Electron microscope

An electron microscope comprising an electron gun for generating an electron beam, an irradiation lens system for irradiating a specimen with the electron beam, means for obtaining an image signal of the specimen, a frame memory for storing the image signal, means for obtaining a total accumulation function, which indicates the sum total of accumulated values in a plurality of one-dimensional directions, and in which each of the accumulated values in the plurality of one-dimensional directions indicates accumulation with respect to a frequency of a function obtained by applying one-dimensional Fourier transform to a function obtained by projecting two-dimensional intensity distribution of said image signal in corresponding one-dimensional direction thereof, and means for obtaining a substantial extreme point of the total accumulation function.

BACKGROUND OF THE INVENTION 
The present invention relates to an electron microscope, and more 
particularly to an electron microscope which is improved in point of 
correction of focusing and/or astigmatism. 
Optimum focusing and astigmatism correction are required in an electron 
microscope in order to observe and photograph an image of high resolution 
(an image of high magnification). However, manipulation, thereof is very 
difficult and requires skill. Desire for an effective automatic focusing 
and automatic astigmatism correction device has increased in recent years 
in order to reduce the burden on the operator, and several techniques have 
been proposed. 
One of these techniques is to examine by utilizing covariance of an image 
obtained with an electron microscope to determine characteristics of the 
image as a function of focal aberration quantity or astigmatism quantity, 
thereby to conduct automatic focusing and automatic astigmatism correction 
(Journal of Microscopy, Vol. 127, Pt Z, August 1982, pp. 185-199). 
Further, another technique is to achieve a similar object utilizing the 
observation quantity of an image produced by inclining an irradiation beam 
(Ultramicroscopy 21 (1987) 209-222)). 
In either case, however, practical use has been difficult particularly in a 
transmission type electron microscope because of detection sensibility, 
following problems in high magnification and so on. Namely, in a high 
magnification of a transmission type electron microscope, phase contrast 
becomes a problem and the configuration of a fine structure which is an 
object of observation is different depending on focal aberration quantity. 
Thus, since focusing while watching the frequency characteristic of an 
image is indispensable, it has been difficult to determine a proper focus 
point. In another example, a method of measuring aberration quantity of a 
focal point and astigmatism quantity by the result of applying 
two-dimensional Fourier transform to an image obtained with an electron 
microscope is presented (Micron 1981, Vol. 12, pp. 105-121). In still 
another example, a two-dimensional image is projected in one-dimension 
thereby to apply one-dimensional Fourier transform thereto for the purpose 
of improvement of processing speed and economization of a memory area in 
place of two-dimensional Fourier transform, which may be effective in 
focusing and astigmatism correction (Scanning Microscopy, Vol. 1, No. 4, 
1987, pp. 1507-1514). 
However, the above-described last two examples are confined to obtaining 
information such as focal aberration quantity and coefficient of 
astigmatism or to comparing an image with Fourier transform by applying 
two-dimensional or one-dimensional Fourier transform, and it is neither 
considered nor mentioned how a current of a focusing electron lens and a 
correction current of an astigmatism correction device are controlled in 
performing automatic focusing or automatic astigmatism correction. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an electron microscope 
which is suitable for performing focusing and/or astigmatism correction 
automatically. 
It is another object of the present invention to provide an electron 
microscope which is suitable for performing focusing and/or astigmatism 
correction efficiently. 
According to the present invention, there is provided an electron 
microscope comprising means for generating an electron beam, means for 
irradiating a specimen with the electron beam, means for obtaining an 
image signal of the specimen, a frame memory for storing the image signal, 
and means for obtaining a total accumulation function, which indicates the 
sum total of accumulated values in a plurality of one-dimensional 
directions, and in which each of the accumulated values in the plurality 
of one-dimensional directions indicates accumulation with respect to a 
frequency of a function obtained by applying one-dimensional Fourier 
transform to a function obtained by projecting two-dimensional intensity 
distribution of the image signal in corresponding one-dimensional 
direction thereof, and for obtaining a substantial extreme point of the 
total accumulation function. 
Other objects and features of the present invention will be apparent from 
the following description with reference to the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
According to a preferred embodiment of the present invention, a 
two-dimensional intensity distribution of an electron microscope image is 
projected in a certain one-dimensional direction, a viz. accumulated only 
in one direction of a certain circular field of the image, one-dimensional 
Fourier transform is applied to the accumulated values, and the functions 
after Fourier transform are accumulated with respect to a frequency. Such 
a processing is repeated in various one-dimensional directions in the 
certain circular field, and the sum total of the integrated values after 
Fourier transform that are obtained in those one-dimensional directions, 
i.e. a mean value is obtained. Direction-change for such accumulation, 
i.e., averaging may be achieved easily by rotating the specimen. 
