JPH0864163A - Charged particle beam device - Google Patents

Abstract

PURPOSE: To provide a charged particle beam device using a superimposed field lens, capable of making aberration on an axis extremely small. CONSTITUTION: When a primary electron beam EB on or near an optical axis reaches near an objective lens 1, by the lens functions of an accelerating part of an electric field constituted with electrodes 6, 7 and a decelerating part of an electric field constituted with electrodes 7, 8, the electron beam EB is deflected in the axial direction. By applying the voltage slightly lower than the accelerating voltage of the electron beam to a specimen 2, electrostatic potential attenuates along the axial, and by deceleration of the electron beam, aberration on axis of spherical aberration Cs and color aberration Cc is decreased by the deceleration of the electron beam. By this constitution, a gap αin the axial direction between the tip of an outside magnetic pole 3 of the objective lens 1 and the tip of an inside magnetic pole 4 is adjusted so that peaks of an electrostatic potential E and a magnetic potential B interfere each other to the specimen and the electron beam is most converged in the axial direction.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a charged particle beam apparatus such as a scanning electron microscope which irradiates a sample with an electron beam and detects secondary electrons generated from the sample to display a sample image.

[0002]

2. Description of the Related Art In a scanning electron microscope, a primary electron beam generated from an electron gun is finely focused on a sample by a condenser lens or an objective lens, and the electron beam is scanned by a deflecting means. Secondary electrons generated by irradiating the sample with the electron beam are detected by the secondary electron detector. Since the detection signal of the detector is supplied to the cathode ray tube synchronized with the scanning of the electron beam, 2
The next electron image is displayed.

In such a scanning electron microscope, when a semiconductor or the like is used as the sample, the sample is damaged when the semiconductor sample is irradiated with an electron beam having a high acceleration voltage. Therefore, the sample should be irradiated with an electron beam having a low acceleration voltage. Is desired. However, an electron beam with a low acceleration voltage has large aberration, and it becomes difficult to observe a secondary electron image with high resolution. For this reason, a superimposed field lens in which a magnetic field type lens and an electrostatic type lens are combined is used as an objective lens to reduce the aberration and distortion of the primary electron beam. For example, as an example using this superposed field lens, the acceleration voltage is 1
An example of achieving a resolution of 4 nm at kV has been reported.

The superposed field lens disclosed in Japanese Unexamined Patent Publication No. 63-160144 has a cylindrical electrode parallel to the optical axis with a total length of 40 mm or more, a snorkel type magnetic lens, and is arranged near the sample to decelerate the electron beam. It is composed of a cylindrical electrode that generates an electric field. The principle of this superposed field lens is
With respect to the electron beam trajectories on the optical axis or paraxial, the hole diameter of the cylindrical electrode is made as small as possible in order to utilize the effect of the cylindrical electric field, and the electron beam is accelerated. The magnetic field lens that overlaps with this electric field can bring the outer magnetic pole closer to the optical axis because the hole diameter of the cylindrical electrode is smaller, and brings the magnetic field peak of strong excitation near the sample to make the electron beam sample in the short focus. Focus on top. An electric field near the sample is used to decelerate the accelerated electron beam and reduce distortion.

[0005]

The superposed field lens, which mainly uses the magnetic field type lens as a focusing factor, applies a high voltage for accelerating the electron beam to the cylindrical electrode, but since the hole diameter is small, an electric field effect is produced. However, since a high voltage is applied to the electrode, the burden on the unit area of the electrode is increased. Therefore, there is a drawback that the electrode is easily damaged. Further, the distance between the magnetic field type lens and the cylindrical electrode must be at least 2 mm or more in order to avoid discharge between vacuums. As a result, the magnetic field type lens requires a certain distance or more from the optical axis, and the effect of the magnetic field on the electron beam passing through the optical axis is reduced. Without strong focusing lens.

The present invention has been made in view of the above points, and an object thereof is to realize a charged particle beam apparatus using a superposed field lens capable of extremely reducing axial aberration.

[0007]

A charged particle beam apparatus according to the present invention includes a superposed field lens including a magnetic lens and an electrostatic lens which is an asymmetric Einzel lens, and a voltage applied to a sample. In a charged particle beam apparatus including a power source, the asymmetric Einzel lens is used as an acceleration type lens, and a gap between an outer magnetic pole and an inner magnetic pole of the magnetic field type lens is a magnetic potential generated in the magnetic field type lens. The electrostatic potential generated by the electrostatic lens is adjusted so as to optimally focus the charged particle beam with which the sample is irradiated.

