JP3280187B2 - Scanning electron microscope - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a scanning electron microscope, and more particularly to a scanning electron microscope capable of performing high-resolution observation even when a sample is tilted.

[0002]

2. Description of the Related Art A conventional scanning electron microscope is disclosed in
As described in JP-A-172643, the main surface of the lens is moved to the sample side by projecting the lower surface of the inner magnetic pole of the objective lens toward the sample side from the lower surface of the outer magnetic pole, thereby reducing the lens aberration. The resolution was improved.

[0003]

Generally, such an objective lens has such a shape that a magnetic field generated from a lens gap is blocked by an inner magnetic pole on an optical axis. Therefore, in order to obtain the on-axis magnetic field intensity necessary for focusing the primary electron beam, it is necessary to generate a very strong magnetic field in the lens gap, and magnetic saturation of the magnetic path material becomes a problem. As the inclination angle of the sample is increased, the lower surface of the inner magnetic pole needs to protrude longer than the lower surface of the outer magnetic pole in order to realize high resolution. Therefore, the problem of magnetic saturation of the magnetic path material becomes more serious.

When the acceleration voltage of the electron microscope is increased, the excitation of the objective lens also needs to be increased. However, as the tilt angle of the sample increases, the area where the excitation coil is arranged in the objective lens becomes smaller. Heat generation due to the exciting current also becomes a major problem. Therefore, conventionally, in order to obtain a high resolution by tilting a sample at a wide angle using such an objective lens, only a very low accelerating voltage of less than 1 kV can be used, and from 1 kV suitable for high-resolution observation of an insulator. Observation at an acceleration voltage of several kV is difficult and has not been practical until now.

[0005]

Means for Solving the Problems The objective lens described above is used in a range of 1 kV to several kV suitable for high-resolution observation of a general insulator sample.
For use at a relatively short focal length in the range up to, consideration must be given to both magnetic saturation of the magnetic path material and heat generation of the magnetic path coil. As shown in FIG. 2, regarding the objective lens 6 having a shape in which the lower surface of the inner magnetic pole projects at the same position as the end surface of the outer magnetic pole or below (toward the sample).
As a result of performing various simulations by changing the inclination angle θ of the outer magnetic path 23 and the distance G to the end face of the outer magnetic path 23 closest to the end face of the lower surface of the inner magnetic pole, the inclination angle θ is 30
If the distance G is set in the range of 5 to 30 mm in the range of ° to 60 °, it becomes optimal for both wide-angle tilt and high-resolution observation of the sample in terms of both magnetic saturation of the magnetic path material and heat generation of the exciting coil. I discovered that. Here, the inclination angle θ of the outer magnetic path is an angle formed between a plane perpendicular to the central axis (optical axis) 20 of the lens 6 and the outer magnetic pole 23.

Further, since the portion where magnetic saturation occurs is concentrated in the region facing the optical axis 20 of the inner magnetic path 21 where the cross-sectional area of the magnetic flux is smallest, at least this portion (the hatched portion shown in FIG. 3). A material having a high saturation magnetization, for example, P
Advantageously, it is composed of ermendur (trade name).
With the objective lens of the present invention having the above-described shape and configuration, the power consumption of the exciting coil can be suppressed, and the highest magnetic field strength can be obtained.

[0007]

FIG. 4 shows a sectional view of the objective lens whose sectional shape is shown in FIG. 2, in which the entire magnetic path is made of pure iron, and the inclination angle θ of the outer magnetic path is 45 °.
The simulation results of the distance G, the maximum magnetic flux density on the optical axis (solid line) obtained at the magnetic saturation point, and the axial magnetic field generation efficiency per unit power of the exciting coil (relative value) are shown under the following conditions. . In the simulation, the distance G is set to 5
The measurement was performed on an objective lens having a diameter of 40 mm to 40 mm.

For example, when observing an insulating material such as a resist, in order to prevent charge-up, the voltage is mainly 1 kV to 3 kV.
Although an acceleration voltage of about V is used, under such conditions, the maximum magnetic flux density on the optical axis is 0.1 V at an acceleration voltage of 1 kV.
If it is 0.5 Tesla and 0.85 Tesla at 3 kV, high-resolution observation with a working distance (WD) of 10 mm or less becomes possible. Focusing on the magnetic saturation of the exciting coil, the on-axis magnetic field generation efficiency per unit power is improved by reducing the distance G, but the magnetic saturation easily occurs.
The maximum magnetic flux density on the optical axis will decrease. Further, if the distance G is too large, the magnetic path material is magnetically saturated, so that the maximum magnetic flux density on the optical axis does not increase. In addition, since the efficiency of magnetic field generation per unit power is reduced, the power consumption of the exciting coil is also increased. This causes a problem of heat generation of the exciting coil, which has an adverse effect.

As a result of considering practical conditions in consideration of these,
It is understood that the distance G should be in the range of 5 mm to 30 mm. When the same simulation is performed by changing the inclination angle θ of the outer magnetic path between 30 ° and 60 °, which is a practical range when observing the sample while tilting the sample, the same results as in FIG. 3 are obtained. can get. If the inclination angle θ of the outer magnetic path is larger than 60 °, the lower surface of the inner magnetic pole needs to protrude longer than the lower surface of the outer magnetic pole to achieve high resolution. Since the shape is cut off by the inner magnetic pole on the axis, it is necessary to generate a very strong magnetic field in the lens gap in order to obtain the on-axis magnetic field intensity necessary for focusing the primary electron beam, which is not preferable. Further, in the above-described objective lens shape, since the area where the exciting coil is disposed becomes small, heat generation due to the exciting current of the coil becomes a problem. If the inclination angle θ of the outer magnetic path is smaller than 30 °, it is not practical because the specimen inclination angle cannot be increased.

