Method of photographing electron microscope images on a single photographic plate and apparatus therefor

This invention relates to a method and apparatus for obtaining a single photograph of electron microscope images, with low gain in magnification, of relatively thick specimen, which photograph is suitable for direct comparison with a corresponding photograph of the same specimen obtained with an optical microscope: wherein either the specimen or the electron beam is so manipulated that the electron beam passes discretely and stepwisely through sequential portions of the specimen, and prior to impinging on the photographic plate the electron beam is screened to eliminate any portion thereof which would produce aberrations or other distortion and is controllably directed to impinge on a portion of the photographic plate which corresponds to the portion of the specimen through which the beam is passing.

FIELD OF THE INVENTION 
This invention relates to a method of photographing electron microscope 
images of different portions of a specimen on corresponding portions of a 
single photographic plate, and apparatus therefor. 
BACKGROUND OF THE INVENTION 
Heretofore, little attention has been given to the formation of low 
magnification photographic images with electron microscope. However there 
is a growing demand to be able to compare the image of a specimen obtained 
with an optical microscope, which has therefore a relatively low 
magnification and wide field of view, with a corresponding image obtained 
with an electron microscope. In order to obtain the latter image it is 
necessary to use, in the electron microscope, a relatively thick specimen 
of the nature suitable for optical microscope. However, it is very 
difficult with a conventional electron microscope to produce an image of 
uniform high quality on a photographic plate, even if the specimen to be 
examined is ultra thin, and needless to say even more so in the case of a 
relatively thick specimen. The reason for this being: (1) the electron 
microscope is subject to several aberrations which are impossible to 
compensate for, since the usual electron microscope is provided with only 
converging lens, (2) the aberrations become abruptly noticeable when the 
converging lenses are used with low gain in magnification, and (3) the 
amount of inelastic scattering electrons increases with the increment 
thickness of the speciment to be examined. Further, the scattering angles 
thereof increase so that the circumferential portion of the image focused 
on the photographic plate is blurred. Therefore, the image photographed 
with low magnification has a satisfactory definition only at its center 
portion. Furthermore, with the conventional electron microscope, 
divergence of the electron beam is limited by the pole pieces of the 
electron lenses or diameter of a fixed aperture, so that only a small 
portion of the object can be photographed at the same time even if the 
magnification is lowered to a considerable extent. Therefore, in order to 
obtain a photograph of a wide field range of object while maintaining 
satisfactory quality, it has been necessary to assemble or neighbor a 
plurality of photographs of somewhat high magnification. Such a case, 
however, requires a tremendous amount of cumbersome work, and encounters 
an additional problem that the plurality of photographs can not be 
assembled precisely due to distortion of the electron lenses as well as 
the shrinkage and/or expansion of photographic paper. 
It is therefore an object of this invention to provide a method of 
photographing images of different portions of a single photographic plate 
using an electron microscope, and apparatus therefor, by which a high and 
uniform quality, wide field of view image of low magnification can be 
recorded on the single photographic plate. 
SUMMARY OF THE INVENTION 
According to one aspect of this invention, a method of photographing 
electron microscope images of different portions of a specimen on 
corresponding portions of a single photographic plate, comprises: 
selecting a portion of said specimen; selecting a portion of a 
photographic plate, which portion corresponds to the selected portion of 
the specimen and on which portion the selected portion of said specimen is 
imaged; exposing an electron beam, emerging from the selected portion of 
said specimen, to the selected portion of said photographic plate; and 
repeating the above steps, whereby the images of the different portions of 
said specimen are photographed on corresponding portions of said single 
photographic plate. 
According to the other aspect of this invention, apparatus for 
photographing electron microscope images of different portions of a 
specimen on different portions of a single photographic plate, comprises: 
an objective lens; first means for selecting a portion of said specimen, 
which means is provided in the vicinity of said objective lens; a shutter 
provided between said objective lens and said photographic plate; second 
means for selecting a portion of said photographic plate, which portion 
corresponds to the selected portion of said specimen and on which portion 
the selected portion of said specimen is imaged; and control means 
electrically connected to said first and second means and said shutter for 
controlling their operation.

