through a focus coil 22. The stigmator coils 21A and 21B
and the focus coil 22 are connected to the column control unit
16.
After passing through the focus coil 22, the beam is directed
through magnetic deflection coils 23, 24, 25, and 26. The magnetic
deflection coils 23 and 24 control the deflection of the beam 11 in
a horizontal or X direction while the magnetic deflection coils 25
and 26 control the deflection of the beam 11 in a vertical or Y
direction. Accordingly, the coils 23-26 cooperate to move the beam
11 in a horizontal scan by appropriately deflecting the beam
11.
While the beam 11 could be moved in a substantially raster fashion
as shown and described in the aforesaid Kruppa et al patent, it is
preferably moved in a back and forth scan so that the beam 11 moves
in opposite directions along adjacent lines as shown and described
in the aforesaid Ryan and Michail et al. applications. Thus, the
negative bucking sawtooth of the type shown in FIG. 3b of the
aforesaid Kruppa et al patent is supplied to the coils 23 and 24
during forward scan while a positive bucking sawtooth, which is of
opposite polarity to the sawtooth shown in FIG. 3b of the aforesaid
Kruppa et al patent, is supplied to the coils 23 and 24 during the
backward scan.
As shown and described in the aforesaid Michail et al application,
the beam 11 then passes between a first set of electrostatic
deflection plates 27, 28, 29, and 30. The electrostatic deflection
plates 27 and 28 cooperate to deflect the beam in a horizontal or X
direction while the electrostatic deflection plates 29 and 30
cooperate to move the beam 11 in the vertical or Y direction. The
plates 27-30 are employed to provide any desired offset of the beam
11 at each of the predetermined positions or spots to which it is
moved. In the aforesaid Kruppa et al patent, the plates 27-30
corrected for linearity, but these correction signals are supplied
to the coils 23-26 in the aforesaid Michail et al application and
in this application.
After passing between the electrostatic deflection plates 27-30,
the beam 11 then passes between a second set of electrostatic
deflection plates 31, 32, 33, and 34. The electrostatic deflection
plates 31 and 32 cooperate to deflect the beam 11 in the horizontal
or X direction while the electrostatic deflection plates 33 and 34
cooperate to deflect the beam 11 in the vertical or y direction.
The plates 31 and 32 deflect the beam 11 in the X direction and the
plates 33 and 34 deflect the beam 11 in the Y direction from each
of the predetermined positions to which it is moved in accordance
with its predetermined pattern so that the beam 11 is applied to
its actual position based on the deviation of the area from its
designed position, both shape and location, in which the beam 11 is
to write as more particularly shown and described in the aforesaid
Michail et al application.
The beam 11 is then applied to a target, which is supported on a
table 35 and can be a semiconductor wafer, for example. The table
35 is movable in the X and Y directions as more particularly shown
and described in the aforesaid Kruppa et al patent.
The beam 11 is moved through A, B, and C cycles as shown and
described in the aforesaid Kruppa et al patent and the aforesaid
Michail et al application. The present invention is particularly
concerned with supplying signals to automatically correct the focus
of the beam 11 during some of the C cycles.
The focus of the beam 11 is controlled by the focus coil 22. A
current flows through the focus coil 22 to regulate the focus of
the beam 11.
When a program in the computer 19 determines that it is time for
the focus of the beam 11 to be automatically corrected, if
necessary, during a C cycle, the table 35 is moved to bring a cross
wire grid 41 (see FIG. 2), which is in the same plane as the
semiconductor wafer supported on the table 35, beneath the
undeflected beam 11. A current collector 42 (see FIG. 7), which is
preferably a photodiode, is disposed beneath the grid 41, which is
preferably formed of tungsten and comprises a plurality of
orthogonal wires extending in the X and Y directions.
After the grid 41 has been properly positioned beneath the beam 11,
the digital control unit 18 causes a FOCUS SERVO signal, which is a
positive pulse as shown in FIG. 15, to be supplied from the analog
unit 17. The analog unit 17 suplies the FOCUS SERVO signal over a
line 43 (see FIG. 14) and through an optical isolator 44 to an OR
gate 45. The optical isolator 44 has a return line 45' to ground.
One suitable example of the otical isolator 44 is sold by
Hewlett-Packard as model 5082-4350.
The OR gate 45 has its output connected as an input to a NAND gate
46. The other input to the NAND gate 46 is from a 300 KHz
oscillator 47.
When the FOCUS SERVO pluse is supplied to the optical isolator 44,
the NAND gate 46 allows the oscillator 47 to supply its frequency
through a dither control 48, which controls the amplitude of the
high frequency jitter supplied through a line 49 to the analog unit
17 to each of the X and Y electrostatic deflection plates 31, 32,
33, and 34 (see FIG. 1). The jitter is preferably two percent of
the width of the beam 11. Since it is applied to both the X and Y
electrostatic deflection plates 31-34 in phase, the jitter is at an
angle of 45.degree. with respect to the X and Y axes so that the
beam 11 oscillates in a single direction. Thus, in FIG. 3, the beam
11 is shown in solid lines in its position when jitter is not
applied and in phantom lines when jitter is applied.
The current, which is produced from the current collector 42, has a
jitter component due to the high frequency jitter. This jitter
component is equal to the product of the current density of the
beam 11 and the area of the jitter of the beam 11.
When the FOCUS SERVO signal is supplied to the optical isolator 44,
the beam 11 is caused to move along a square-shaped path 60 (see
FIG. 2) so as to cross portions 61, 62, 63, and 64 of the wire grid
41 in this order. Thus, the beam 11 is moved in a predetermined
closed path around the wire grid 41.
The beam 11 is initially moved in a +X direction as indicated by an
arrow 65 so as to cross the vertical portion 61 of the wire grid 41
during this scan. This initial movement of the beam 11 is caused by
a +X SCAN signal being supplied from the analog unit 17 due to the
digital control unit 18 supplying a digital signal thereto. This +X
SCAN pulse starts at the same time that the FOCUS SERVO signal
begins as shown in FIG. 15.
