JPS6010419B2 - electronic lens - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides an optimal S
This invention relates to an electronic lens with little font distortion aberration.

A so-called three-pole lens, in which two magnetic pole gaps are formed by three magnetic poles, and the polarities of the magnetic fields generated in both gaps are opposite to each other, has been proposed as a lens that can eliminate rotational chromatic aberration, and then, under certain excitation conditions, lateral chromatic aberration. She attracted attention as a lesbian who could reduce the amount of money to zero.

Furthermore, it has been confirmed that the distortion bride aberration can also be reduced to zero. The inventor of the present invention has previously conducted research on three-pole lenses, and has developed a lens that can significantly reduce S-shaped distortion aberration compared to ordinary two-pole lenses. I couldn't make it zero. Therefore, it is an object of the present invention to propose an electron lens with zero S-shaped distortion aberration, which is important for miniaturizing electron microscopes.

FIG. 1 shows the structure of a three-magnetic pole lens, where 1 is a yoke, which is magnetically connected to a first magnetic pole 2, a second magnetic pole 3, and a third magnetic pole 4.

The respective magnetic poles face each other with a constant distance S between them, forming two magnetic pole gap axes and a bag. The thickness of the intermediate magnetic pole 3 is t, and a vertically symmetrical magnetic pole structure is created with respect to the center of this intermediate magnetic pole. 5 and 6 are excitation coils, which supply current V to the magnetic pole gap g and the bag so as to obtain a magnetic field of opposite polarity (the direction of magnetic flux is opposite).

In Figure 2, such a three-pole lens is used as a projection lens,
Focal length fp (side) when the strength of the magnetic fields generated in the first gap seal and the second gap g2 are made equal, distorted image aberration △r/r (%) at a position of 100 feet in diameter during the final image, and S-shape This is a calculated value of distortion aberration Δs/r (%). In the same figure, the horizontal box clam Bu*ichisu (skin/T/KV-su) shows the excitation parameters of the shins, where S is the aforementioned magnetic pole gap, B is the peak value of the magnetic field strength, and u* is the relative correction value. The accelerating voltage is also S=5
This is a case of skin, 1 = 3 winds, and magnetic pole hole d = 3 candles.
Curve a is the focal length and shows a minimum value at a point where the excitation intensity is slightly less than 0.2. The curve b shows distortion and image aberration, which is pincushion distortion on the low excitation side and barrel-shaped on the high excitation side. - It can be seen that the conditions for this distortion and image aberration to be zero match the conditions for the focal length to be minimum. The curve c shows S-shaped distortion aberration, which increases rapidly as the excitation intensity increases.
Don't cross zero. By the way, the S-shaped distortion aberration ○sp becomes DSp=
Reed (point r) elbow iB3Y2dt Zo Chapter 10 (represented by red r adjudicial words BY2dZ).

Here, B is the strength of the magnetic field, Y is the coordinate representing the trajectory of the adoptive line, Y' is its gradient, Zo is the position of the price, and Zi is the position of the bride face. Since this equation is expressed by the odd-order term of B, in a lens in which there are two magnetic field regions with opposite polarities, that is, in a triode lens as shown in Fig. 1, in the first gap & The S-shaped distortion aberration caused by the magnetic field and the second
This means that the S-shaped distortion aberration caused by the magnetic field in the gap saddle cancels each other out. However, in reality, the S-shaped distortion aberration does not become zero, but increases rapidly as shown in FIG.

This point will be qualitatively explained using FIG. 3. This figure shows the magnetic field distribution in the gap between two magnetic poles and the trajectory of an electron beam incident on this magnetic field at a height of 1 (Y = 1) and a gradient of 0 (Y = 0) along the Z-axis. be. The magnetic field strength B is the first
Since the gap and the second gap are equal, the occurrence of aberration is due to the fact that Y and Y' are different between the first gap and the second gap. First, regarding the first term of the above equation, it can be seen from the figure that the contribution of the magnetic field in the first gap is overwhelmingly higher than that in the second gap. Regarding the second term, although it is difficult to understand from the figure, Y′
is maximum when the electron beam crosses the optical axis, so the contribution of the magnetic field in the second gap becomes larger. In one example of calculation by the inventor for the trajectory shown in Figure 3, Dsp is 696 for the first term.
2, and the second term was -1621. The directions of rotation are different between the two, and the first term (positive rotation) is much larger, and as a result, a large Dsp exists. From the above considerations, if the contribution of the magnetic field of the second gap is made significantly larger than the contribution of the magnetic field of the first gap, the magnitudes of the first and second terms in the above equation can be made approximately equal ( It was found that the S-shaped distortion aberration coefficient Dsp could be made koto or almost zero. Based on this principle, the inventor conducted various experiments and found that the S-shaped distortion can be reduced to zero by making the magnetic field strength (peak value) in the second gap higher than the magnetic field strength in the first gap. I confirmed that I would get it. Figure 4 is the first
The second value for the peak value B of the magnetic field strength in the magnetic pole gap 9
The ratio of the peak value & of the magnetic field strength at the magnetic pole gap g2 (&/
B, ) is used as a parameter to show the change in focal length.

