U.S. patent number 3,715,582 [Application Number 05/065,996] was granted by the patent office on 1973-02-06 for method of and apparatus for attaining focusing following variation in magnification in electron microscope.
This patent grant is currently assigned to Hitachi, Ltd.. Invention is credited to Hiroshi Akahori, Toshihiro Furuya, Morioki Kubozoe, Sadayasu Ueno.
United States Patent |
3,715,582 |
Akahori , et al. |
February 6, 1973 |
METHOD OF AND APPARATUS FOR ATTAINING FOCUSING FOLLOWING VARIATION
IN MAGNIFICATION IN ELECTRON MICROSCOPE
Abstract
A method of and apparatus for attaining the focusing following a
variation in the magnification in an electron microscope having an
image producing lens system consisting of three or more lenses or
generally n lenses including at least an objective lens, an
intermediate lens and a projective lens. Means are provided in the
apparatus so that, when the value of excitation current for the
magnification varying lens is varied to vary the magnification, the
excitation current supplied to the focus adjusting lens can be set
at a value corresponding to the variation in the magnification,
whereby the focusing following the variation in the magnification
by the magnification varying lens as well as the adjustment of the
intensity of illumination can be achieved by a single regulating
operation.
Inventors: |
Akahori; Hiroshi (Katsuta,
JA), Kubozoe; Morioki (Katsuta, JA),
Furuya; Toshihiro (Katsuta, JA), Ueno; Sadayasu
(Katsuta, JA) |
Assignee: |
Hitachi, Ltd. (Tokyo,
JA)
|
Family
ID: |
26347983 |
Appl.
No.: |
05/065,996 |
Filed: |
August 21, 1970 |
Foreign Application Priority Data
|
|
|
|
|
Feb 13, 1970 [JA] |
|
|
45/40945 |
May 15, 1970 [JA] |
|
|
45/12377 |
|
Current U.S.
Class: |
250/396R; 850/10;
850/6 |
Current CPC
Class: |
H01J
37/21 (20130101) |
Current International
Class: |
H01J
37/02 (20060101); H01J 37/21 (20060101); H01j
037/10 () |
Field of
Search: |
;250/49.5A,49.5C,49.5D |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Borchelt; Archie R.
Assistant Examiner: Church; C. E.
Claims
We claim:
1. In an electron microscope comprising a three-stage image forming
electron lens system consisting of an objective lens, an
intermediate lens and a projection lens arranged in this order in
the direction of an electron beam axis, and excitation current
sources each connected to each of said electron lenses for
supplying excitation currents thereto and for stabilizing said
excitation currents, each of said excitation current sources
including means for adjusting said excitation currents; a focusing
apparatus comprising means for mechanically coupling said
excitation current adjusting means to simultaneously control said
excitation currents, so that when the magnification of said
electron microscope is varied by varying the excitation current of
said intermediate lens while keeping the excitation current, of
said projection lens constant, the excitation current of said
objective lens is set at the value compensating for the variation
of said excitation current of said intermediate lens to provide
proper focusing.
2. In an electron microscope having a three-stage image producing
electron lens system consisting of an objective lens, an
intermediate lens and a projective lens which are disposed in the
above order in the direction of the electron beam axis, an
apparatus for attaining the focusing following a variation in the
magnification comprising an excitation power supply connected to
each of said electron lenses for supplying excitation current to
said electron lenses and stabilizing the excitation current, each
said excitation power supply including an excitation current
control means for controlling the excitation current flowing
through the lens coil, a detecting resistor for detecting the
excitation current flowing through said lens coil, means for
varying the resistance of said resistor, a source of reference
voltage for supplying a reference voltage for comparison between it
and the voltage detected by said detecting resistor, and an
amplifier for detecting and amplifying the difference between said
detected voltage and the voltage of said reference voltage source,
said amplifier having its first input terminal connected to said
detecting resistor, its second input terminal connected to said
reference voltage source and its output terminal connected to said
excitation current control means, and means mechanically coupling
said resistance varying means for said detecting resistors in said
excitation power supplies for the simultaneous control of the
resistances of said detecting resistors so as to set the excitation
current for said objective lens at a value corresponding to a
variation of the value of excitation current for said intermediate
lens when the value of excitation current for said intermediate
lens is varied while maintaining the excitation current for said
projective lens at a constant value.
3. An apparatus for effecting focusing dependent upon the variation
in magnification of an electron microscope, comprising: an electron
source for producing electrons; an irradiation electron lens system
including a plurality of condenser lenses for converging said
electrons into an electron beam to direct said electron beam to a
specimen; a three-stage image forming electron lens system
consisting of an objective lens, an intermediate lens and a
projection lens; excitation current sources each connected to each
of said lenses for supplying excitation currents thereto and for
stabilizing said excitation currents, each of said excitation
current sources including means for adjusting each respective
excitation current; and means for mechanically coupling said
excitation current adjusting means to simultaneously control said
excitation currents, so that when the magnification of said
electron microscope is varied by varying the excitation current of
said intermediate lens while keeping the excitation current of said
projection lens constant, the excitation current of said objective
lens is set at the value compensating for the variation of said
excitation current of said intermediate lens to provide proper
focusing necessitated by the variation in the magnification and at
the same time the excitation currents of said condenser lenses are
set at the values corresponding to the variation of said
magnification to adjust the brightness dependent upon said
variation in magnification.