Thus, as shown in the timing diagram of FIG. 15, an increasing X
magnetic deflection voltage is initially supplied to the X magnetic
deflection coils 23 and 24 to move the beam 11 in the +X direction.
At the end of the increase in the X magnetic deflection voltage, a
decreasing Y magnetic deflection voltage is supplied to the Y
magnetic deflection coils 25 and 26 so that the beam 11 moves in a
-Y direction as indicated by an arrow 66 in FIG. 2 to pass over the
horizontal portion 62 of the wire grid 41. When the Y magnetic
deflection voltage ceases to fall to stop movement in the -Y
direction, a decreasing X magnetic deflection voltage is supplied
to the X magnetic deflection coils 23 and 24 to move the beam 11 in
a -X direction as indicated by an arrow 67 in FIG. 2 to pass over
the vertical portion 63 of the wire grid 41. When the X magnetic
deflection voltage stops falling so that movement in the -X
direction stops, an increasing Y magnetic deflection voltage is
supplied to the Y magnetic deflection coils 25 and 26 to move the
beam 11 in a +Y direction as indicated by an arrow 68 in FIG. 2 to
pass over the horizontal portion 64 of the wire grid 41. As a
result, the beam 11 is moved along the square-shaped predetermined
path 60.
As shown in FIG. 4, the output of the current collector 42 is at a
maximum when the beam 11 is completely off the wire grid 41,
decreases from its maximum to its minimum as the beam 11 moves over
the wire grid 41, is at a minimum when the beam 11 is completely
disposed above one of the portions 61-64 of the wire grid 41, and
increases from its minimum to its maximum as the beam 11 moves off
of one of the portions 61-64 of the wire grid 41.
During the time that the beam 11 is moving onto or off of one of
the portions 61-64 of the wire grid 41, the current at the current
collector 42 includes the oscillations produced by a high frequency
jitter from the dither control 48. The high frequency jitter
produced by the beam moving onto one of the portions 61-64 of the
wire grid 41 is indicated on the pulse of FIG. 4 by 68A and the
high frequency jitter produced by the beam 11 moving off of one of
the portions 61-64 of the wire grid 41 is indicated on the pulse of
FIG. 4 by 68B. This high frequency jitter produces an envelope
which is indicative of the quality of the focus of the beam 11.
Two different envelopes 69 and 70 of the high frequency jitter are
shown in FIG. 5. Each of the envelopes 69 and 70 has the same area
thereunder since the beam 11 produces the same current.
If the beam 11 were free of aberrations, it would have a
square-shaped envelope since the current density would be the same
in all parts of the beam 11. However, when the beam 11 goes out of
focus, the edge definitions thereof become smeared so that the
current density profile, which is represented by each of the
envelopes 69 and 70, for example, of the beam 11 becomes smeared.
If the beam 11 is astigmatic, the envelope produced by the beam 11
moving in one of the X and Y scans may be sharp but movement of the
beam 11 in the other scan direction would produce a smeared
envelope.
It should be understood that each of the envelopes 69 and 70 is
formed by voltages which are directly related to the jitter
component of the current from the current collector 42. This is
because the output of the current collector 42 is supplied to a
current to voltage converter 71 (see FIG. 7), which can be an
operational amplifier, for example, and the envelopes 69 and 70
represent the output of the voltage converter 71, which supplies an
inverted output.
By comparing the number of times that the voltage reaches some
level in each of the envelopes 69 and 70, a determination can be
made as to which of the two envelopes 69 and 70 produces the best
focus. Thus, for example, if a count is made each time that the
voltage in the envelope 69 or 70 exceeds ten percent of the peak
voltage, a comparison can be made as to which of the two envelopes
69 and 70 is producing the better focus since the envelope having
the lowest number of times that the voltage exceeds ten percent of
the maximum is a better envelope for focus.
A total of eight envelopes is produced during each movement of the
beam 11 along the entire path 60 since an envelope is produced each
time that the beam 11 moves on or off of the wire grid 41. By
comparing the total number of times that the voltage exceeds ten
percent of the peak voltage in two consecutive movements of the
beam 11 around the wire grid 41 along the path 60, a comparison
between the two cycles of movement of the beam 11 along the entire
path 60 is obtained so that it can be ascertained whether a
correction signal to the focus coil 22 in the time between the two
cycles improved or lowered the quality of the focus of the beam
11.
When the beam 11 starts to cross the vertical portion 61 of the
wire grid 41, for example, the current collector 42 produces a
varying current with the dither thereon. The signal from the
current collector 42 is supplied to the current to voltage
converter 71, which supplies an inverted output voltage to a
bandpass filter and amplifier 72 (see FIG. 7). The bandpass filter
and amplifier 72 allows only the envelope voltage, which is
produced when the beam 11 is crossing one of the portions 61-64 of
the wire grid 41, to pass therethrough to form the envelopes.
As shown in FIG. 8, the bandpass filter and amplifier 72 includes a
pair of differential amplifiers 73 and 74 cooperating to produce a
Q of ten and a gain of 100 at 300 KHz. The output of the bandpass
filter and amplifier 72 is supplied over a line 75 to an automatic
gain control 76 (see FIG. 7). The output of the bandpass filter and
amplifier 72 also is supplied over a line 77 to an oscilloscope 78
located within the analog unit 17. The oscilloscope 78 enables any
of the eight envelopes produced by movement of the beam 11 over the
wire grid 41 to be viewed.
The automatic gain control 76 maintains the amplitude of the
envelope at fourteen volts peak to peak to compensate for slow
drifts in the brightness of the beam 11. Thus, the maximum peak to
peak voltage produced on an output line 79 of the automatic gain
control 76 is fourteen volts.
The focus envelope wave form can be described as a function of time
by an expression of the form envelope = V.sub.p E(t)sin 2IIft where
sin 2IIft represents a unity amplitude sine wave of frequency f.
