JPS5940448A - Electrostatic acceleration lens - Google Patents

Abstract

PURPOSE:To reduce chromatic aberration by constituting lens electrodes in the order of the first - fourth electrode having the center opening, applying a charged-particle acceleration voltage across the first and fourth electrodes, the same potential to the second and fourth electrodes, and a negative or positive potential across the third and fourth electrodes. CONSTITUTION:Electrodes 6-9 are arranged in rotational symmetry with respect to a light axis to constitute lens electrodes. A beam having the energy of a volage V1 in relation to the electrode 6 is generated from a charged beam generation source. A voltage V2 is applied across the electrodes 6, 9, a voltage V3 across the electrodes 8, 9, and the same potential to the electrodes 8, 9. The polarity of the voltage V3 is selected so that a beam is accelerated between the electrodes 7, 8 and decelerated between the electrodes 8, 9. When the distance S between a light source and the electrode 9 is limited to Sm, a distance Lm between the electrodes 6, 7 or a diameter d1m of the electrode 6 is calculated, while the focal distance is kept constant and S=Sm with the distance L between the electrodes 6, 7 or the diameter d1 of the electrode serving as a parameter, than L and d1 are set to 50-100% of Lm and d1m respectively. Accordingly, chromatic aberration can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention focuses on an electrostatic acceleration lens that accelerates and focuses a charged beam, particularly an ion beam, in an optical system of a beam focusing device that focuses a charged beam, particularly an ion beam, to form a fine spot and performs fine processing of LSI, etc. It is related to.

An electrostatic accelerating lens is used to accelerate a charged beam generated from a field emission electron gun, field ionization, or field emission ion source to the required energy. Also,
In addition to accelerating the beam, it often functions as a condenser lens. Conventionally, a spy potential lens shown in FIG. 1 has been often used as an electrostatic lens for this purpose. This is a two-electrode configuration. Reference numeral 1 denotes a charged beam generation source which applies a voltage 1 between it and an electrode 2 to draw out the beam. ■1 corresponds to the initial velocity energy of the beam. Apply voltage ■2 between electrode 2 and electrode 6,
Accelerating the beam. The accelerating voltage becomes ■1+■2.

Also, by multiplying by 2, a lens effect is created between electrode 2 and B. The focal point of the lens can be varied by changing the ratio 2/■1, and it can be used as a condenser lens (or this lens alone can focus the beam on a spot). By the way, since v1 is usually determined by the beam emission characteristics, it is often fixed at a certain voltage. Changing vl generally causes a large change in beam current. Therefore, to change the focal length of the condenser lens, you need to change (2), but changing (2) changes the accelerating voltage at the same time, which is inconvenient. Therefore, the configuration shown in FIG. 1 only accelerates the lens, and does not have much advantage as a condenser lens. In order to eliminate this drawback, the second
A lens with a six-electrode configuration as shown in the figure has been proposed.

This lens has an electrode 4 inserted between the electrodes 2 and 2, and a voltage v3 is applied to the electrode 4. Here, ■1, ■2
By fixing V3 and changing V3, it is possible to make the focal length of the condenser lens variable while keeping the accelerating voltage constant.

By the way, in a field emission type liquid metal ion source, compared to an electron gun, the beam energy width ΔV is usually 10 times larger.
It is said to be about double that. Therefore, when focusing the ion beam, the influence of chromatic aberration becomes a problem, and the conventional lenses shown in I8 Figures 1 and 2, in which reduction of chromatic aberration is desired, cannot be used when using an electron gun. In this case, chromatic aberration is small and spherical aberration is dominant, but when an ion source is used, chromatic aberration becomes larger than spherical aberration. Conventional lenses are insufficient to obtain a fine ion beam spot, and a lens with small chromatic aberration is required.

If the magnification of the focusing optical system is small and it is a reduction system, the aberrations of the accelerating and condenser lenses will be reduced and their influence will be small. However, in field emission type ion sources and electron guns, the light source diameter is very small, so the magnification is large, and magnification from the same degree is often used. Therefore, since the aberrations of the acceleration and condenser lenses also become large, it is particularly important to reduce the above-mentioned chromatic aberration in a field emission type ion source.

