JP4537191B2 - Electron gun - Google Patents

Description

  The present invention relates to an electron gun used in an electron beam utilization apparatus such as an electron microscope.

  In an electron gun used in an electron beam utilization apparatus such as an electron microscope, a high voltage insulation technique capable of stably applying a high voltage for a long time is required. High voltage insulation technology with vacuum has existed for a long time, and various withstand voltage insulation methods have been devised. Many of them have been found empirically and experimentally, and they are still applied to high voltage insulation designs. Recently, simulation analysis has become possible, and high-voltage insulation has been optimized. As an example, Japanese Patent Application Laid-Open No. 2002-313269 discloses an analysis result of an insulator and an electrode shape when a high voltage is applied to an emitter. Techniques for suppression are disclosed.

JP 2002-313269 A

  In order to apply a high acceleration voltage for a long time in an electron gun used in an electron beam analyzer (EBSP, CL), an electron beam drawing apparatus, etc. that require high brightness with high acceleration voltage, the inside of the electron gun chamber is subjected to a high vacuum. It is necessary to perform cleaning to prevent discharge due to Wehnelt contamination, but first, a high voltage is applied by designing the shape of the insulator and homogenizing the electric field on the surface along the insulator. However, it is necessary to prevent discharge from occurring. In an electron gun placed in a vacuum, electrons jump out due to electric field concentration at the triple junction (see 13 in FIG. 1) where the vacuum-insulator-metal (electrode) is in contact. It is considered that creeping discharge that causes an avalanche phenomenon due to the electric field on the surface of the insulator and moves in the ground direction through the creeping discharge path (see 14 in FIG. 1) hinders the withstand voltage. The more electrons move in the direction of the earth, the weaker the withstand voltage and the easier to discharge.

  As a technique for relaxing the electric field of the triple junction, a method of crafting and relaxing the shape of the insulator and the shape of the electrode is generally used. By making the insulator shape to hide the triple point from the electrode side in the vacuum, or concealing the triple point from the electrode side in the vacuum by conversely the electrode shape, the diameter of the electrode is made larger than the diameter of the insulator, The electric field strength was relaxed. In addition, in order to suppress creeping discharge in which electrons after jumping out move toward the ground due to the surface electric field of the insulator, the creepage distance of the insulator is increased, and the electric potential gradient on the insulator surface is relaxed to make the electric field uniform. On the other hand, if the insulator is made of ceramic, a technique such as vacuum degassing has been employed. Here, the creeping distance of a ceramic insulator is generally converted by a surface electric field of 1 kV / mm.

  Since suppression of these discharges is often determined almost at the stage of shape design, it is common to increase the creepage distance for high withstand voltage, while the irradiation device itself is increased in size. In addition, the cost increases due to the complexity of the insulator and electrode shape. Therefore, the insulator design and electrode design that take into account the relaxation of the electric field strength of the triple junction and the suppression of creeping discharge at the same time, and the irradiation device that is small and has excellent high voltage resistance characteristics with the minimum necessary insulation creepage distance. It is considered to be a high device.

  An object of the present invention is to provide such an electron gun that is small in size and excellent in high voltage resistance.

  In order to achieve the above object, an electron gun of the present invention includes a vacuum chamber, an insulator having an inner surface facing the vacuum side of the vacuum chamber and an outer surface facing the atmosphere side, a filament for thermionic emission, and a filament. Wenelt, which is biased to a negative voltage, a guard ring connected to Wenelt and located on the insulator side of Wenelt, an anode placed outside Wenert, and an insulator placed through the insulator. A first high-voltage introduction connection pin that feeds power and a second high-voltage introduction connection pin that is installed through the insulator and applies a potential to Wehnelt are electrically connected to the second high-voltage introduction connection pin on the outer surface of the insulator A metallized electrode is provided. With this configuration, it is possible to reduce the location where the potential gradient is locally localized on the surface of the insulator, and the surface electric field can be made uniform.

  The guard ring is preferably shaped so as to cover the triple junction on the vacuum side. In order to relieve the electric field at the triple point (vacuum / electrode / insulator junction), the insulator is encased. Further, by making the diameter of the metallized electrode on the insulator atmosphere side and the diameter of the vacuum side electrode (guard ring electrode) the same, the potential gradient on the insulator surface can be made uniform.

  The major difference between Patent Document 1 and the present invention is that metallization is performed on the vacuum side in Patent Document 1 whereas metallization is performed on the atmosphere side in the present invention. In Patent Document 1, since the metallization is directly performed on the creeping portion of the insulator on the vacuum side, which is the most important, the arrangement configuration of the components tends to be tight, and the metallization cannot be performed (particularly sharp projections) from sharp projections. However, there is no such technical difficulty in the present invention.

  According to the present invention, it is possible to achieve both improvement of the withstand voltage performance and miniaturization design of the electron gun.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a schematic sectional view showing an embodiment of an electron gun according to the present invention, and FIG. 2 is a top view of the insulator.