DETAILED DESCRIPTION OF THIS INVENTION 
The invention will be described by way of example with reference to the 
accompanying drawings. First, in connection with FIG. 1, the principle of 
the method of photographing different portions of a specimen on 
corresponding portions of a single photographic plate using an electron 
microscope is discussed. It is firstly necessary to select a portion, as a 
section to be observed or examined, of a given specimen fixedly mounted on 
a specimen stage. The portion to be examined can be selected by 
mechanically moving the specimen stage in a plane perpendicular to the 
longitudinal axis of the electron microscope. As an alternative, the 
portion can be selected, without any mechanical movement of the specimen 
stage, by deflecting the electron beam which has passed through the 
selected portion but which does not include the axis of an objective lens. 
In this case, the electron beam is so deflected as to travel on or near 
the axis of an electron lens next to the objective lens. FIG. 1 shows a 
specimen 1 divided into a plurality of portions 2, which are to be 
separately observed and conveniently labelled A through Y as shown. As the 
portions 2 are selected individually for observation in, for example, 
alphabetical order, the final images thereof are caused to form on 
corresponding portions 4 of a single photographic plate 3 in the order of 
primed alphabets, as shown in FIG. 2. It is as a matter of course that the 
size of each image 4 on the plate 3 is smaller than that of the latter. 
Selecting the portions 4 is carried out by deflecting the electron beam 
impinging on the photographic plate 3, or alternatively by mechanically 
and intermittently moving the plate 3. It is thus understood that the 
different portions 2 of the specimen 1 can be formed and then recorded on 
the corresponding portions 4 of the photographic plate 3, thereby a 
photographic image with a wide field of view can be attained. 
In FIG. 3, a grid 5 is placed on the specimen 1, and apertures of the grid 
5 define respectively the portions 2 each to be observed separately. The 
portions 2 are selected, for example, in alphabetical order as referred to 
in connection with FIG. 1. With this arrangement, a metalic frame 7 of the 
nature blocking an electron beam serves to clearly define the peripheries 
of the portions 2 as well as those of the images formed on the plate 3. As 
a result, the grid 5 enables the neighboring or assembling of the images 
on the plate 3 to be easy, so that, as compared with the absence of the 
grid 5, high precision is no longer required in neighboring the final 
images and hence this arrangement is practically suitable. Although each 
of the apertures 6 of the grid 5 has a square configuration in FIG. 3, it 
is not restricted thereto and may be rectangular, hexagonal or octagonal, 
for example. 
Referring now to FIGS. 4 and 5, wherein a first embodiment of this 
invention is schematically illustrated. A selector 8 comprises a specimen 
stage 9 for displacing the specimen 1, two drivers 10 and 11, and two 
amplifiers 12 and 13. The drivers 10 and 11 moves the specimen stage 9 
under control such as to be parallel with two orthogonal axes in a plane 
normal to the longitudinal axis of the microscope, and then brings the 
portion 2 (FIGS. 1 and 3), to be observed, into a position perpendicular 
to the axis of an objective lens 14. FIG. 5 shows a condition where the 
portion B is selected. As previously described, the positioning or 
neighboring of the images on the plate 3 becomes easier with the grid 5. 
A field view limiting aperture 15 which operates as an electron regulation 
means is provided under the objective lens 14 and allows only the electron 
beam, emerging from or passing through the portion B (FIG. 5), to travel 
through the intermediate lens 16 and projector lens 19. Then, the electron 
beam from the aperture 15 passes through an intermediate lens 16. The 
aperture 15 serves the purpose of precisely neighboring the images formed 
on the plate 3 by limiting each field view of the images. The aperture 15 
is inserted into or withdrawn from a position normal to the axis of the 
microscope by means of an associated driver 17 which is energized by a an 
amplifier 18. However, an an alternative, the aperture 15 can be operated 
manually and positioned in front of a projector lens 19. The configuration 
of the aperture 15 should be in analogy with that of the aperture 6 of the 
grid 5, which may be rectangular, hexagonal or octagonal, for example. 