In order to compare the aberration characteristics, consider the imaging conditions shown in FIG. 5. This is an imaging condition in which the beams emitted from the object point position zo are parallel.

That is, the image point Z1 is at a position of +■, and the magnification M is ω.

It is. fo is the focal length. In an actual optical system,
The configuration is such that the parallel beam is focused by an objective lens, but here we will consider the acceleration and condenser parts. Aberration is expressed as follows.

d − d + d S where ds and do are the spherical aberration and chromatic aberration respectively converted to the object side, and Cso and Cco are the spherical aberration coefficient and chromatic aberration coefficient defined on the object side, α. is the aperture half angle on the object side. d
2 is the sum of squares of spherical aberration and chromatic aberration. In addition, Δ■ is the energy width mentioned above, and ■o is the beam energy on the object side (
For field emission light sources, this corresponds to the beam extraction voltage)
It is.

An object of the present invention is to provide an electrostatic accelerating lens that can reduce the chromatic aberration coefficient C60, thereby reducing the overall aberration.5 Features of the present invention are as follows:
In order to achieve the above object, a lens electrode is constructed by arranging a first electrode, a second electrode, a sixth electrode, and a fourth electrode, each having an opening in the center, rotationally symmetrically with respect to the optical axis in this order, and・Assuming that the particles are incident through the first electrode, a charged particle accelerating voltage is applied between the first electrode and the fourth electrode, and the charged particle acceleration voltage is applied between the second electrode and the fourth electrode.
The electrodes are of equal potential, and either a negative or positive voltage is applied between the sixth electrode and the fourth electrode.

In the second aspect of the present invention, in addition to the above configuration, further:
When the distance S between the object point and the first electrode is restricted to a certain distance Sm or more, the distance between the first electrode and the second electrode or the aperture d1 of the first electrode is set as a parameter while keeping the focal length constant. The first electrode and the second electrode when S becomes equal to Sm
Determine the electrode spacing Lm or the first electrode diameter d1m, and calculate L
in the range of 50 to 100% of Lm, or dl to dil
The configuration is such that n is set in the range of 50 to 100 inches.

The present invention will be explained below with reference to the drawings.

FIG. 4 is a diagram showing the structure of the electrostatic accelerating lens of the present invention.
is a first electrode, 7 is a second electrode, 8 is a sixth electrode, and 9 is a fourth electrode, and the lens electrodes are arranged rotationally symmetrically with respect to the optical axis in the order of 1st to 4th. , and the charged particles are incident from the first electrode 6 side. vl is the voltage between the charged beam generation source (hereinafter referred to as the light source) and the first electrode B. Here, it is assumed that a beam with energy of Vl is emitted from the light source, and the drift space and the voltage at the first electrode 6''i! think.

A voltage V2 is applied between the first electrode 6 and the fourth electrode 9, and a voltage V5 is applied between the third electrode 8 and the fourth electrode 9.

The second electrode 7 and the fourth electrode 9 are at the same potential, and here they are at ground potential. The polarity of the voltage ■ is such that the beam is connected to the second electrode 7.
and the sixth electrode 80, and the sixth electrode 8 and the fourth electrode 9
The speed will be reduced between 0 and 0.

That is, when the beam is a positive ion beam, the voltage at the 7th port is negative. Further, the accelerating voltage V is V3==V1+■2, and the beam energy ■o on the object side is ■.

=v1.

FIG. 5 shows the axial potential distribution of the lens shown in FIG. 4 when the accelerating voltage Va is 3Q kV and 3 is -33 kV. Region (a) in FIG. 5 corresponds to between the first electrode 6 and second electrode 70, and region (b) corresponds to the part of ε region (a) between second electrode 7 and fourth electrode 90. focal length f. is 53 mm, but areas (a) and (b
), the focal length is 25171171. The shortened focal length corresponds to the lens action in region (b).

