Electromagnetic lens

There is disclosed a conical electromagnetic lens whose spherical aberration and chromatic aberration coefficients can be made small if the apex angle of the conical lens is small. The lens comprises a conical bobbin through which a beam of charged particles passes and a coil wound on the outer periphery of the bobbin. The number of turns of the coil per unit length along the axis of the lens is increased from the inlet end toward the exit end of the lens.

BACKGROUND OF THE INVENTION 
The present invention relates to an electromagnetic lens for focusing a 
charged-particle beam such as an electron beam and, more particularly, to 
improvements in a conical electromagnetic lens. 
U.S. Pat. No. 3,707,628 discloses a conical electromagnetic lens of the 
construction shown in FIG. 1. This lens comprises a conical bobbin 10, a 
coil 11 wound around the bobbin, and a shroud 12 surrounding the outside 
of the coil. A beam of charged particles passes through the bobbin 10 
along its axis Z. Where this lens is used as the objective lens of a 
scanning electron microscope, a specimen 13 is placed below the lens. The 
bobbin 10 is made from a nonmagnetic substance, while the shroud 12 is 
made from a ferromagnetic substance. 
In the conical lens of this structure, a space for installing a detector 
acting to detect secondary electrons, reflected electrons, or X-rays 
emitted from the specimen is created close to the lens and, therefore, it 
is easy to detect the above-described secondary electrons, and so on. Even 
if the specimen 13 is tilted at the maximum angle, the specimen can be 
brought close to the principal plane of the lens, thus enabling 
high-resolution observation. Consequently, this lens is adapted for use in 
a scanning electron microscope where a flat specimen having a large area, 
such as a silicon wafer, is tilted for observation. 
The coil 11 described above is composed of a conductor wire that is wound 
on the bobbin with a uniform radial thickness d. That is, the number of 
turns per unit length along the axis Z of the coil is constant. The lens 
having this coil 11 shows an axial magnetic field distribution as shown in 
FIG. 2(a), where the broken line shows the orbit of an electron beam 
incident on the lens parallel to the axis Z. 
In this conical lens, if the half conical angle .alpha. is made small to 
permit the tilt angle .theta. of the specimen to be made larger, e.g., 
larger than 60.degree., then it is impossible to reduce the distance 
Z.sub.o between the position of the principal plane of the lens and the 
specimen below a certain value. This, in turn, makes it impossible to 
decrease the spherical aberration coefficient C.sub.s and the chromatic 
aberration coefficient C.sub.c of the lens below certain values. 
These problems are described now in detail. In order to investigate the 
spherical aberration coefficient C.sub.s of the electromagnetic lens, the 
present inventors calculated the relation of the spherical aberration 
coefficient C.sub.s of the lens to the half-value width D of the axial 
magnetic field distribution, using the distance Z.sub.o between the 
principal plane of the lens and the focal point, or the position of the 
specimen, as a parameter. FIG. 3 is a graph diagrammatically showing the 
results of the calculation. It can be seen from FIG. 3 that (1) the 
distance Z.sub.o should be made as small as possible to reduce the 
spherical aberration coefficient C.sub.s and that (2), where the distance 
Z.sub.o assumes a certain value, the half-value width D should be so 
selected as to minimize the value of the coefficient C.sub.s. 
With respect to the requirement (1), the chromatic aberration coefficient 
C.sub.c corresponds to the distance Z.sub.o and so reducing the distance 
Z.sub.o is also effective in reducing the chromatic aberration coefficient 
C.sub.c With respect to requirement (2), if the distance Z.sub.o is made 
small, then it is necessary to reduce the half-value width D, as can be 
seen from FIG. 3. 
We now discuss the conical lens shown in FIG. taking account of these 
facts. Since the position at which the axial magnetic field strength 
assumes its maximum value is located at a relatively high position, it is 
difficult to reduce the distance Z.sub.o. Therefore, a limit is set to 
reduction in the coefficient C.sub.c. In order to adjust the half-value 
width D, it is desired to make the distribution of the axial magnetic 
field adjustable, but this is achieved only by adjusting the length of the 
coil, or the dimension taken along the Z axis. Thus, there exists only a 
small degree of freedom. Furthermore, it is difficult to reduce the width 
D itself below a certain value. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a conical 
electromagnetic lens whose spherical aberration and chromatic aberration 
coefficients can be made small if the half conical angle .alpha. is 
rendered small. 
The above object is achieved by a conical electromagnetic lens comprising a 
conical bobbin through which a beam of charged particles passes and a coil 
wound on the outside of the bobbin, the coil being so designed that the 
number of turns per unit length along the axis of the lens is increased 
from the inlet end of the lens toward the exit end. 