E(t) is a voltage that is a function of time that follows the
positive peaks of the envelope and is normalized to a maximum peak
amplitude of one, and V.sub.p is the actual positive peak amplitude
of the envelope.
As shown in FIG. 9, the automatic gain control 76 has the line 75
connected to lines 80 and 81. The line 80 supplies one of the two
inputs to a multiplier 82. The line 81 is connected to a negative
peak and hold circuit through a diode 83. The output of the
negative peak and hold circuit at the output of a differential
amplifier 84 is the negative peak voltage, -V.sub.p. This output is
supplied to a divider 85, which receives a constant input voltage
on a line 86. The divider 85 inverts the negative output of the
differential amplifier 84 and converts the signal on its output
line 87 to a positive output of 10C/v.sub.p where C is the constant
voltage input on the line 86 and V.sub.p is the positive peak
voltage.
The multiplier 82 multiplies the input on the line 80 from the
bandpass filter and amplifier 72 and the input from the divider 85
on the line 87 to supply an output voltage on the line 79. The
output voltage on the line 79 is the envelope produced by the beam
11 entering or leaving one of the portions 61-64 of the wire grid
41 and is one-tenth of the product of the two multiplicands
10C/V.sub.p and V.sub.p E(t) sin 2IIft; thus, the output voltage on
the line 79 is C[E(t) sin 2IIft]. This means that each of the focus
envelopes at the line 79 has a constant amplitude determined by C
while its shape, determined by E(t) sin 2IIft, is not
distorted.
The output voltage of the automatic gain control 76 is connected
through the output line 79 and a line 88 (see FIG. 7) to a
detecting and counting means 89. The detecting and counting means
89 includes comparators 90 and 91 (see FIG. 10) with each of the
comparators producing an output when its threshold signal is
crossed.
The threshold signal for the positive comparator 90 is ten percent
of the positive peak voltage on the output line 79. Since the
positive peak voltage is 7 volts, the positive comparator 90
produces an output signal each time that the voltage within the
envelope exceeds 0.7 volt. Similarly, the negative comparator 91
produces an output each time that the voltage is more negative than
ten percent of the negative peak voltage. Thus, each time that the
negative voltage within the envelope exceeds -0.7 volt, the
comparator 91 produces an output pulse.
The output of the positive comparator 90 is connected to a single
shot 92 which produces a positive pulse whenever the comparator 90
produces an output, which is a positive pulse. The positive output
of the single shot 92 is inverted by an inverter 93 and supplied as
a negative input pulse to a NAND gate 94.
Similarly, the output of the negative comparator 91 causes a single
shot 95 to have a positive output pulse whenever the negative
comparator 91 has its threshold voltage crossed to produce a
positive output. The positive pulse of the single shot 95 is
inverted by an inverter 96 and supplied as a negative input pulse
to the NAND gate 94.
Accordingly, whenever either of the comparators 90 and 91 has its
threshold voltage crossed to supply a negative input to the NAND
gate 94, a positive pulse occurs at the output of the NAND gate 94
since it produces a negative pulse only when both of its inputs are
positive. Thus, each output pulse from the comparators 90 and 91
appears as a positive pulse at the output of the NAND gate 94.
Therefore, all of the positive output pulses at the NAND gate 94
are a total of the number of times that the voltage within the
envelope exceeds ten percent of the peak positive or negative
voltage when the beam 11 enters or leaves one of the portions 61-64
of the wire grid 41.
The output of the NAND gate 94 is supplied through a line 97 to
input lines 98 and 99 (see FIG. 7) of an up down counting means
100. The input line 98 supplies one of the inputs to a NAND gate
101 (see FIG. 11), which has its other input supplied from Q output
of a clocked JK flip flop 102. The Q output of the flip flop 102 is
tied to K input of the flip flop 102.
Thus, when the flip flop 102 is supplying a positive signal at the
Q output, the NAND gate 101 produces a negative pulse on its output
line 103 each time that a positive pulse appears on the input line
98 to the NAND gate 101 from the output of the NAND gate 94.
Accordingly, each pulse counted by the NAND gate 94 from each of
the comparators 90 and 91 is supplied on the output line 103 when
the Q output of the flip flop 102 is positive.
The input line 99 of the up down counting means 100 comprises one
of the inputs to a NAND gate 104, which has its other input
supplied from Q output of the flip flop 102. The Q output is tied
to J input of the flip flop 102.
Thus, when the Q output of the flip flop 102 is positive, the NAND
gate 104 produces a negative pulse on its output line 105 each time
that the NAND gate 94 has a positive pulse on the output line 97.
Therefore, if the flip flop 102 is set in the state in which the Q
output is positive, then the NAND gate 104 counts the pulses from
the NAND gate 94 and supplies these on the line 105.
The flip flop 102 has its clock input C connected by a line 106 to
a single shot 107 (see FIG. 7). The single shot 107 is connected
through a line 108 to receive the +X SCAN signal from the analog
unit 17 (see FIG. 1). Thus, each time that the beam 11 starts to
move along the path 60 (see FIG. 2), the single shot 107 (see FIG.
7) is fired to supply a pulse to the C input of the flip flop 102
(see FIG. 11).
The negative going portion of the pulse from the single shot 107
trips the flip flop 102 to change its state. Therefore, each time
that the beam 11 is to start movement along the path 60, the state
of the flip flop 102 is changed to cause the opposite of the NAND
gates 101 and 104 from that in the prior cycle of movement of the
beam 11 along the path 60 to be responsive to the positive pulses
on the line 97.
The lines 103 and 105 are connected to a twelve bit up down counter
110. When the NAND gate 101 is responsive to the positive pulses
from the NAND gate 94 due to the Q output of the flip flop 102
being positive, the counter 110 counts up. When the NAND gate 104
is responsive to the positive pulses from the NAND gate 94 due to
the Q output of the flip flop 102 being positive, the counter 110
counts down.
