JPH11297248A - Electron beam accelerator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electron beam accelerator causing no axial dislocation without bending an electron beam up to low accelerating voltage from high accelerating voltage by axially symmetrically installing an odd number of ion pumps in an electron beam passage so as to cancel a component vertical to the electron beam advancing direction. SOLUTION: The inside of a vessel 3 of an electron beam accelerator is filled with inert insulating gas, and the inside of an accelerating tube 4 is exhausted to a prescribed degree of vacuum by a first ion pump 1 and a second ion pump 2 symmetrically placed in an electron beam passage as the center. Permanent magnets 11a, 11b for the first ion pump and the second ion pump are symmetrically placed relative to the Z axis, and when intensity of the respective permanent magnets is entirely the same, they cancel each other. Therefore, a leakage magnetic field of the ion pumps is only a Z directional component on the electron beam passage, so that an electron beam is not bent by the leakage magnetic field of the ion pumps even in any accelerating voltage.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electron gun used for an electron microscope using a field emission type cathode whose inside needs to be evacuated to an ultra-high vacuum, and particularly to a high acceleration voltage of 100 kV or more and a 10 kV or higher. The present invention relates to an electron gun characterized in that the following low acceleration voltage can be handled.

[0002]

2. Description of the Related Art As described in Japanese Patent Application Laid-Open No. 7-296759, when an insulator sample is irradiated with an electron beam, the amount of secondary electrons and reflected electrons generated from the sample is reduced by the amount of electron irradiation applied to the sample. Using a low-energy electron beam that balances allows observation of the surface without charging, while using a high-energy electron beam allows observation of the bottom surface of a concave portion such as a deep hole in a semiconductor device. it can. As described above, a device that can be used continuously from a high energy electron beam to a low energy electron beam is very useful in industry.

Meanwhile, in recent electron microscopes, cold cathode or Schottky emission field emission electron sources are frequently used as electron sources due to demands for high resolution images and minute part analysis capabilities. Since electron guns using these cathodes extract electrons by the tunnel effect, in order to reduce the ion bombardment of the cathode due to gas around the cathode, the area around the cathode or the entire electron gun is over 1 × 10 ⁻⁷ Pa or more. High vacuum is required.

A vacuum pump is used to achieve an ultra-high vacuum. However, since the amount of gas discharged from the pump is small and the pump can be evacuated to an ultra-high vacuum, vacuum pumps such as ion pumps and noble pumps are usually used. A pump is used. These pumps evacuate the gas by ionizing and ionizing the gas at a high voltage of about 5 kV and attracting it to the electrodes by a magnetic field created by a permanent magnet. There are various types of pumps having such a principle. In this specification, pumps having such a principle are collectively referred to as an ion pump.

[0005] In the case of the above-described ion pump, for example, a 20 liter / S ion pump of Nidec Anelva, according to the Nikkei Anelva Vacuum Equipment General Catalog, a magnetic field of about 1 Gauss leaks at a position 200 mm from the pump exhaust port. Is described in the catalog. When this ion pump is used in an electron gun, this leakage magnetic field is usually easily shielded by installing a shield plate between the ion pump inlet and the electron beam passage, or by moving the ion pump away from the electron beam passage. Can be. Further, if the acceleration voltage is sufficiently high, the energy of electrons is large, so that the leakage magnetic field of the ion pump does not matter.

[0006]

However, as described in the above prior art, an ion pump usually has a leakage magnetic field. If the electron beam is accelerated at a high accelerating voltage, the leakage magnetic field is not particularly a problem. However, if the accelerating voltage is kept low and the electron beam is bent by the ion pump, the leakage magnetic field is bent. Here, the extent to which the accelerated electron beam bends due to the stray magnetic field is quoted from "Basics of Electron Microscopy" published by Kyoritsu Shuppan Co., Ltd.
I will make a trial calculation.

When an electron beam is incident on a uniform magnetic field having a magnetic flux density B, if the direction of the magnetic field is the y-axis and the x-axis and the z-axis are orthogonal to the y-axis, the electrons are in the xz plane. Draw a circular orbit. The radius R is determined by the balance between the centrifugal force mv ² / R of the electron and the Lorentz force eVB.