4. In an electron microscope comprising a three stage image forming
electron lens system including an objective lens, an intermediate
lens, and a projection lens arranged sequentially in the direction
of the electron beam axis, and excitation sources connected to each
respective electron lens for supplying excitation currents thereto
and for stabilizing said excitation currents, each of said
excitation sources including means for adjusting said excitation
currents, an improved focusing apparatus comprising:
control means, coupled to each of said current sources and said
lenses, for simultaneously maintaining the excitation current of
said projection lens constant,
for varying the excitation current of said intermediate lens,
and
for adjusting the excitation current of said objective lens to a
predetermined value to provide proper focusing in response to said
variation of the excitation current of said intermediate lens,
wherein each of said excitation sources comprises a power supply, a
switching transistor connected between said power supply and a
respective electron deflection element associated with each lens
and a control circuit responsive to said control means for
appropriately energizing said switching transistor to control the
flow of current to said deflection element, and wherein
said deflection element comprises a lens coil and wherein said
control circuit comprises a voltage dividing circuit, connected to
said lens coil, and a differential amplifier having one input
connected to a respective source of reference potential and another
input adjustably connected to said voltage dividing circuit, the
output of said differential amplifier being connected to control
the operation of said switching transistor.
5. An improved focusing apparatus according to claim 4, wherein
said control means further includes a mechanically ganged switching
arrangement having respective switches connected between each
voltage divider and the corresponding input to each of said
differential amplifiers.
6. A method of varying the magnification of an electron microscope
having a four-stage image forming electron lens system consisting
of an objective lens, intermediate lens, and first and second
projection lenses arranged in this order in the direction of an
electron beam axis, excitation current sources each connected to
each of said electron lenses for supplying an excitation current
thereto and for stabilizing said excitation current, each of said
excitation current sources including means for adjusting said
excitation current, and means for mechanically coupling said
excitation current adjusting means to simultaneously control said
excitation currents in a predetermined relation therebetween,
comprising the steps of:
a. dividing the entire magnification range into a plurality of
sections and varying the excitation current of said intermediate
lens discontinuously at the boundaries of said sections while
keeping said excitation current of said intermediate lens constant
within each section;
b. varying the excitation current of one of the electron lenses
disposed downstream of said intermediate lens within said each
section to vary the magnification of said electron microscope;
and
c. keeping the excitation current of said objective lens, within
said each section, substantially constant at a value compensating
for the variation of the excitation current of said intermediate
lens.
7. A method of varying the magnification of an electron microscope
according to claim 6, in which said one electron lens of the step
(b) is said first projection lens.
8. A method of varying the magnification of an electron microscope
according to claim 6, in which said one electron lens of the step
(b) is said second projection lens.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to electron lens systems of electron
microscopes. More particularly, it relates to a method and
apparatus for attaining the focusing and adjustment of the
intensity of illumination in response to a variation in the
magnification when the magnification is varied by varying the value
of excitation current supplied to the magnification varying lens in
an electron microscope.
2. Description of the Prior Art
In an electron microscope, the magnification must be suitably
varied for the observation of various kinds of specimens or for the
observation of those specimens under various states. The
manipulation for the variation in the magnification in an electron
microscope includes varying the value of excitation current
supplied to the magnification varying lens for varying the
magnification and then varying the value of excitation current
supplied to the focus adjusting lens for attaining the focusing
thereby to produce an enlarged image of a specimen on a final image
screen. In the electron microscope, the intensity of illumination
of the final image must be kept constant since the intensity of
illumination of the image varies in inverse proportion to the
square of the magnification with an increase or decrease in the
magnification.
By way of example, in an electron microscope having a three-stage
image producing lens system consisting of an objective lens, an
intermediate lens and a projective lens, the intermediate or
projective lens acts as the magnification varying lens, while the
objective lens acts as the focus adjusting lens. Heretofore, a
microscopist has manually adjusted the value of excitation current
supplied to the focus adjusting lens and condenser lens while
observing the final image produced on the fluorescent screen.
However, such a focusing system has been defective in that an
elongated period of time is required for the focus adjusting
manipulation and, since the specimen is exposed to the electron
beam for such a long period of time, the surface of the specimen is
damaged to such an extent that it is no longer possible to observe
the specimen under ordinary conditions. This defect has been
especially marked in the case of a specimen such as a high
molecular compound which is easily affected by heat. Further, it
has been very difficult with the prior art focusing system to
attain the focusing following a variation in the magnification in
the case of observation of a specimen such as a piece of tissue
with a so-called low magnification of the order of several thousand
times or less, and the manipulation for the proper focusing has
required an elongated period of time. Thus, this defect has
aggravated the above-described defect.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a
novel method of attaining the focusing following a variation in the
magnification in an electron microscope which overcomes the defects
described above, and an apparatus for carrying out such a
method.
Another object of the present invention is to provide an apparatus
for attaining the focusing following a variation in the
magnification in an electron microscope in which means are provided
so that, when the value of excitation current supplied to the
magnification varying lens is varied to vary the magnification, the
excitation current supplied to the focus adjusting lens can be set
at a value corresponding to the above variation in the
magnification to attain the focusing in simultaneous relation with
the manipulation for the variation in the magnification, and at the
same time, the excitation current supplied to the condenser lens
can also be set at the value corresponding to the above variation
in the magnification to adjust the intensity of illumination,
whereby the variation in the magnification can be always carried
out in the state in which the focusing is properly attained and the
intensity of illumination is completely adjusted.
