JPH07240168A - Scanning electron microscope - Google Patents

Abstract

PURPOSE:To improve the detecting efficiency of secondary electron in a scanning electron microscope for introducing the secondary electron onto the upper part of an objective lens to detect the secondary electron. CONSTITUTION:The secondary electron (e) ascending in the passage of electron beam EB of an objective lens 11 is moved to be wound on the electron beam optical axis by the axial magnetic field of the objective lens 11. The secondary electron directing an inside magnetic pole 12 is attracted by a mesh second cylindrical electrode 18, and passed through the electrode 18. The secondary electron passed through the electrode 18 is drawn back in the second electrode 18 direction by the potential between a first cylindrical electrode 17 having an earth potential and the second cylindrical electrode 18 to which a positive voltage is applied. The secondary electron drawn back in the optical axial direction is again passed through the mesh electrode 18. Thus, the secondary electron (e) reaches the upper part of the objective lens 11, and is incident to a secondary electron detector 15 and detected thereby.

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 in which a sample is irradiated with an electron beam and secondary electrons generated from the sample are detected by a secondary electron detector arranged above an objective lens.

[0002]

2. Description of the Related Art In a scanning electron microscope, a secondary electron detector is usually arranged under an objective lens, that is, close to a sample. However, recently, for high-resolution image observation, the sample is placed inside the objective lens or the sample and the lower end surface of the objective lens magnetic pole are made extremely close to each other to shorten the so-called differential distance (working distance) to observe the sample. There are cases to do. In this case, it is common to arrange a secondary electron detector above the objective lens and detect the secondary electrons above the objective lens through an electron beam passage inside the objective lens.

FIG. 1 shows an objective lens portion of an in-lens type scanning electron microscope in which a sample is arranged inside an objective lens. In the figure, reference numeral 1 denotes an objective lens, and a sample 2 is arranged inside the objective lens 1. The sample 2 is irradiated with the primary electron beam EB. A secondary electron detector 3 is provided above the objective lens 1 at a position away from the optical axis of the electron beam. The secondary electron detector 3 is a detector including a scintillator and a photomultiplier tube. A ring-shaped electrode 4 is attached to the front surface of the scintillator of the detector 3, and the secondary electrode 4 is attached to the electrode 4. A positive voltage is applied to attract the electrons.

With such a structure, the electron beam EB is finely focused by a focusing lens (not shown) and the objective lens 1, and is irradiated onto the sample 2. The irradiation position of the electron beam on the sample 2 is two-dimensionally scanned by supplying a scanning signal to a scanning coil (not shown). Electron beam E
When the sample 2 is irradiated with B, a secondary electron e is generated. In the case of the in-lens scanning electron microscope, the secondary electron e exists in the objective lens 1 because the strong magnetic field exists above the lower surface of the objective lens. Constrained by the magnetic field, it wraps around the optical axis of the electron beam and rises without spreading much in the radial direction. The secondary electrons e reaching the upper part of the objective lens 1 are attracted toward the detector 3 by the electric field based on the voltage applied to the electrode 4 of the secondary electron detector 3, and are detected by the detector 3. Although not shown, the detected signal is supplied to the cathode ray tube to display a secondary electron scanning image.

FIG. 2 shows an objective lens portion of a scanning electron microscope in which the differential distance is shortened. In this figure, the same or similar components as those of the scanning electron microscope of FIG. 1 are designated by the same reference numerals. In the figure, reference numeral 5 denotes an objective lens, and in this objective lens 5, the inclination angle θ of the lower magnetic pole piece 6 is made large in order to reduce the restriction of the movement of the sample 2. The secondary electron detector 3 is arranged above the objective lens 5 as in the case of FIG.

In the configuration of FIG. 2 as well, similar to the in-lens scanning electron microscope, the secondary electrons e from the sample 2 are constrained by the magnetic field of the objective lens 5 and the upper part of the objective lens 5 while the optical axis of the electron beam EB is wound. To reach. The secondary electrons e reaching the upper part of the objective lens 5 are attracted toward the detector 3 by the electric field based on the voltage applied to the electrode 4 of the secondary electron detector 3, and are detected by the detector 3.

