JPH0136230B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention uses a three-magnetic pole lens with almost zero S-shaped distortion aberration as a projection lens, and allows variable magnification with extremely small or almost no distortion aberration. This invention relates to a new imaging lens system for an electron microscope.

Normally, variable magnification in a three-stage imaging lens _system consisting of an objective lens, a projection lens, and an intermediate _lens placed between them is achieved in a region (M ₁ range), a region where the excitation of the intermediate lens NI _i is decreased with the excitation of the projection lens fixed at the maximum value (M ₂ range), and an area where the excitation of the intermediate lens is reduced with the excitation of the projection lens NI _p fixed at the maximum value (M 2 range). Area to be increased from excitation in ₁ range (M ₃ range)
It consists of three areas. Among these, in the _M1 range, a system that satisfies the distortion-free condition has already been established (see, for example, Japanese Patent Publication No. 7569/1983), and a sufficiently small distortion aberration can be obtained. However, in the _M2 range, because the excitation of the intermediate lens is reduced, the pincushion distortion created by the projection lens cannot be sufficiently canceled by the barrel distortion of the intermediate lens, and in the _M3 lens, the projection Both the lens and the intermediate lens suffer from pincushion distortion, and distortion-free conditions do not exist. Furthermore, in conventional lens systems, no correction is made for S-shaped distortion aberration, and therefore, straight images are unsightly distorted into an S-shape.

By the way, the present inventor has recently developed a three-magnetic pole lens that can eliminate the S-shaped distortion aberration, and has also discovered that there is a magnetic pole shape that can simultaneously eliminate the distortion aberration and the S-shaped distortion aberration. However, such a condition exists only under a specific magnetomotive force, and even if the excitation of the projection lens is made larger or smaller than the specific magnetomotive force, a significantly large distortion aberration will occur. It was only available at fixed magnification.

Therefore, with ordinary electron microscopes, it is essential to observe images at various magnifications.
Therefore, a method has emerged in which the magnification can be varied while keeping both distortion and S-shaped distortion at zero or significantly small using a three-pole lens.

The present invention provides an imaging lens system for an electron microscope that can satisfy these requirements.

FIG. 1 shows an example of the magnetic pole shape of a three-pole lens that can reduce distortion aberration and S-shaped distortion aberration to zero or significantly. 1, 2, and 3 are the first magnetic pole,
A second magnetic pole and a third magnetic pole are shown. The hole diameters b ₂ and b ₃ of the second magnetic pole and the third magnetic pole are made equal, and the hole diameter of the first magnetic pole is
The ratio b ₁ /b ₂ of b 1 to the hole diameter b ₂ of the second magnetic pole is set in _the range of 1.5 to 5. Figure 2 shows the distortion aberration ΔX/X and S-shaped distortion aberration ΔY/X when b ₁ /b ₂ = 3.3.
and is a graph showing the dependence of focal length f _p on magnetomotive force. As can be seen from the figure, distortion aberration and S-shaped distortion aberration both become zero when the magnetomotive force is NI ₁ . The above NI ₁ had a power of just under 2500 AT in experiments. However, if the excitation is weakened to reduce the magnification, the focal length f _p at NI ₂ (approximately 1500 AT) is only 1.6 times that at NI ₁ , but the distortion increases by as much as 10%. Therefore, such a three-pole lens cannot change its excitation to change the magnification.

Therefore, the present inventor proposed a method of using such a three-magnetic pole lens as a projection lens, and providing an intermediate lens with two magnetic poles between the projection lens and the objective lens, and changing the magnification by variable excitation of this intermediate lens. I tried. FIG. 3 is a diagram schematically showing the apparatus, and 4 is an objective lens. The objective lens is excited by a power source 5 and generates a strong lens magnetic field within the magnetic pole gap 6. Reference numeral 7 denotes a sample, which is actually inserted into the magnetic pole gap 6. 8 is a 3 magnetic pole projection lens,
Excited by power source 9, magnetic pole gaps 10a, 10b
generate magnetic fields of opposite polarity. Objective lens 4
A two-pole intermediate lens 11 is placed between the projection lens 8 and the projection lens 8, and is excited by a power source 12. 13 is a fluorescent screen on which the final image is projected.