A further object of the present invention is to provide an electron
microscope having an image producing lens system consisting of
three or more lenses or generally n lenses including at least an
objective lens, an intermediate lens and a projective lens, in
which means are provided to set the excitation current supplied to
the focus adjusting lens at a value corresponding to a variation in
the value of excitation current supplied to the magnification
varying lens so that the focusing following the variation in the
magnification can be carried out by a single regulating operation
thereby to eliminate the troublesome manipulation involved in the
handling of the electron microscope.
A yet further object of the present invention is to provide, in an
electron microscope having an image producing lens system
consisting of three or more lenses or generally n lenses including
at least an objective lens, an intermediate lens and a projective
lens, a method of varying the magnification of the electron lens
comprising fixing the focal length of the intermediate lens and
varying the value of excitation current supplied to the lens or
lenses disposed in the stage or stages lower than the intermediate
lens for carrying out the variation in the magnification.
A still further object of the present invention is to provide, in
an electron microscope having an image producing lens system
consisting of three or more lenses or generally n lenses including
at least an objective lens, an intermediate lens and a projective
lens, a method and apparatus for varying the magnification of the
electron microscope in which the value of excitation current
supplied to the intermediate lens is varied stepwise while the
value of excitation current supplied to the lens or lenses disposed
in the stage or stages lower than the intermediate lens is
continuously varied at a portion corresponding to the stepped
portion of the intermediate lens excitation current so as to carry
out the variation in the magnification.
A common object of the present invention is to provide a novel
method of varying the magnification of an electron microscope and
an apparatus capable of attaining the focusing following the
variation in the magnification by a single regulating operation in
simultaneous relation with the above variation in the
magnification.
Other objects, features and advantages of the present invention
will be readily apparent from the following detailed description
taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagrammatic view showing the relative position of
electron lenses and the positional relation between image planes in
an electron microscope having a four-stage image producing lens
system consisting of an objective lens, an intermediate lens, a
first projective lens and a second projective lens.
Fig. 2 is a block diagram of an embodiment of the present invention
having interlocking control means for the simultaneous control of
the values of excitation current supplied to electron lenses in an
electron microscope having an image producing lens system
consisting of four or more lenses or generally n lenses including
at least an objective lens, an intermediate lens, a first
projective lens and a second projective lens.
FIG. 3 is a graph showing the relation between objective lens
excitation current and intermediate lens excitation current when,
in an electron microscope having an image producing lens system
consisting of three or more lenses or generally n lenses including
at least an objective lens, an intermediate lens and a projective
lens, the value of excitation current supplied to the electron lens
or lenses disposed in the stage or stages lower than and including
the projective lens is kept constant.
FIG. 4 is a graph showing the manner of variation in the values of
excitation current supplied to electron lenses for illustrating an
embodiment of the method of varying the magnification according to
the present invention when it is applied to an electron microscope
having a four-stage image producing lens system consisting of an
objective lens, an intermediate lens, a first projective lens and a
second projective lens.
FIG. 5 shows a modification of the embodiment shown in FIG. 2.
FIGS. 6 and 7 are modifications of parts of the embodiment shown in
FIG. 5.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The relative position of electron lenses and the positional
relation between image planes in an electron microscope having a
four-stage image producing lens system consisting of an objective
lens, an intermediate lens, a first projective lens and a second
projective lens will be described with reference to FIG. 1. In FIG.
1, a specimen to be observed is designated by the reference numeral
1. The objective lens O.sub.bj has a principal plane 2 and an image
forming plane 3. The intermediate lens I.sub.nt has a principal
plane 4 and an image forming plane 5. The image forming plane 5 of
the intermediate lens Int corresponds at the same time to the
object plane for the first projective lens P.sub.1 disposed in the
next stage. The first projective lens P.sub.1 has a principal plane
6 and an image forming plane 7. The image forming plane 7 of the
first projective lens P.sub.1 corresponds at the same time to the
object plane for the second projective lens P.sub.2 disposed in the
next stage. The second projective lens P.sub.2 has a principal
plane 8 and an image forming plane 9. Generally, a fluorescent
screen for the image observation or the emulsion surface of a
photographic plate or film is disposed at the image forming plane 9
of the second projective lens P.sub.2.
Suppose that f.sub.o, a.sub.o and b.sub.o are the focal length of
the objective lens O.sub.bj, the distance between the specimen 1
and the principal plane 2 of the objective lens O.sub.bj, and the
distance between the principal plane 2 and the image forming plane
3 of the objective lens O.sub.bj, respectively. Suppose further
that f.sub.I, a.sub.I, b.sub.I ; f.sub.P1, a.sub.P1, b.sub.P1 ; and
f.sub.P2, a.sub.P2, b.sub.P2 are those of the intermediate lens
I.sub.nt, first projective lens P.sub.1 and second projective lens
P.sub.2, respectively, and u, v and w are the distances between the
principal planes of the four lenses. Then, the following equations
hold:
1/a.sub.o + 1/b.sub.o = 1/f.sub.o (1)
1/a.sub.I + 1/b.sub.I = 1/f.sub.I (2)
1/a.sub.P1 + 1/b.sub.P1 = 1/f.sub.P1 (3)
1/a.sub.P2 + 1/b.sub.P2 = 1/f.sub.P2 (4)
a.sub.I + b.sub.o = u (5)
a.sub.P1 + b.sub.I = v (6)
a.sub.P2 + b.sub.P1 = w (7)
Suppose further that M.sub.o, M.sub.I, M.sub.P1 and M.sub.P2 are
the magnification of the objective lens O.sub.bj, that of the
intermediate lens I.sub.nt, that of the first projective lens
P.sub.1 and that of the second projective lens P.sub.2,
respectively. Then, the total magnification M.sub.T is given by
M.sub.T = b.sub.o /a.sub. o .times. b.sub.I /a.sub. I .times.
