Scanning electron beam exposure system

A scanning electron beam exposure system includes two apertures (39a, 48a) for forming a rectangular beam (31). The cross section of the rectangular beam is changed by a deflection unit (47X, 47Y) arranged between the two apertures. The rectangular beam is refocused by a refocusing coil (51) to improve the peripheral sharpness of a projected image of the beam. The refocusing coil is controlled in accordance with the cross section (X.sub.1, Y.sub.1) of the beam.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an electron beam exposure system. More 
particularly, it relates to a scanning electron beam exposure system in 
which the shape of an electron beam is modified. 
2. Description of the Prior Art 
In recent years, in a scanning electron beam exposure system, a shaped, or 
rectangular beam has been used to enhance the throughput of the exposure 
system, as compared with a round beam. 
In this system, the total current of the rectangular beam increases as the 
cross section thereof increases, resulting in the generation of excessive 
electron-electron interactions or repulsive forces, that is, so-called 
Coulomb scattering effects. Such electron-electron interactions result not 
only in increased energy point distribution around the mean beam energy 
but also result in deterioration of peripheral sharpness due to electron 
displacement and disorientation. 
One approach to weakening the above-mentioned electron-electron 
interactions is to reduce the current density of an electron beam when the 
cross section thereof is large. This approach, however, is disadvantageous 
in regard to the throughput of the exposure system since the beam current 
itself is also reduced. Another approach is to shorten the length of the 
electron optical systems, such the magnifying and demagnifying lens 
systems. This, however, is difficult. 
In addition, in the rectangular-beam exposure system, a first rectangular 
beam-shaping aperture and a second rectangular beam-shaping aperture are 
provided, and superposition of the two apertures is modified by a 
deflection unit so as to form an arbitrary rectangular beam. That is, a 
fluctuation in the cross section of the rectangular beam generates a 
fluctuation in the mode of electron-electron interactions so that the 
point of focus of a projected image is shifted. As a result, the sharpness 
of the projected image is changed in accordance with the fluctuation of 
the cross section of the rectangular beam. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to provide a scanning 
electron beam exposure system in which the peripheral sharpness of a 
projected image is substantially unchanged. 
In view of the above-mentioned object, according to the present invention, 
an auxiliary converging means or a refocusing means is provided to 
converge or refocus a shaped beam in accordance with the cross section 
thereof. Compensation for the shifting of the position of the shaped beam 
due to refocusing also occurs. Further, in order to enhance the throughput 
of the exposure system, means are provided for dividing a rectangular 
pattern into a plurality of patterns each having the same size when the 
rectangular pattern is of a size larger than a predetermined size. 
The present invention will be more clearly understood from the description 
as set forth below with reference to the attached drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1, which is a schematic diagram of a prior art electron 
beam-forming system, reference numeral 1 designates an electron beam. A 
first mask 2 having a square or rectangular aperture 3 and a second mask 4 
having a square or rectangular aperture 5 are arranged in the path of the 
electron beam 1. Between the two masks 2 and 4, there is arranged a 
demagnifying lens 6 and a deflection unit 7 which comprises electrostatic 
plates. 
In FIG. 1, the state of superposition of the two apertures 3 and 5 is 
modified by the deflection unit 7, which deflects a rectangular electron 
beam passing through the aperture 3 of the mask 2 so that an arbitrarily 
rectangular electron beam, indicated by reference numeral 8, is obtained. 
In FIG. 2A, which shows the peripheral sharpness deterioration caused by 
electron-electron interactions, reference numeral 21 indicates a 
de-magnifying lens, and reference numerals 22 and 23 indicate electron 
beams. If the current density is definite, the current of the electron 
beam 22 is small while the current of the electron beam 23 is large. In 
such a case, the point of focus F.sub.2 of the electron beam 23 is shifted 
by F from the point of focus F.sub.1 of the electron beam 22. This means 
that the peripheral sharpness of the large-current electron beam 23 is 
deteriorated as compared with that of the small-current electron beam 22. 
That is, as the line A in FIG. 2B shows, the periphery of an image is 
blurred in accordance with the beam cross section or the beam current. In 
the present invention, a refocusing operation is performed with respect to 
the deteriorated peripheral sharpness of an electron beam in accordance 
with the magnitude of the beam cross section. As can be seen from the 
dashed line B, which shows the peripheral blurring according to the 
present invention, the peripheral sharpness is improved since the 
peripheral blurring is small, and the amount of shifting .DELTA.F in FIG. 
