JPH0773836A - Electron microscope - Google Patents

Abstract

PURPOSE:To improve the response characteristics of a large-sized electron lens for use, for example, in an ultra-high voltage electron microscope. CONSTITUTION:Coil cooling plates 23, 24 are provided to absorb and radiate the Joule's heat generated by the current flowing in the coil 11 of an electron lens and are equipped with a cut part 25 so as not to form an electric closed loop circuit which is in electromagnetic induction coupling with the coil 11. The yoke, pole piece, etc., of magnetic substance to constitute the electron lens are alike provided with a cut part so as not to form an electric closed loop circuit.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electron microscope, and more particularly to a magnetic field type electron lens used in an electron microscope.

[0002]

2. Description of the Related Art Electron lenses used in electron microscopes are classified into a magnetic field type and an electrostatic type, but it is preferable to use a magnetic field type lens because it is easy to obtain a high magnification and lens aberration is small. Many. The magnetic field type lens utilizes the magnetic field generated in the air gap obtained by cutting a part of the yoke of the electromagnet as an electron lens.

FIG. 5 shows the configuration of a general electron microscope. The electron beam 2 from the electron gun and the accelerating tube 1 is formed into a desired beam spot by the condenser lens 3 and is irradiated on the sample 4 to be observed. The electron beam 2 transmitted through the sample 4 is focused by the objective lens 5, and the intermediate lens 6
The image is enlarged by the projection lens 7 and is formed on the fluorescent plate 8 as a sample transmission image 9, that is, an electron microscope image.

FIG. 6 shows a sectional view of the structure of a typical electron lens. As shown in the figure, the electron lens is composed of a magnetic yoke 10, a coil 11, a cooling unit 12, a pole piece unit 13 and the like. FIG. 7 is an enlarged sectional view of the pole piece portion 13. The upper piece 14 and the lower piece 15 are magnetically connected to the yoke 10, and the upper and lower pieces are the spacer 1
6, the mechanical air gap 17 is accurately formed. What actually functions as an electron lens is the intensities of the radial direction component and the axial direction component of the magnetic flux density vector generated in the air gap 17. The focal position can be changed by forming a convex lens with this magnetic flux density vector and controlling the current flowing through the coil.

FIG. 8 shows the structure of the coil 11 and the cooling unit 12. When a current is passed through the coil 11, Joule heat is generated that is proportional to the product of the electric resistance of the coil 11 and the square of the current, and the temperature of the peripheral portion including the coil 11 rises. In order to efficiently absorb the heat generated by the coil 11, the cooling unit 12
As the upper cooling plate 18 and the lower cooling plate 19 constituting the above, a block material made of a metal having a high thermal conductivity, usually copper, is used, and an extremely thin electric insulator is provided between the coil 11 and the upper cooling plate 18 and the lower cooling plate 19. It is tightly joined with a bolt 21 with 20 interposed. Cooling pipes (not shown) on the upper and lower cooling plates 18 and 19
Is buried and is cooled by flowing cooling water.

In the objective lens for focusing the electron microscope image, a strong magnetic field lens is used to stabilize the operation, and a large current is passed through the coil. As a result, a large amount of Joule heat is generated due to the resistance of the coil itself, which causes a rise in the temperature of the coil and, in turn, a rise in the temperature of the electron lens and the electron microscope device itself. If this temperature rise is left as it is, it causes a microscopic dimensional change in the electron lens and the like, and it is difficult to maintain stable device performance. Therefore, in the case of an electron lens whose temperature rises sharply, the cooling plate is closely attached to the coil that constitutes the electron lens as described above, and the heat of the cooling plate is absorbed by the cooling water whose temperature is constant to stabilize the temperature of the electron lens. I am trying.

[0007]

In recent years, development of an ultra-high voltage electron microscope has progressed, and an electron lens that can generate a strong magnetic field as the acceleration voltage of an electron beam rises has been used. When a large current is passed through the coil of the electron lens to generate a strong magnetic field, the amount of heat generated from the coil inevitably increases, and the cooling plate for cooling it also increases in size. It will be big.

By the way, if an ultra-high voltage electron microscope is constructed by using an electron lens having a structure in which such a conventional electron lens is simply increased in size, it is apparent that the response characteristics are significantly deteriorated as compared with the conventional electron microscope. Became. For example,
When the current flowing through the coil of the objective lens is changed to adjust the focus, it may take 1 second or more for the focus to stabilize, which considerably deteriorates the operability of the electron microscope.
It is an object of the present invention to improve the response characteristics of a large-sized electron lens used for an ultra high voltage electron microscope.

[0009]

As a result of research on the cause of the deterioration of the response characteristic of the electron lens, the present inventors have found that the conventional electron lens is Several electric closed loop electric circuits are formed in the peripheral part such as a cooling plate that absorbs generated heat and a magnetic circuit that passes magnetic flux, and each electric closed loop circuit forms an electromagnetic inductive coupling with the coil of the electron lens. None,
It has been found that it acts as a load on the coil while acting to prevent changes in the magnetic flux density generated by the coil.

