Charged particle beam lithograph apparatus

A charged particle beam lithograph apparatus of the present invention projects a charged particle beam onto a sample through a mask and lithographs a mask pattern on the sample through the movement of the charged particle beam. In order to focus the charged particle beam, the apparatus creates an electromagnetic field, from the magnetic lens, symmetric with respect to an optical axis of the charged particle beam. An aberration of the charged particle beam is created under the symmetric electromagnetic magnet. The aberration is compensated for under an electromagnetic field nonsymmetric with respect to the optical axis which is created by a deflection unit.

BACKGROUND OF THE INVENTION 
The present invention relates to an apparatus for projecting a charged 
particle beam, such as an electron or ion beam, onto a sample through a 
mask and, in particular, to a charged particle beam lithograph apparatus 
for lithographing a mask pattern on a sample by moving such a beam. 
This type of apparatus is equipped with an optical system for focusing or 
deflecting a beam. The focusing is done by an axisymmetric electromagnetic 
field created by a magnetic lens, electrostatic lens, etc., and the 
deflection is effected by controlling non-symmetry in an electromagnetic 
field. 
Such an optical system presents an aberration problem. The aberration 
handling method is, for example, (1) by curving a mask or (2) by varying 
the intensity of an electromagnetic field by an electrostatic lens and 
hence a focal distance involved and utilizing only a crosspoint between a 
focus plane and a sample surface which is created by moving the focus 
plane up and down. 
However, these methods (1) and (2) are effective to a curvature aberration 
caused by a curved focal plane involved, but cannot eliminate, for 
example, a coma aberration. 
The former method (1) cannot be applied to the case where the mask cannot 
be curved, while, on the other hand, the latter method (2) presents the 
problem in that the inclination of the focus plane is made greater, as the 
distance is moved away from an optical axis, with the consequent lowered 
production efficiency. 
BRIEF SUMMARY OF THE INVENTION 
It is accordingly the object of the present invention to provide a charged 
particle beam lithograph apparatus which can effectively compensate for 
the aberration of a charged particle beam resulting from a symmetric 
electromagnetic field, such as a coma aberration and curvature aberration. 
A charged particle beam lithograph apparatus of the present invention 
lithographs a mask pattern onto a sample through the projection of a 
charged particle beam onto a sample through a mask and the movement of the 
charged particle beam. In order to focus the charged particle beam, an 
electromagnetic field is defined, by a magnetic lens, symmetric with 
respect to an optical axis. An aberration of the charged particle beam is 
produced under this symmetric electromagnetic field and it is compensated 
for under an electromagnetic field nonsymmetric with respect to the 
optical axis which is defined by the deflection unit. 
Additional objects and advantages of the invention will be set forth in the 
description which follows, and in part will be obvious from the 
description, or may be learned by practice of the invention. The objects 
and advantages of the invention may be realized and obtained by means of 
the instrumentalities and combinations particularly pointed out in the 
appended claims.

DETAILED DESCRIPTION OF THE INVENTION 
A charged particle beam lithograph apparatus of the present invention will 
be explained below with reference to a preferred embodiment. Here, 
explanation is given below, by way of example, about a charged particle 
beam lithograph apparatus for projecting a charged particle beam, such as 
an electron beam or ion beam, past a very fine pattern mask onto a sample, 
transferring the beam and lithographing the pattern on the sample. 
FIG. 1 is a cross-sectional view showing a column of a charged particle 
beam lithograph apparatus according to one embodiment of the present 
invention. A beam 6 generated from an electron gun 25 illuminates a mask 1 
past an electromagnetic field symmetric to an optical beam 8 which is 
produced by a primary magnetic lens 22 and an electromagnetic field 
nonsymmetric to the optical axis 8 which is produced by a scanning 
deflection unit (deflector) 24. Any given LSI pattern is written at a time 
or cyclically on a mask 1. 
The beam shaped at the mask 1 is imaged on the sample 7 past an 
electromagnetic field symmetric to the optical axis 8 which is defined by 
secondary magnetic lenses 2, 3 and an electromagnetic field nonsymmetric 
to the optical axis 8 which is defined by deflection units (deflectors) 4, 
5. The deflection units 4, 5, each, comprise three deflection coils 
arranged parallel to the optical axis 8. 
It is possible to write the pattern of the mask 1 on the sample 7 by 
controlling the nonsymmetry of the electromagnetic field by the scanning 
deflection unit 24. 
By dynamically controlling the nonsymmetry of the electromagnetic field 
defined by the deflection units 4, 5 in accordance with the position of 
the beam on the mask 1, that is, with the use of a coma aberration 
intentionally produced by the deflectors, the coma aberration resulting 
from the magnetic lenses 2, 3 is canceled. That is, a total of the coma 
aberration resulting from the magnetic lenses 2, 3 and coma aberration 
resulting from the deflection units 4, 5 is set, at all times, to be zero 
or a very small level at a sample surface. If, at this time, the 
deflection sensitivity is set to be zero, the characteristic becomes equal 
to an axial imaging characteristic in the case of those axisymmetric 
magnetic lenses 2, 3 only being involved. 