2A is reduced. 
The principle of the present invention will now be explained with reference 
to FIGS. 3A through 3C. 
In FIG. 3A, two rectangular image patterns P and P', which are projected by 
utilizing the rectangular electron beam 8 of FIG. 1, are illustrated. The 
pattern P is small in size, as is indicated by size information X.sub.1 
and Y.sub.1, and the pattern P' is large in size, as is indicated by size 
information X.sub.1 ' and Y.sub.1 '. Generally, the peripheral sharpness 
of the large pattern P' is deteriorated as compared with that of the small 
pattern P. In this case, however, the origin O of the pattern P is 
substantially the same as the origin O' of the pattern P'. Note that the 
origins O and O' are determined by the periphery of the aperture 5 of the 
second mask 4. 
In FIG. 3B, a refocusing operation according to the present invention is 
performed so as to improve the peripheral sharpness of the image patterns 
P and P'. In this case, however, the origin O' of the pattern P' is far 
away from the origin O of the pattern P since the axis of a refocusing 
coil for performing the above-mentioned refocusing operation does not 
always coincide with the electron beam axis. 
In the present invention, the amount of shifting .DELTA.X and the amount of 
shifting .DELTA.Y of FIG. 3B are detected and, as a result, are 
proportional to the area of the corresponding image pattern, such as P'. 
In FIG. 3C, a flyback operation is performed so as to correct the shifting 
of position indicated by .DELTA.X and .DELTA.Y in FIG. 3B. In such a 
flyback operation, a deflection unit for controlling the position of a 
projected beam is used. 
FIG. 4, including 4A and 4B, which is an embodiment of the present 
invention, is roughly divided into a beam-forming and beam-deflecting 
portion and a pattern-generating and pattern-control portion. 
First, the beam-forming and beam-deflecting portion is explained. Reference 
numeral 31 indicates an electron beam emitted from an electron gun 32, 33 
indicates an anode, 34 indicates a cathode, 35 through 38 indicate 
aligning coils, 39 indicates a first mask, having a first rectangular 
aperture 39a,which corresponds to the mask 2 of FIG. 1, and 40 through 45 
indicate de-magnifying lenses (coils). The last de-magnifying lens 45 is 
also called a projecting lens for projecting the electron beam 31 to a 
target plate 46, such as a wafer or a glass plate. 
Reference numerals 47X and 47Y indicate deflection units (plates) for 
determining the size of the electron beam 31 on the basis of the size 
information X.sub.1 and Y.sub.1. The deflection units 47X and 47Y 
correspond to the deflection unit 7 of FIG. 1. Note that the deflection 
units 47X and 47Y are, of course, separated from each other; however, only 
one unit is illustrated so as to simplify the illustration. 
A second mask 48, having a second rectangular aperture 48a,which 
corresponds to the second mask 4 of FIG. 1 is provided within the 
de-magnifying lens 42. 
The beam 31 shaped by the first aperture 39a, the second aperture 48a, and 
the deflection units 47X and 47Y, passes through a pair of blanking plates 
49 which determine whether the beam will be projected onto the plate 46 or 
will be blanked. 
After the beam 31 passes through the demagnifying lens 43 it through round 
aperture 50a of mask 50. The aperture 50a passes only the electrons 
passing through the center of the above-mentioned electron optical system, 
including the de-magnifying lenses. 
The de-magnifying lens 44 projects the rectangular beam 31 onto the center 
of the projecting lens 45. Within the de-magnifying lens 44, an auxiliary 
converging coil, or a refocusing coil, 51, which is a coreless coil is 
provided as to improve the peripheral sharpness of the beam 31. As is 
shown in FIG. 5, the lens 44 comprises coils 44a, an iron frame 44b 
enclosing the coils 44a,and ferrite pole pieces 44c. The ferrite pole 
pieces 44c prevent eddy currents and thereby enhance the response speed of 
the refocusing coil 51. 
Within the projecting lens 45, deflection units 52X and 52Y are provided to 
deflect the beam 31 on the basis of the position information X.sub.2 and 
Y.sub.2. The deflection units 52X and 52Y are electrostatic plates. Also, 
only one of the deflection units 52X and 52Y is illustrated to simplify 
the illustration. 
Reference numerals 53 and 54 indicate a dynamic focusing coil and a 
stigmatic coil, respectively, for dynamic correction, which is required 
when deflection of the beam is increased and the beam is far away from the 
optical axis. 