b.sub.P1 /a.sub. P1 .times. b.sub.P2 /a.sub. P2
= m.sub.o .sup.. M.sub.I .sup.. M.sub.P1 .sup.. M.sub.P2
In the electron microscope, the variation in the magnification is
generally carried out by varying the value of excitation current
supplied to the intermediate lens I.sub.nt thereby varying the
focal length f.sub.I of the intermediate lens I.sub.nt, that is, by
shifting the position of the object plane 3 for the intermediate
lens I.sub.nt. More precisely, when the distance a.sub.I is varied
to a.sub.I + .alpha..sub.I, the magnification of the intermediate
lens I.sub.nt is now given by M.sub.I ' = b.sub.I /(a.sub.I +
.alpha..sub.I), and the magnification of the objective lens
O.sub.bj is given by M.sub.o ' = (b.sub.o - .alpha..sub.I)/a.sub.o.
Thus, the magnification at the portion including the objective lens
O.sub.bj and the intermediate lens I.sub.nt is varied from
M.sub.o.sup.. M.sub.I to M.sub.o '.sup. .M.sub.I ' = (b.sub.o -
.alpha..sub.I)/ a.sub.o .sup.. b.sub.I /(a.sub.I + .alpha..sub.I).
However, due to the fact that the image forming plane 3 of the
objective lens O.sub.bj is shifted by -.alpha..sub.i, the image
would be out of focus unless the focal length f.sub.o of the
objective lens O.sub.bj is varied correspondingly provided that the
distance a.sub.o is fixed.
Thus, with the system described above, the variation in the
magnification by the intermediate lens results in blurring of the
image, and therefore, the value of excitation current supplied to
the objective lens must be adjusted for attaining the proper
focusing each time the magnification is varied. Modern electron
microscopes have a widened range of variation in the magnification.
In such an electron microscope, the focusing operation must be done
quite frequently and a lot of time is required for the focusing
operation, resulting in troublesome and inconvenient handling of
the electron microscope. Other drawbacks involved in the prior art
system include damage to the specimen due to exposure to the
electron beam for a long period of time.
With a view to overcome the above drawbacks, the present invention
contemplates the provision of an electron microscope which is
provided with means for setting the excitation current supplied to
the focus adjusting lens at a value corresponding to a variation in
the magnification in simultaneous relation with the variation in
the magnification by the magnification varying lens. The present
invention contemplates also the provision of a novel method of
carrying out the variation in the magnification.
The method of varying the magnification according to the present
invention will be described in detail hereunder.
1. At first, the variation in the focal length f.sub.o of the
objective lens O.sub.bj when the magnification is varied by varying
the value of excitation current supplied to the intermediate lens
I.sub.nt in the relative position shown in FIG. 1 will be
discussed. In this case, the focal length f.sub.I of the
intermediate lens I.sub.nt is solely subject to variation. Thus,
the focal lengths f.sub.P1 and f.sub.P2 are fixed and the distances
a.sub.o, a.sub.I, a.sub.P1, b.sub.P1, a.sub.P2 and b.sub.P2 are
kept constant. The total magnification M.sub.T in this case is
given by
M.sub.T = M.sub.o .sup.. M.sub.I .sup.. M.sub.P1 .sup..
M.sub.P2
= (b.sub.o /a.sub. o) .sup.. (b.sub.I /a.sub. I) .sup.. (b.sub.P1
/a.sub. P1) .sup.. (b.sub.P2 a.sub. P2)
= k.sub.1 (b.sub.o /a.sub. I) (8)
where K.sub.1 = (b.sub.I /a.sub. o) .sup.. (b.sub.P1 /a.sub. P1)
.sup.. (b.sub.P2 /a.sub. P2).
Substituting a.sub.I and b.sub.o in the equation (8) by those
obtained from the equations (2) and (5), M.sub.T is expressed
as
M.sub.T = K.sub.1 [ u(b.sub.I - f.sub.I) - b.sub.I f.sub.I /b.sub.]
I f.sub.I (9)
The focal length f.sub.I is sought from the equation (9) as
where u+b.sub.I = K.sub.2 and b.sub.I /K.sub. 1 = K.sub.3. From the
equation (1), f.sub.o is given by
f.sub.o = a.sub.o b.sub.o (a.sub.o + b.sub.o)
Thus, from this equation and the equations (2) and (5), f.sub.o is
expressed as
Substituting f.sub.I in the equation (11) by that in the equation
(10), f.sub.o is expressed as
The equation (12) is differentiated with M.sub.T to find the
variation .DELTA.f.sub.o of f.sub.o when f.sub.I is varied to vary
M.sub.T. The result is given by
(.DELTA.f.sub.o).sub. fI = (K.sub.1 /u ) .sup.. f.sub.o.sup.2.