In the present invention, the gap is adjusted so that both the peak of the magnetic potential generated by the magnetic field type lens and the peak of the electrostatic potential generated by the electrostatic type lens are located in front of the sample. ing

[0009]

In the charged particle beam system according to the present invention, the asymmetric Einzel lens used in the superposed field lens is used as the acceleration type lens, and the gap between the outer magnetic pole and the inner magnetic pole of the magnetic field type lens is formed by the magnetic field type lens. The magnetic potential generated and the electrostatic potential generated by the electrostatic lens are adjusted to optimally focus the charged particle beam with which the sample is irradiated, and aberration is made extremely small.

[0010]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 shows a main part of a scanning electron microscope according to the present invention, and 1 is a magnetic field type objective lens. The primary electron beam EB generated and accelerated by an electron gun (not shown) is finely focused on the sample 2 by the magnetic field type objective lens 1. The objective lens 1 is composed of an outer magnetic pole 3, an inner magnetic pole 4, a coil 5, and the like. An asymmetric Einzel lens 9 composed of three electrodes 6, 7 and 8 is arranged in the electron beam passage in the center of the objective lens 1. A negative voltage is applied to the sample 2 from the power source 10. A secondary electron detector 11 is provided at a position above the optical axis above the objective lens 1. A weak positive voltage for attracting secondary electrons is applied to the front surface of the detector 11. The operation of such a configuration will be described below.

To obtain a secondary electron image, the primary electron beam E
B is finely focused by the magnetic field type objective lens 1 and the electrostatic type Einzel lens 9 composed of the electrodes 6, 7, and 8, and is irradiated onto the sample 2. The irradiation position of the electron beam on the sample 2 is scanned by a deflecting unit (not shown). Secondary electrons are generated from the sample as the sample 2 is irradiated with the electron beam. The secondary electrons are restrained by the magnetic field of the objective lens 1 and the electric field of the Einzel lens 9, and are taken out above the objective lens 1. Be done. Objective lens 1
The secondary electrons above is detected by being attracted to the detector 10 by the positive electric field applied to the secondary electron detector 11.
Since the detection signal of the detector 11 is supplied to a cathode ray tube (not shown) synchronized with the scanning of the primary electron beam EB, a scanning secondary electron image of the sample is displayed on the cathode ray tube.

With the above structure, when the primary electron beam EB on the optical axis or near the axis arrives near the objective lens 1, first, the accelerating part of the electric field constituted by the electrodes 6 and 7, and the electrodes 7 and 8. The electron beam EB is deflected in the axial direction by the lens action of the deceleration part of the electric field constituted by. Further, by applying a voltage that is slightly lower than the acceleration voltage of the electron beam to the sample 2, the electrostatic potential is strongly attenuated along the axis, and the spherical aberration Cs and the chromatic aberration Cc due to the deceleration of the electron beam.
On-axis aberration is reduced.

Further, in the configuration of FIG. 1, the axial gap α between the tip of the outer magnetic pole 3 and the tip of the inner magnetic pole 4 of the objective lens 1 is the peak of the electrostatic potential E and the magnetic potential B with respect to the sample. Interfere with each other and the electron beams are adjusted to be most focused in the axial direction. That is, the outer magnetic pole 3 and the inner magnetic pole 4 retract the inner magnetic pole 4 inward when viewed in the axial direction.

Here, the aberration in the configuration of FIG. 1 will be considered. The acceleration voltage of the primary electron beam EB in the electron gun is set to −10 kV, the electrodes 6 and 8 are applied to the ground potential, and the electrode 7 is applied to, for example, 7 kV. In addition, the sample 2 has a power source 10
To -9.99 kV negative voltage is applied. As a result,
Energy Vs of the electron beam that finally reaches the sample 2
Is 10V. Furthermore, the excitation of the objective lens 1 is set to 2000
Assuming AT and the distance (working distance) between the sample 2 and the objective lens 1 is 3 mm, computer simulation shows that only the electric field in which only the Einzel lens 9 is operated and the Einzel lens 9 and the objective lens 1 are used. The following table 1 shows the spherical aberration (Cs), the chromatic aberration (Cc), and the voltage (VCEN) applied to the electrode 7 in the case of a superimposed field in which both and are simultaneously operated.

