U.S. patent number 3,927,320 [Application Number 05/493,878] was granted by the patent office on 1975-12-16 for method and apparatus for deriving from a scanning electron microscope signals that can be displayed stereoscopically.
This patent grant is currently assigned to Ontario Research Foundation. Invention is credited to Eric John Chatfield, Verner Henrick Nielsen.
United States Patent |
3,927,320 |
Chatfield , et al. |
December 16, 1975 |
Method and apparatus for deriving from a scanning electron
microscope signals that can be displayed stereoscopically
Abstract
A scanning electron microscope having line deflection coils
ahead of its final aperture is modified by the addition of an ALD
(after lens deflection) coil. Rectangular waveforms are supplied to
these coils with the polarities of the waveforms being reversed in
a predetermined fashion dependent upon the scanning sequence, thus
enabling a given area on the object being scanned to be "seen" from
two different angles. The derived signals may be supplied to a
colour television receiver or monitor along with appropriate
switching signals to enable the image of the object to be shown
stereoscopically.
Inventors: |
Chatfield; Eric John (Oakville,
Ontario, CA), Nielsen; Verner Henrick (Oakville,
Ontario, CA) |
Assignee: |
Ontario Research Foundation
(Ontario, CA)
|
Family
ID: |
10403751 |
Appl.
No.: |
05/493,878 |
Filed: |
August 2, 1974 |
Foreign Application Priority Data
|
|
|
|
|
Aug 14, 1973 [UK] |
|
|
38483/73 |
|
Current U.S.
Class: |
250/307; 250/310;
250/311; 250/399 |
Current CPC
Class: |
H01J
37/15 (20130101); H01J 37/28 (20130101); H01J
37/1478 (20130101) |
Current International
Class: |
H01J
37/147 (20060101); H01J 37/28 (20060101); H01J
37/15 (20060101); G01N 023/00 () |
Field of
Search: |
;250/307,310,311,398,399 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Willis; Davis L.
Attorney, Agent or Firm: Sim & McBurney
Claims
The embodiments of the invention in which an exclusive property or
privilege is claimed are defined as follows:
1. In a scanning electron microscope of the type having an electron
gun for producing an electron beam, a final aperture through which
at least a part of said electron beam passes prior to impingement
on an object being scanned by said electron beam, a longitudinal
axis extending from said electron gun through said final aperture
and through the object being scanned by said electron beam, and an
electron beam deflecting system for scanning said electron beam
across the object being scanned in adjacent parallel lines, said
electron beam deflecting system including a plurality of electron
beam line deflection devices and means for supplying a scanning
waveform to at least one of said electron beam line deflection
devices to scan said electron beam in a line across the object
being scanned by said electron beam, said plurality of electron
beam line deflection devices including at least two electron beam
line deflection devices located between said electron gun and said
final aperture and a third electron beam line deflection device
located between said final aperture and the object being scanned by
said electron beam; the improvement comprising apparatus for
enabling said scanning electron microscope to generate signals that
enable an image of the object being scanned by said electron beam
to be viewed in three dimensions, said apparatus comprising means
for supplying to at least two of said electron beam line deflection
devices located between said electron gun and said final aperture a
waveform which (a) during the scanning of a first set of said
lines, and in addition to any deflection of said electron beam
caused by said scanning waveform, creates a field which deflects an
electron beam that is travelling from said electron gun through
said final aperture on said longitudinal axis in a first path
firstly away from said longitudinal axis and to one side of said
longitudinal axis and lastly towards said longitudinal axis to
intersect said longitudinal axis in the immediate vicinity of said
final aperture and then to proceed away from said longitudinal axis
but on the opposite side thereof and (b) during the scanning of a
second set of said lines, and in addition to any deflection of said
electron beam caused by said scanning waveform, creates a field
which deflects an electron beam that is travelling from said
electron gun through said final aperture on said longitudinal axis
in a second path firstly away from said longitudinal axis and to
said opposite side thereof and lastly towards said longitudinal
axis to intersect said longitudinal axis in the immediate vicinity
of said final aperture and then to proceed away from said
longitudinal axis but on said one side thereof, and means for
supplying to said third electron beam line deflection device an
electron beam deflecting waveform, said electron beam deflecting
waveform being applied to said third electron beam deflection
device during the scanning of said respective sets of lines and
deflecting said electron beam in addition to any deflection thereof
caused by said scanning waveform, the nature of said electron beam
deflecting waveform being such that said electron beam deflecting
waveform creates a field which, when acting on an electron beam
travelling in the portions of said first and second paths that are
beyond said final aperture, deflects an electron beam travelling in
the portions of said first and second paths respectively that are
beyond said final aperture back towards said longitudinal axis to
intersect said longitudinal axis at the object being scanned by
said electron beam from both said one and said opposite sides
thereof.