In such a device, the sum of the magnetic pole gap length Si and the magnetic pole hole diameter b _i is S _i +b _i =45 mm as the intermediate lens, and the distance between the lens center of the intermediate lens 11 and the lens center of the projection lens 8 is 130 mm. Axial magnetic field distribution
B _z , the trajectory r〓 of the electron beam incident parallel to the optical axis Z, the trajectory r〓 of the electron beam emitted parallel to the optical axis Z, the distortion aberration coefficient D _r and the S-shaped distortion aberration coefficient D _sp as the fourth It is shown in the figure. In the same figure a, when the excitation NI _i of the intermediate lens is zero,
b is the case when NI _i =600AT, and the excitation NI _p of the projection lens is about 2250AT in both cases. a
As shown in the figure, when the excitation of the intermediate lens is zero, distortion is created only by the projection lens, and the aberration coefficient D _r takes a negative value at the first gap and increases, but the electron beam trajectory r〓
reaches a maximum at the point where it intersects the optical axis Z, gradually decreases, and then increases to a positive value after crossing zero. Then, it reaches a maximum at the point where the r = orbit intersects the optical axis, and then decreases again. The distortion aberration ΔX/X is connected to the distortion aberration coefficient D _r by ΔX/X=D _r (Xf/L) ² . Here, X is the distance from the center of the fluorescent screen (or photographic film), and is usually measured at X=50 mm. f is the focal length of the system including the intermediate lens and the projection lens, and L is the distance between the projection lens and the fluorescent screen. In the case of figure a, D _r was finally 0.0178 mm ^-2 and the aberration ΔX/X was a little over 0.8%.

Then, when the intermediate lens is excited to 600AT,
Due to the presence of this intermediate lens, as is clear from Figure b, D _r already takes a negative value when it enters the projection lens. However, compared to the case where NI _i is zero, both the negative maximum and the positive maximum D _r decreases, and finally reaches 0.003 mm ^-2 , which is 1/6 of that in the case of diagram a. In contrast to this decrease in the aberration coefficient, since the focal length conversely becomes longer, as is clear from the above aberration formula, the degree of decrease in the distortion aberration ΔX/X was small, at just over 0.3%.

From the above, as long as the aberration coefficient D _r of the intermediate lens is not too large, that is, unless the magnetic pole gap length and magnetic pole hole diameter of the intermediate lens are extremely small, the aberration coefficient of the three-pole projection lens can be improved by adding the intermediate lens. D _r decreases significantly, and the distortion aberration ΔX/X decreases despite the increase in focal length. This means that on the projection lens side, the r〓 orbit is the same regardless of the presence of the intermediate lens, so the reason for the decrease in D _r is that the r〓 orbit is tilted somewhat by the addition of the intermediate lens, and the projection lens This is due to the fact that the electrons were closer to the optical axis when they entered the optical axis.

On the other hand, regarding S-shaped distortion aberration, this is also r〓
It is reduced by the reduction on the projection lens side of the trajectory.
That is, the S-shaped distortion aberration ΔY/X is 0.1 in the case of Fig. 4a.
%, but in the case of Figure 4b, it decreased to 0.07%. Therefore, in this lens system, it is sufficient to set conditions to reduce or eliminate distortion.

FIG. 5 shows the change in the distortion aberration ΔX/X with respect to the excitation NI _i of the intermediate lens, using the excitation NI _p of the projection lens as a parameter. In the figure, a,
b, c, d, e, f, and g show changes when the excitation of the projection lens is different, and a is NI _p =
In the lowest case of 1500AT, the magnetomotive force increased as b, c, d..., and at f, NI _p = 2390AT.
As can be seen from this figure, the distortion aberration ΔX/X decreases in any case where the projection lens magnetomotive force increases as the excitation of the intermediate lens increases. Also, a particularly useful feature is that it is better to set the excitation conditions of the projection lens so that NI _i = 0, that is, when the intermediate lens is turned off, the distortion aberration ΔX/X is close to zero, because the increase in the intermediate lens magnetomotive force It can be seen that the change in distortion aberration is small.
Note that under the excitation condition (g) in which ΔX/X is negative (barrel distortion) when NI _i =0, the negative value becomes slightly larger as NI _i increases.

From the above, at NI _i = 0, ΔX/X=
If the intermediate lens current NI _i is increased while the value of the projection lens current is fixed near the excitation condition where it becomes 0,
Adjust the intermediate lens magnetomotive force NI _i to NI _i = 0 to 0 while keeping the final distortion within 0.5% or even 0.2%.
It turned out that it can be varied within the range of 700AT. Therefore, the focal length of the system can be varied significantly while keeping distortion small.

