Automatic focusing and astigmatism correction for electron beam apparatus

A method of automatically and accurately accomplishing focusing and astigmatism correction in an electron beam apparatus such as a scanning electron microscope. The electron beam is raster scanned to scan a specimen in two dimensions. If the obtained signal intensity distribution curve has only one peak, the peak position P2 of a curve obtained by raster scanning the beam vertically is detected. The peak position P1 of a curve obtained by raster scanning the beam horizontally is detected. An intermediate position P0 is calculated, using the formula EQU P0=(P1+P2)/2 The intensity of excitation of the objective lens is adjusted to this intermediate position P0. In this way, if two peaks cannot be obtained by a horizontal raster scan of the beam, depending on the state of the surface of the specimen, it is possible to accurately bring the beam into focus at the position of the circle of least confusion. After this focusing operation, an automatic astigmatism correction is made.

FIELD OF THE INVENTION 
The present invention relates to an electron beam apparatus such as a 
scanning electron microscope and, more particularly, to automated focusing 
and astigmatism correction used in an electron beam apparatus. 
BACKGROUND OF THE INVENTION 
Where an electron beam apparatus such as a scanning electron microscope 
automatically effects both focusing and astigmatism correction, an 
automatic focusing operation is first performed. In this focusing 
operation, the focus is brought into the position of the circle of least 
confusion. Then, an astigmatism correction is made. To bring the focus 
into the position of the circle of least confusion, an electrical current 
supplied to the objective lens or to a lens auxiliary to the objective 
lens is varied in a stepwise fashion. The position of the focus is shifted 
for each different value of the lens current so that the electron beam 
scans the surface of a specimen in two dimensions. During the 
two-dimensional scan, secondary electrons or reflected electrons emanating 
from the specimen are detected by a detector. The output signal from the 
detector is integrated for one frame of image. The signal distribution 
curve shown in FIG. 1 is obtained from the integrated values derived from 
every value of lens current. The integrated values are plotted on the 
vertical axis of FIG. 1. The intensity of excitation of the objective lens 
is plotted on the horizontal axis. It is known that the position of the 
peak of this distribution curve is the position of the circle of least 
confusion. The objective lens current is so controlled that the intensity 
of excitation of the lens corresponds to the position of the peak. Then, 
an automatic astigmatism correction is made. Subsequently, a second 
automatic focusing operation is performed. FIG. 2 is a flowchart 
illustrating these focusing operations and astigmatism correction 
operations. 
Where the electron beam involves astigmatism, the above-described signal 
distribution curve has two peaks as shown in FIG. 3. In this case, it is 
known that the position of least confusion lies midway between the two 
peaks P1 and P2. If the difference in height between the two peaks is 
great, then there is the possibility that the control system erroneously 
regards the maximum peak position P1 as the position of least confusion. 
At this time, if the focus is brought to an overlying position that is not 
the position of least confusion, and if an astigmatism correction is made 
at that position, then the correction is not carried out accurately. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a method of 
automatically performing both focusing operation and astigmatism 
correction accurately in an electron beam apparatus. 
This object is achieved in an electron beam apparatus having an objective 
lens, a condenser lens for sharply focusing an electron beam directed to 
the surface of a specimen, deflecting means for scanning the electron beam 
in two dimensions in the x- and y-directions, and an astigmatic corrector 
disposed in the electron beam path. The inventive method comprises the 
steps of: scanning the electron beam horizontally on the specimen and 
shifting the scanning line vertically to scan the specimen in two 
dimensions; detecting electrons produced from the specimen by a detector 
and integrating the output signal from the detector by an integrator; 
repeating the scan of the electron beam many times with different 
intensities of excitation of the objective lens; finding a curve 
representing variations in the output signal from the integrator caused by 
variations in the intensity of excitation of the objective lens. If the 
curve has two peaks, the intensity of excitation of the objective lens is 
adjusted to a position midway between the two peaks of the curve, and then 
an astigmatism correction is made. If only one peak is found in the curve, 
the electron beam is raster scanned on the specimen vertically to scan the 
specimen in two dimensions. Electrons produced from the specimen in 
response to the two-dimensional scan are detected by the detector. The 
scan is repeated many times with different intensities of excitation of 
the objective lens. A curve representing variations in the output signal 
from the integrator caused by variations in the intensity of excitation of 
the objective lens is found. The intensity of excitation of the objective 
lens is adjusted to a position midway between the peak of this curve and 
the peak of the curve already obtained by the first twodimensional scan of 
the beam, and then an astigmatic correction is made. 