That is, in general, a material having a high thermal conductivity also has a large electric conductivity, and when a coil cooling plate is manufactured from a material having a high thermal conductivity, an electric closed loop circuit is constructed. Moreover, in a large-sized electron lens, the amount of heat generated by the coil is large, and the cooling plate for cooling the coil has a large volume, so the resistance is extremely small and the circuit time constant (L / R) is extremely large.
As a result, a large induction current flows through the coil cooling plate, and it takes a long time to disappear, which significantly deteriorates the response characteristics of the device. At the same time, the magnetic material used for the magnetic circuit of the electron lens has a large electric resistance compared to a good conductor, but the electric resistance forming the magnetic path is small because the volume is extremely large,
The circuit time constant is as large as that of the cooling plate, which causes the response characteristics of the device to deteriorate.

The structure of the coil and cooling unit of the conventional electron lens will be described with reference to FIG. 8. The structure of the coil 11 and the upper and lower cooling plates 18 and 19 has strong electromagnetic induction coupling.
The induced current 22 in the direction that disturbs the magnetic flux generated by the upper and lower cooling plates 1
Since the resistances of 8 and 19 are extremely small, and the circuit time constant is extremely large, the response speed of the magnetic flux density when the lens current changes is extremely slow.

In the present invention, based on the above consideration, the electric closed loop circuit around the coil of the electron lens is reduced as much as possible, and the electromagnetic inductive coupling between the coil and the electric closed loop circuit is weakened to reduce the induced current. The above object is achieved by making the electric resistance of the circuit as small as possible and making the time constant small to shorten the disappearance time of the induced current and suppress the generation of eddy current.

[0013]

The response characteristics of the electron lens are improved by minimizing the induced current due to the electric closed loop circuit which becomes the load of the coil and at the same time reducing the time constant of the circuit. The induced current is interrupted by disconnecting the various electrical closed loop circuits. Also, the circuit time constant of the coil cooling plate is determined by the inductance, electrical resistance, and the capacitance of the capacitor, which mainly consists of stray capacitance, but the main factor in this case is that the electrical resistance is small, so cooling with a high-resistance material The time constant can be reduced by manufacturing the plate.

[0014]

EXAMPLES Examples of the present invention will be described below. [Embodiment 1] FIG. 1 shows the structure of a coil and a cooling plate according to an embodiment of the present invention. Upper cooling plate 23 and lower cooling plate 2
4 generally uses a metallic material having a high thermal conductivity to absorb the heat generated by the coil 11, but these cooling plates 23, 24
A cutting part 25 is made in the
Shut off. Cooling pipes (not shown) are embedded in the upper cooling plate 23 and the lower cooling plate 24, and cooling water is caused to flow to perform water cooling.
The same effect can be obtained even if the cutting portion 25 is made of an electrically insulating material.

The upper cooling plate 23 and the lower cooling plate 24 are joined by bolts 21 with a thin electric insulator 20 interposed therebetween in order to enhance the degree of adhesion with the coil 11 and efficiently absorb the heat generated by the coil 11. To do. At this time, the upper and lower cooling plates 23, 24
In order to prevent the bolt 21 from forming an electrically closed loop circuit, an insulator spacer 26 is interposed between the bolt 21 and the upper cooling plate 23 to cut off the conduction. This insulation can be achieved by forming the bolt tap portion of the lower cooling plate 24 with an insulator. Further, the upper cooling plate shown in FIG. 8 has an L-shaped cross section, and the ring-shaped cooling plate portion in contact with the inner peripheral portion of the coil of the upper cooling plate 18 is in the same state in that it blocks the induced current. The same effect can be obtained by forming the cut portion 25 with. However, since this portion does not contribute much to the cooling effect and eddy current is generated because it is adjacent to the coil 11, it is possible to further improve the response characteristics by deleting it as in this embodiment.

[Embodiment 2] FIG. 2 shows a modification of FIG. 1 in which the joining means for the upper and lower cooling plates is changed from bolts to support plates. In this embodiment, as shown in FIG. 1, the upper and lower cooling plates 27 having the cut portions 25 are pressed by the support plates 28 which are electrical insulators, and the coil 11 and the upper and lower cooling plates are cooled by the tightening bolts 29 and the nuts 30. The plate 27 is tightened and tightly joined. Also, the same result can be obtained by forming the support plate 28 from a metal material and inserting an electrical insulator between the support plate 28 or the tightening bolt 29 and the cooling plate 27.