2. The invention according to claim 1 wherein said scanning
electron microscope employs interlaced scanning and said first and
second sets of lines are interlaced.
3. The invention according to claim 1 wherein said electron beam is
scanned across the object following each adjacent line in sequence
and said first and second sets of lines are the same sets of
lines.
4. The invention according to claim 1 wherein said scanning
electron microscope includes a final electromagnetic lens, said
third electron beam line deflection device also being located
between said final electromagnetic lens and the object being
scanned by said electron beam, and means for rotating the electron
beam deflecting field of said third electron beam line deflection
device relative to the electron beam deflecting fields of said
electron beam line deflection devices located between said electron
gun and said final aperture.
5. The invention according to claim 1 wherein said scanning
waveform is a sawtooth waveform, said means for supplying said
scanning waveform to at least one of said electron beam line
deflection devices comprising means for supplying said scanning
waveform to at least two electron beam line deflection devices.
6. The invention according to claim 1 wherein said scanning
waveform is a sawtooth waveform, said means for supplying said
scanning waveform to at least one of said electron beam line
deflection devices comprising means for supplying said scanning
waveform to at least said third electron beam line deflection
device.
7. The invention according to claim 1 wherein said scanning
waveform is a sawtooth waveform, said means for supplying said
scanning waveform to at least one of said electron beam line
deflection devices comprising means for supplying said scanning
waveform to said two and said third electron beam line deflection
devices.
8. The invention according to claim 1 wherein said electron beam
line deflection devices comprise electromagnetic devices for
deflecting said electron beam.
9. The invention according to claim 1 wherein said supplying means
comprise a rectangular waveform generator and means connecting said
rectangular waveform generator to supply a rectangular waveform to
said two and said third electron beam line deflection devices.
10. The invention according to claim 9 wherein said scanning
electron microscope employs interlaced scanning and said first and
second sets of lines are interlaced.
11. The invention according to claim 10 wherein said scanning
waveform is a sawtooth waveform, said means for supplying said
scanning waveform to at least one of said electron beam line
deflection devices comprising means for supplying said scanning
waveform to at least said two electron beam line deflection
devices.
12. The invention according to claim 11 wherein said scanning
electron microscope includes a final electromagnetic lens, said
third electron beam line deflection device also being located
between said final electromagnetic lens and the object being
scanned by said electron beam, and means for rotating the electron
beam deflecting field of said third electron beam line deflection
device relative to the electron beam deflecting fields of said two
electron beam line deflection devices.
13. The invention according to claim 12 wherein said electron beam
line deflection devices comprise electromagnetic devices for
deflecting said electron beam.
14. The invention according to claim 12 wherein said third electron
beam line deflection device is the only electron beam line
deflection device between said final aperture and the object being
scanned by said electron beam.
15. The invention according to claim 1 wherein said third electron
beam line deflection device is the only electron beam line
deflection device between said final aperture and the object being
scanned by said electron beam.
16. A method for operating a scanning electron microscope of the
type having an electron gun, a final aperture through which at
least a part of said beam passes prior to impingement on an object
being scanned by said electron beam and a longitudinal axis
extending from said electron gun through said final aperture and
through the object being scanned by said electron beam to enable
said scanning electron microscope to generate signals that enable
an image of the object being scanned by said electron beam to be
viewed in three dimensions, said method comprising: (a) scanning
said electron beam across the object being scanned in adjacent
parallel lines, (b) between said electron gun and said final
aperture and in addition to any deflection of said electron beam by
said scanning (i) providing a field during the scanning of a first
set of said lines which deflects an electron beam that is
travelling from said electron gun through said final aperture on
said longitudinal axis in a first path firstly away from said
longitudinal axis and to one side of said longitudinal axis and
lastly towards said longitudinal axis to intersect said
longitudinal axis in the immediate vicinity of said final aperture
and then to proceed away from said longitudinal axis but on the
opposite side thereof and (ii) providing a field during the
scanning of a second set of said lines which deflects an electron
beam that is travelling from said electron gun through said final
aperture on said longitudinal axis in a second path firstly away
from said longitudinal axis and to said opposite side thereof and
lastly toward said longitudinal axis to intersect said longitudinal
axis in the immediate vicinity of said final aperture and then to
proceed away from said longitudinal axis but on said one side
thereof, and (c) between said final aperture and the object being
scanned by said electron beam, and in addition to any deflection of
said electron beam by said scanning, during said scanning of said
respective sets of lines providing fields which are of such a
nature that, when acting upon electron beams travelling in the
portions of said first and second paths that are beyond said final
aperture, deflect electron beams travelling in the portions of said
first and second paths respectively that are beyond said final
aperture back towards said longitudinal axis to intersect said
longitudinal axis at the object being scanned by said electron beam
from both said one and said opposite sides thereof.