Other objects and features of the invention will appear in the course of 
the description thereof which follows.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIG. 4, there is shown a scanning electron microscope for 
carrying out a method of correcting astigmatism in accordance with the 
present invention. The microscope has an electron gun 1 emitting an 
electron beam EB which is focused onto a specimen 4 by a condenser lens 2 
and an objective lens 3. Beam-deflection coils 5 scan the electron beam EB 
on the specimen 4 according to scanning signals produced by a scanning 
signal-generating circuit 6. Coils 7 and 8 are provided to correct 
astigmatism in the x- and y-directions, respectively. The x-direction 
astigmatism correction coil 7 is excited by a coil driver portion 9, which 
is supplied with data used to correct astigmatism in the x-direction from 
a register 10 via a D/A converter 12. The y-direction astigmatism 
correction coil 8 is excited by a coil driver portion 13, which is 
supplied with data used to correct astigmatism in the y-direction from a 
register 14 via a D/A converter 15. The objective lens 3 is excited by a 
driver portion 16, which is supplied with data about objective lens 
current values from a register 17 via a D/A converter 18. A detector 19 
detects secondary electrons produced by the irradiation of the electron 
beam to the specimen 4. The output signal from the detector 19 is supplied 
to an integrator 22 via a filter circuit 20 and via an absolute-value 
circuit 21. The integrated value obtained by the integrator 22 is supplied 
via an A/D converter 23 to a memory 24, where the value is stored. A CPU 
26 creates a signal intensity distribution from the signal stored in the 
memory 24, and detects peak values. The CPU 26 controls the scanning 
signal-generating circuit 6, the integrator 22, and other components. The 
CPU 26 also acts to read values from a data memory 27, to place data used 
to correct astigmatism into the registers 10 and 14, and to place data 
about objective lens current values into the register 17. 
FIG. 5 particularly shows the beam-deflecting coils 5 and the scanning 
signal-generating circuit 6 shown in FIG. 4. One of the coils 5 is a 
horizontal deflecting coil 5x, while the other is a vertical deflecting 
coil 5y. The scanning signal-generating circuit 6 comprises a horizontal 
scanning signalgenerating circuit 28, a vertical scanning signalgenerating 
circuit 29, amplifiers 30, 31, 32, 33, and switches 34, 35. The switch 34 
selectively supplies the horizontal and vertical scanning signals to the 
deflecting coil 5x. The switch 35 selectively supplies the horizontal and 
vertical scanning signals to the deflecting coil 5y. 
The operation of the structure shown in FIGS. 4 and 5 is now described. 
When an ordinary secondary electron image of the specimen 4 should be 
obtained, the electron beam EB is sharply focused onto the specimen 4 by 
the condenser lens 2 and the objective lens 3. The scanning 
signal-generating circuit 6 supplies signals to the beam-deflecting coils 
5 to scan the electron beam in two dimensions. The output signal from the 
secondary electron detector 19 is supplied to a cathode-ray tube (not 
shown). Normally, the switches shown in FIG. 5 are connected as indicated 
by the solid lines. The horizontal scanning signal from the horizontal 
scanning signal-generating circuit 28 is supplied to the horizontal 
deflecting coil 5x, whereas the vertical scanning signal from the vertical 
scanning signalgenerating circuit 29 is furnished to the vertical 
deflecting coil 5y. The specimen 4 is raster scanned horizontally by the 
electron beam. The raster line is shifted vertically successively. 