[0007]

In both the configuration shown in FIG. 1 and the configuration shown in FIG. 2, the secondary electrons e are wound around the axial magnetic field of the objective lens 1 or 5 and rise, but in the portion where the axial magnetic field becomes smaller as it rises. When approaching the inner pole piece of the objective lens 1 or 5, the secondary electrons travel toward the pole piece and collide with the pole piece. Therefore, the detection efficiency of the secondary electrons e is not good. Further, in the configuration of FIG. 2, since the inclination angle θ of the lower pole piece 6 of the objective lens 5 is made large, it is necessary to consider the magnetic saturation of the objective lens, so that the inner diameter of the passage of the electron beam becomes small, and The distance from the secondary electron generation point (sample 2) to the detector 3 becomes long. Therefore, more secondary electrons e
Has a high probability of colliding with the pole piece and disappearing. Further, in both the configurations shown in FIGS. 1 and 2, the inner diameter of the electron beam passage inside the objective lens is often small due to the arrangement of the astigmatism correction coil and the dynamic focus coil inside the objective lenses 1 and 5. .

The present invention has been made in view of the above circumstances, and an object thereof is to provide a scanning electron microscope which guides secondary electrons to the upper part of the objective lens by a magnetic field of the objective lens and detects the secondary electrons. It is to realize a scanning electron microscope capable of improving the detection efficiency of secondary electrons.

[0009]

A scanning electron microscope according to the present invention is a scanning electron microscope adapted to display a scanning image based on a signal detected by a secondary electron detector arranged above an objective lens. A first cylindrical electrode is arranged inside the objective lens along the optical axis of the electron beam, and a second cylindrical electrode having a passage portion for secondary electrons is arranged inside the second cylindrical electrode. It is characterized in that a positive voltage is applied to the first cylindrical electrode and the potential of the first cylindrical electrode is kept lower than the potential of the second cylindrical electrode.

[0010]

In the scanning electron microscope according to the present invention, the first and second cylindrical electrodes are arranged inside the objective lens, and the secondary electrons are directed in the optical axis direction by the electric field formed by the first and second cylindrical electrodes. Return to.

[0011]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 3 shows an objective lens portion of a scanning electron microscope which is an embodiment of the present invention. Reference numeral 11 denotes an objective lens, which is composed of an inner magnetic pole 12, an outer magnetic pole 13, and a coil 14. A lens magnetic field is formed below the lower end surfaces of the magnetic poles 12 and 13 by the objective lens 11, and the sample 30 is placed in the lens magnetic field.

A secondary electron detector 15 is provided above the objective lens 11 at a position away from the optical axis. The secondary electron detector 15 has a structure in which a scintillator and a photomultiplier tube are combined, and a ring-shaped electrode 16 is provided around the circular scintillator.
Is applied with a positive voltage that attracts secondary electrons. This voltage value is selected from the range of several kV to several tens of kV.
The detection signal of the secondary electron detector 15 is not shown,
It is supplied to a cathode ray tube synchronized with the scanning of the electron beam EB.

A first cylindrical electrode 17 is arranged in the passage of the electron beam in the central portion of the objective lens 11. A mesh-shaped second cylindrical electrode 18 is arranged inside the first cylindrical electrode 17, and both the first and second cylindrical electrodes 17 and 18 emit light of the electron beam EB. They are arranged symmetrically about the axis. The first cylindrical electrode 17 is set to the ground potential, and the second cylindrical electrode 18 is applied with a positive voltage from the power source 19, for example, about + 50V. A dynamic focus coil 20 and an astigmatism correction coil 21 are provided outside the first cylindrical electrode 17. The operation of such a configuration is as follows.

When observing a secondary electron image with the above-mentioned structure, a predetermined scanning signal is supplied from a scanning signal generating circuit (not shown) to the scanning coil, and an arbitrary two-dimensional area on the sample 30 is irradiated with the electron beam EB. Raster-scanned by. Here, the objective lens 11 is configured so that a single lens magnetic field is formed below the lower end surfaces of the inner magnetic pole 12 and the outer magnetic pole 13, and the sample 30 is placed in this lens magnetic field. Secondary electrons e generated by irradiating the sample 30 with the electron beam are constrained by the lens magnetic field and travel upward.