17. A method according to claim 16 wherein said scanning is
interlaced and said first and second sets of lines are
interlaced.
18. A method according to claim 16 wherein said scanning electron
microscope includes at least one electron beam line deflection
device and said scanning is accomplished by applying a sawtooth
waveform thereto.
19. A method according to claim 16 wherein said scanning electron
microscope includes a plurality of electron beam line deflection
devices, at least a first and a second of said devices being
located between said electron gun and said final aperture and a
third of said devices being located between said final aperture and
the object being scanned by said electron beam, said fields being
provided by applying rectangular waveforms to said first, second
and third electron beam line deflection devices.
20. A method according to claim 19 wherein said scanning is
accomplished by applying a sawtooth waveform to at least said first
and second electron beam line deflection devices.
21. A method according to claim 19 wherein said scanning is
accomplished by applying a sawtooth waveform to at least said first
and second electron beam line deflection devices by applying a
sawtooth waveform to at least said third electron beam line
deflection device.
22. A method according to claim 19 wherein said scanning is
accomplished by applying a sawtooth waveform to at least said first
and second electron beam line deflection devices by applying a
sawtooth waveform to said first, second and third electron beam
line deflection devices.
Description
This invention relates to improvements in scanning electron
microscopes of the type having deflection coils located or
deflector plates located ahead of the final aperature. By the
practice of this invention signals can be derived from such
scanning electron mocroscopes that can be translated to permit the
object under examination to be seen in three dimensions, i.e.,
stereoscopically.
The production of stereo pictures using a scanning electron
microscope (SEM) has been achieved in the past. One technique that
has been employed is to mechanically tilt the specimen, make two
exposures thereof at two different viewing angles, photograph the
images for each exposure (these photographs being referred to as
stereo pairs), and view the stereo pairs through a conventional
stereo viewer. This technique is very slow, and only one person at
a time can look at a stereo pair through a viewer.
It is also known to modify an SEM by adding to it what is called an
after lens deflection (ALD) coil and applying a deflection signal
thereto to deflect the electron beam after it has passed through
the final aperture so that it impinges on the specimen from two
different angles. In this manner stereo pairs can be made without
mechanically tilting the specimen. This system has a number of
inherent disadvantages that will be discussed at a later point
herein.
Further, it is known to employ a double ALD system to permit
viewing in stereo of a specimen being examined by an SEM. The major
problem with a double ALD system is the poor resolution that
results from the necessarily larger working distance between the
final aperature and the specimen as compared with a single ALD
system where only one ALD coil is required, or the system where
there are no ALD coils and the specimen is tilted mechanically. In
addition, a double ALD system has its own inherent astigmatism
problems.
In accordance with one aspect of this invention there is provided,
in a scanning electron microscope of the type having an electron
gun for producing an electron beam, a final aperture through which
at least a part of said electron beam passes prior to impingement
on an object being scanned by said electron beam, a longitudinal
axis extneding from said electron gun through said final aperture
and through the object being scanned by said electron beam, and an
electron beam deflecting system for scanning said electron beam
across the object being scanned in adjacent parallel lines, said
electron beam deflecting system including a plurality of electron
beam line deflection devices and means for supplying a scanning
waveform to at least one of said electron beam line deflection
devices to scan said electron beam in a line across the object
being scanned by said electron beam, said plurality of electron
beam line deflection devices including at least two electron beam
line deflection devices located between said electron gun and said
final aperture and a third electron beam line deflection device
located between said final aperture and the object being scanned by
said electron beam; the improvement comprising apparatus for
enabling said scanning electron microscope to generate signals that
enable an image of the object being scannned by said electron beam
to be viewed in three dimensions, said apparatus comprising means
for supplying to at least two of said electron beam line deflection
devices located between said electron gun and said final aperture a
waveform which (a) during the scanning of a first set of said
lines, and in addition to any deflection of said electron beam
caused by said scanning waveform, creates a field which deflects an
electron beam that is travelling from said electron gun through
said final aperture on said longitudinal axis in a first path
firstly away from said longitudinal axis and to one side of said
longitudinal axis and lastly towards said longitudinal axis to
inersect said longitudinal axis in the immediate vincity of said
final aperture and then to proceed away from said longitudinal axis
but on the opposite side thereof and (b) during the scanning of a
second set of said lines, and in addition to any deflection of said
electron beam caused by said scanning waveform, creates a field
which deflects an electron beam that is traveling from said
electron gun through said final aperture on said longitudinal axis
in a second path firstly away from said longitudinal axis and to
said opposite side thereof and lastly towards said longitudinal
axis to intersect said longitudinal axis in the immediate vicinity
of said final aperture and then to proceed away from said
longitudinal axis but on said one side thereof, and means for
supplying to said third electron beam line deflection device an
electron beam deflecting waveform, said electron beam deflecting
waveform being applied to said third electron beam deflection
device during the scanning of said respective sets of lines and
deflecting said electron beam in addition to any deflection thereof
caused by said scanning waveform, the nature of said electron beam
deflecting waveform being such that said electron beam deflecting
waveform creates a field which, when acting on an electron beam
travelling in the portions of said first and second paths that are
beyond said final aperture, deflects an electron beam travelling in
the portions of said first and second paths respectively that are
beyind said final aperture back towards said longitudinal axis to
intersect said longitudinaal axis at the object being scanned by
said electron beam from both said one and said opposite sides
thereof.