If the down count is greater than the up count, the counter 110
supplies a negative pulse on its output line 111. No signal is
supplied on the output line 111 if the down count does not exceed
the up count. One suitable example of the counter 110 is sold by
Fairchild Semiconductor as model 9366.
When the flip flop 102 has its state changed so that the NAND gate
101 causes the counter 110 to count up, the signal from the Q
output of the flip flop 102 is supplied not only to the NAND gate
101 but also to a single shot 112, which produces a positive pulse
on its output line 113 when the Q output of the flip flop 102 goes
positive. The output line 113 is connected to the master reset of
the counter 110 to reset the counter 110 to zero prior to counting
up.
When the flip flop 102 changes state to cause count down in the
counter 110, the Q output, which enables the NAND gate 104 to be
activated by any positive pulse on the line 99, also causes a
single shot 114 to produce a positive pulse on its output line 115.
The single shot 114 produces the positive pulse when the Q output
of the flip flop 102 goes positive to enable the NAND gate 104 for
the pulses from the NAND gate 94 to cause down count in the counter
110.
The output line 111 of the counter 110 is connected as one input to
a NAND gate 116 (see FIG. 12) of a focus control unit 117. The
other input to the NAND gate 116 is from a flip flop 118, which is
the same type as the flip flop 102. The flip flop 118 receives a
signal from the analog unit 17 at the start of the FOCUS SERVO
signal to change its state. The pulse, which is identified as FOCUS
SERVO INITIATE in the timing diagram of FIG. 15, is applied through
a line 119 from the analog unit 17. An inverter 120 causes a
negative pulse to be supplied to input S of the flip flop 118
whereby output Q of the flip flop 118 goes positive when the FOCUS
SERVO INITIATE pulse is supplied on the line 119. Thus, a positive
input is supplied to the NAND gate 116 from the Q output of the
flip flop 118 at the start of the cycle for automatically focusing
the beam 11.
The Q output of the flip flop 118 also is connected as one input to
a NAND gate 121, which receives its other input from the output
line 115 of the single shot 114. Thus, after the up count is
completed and down count is about to start so that the single shot
114 provides a positive pulse, the NAND gate 121 has its output
change state to supply a negative pulse on its output line 122.
The line 122 is connected as one of the inputs to each of NAND
gates 123 and 124. The other input to the NAND gate 123 is from a
flip flop 125, which is the same type as the flip flop 102. Thus,
the NAND gate 123 is connected to output Q of the flip flop 125.
The other input to the NAND gate 124 is from output Q of the flip
flop 125.
The flip flop 125 has its clock input C connected by a line 126 to
the output of the NAND gate 116. Thus, each time that there is a
positive pulse from the NAND gate 116, the flip flop 125 changes
state to cause the other of the NAND gates 123 and 124 to have its
output responsive to signals supplied from the NAND gate 121.
Each time that the NAND gate 121 provides a negative pulse on the
output line 122, the counter 110 has counted up. Thus, at the
completion of each up count, which is a counting of all the times
that the voltage exceeds ten percent of the peak voltage in the
eight envelopes produced during movement of the beam 11 one time
around the path 60, one of the NAND gates 123 and 124, depending on
the state of the flip flop 125, produces a pulse to an eight bit up
down counter 127. One suitable example of the eight bit up down
counter 127 is sold by Fairchild Semiconductor as model 9366.
The output of the counter 127 is connected to an eight bit digital
to analog converter (DAC) 128. One suitable example of the DAC 128
is sold by Beckman Instruments as model 845. The DAC 128 supplies a
voltage through its output line 129 and a coil driver 130 (see FIG.
7) to the focus coil 22.
If the counter 110 (see FIG. 11) produces a signal on the output
line 111 at the completion of the down count, the NAND gate 116
(see FIG. 12) provides a positive pulse on the output line 126 to
change the state of the flip flop 125. As a result, the other of
the NAND gates 123 and 124 produces an output in response to the
output of the NAND gate 121 at the completion of the next cycle of
the beam 11 along the path 60.
Each of the positive output pulses from the NAND gate 123 causes an
up count in the counter 127. Each of the positive output pulses
from the NAND gate 124 produces a down count in the counter 127.
Thus, if the pulse supplied to the counter 127 is from the NAND
gate 123, the voltage from the eight bit DAC 128 increases to cause
an increase in the current to the focus coil 22. If the NAND gate
124 supplies the pulse to the counter 127, then the eight bit DAC
128 decreases the output voltage on the line 129 to cause the
current to the focus coil 22 to decrease.
The counter 110 (see FIG. 11) produces a negative pulse on the
output line 111 only when the number of pulses in the down count
exceeds that during the up count. This means that a worse focus
exists during the second scan, which is when the down count is
occurring, than existed prior to change in the current in the focus
coil 22 at the end of the first scan (an up count). Therefore, it
is necessary to change the direction in which the counter 127 (see
FIG. 12) is being stepped or counted so that the direction of
change of the current to the focus coil 22 is reversed.
Thus, when the next scan occurs after the down count so that the
counter 110 is counting up, the output on the line 122 of the NAND
gate 121 at the completion of the up count would now be supplied to
the opposite of the NAND gates 123 and 124 to shift the direction
in which the counter 127 counts. This reverses the direction of
change of the current to the focus coil 22. That is, if the current
had been increasing, it will be decreased. Similarly, if the
current had been decreasing, it will be increased.
Of course, if there is no output on the line 111 at the completion
of the down count of the counter 110, then this means that the
focus has been improved by the change in the current to the focus
coil 22 at the end of the prior scan (up count). Therefore, the
state of the flip flop 125 is not altered, and the counter 127
continues to count in the same direction.
Since it is desired for a minimum number of the pulses exceeding
ten percent of the negative or positive peak voltage to exist
during movement of the beam 11 through a cycle along its path 60 to
indicate the best quality of the focus of the beam 11, there is a
focus coil current at which the number of pulses will be at a
minimum. Referring to FIG. 16, there is shown a relationship
between the number of pulses and the current in the focus coil 22.