When it is assumed that the sum total, i.e. a mean value thus derived is I, 
and N.sub.z is the number of times of changing a focal point of an 
electron beam irradiating the specimen by .DELTA..sub.Z at a time n, it 
has been found through experiments that I appears as shown in FIG. 1 
against n. In FIG. 1, however, 1.0 is shown as the maximum value along the 
ordinate, i.e. l is normalized. The data in FIG. 1 is obtained in such a 
manner that only signals in a certain frequency range are obtained by 
applying signals after Fourier transform by projected values (accumulated 
values) in N.sub.z 24 directions at intervals of .DELTA..sub.Z 15.degree. 
to a band-pass filter and these signals are accumulated further. Namely, 
when it is assumed that the value obtained by accumulating the functions 
subjected to one-dimensional Fourier transform in a range of 12% to 56% of 
the maximum frequency with respect to a space frequency at a certain 
rotation angle .theta..sub.i is .intg..sup.q2.sub.q1 I(q) q dq, FIG. 1 
shows the total accumulation function: 
##EQU1## 
where, .DELTA..theta.=.pi./N, N=24 and q is a space frequency. 
The position of a local minimum value is located approximately at a central 
position of symmetry of the function curve in FIG. 1, but is has been 
found that the contrast is substantially minimum locally at this position. 
As known, the contrast becomes minimum locally at a proper focus position 
in a transmission type electron microscope image (the contrast becomes 
maximum locally at a proper focus position in a scanning type electron 
microscope image). Accordingly, the center position of symmetry in FIG. 1, 
i.e., the position of the local minimum value is the regular focal 
position. 
FIG. 2 shows data showing the relationship of I with n.sub.X which is 
similar to FIG. 1, when N.sub.x is the number of times of changing the 
focal point of an electron beam in the X-direction by .DELTA.A.sub.X at a 
time. In FIG. 2, the position of a local minimum value, i.e., a center 
position of symmetry is a position where the astigmatism quantity becomes 
minimum. The relationship of I with n.sub.Y when the number of times of 
changing the focal point of an electron beam in Y-direction by 
.DELTA.A.sub.Y at a time is n.sub.Y is similar to that shown in FIG. 2. In 
this case, the position of the local minimum value, i.e., the center 
position of symmetry is also a position where the astigmatism quantity 
becomes minimum. 
Focusing can be achieved automatically in such a manner that I shown in 
FIG. 1 is obtained for every change by changing an exciting current of an 
electron lens which determines a focal point by a fixed quantity at a 
time, a center position of symmetry showing a local minimum value of the 
curve I is obtained, and the exciting current value at that position is 
set as the exciting current of the electron lens. 
Astigmatism can be corrected automatically in a similar manner by obtaining 
I shown in FIG. 2 for every change by changing a correction current of an 
astigmatism correction device by a fixed quantity at a time, obtaining a 
center position of symmetry showing a local minimum value of the curve I, 
and setting a correcting current value at that position as the correcting 
current of the astigmatism correction device. 
Now, referring to FIG. 3, selection of a transmission type electron 
microscope 1 and a scanning type electron microscope 2 is made with a 
selector 4. In the former case, a final image is formed on a transmission 
type fluorescent plate, and this image is picked up using a TV camera and 
converted into an electrical signal, and is taken into a frame memory 
(RAM) 5 as an image signal through an A/D converter 3a. In the latter 
case, an image signal in a direct TV mode or an image signal by slow 
scanning is taken into the frame memory (RAM) 5 through an A/D converter 
3b. The input images are accumulated by means of an accumulator 6 as 
needed for the purpose of removing noise, and the values are stored in the 
frame memory 5 as a still image. 
A reference numeral 7 denotes a buffer (VIDEO RAM) for displaying an image 
on a CRT 8, in which image data which is taken into the frame memory 5 or 
processed is stored, and this data is displayed on the CRT 8. A circular 
image having a vector direction designated by a sampling circuit 10 and 
enlargement or reduction factor and center position of a designated image 
is sampled from images in the frame memory 5. Such designation is made by 
a computer 9c. For the sampled image, window processing is executed by a 
logical circuit 11 for the purpose of removing artefacts by an edge 
portion of the image. Then, the image is projected in a one-dimensional 
direction by means of a one-dimensional projector 12. Namely, image data 
in the form of two-dimensional arrays are accumulated in a column or row 
direction and projected as one-dimensional data in the one-dimensional 
projector. The data projected in the one-dimensional direction is applied 
to a one-dimensional fast Fourier transform circuit, and computed, data 
(frequency spectrum) is stored in a memory 14 for display on the CRT 
similarly to the input image. Further, it is possible to read the spectrum 
any time by means of the computer 9c. 