In accordance with another aspect of this invention there is
provided a method for operating a scanning electron microscope of
the type having an electron gun, a final aperture through which at
least a part of said beam passes prior to impingement on an object
being scanned by said electron beam and a longitudinal axis
extending from said electron gun through said final aperture and
through the object being scanned by said electron beam to enable
said scanning electron microscope to generate signals that enable
an image of the object being scanned by said electron beam to be
viewed in three dimensions, said method comprising: (a) scanning
said electron beam across the object being scanned in adjacent
parallel lines, (b) between said electron gun and said final
aperture and in addition to any deflection of said electron beam by
said scanning (i) providing a field during the scanning of a first
set of said lines which deflects an electron beam that is
travelling from said electron gun through said final aperture on
said longitudinal axis in a first path firstly away from said
longitudinal axis and to one side of said longitudinal axis and
lastly towards said longitudinal axis to intersect said
longitudinal axis in the immediate vicinity of said final aperture
and then to proceed away from said longitudinal axis but on the
opposite side thereof and (ii) providing a field during the
scanning of a second set of said lines which deflects an electron
beam that is travelling from said electron gun through said final
aperture on said longitudinal axis in a second path firstly away
from said longitudinal axis and to said opposite side thereof and
lastly toward said longitudinal axis to intersect said longitudinal
axis in the immediate vicinity of said final aperture and then to
proceed away from said longitudinal axis but on said one side
thereof, and (c) between said final aperture and the object being
scanned by said electron beam, and in addition to any deflection of
said electron beam by said scanning, during said scanning of said
respective sets of lines providing fields which are of such a
nature that, when acting upon electron beams travelling in the
portions of said first and second paths that are beyond said final
aperture, deflect electron beams travelling in the portions of said
first and second paths respectively that are beyond said final
aperture back towards said longitudinal axis to intersect said
longitudinal axis at the object being scanned by said electron beam
from both said one and said opposite sides thereof.
This invention will become more apparent from the following
detailed description of a preferred embodiment and other
embodiments, taken in conjunction with the appended drawings, in
which:
FIG. 1 is a schematic diagram of a conventional SEM of a type that
may be modified in accordance with this invention,
FIG. 2 is a schematic diagram of a prior art system that enables a
specimen in an SEM to be viewed stereoscopically,
FIGS. 3 and 4 are schematic diagrams of an SEM modified in
accordance with this invention,
FIGS. 5-7 show the waveforms of signals that are applied to an SEM
modified in accordance with this invention,
FIGS. 8 and 9 are other schematic diagrams of an SEM modified in
accordance with this invention,
FIG. 10 shows, in block form, circuitry useful in practising this
invention,
FIG. 11 shows, in block form, another system embodying this
invention, and
FIG. 12 shows one of the blocks of FIG. 10 in greater detail.
It will be understood, of course, that FIGS. 1-4 do not purport to
show a complete scanning electron microscope. Scanning electron
microscopes are well known and are commercially available, so only
those components thereof that are necessary for an understanding of
this invention are shown and will be discussed. In fact FIG. 1 is
intended to be a schematic representation of a Cambridge STEREOSCAN
(trade mark) S4 scanning electron microscope that is particularly
suited for use in the practice of this invention.
Referring to FIG. 1, the electron gun of an SEM is shown at 10,
while 11 is the final aperture, 12 the specimen and 13 the normal
line deflection coils of the SEM. In TV parlance the line
deflection coils are the horizontal deflection coils. The vertical
or field deflection coils are not shown. In the particular SEM
illustrated there are two line deflection coils 13. The two line
deflection coils may be connected together in series circuit to
form an integral double deflection system. In this case coils 13
are oppositely wound to provide the required opposite polarity
deflection fields. Alternatively, coils 13 may be electrically
independent and wound in the same way. In this case they are
supplied from the same sawtooth generator but with 180.degree. out
of phase sawtooth waveforms to provide the required opposite
polarity deflection fields. In any event, the field magnitudes of
coils 13 always are adjusted so that the electron beam passes
through final aperture 11. Reference numeral 50 designates the lens
plate of the final lens of the SEM. Not shown is the detector which
detects secondary electrons emitted by specimen 12 to provide the
output signal of the SEM.