M.sub.T.sup..sup.-2. .DELTA. M.sub.T (13)
The above equation represents the variation .DELTA.f.sub.o of the
focal length f.sub.o of the objective lens O.sub.bj when the total
magnification is varied by .DELTA.M.sub.T by varying the value of
excitation current supplied to the intermediate lens I.sub.nt.
2. The next discussion is directed to the case in which the
magnification is varied by varying the value of excitation current
supplied to another lens, for example, the first projective lens
P.sub.1 instead of the intermediate lens I.sub.nt. In this case,
the focal length f.sub.P1 is solely subject to variation. Thus, the
focal lengths f.sub.I and f.sub.P2 are fixed and the distances
a.sub.o, b.sub.P1, a.sub.P2 and b.sub.P2 are kept constant. The
total magnification M.sub.T in this case is given by
M.sub.T = M.sub.o.sup.. M.sub.I.sup.. M.sub.P1.sup.. M.sub.P2 =
(b.sub.o /a.sub. o).sup. . (b.sub.I /a.sub. I).sup. . (b.sub.P1
/a.sub. P1).sup. . (b.sub.P2 /a.sub. P2)
= k.sub.1 (b.sub.o /a.sub. I).sup. . (b.sub.I /a.sub. P1) = k.sub.1
(u-a.sub.I /a.sub. I).sup. . (v-a.sub.P1 /a.sub. P1) (14) where
k.sub.1 = (b.sub.P1 /a.sub. o).sup. . (b.sub.P2 /a.sub. P2).
From the equations (1) through (7), M.sub.T is expressed as
where uv-(u+v)f.sub.I = K.sub.2, (u-f.sub.I) b.sub.P1 = k.sub.4 and
f.sub.I.sup.. b.sub.P1 /h.sub. 1 = k.sub.3.
The focal length f.sub.P1 is sought from the equation (15) as
f.sub.P1 = k.sub.2 b.sub.P1 /(k.sub.2 + k.sub.4 + k.sub.3 M.sub.T)
(16)
The focal length f.sub.o is given by
Substituting f.sub.P1 in the above equation by that in the equation
(16), f.sub.o is expressed as
The equation (17) is differentiated with M.sub.T to find the
variation .DELTA.f.sub.o of f.sub.o when M.sub.T is varied by
varying f.sub.P1. The result is given by
(.DELTA.f.sub.o).sub.f = (k.sub.1 /h.sub. 2) .sup.. uf.sub.o.sup.2
. M.sub.T.sup..sup.-2 . .DELTA. M.sub.T (18)
The above equation represents the variation .DELTA.f.sub.o of the
focal length f.sub.o of the objective lens O.sub.bj when the
magnification is varied by varying the value of excitation current
supplied to the first projective lens P.sub.1.
3. The ratio between the equations (13) and (18) is sought to seek
the ratio between the variations .DELTA.f.sub.o of the focal length
f.sub.o of the objective lens O.sub.bj when the magnification is
varied to the same value.
For the sake of simplicity, it is assumed that u .apprxeq. v and
(b.sub.P1).sub.f .apprxeq. (b.sub.I).sub.f since such conditions
hold in most cases.
The above equation can be simplified as
when u .apprxeq. v, f.sub.P2 is constant and (b.sub.P1).sub.f
.apprxeq. (b.sub.P1).sub.f . Since the focal length f.sub.I of the
intermediate lens I.sub.nt is used in the range in which it is
smaller than u and the relation f.sub.I << u holds at a high
magnification, the following equation can be obtained:
(.DELTA.f.sub.o).sub.f /(.DELTA.f.sub.o).sub.f .apprxeq.
(M.sub.P1).sub.f (20)
The above equation shows the fact that, when the magnification is
varied by varying the focal length f.sub.I of the intermediate lens
I.sub.nt, the variation .DELTA.f.sub.o of the focal length f.sub.o
of the objective lens O.sub.bj is greater by the magnification of
the first projective lens P.sub.1 than when the magnification is
varied by varying the focal length f.sub.P1 of the first projective
lens P.sub.1.
In other words, the above equation implies that the variation
f.sub.o in the focal length f.sub.o of the objective lens for the
variation in the magnification is smaller when the magnification is
varied by varying the focal length f.sub.P1 of the first projection
lens than when the magnification is varied by varying the focal
length f.sub.I of the intermediate lens. The ratio between the two
cases is about 1/M.sub.P1. Ordinarily the value of M.sub.P1 is 10
to 20. Consequently, when the magnification is varied by varying
the focal length f.sub.P1 of the first projection lens, the image
falls within the depth of focus of the objective lens O.sub.bj to
become a focused or distinct image even without varying the focal
length f.sub.o of the objective lens.
This applies also to an electron microscope having a three-stage
image producing lens system consisting of an objective lens, an
intermediate lens and a projective lens. In this case too, the
variation in the magnification can be carried out in a
substantially sharply focused state by varying the magnification by
the projective lens while fixing the value of excitation current
supplied to the intermediate lens.
However, the variation in the magnification solely by the first
projective lens is not practical because its magnification varying
range is narrow.
In such a case, as will be described later with reference to FIG.