[Embodiment 3] FIG. 3 shows a structural example of an electron lens. Also in this example, since the coil 11 and the yoke 10 are electromagnetically inductively coupled, a cut portion is formed in the yoke 10 to interrupt the induced current. The same effect can be obtained by forming the cut portion with an electrical insulator. [Embodiment 4] FIG. 4 shows an example of the construction of a pole piece which is the center of an electron lens. Since the pole pieces 14 and 15 also form electromagnetic induction coupling with the coil 11, the pole pieces 14 and 1
A cut portion 25 is formed in 5 to cut off the induced current 22. Similarly, a cut portion 25 is also formed in the spacer 16 that determines the gap 17 of the pole piece, or the spacer 16 is made of a material that is a non-magnetic material and has a high electric resistance to interrupt the induced current.

The method for cutting off the induced current by making a cut in the electrically closed loop circuit around the coil has been mainly described above. However, if the induced current can be reduced, the same effect can be achieved. The member forming the electrically closed loop may be made of a material having high resistance.

[0019]

The induced current of the closed loop circuit around the coil of the electron lens is minimized, the time constant of the circuit is reduced, and the induced current disappears in a short time. Therefore, when the magnetic flux density of the electron lens changes. The response characteristic is remarkably improved, and the response speed is increased by one digit or more as compared with the case where the induced current is not taken into consideration.

[Brief description of drawings]

FIG. 1 is a partially cutaway view illustrating a structure of a coil cooling plate of an electron lens according to an exemplary embodiment of the present invention.

FIG. 2 is a partial cross-sectional view showing a modified example of FIG.

FIG. 3 is a partially cutaway view illustrating a structure of an electron lens according to an embodiment of the present invention.

FIG. 4 is a partially cutaway view showing the structure of a pole piece according to an embodiment of the present invention.

FIG. 5 is a diagram illustrating a configuration of a general electron microscope.

FIG. 6 is a diagram showing a structure of an electron lens of a conventional device.

FIG. 7 is a partially enlarged cross-sectional view showing the structure of a pole piece portion of an electron lens of a conventional device.

FIG. 8 is a partially cutaway view showing a structure of a coil cooling plate of an electron lens of a conventional device.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Electron gun and accelerating tube, 2 ... Electron beam, 3 ... Condenser lens, 4 ... Sample, 5 ... Objective lens, 6 ... Intermediate lens, 7 ... Projection lens, 8 ... Fluorescent plate, 9 ... Sample transmission image, 1
0 ... Yoke, 11 ... Coil, 12 ... Coil cooling part, 13
... pole piece part, 14 ... upper pole piece, 15 ... lower pole piece, 16 ... spacer, 17 ... air gap, 18 ... upper cooling plate, 19 ... lower cooling plate, 20 ... electrical insulator, 21 ... bolt, 22 ... Induced current, 23 ... Upper cooling plate, 24 ... Lower cooling plate, 25 ... Cutting part, 26 ... Insulating spacer, 27 ... Upper and lower cooling plates, 28 ... Support plate, 29 ... Tightening bolt, 30 ... Nut

Claims (9)

[Claims] 1. A coil, a coil cooling plate formed of a material having high thermal conductivity and arranged in thermal contact with the coil, a yoke magnetically coupled to the coil, and connected to the yoke. In the electronic lens including the pole piece, a cutting portion is provided in the coil cooling plate so as not to form an electrical closed loop circuit that electromagnetically couples with the coil. 2. A cutting portion is provided on at least one of the yoke, the pole piece and a non-magnetic spacer for fixing the pole piece so as not to form an electric closed loop circuit for electromagnetically coupling with the coil. The electronic lens according to claim 1, wherein: 3. The electron lens according to claim 1, wherein the cut portion is made of an electrically insulating material. 4. A circuit time constant of an electrically closed loop circuit for electromagnetically inductively coupling at least one of the yoke, the pole piece and a non-magnetic spacer fixing the pole piece to the coil, thereby reducing the closed loop. The electron lens according to claim 1, wherein the electron lens is made of a material having a relatively high electric resistance in order to reduce an induced current flowing through the circuit. 5. A coil, a coil cooling plate formed of a material having high thermal conductivity and arranged in thermal contact with the coil, a yoke magnetically inductively coupled to the coil, and connected to the yoke. In the electronic lens including the pole piece, the coil cooling plate is heated in order to reduce the circuit time constant of the electrical closed loop circuit formed by the coil cooling plate and reduce the induced current flowing in the closed loop circuit. An electronic lens made of a material having a high conductivity and a relatively high electric resistance. 6. A circuit time constant of an electric closed loop circuit for electromagnetically inductively coupling at least one of the yoke, the pole piece and a non-magnetic spacer fixing the pole piece to the coil to reduce the closed loop. The electron lens according to claim 5, wherein the electron lens is made of a material having a relatively high electric resistance in order to reduce an induced current flowing through the circuit. 7. A cutting portion is provided on at least one of the yoke, the pole piece and a non-magnetic spacer for fixing the pole piece so as not to form an electric closed loop circuit electromagnetically inductively coupled with the coil. The electronic lens according to claim 5, wherein: 8. The electron lens according to claim 7, wherein the cut portion is made of an electrically insulating material. 9. An electron microscope comprising the electron lens according to claim 1.