The pattern-generating and pattern-control portion will now be explained. 
Reference numeral 61 designates a central processing unit (CPU) which 
controls the entire system, 62 designates a data memory for storing a 
plurality of rectangular exposure patterns, and 63 designates a pattern 
generator. The CPU 61, the data memory 62, and the pattern generator 63 
are connected to each other by a data bus 64. When a rectangular exposure 
pattern stored in the data memory is larger than a predetermined size, the 
rectangular exposure pattern is divided into a plurality of equal patterns 
by the pattern generator 63, which, in turn, generates equal patterns 
defined by the size information X.sub.1 and Y.sub.1 and the position 
information X.sub.2 and Y.sub.2, shown in FIG. 6. In this case, X.sub.1 
.ltoreq.S.sub.0 and Y.sub.1 .ltoreq.S.sub.0, wherein S.sub.0 is a 
predetermined value. The pattern generator 63 will be explained in more 
detail. 
The size information X.sub.1 is supplied through a digital-to-analog 
converter (DAC) 65 and an amplifier 66 to the deflection unit 47X while 
the size information Y.sub.1 is supplied through a DAC 67 and an amplifier 
68 to the deflection unit 47Y, with the result that the rectangular beam 
31 is shaped or modified in accordance with the size information X.sub.1 
and Y.sub.1. The information X.sub.1 and Y.sub.1 is supplied to a 
multiplier 69, which calculates S=X.sub.1 .multidot.Y.sub.1, S being the 
beam cross-section. 
A register 70 stores a compensation coefficient .alpha., which is 
periodically renewed by the CPU 61 and which compensates for a focus shift 
generated due to a change in the cross-section of the electron beam. The 
coefficient .alpha. is determined by .alpha.=I/S, where I is the current 
flowing through the refocusing coil 51 and S is the beam cross-section. 
The values .alpha. and S are supplied to a multiplier 71, which calculates 
a value .alpha..multidot.S. The value .alpha..multidot.S is supplied 
through a DAC 72 and an amplifier 73 to the refocusing coil 51, with the 
result that the refocusing operation is performed in accordance with the 
magnitude of the cross section of the beam 31. That is, when the size S of 
the beam 31 is small, the refocusing coil 51 is controlled so as to weaken 
the converging effect thereof. However, when the size S of the beam 31 is 
large, the refocusing coil 51 is controlled so as to strengthen the 
converging effect thereof. As a result, the beam 31 is converged at the 
same point of focus, regardless of the size S thereof. 
Registers 74 and 75 and multipliers 76 and 77 are used for the flyback 
operation, that is, correction of the axis of the beam 31. Also, the 
constants .gamma.X and .gamma.Y are stored in the registers 74 and 75, 
refer to deflection amounts which hold the position of the beam at a 
definite position when the beam size is changed, and are periodically 
renewed by the CPU 61. The constant .gamma.X is determined by 
.gamma.X=.DELTA.X/S, and the constant .gamma.Y is determined by 
.gamma.Y=.DELTA.Y/S. The values .gamma.X and S are supplied to the 
multiplier 76, which calculates .DELTA.X=.gamma.X.multidot.S, and the 
values .gamma.Y and S are supplied to the multiplier 77, which calculates 
.gamma.Y=.gamma.Y.multidot.S. Thus, the amount of flyback .DELTA.X and the 
amount of flyback .DELTA.Y are calculated in accordance with the magnitude 
of the cross section S of the beam 31. 
The position information X.sub.2 and Y.sub.2 is processed by a correction 
circuit 78, which performs the following well-known correction 
calculations: 
EQU X.sub.2 '=X.sub.2 +g.sub.x .multidot.X.sub.2 +r.sub.X .multidot.Y.sub.2 
+h.sub.X .multidot.X.sub.2 .multidot.Y.sub.2 +O.sub.x 
EQU Y.sub.2 '=Y.sub.2 +g.sub.Y .multidot.Y.sub.2 +r.sub.Y .multidot.X.sub.2 
+h.sub.y .multidot.X.sub.2 Y.sub.2 +O.sub.x 
where g.sub.X and g.sub.Y are gain coefficients, r.sub.X and r.sub.y are 
rotational coefficients, h.sub.X and h.sub.Y are trapezoidal coefficients, 
and O.sub.X and O.sub.Y are offset coefficients. 
The amount of flyback .DELTA.X is added to the corrected position data 
X.sub.2 ' by an adder 79. The result, X.sub.2 '+.DELTA.X, is supplied 
through a DAC 80 and an amplifier 81 to the deflection unit 52X. 