  The vacuum chamber 15 evacuated from the exhaust port 11 constitutes the electron gun chamber 6. The filament 1 installed in the electron gun chamber 6 maintained at a high vacuum is heated to about 2700 K to emit thermoelectrons, and the emitted electrons accumulate in the Wehnelt 2 biased to a negative potential. An anode 3 having a ground potential is disposed outside the Wehnelt 2. The high-voltage insulator 4 made of a hard insulating material such as porcelain or ceramic is penetrated by high-voltage introduction connection pins 7a and 7b for feeding the filament 1 and a high-voltage introduction connection pin 8 for feeding the Wehnelt 2, A high voltage is applied from the atmosphere side to the vacuum side via the high-pressure introduction cable 10. The electrode called the guard ring 5 acts to make the electric field distribution on the vacuum side creeping surface of the high voltage insulator uniform. The guard ring 5 may be formed integrally with the Wehnelt 2.

FIG. 3 shows the connection relationship between the filament 1, Wehnelt 2, anode 3 and the power source. The filament 1 is applied with a current of about 3 A and an acceleration voltage of a maximum of −30 kV, and thermoelectrons are emitted. The emission current is proportional to the filament current, but becomes saturated under certain conditions (self-bias effect or variable bias) due to the negative potential of about several hundred V of the Wehnelt 2 potential difference. This saturation depends on the distance between the filament 1 and the Wehnelt 2, the hole diameter of the Wehnelt 2, and the filament current. In the required analysis (EBSP, CL) and electron beam drawing, the filament current is adjusted so that the emission current is saturated with respect to the filament current. The electron gun chamber 6 is maintained in a high vacuum (about 10 ⁻³ to 10 ⁻⁴ Pa), a maximum acceleration voltage of −30 kV is applied from the high voltage introduction cable 10, and the potential of the Wehnelt 2 is about −29.5 kV to −− It becomes about 29.9 kV. This may be considered as substantially the same potential as the acceleration voltage −30 kV applied to the filament 1.

  The electron gun according to the present invention has a metallized electrode 16 electrically connected to the high-voltage introduction connection pin 8 on the atmosphere side of the insulator 4 as means for relaxing the potential gradient on the surface of the insulator 4 and making the surface electric field uniform. Provide. The metallized electrode 16 and the Wehnelt 2 are at the same potential via the high voltage introduction connection pin 8. The outer diameter of the metallized electrode 16 is made substantially the same as the outer diameter of the guard ring 5. The high-voltage introduction connection pins 7 a and 7 b that supply power to the filament 1 are not electrically connected to the metallized electrode 16. The metallization can be formed by a known Mo-Mn method used for fixing a metal to an insulator, and the formed metallization is used as an electrode. In view of the potential around the metallized electrode 16, there is no possibility of discharge due to the installation of the metallized electrode 16. Further, since the metallization is on the atmosphere side, the metallization failure (especially sharp protrusions, etc.) does not cause the discharge that occurs on the vacuum side. The possibility of discharge due to insufficient creepage on the atmosphere side can be avoided by insulating the atmosphere side integration such as the metallized electrode 16, the high voltage introduction cable 10, the cable connection pin of the insulator 4 with the silicon rubber mold 9 by vacuum deaeration. In the figure, 20 is an electron gun cover (ground potential), and 12 is a vacuum seal (O-ring).

  FIG. 4 is a schematic sectional view showing another embodiment of the electron gun according to the present invention. The only difference from the embodiment shown in FIG. 1 is the shape of the guard ring 17 having the same potential as the Wehnelt 2. In the electron gun of this example, a shape that covers the triple junction 13 as the guard ring 17 was adopted. By covering the triple junction 13 with the guard ring 17, electric field concentration at the triple junction 13 can be reduced. Considering that the cause of creeping discharge is the electric field concentration at the triple junction, it is considered that the effect of reducing the surface electric field at the triple junction is great.

  A potential distribution was simulated for the electron gun of the present invention shown in FIGS. In addition, the potential distribution of an electron gun without a metallized electrode on the insulator was also simulated. The condition was that an acceleration voltage of 30 kV was applied, the insulator was made of ceramics, and the dielectric constant was 9.0. The results are shown in FIG. FIG. 5A shows the potential distribution of the electron gun shown in FIG. 1, and FIG. 5B shows the potential distribution of the electron gun shown in FIG. FIG. 5C shows the potential distribution of an electron gun having the same structure as in FIG. 1 and no metallized electrode, and FIG. 5D shows the potential distribution of an electron gun having the same structure as in FIG. 4 and no metallized electrode. The broken arrow represents the direction of the potential line.

