JP2001357810A - Method of using electron emitting cathode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method of using an electron emitting cathode holding a reservoir so as not to easily drop by enduring repeated temperature rises and drops, and having a long life and high reliability. SOLUTION: In this method of using an electron emitting cathode, an electron emitting cathode having a metallic base, a coated layer to lower the work function of the metallic base by coating the surface of the metallic base, and a supply source to supply a substance to compose the coated layer, is operated at a lower temperature than the transformation temperature of the substance composing the supply source.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electron emission cathode used as an electron beam source for an electron microscope, an electron beam exposure device, an electron beam tester, a length measuring device, and the like, and more particularly, to achieving a long life. About the method.

[0002]

2. Description of the Related Art In recent years, an electron emission cathode using a needle-shaped electrode made of tungsten single crystal has been used in order to obtain an electron beam with higher luminance. This electron emission cathode is provided with a coating layer made of zirconium and oxygen (hereinafter, called a ZrO coating layer) on a tungsten single crystal chip (hereinafter, simply called a W chip) having an axial orientation of <100>. The work function of the (100) plane of the tungsten single crystal was reduced to about 2.8 eV due to the presence of the layer, and was formed at the tip of the W chip.
Since only the minute crystal plane corresponding to the plane is the electron emission area, an electron beam with higher brightness than the conventional hot cathode can be obtained,
Moreover, it has a feature of long life. It is also more stable than a cold field emission cathode, operates even at a low vacuum, and is easy to use.

The electron emission cathode has a W tip for emitting an electron beam fixed to a predetermined position of a tungsten wire provided on a metal support fixed to an insulator by welding or the like, and thermoelectrons from the tungsten wire or the like. And a suppressor electrode for forming an electric field for suppressing the emission of light.

[0004] In the electron emission cathode, a supply source of zirconium and zirconium made of zirconium oxide, that is, a reservoir is provided in a part of the W chip.
The surface of the W chip is covered with a ZrO coating layer. The W chip is electrically heated by a tungsten wire and is generally used at a temperature of about 1800 K, and the ZrO adsorption layer on the W chip surface is consumed by evaporation. However, since zirconium and oxygen diffuse from the reservoir, W
Since it is continuously supplied to the surface of the chip, as a result, Zr
The O adsorption layer is maintained.

The structure and operation of the above-mentioned electron emission cathode are described below.
The present invention relates to an electron emission cathode provided with a ZrO coating layer made of zirconium and oxygen (referred to as a ZrO / W emitter). There is known an electron emission cathode (referred to as HfO / W emitter) provided with.

[0006]

A ZrO / W emitter having a high luminance and a long life is widely used in a semiconductor inspection apparatus and the like. When applied to the above-mentioned application field, a ZrO / W emitter is also used in a commercially available ZrO / W emitter. At an operating temperature of 1800K, a life of about one year is obtained. However, when the same electron emission cathode as described above is applied to an SEM or TEM for analytical use, its life is as short as about 2,000 to 5,000 hours, and the dispersion is large, so that satisfactory results cannot be obtained.

The inventor of the present invention has conducted various studies on the problem of short life and variation in the analytical application field, and found that the transformation point of the source of the coating layer is lower than the operating temperature. When setting the temperature, the temperature of the material constituting the coating layer passes through the transformation point, and the volume change that occurs at that time causes cracks in the supply source,
This problem is caused by the fact that the source that supplies the material constituting the coating layer always operates the electron emission cathode so as to maintain the temperature below the transformation point of the material. The inventors have found that the present invention can be solved, and have reached the present invention.

That is, even if the present invention is applied to an electron beam utilizing device having a strong ON-OFF operation, such as an analysis application such as a SEM or a TEM, the reservoir is resistant to repeated temperature rising / falling operations and the reservoir is hardly dropped. As a result, it is an object of the present invention to provide a method of using an electron emission cathode which has a long life and can use the electron emission cathode with high reliability.