The path of the undeflected electron beam is shown at 14 in FIG. 1,
which also designates the longitudinal axis of the SEM, the latter
extending from gun 10 through aperture 11 and specimen 12. By
applying a conventional sawtooth sweep signal at TV scan rate to
coils 13, the electron beam is caused to scan across specimen 12 in
parallel lines. Odd numbered lines are scanned to create one field.
Even numbered lines then are scanned to create another field. This
is known as interlaced scanning. The two fields together make a
frame or picture, all as is well known. The odd and even lines are
interlaced, i.e. even lines occur between odd lines and vice versa.
The extreme limits of the paths of the electron beam (the
beginnings and ends of the lines) are shown at 15a and 15b in FIG.
1. Double deflection is required because it is necessary for the
beam to pass through final aperture 11.
Referring now to FIG. 2, shown therein is what is believed to be a
system described by A. R. Dinnis in a paper delivered at the Fourth
Annual Scanning Electron Microscope Symposium in Chicago in April,
1971 for obtaining stereo pairs from an SEM. In this system there
is added between final aperture 11 and specimen 12 a deflection or
ALD coil 16. The paths 15a and 15b of the electron beam
corresponding to those paths shown in FIG. 1 and which the electron
beam would follow were it not for deflection coil 16 are also shown
in FIG. 2. During, say, the scanning of odd numbered lines, Dinnis
applied a positive deflection current to coil 16 to deflect the
electron beam along paths having the extreme limits 15a' and 15b'
creating what can be referred to as picture A. During the scanning
of even numbered lines Dinnis applied a negative deflection current
to deflection coil 16 to deflect the electron beam along paths
having the extreme limits 15a" and 15b" creating what can be
referred to as picture B. Where pictures A and B overlap, picture C
is created, and by photographing picture C during the scanning of
odd numbered lines and again during the scanning of even numbered
lines, stereo pairs are created. The limitations of this system are
obvious from FIG. 2. Only a very small portion of specimen 12 can
be seen in stereo. Moreover, Dinnis reported "serious aberrations
which cannot be adequately corrected" and rejected this system in
favour of one employing a double deflection arrangement between
final aperture 11 and specimen 12. The disadvantages of such a
double deflection arrangement have been previously discussed.
Referring now to FIG. 3, the same SEM as is illustrated in FIG. 1
is shown. However, in addition to the normal sawtooth waveform 17
(FIG. 5) which is applied to deflection coils 13, there is
additionally applied to deflection coils 13 the rectangular
waveform 18 shown in FIG. 6. It should be understood, however, that
in this embodiment coils 13 are considered to be series connected
and oppositely wound. Thus the rectangular waveform 18 shown in
FIG. 6 is supplied to the upper one of coils 13 and, in effect, the
waveform that is supplied to lower coil 13 thus is 180.degree. out
of phase with waveform 18. It is shown at 19 in FIG. 7. Referring
to FIG. 6, the interval T1-T2 and the interval T3-T4 represent the
periods of time taken to scan, say, the odd numbered lines of a
field, while the interval T2-T3 represents the period of time taken
to scan the even numbered lines of a field. The result of applying
waveform 18 to deflection coils 13 is that the electron beam which
otherwise would follow paths within the area bounded by paths 15a
and 15b is caused to follow paths within the area bounded by paths
19a and 19b during intervals T1-T2 and T3-T4 and paths within the
area bounded by paths 20a and 20b during the interval T2-T3. In
other words, the electron beam is deflected to the left and to the
right of the paths that it normally would follow if only waveform
17 were applied to coils 13. This modification alone does not
permit stereo pictures to be obtained because, as is readily
apparent from FIG. 3, neither the electron beams 19b and 20b nor
the electron beams 19a and 20a converge at the same point on
specimen 12. It is well known that to produce stereo pictures it is
necessary for the same points on specimen 12 to be viewed from two
slightly different angles. This result is achieved by the addition
of a deflection coil 21 between final aperture 11 and specimen 12,
as shown in FIG. 4, and the application thereto of a waveform of
the same general configuration, i.e. rectangular, as waveform 18.