[0015]

[Table 1]

As is clear from the above table, the aberration can be made extremely small by using the superimposed field as compared with the case where only the electric field is used. Further, the voltage applied to the electrode 7 can be lowered by setting the superimposed field.

Next, when the opening angle θ with respect to the axis when the electron beam collides with the sample 2 is about 8.44 mrad from the superposed field data in the above table, the resolution is 2.79 n.
m can be obtained.

Table 2 shows the data of the axial aberration when the gap α between the outer magnetic pole 3 and the inner magnetic pole 4 of the objective lens 1 in FIG. 1 is changed, and the gap α is the case 1
2 and 3 show the superposed field potentials in case 2 and case 2, respectively. 2 and 3, the horizontal axis represents the position in the optical axis direction, and the vertical axis represents the electric field and magnetic field potentials.

[0019]

[Table 2]

As shown in Table 2 above, the aberration can be made smaller by appropriately adjusting the value of the gap α. FIG. 2 shows the case 1 in which the gap α is 0, and the peak Pe of the electric field potential E is on the left side (electron gun side) with respect to the sample position S shown by the dotted line, but the peak of the magnetic field potential B is. Pb is on the right side, and the rising part of the magnetic field is on the sample. As a result, the degree of interference between the magnetic field and the electric field potential is low, and the electron beam cannot be sufficiently focused. Compared with this, in case 2 of FIG. 3, that is, when the gap α is 2 mm, the sample position S
On the other hand, the peak Pb of the magnetic field potential B is on the left side like the peak Pe of the electric field potential E, the potentials of the magnetic field and the electric field interfere with each other, and the electron beam is most efficiently focused in the optical axis direction with a small aberration. To work.

Although the embodiment of the present invention has been described above, the present invention is not limited to this embodiment. For example, although the scanning electron microscope has been described as an example, it may be used in an apparatus for scanning an ion beam such as an ion beam apparatus. In that case, a positive voltage is applied to the sample. Further, the surface of each electrode (6, 7, 8) forming the Einzel lens, which faces the optical axis, is not flat but has a curvature, so that the discharge can be prevented. Furthermore, it is effective to be able to arbitrarily change the voltage applied to the sample.

[0022]

As described above, the charged particle beam system according to the present invention uses the asymmetric Einzel lens used in the superimposed field lens as the acceleration type lens, and further includes the outer magnetic pole and the inner magnetic pole of the magnetic field type lens. The gap between them was adjusted so that the magnetic potential generated by the magnetic field type lens and the electrostatic potential generated by the electrostatic type lens optimally focused the charged particle beam with which the sample was irradiated. As a result, the axial aberration can be made extremely small. Further, since the magnetic field is superimposed on the electric field, the voltage applied to the central electrode of the Einzel lens is reduced, and the deflection effect of the charged particle beam by this lens can be fully utilized.
As the voltage can be reduced, the load of the electric field applied to each electrode is reduced, and the risk of discharge between vacuums is reduced.

[Brief description of drawings]

FIG. 1 is a diagram showing an embodiment of a scanning electron microscope according to the present invention.

FIG. 2 is a diagram showing a superimposed field potential.

FIG. 3 is a diagram showing a superimposed field potential.

[Explanation of symbols]

 1 Objective Lens 2 Sample 3 Outer Magnetic Pole 4 Inner Magnetic Pole 5 Coil 6, 7, 8 Electrode 9 Einzel Lens 10 Power Supply 11 Secondary Electron Detector

Claims (2)

[Claims] 1. A charged particle beam apparatus comprising a superposed field lens comprising a magnetic field type lens and an electrostatic type lens which is an asymmetrical Einzel lens, and a power source for applying a voltage to a sample. The Zell lens is used as an acceleration type lens, and the gap between the outer magnetic pole and the inner magnetic pole of the magnetic field type lens has a magnetic potential generated by the magnetic field type lens and an electrostatic potential generated by the electrostatic type lens on the sample. A charged particle beam device, which is adjusted to optimally focus an irradiated charged particle beam. 2. The gap is adjusted so that both the peak of the magnetic potential generated by the magnetic lens and the peak of the electrostatic potential generated by the electrostatic lens are located in front of the sample. Charged particle beam device as described.