As shown in FIG. 2, a region (hatched portion) facing the optical axis of the inner magnetic path where magnetic saturation easily occurs is made of a material having a higher saturation magnetization than pure iron, for example, Permendu.
In the case of the configuration of r, the maximum magnetic flux density on the optical axis obtained at the distance G and the magnetic saturation point under the condition that the inclination angle θ of the outer magnetic path is 45 ° is shown by a broken line in FIG. The maximum on-axis magnetic flux density is larger than when the entire magnetic path is composed of pure iron (solid line), enabling high-resolution tilt observation up to several kV of acceleration voltage with lower power consumption (lower heat generation). Become. For example, when an X-ray analysis is performed, an acceleration voltage of 5 kV is used.
Satisfies 11 Tesla or more, enabling high-resolution observation and high-sensitivity analysis.

[0011]

FIG. 1 is a schematic structural view of one embodiment of a scanning electron microscope according to the present invention. The primary electron beam 2 emitted from the cathode 1 by the voltage V1 applied to the cathode 1 and the extraction electrode 3 is accelerated by the applied voltage Vacc applied between the cathode 1 and the acceleration electrode 4. The primary electron beam 2 is narrowed down by the focusing lens 5 and the objective lens 6 controlled by the lens control power supply 14 and is irradiated onto the sample 8 to be deflected.
The sample is scanned two-dimensionally at b.

The scanning signals of the deflectors 7a and 7b are controlled by a deflection control circuit 12 in accordance with the observation magnification. Secondary signal 15 generated from sample 8 by irradiation of primary electron beam 2
Is detected by the secondary signal detector 9 and is displayed as an enlarged image of the sample on an image display device 13 such as a CRT. The sample stage 10 has a mechanism for moving in the horizontal and vertical directions with respect to the objective lens 6 and a mechanism for tilting. As shown in FIG. 2, the objective lens 6 sets the inclination angle θ of the outer magnetic path in the range of 30 ° ≦ θ ≦ 60 °, and the lower surface of the inner magnetic pole is at the same position as the lower surface of the outer magnetic pole or below ( The distance G between the end face of the lower surface of the inner magnetic pole and the end face of the outer magnetic pole closest to the outer magnetic pole is 5 to 30 mm.
mm.

As shown by hatching in FIG. 3, at least a region of the magnetic path of the objective lens 6 including the inner magnetic pole where the magnetic saturation is likely to occur and facing the optical axis is made to have a saturation magnetization higher than that of pure iron. Construct with high material. As a result, in the objective lens of the same shape, as shown by the broken line in FIG. 4, the maximum axial magnetic flux density becomes larger than when the entire magnetic path is made of pure iron, and the state of lower power consumption (lower heat generation) is achieved. Thus, high-resolution tilt observation can be performed up to an acceleration voltage of several kV.

FIG. 5 is an enlarged view near the objective lens 6 and the sample 8. Objective lens 6 has lower inner magnetic pole (sample side)
As shown in the figure, a magnetic field distribution 17 is formed on the sample 8 side from the objective lens 6. Therefore, even when observing the sample 8 while tilting it, the focal length f of the objective lens 6 becomes extremely short, so that high-resolution observation becomes possible.

[0015]

According to the present invention, the power consumption of the objective lens exciting coil can be suppressed, and the highest magnetic field strength can be obtained. In addition, since the lower surface of the inner magnetic pole protrudes below the lower surface of the outer magnetic pole (toward the sample), a high-resolution image can be obtained even when observing the sample while tilting it.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a scanning electron microscope.

FIG. 2 is a sectional view showing the shape of an objective lens.

FIG. 3 is a cross-sectional view of an objective lens in which a part of a magnetic path is made of a material having high saturation magnetization.

FIG. 4 is a diagram showing a magnetic saturation characteristic and a magnetic field generation efficiency of an objective lens.

FIG. 5 is a diagram showing an objective lens, its magnetic field, and a sample position.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Cathode, 2 ... Electron beam, 3 ... Extraction electrode, 4 ... Acceleration electrode, 5 ... Focusing lens, 6 ... Objective lens, 7a, 7b ... Deflector, 8 ... Sample, 9 ... Secondary electron detector, 10 ... Sample stage, 11 ... Aperture, 12 ... Deflection control circuit, 13 ... Image display device, 14 ... Lens control power supply, 15 ... Secondary signal, 16 ...
Material with high saturation magnetization, 17: magnetic field distribution, 20: optical axis, 2
1: inner magnetic path, 23: outer magnetic path

Claims (2)

(57) [Claims] 1. A scanning electron microscope which two-dimensionally scans a narrowly focused electron beam on a sample and detects a signal generated from each scanning point of the sample. The inclination angle θ of the magnetic path is in the range of 30 ° to 60 °, the lower surface of the inner magnetic pole is projected at the same position as the lower surface of the outer magnetic pole, or below (to the sample side), and the lower surface of the inner magnetic pole is Distance G to the end face of the close outer magnetic pole is 5m
A scanning electron microscope comprising an objective lens in a range from m to 30 mm. 2. The scanning electron microscope according to claim 1, wherein the objective lens has a region facing the optical axis of the magnetic path made of a material having a higher saturation magnetization than a material of another region. .