Conversely, if the electron beam incident on the specimen 1 is adjusted in 
diameter and cross-sectional configuration so as to match the shape of 
each portion 2, then the aperture 15 can be omitted as the field view in 
question can be desirably limited without a provision of an aperture. 
The projector lens 19, which is provided under the intermediate lens 16, is 
to focus a magnified final image on the photographic plate 3. 
A shutter 20 is provided under the projector lens 19 and opens and closes, 
in a position normal to the axis of the microscope, by means of an 
associated driver 21 thereof which is energized by an amplifier 22. The 
shutter 20 is not necessarily provided under the projector lens 19, but 
may be provided in any position if in front of the plate 3. 
A selector 23 comprises an amplifier 25 and an electron beam deflecting 
means 24 which is provided under the shutter 20 and includes two pairs of 
orthogonal deflectors although only one pair is shown in FIGS. 4 and 5. 
The selector 23 is to control the direction of the electron beam such as 
to selectively impinge on the portions of the photographic plate 3, as 
schematically shown in FIG. 5. The adjustment of the travelling direction 
of the electron beam is implemented by controlling currents or voltage 
applied to the deflecting means 24 from the amplifier 25. Although the 
deflecting means 24 is provided under the shutter 20 in this particular 
embodiment, it can be provided in any position between the view field 
limiting aperture 15 and the plate 3. 
A controller 26 is electrically connected to the amplifiers 12, 13, 18, 22 
and 25, and amplifies the control signals applied to the drivers 10, 11, 
17, 21 and the deflection means 24, respectively, in order to properly 
photograph the images of different portions of the specimen 1 on the 
corresponding portions on the plate 3. More specifically, the drivers 10, 
11, under the control of the controller 26, adjusts the horizontal 
positions of the specimen stage 9 for selecting in order the portions A 
through Y of the specimen 1. The controller 26 controls the driver 17 
through the amplifier 18, and causes the field view limiting aperture 15 
to be inserted into and withdrawn from the preset position. On the other 
hand, the driver 21 makes the shutter 20 open and close under the control 
of the controller 26 as the portions A through Y are selected one by one. 
While, the deflection means 24 under the control of the controller 26 
establishes different electron paths toward the selected portions of the 
photographic plate 3. 
Reference is now made to FIGS. 6 and 7, wherein the second embodiment of 
this invention is schematically illustrated. This embodiment is similar to 
the first one, except that the specimen stage 9 is fixed and the portions 
of the specimen 1 are separately selected by allowing the electron beam 
passing through the particular portion, to be examined, to proceed on or 
very near the axis of the microscope, thereby including therein said axis 
in the former. To this end, a selector 8', comprising an amplifier 28 and 
an electron beam deflection means 27, is provided between the object lens 
14 and the intermediate lens 16. The deflection means 27 consists of two 
pairs of orthogonal deflectors, operates as an electron regulation means, 
although only one pair is shown in FIGS. 6 and 7. More specifically, the 
deflection means 27 is supplied with currents or voltages from the 
amplifier 28 under the control of the controller 26, and then deflects the 
electron beam, emerging from the selected portion (in FIG. 7, the portion 
D) and which does not include therein the axis of the objective lens 14, 
in such a manner as to travel on or very near the axis of the intermediate 
lens 16. Inasmuch as the specimen stage 9 is fixedly positioned in this 
embodiment, positioning errors of the specimen arising from the mechanical 
movement of the stage 9 and also the unwanted drifting of the specimen 1 
due to the breaking of thermal equilibrium can advantageously be 
prevented. Therefore, the final images formed on the plate 3 are 
photographed stably and precisely. The electron beam deflection means 27 
is not necessarily provided as shown in FIGS. 6 and 7, and may 
alternatively be positioned between the intermediate lens 16 and the 
projector lens 19. The other elements of the second embodiment is the same 
as those of the first (FIGS. 4 and 5), so that further description will be 
omitted for brevity. 