[0008]

(Equation 1)

[0009] Here, v is the electron incident velocity, e is the charge of the electron (1.6 × 10 ⁻¹⁹ [C]), and m is the mass of the electron (9.1 × 10 ⁻³¹ [kg]). Also, the angular velocity ω (cyclotron frequency) that goes around this orbit is

[0010]

(Equation 2)

[0011] is given.

As shown in FIG. 2, if it is bent by a magnetic field in the range of d and then goes straight by L, the deflection amount a
Is

[0013]

A = R−R cos θ + L tan θ (Equation 3) Where θ is

[0014]

(Equation 4)

[0015]

Now, as shown in FIG. 2, d = 0.1 [m], L = 0.5 [m], B = 1 × 10
^-6 [T], if the incident electrons are accelerated at 1 [kV], the electron velocity v is 1.88 × 10 ⁷ [m / s], and the deflection amount a is 5.1. [Mm].

On the other hand, assuming that the acceleration voltage is 100 [kV], the electron velocity becomes 1.64 × 10 ⁸ [m / s], and the deflection amount a becomes 0.6 [mm].

As described above, when the acceleration voltage of the electrons is low, only one electron beam is perpendicular to the traveling direction of the electron beam on the electron beam path.
Electrons are greatly bent only by the magnetic field of 00 [mG].

Furthermore, if the bending of the electrons is changed by changing the accelerating voltage, the axis of the electron beam must be re-adjusted every time the accelerating voltage is changed, making the device extremely difficult to use. I will.

The leakage magnetic field of the ion pump can be reduced by enclosing the electron beam path with a magnetic shield made of a ferromagnetic material. However, by inserting an extra member such as a magnetic shield in a vacuum, the vacuum container can be reduced. Surface area increases. In an ultra-high vacuum apparatus, efforts are made to minimize the adsorbed gas on the surface of the vacuum vessel, but the insertion of a magnetic shield goes against this effort.

The present invention has been made in view of the above, and an object of the present invention is to provide a leakage magnetic field from an ion pump, which significantly deteriorates the operability of a continuously usable device from high energy electron beams to low energy electron beams. Of the present invention is to provide an electron beam accelerator and an electron microscope using the same, wherein the electron beam is reduced without using a magnetic shield made of a ferromagnetic material.

[0022]

An electron beam accelerator according to the present invention and an electron microscope using the same are provided with an electron beam generating cathode, an extraction electrode, a multi-stage accelerating electrode, and an electron beam generating device for extracting an electron beam from the electron beam generating cathode. A power supply for applying an extraction voltage to the extraction electrode, a power supply for applying an acceleration voltage to the cathode so as to accelerate the electron beam, and a division resistor for dividing the acceleration voltage applied to the extraction electrode and applying the divided acceleration voltage to the remaining acceleration electrodes And an even number of ion pumps capable of evacuating the inside of the electron gun to a predetermined degree of vacuum.

[0023]

FIG. 1 is a conceptual diagram of an embodiment of an electron gun portion of an electron microscope according to the present invention. This embodiment is a cold cathode field emission multi-stage acceleration type electron gun.

An output voltage from an accelerating voltage generating power source 8 capable of outputting an arbitrary voltage is applied to the field emission type cathode 7, and the accelerating voltage generating power source 8 for stabilizing the accelerating voltage is applied to the extraction electrode 14. An output voltage from a field emission extraction voltage generating power supply 10 connected in a form hanging from the output is applied. Further, a heating power supply 9 for heating or flashing the field emission cathode 7 is connected to the field emission cathode 7. The voltage applied to the extraction electrode 14 is divided by the division resistor 6 and is sequentially applied to the acceleration electrode 5. The inside of the container 3 is filled with an inert insulating gas, and the inside of the accelerating tube 5 is brought to a predetermined vacuum degree by the first ion pump 1 and the second ion pump 2 placed on the electron beam path. Exhausted.