It will be assumed that the number of pulses is being reduced by
each increase in the focus coil current at the end of each up count
by the counter 110 (see FIG. 11) with the counter 127 (see FIG. 12)
counting up because of pulses from the NAND gate 123 to increase
the voltage from the DAC 128.
When the minimum number of pulses occurs, the next scan (a down
count) will result in the number of pulses increasing as indicated
by step 131 in FIG. 16. When this occurs, the counter 110 (see FIG.
11) produces a negative pulse on the output line 111 because the
number of pulses counted during the down count exceeds those
counted during the up count whereby the flip flop 125 (see FIG. 12)
has its state changed. As a result, the current to the focus coil
22 will be decreased by the DAC 128 producing a lower voltage since
the counter 127 is now counted down at the end of the up count in
the counter 110 by the NAND gate 124.
With the focus coil current now decreased during the next down
count in the counter 110, the number of pulses will be a minimum
during this cycle of movement of the beam 11 along the path 60.
This is during a down count in the counter 110. Thus, there is no
change in the state of the flip flop 125 at the end of the down
count in the counter 110.
As a result, the current to the focus coil 22 is decreased at the
end of the scan producing the next up count in the counter 110.
This increases the number of pulses beyond the minimum during the
next down count in the counter 110 so that the counter 110 again
produces a negative pulse on the output line 111 at the end of the
down count in the counter 110. This increase in the number of the
pulses during the down count is indicated by step 132 in FIG.
16.
As a result of the negative pulse on the output line 111, the flip
flop 125 again changes state. This causes the current to the focus
coil 22 to again be increased until the step 131 is again reached.
At this time, the counter 110 again produces a pulse on the line
111 to again shift the state of the flip flop 125.
When three of these pulses have occurred on the line 111, the
minimum number of pulses within the envelope will have been
produced since these three changes indicate that the valley of the
number of pulses has been reached as indicated by curved line 133
in FIG. 16. When this occurs, it is desired to stop the focus
control unit from functioning.
Accordingly, the output line 126 of the NAND gate 116 is not only
connected to the flip flop 125 but also to a three state counter
135 (see FIG. 12). Thus, the counter 135 counts each time that the
flip flop 125 changes state. The counter 135 is actually a four bit
counter with its third output being employed to activate a single
shot 136. One suitable example of the counter 135 is sold by
Fairchild Semiconductor as model 9366.
The single shot 136 produces a negative output pulse on its line
137 to reset input R of the flip flop 118. This causes the Q output
of the flip flop 118 to become negative whereby the NAND gate 121
can no longer change state when a positive pulse is supplied from
the line 115. Therefore, when the flip flop 125 has changed state
three times so as to indicate that the minimum number of pulses are
within the eight envelopes, the counter 135 prevents further
activation of the NAND gate 121 so that there is no further change
to the DAC 128. The DAC 128 holds its final output until the next
time a focus servo cycle is activated to supply the FOCUS SERVO
INITIATE pulse to the flip flop 118.
The single shot 136 also provides the negative pulse on its output
line 137 by a line 138 to the master reset of the counter 135. This
negative pulse sets the counter 135 to zero.
As previously mentioned, there are eight envelopes produced by the
beam 11 crossing the four portions 61-64 (see FIG. 2) of the wire
grid 41. The total times that the voltage in these eight envelopes
exceeds ten percent of the positive or negative peak voltage is
what is counted in the counter 110.
Furthermore, the beam 11 passes completely around the wire grid 41
sixteen times. Even though two of these cycles are required for the
up and down count to the counter 110, the eight times that there
can be control of the current to the focus coil 22 are sufficient
to focus the beam 11 satisfactorily.
In addition to automatically correcting the focus of the beam 11,
the present invention also allows manual correction for quality of
the focus of the beam 11. Accordingly, the output line 97 (see
FIGS. 7 and 10) of the NAND gate 94 (see FIG. 10) also supplies the
positive output pulses of the NAND gate 94 through a line 150 (see
FIG. 7) to an eight-twelve bit counter 151. One suitable example of
the eight-twelve bit counter 151 is sold by Fairchild Semiconductor
as model 9366.
The eight-twelve bit counter 151 counts all of the pulses from the
output of the NAND gate 94 and obtains an average of the eight
envelopes produced during one cycle of movement of the beam 11
around the path 60 by dividing by two or four depending on the size
of the beam 11. By dividing by two or four, the average quality of
the focus of the beam 11 in both the X and Y axes is obtained.
The output of the eight-twelve bit counter 151 is supplied to an
eight-bit digital to analog converter (DAC) 152. One suitable
example of the DAC 152 is sold by Analog Devices Inc. as model DAC
8QM.
When the scan of the beam 11 in the +X direction starts as
indicated by the +X SCAN pulse on the line 108 to cause the single
shot 107 to produce a positive pulse, the positive pulse from the
single shot 107 is not only transmitted over the line 106 to the
flip flop 102 (see FIG. 11) but also is supplied through a line 153
(see FIG. 7) to the eight bit DAC 152 to strobe it. When this
occurs, the eight bit DAC 152 stores the value of the counter 151
until the next strobe and supplies an output voltage, which is
representative of the average quality of the focus of the beam 11
in both the X and Y axes at the completion of a cycle of movement
of the beam 11 along its entire path 60, on its output line
154.
A line 155 transmits this signal from the line 154 to a voltmeter
156. The output voltage on the line 154 is proportional to the
average width of the eight envelopes. Through applying the voltage
from the eight bit DAC 152 to the voltmeter 156, the quality of the
focus of the beam 11 can be readily ascertained from viewing the
voltmeter 156. The voltage, which produces a satisfactory focus, is
known and compared to the voltage at the voltmeter 156.
The negative going portion of the single shot 107 is supplied to a
single shot 157 to produce a positive pulse on its output line 158.