Next, the influence on the aberrations in regions (a) and (b) will be explained. When L is the distance between the first electrode 6 and the second electrode 70, the larger L is, the weaker the lens action is in the area (a), and the longer the focal length of this area is. If the lens action in the part (a) is reduced, aberrations tend to be reduced. Focal length f. If L is fixed, the larger L is, the smaller the chromatic aberration coefficient will be. However, in this case, if fo is fixed, it is necessary to strengthen the lens action in region (b), and IV31 is set too high.Also, even if the aperture of the first electrode 6 is increased, the chromatic aberration coefficient In order to explain this, Fig. 6 shows a noyola meter that defines the specific dimensions of the lens of the present invention.For simplicity, all electrode thicknesses are assumed to be the same and are assumed to be t. The distance between the second and sixth electrodes and the distance between the sixth and fourth electrodes are also the same, and are assumed to be l.The aperture of the first electrode, dl, and the aperture of the 2.6.4th electrode are the same, and are assumed to be d2. Z is the distance from the outer surface of the lens of the first electrode to the object point (light source). First, in Fig. 7, Vo = 5kV', Va- = 3[] kV, fo
=25rnm, t =2mm, 145mm, d1
= 15mm, Z5, Cco when d2 = 4mm
The results of calculating the change in L with respect to L are shown. It can be seen that as L becomes larger, the chromatic aberration coefficient Cco becomes smaller. However, the focal length f. Since is fixed, as L increases, Z5 decreases and the light source approaches the lens.

Due to the structure of the light source (such as an ion source), it is actually not possible to bring the light source very close to the lens. For this reason, the working distance S
There is a restriction that (distance between the light source and the lens surface) is greater than or equal to a certain value Sm. For example, if Sln is 10 mm, the seventh
In the illustrated example, L≦3QIT1nl may be satisfied. Lm3
When 0mm, s=1omm. However, the larger L is, the larger the chromatic aberration coefficient C6° is, so it is desirable that it be closer to 33 mm. In reality, this maximum OL is 50 to 100.
If it is within this range, better characteristics than the conventional lens described later can be obtained; therefore, L may be set within this range. Next, FIG. 8 shows the effect of the diameter d1 of the first electrode. this is,
ve=5 kV, Va=roQkV, f O=”15
mm, t=2man, 7==5mm, d2=
= 4 mm, L = 3 [] mm, Z5, C
This is a calculation of the change in oo with respect to dl. From this result, it can be seen that as dl increases, the chromatic aberration coefficient C60 decreases. shi power shi.

Similarly to L, when dl becomes thicker, Z5 becomes smaller and it is necessary to consider the working distance. In this case, assuming that the maximum working distance is 1[]mm, dl and 17mm should be 1-1. Similarly to setting L, 50 to 1 of the maximum d1
It is sufficient to set dl in a range of about 00%. however,
When taken equal to the maximum dl, coo becomes the minimum.

To determine the dimensions d2 and l of the 2.6.4 electrode, consider this as a single lens and use the usual Einzel lens design method. Furthermore, l, d2 beam, and dl do not affect the chromatic aberration characteristics.

In the examples shown in FIGS. 7 and 8, a negative voltage (in the case of a positive charge beam) is applied to the sixth electrode, but a positive voltage may also be applied. However, for example, when L-3Qmm is used in the example of Fig. 7, 1
, Coo=12mm, but when a positive voltage is applied,
co. -16 mm, indicating that the chromatic aberration coefficient becomes smaller when a negative voltage is applied. At either voltage, a lens with smaller chromatic aberration than conventional lenses can be obtained, so both can be said to be effective.

A comparison of chromatic aberration characteristics between the electrostatic accelerating lens of the present invention and a conventional lens is shown. However, Vo== 5 kV, ■8=3Q
kV, fo-25m January. In addition, the imaging condition is set to a magnification of ■. Figure 9 shows the dimensions of the lenses for comparison. (
A) is a lens with two electrodes, (1)) is a lens with six electrodes, and (C) is a lens with four electrodes of the present invention. (a) and (b) are selected from those with relatively good characteristics. Table 1 shows the calculated values of the chromatic aberration coefficients of these lenses.