As shown in FIG. 4, the application of this rectangular waveform to
coil 21 causes electron beams 19a and 20a to impinge on specimen 12
at the same point but from two slightly different angles. The same
is true of electron beams 19b and 20b. Consequently the condition
for providing stereo images has been achieved. In fact, beams 19a
and 20a do not converge at precisely the same point on specimen 12,
but rather one line apart. This does not materially detract from
the quality of the stereo picture that can be obtained.
Reference may be made to FIG. 8 which illustrates the system
described in the preceding paragraph in greated detail. In this
Figure the sawtooth generator which provides waveform 17 is shown
at 51 and is connected to upper coil 13. The internal connection of
oppositely wound coils 13 is shown by the dotted line 70. Reference
numeral 52 designates a "square" wave generator that provides the
rectangular waveform shown at 18. Generator 52 is connected to
upper coil 13 and to ALD coil 21. Lines 56 and 57 schematically
indicate the paths followed by the electron beam during times T1 to
T2 and T2-T3 respectively due solely to the output of generator
52.
In another embodiment shown in FIG. 9 coils 13 are not connected in
series and are wound in the same direction. In this case both
generators 51 and 52 are connected via phase inverters 53 and 54
respectively to lower coil 13.
In another embodiment of the invention also shown in FIG. 9,
sawtooth generator 51 also may be connected to ALD coil 21, as
shown by the dotted line 55. This connection may be made via an
amplifying or attenuating device, neither of which is shown, so as
to permit the amplitude of the sawtooth signal supplied to coil 21
to be varied.
In a less preferred embodiment of the invention which is not
illustrated, sawtooth generator 51 may be connected only to ALD
coil 21. This embodiment is not preferred because of the increased
angle of scan that necessarily results when scanning a line of a
given length as the scanning coil, i.e., the coil furthest from
specimen 12 to which waveform 17 is applied, moves closer to
specimen 12.
Those skilled in the art will understand that the times T1, T2, T3
and T4 etc. are one-sixtieth of a second apart and that during
these one-sixtieth second intervals there occur sufficient cycles
of sawtooth waveform 17 to scan first the odd numbered lines, then
the even numbered lines etc. While this is the preferred embodiment
of the invention, since the derived signal can be displayed using a
conventional colour television receiver or monitor, as will be
outlined in greater detail hereafter, it also would be possible for
the switching to take place after each frame. Another alternative
would be to scan in a non-interlaced fashion with switching taking
place after each line. Other types of systems could be used to meet
the requirement that any given area on the specimen be viewed from
two different angles.
It will be seen from the foregoing that in practising this
invention a biasing signal is supplied to coils 13 that has the
effect, during the scanning of certain lines, and in addition to
the scan, of deflecting the electron beam from gun 10 in a first
path, say 56 (FIG. 8), that is firstly away from longitudinal axis
14 and to the left side thereof and lastly towards axis 14 to
intersect axis 14 in the immediate vicinity of final aperture 11,
the beam then proceeding away from axis 14 on the right hand side
thereof. The biasing signal supplied to coils 13 also has the
effect, during the scanning of certain lines, which may or may not
be the same lines as previously mentioned, depending on the
scanning sequence which is used, and in addition to deflection
caused by the scan, of deflecting the electron beam from gun 10 in
a second path 57 that is firstly away from longitudinal axis 14 and
to the right side thereof and lastly towards axis 14 to intersect
axis 14 in the immediate vicinity of final aperture 11, the beam
then proceeding away from axis 14 on the left side thereof. While
there may be more than two deflections of the beam ahead of final
aperture 11, it is essential that the final deflection ahead of
final aperture 11 be towards axis 14 and that the beam intersect
axis 14 at or substantially at final aperture 11. In addition a
biasing signal is supplied to coil 21 that, in addition to the
deflection resulting from scanning, deflects the electron beams
travelling in the portions of paths 56 and 57 that are beyond
aperture 11 back towards axis 14 to intersect axis 14 at specimen
12 from both the right and left hand sides thereof. Finally, a
scanning waveform is provided. It may be supplied to both coils 13
and not to coils 21 or to coil 21 only or to all three coils. If
desired the scanning waveform could be applied to coils that are
completely independent of coils 13 and 21 and the latter used
solely for stereo deflection.
It also should be noted that in practising this invention
electrostatic rather than electromagnetic deflection may be used.
Consequently coils 13 and 21 could be replaced by plates and
generators 51 and 52 by generators of suitable voltage waveforms
for electrostatic deflection.