The positive pulse on the output line 158 is supplied by a line 159
to master reset of the eight-twelve bit counter 151 to reset it to
zero. Since this occurs after the eight bit DAC 152 has stored the
value of the counter 151 and from this supplies the output voltage
over the line 154, the counter 151 is ready to begin to count
again. Since the output pulse of each of the single shots 107 and
157 is very short, the eight-twelve bit counter 151 is set to zero
before the beam 11 reaches the vertical portion 61 (see FIG. 2) of
the wire grid 41.
To cause the jitter frequency to be supplied over the line 49 (see
FIG. 14) when there is not to be automatic adjustment of the
quality of the focus of the beam 11, a positive input voltage must
be supplied to the NAND gate 46. This can be supplied from closing
a manual switch 160 to supply a positive input voltage to the OR
gate 45. Similarly, a MONITOR ENABLE signal can be supplied to the
OR gate 45 through an optical isolator 161, which is the same as
the optical isolator 44, to the OR gate 45. The input of the
MONITOR ENABLE signal from the analog unit 17 is over a line 162 to
the optical isolator 161. A line 163 from the optical isolator 161
is grounded.
Thus, by either closing the manual switch 160 or producing the
MONITOR ENABLE signal from the analog unit 17, a positive input
voltage is supplied to the NAND gate 46 whereby the jitter
frequency appears on the line 49. Of course, it should be
understood that the voltmeter 156 also visually presents the
quality of the beam during the automatic adjustment.
In addition to manual correction for quality of the focus of the
beam 11, the present invention also allows a determination of
whether the brightness of the beam 11 is sufficient to enable
manual correction of the focus of the beam 11.
As shown in FIG. 6, an envelope 170 is produced when the focus of
the beam 11 is satisfactory but there is poor illumination. The
envelope 170 has a non-flat or slanted top. Accordingly, the area
beneath the top and above ninety percent of the peak voltage is
smaller, for example, in comparison with that shown in the envelope
69, which has good focus and illumination in FIG. 5. Thus, the
number of times that the voltage exceeds ninety percent of the peak
voltage in the envelope 170 is less than in the envelope 69; this
indicates that the envelope 69 is produced by the beam 11 having
better illumination than when the envelope 170 is produced by the
beam 11.
The poor illumination is due to the current density not being the
same in all portions of the beam 11. Correction for illumination is
obtained through correcting the alignment of the beam 11 as more
particularly shown and described in the copending patent
application of Hans C. Pfeiffer et al for "Method And Apparatus For
Aligning Electron Beams," Ser. No. 393,365, filed Aug. 31, 1973,
now U.S. Pat. No. 3,894,271, and assigned to the same assignee as
the assignee of this application.
To ascertain whether the illumination of the beam 11 is poor, an
illumination detecting means 171 (see FIG. 7) is connected to the
output line 79 of the automatic gain control 76 by a line 172. The
line 172 is connected to a squarer 173 (see FIG. 13), which is a
multiplier used as a squarer, of the illumination detecting means
171. The squarer 173 makes each of the envelopes produced by the
beam 11 passing over the wire grid 41 entirely positive.
To ascertain each time that the voltage exceeds ninety percent of
the peak voltage as the beam 11 passes over the wire grid 41, the
output of the squarer 173 is connected to a comparator 174, which
produces an output each time that its threshold voltage is crossed
by the output voltage of the squarer 173 exceeding eighty-one
percent of the peak voltage. This is because the squarer 173
reduces the voltage of ninety percent of the peak voltage to
eighty-one percent of the peak voltage. The output of the
comparator 174 is connected to an eight-twelve bit counter 175,
which is preferably the same as the eight-twelve bit counter 151.
The counter 175 counts each time that the comparator 174 produces
an output due to its threshold being crossed.
The eight-twelve bit counter 175 counts all of the pulses from the
output of the comparator 174 and obtains an average of the eight
envelopes produced during one cycle of movement of the beam 11
around the path 60 (see FIG. 2) by dividing by two or four
depending on the size of the beam 11. By dividing by two or four,
the average quality of illumination of the beam 11 in both the X
and Y axes is obtained.
The output of the eight-twelve bit counter 175 (see FIG. 13) is
supplied to an eight bit digital to analog converter (DAC) 176.
When the beam 11 starts to move in the +X direction as indicated by
the +X scan signal on the line 108 (see FIG. 7) to cause the single
shot 107 to produce a positive output pulse, the positive pulse is
not only transmitted to the flip flop 102 and the eight bit DAC 152
but also is transmitted by a line 177 to the eight bit DAC 176 (see
FIG. 13) to strobe it. When this occurs, the eight bit DAC 176
supplies an output voltage, which is representative of the
illumination of the beam 11 in both the X and Y axes at the
completion of a cycle of movement of the beam 11 around its closed
path 60, on its output line 178.
The output voltage on the line 178 is transmitted to a comparator
180. The output voltage on the line 178 is proportional to the
average width of the eight envelopes at the ninety percent level of
the peak voltage.
If the output voltage from the eight bit DAC 176 is high enough to
indicate that the illumination of the beam 11 is satisfactory, then
the threshold voltage of the comparator 180 is crossed and a
positive output pulse occurs on a line 181. However, if the output
voltage from the eight bit DAC 176 is not sufficient to indicate
satisfactory illumination, then the threshold voltage of the
comparator 180 is not crossed and a positive output voltage does
not appear on the output line 181 from the comparator 180.
The output line 181 of the comparator 180 is connected to an
inverter 182, which is connected by a line 183 as one input to a
NAND gate 184. The other input to the NAND gate 184 is from an
inverter 185, which receives an input from a comparator 186.
The comparator 186 is connected to the line 154 (see FIG. 7) by a
line 187 to receive the output voltage of the eight bit DAC 152. As
previously mentioned, this voltage is proportional to the average
width of the eight envelopes at the ten percent level of the peak
voltage of the envelope.
If the voltage from the eight bit DAC 152 exceeds the threshold
voltage of the comparator 186 (see FIG. 13), then a pulse appears
on an output line 189, which connects the comparator 186 to the
inverter 185. This is indicative of the quality of the focus of the
beam 11 being poor.