The magnetic field leaking from the first ion pump is 12a, and the magnetic field leaking from the second ion pump is 12b.
And The directions of the magnetic flux are the directions of B _IP1 and B _{IP2 in} FIG. 1, respectively.

As shown in FIG. 3, the synthesis of the magnetic field is a vector synthesis in the direction of each magnetic flux. In FIG.
Letting the electron beam path be Z, electrons proceed in the Z direction. The permanent magnet for the first ion pump and the permanent magnet for the second ion pump are indicated as 11a and 11b for simplicity, respectively.
The magnetic field leaking from the permanent magnet for the first ion pump draws lines of magnetic force like 12a, and the magnetic field leaking from the ion pump 2 draws lines of magnetic force like 12b. In terms of the magnetic field lines 12a of the magnetic field leaking from the first ion pump intersects with Z-axis, the vector of the magnetic field strength is the B _IP1 in Fig. Therefore, the component of the magnetic field perpendicular to the traveling direction of the electron beam is B _IP1Y .

Similarly, the component of the magnetic field line 12b of the magnetic field leaking from the second ion pump and perpendicular to the traveling direction of the electron beam is B _IP2Y . Here, each permanent magnet 1
If 1a and 11b are placed symmetrically with respect to the Z axis and the strengths of the respective permanent magnets are exactly the same, B _IP1Y and B _IP2Y have opposite directions and the same size.
Cancel each other out. Therefore, on the path of the electron beam, the leakage magnetic field of the ion pump has only a Z-direction component, and the electron beam is not bent by the leakage magnetic field of the ion pump at any accelerating voltage.

In this embodiment, an example in which two ion pumps are attached to the object with respect to the Z-axis has been described. , The components in the direction horizontal to the Z-axis are similarly canceled.

[0029]

According to the present invention, the component of the magnetic field perpendicular to the direction in which the electron beam travels on the electron beam path is reduced.
Since the electron beam is not bent from a high acceleration voltage to a low acceleration voltage, an electron beam accelerator that does not cause an axis shift from a high acceleration voltage to a low acceleration voltage is provided.

Further, since it is not necessary to provide a magnetic shield which increases the exhaust resistance or increases the surface area in the vicinity of the electron beam path, an electron beam accelerating device having an interior having an almost ideal vacuum degree is provided.

[Brief description of the drawings]

FIG. 1 is a schematic view of an embodiment of an electron beam accelerator according to the present invention.

FIG. 2 is a model diagram illustrating a conventional electron trajectory when a magnetic field is present in a direction perpendicular to a traveling direction of an electron beam.

FIG. 3 is a model diagram illustrating the operation principle of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... First ion pump, 2 ... Second ion pump, 3 ... Container, 4 ... Acceleration tube, 5 ... Acceleration electrode, 6 ... Voltage division resistance, 7
... field emission type cathode, 8 ... acceleration voltage generation power supply, 9 ... cathode heating power supply, 10 ... field emission extraction voltage generation power supply, 11a ... permanent magnet for first ion pump, 11b ... permanent magnet for second ion pump, 12a ... Magnetic field lines of the magnetic field leaking from the first ion pump, 12b: Magnetic field lines of the magnetic field leaking from the second ion pump, 13: electron beam, 14: extraction electrode.

Continued on the front page (72) Inventor Shunichi Watanabe 882 Mage, Oaza-shi, Hitachinaka-shi, Ibaraki Pref.

Claims (1)

[Claims] An electron beam source, an extraction electrode for extracting electrons from the electron beam source, an acceleration tube for applying an acceleration voltage, and an extraction electrode for extracting an electron beam from the electron beam source. An electron beam accelerator including a power supply for applying a voltage, a power supply for applying an acceleration voltage to the accelerator tube so as to accelerate the electron beam, and an ion pump or a similar pump for evacuating the electron beam passage and the inside of the accelerator tube. At
An even number of ion pumps or similar pumps installed to evacuate the electron beam path of the electron beam accelerator to ultra-high vacuum include the magnetic field leaking from the pump to the electron beam path in the direction of electron beam travel. On the other hand, an electron beam accelerator is mounted on an electron beam path so as to be axially symmetric so as to cancel a vertical component.