Other objects and features of the invention will appear in the course of 
the description thereof which follows.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 4, there is shown a conical lens according to the 
invention. This lens comprises a conical bobbin 10, a coil 14 wound around 
the bobbin, and a shroud 12 surrounding the outside of the coil. The 
bobbin 10 has a port P through which a beam of charged particles enters 
the bobbin. The incident beam passes through the bobbin along the axis Z 
of the conical bobbin. A specimen 13 is placed in the focal point lying 
below the lens when this lens is used as the objective lens of a scanning 
electron microscope. The bobbin 10 is made from a nonmagnetic substance, 
while the shroud 12 is made from a ferromagnetic substance. 
When the coil 14 is wound around the bobbin 10, the coil can be wound 
around the bobbin from a high position to a low position or vice versa to 
obtain the cross section shown in FIG. 4. If the coil is wound in plural 
layers as shown in FIG. 5, then it is easy to fabricate the winding. In 
particular, a conductor wire L.sub.1 is wound from a low position to a 
high position to form the first layer. Then, less turns of a conductor 
wire L.sub.2 are wound from the low position to a less high position to 
form a second layer L.sub.2 The upper end of the second layer L.sub.2 is 
shifted downward from the upper end of the first layer L.sub.1 Similarly, 
conductor wires are coiled to form the third layer L.sub.3,the fourth 
layer L.sub.4, and so on such that the number or turns of each succeeding 
layer is reduced and that the upper end of each succeeding layer is 
shifted downward. The upper end of each layer is electrically connected 
with the lower end of the following layer through the gap between the coil 
and the shroud, so that the wires forming the layers are connected in 
series to thereby form one coil. When the coil produces a large amount of 
heat, the gap can be used as a passage through which a coolant for cooling 
the coil is caused to flow. 
The number of turns per unit length of the coil 14 wound in this way along 
the axis Z of the coil as shown in FIG. 4 is increased in a stepwise 
fashion from the inlet end toward the exit end of the lens. 
Expressed differently, the coil 14 is wound on the bobbin in such a way 
that the angle (.alpha..sub.1) between the inner surface of the coil and 
the axis Z of the coil is larger than the angle (.alpha..sub.2) between 
the outer surface of the coil and the axis Z. 
FIG. 2(b) shows the axial magnetic field distribution of the 
electromagnetic lens equipped with the coil 14 wound in this manner. 
Because the number of turns per unit length along the axis Z of the coil 
increases from the inlet end of the lens toward the exit end, the position 
at which the strength of the magnetic field is greatest is closer to the 
specimen that in 
FIG. 2(a). This enables the distance Z.sub.o to be reduced. Hence, the 
spherical aberration coefficient C.sub.s and the chromatic aberration 
coefficient C.sub.c can be decreased. 
The angle .alpha..sub.2 that the outer surface of the coil 14 forms with 
the axis Z can be adjusted by varying the manner in which the coil is 
wound. As a result, the distribution of the axial magnetic field, i.e., 
the position at which the half-value width and the axial magnetic field 
strength assume their maximum values, can be changed. Thus, the number of 
parameters capable of adjusting the distance Z.sub.o between the principal 
plane of the lens and the position of the specimen is increased. In 
addition, the half-value width is reduced compared with the case of FIG. 
2(a), because the number of turns per unit length along the axis Z of the 
coil 14 is increased from the inlet end of the lens toward the exit end. 
Furthermore, the position at which the strength of the axial magnetic 
field is greatest is closer to the exit end of the lens than in the case 
of FIG. 2(b). Therefore, the distance Z.sub.o can be reduce further. 
Consequently, an electromagnetic lens having a further reduced spherical 
aberration coefficient C.sub.s can be realized. 
Referring to FIG. 6, there is shown another conical lens according to the 
invention. This lens is similar to the lens already described except that 
the bobbin 10 has a cylindrically protruding front end portion 101 
surrounding the axis Z. The coil 14 is wound up to this front end portion. 
The coil 14 is wound in the same way as in the previous example. 
In this example, a coil 141 is wound on the front end portion 101. That is, 
the coil 141 is added to the lens shown in FIG. 4. Because of the 
contribution of the added coil 141, the axial magnetic field is 
distributed even closer to the exit end of the lens, which makes the 
distance Z.sub.o further smaller than in the case of FIG. 2(b). 
Consequently, the spherical aberration coefficient C.sub.s and the 
chromatic aberration coefficient C.sub.c can be reduced for the 
above-described reasons. 
It is to be noted that the invention is not limited to the foregoing 
examples and that various modifications and changes are possible. For 
example, the cross sections of the conductor wires forming the coil may be 
circular or square in shape, or may take any other form. Also, the coil 
can be fabricated by winding a conductor in the form of tape. 
Having thus described our invention with the detail and particularity 
required by the Patent Laws, what is claimed and desired to be protected 
by Letters Patent is set forth in the following claims.