Simultaneously, the amount of flyback .DELTA.Y is added to the corrected 
position data Y.sub.2 ' by an adder 82. The result, Y.sub.2 '+.DELTA.Y, is 
supplied through a DAC converter 83 and an amplifier 84 to the deflection 
unit 52Y. Thus, the operation for flying back the shift in position due to 
the refocusing operation is performed in accordance with the magnitude of 
the cross-sections of the beam 31. 
It should be noted that deflection units exclusively for the flyback 
operation can also be provided. If such deflection units are provided, the 
values .DELTA.X and .DELTA.Y are supplied thereto. 
The arrangement of the refocusing coil 51 will now be explained with 
reference to FIG. 7, which shows the distribution of the magnetic flux 
density in the path of the beam 31. The efficacy of the refocusing coil 51 
is dependent on the magnetic field intensity or the magnetic flux density 
of the coil 51. That is, the focus distance f is represented by 
##EQU1## 
Therefore, the amount of shift of the focus 
##EQU2## 
due to the refocusing coil 51 is represented by 
##EQU3## 
where .DELTA.B is the change of the magnetic flux density due to the 
refocusing coil 51. As a result, when the magnetic flux density B at the 
refocusing coil 51 is large, the ability of the refocusing coil 51 to 
shift the point of focus of the beam 31 is large. Therefore, it is 
preferable that the refocusing coil 51 be positioned within the lens 43, 
44 or 45. 
Assume that the refocusing coil 51 is positioned within the de-magnifying 
lens 43. In this case, the refocusing coil 51 does not have a sufficient 
effect on the image projected onto the plate 46 since the distance between 
the refocusing coil 51 and the plate 46 is too great. 
Assume that the refocusing coil 51 is positioned within the last, or 
projecting, lens 45. In this case, since many other deflection units, such 
as 53 and 54 are positioned within the projecting lens 45, large magnetic 
interactions between the refocusing coil 51 and the other deflection units 
are generated. This is not preferable in regard to the control of the 
refocusing coil 51. In addition, as illustrated in FIG. 8 which shows 
projected images at the plate 46 after the refocusing operation, the 
relative shift in position of a small beam and a large beam in each 
sub-field SF fluctuates largely due to the large aberration of the 
projecting lens 45. In FIG. 8, MF indicates a main field determined by, 
for example, electromagnetic deflection means, SF indicates a sub-field 
determined by, for example, electrostatic deflection means, P.sub.1, 
P.sub.2, . . . , P.sub.5 indicate relatively small patterns, and P.sub.1 
', P.sub.2 ', . . . , P.sub.5 ' indicate relatively large patterns. In 
this case, the small patterns P.sub.1, P.sub.2, . . . , P.sub.5 superpose 
the large patterns P.sub.1 ', P.sub.2 ', . . . , P.sub.5 ', respectively, 
superpose the large patterns P.sub.1 ', P.sub.2 ', . . . , P.sub.5 ', 
respectively, before the refocusing operation. Thus, if the refocusing 
coil 51 is positioned within the projecting lens 45, it is difficult to 
perform a flyback operation due to fluctuation of the relative shift in 
position between a small beam (pattern) and a large beam (pattern). 
When the refocusing coil 51 is positioned within the lens 44, as is 
illustrated in FIG. 4, the relative shift in position between a small beam 
and a large beam after the refocusing operation is uniform in each 
sub-field, as is illustrated in FIG. 9A, since the aberration of the 
refocusing lens 51 is, in this case, small. Therefore, after the flyback 
operation is performed, the small patterns P.sub.1, P.sub.2, . . . , 
P.sub.5 superpose the large patterns P.sub.1 ', P.sub.2 ', . . . , P.sub.5 
', respectively, as is illustrated in 9B. 
Thus, it is preferable that the refocusing coil 51 be positioned within the 
lens 44, not within the lens 43 or 45. 