In the case where a position in a focus plane varies by the deflection 
units 4, 5 in the magnetic field, it is only necessary to variably control 
a drive current also in the magnetic lenses 2, 3 in accordance with the 
control of the deflection units 4, 5. Further, a mask stage with the mask 
1 placed thereon and sample stage with the sample placed thereon, that is, 
with the sample on which the pattern on the mask 1 is projected, are 
synchronously driven/controlled so as to enable their speed ratio to be 
set equal to a magnification of an electronic beam optical system. 
If, on the other hand, the mask 1 has a pattern 1 of cyclic holes, the same 
control as that on a conventional variable shaping beam exposure apparatus 
is carried out. In the conventional variable shaping beam exposure 
apparatus, a rectangular beam which is shaped on a first aperture is 
shaped by a second aperture into a rectangular or triangular beam of any 
size. At this time, a single opening was formed, normally on the optical 
axis, in the first and second apertures and a shaped beam was projected, 
under a deflection unit, on a projection plane at any given position. In 
order to secure a broader illumination area, this system has required, as 
a power supply for the deflection unit, a DA converter for more bits and a 
high speed amplifier of a greater amplification factor. 
If, with the use of an aperture mask with openings of the same size 
cyclically arranged as the mask 1, they are selectively used in accordance 
with a position on the projection plane, it is possible to project a 
shaped beam of a given pattern in a broader range. It is, therefore, 
possible to illuminate a relatively broader area even with the use of a DA 
converter of less bits and amplifier of a smaller amplification factor. At 
this time, since an area greatly distant from the optical axis can be used 
relative to the aperture mask, the aberration of the lens becomes greater 
in the conventional structure, so that a given pattern cannot be printed 
with a better image resolution. According to the aberration correction 
method of the present invention, on the other hand, it is possible to 
solve this problem encountered in the conventional structure. 
According to the present invention, as set out above, the coma aberration 
of the above-mentioned electromagnetic lenses 2, 3 is dynamically canceled 
by the coma aberration of the magnetic lenses 2, 3 and it is possible to 
perform a lithograph process without a beam spot being minimized due to a 
curvature aberration caused by the optical system involved. 
FIG. 2 shows an arrangement of a major section for realizing these 
features. The major section is so arranged that a system controller 31 
serves as a control center. In this arrangement, a focusing signal 
generator 33 generates a focusing signal in accordance with a control 
signal from the system controller 31. The focusing signal is amplified by 
an amplifier 45 and supplied to the primary magnetic lens 22. The scanning 
signal generator 35 generates a scanning signal in accordance with a 
scanning control signal from the system controller 31. The scanning signal 
is amplified by an amplifier 47 and supplied to a scan deflector 
(deflection unit) 24. 
A coma aberration cancel signal generator 37 takes in the scan control 
signal and a coma aberration cancel signal is dynamically generated in 
accordance with a position on the mask 1 represented by this signal. A 
deflection magnetic field is created by the coma aberration cancel signal 
from the deflection unit (deflectors 3, 4) and the coma aberration of the 
magnetic lenses 2, 3 is dynamically canceled by the coma aberration 
intentionally produced through the deflection. 
A focus plane shift circuit 39 takes in a scanning control signal and a 
shift signal for shifting sideways (the orthogonal direction for the 
optical axis) a focus plane is dynamically formed in accordance with a 
position on the mask 1 represented by that signal and added to the coma 
aberration cancel signal. The addition signal is supplied to the 
deflection units (deflectors 4, 5). 
A focusing signal generator 41 generates a focusing signal in accordance 
with a control signal from the system controller 31. A focusing distance 
correcting circuit 43 takes in the scanning control signal and the focal 
distance of the focusing signal from the focusing signal generator 41 is 
dynamically corrected (varied) in accordance with a position on the mask 1 
represented by this signal. The corrected focusing signal is amplified by 
the amplifier 51 and supplied to the secondary magnetic lenses 2, 3. 
By the shifting of the focus plane and correction of the focal distance it 
is possible to effect a lithograph process by a bottom portion of a beam 
focus plane curved by the curvature aberration, that is, by the effective 
greatest portion of a beam spot. 