The lens of an SEM commonly is electromagnetic in nature, but
electrostatic lenses also are known. In the case where an
electromagnetic lens is employed, an important feature of the
instant invention is the fact that the field of ALD coil 21 is
rotatable relative to the fields of coils 13. This is schematically
shown in FIG. 8 by a worm gear 58 that can be manually turned to
rotate coil 21 about axis 14. The field rotation is necessitated by
the fact that the electron beam emerging from final aperture 11
moves in a spiral path, rather than in the x plane, the actual path
being determined by the degree of final lens excitation. Under
these circumstances, if the fields of coils 13 and 21 lay in the
same plane, vertical convergence would not be possible. It is
achieved by rotating coil 21 until vertical convergence is
obtained. Generally only a slight rotation of the order of
2.degree. or 3.degree. is required.
It should be noted that ALD coil 21 can be mechanically rotated or
its field electrically rotated. In the latter case a quadrupole
deflection coil system could be used, for example. Likewise, if
electrostatic deflection is employed together with electromagnetic
focussing, provision should be made for electrical field rotation
or mechanical plate rotation. It also would be possible to achieve
the same effect by electrical field rotation or mechanical rotation
of coils 13.
An important feature of the invention is the fact that only the ALD
coil for x deflection is required. In this respect, the greater the
number of ALD coils that are required, the greater is the
astigmatism problem.
If desired, in any embodiment of the invention an amplifying or
attenuating device may be connected between generator 52 and ALD
coil 21. By varying the setting of this device and hence the drive
to ALD coil 21, convergence of beams 56 and 57 on specimen 12 can
be accomplished. If, as previously noted in connection with FIG. 9,
an amplifying or attenuating device is connected between generator
51 and ALD coil 21, it may be varied so that during scanning the
electron beam impinging on specimen 12 at various points from an
angle upwardly and to the right follows parallel paths in moving
towards axis 14 and likewise for the beam coming in at various
points from an angle upwardly and to the left. In other words, the
angle of view can be kept constant.
In the embodiment of the invention shown in FIG. 8, coils 13 may be
properly called electron beam line deflection devices as a scanning
waveform which deflects the beam in a line across specimen 12 is
applied thereto. No such scanning waveform is applied to coil 21 in
FIG. 8. Notwithstanding this, it still may be termed an electron
beam line deflection device as it does deflect the beam in the same
plane (below lens 50) as the one in which it is scanned. Thus,
where herein and in the claims reference is made to an electron
beam line scanning device, coil or plate, this is intended to
include both such a device to which a line scanning waveform is
supplied and one that is adapted to deflect an electron beam that
is being line scanned in the plane in which such scanning is taking
place.
Referring now to FIG. 10, the TV scanning system of the SEM
modified in accordance with FIGS. 3 and 4 is shown at 22. A
composite video output signal is derived in a conventional manner
and is supplied by a line 23 to a network 24 that incorporates a
video signal inverter and an AGC defeat circuit. The composite
video signal is like any normal transmitted TV signal and contains
video information and sync pulses. The video information, of
course, constitutes the information derived from the SEM by
scanning specimen 12. Thus, during time intervals T1-T2 and T3-T4,
the electron beam scans the lines of an odd field and moves between
extreme positions 19a and 19b (FIG. 4), the lines of the even field
of the frame being scanned during time interval T2-T3 while the
electron beam moves between the extreme positions 20a and 20b, and
the resultant field information constitutes the video information
of the composite video signal. The composite video signal is
inverted by the video inverter and supplied via a line 25 to a
conventional colour television receiver 26. This colour television
receiver is of a type that accepts positive-going video, whereas
the output of network 22 is a composite video signal having
negative-going polarity. This is the reason, of course, for the
video inverter. If the scanning system of the SEM were to produce a
composite video signal having positive-going polarity, the video
inverter would not need to be employed.
The function served by the AGC defeat network is to turn off the
tuner of television receiver 26 so that no external signal
transmitted on the channel to which the television receiver is
tuned can be picked up to interfere with the stereo display of an
image of specimen 12. It is not required if a colour monitor is
used.
Also derived from network 22 are certain sync signals. Thus,
vertical sync signals signifying the start of a new field are
obtained on line 27, while a composite sync signal containing both
horizontal and vertical sync pulses is obtained on line 28. From
the latter signal information can be obtained as to whether the
field is an even field or an odd field. The two sync signals are
supplied to a network 29 designated sync and switching circuits
which incorporates a monostable multivibrator and a set-reset flip
flop. Network 29 produces pulses at each time T1, T2, T3 etc. These
pulses are supplied via lines 30a, 30b and 31a, 31b to square wave
generator 52 that provides the waveform 18 shown in FIG. 6 and
which is supplied to coil 21 via lines 50. The output of square
wave generator 52 also is supplied via lines 33 to network 22 where
it is supplied to line deflection coils 13.