FIG. 8 shows a preferable positioning of the deflection means 24 relative 
to the projector lens 19, which is highly advantageous in the first and 
the second embodiments of this invention. The deflection means 24 is so 
provided that the main deflection plane 29 is at a position 30 where the 
cross-section area of the electron beam from the projector lens 19 is 
substantially the smallest value. As a result, the electron beam is 
uniformly deflected in that the cross-sectional area is very small at the 
main deflection plane 29. This means that the final images on the plate 3 
is not subject to distortion of deflection with attendant image formation 
of high precision. 
Each of the above described embodiments of this invention uses a three 
stage electron lens system, viz, the objective lens 14, the intermediate 
lens 16 and the projector lens 19. However, this invention is not limited 
to the three stage type electron microscope and may be of a two stage type 
with the exclusion of the intermediate lens 16, or, of four or more stage 
type with two or more intermediate lenses in addition to the objective 
lens 14 and the projector lens 19. 
The method of individually photographing the final images formed on the 
plate 3 will be discussed, by way of example, in more detail. Firstly, the 
field view limiting aperture 15 is controlled by the controller 26 and set 
in a position predetermined and normal to the axis of the instrument, and 
once initially positioned, usually retained therein until the 
photographing is completed. The aperture 15, however, may alternatively be 
positioned manually. Then, the shutter 20 closes in response to the 
control signal from the controller 26, and the photographic plate or film 
3 is loaded in a preset position. In the latter case, if a photo-loading 
device (not shown) of a photographic plate (not shown) is designed to be 
controlled by the controller 26, the loading of the plate 3 can be 
automated. Following, the portion A of the specimen 1 is selected: by 
moving the specimen stage 9 so as to center the portion A with the axis of 
the objective lens 14 in the case of the first embodiment (FIGS. 4 and 5), 
or by bending the electron beam, under the control of the controller 26, 
in a manner to include therein the axis of the intermediate lens 16 in the 
case of the second embodiment (FIGS. 6 and 7). The deflection means 24 of 
the selector 23 is energized or magnetized, under the control of the 
controller 26, such that the electron beam passing through the deflection 
means 24 is directed to impinge on the portion A' of the plate 3. After 
these steps, the controller 26 opens the shutter 20, through the power 
supply 18 and the driver 17, for a time sufficient to attain proper 
photographic density on the portion A', and thereafter closes the shutter 
20. The image of the portion A is thus photographed on the corresponding 
portion A' of the plate 3. Then, the next portion B is selected and the 
above steps are repeated until the final portion Y is imaged and 
photographed on its corresponding portion Y' of the plate 3. Thereafter, 
the plate 3 is accommodated in the photographic plate container (not 
shown) while closing the shutter 20. 
It is understood from the foregoing that even if the field of view of 
specimen 1 is small (for example, 40.mu..times.40.mu.), the entire 
relatively wide specimen (for example, an area of 9 mm.sup.2 -25 mm.sup.2) 
can be photographed on a single photographic plate 3 by correspondingly 
changing the portions of the specimen 1 and the plate 3. The electron beam 
impinging on the intermediate lens 16 as well as the projector lens 19 is 
limited to be very near or include the longitudinal axis of the 
microscope, so that various aberrations arising from these lenses can be 
reduced considerably with great increase of photographic quality. 
Furthermore, when the deflection means 27 is employed for selecting the 
portions of the specimen 1, the unwanted drift of the specimen 1 can be 
removed with attendant high quality of photography. Still furthermore, 
when the main deflection plane of the projector lens 24 is positioned 
where the cross-sectional area of the electron beam is the smallest or 
close to said value, the aberration of deflection can be reduced to a 
considerable extent with the result of image formation of high precision.