First, the canceling of the coma aberration will be explained below. FIG. 3 
shows the positional dependency of the coma aberration by the magnetic 
lenses 2 and 3. A line 9 represents a coma aberration by the magnetic 
lenses 2 and 3 and a line 10 represents a coma aberration produced by the 
deflection units 4 and 5. The sum of both the coma aberrations is 
represented by a line 11. For example, when a beam is on a position 12 on 
the mask 1, the coma aberration is so created by the deflection units 4, 5 
as to set the sum of coma aberration to be zero at that position 12. This 
is represented by the following equation. With L representing the distance 
from the optical axis of a charged particle beam and A, the convergence 
semiangle, the coma aberration by the axisymmetric lens becomes 
L.multidot.A.sup.2, while, on the other hand, the coma aberration by the 
deflection units 4, 5 becomes D.multidot.A.sup.2 where the strength of the 
deflection magnetic field is represented by D. If, therefore, L takes any 
given value L.sub.0, 
EQU L.multidot.A.sup.2 +D.multidot.A.sup.2 =(L+D).multidot.A.sup.2 =(L.sub.0 
-L.sub.0).multidot.A.sup.2 
where D=-L.sub.0, and the coma aberration by the magnetic lenses 2, 3 and 
that by the deflection units 4, 5 cancel each other. 
In the case where the charged particle beam illuminates an area near the 
position 12, a deflection coma aberration as at the line 10 is produced by 
the deflection units 4, 5, whereby the coma aberration is canceled. By 
adjusting a drive force (drive voltage, drive current) of the deflection 
units 4, 5 by a coma aberration cancel signal in synchronization with the 
scanning of the charged particle beam and controlling the deflection coma 
aberration it is possible to dynamically remove the coma aberration of the 
magnetic lenses 2, 3 varying in accordance with the scanning position. 
Explanation will be given about preventing the minimization of the beam 
spot by the curvature aberration. FIG. 4 shows the pattern of the focusing 
plane near the surface of a sample 7. The line 13 shows a sample surface 
on which a mask pattern is to be projected and the curve 14 shows a focus 
plane curved due to the curvature aberration by the magnetic lenses 2, 3. 
A deflection electromagnetic field 15 created by a shift signal under the 
deflection units 4, 5 have its focus plane 14 shifted to a position of a 
curve 16. 
When the focal distance of the magnetic lenses 2, 3 is varied by a focusing 
signal whose focal distance is corrected, the focal plane is moved to a 
position of a curve 18 where a bottom portion of the curved beam focus 
plane 18 is set in contact with the sample surface 13 at a position 17. 
The bottom portion of the curved beam focus plane is relatively flattened 
and it is, therefore, possible to effect a lithograph process, with a 
great spot, at and near the position 17. 
In order to eliminate the coma aberration or shift the focus plane, the 
deflection units 4, 5 are so controlled as to have a sum of their 
deflection amounts set to be substantially zero at the focus plane (or the 
sample surface). By doing so, it is possible to eliminate only the 
aberration without varying the basic characteristics of the focusing by 
the magnetic lenses 2, 3. For this purpose, requisite information is 
supplied from the system controller 31 to the coma aberration cancel 
signal generator 37. With this information additively combined, the coma 
aberration cancel signal generator 37 generates a coma aberration cancel 
signal. 
Although, irrespective of the movement of the charged particle beam, the 
beam is focused on the same plane, when control is so made as to allow a 
deflection amount to be set to be zero at a crossover plane, it is 
possible to control the convergence semi-angle of the charged particle 
beam with an aperture of a circular opening set relative to the crossover 
plane. For this reason, the requisite information is supplied from the 
system controller 31 to the coma aberration cancel signal generator 37 and 
the coma aberration cancel signal is produced, with the information 
additively combined, from the coma aberration cancel 
According to the present embodiment as set out in detail above, it is 
possible to, by dynamically producing the deflection coma aberration in 
accordance with the position of the charged particle beam illuminated on 
the mask, dynamically cancel the coma aberration by the magnetic lens 
varying in accordance with that position. By dynamically shifting the 
focus plane, by deflection, in accordance with the position of the charged 
particle beam illuminated on the mask and varying the focal distance of 
the lens, it is possible to perform the lithograph process with a great 
beam spot and improve the production efficiency. 
The present invention is not restricted to the above-mentioned embodiment. 
In the present embodiment, the arrangement of the optical system is not 
restricted thereto and may be properly modified in accordance with its 
specification. Further, the lens may be not only of an magnetic type but 
also of an electrostatic type. The deflection unit may be comprised of not 
only an electrostatic deflection unit but also magnetic deflection unit. 
The present invention is not restricted to the lithograph apparatus only 
and can also be applied to an apparatus for projection-focusing a mask 
image on the sample surface with an electron beam or ion beam. 
Additional advantages and modifications will readily occur to those skilled 
in the art. Therefore, the invention in its broader aspects is not limited 
to the specific details and representative embodiments shown and described 
herein. Accordingly, various modifications may be made without departing 
from the spirit or scope of the general inventive concept as defined by 
the appended claims and their equivalents.