The pulse output signal of network 29 also is supplied via lines
30a, 30b and 34a, 34b to the stigmator drive network 35 of the SEM,
the output of this network being supplied via lines 36 to the
stigmator coils 37 of the SEM. In this manner additional dynamic
correction for astigmatism resulting from the additional deflection
of the electron beam in accordance with this invention is applied
to stigmator coils 37. For proper correction it has been found that
about ten times the current normally used for astigmatism
correction in the S4 SEM should be used. Of course the astigmatism
correction is switched to allow correction of both left and right
images.
The output pulses of network 29 also are supplied via lines 30a,
30b and 38a, 38b to a colour selection network 39. The colour
selection network performs the necessary switching of the electron
guns of the colour picture tube of television receiver 26. The blue
gun is turned off permanently. During scanning of odd numbered
fields the output signal of network 39 turns off the red gun of the
television receiver, so that the odd fields appear as green images
on the screen of the television receiver picture tube. During the
scanning of even fields the signal from network 39 turns off the
green gun, so that even fields appear as red images on the screen
of the colour picture tube. Of course, this arrangement could be
completely reversed without departing from the invention. In other
words, the green gun could be turned off during the scanning of odd
fields and the red gun turned off during the scanning of even
fields. When the picture appearing on the screen of the television
receiver is viewed by an observer wearing glasses having a red
filter in front of one eye and a green filter in front of the other
eye, an image of specimen 12 in three dimensions will be seen.
Referring to FIG. 12, block 29 shown in FIG. 10 may be seen in
greater detail. It is composed of three integrated circuits I.C.
7400, I.C. 74121 and I.C. 7408. Incorporated in block 22 of FIG. 8
is a ZENITH (trade mark) TV module sync board that is sold as an
accessory with a Stereoscan S4 SEM. Line 27 is connected to pin
number 1 of module Z319 on this board, while line 28 is connected
to pin D on the sync board.
Those skilled in the art will understand that the selection of red
and green is not essential to the obtaining of stereo images. Thus,
the green gun could be permanently turned off and even and odd
fields displayed in red and blue respectively, in which event
stereo images will be seen when using glasses having a red filter
and a blue filter. It also would be possible, although rather
impractical, considering the other alternative which is available
namely the system just disclosed, to place on the screen of the
picture tube alternate strips of vertically and horizontally
polarized material corresponding to and in registry with the odd
and even numbered lines and then view the screen of the picture
tube through glasses having one lens vertically polarized and the
other lens horizontally polarized. It also would be possible to
supply the odd field information to a first picture tube, which may
be a black and white picture tube, and the even field information
to a second picture tube, also of the monochrome type, and a stereo
image will be seen when the two picture tubes are viewed through a
stereo viewer. Of course, this system suffers the disadvantage that
only one person at a time can look through the stereo viewer.
While an embodiment of the invention has been disclosed in which
odd field information is displayed in one colour and even field
information is displayed in another colour, those skilled in the
art will appreciate that the whole of one frame may be displayed in
one colour, the whole of the next frame being displayed in a
different colour. This simply involves altering the waveform 18 so
that switching occurs at intervals T1, T3 etc. rather than at
intervals T1, T2, T3 etc. The switching speed in this case is half
as fast, and the net result is that a flicker appears on the screen
of the colour picture tube, but this flicker is tolerable.
An important advantage of the system disclosed herein is that the
composite video signal which is derived from network 22 can be
recorded on a monochrome tape recorder, since only one image is
transmitted at any time. A colour tape recorder is not required.
Referring to FIG. 11, the composite video signal on line 23 is
supplied to a monochrome tape recorder 40 and recorded for
subsequent playback. During playback the composite video signal is
supplied to network 24 and then to colour television receiver 26 in
the same manner as has been disclosed in connection with FIG. 10.
By utilizing the sync circuits of the television receiver itself,
composite sync pulses (containing both the vertical and horizontal
pulses) signifying the start of a new field are supplied via line
41 to a colour switching network 39a that performs the same
function as colour selection network 39 of FIG. 10. Thus, colour
switching network 39a turns off the red gun during one field and
then turns off the green gun during the next field. The network 39a
determines whether any given vertical sync pulse on line 41
signifies the start of an odd field or an even field, and
eliminates an ambiguity which otherwise might require reversal of
the coloured viewing lenses.
Important advantages of the preferred embodiment of this invention
are that the display is continuous, is seen in real time, i.e.,
while the specimen is in the microscope, and can be viewed by
several people simultaneously.
Notwithstanding the foregoing, while for the sake of completeness
systems have been disclosed whereby the stereo signals that are
derived in accordance with the instant invention can be displayed,
and while both together constitute a preferred embodiment of the
invention, the essence of this invention is the system and method
for deriving the stereo signals, and other display systems may be
employed as desired without departing from the basis of this
invention.
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