  5 (c) and 5 (d), the potential line is directed in the direction of the pin 8 of the high voltage introduction portion of the insulator 4, and in the region 19 on the vacuum insulator surface surrounded by a circle in the figure, It can be seen that the potential lines are dense. From this, it can be inferred that when the surface electric field is locally about 1.5 to 2.0 kV / mm, the avalanche phenomenon is promoted and the discharge is activated. On the other hand, in the case of the electron gun of the present invention in which the metallized electrode 16 is provided on the atmosphere side surface of the insulator 4 as shown in FIGS. Concentration has been relaxed. As described above, the metallized electrode 16 having the same potential as the Wehnelt 2 is provided on the atmosphere side of the insulator 4, and the outer diameter of the metallized electrode 16 and the outer diameter of the guard rings 5 and 17 are made substantially the same. It is possible to reduce the number of places where the potential lines on the four surfaces are dense. This indicates that the insulator design can be performed in consideration of both the electric field relaxation of the triple junction 13 and the electric field uniformity of the insulator surface.

  FIG. 6 shows the surface electric field of the insulator from the triple junction 13 to the region 19. The position indicated as point A in FIG. 6 corresponds to the triple junction 13, and the position indicated as point B corresponds to the region 19 where the potential gradient tends to be high. 5A shows a uniform electric field distribution from the point A to the point B. When the electric field strength is compared at the point A (triple junction) in FIGS. 5B and 5D, FIG. 5 (d) and FIG. 5 (d) are 0.01 kV / mm, whereas FIG. 5 (a) is about 10 times as large as 0.1 kV / mm. This is considered to be because electrons, which are the beginning of creeping discharge, are likely to jump out about 10 times. Therefore, it can be seen that the effect of relaxing the electric field concentration at the point A (triple junction) in FIGS. 5B and 5D appears due to the effect of the shape of the guard ring.

  Further, as shown in the results of FIG. 5, in the conventional electron gun shown in FIG. 5C or FIG. 5D, the potential lines are dense along the insulator (where the potential gradient is tight). ) And the surface electric field at that point greatly exceeds 1.5 kV / mm. For example, in the conventional electron gun shown in FIG. 5C and the electron gun of the present invention shown in FIG. Indicates a larger value. In particular, at the surface B of the insulator, the value of the conventional surface electric field is about twice that of the present invention. As is clear from the above, the structure of the present invention in which the metallized electrode having the same potential as Wehnelt is provided on the atmosphere side of the insulator is an effective structure for withstand voltage characteristics.

  Actually, an electron gun having the structure of the present invention shown in FIG. 5B and an electron gun having a conventional structure shown in FIG. The creepage distance from point A to point B is 24 mm. In an experiment to evaluate the high voltage resistance characteristics with the maximum accelerating voltage that can be applied (AC voltage in the experiment), the conventional electron gun is short-circuited at about 36 kV, whereas the electron gun of the present invention is short-circuited at about 45 kV. I did not. It was proved that the structure of the present invention has a pressure resistance effect about 1.25 times that of the conventional structure. In addition, when comparing the size of the insulator, the insulator of the current type electron gun that is mass-produced has an outer diameter of φ108 mm and a height of 35 mm (equivalent). The diameter was about 17% smaller at almost the same height. As described above, according to the present invention, it is possible to realize high voltage resistance improvement and downsizing of the apparatus.

1 is a schematic sectional view showing an embodiment of an electron gun according to the present invention. The top view of an insulator. The figure which shows the connection relation of a filament, Wehnelt, an anode, and a power supply. FIG. 6 is a schematic sectional view showing another embodiment of the electron gun according to the present invention. The potential distribution simulation diagram. The figure which shows the surface electric field of the insulator from a triple junction part (A point) to the B point where an electric potential gradient tends to become high.

Explanation of symbols

1: Filament, 2: Wehnelt, 3: Anode (ground potential), 4: Insulator (made of ceramic), 5: Electrode (guard ring), 6: Electron gun chamber (high vacuum side), 7: High-pressure introduction connection pin (Filament side), 8: high-pressure introduction connection pin (Welner side), 9: silicon rubber mold (atmosphere side), 10: high-pressure introduction cable, 11: electron gun chamber exhaust port, 13: triple junction (point A), 14: Creeping discharge path, 15: Electron gun chamber, 20: Electron gun cover (ground potential), 16: Metallized electrode, 19: Point B (point where potential gradient tends to increase), 17: Guard ring

Claims (2)

A vacuum chamber;
An insulator having an inner surface facing the vacuum side of the vacuum chamber and an outer surface facing the atmosphere;
A filament for thermionic emission held at the lower vacuum side of the insulator ;
Wehnelt below the insulator and around the filament, biased to a negative voltage with respect to the filament;
A guard ring disposed between the insulator and Wehnelt ;
An anode located at the bottom of the Wehnelt;
A first high-pressure introduction connecting pin that is installed vertically through the insulator and feeds power to the filament;
A second high-voltage introduction connection pin that is installed vertically through the insulator and applies a potential to the Wehnelt ;
Wherein the outer surface upper portion of the insulator, the second is the high-pressure supply connection pin is electrically connected, and has a arranged metallized electrodes so as to face the guard ring through the insulator,
The electron gun, wherein the metallized electrode and the guard ring have the same outer diameter . The electron gun according to claim 1,
The guard gun has a shape that covers a triple junction on the vacuum side.

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