An operation for focusing the electron beam is next described. The 
intensity of excitation of the objective lens 3 is varied in small 
increments. A desired region on the specimen 4 is scanned by the electron 
beam at each different value of the intensity of excitation. In this case, 
the objective lens current placed into the register 17 from the CPU 26 is 
varied in small increments. The CPU 26 controls the scanning 
signal-generating circuit 6 in such a way that one twodimensional scan is 
made for each different objective lens current. 
The secondary electrons produced in response to the irradiation of the 
electron beam EB to the specimen 4 are detected by the detector 19. The 
output signal from the detector 19 is fed to the integrator 22 via the 
filter circuit 20 and via the absolute-value circuit 21. The integrator 22 
integrates its input signal during each two-dimensional scan of the beam. 
After each two-dimensional scan, the value obtained by the integrator 22 
is supplied to the memory 24, where the value is stored together with the 
corresponding intensity of excitation of the objective lens 3. Where the 
electron beam is scanned in two dimensions and the output signal from the 
secondary electron detector is integrated for each different value of the 
intensity of excitation of the objective lens 3, if astigmatism is 
present, the CPU 26 creates a signal intensity distribution curve as shown 
in FIG. 6. In FIG. 6, the intensity of excitation of the objective lens is 
plotted on the horizontal axis, and the integrated value is plotted on the 
vertical axis. 
If the CPU 26 detects two peaks in the found signal intensity distribution 
curve, the CPU finds the intensity of excitation of the objective lens at 
the position midway between the two peaks. The CPU 26 places the value 
corresponding to this intensity of excitation into the register 17. 
Therefore, the intensity of excitation of the objective lens 3 is such 
that the circle of least confusion for the electron beam is formed on the 
specimen. Thus, the first focusing operation is finished. Thereafter, a 
well-known automatic astigmatism correction is made. In particular, a 
plurality of values which are used to correct the astigmatism and are to 
be supplied to the x-direction astigmatism correction coil 7 are read from 
the data memory 27 and supplied to the register 10. The electron beam is 
scanned for each different one of these correcting values. Secondary 
electrons produced during each scan are detected. The output signal from 
the detector corresponding to each correcting value is integrated. As a 
result, a curve is obtained from the correcting values. The 
astigmatism-correcting value which gives the peak of the curve is placed 
into the register 10. As a result, the astigmatism is corrected in the 
x-direction. Similarly, a plurality of values which are used for 
correcting the astigmatism and are to be supplied to the y-direction 
astigmatism correction coil 8 are read from the data memory 27 and 
supplied to the register 14. The electron beam is scanned for each 
different correcting value. Secondary electrons produced by each scan are 
detected. The output signal from the detector corresponding to each 
correcting value is integrated. As a result, a curve is derived from the 
output values from the integrator which correspond to the various 
correcting values. The correcting value which gives the peak of this curve 
is placed into the register 14. In this way, the astigmatism is corrected 
in the y-direction. 
A method of discriminating between peak and noise when a peak is detected 
from the integral curve (shown in FIG. 6) obtained from the secondary 
electron detector for each value of the intensity of excitation of the 
objective lens is now described. In FIG. 6, a indicates the minimum value 
of the curve. Indicated by b is the maximum value. Indicated by c is the 
second greatest value. Indicated by d is the minimum value between a and 
c. Indicated by e is the intensity of excitation when the maximum value b 
is obtained. Indicated by f is the intensity of excitation when the second 
greatest value c is obtained. Indicated by g is the intensity of 
excitation when the minimum value between a and c is obtained. 
In the case of FIG. 6, two peaks are discriminated from noise if all of the 
following three conditions are met: 
(1) signal intensities: 
EQU a&lt;b&gt;d&lt;c&gt;a 
(2) the number of samples: 
EQU e-g.gtoreq.two points 
EQU f-g.gtoreq.two points 
(3) relations among signal intensities: 
EQU (c-d)/(b-d)&gt;R if (b-d)&gt;(c-d) 
EQU (b-d)/(c-d)&gt;R if (b-d)&lt;(c-d) 
where R is a threshold value. For example, R is selected equal to 0.6. 