Table 1 As can be seen from the results, the chromatic aberration coefficient of the lens of the present invention is about 1/3 smaller than that of the conventional lens. Also, the chromatic aberration d defined by equation (1)
. some α. Table 2 shows examples of calculations for However, the energy width ΔV unit (μm) was set to 10e■. Although spherical aberration d has been carefully considered here, it is smaller than chromatic aberration. That is, in the case of the lens of the present invention, α. = 5 mrad (milliradians), d8 = 0.04 μm and can be ignored. As can be seen from the example in Table 2, in the lens of the present invention, α.

= 4 mrad, a fine probe of 0.1 μm or less can be obtained, realizing extremely advantageous characteristics for microfabrication applications.

As explained above, the electrostatic accelerating lens of the present invention has the advantage of being able to reduce chromatic aberration, which has been a problem in the past, and of being able to easily adjust the focus without changing the accelerating voltage.

Therefore, in an optical system that focuses an ion beam or the like having a large energy width into a fine spot, it is highly effective when used as an electrostatic lens for an accelerator or a condenser. Further, even if a multi-stage optical system is configured by combining the lens of the present invention and other electrostatic lenses, the effect of the lens of the present invention is significant.

[Brief explanation of drawings]

Fig. 1 is a block diagram of a conventional two-electrode lens, Fig. 2 is a block diagram of a conventional six-electrode lens, Fig. 6 is a diagram showing the imaging relationship and optical parameters, and Fig. 4 is a diagram showing the implementation of the present invention. Example configuration diagram,
Figure 5 is an axial potential distribution diagram of the lens configured in Figure 4.
Fig. 6 is a diagram showing the dimensional parameters of the lens of the present invention, Fig. 7 is a diagram showing the distance from the object point to the lens surface and the dependence of the chromatic aberration coefficient on the electrode spacing, and Fig. 8 is a diagram also showing the dependence on the electrode aperture diameter. , FIG. 9 is a diagram showing a comparison between a conventional lens and a lens of the present invention. Explanation of symbols 1...Charged beam source 2, ro, 4...electrode 6...
-First electrode 7...Second electrode 891. 6th electrode? ...Fourth electrode patent applicant Nippon Telegraph and Telephone Public Corporation patent attorney Junnosuke Nakamura 1 Figure 9 ・ Device (?, Im) Figure 6 L (?M) Figure 8 A (Qn side) cl, ( πfp1)

Claims (2)

[Claims] (1) A first electrode, a second electrode, a sixth electrode, and a fourth electrode each having an opening in the center are arranged in this order rotationally symmetrically with respect to the optical axis to form a lens electrode, and the charged particles are placed in the first electrode.
A charged particle accelerating voltage is applied between the first and fourth electrodes assuming that the incident is from the electrode side, the second and fourth electrodes are at equal potential, and either a negative or positive voltage is applied between the sixth and fourth electrodes. An electrostatic accelerating lens characterized by giving. (2) A lens electrode is constructed by arranging a first electrode, a second electrode, a sixth electrode, and a fourth electrode, each having an opening in the center, in this order rotationally symmetrically with respect to the optical axis, and the charged particles are placed in the first electrode.
A charged particle accelerating voltage is applied between the first and fourth electrodes assuming that the incident is from the electrode side, the second and fourth electrodes are at equal potential, and the sixth and fourth electrodes are set at a negative or positive voltage between the electrodes. When any voltage is applied and the distance between the object point and the first electrode is limited to a range of s-7,5 or more than a certain distance Sm, the distance between the first and second electrodes is determined by keeping the focal length constant. However, using the diameter d1 of the first electrode as a parameter, calculate the distance L111 between the first and second electrodes or the diameter d1m of the first electrode when S becomes equal to Sn1, and calculate L. 1. An electrostatic accelerating lens characterized in that dl is set in a range of 50 to 100 of 1, or dl is set in a range of 50 to 100% of dlm.