The secondary electrons e ascending in the passage of the electron beam EB of the objective lens 11 move so as to be wrapped around the electron beam optical axis by the axial magnetic field of the objective lens 11. Of these, the secondary electrons toward the inner magnetic pole 12 are attracted to the mesh-shaped second cylindrical electrode 18 and pass through the electrode 18. The secondary electrons that have passed through the electrode 18 are generated by the potential between the first cylindrical electrode 17 at ground potential and the second cylindrical electrode 18 to which a positive voltage is applied.
It is pulled back in eight directions. The secondary electrons pulled back in the direction of the electrode 18 (optical axis direction) pass through the mesh electrode 18 again.

In this way, the secondary electrons e rise in the passage of the electron beam EB inside the objective lens 11 and reach the upper portion of the objective lens 11 without being absorbed almost anywhere. Above the objective lens 11, the secondary electron detector 1
An electric field based on the voltage applied to the front surface of No. 5 is formed. This electric field causes the objective lens 13 generated from the sample 30.
The secondary electrons that have gone upward inside are bent toward the detector 15, and then enter the secondary electron detector 15 to be detected. Although not shown, the detection signal is supplied to the cathode ray tube through an amplifier, and the secondary electron image of an arbitrary region of the sample is displayed on the cathode ray tube.

Although one embodiment of the present invention has been described above, the present invention is not limited to this embodiment. For example, as the structure of the objective lens, the complete in-lens method of FIG. 1 or the structure of FIG. 2 may be used in addition to the structure of FIG. Further, although the first cylindrical electrode 18 is formed of a mesh, it may be a cylindrical electrode formed by winding a thin wire in a spiral shape, or a cylindrical electrode having a large number of minute holes may be used. . Furthermore, although the second cylindrical electrode has a ground potential, it may have a positive or negative potential. In that case, it suffices if the secondary electrons can be returned in the optical axis direction by the first and second cylindrical electrodes.

[0018]

As described above, in the scanning electron microscope according to the present invention, the first and second cylindrical electrodes are arranged inside the objective lens, and the electric field formed by the first and second cylindrical electrodes. Since it is configured to return the secondary electrons in the optical axis direction by
In a scanning electron microscope that guides secondary electrons to the upper part of the objective lens by the objective lens magnetic field and detects the secondary electrons,
The detection efficiency of secondary electrons can be improved. Also,
The first and second cylindrical electrodes are arranged symmetrically with respect to the optical axis of the electron beam, and therefore do not adversely affect the primary electron beam EB. Furthermore, by arranging an astigmatism correction coil and a dynamic focus coil inside the objective lens, even if the inner diameter of the objective lens is substantially reduced, secondary electrons will not collide with any member inside the objective lens and disappear. To be prevented.

[Brief description of drawings]

FIG. 1 is a diagram showing an objective lens portion and a secondary electron detector in a conventional in-lens type scanning electron microscope.

FIG. 2 is a diagram showing an objective lens portion and a secondary electron detector in a conventional scanning electron microscope.

FIG. 3 is a diagram showing an embodiment of the present invention.

[Explanation of symbols]

 11 Objective Lens 12 Inner Magnetic Pole 13 Outer Magnetic Pole 15 Secondary Electron Detector 17 First Cylindrical Electrode 18 Second Cylindrical Electrode 19 Power Supply 20 Dynamic Focus Coil 21 Astigmatism Correction Coil 30 Sample

Claims (2)

[Claims] 1. A scanning electron microscope configured to display a scanning image on the basis of a signal detected by a secondary electron detector arranged above an objective lens, wherein a scanning image is displayed inside the objective lens along an optical axis of an electron beam. A cylindrical electrode of No. 1 and a second cylindrical electrode having a passage of secondary electrons inside thereof are arranged, and a positive voltage is applied to the second cylindrical electrode and Scanning electron microscope configured to keep the potential of the cylindrical electrode lower than the potential of the second cylindrical electrode. 2. The scanning electron according to claim 1, wherein the objective lens has an inner magnetic pole and an outer magnetic pole, forms a single lens magnetic field below the lower end surface of the magnetic pole, and arranges the sample in the lens magnetic field. microscope.