If the output voltage from the eight bit DAC 152 does not exceed
the threshold voltage of the comparator 186, then there is no
output from the comparator 186 so that the output line 189 does not
have a positive pulse thereon. This indicates that the focus of the
beam 11 is satisfactory.
Accordingly, when the focus of the beam 11 is satisfactory, the
inverter 185 provides a positive pulse as the other input to the
NAND gate 184. If the illumination is poor, then a positive pulse
also appears on the line 183 from the inverter 182 because the
threshold voltage of the comparator 180 was not crossed by the
output of the eight bit DAC 176. As a result of two positive or
high signals as the inputs to the NAND gate 184, a low output
occurs on output line 190 of the NAND gate 184. An inverter 191
changes this signal to a high for supply to a status decoder 192.
This high to the status decoder 192 is indicative of poor
illumination and causes the status decoder 192 to produce a signal
to illuminate a warning light. As a result, the alignment of the
beam 11 must be corrected for the focus and astigmatism of the beam
11 to be manually corrected.
The output line 158 (see FIG. 7) of the single shot 157 also is
connected by a line 193 to the master reset of the counter 175 (see
FIG. 13) to reset it to zero. Since this occurs after the eight bit
DAC 176 has supplied the output voltage over the line 178, the
counter 175 is ready to begin to count again at the same time that
the counter 151 is. Since the output pulse of each of the single
shots 107 and 157 is very short, the counter 175 is set to zero
before the beam 11 reaches the vertical portion 61 (see FIG. 2) of
the wire grid 41 in the same manner as discussed with respect to
the counter 151.
Considering the operation of the present invention, the beam 11 is
disposed at the center of its deflection field for manual
correction for focus and astigmatism of the beam 11. Either the
MONITOR ENABLE signal is supplied to the optical isolator 161 (see
FIG. 14) or the manual switch 160 is closed to cause a jitter
frequency to be supplied to the X and Y electrostatic deflection
plates 31-34 (see FIG. 1) in phase from the oscillator 47 (see FIG.
14). Manual controls on the column control unit 16 are turned to
adjust the focus of the beam 11 and its astigmatism in accordance
with the voltage on the voltmeter 156. The manual controls change
the resistances of potentiometers in the column control unit
16.
After the beam 11 has been satisfactorily manually adjusted at the
center of the field, the beam 11 is deflected to each of the
corners of the field with the focus coil 22 being adjusted
initially and then the stigmator coils 21A and 21B being adjusted.
Thereafter, the beam 11 is deflected to the middle of each of the
sides of the field and the stigmator coils 21A and 21B are manually
adjusted to further correct for astigmatism. This manual adjustment
is not necessary for about a year unless some portion of the
electron beam assembly is disassembled.
Whenever there is to be automatic adjustment of the focus of the
beam during one of the C cycles, the FOCUS SERVO signal, which is a
positive pulse as shown in the timing diagram of FIG. 15, is
supplied through the optical isolator 44 (see FIG. 14) to the OR
gate 45 to cause a jitter frequency to appear on the output line 49
of the dither control 48. The output line 49 supplies this voltage
through the analog unit 17 (see FIG. 1) to the X and Y
electrostatic deflection plates 31-34 in phase.
As shown in the timing diagram of FIG. 15, the X magnetic
deflection voltage to the X magnetic deflection coils 23 and 24
increases from its minimum to its maximum to move the beam 11 in
the +X direction as indicated by the arrow 65 in FIG. 2. The Y
magnetic deflection voltage to the Y magnetic deflection coils 25
and 26 starts to decrease from its maximum to its minimum as soon
as the X magnetic deflection voltage reaches its maximum. Thus,
this causes the beam 11 to move in the -Y direction as indicated by
the arrow 66 in FIG. 2.
At the time that the FOCUS SERVO signal is supplied, the +X SCAN
signal, which is a positive pulse, is supplied from the analog unit
17 (see FIG. 1) on the line 108 (see FIG. 7). As shown in the
timing diagram of FIG. 15, the +X SCAN pulse remains up for the
entire time that the beam 11 moves in the +X direction as indicated
by the arrow 65 in FIG. 2.
The +X SCAN pulse causes the timing pulse from the single shot 107
(see FIG. 7) to be supplied through the line 106 to change the
state of the flip flop 102 (see FIG. 11). It also strobes the eight
bit DAC 152 (see FIG. 7) and the eight bit DAC 176 (see FIG. 13) to
cause their outputs to be supplied at the same time to the
voltmeter 156 (see FIG. 7) and the comparator 186 (see FIG. 13)
from the eight bit DAC 152 and to the comparator 180 (see FIG. 13)
from the eight bit DAC 176. However, illumination is not utilized
during the automatic adjustment. Furthermore, the voltmeter 156 is
normally not utilized during automatic adjustment of the focus of
the beam 11.
It should be understood that the timing pulse from the single shot
107 and the timing pulse from the single shot 157 are very short in
time in comparison with the +X SCAN as shown in FIG. 15. Thus, the
flip flop 102 changes state almost as soon as the +X SCAN signal is
supplied.
As shown in the timing diagram of FIG. 15, the state of the flip
flop 102 (see FIG. 11) is set so that the counter 110 will count up
during the deflection of the beam 11 around its path 60 (see FIG.
2). Thus, the NAND gate 101 (see FIG. 11) will supply pulses from
the output of the NAND gatee 94 (see FIG. 10) to the counter 110
(see FIG. 11).
As shown in the timing diagram of FIG. 15, the pulse from the
output of the single shot 112 (see FIG. 11) to the master reset of
the counter 110 is supplied when the flip flop 102 changes state.
Thus, the counter 110 is set at zero just after the flip flop 102
changes state to allow pulses from the NAND gate 94 to be supplied
through the NAND gate 101 to the counter 110.