Here, focal points of taken-in images are made variable, viz., the current 
of focusing electron lenses is controlled by means of respective control 
computers 9a and 9b in the transmission type electron microscope 1 and the 
scanning type electron microscope 2, and the control computers 9a and 9b 
are connected with a computer 9c by communication. Further, respective 
computers 9a and 9b also control the exciting current of the astigmatism 
correction device. Thus, it is possible to control focal points and 
astigmatism correction of the transmission type electron microscope 1 and 
the scanning type electron microscope 2 by the instructions of the 
computer 9c. 
In the construction described above, automatic focus point or astigmatism 
correction will be described hereafter. The exciting current of an 
electron lens or an astigmatism device which determines a focal point is 
varied by .DELTA.Z or .DELTA.A.sub.n and .DELTA.A.sub.Y at a time, and a 
value I is obtained each time by accumulating the functions after 
one-dimensional Fourier transform obtained as described above, thus 
obtaining curves as shown in FIG. 1 or FIG. 2. n.sub.Z or n.sub.X and 
n.sub.Y where a cross-correlation function W(y) such as shown in the 
following expression becomes the maximum are obtained from these curves, 
and respective currents are set taking the position thereof as a proper 
focus position or a position where astigmatism correction becomes the 
minimum. 
EQU W(y)=.intg..sup..infin..sub.-.infin. 
[I(z)-I.vertline..multidot..vertline.I(y-z)-I]dZ 
Here, when it is assumed that Z is n.sub.z x .DELTA.Z, n.sub.X x 
.DELTA.A.sub.X and n.sub.Y x .DELTA.A.sub.Y, and I is a mean value of I, 
and the center of symmetry is Z' (or A.sub.X ' and A.sub.Y '), W(y) 
becomes the maximum when u=2Z' (or 2A.sub.x ', A.sub.Y '). 
By obtaining n.sub.Z, n.sub.X and n.sub.Y with the steps of procedures 
described above and setting a focus current and an astigmatism correction 
current by communication, it is possible to correct the focus point or 
astigmatism automatically. 
In case there is astigmatism, the accumulated value I of the functions 
after one-dimensional Fourier transform is different by a great margin 
with respect to every rotation of the image. With this, it is only 
required to correct astigmatism so that the maximum value I.sub.max and 
the minimum value I.sub.min are almost equal to each other. Here, 
astigmatism may also be corrected so that .xi. becomes the minimum, where 
.xi.=(I.sub.max -I.sub.min)/I.sub.mean. I.sub.mean indicates the mean 
value of accumulated values with respect to every rotation. 
Similarly, when there is astigmatism, space frequency components of a 
function after one-dimensional Fourier transform are different with 
respect to every rotation. Accordingly, astigmatism correction can be made 
automatically similarly to that described above by also making astigmatism 
correction so that .xi.=(l.sub.max -l.sub.min)/l.sub.mean becomes the 
minimum with the range l of space frequencies where values of respective 
functions become a certain fixed value and more, the maximum value 
l.sub.max of l, the minimum value l.sub.min of l, and a mean value 
l.sub.mean of l with respect to every rotation. 
Referring to FIG. 4, an electron beam 52 emitted from an electron gun 41 is 
converged by means of an irradiation lens system 42 and irradiates a 
specimen 44. The electron beam which has transmitted through the specimen 
44 forms an image on a fluorescent plate 46 by an image-forming lens 
system 45. A formed final image is picked up by a TV camera 40. The 
astigmatism of the electron beam 52 irradiating the specimen is corrected 
by an astigmatism correction device 43. 
An accelerating power source 58 is connected to the electron gun 41, a lens 
power source 59 is connected to the lens system, and a power source 81 is 
connected to the astigmatism correction device 43, respectively. These 
power sources are controlled by a computer 9a through D/A converters 67 
and 82. A specimen rotating device 93 which rotates the specimen 44 is 
provided and is controlled by a computer 9a through a D/A converter 94. 
Besides, 90 denotes a memory composed of a ROM and a RAM. 
Referring to FIG. 5, an electron beam emitted from an electron gun 100 is 
converged by a convergent lens system 101 and irradiates a specimen 102. 
The electron beam irradiating the specimen 102 is deflected 
two-dimensionally on the specimen 102 by means of an electron beam 
deflecting device 103. The astigmatism of the electron beam irradiating 
the specimen 102 is corrected by an astigmatism correction device 104. 
Power sources 105, 106 and 107 are connected to the convergent lens system 
101, the deflecting device 103 and the astigmatism correction device 104, 
respectively. These power sources are controlled by means of a computer 9b 
through D/A converters 108, 109 and 110. 
Secondary electrons are generated from the specimen, and are detected by 
means of a secondary electron detector 111. The output signal of this 
detector is input to the frame memory 5 through the A/D converter 3b (FIG. 
3). Besides, 112 denotes a memory composed of a ROM and a RAM. A specimen 
rotating device 130 which rotates the specimen 102 is provided, and is 
controlled by a computer 9b through a D/A converter 131.