4, the entire magnification range of the electron microscope is
divided into N sections, at the boundaries of which the excitation
current of the intermediate lens is varied discontinuously, but
within each of which it is kept constant. Within each section the
excitation current of the lens of a subsequent stage to the
intermediate lens, that is, of the first or second projection lens
is finely varied so that the magnification is varied without a
substantial variation in the focal length f.sub.o of the objective
lens, that is, in the substantially focused state.
In order to keep the focal length of the objective lens
substantially constant irrespective of the variation in the
magnification, the variation in the excitation current of the
objective lens corresponding to the variation in the excitation
current of the intermediate lens, which has been calculated or
measured in advance, is imparted to the excitation current of the
objective lens in coordination with the switching of the excitation
current of the intermediate lens. In this manner the magnification
can be varied over its entire range in a substantially focused
state.
The above description has referred to the method of varying the
magnification of an electron microscope having a three-stage image
producing lens system consisting of an objective lens, an
intermediate lens and a projective lens.
FIG. 2 is a block diagram of means for controlling the excitation
current supplied to electron lenses in an electron microscope
having an image producing lens system consisting of four lenses or
generally n lenses including at least an objective lens, an
intermediate lens, a first projective lens and a second projective
lens. As for the means for controlling the excitation current
supplied to electron lenses in an electron microscope having a
three-stage image producing lens system, the case in which the
excitation current supplied to the lenses disposed in the stages
lower than the third stage is fixed at a predetermined value in
FIG. 2 may be considered.
Referring to FIG. 2, the reference numerals 20, 21, 22 and 23
designate an objective lens excitation current control system, an
intermediate lens excitation current control system, a first
projective lens excitation current control system, and a second
projective lens excitation current control system, respectively.
The reference numeral 24 designates generally an nth projective
lens excitation current control system. The electron lens control
systems 20, 21, 22, 23 and 24 are supplied with current from a
common stabilized power supply 25.
The objective lens excitation current control system 20 includes an
objective lens coil O.sub.bj, a detecting resistor R.sub.o for
detecting the excitation current flowing through the objective lens
coil O.sub.bj, a voltage-dividing resistor R.sub.o ' for the
detecting resistor R.sub.o, an error amplifier D.sub.o such as a
balanced DC amplifier for detecting and amplifying the difference
between a voltage detected by the voltage-dividing resistor R.sub.o
' and a reference voltage V.sub.Ro, and an output control means
C.sub.o which is controlled by the output from the amplifier
D.sub.o for controlling the excitation current flowing through the
coil O.sub.bj. The lens excitation current flowing through the
objective lens coil O.sub.bj is converted into a voltage by the
combination of the current detecting resistor R.sub.o and the
voltage-dividing resistor R.sub.o ', and the difference between
this voltage and the reference voltage V.sub.Ro is detected and
amplified by the error amplifier D.sub.o. The output from the error
amplifier D.sub.o is supplied to the output control means C.sub.o
for stabilizing the current flowing through the excitation coil
O.sub.bj. That is, the lens excitation current is maintained at a
predetermined value depending on the value of the detecting
resistor R.sub.o. The setting of the lens excitation current
flowing through the objective lens coil O.sub.bj can be varied by
varying the resistance of the detecting resistor R.sub.o or the
voltage division ratio of the voltage-dividing resistor R.sub.o '.
In the present embodiment the value of the excitation current
flowing through the objective lens coil O.sub.bj is varied by
varying the voltage division ratio of the voltage-dividing resistor
R.sub.o ' connected in parallel with the detecting resistor
R.sub.o. For this purpose, the voltage-dividing resistor R.sub.o '
may have a variable resistance so that it may be varied
continuously or the voltage-dividing resistor R.sub.o ' may be
divided stepwise as shown in FIG. 2 so that its resistance may be
changed over by a changeover switch 30.sub.o.
The operation of the remaining electron lens excitation current
control systems 21, 22, 23 and 24 is similar to the operation of
the objective lens excitation current control system 20, and
suffixes I, P.sub.1, P.sub.2 and P.sub.n are attached to the
corresponding components of the intermediate lens excitation
current control system 21, first projective lens excitation current
control system 22, second projective lens excitation current
control system 23, and nth projective lens excitation current
control system 24, respectively.
The variation in the values of excitation current in the electron
lens excitation current control systems is carried out by an
interlocking change-over means 40 such as a rotary switch which
changes over the voltage division ratios R' of the detecting
resistors which are preset to supply required current values in a
fixed relationship with each other. As an example, when the
excitation currents for the electron lenses disposed in the stages
lower than and including the first projective lens P.sub.1 are kept
at respective fixed values and the focal length f.sub.I of the
intermediate lens I.sub.nt is varied to vary the magnification of
the electron micriscope shown in FIG. 2, a corresponding variation
.DELTA.f.sub.o of the focal length f.sub.o of the objective lens
O.sub.bj will be as shown in the equation (13). It is known that
the focal length of a magnetic lens generally varies in inverse
proportion to the square of the excitation current. Suppose, in
this case, that I.sub.o and I.sub.I are the excitation current for
the objective lens O.sub.bj and the excitation current for the
intermediate lens I.sub.nt, respectively. Then, the following
equation is derived from the equation (12):
I.sub.o.sup.2 = C.sub.1 + [ 1/(C.sub.2 I.sub.I.sup.2 + C.sub.3)]
(21)
where C.sub.1, C.sub.2 and C.sub.3 are constants. This relation is
illustrated in FIG. 3.