If these three conditions are satisfied, the position of the circle of 
least confusion is found from (e+f)/2. 
If any one of the above-described conditions is not catered for, or if the 
signal intensity distribution curve has only one peak, then the position 
of the circle of least confusion is detected in the manner described 
below. FIG. 7(a) shows the signal intensity distribution curve obtained 
from the integrator when the scanning signals from the horizontal scanning 
signal-generating circuit 28 and from the vertical scanning 
signalgenerating circuit 29 of the scanning signal-generating circuit 6 
shown in FIG. 5 are supplied to the horizontal deflecting coil 5x and the 
vertical deflecting coil 5y, respectively. If the CPU 26 finds one peak 
(P1) from the distribution curve, the CPU switches the states of the 
switches 34 and 35 to the states indicated by the broken lines. Under this 
condition, the horizontal scanning signal and the vertical scanning signal 
are supplied to the vertical deflection coil 5y and the horizontal 
deflection coil 5x, respectively. As a result, the electron beam is raster 
scanned vertically on the specimen. The raster line is shifted 
horizontally. Whenever such a two-dimensional scan is made, the output 
signal from the detector is integrated by the integrator 22. At the same 
time, the intensity of excitation of the objective lens 3 is changed in a 
stepwise fashion. Consequently, the CPU 26 creates the signal intensity 
distribution curve shown in FIG. 7(b). Then, the CPU 26 finds from the 
distribution curve of FIG. 7(b) the peak position P2 at which the maximum 
value is obtained. The CPU 26 calculates P0=(P1+P2)/2, where P1 is the 
peak position P1 when the raster line is shifted horizontally, and P2 is 
the peak position wherein the raster line is shifted vertically. The 
intensity of excitation of the objective lens 3 which corresponds to P0 is 
found. In this way, if two peaks are not obtained in spite of 
two-dimensional scan of the electron beam according to the state of the 
surface of the specimen, the electron beam can be brought into focus at 
the position of the circle of least confusion. After this focusing 
operation, automatic astigmatic correction similar to the above-described 
correction is made. 
FIG. 8 is a flowchart illustrating the automatic focusing and the automatic 
astigmatism correction operations described above. Since the second 
automatic focusing operation is performed after an astigmatic correction, 
the specimen may be scanned in two dimensions while the beam is being 
raster scanned either horizontally or vertically. 
While one embodiment of the invention has been described in detail, it is 
to be understood that the invention is not limited thereto. In the above 
embodiment, secondary electrons are detected. It is also possible to 
detect reflected electrons. Also in the above embodiment, the intensity of 
excitation of the objective lens is varied during the automatic focusing 
operation. Alternatively, the intensity of excitation of a lens auxiliary 
(see 3 in FIG. 4) to the objective lens could be varied. 
As described above, the novel method of automatically achieving focusing 
and astigmatism correction in an electron beam apparatus involves scanning 
an electron beam horizontally on the surface of a specimen, moving the 
scanning line vertically to scan the specimen in two dimensions, and 
varying the intensity of excitation of the objective lens in a stepwise 
manner. The output signal from a secondary electron detector is integrated 
by an integrator. Variations in the intensity of the objective lens 
produce variations in the curve obtained by the integrator. If only one 
peak is found in the curve, the beam is scanned on the specimen 
vertically. The scanning line is shifted horizontally and thus the 
specimen is scanned in two dimensions The resulting signal is integrated. 
This scan of the electron beam is repeated many times while varying the 
intensity of excitation of the objective lens. A curve representing 
variations in the output signal from the integrator which are caused by 
the variations in the intensity of excitation of the objective lens is 
found. The objective lens is adjusted to the intensity of excitation lying 
midway between the peak of this curve and the peak of the curve already 
obtained by the first scan. Then, the astigmatism is corrected. 
Consequently, focusing and astigmatism correction can be automatically and 
accurately accomplished. 
Having thus described my invention with the detail and particularity 
required by the Patent Laws, what is desired and claimed to be protected 
by Letters Patent is set forth in the following claims.