The pattern generator 63 of FIG. 4 will now be explained in more detail 
with reference to FIGS. 10, 11, and 12. If a rectangular exposure pattern 
stored in the data memory 62 of FIG. 4 is smaller than or equal to a 
predetermined size, the pattern generator 63 generates the same pattern as 
that stored in the data memory 62. However, if a rectangular exposure 
pattern stored in the data memory 62 is larger than the above-mentioned 
predetermined size, the pattern generator 63 generates a plurality of 
patterns having the same size. That is, as is shown in FIG. 11, if a large 
rectangular pattern stored in the data memory is defined by the size 
information X.sub.10 and Y.sub.10 and the position information X.sub.20 
and Y.sub.20, the pattern generator 63 generates six patterns defined by 
the size information X.sub.1 and Y.sub.1. The position information 
concerning the six patterns is defined by (X.sub.20, Y.sub.20), (X.sub.20 
+X.sub.1, Y.sub.20), . . . , (X.sub.20 +2X.sub.1, Y.sub.20 +Y.sub.1), 
respectively. 
That is, after the CPU 61 reads the information X.sub.10, Y.sub.10, 
X.sub.20, and Y.sub.20 out of the data memory 62, the CPU 61 performs the 
operation illustrated in FIG. 12. Then the CPU 61 transmits the 
information X.sub.1 and Y.sub.1 in addition to the information X.sub.10, 
Y.sub.10, X.sub.20, and Y.sub.20. 
The operation illustrated in FIG. 12 will now be explained. The operation 
starts at step 1201. Step 1202 determines whether or not X.sub.10 is 
larger than a predetermined value S.sub.0. If the answer at step 1202 is 
affirmative, the control proceeds to step 1203, in which N.sub.x 
=]X.sub.10 /S.sub.0 ]+1 is calculated. Here, N.sub.x represents the number 
of divided patterns in the X direction. Next, at step 1204, the CPU 61 
calculates X.sub.1 =[X.sub.10 /N.sub.x ], i.e., the size of each divided 
pattern in the X direction. The control then proceeds to step 1206. 
However, if the answer at step 1202 is negative, the control proceeds to 
step 1205, in which X.sub.1 .rarw.X.sub.10 is performed. At step 1206, the 
CPU 61 determines whether or not Y.sub.10 is larger than the value 
S.sub.0. If the answer at step 1206 is affirmative, the control proceeds 
to steps 1207 and 1208, in which the number N.sub.y of divided patterns 
and the size Y.sub.1 of each divided pattern in the Y direction are 
calculated. Then the control proceeds to step 1210. However, if the answer 
at step 1206 is negative, the control proceeds to step 1209 in which 
Y.sub.1 .rarw.Y.sub.10 is performed. Next, the control proceeds to step 
1210, in which the operation of FIG. 12 is completed. 
Referring back to FIG. 10, the structure of the pattern generator 63 will 
now be explained. In FIG. 10, reference numerals 1001 through 1006 
designate registers divided into first and second sets for receiving the 
information X.sub.1, X.sub.10, Y.sub.1, Y.sub.10, X.sub.20, and Y.sub.20, 
respectively, from the CPU 61; 1007 and 1008 designate registers for 
storing the remainder of divided patterns in the X direction and the Y 
direction, respectively; 1009 and 1010 designate registers for storing the 
position information X.sub.2 and Y.sub.2 of each divided pattern; 1011 and 
1012 designate subtracters; 1013 and 1014 designate adders; 1015 and 1016 
designate comparators; and 1017 and 1018 designate AND circuits. It is 
assumed that small values such as X.sub.1 /2 and Y.sub.1 /2 are applied by 
the CPU 61 to the (+) inputs of the comparator 1015 and the comparator 
1016, respectively. 
The operation of the circuit of FIG. 10 will now be explained with 
reference to FIG. 11. When the CPU 61 generates a clock signal CK1, the 
values X.sub.10, Y.sub.10, X.sub.20, and Y.sub.20 are set in the registers 
1007, 1008, 1009, and 1010, respectively. As a result, the position 
parameters of the first pattern P.sub.1 are 
EQU (X.sub.2, Y.sub.2)=(X.sub.20, Y.sub.20). 
In this case, the value of the register 1007 is X.sub.10 (&gt;X.sub.1 /2), the 
output of the comparator 1015 remains low, the value of the register 1008 
is Y.sub.20 (&gt;Y.sub.1 /2), and the output of the comparator 1016 remains 
low. After exposure of the first pattern P.sub.1 is completed, a clock 
signal CK2 from the CPU 61 is generated to operate the adder 1013. As a 
result, the value of the register 1019 is changed from X.sub.20 to 
X.sub.20 +X.sub.1. Therefore, the position parameters of the second 
pattern P.sub.2 are 
EQU (X.sub.2, Y.sub.2)=(X.sub.20 +X.sub.1, Y.sub.20). 