Therefore, the voltage division ratio of the voltage-dividing
resistor R.sub.I ' for the detecting resistor R.sub.I in the
intermediate lens excitation current control system 21 and the
corresponding voltage division ratio of the voltage-dividing
resistor R.sub.o ' for the detecting resistor R.sub.o in the
objective lens excitation current control system 20 may be
determined on the basis of the relation between the intermediate
lens excitation current I.sub.I and the objective lens excitation
current I.sub.o shown in FIG. 3, and the change-over switches
30.sub.I and 30.sub.o for the change-over of these voltage division
ratios may be mechanically coupled to each other by the
interlocking change-over means 40 as to attain the focusing
following the variation in the magnification by a single regulating
operation. The interlocking change-over means 40 may be disposed
between the objective lens excitation current control system 20 and
the intermediate lens excitation current control system 21 or it
may be arranged to simultaneously control the values of all the
electron lens excitation currents as shown in FIG. 2. In this
latter case, the change-over of the values of excitation current
for the lenses disposed in the stages lower than the intermediate
lens I.sub.nt may be such that the excitation currents are kept at
the same values in spite of change-over of the steps for varying
the voltage division ratio of the voltage-dividing resistors R'.
For example, the change-over steps may be connected to the same
voltage-dividing point so as to show the same resistance.
A condenser lens excitation current control system 26 is so
arranged that the excitation current for a condenser lens coil C is
set at a value corresponding to the variation in the value of
excitation current for the intermediate lens coil O.sub.bj in the
intermediate lens excitation current control system 21 in order to
adjust the intensity of illumination of the image following the
variation in the magnification. Since the intensity of illumination
of the final image can be kept always constant in spite of the
variation in the magnification, photographing, especially
continuous photographing can be done without any fear of
underexposure or overexposure.
The above description has referred to the simultaneous control
means for the electron lens excitation currents on the basis of the
discussion given in paragraph 1.
The following description is directed to a method and apparatus for
varying the magnification on the basis of the discussion given in
paragraphs 2 and 3.
FIG. 4 is a graphic illustration of one form of the method of
varying the magnification in an electron microscope having a
four-stage image producing lens system consisting of an objective
lens O.sub.bj, an intermediate lens I.sub.nt, a first projective
lens P.sub.1 and a second projective lens P.sub.2. In FIG. 4, the
horizontal and vertical axes represent the magnification and the
lens excitation current, respectively. As will be apparent from
FIG. 4, the value of excitation current for the intermediate lens
I.sub.nt is varied stepwise and the value of excitation current for
the first projective lens P.sub.1 or second projective lens P.sub.2
is varied continuously or stepwise at portions corresponding to the
stepped portions of the intermediate lens excitation current for
varying the magnification. Points bearing the marks on the
characteristic curves for the first and second projective lenses
P.sub.1 and P.sub.2 represent the current values varied
stepwise.
In the magnification range A of from 600 to 3,000 times, the values
of excitation current for the intermediate lens I.sub.nt and first
projective lens P.sub.1 are fixed at about 2.17A and 0.83A,
respectively, while the value of excitation current for the second
projective lens P.sub.2 is varied continuously or discontinuously
(stepwise) from about 2.67A to 4.17A. P.sub.20, P.sub.21, . . .
P.sub.26 represent the stepwise varying values of excitation
current for the second projective lens P.sub.2, and P.sub.oo,
P.sub.o1, . . . P.sub.o6 represent the settings of excitation
current for the objective lens O.sub.bj corresponding to the
stepwise varying values of excitation current for the second
projective lens P.sub.2.
Therefore, the voltage division ratios of the voltage-dividing
resistors R' for the detecting resistors R in the respective
electron lens excitation current control systems may be set to have
a relation as described above and these voltage-dividing resistors
may be mechanically coupled to each other by the interlocking
change-over means 40 shown in FIG. 2. The above arrangement applies
also to the magnification ranges described below.
In the magnification range B of from 3,000 to 30,000 times, the
values of excitation current for the intermediate lens I.sub.nt and
second projective lens P.sub.2 are kept constant, while the value
of excitation current for the first projective lens P.sub.1 is
varied continuously or stepwise. In this range, the value of
excitation current for the objective lens O.sub.bj is varied in a
manner as shown.
In the magnification range C of from 30,000 to 300,000 times, the
values of excitation current for the intermediate lens I.sub.nt and
second projective lens P.sub.2 are kept constant, while the value
of excitation current for the first projective lens P.sub.1 is
varied continuously or stepwise. In this range, the value of
excitation current for the objective lens O.sub.bj is varied in a
manner as shown.
It will be apparent from FIG. 4 that the value of excitation
current for the objective lens O.sub.bj is varied stepwise in a
relation similar to the stepped variation of the value of
excitation current for the intermediate lens I.sub.nt, but the
variation in the value of excitation current for the objective lens
O.sub.bj at the portions corresponding to the stepped portions of
the excitation current for the intermediate lens I.sub.nt is very
little compared with the variation in the value of excitation
current for other electron lenses.
It will be understood that the variation in the magnification
according to the so-called Zoom lens system can be carried out by
an arrangement in which the values of excitation current for the
respective electron lenses are varied in a relation as shown in
FIG. 4 and an interlocking change-over means 40 as shown in FIG. 2
is used for the simultaneous control of the variation of these
current values.