The machine axis in the figure indicates the excitation intensity of the magnetic field within the first magnetic pole gap. From this figure, as &/B increases, the minimum value of the focal length nap decreases and shifts to the low excitation side. Of course, along with this shift in (nap)min, the condition for zero distortion image aberration Δr/r also shifts to the low excitation side. Figure 5 shows
This figure shows changes in the S-shaped distortion aberration ΔS/r corresponding to FIG. 4.

As is clear from the figure, when B2/B, becomes larger than 1,
Each curve crosses zero and reverses sign on the low excitation side. That is, it can be seen that there exists a condition in which the S-shaped distortion aberration becomes zero if B2/B>1. However,
Since other aberrations must also be taken into account when designing the lens, the usable area is naturally limited. As mentioned above, the distortion image aberration becomes zero under the excitation condition where the nap is minimized, and this condition is also advantageous for the chromatic aberration of magnification. Therefore, under this excitation condition of (fp) min, the S-shaped distortion becomes zero or approximately zero, which is extremely preferable. Figure 6 is based on Figures 4 and 5.
This figure shows the excitation conditions under which each of n and ΔS/r=0 appears, and the vertical koji indicates the excitation intensity of the magnetic field in the first gap, and the axis indicates B2/B.

In this figure, the curve of (nap)min and △S/r=0
The most preferable condition is the point where the curves intersect. And this intersection is B/B, = 1.9〆 Excitation intensity S of the first gap
Bu*-su=1.55 skin/TKv-su. The above condition B2/B=1.58 is the most preferable case, and it was confirmed that values around this can be used, and that 1.4 to 1.8 is a practical range. Beyond this range, Figure 4,
As can be seen from the curve of B2/B=2.0 in FIG. 5, with the excitation intensity of ΔS/r=0, the focal length nap becomes very long (approximately 11 side) and (mep)min Since the position deviates greatly from the position of , distortion and image aberration etc. become large, making it impractical. The same holds true when &/B, is 1.4 or less; as can be seen from the comparison of the curves for &/B=1.25 in Figures 4 and 5, when △S/r=0, fp is More than 11 ribs. As described in detail above, the present invention provides a three-pole lens with a second
By setting the ratio B2/B of the peak value B2 of magnetic field strength in the gap to be larger than 1, especially within 1.4 to 1.8 (near 1.58), distortion aberrations and lateral chromatic aberrations can be prevented from increasing. In addition, S-shaped distortion can be reduced to zero without sacrificing the focal length, so when used as a projection lens for an electron microscope, the distance between the lens and the camera can be significantly shortened. It is particularly effective for miniaturizing high-acceleration electron microscopes.

[Brief explanation of drawings]

Figure 1 is a diagram showing the structure of a three-pole lens, Figure 2 is a diagram showing the characteristics of a conventional lens, Figure 3 is a diagram to explain the principle of the present invention, and Figures 4 to 6 are diagrams showing the principle of the present invention. This is a graph to explain. 1: Yoke, 2, 3 and 4: Magnetic poles, 5 and 6: Excitation cord

Claims (1)

[Claims] 1 A three-pole lens in which two gaps are formed by three magnetic poles and the polarity of the magnetic field generated in both gaps is reversed, and the peak strength of the magnetic field in the first magnetic pole gap (electron incidence side) is B_1
, the magnetic field peak strength of the second magnetic pole gap (electron emission side) is B
An electron lens characterized in that the magnetic field strength of the gap between the two magnetic poles is set so that when B_2/B_1 is in the range of 1.4 to 1.8.