FIG. 6 shows the change in the focal length f of the system with respect to the change in the magnetomotive force NI _i of the intermediate lens, in the case where the magnetomotive force NI _p of the projection lens is 2390AT. Under these conditions, ΔX/X should be very small (0.2% or less)
The focal length f can be doubled from approximately 5mm to 10mm while maintaining the same focal length. Furthermore, if ΔX/X is set to about 0.5%, f can be increased by about 3 times. Conversely, the magnification can be reduced by about 1/2 to 1/3. In a normal electron microscope, the magnification in the _M2 range is
Since it is approximately 5,000 times to 10,000 times, the variable range of the focal length is within a sufficiently practical range.

As mentioned above, in the lens system of the present invention using a three-magnetic-pole projection lens, the S-shaped distortion aberration does not increase due to the addition of the intermediate lens, and when NI _p = 2390AT,
At NI _i = 0, S-shaped distortion ΔY/X is -0.023%
However, it decreases to +0.006% when NI _i =600AT, and changes to +0.026% when NI _i =700AT.
In other words, the S-distortion aberration is well below 0.1% in the entire range, which is an extremely excellent value compared to the S-distortion aberration of a normal lens system, which varies between 1.3 and 1.5%. I understand that there is something.

As described above, in the present invention, the S-shaped distortion aberration is small.
By using a magnetic pole lens as a projection lens and providing an intermediate lens in front of the projection lens, S
The focal length can be varied by 2 to 3 times while maintaining the distortion aberration to a small value of about 1/2 to 1/3 of a normal two-pole lens without increasing the character distortion, making it possible to construct a good _M2 range. .

In the present invention, it is also possible to change the focal length while keeping the distortion at zero. This can be done by increasing the excitation of the intermediate lens and simultaneously decreasing the excitation of the projection lens continuously (or intermittently). FIG. 7 is a graph showing the method of control, using NI _p as a parameter and showing changes in focal length with respect to NI _i . In the figure b′, c′,
The curves d' and f' are the curves b, c, d in Fig.
and the projection lens magnetomotive force is the same as f.
Also, f' is the same as the curve in Figure 6, and NI _p is
This is the case for 2390AT. In the figure, the dotted line indicates the control method in this example, and _{NI p}
is changing. That is, as NI _i increases,
NI _p must be gradually reduced. For this purpose,
In the apparatus shown in FIG. 3, it is necessary to configure the power source 9 for the projection lens and the power source 12 for the intermediate lens so that they can be controlled in conjunction with each other as shown by dotted lines. Furthermore, the above control is
In response to changes in NI _i , NI _p is continuously changed, but the intermediate lens magnetomotive force NI _i is divided into several parts, and the projection lens magnetomotive force NI is adjusted to maintain a constant projection lens magnetomotive force in each region. _p may be configured to change intermittently.

By performing the above control, it is possible to greatly vary the focal length from 5 mm to 20 mm or more while keeping the distortion to zero or significantly reducing it.
The variable magnification range can be further expanded.

In addition, in the above explanation, 3 magnetic pole lens b ₁ /b ₂ =
Although an asymmetric lens with a diameter of 1.5 to 5 was used, in the present invention, for example, a lens in which the first gap length S ₁ is approximately 3 times larger than the second magnetic pole gap length S ₂ is used to reduce S-shaped distortion aberration and distortion aberration. It goes without saying that any lens that can be made small or zero can be used.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the magnetic pole shape of a three-pole lens proposed prior to the present invention, Fig. 2 is a diagram showing the characteristics of the lens, and Figs. 3 to 6 illustrate an embodiment of the present invention. FIG. 7 is a diagram for explaining another embodiment of the present invention. 4: Objective lens, 8: 3 magnetic pole projection lens, 1
1: intermediate lens, 5, 9 and 12: lens power supply.

Claims (1)

[Claims] 1. An objective lens, a projection lens that has three magnetic poles and generates lens magnetic fields of different polarities within the gap between the two magnetic poles, and a lens that is placed in front of the projection lens. and one intermediate lens, and the magnetomotive force of the intermediate lens is set with the magnetomotive force of the projection lens substantially fixed at a value such that distortion aberration is reduced to a desired value when the magnetomotive force of the intermediate lens is zero. An imaging lens system for an electron microscope that features a variable configuration. 2 In relation to the variation of the magnetomotive force of the intermediate lens,
2. The imaging lens system for an electron microscope according to claim 1, wherein the magnetomotive force of the projection lens is varied so that the value of the magnetomotive force of the intermediate lens decreases as the magnetomotive force of the intermediate lens increases.