While the above description has referred to the case of varying the
values of excitation current by varying the resistances of the
detecting resistors or the voltage division ratios of the
voltage-dividing resistors in the respective excitation current
control systems, such may be carried out by varying the voltage
value of the reference voltage source V.sub.R.
Another embodiment of the present invention in which the value of
reference voltage is varied to vary the value of excitation current
will be described with reference to FIG. 5. Although a three-stage
image producing lens system consisting of an objective lens
O.sub.bj, an intermediate lens I.sub.nt and a projective lens P is
shown in FIG. 5 by way of example, it will be apparent for those
skilled in the art that the present embodiment is also applicable
to an n-stage lens system. The suffixes O, I and P are attached to
components belonging to excitation current control systems 200, 210
and 220 for the objective lens O.sub.bj, intermediate lens I.sub.nt
and projective lens P, respectively. The reference numerals 50 and
60 designate a source of reference voltage and a voltage-dividing
resistor group, respectively, which are common to the electron lens
excitation current control systems 200, 210 and 220. A change-over
means S.sub.o for the objective lens excitation current control
system 200 includes a change-over switch 70.sub.o and a plurality
of stationary contacts O.sub.1, O.sub.2, . . . O.sub.i. A
change-over means S.sub.I for the intermediate lens excitation
current control system 210 includes a change-over switch 70.sub.I
and a plurality of stationary contacts I.sub.1, I.sub.2, . . .
I.sub.i. Similarly, a change-over means S.sub.P for the projective
lens excitation current control system 220 includes a change-over
switch 70.sub.P and a plurality of stationary contacts P.sub.1,
P.sub.2, . . . P.sub.i. The voltage-dividing resistor group 60 has
a plurality of voltage-dividing terminals T.sub.1, T.sub.2, . . .
T.sub.n.sub.-1, T.sub.n.
The precision of the setting of the value of excitation current is
different in each electron lens, but a very high degree of
precision is required for every electron lens. For example, in
order to eliminate any fluctuation in the intensity of illumination
of the image as well as in the focal point when the magnification
is varied in a lens system consisting of a plurality of stages, the
values of excitation current must not deviate beyond 0.001 percent
in the case of the objective lens and 1 percent in the case of the
condenser and projective lenses. Therefore, the voltage-dividing
resistor group 60 used for the setting of the current value
requires such a number of resistors which will correspond to the
very finely divided units of the current value and the individual
resistors must also be of high precision. Taking the above point
into consideration, the voltage-dividing resistor group 60 is
composed of a series connection of n unit resistors having the same
rated resistance R, and the stationary contacts O.sub.1, O.sub.2, .
. . O.sub.i ; I.sub.1, I.sub.2, . . . I.sub.i ; and P.sub.1,
P.sub.2, . . . P.sub.i of the respective change-over means S.sub.o,
S.sub.I and S.sub.P are connected to the voltage-dividing terminals
corresponding to the desired current settings in a manner as, for
example, shown in FIG. 5. When, for example, the change-over switch
70.sub.P of the change-over means S.sub.P for the projective lens
excitation current control system 220 is brought into contact with
the stationary contact P.sub.2, the input terminals of an error
amplifier D.sub.P is connected through the change-over switch
70.sub.P and stationary contact P.sub.2 to the voltage-dividing
terminal T.sub.5 of the voltage-dividing resistor group 60
connected to the reference voltage source 50. Thus, a reference
voltage which is 5/n of the voltage V.sub.R of the reference
voltage source 50 is applied to the error amplifier D.sub.P.
Similarly, a voltage which is any desired division of the reference
voltage V.sub.R can be obtained when each of the change-over
switches 70.sub.o and 70.sub.I is brought into contact with a
suitable stationary contact. Therefore, the variation in the
magnification can be always carried out in a sharply focused state
when these stationary contacts are previously connected to give
respective fixed voltages as described in the first embodiment and
the change-over switches 70.sub.o, 70.sub.I and 70.sub.P are
simultaneously changed over by an interlocking change-over means
40. Two or three of the stationary contacts in the change-over
means S.sub.o, S.sub.I and S.sub.P are connected to the same
voltage-dividing terminal of the voltage-dividing resistor group 60
as shown in FIG. 5. In order to deal with possible fluctuation in
the reference voltage value applied to the error amplifier D.sub.o,
D.sub.I or D.sub.P when these stationary contacts are connected to
the same terminal, the input resistors for the error amplifiers
must have a sufficiently large resistance.
When it is required that the divided voltage of the reference
voltage V.sub.R applied to the error amplifier has a more precise
value lying intermediate between the divided voltage values
appearing at the adjacent terminals T.sub.i and T.sub.i.sub.+1 of
the voltage-dividing resistor group 60, a series circuit including
ten resistors each having a resistance of 10R may be connected in
parallel with the resistor R lying between the voltage-dividing
terminals T.sub.i and T.sub.i.sub.+1 as shown in FIG. 6 or a series
circuit including ten resistors each having a resistance of R/10
may be connected in series across the terminals T.sub.i and
T.sub.i.sub.+1 as shown in FIG. 7. By this arrangement, the
excitation current can be set at a value which is one place higher
in precision than that obtained with the arrangement shown in FIG.
5.
* * * * *