Simultaneously, the subtracter 1011 is operated so that the value of the 
register 1007 is changed from X.sub.10 to X.sub.10 -X.sub.1 
(.congruent.2X.sub.1). Therefore, the output of the comparator 1015 does 
not change. Similarly, after exposure of the second pattern P.sub.2 is 
completed, a clock signal CK2 from the CPU 61 is generated to operate the 
adder 1013. As a result, the value of the register 1009 is changed from 
X.sub.20 +X.sub.1 to X.sub.20 +2X.sub.1. Therefore, the position 
parameters of the third pattern P.sub.3 are 
EQU (X.sub.2, Y.sub.2)=(X.sub.20 +2X.sub.1, Y.sub.20). 
Simultaneously, the subtracter 1011 is operated so that the value of the 
register 1007 is changed from X.sub.10 -X.sub.1 to X.sub.10 -2X.sub.1 
(.congruent.X.sub.1). Therefore, the output of the comparator 1015 does 
not change. Similarly, when exposure of the third pattern P.sub.3 is 
completed, a clock signal CK2 from the CPU 61 is generated to operate the 
adder 1013. As a result, the value of the register 1009 is changed from 
X.sub.20 +2X.sub.1 to X.sub.20 +3X.sub.1. However, in this case, the 
subtracter 1011 is operated so that the value of the register 1007 is 
changed from X.sub.10 -2X.sub.1 to X.sub.10 -3X.sub.1 (.congruent.0), and, 
accordingly, the output of the comparator 1015 is changed from low to 
high. As a result, the values X.sub.10 and X.sub.20 are again set in the 
registers 1007 and 1009, respectively. Further, the subtracter 1012 is 
operated so that the value of the register 1008 is changed from Y.sub.10 
to Y.sub.10 - Y.sub.1 (.congruent.1), and, in addition, the adder 1014 is 
operated so that the value of the register 1010 is changed from Y.sub.20 
to Y.sub.20 +Y.sub.1. Therefore, the position parameters of the fourth 
pattern P.sub.4 are 
EQU (X.sub.2, Y.sub.2)=(X.sub.20, Y.sub.20 +Y.sub.1). 
Subsequently, exposure of the patterns P.sub.4, P.sub.5, and P.sub.6 is 
completed, the output of the comparator 1015 and the output of the 
comparator 1016 both become high, and, accordingly, the AND circuit 1018 
transmits an exposure end signal ED to the CPU 61. Thus, exposure of the 
pattern defined by the information X.sub.10, Y.sub.10, X.sub.20, and 
Y.sub.20 of the data memory 62 is completed. 
As was explained above, when a rectangular pattern stored in the data 
memory 62 is larger than the predetermined size (S.sub.0 .times.S.sub.0), 
the pattern is divided into a plurality of patterns having the same size. 
As a result, the throughput of the electron beam exposure system is 
improved, which will be explained with reference to FIGS. 13A, 13B, 14A, 
and 14B. 
In FIG. 13A, patterns 131 and 132 are smaller than the predetermined size, 
and a pattern 13 is larger than the predetermined size so that the pattern 
13 is divided into a plurality of identical patterns 133 through 150. When 
the exposure operation is performed on the patterns 131, 132, . . . , 150 
in that order, the current I.sub.f flowing through the refocusing coil 51 
changes, as is shown in FIG. 13B. As a result, the number of current 
transitions is small, and, accordingly, the entire exposure time is small. 
The shaded portions indicate transitions of the current I.sub.f, in which 
the beam 31 is blanked to stop the exposure operation. 
In FIG. 14A, the pattern 13 is divided into a plurality of patterns 133' 
through 150'. However, in this case, only the patterns 133' through 137' 
and the patterns 139' through 143' have the same size (S.sub.0 
.times.S.sub.0), the other patterns 138' and 144' through 150' being 
smaller than the predetermined size. Therefore, when the exposure 
operation is performed, the patterns 131, 132, and 133' through 150', in 
that order, the current I.sub.f flowing through the refocusing coil 51 
changes, as is shown in FIG. 14B. As a result, the number of current 
transtions is large, and, accordingly, the entire exposure time is 
increased as compared with the case illustrated in FIGS. 13A and 13B. 
Thus, in respect to the throughput of the exposure system, it is necessary 
that a large pattern be divided into a plurality of patterns each having 
the same size. 
As was explained hereinbefore, according to the present invention, the 
peripheral sharpness of a projected image is improved by a refocusing 
operation in accordance with the magnitude of the cross-section of a beam.