While the beam 11 starts to scan in the +X direction as soon as the
+X SCAN signal is supplied to increase the magnetic deflection
voltage to the coils 23 and 24 (see FIG. 1), the flip flop 102 (see
FIG. 11) does not change state until the negative going portion of
the output of the single shot 107. When the Q output of the flip
flop 102 goes positive, the single shot 112 supplies a positive
pulse on the line 113 to reset the counter 110 to zero. All of this
occurs before the beam 11 reaches the vertical portion 61 (see FIG.
2) of the wire grid 41 during its movement in the +X direction.
At the completion of the first deflection cycle of the beam 11
around its path 60, another +X SCAN pulse is supplied from the
analog unit 17. The counter 110 has counted each time that the
voltage in each of the eight envelopes has exceeded ten percent of
the positive or negative peak voltage. These pulses are stored in
the counter 110.
When the +X SCAN pulse occurs to cause the next deflection cycle of
the beam 11 around its closed path 60, the state of the flip flop
102 is changed by the pulse from the single shot 107 (see FIG. 7)
so that the Q output of the flip flop 102 goes positive. This
results in the output pulses from the NAND gate 94 (see FIG. 10)
being supplied through the NAND gate 104 (see FIG. 11) to the
counter 110. This causes the counter 110 to count down from the
count stored in the counter 110 during the prior deflection cycle
of the beam 11 around its closed path 60.
If the down count exceeds the up count, then a negative pulse
appears on the line 111 as indicated in the timing diagram of FIG.
15 as "BORROW WRONG DIRECTION ON LINE 111." This negative signal
occurs during or at the end of the down count depending on when the
down count exceeds the up count. If the down count does not exceed
the up count, then no signal appears on the line 111.
When the FOCUS SERVO signal starts, the FOCUS SERVO INITIATE signal
(see FIG. 15) is supplied from the analog unit 17 (see FIG. 1) to
the line 119 (see FIG. 12) to set the flip flop 118 in the
condition in which the Q output of the flip flop 118 is positive.
The Q output of the flip flop 118 is supplied as one input to each
of the NAND gates 116 and 121.
When a positive pulse from the single shot 114 (see FIG. 11) occurs
due to the flip flop 102 changing state so that the Q output goes
positive to start the down count, the NAND gate 121 (see FIG. 12)
supplies a negative pulse on the line 122 since both of its inputs
are positive. This causes one of the NAND gates 123 and 124,
depending on the state of the flip flop 125, to have its output
supply a positive pulse to the counter 127 whereby the eight bit
DAC 128 supplies a change in voltage to the focus coil driver 130
(see FIG. 7) to change the current to the focus coil 22.
If the down count in the counter 110 (see FIG. 11) exceeded the up
count, then the negative pulse on the line 111 causes the NAND gate
116 (see FIG. 12) to produce a positive pulse on the line 126 to
change the state of the flip flop 125. Accordingly, when the
counter 110 completes the up count during the next cycle of
deflection of the beam 11 around its closed path 60 (see FIG. 2)
over the wire grid 41, the other of the NAND gates 123 (see FIG.
12) and 124 will supply the signal to the counter 127 to reverse
the direction of count in the counter 127 whereby the eight bit DAC
128 changes the direction in which the magnitude of the output
voltage is being altered. If this change in voltage results in the
down count in the counter 110 (see FIG. 11) again exceeding the up
count during the next down count cycle, then the flip flop 125 (see
FIG. 12) again changes state. This is shown in FIG. 15 by a second
negative pulse on the line 111 (see FIG. 11) no later than
completion of the down count. If at the end of the third down count
cycle, another negative pulse appears on the line 111 to indicate
that the down count exceeded the up count in the counter 110, then
the focus is at its best adjustment as previously mentioned. As a
result, the three state counter 135 (see FIG. 12), which has
counted each time that there was a positive pulse on the line 126,
causes the flip flop 118 to change state through a negative pulse
from the single shot 136 to the R input of the flip flop 118.
The changing of the state of the flip flop 118 prevents any further
negative pulse from being supplied from the NAND gate 121. It also
prevents the NAND gate 116 from supplying positive pulses as an
output since the output of the NAND gate 116 remains up. This is
because the input to the NAND gates 116 and 121 from the Q output
of the flip flop 118 remains negative.
The FOCUS SERVO signal is supplied to the optical isolator 44 (see
FIG. 14) throughout the C cycle. During the time that the optical
isolator 44 is supplying an input to the OR gate 45 to activate the
dither control 48, this signal also is supplied through an inverter
194 and a line 195 to the analog unit 17 (see FIG. 1) to inhibit
any other operations which occur during the C cycle such as
correcting for alignment of the beam 11, for example, as shown and
described in the aforesaid Pfeiffer et al application.
While the present invention has shown and described a change in the
voltage to the focus coil driver 130 occurring only after the end
of an up count to the counter 110 and a reversal in the direction
in which the magnitude is being changed only during the down count
or at the end thereof, it should be understood that such is not a
requisite of the present invention. Thus, if desired, each cycle of
deflection of the beam 11 along its closed path 60 could be
compared with the prior cycle and correction made to the focus of
the beam 11 after each cycle rather than after every other cycle.
Of course, this would require a different circuit arrangement.
While the present invention has shown and described the beam 11 as
being moved in the square path 60, it should be understood that
such is not a requisite for satisfactory operation. Thus, the beam
11 could be deflected in any closed path such as a circle or a
rectangle, for example. Furthermore, it is not even necessary for
the beam 11 to be deflected or moved in a closed path but only for
the beam 11 to move along the same predetermined path during each
cycle of movement.
An advantage of this invention is that the focus of a beam of
charged particles can be automatically adjusted. Another advantage
of this invention is that the quality of the beam focus is visually
obtainable. A further advantage of this invention is that the
quality of illumination of the beam is visually obtainable.
While the invention has been particularly shown and described with
reference to a preferred embodiment thereof, it will be understood
by those skilled in the art that various changes in form and
details may be made therein without departing from the spirit and
scope of the invention.
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