[0009]

The present invention comprises a metal substrate,
A method for using an electron emission cathode having a coating layer for coating the surface of the metal substrate and reducing a work function of the metal substrate, and a supply source for supplying a material constituting the coating layer, A method of using an electron emission cathode, wherein the electron emission cathode is operated at a temperature lower than the transformation point of a material constituting the supply source. Preferably, the metal substrate is selected from tungsten, molybdenum, tantalum, and rhenium. The use of the above-mentioned electron emission cathode, and more preferably the use of the above-mentioned electron emission cathode, wherein the supply source is zirconium oxide or hafnium oxide.

[0010]

According to the present invention, by operating at a temperature lower than the transformation point of the material constituting the supply source (reservoir), the material does not pass through the transformation temperature when the temperature rises and falls. Even if the temperature is repeatedly raised and lowered, the reservoir can be prevented from falling without cracking.

In the case of the ZrO / W emitter, the transformation point of the material constituting the reservoir is 1200-1500K, and in the case of the HfO / W emitter, the transformation point of the substance constituting the reservoir is 1900-2100K. Therefore, as an embodiment of the present invention, by setting the operating temperature to less than 1200K for the ZrO / W emitter and to less than 1900K for the HfO / W emitter,
Even if the temperature is repeatedly raised and lowered, it is possible to prevent the reservoir from falling off, and to obtain high reliability. Also, Zr,
Since the oxides of Hf are mutually dissolved at a total rate, they can be used as a mixture, but from the viewpoint of uniformity of the electron emission characteristics,
It is desirable to use it alone as much as possible.

As the metal substrate used in the present invention, tungsten is generally selected, but molybdenum, tantalum and rhenium can also be used in addition to tungsten. In the case where the coating layer of the metal base is provided with tungsten or molybdenum, its portion (100) is generally used.

[0013]

Examples [Examples 1 and 2 and Comparative Examples 1 and 2] After fixing a tungsten wire to a metal support brazed to an insulator by spot welding, W was cut into single-crystal tungsten thin wires of <100> orientation. The tip was attached to the tungsten wire by spot welding, and the tip of the W tip was electrolytically polished to a sharp tip to obtain an electron emission cathode intermediate.

A commercially available zirconium hydride powder was pulverized and mixed in a mortar with isoamyl acetate as a dispersion medium to obtain a slurry. The slurry is mixed with W of the electron emission cathode intermediate.
It was applied to the chip (the position where the W chip was fixed to the tungsten wire and the center of the tip of the W chip) to form a reservoir in advance. After the isoamyl acetate in the slurry evaporates, the tungsten tip is heated to 1800K by applying a current to the tungsten wire in an ultra-high vacuum of 1 × 10 ⁻⁹ Torr (1.3 × 10 ⁻⁷ Pa) to remove zirconium hydride. The reservoir was calcined by thermal decomposition into zirconium and hydrogen, and solidified. Further, under an oxygen atmosphere, 3 × 10 ⁻⁶ Torr (4.0 × 10 ⁻⁴ Torr)
Pa) for 20 hours to oxidize, bake and diffuse the zirconium in the reservoir, and to deposit Zr on the surface of the W chip.
An O coating layer was formed to produce a ZrO / W emitter.

For each of the five electron emission cathodes obtained by the above procedure, 1 × 10 ⁻⁹ Torr (1.3 × 10
Under an ultra-high vacuum of ^-7 Pa), heating and cooling (stopping the current supply) up to 1800 K by current heating were repeated 200 times, and then the state of the reservoir was observed (Comparative Example 1). Also,
1 × for 5 electron emission cathodes obtained by the same operation as above
Under an ultra-high vacuum of 10 ⁻⁹ Torr (1.3 × 10 ⁻⁷ Pa), heating and cooling to 1150 K by applying current and repeating cooling (stopping the current) were repeated 200 times, and the state of the reservoir was observed ( Example 1).

Further, an HfO / W emitter is manufactured by replacing zirconium hydride with hafnium hydride.
A similar experiment was performed. However, the temperatures at the time of heating when forming the coating layer and heating and cooling were 2150 K (Comparative Example 2) and 1800 K (Example 2), respectively.

Table 1 shows the results of Examples 1 and 2 and Comparative Examples 1 and 2. In Comparative Example 1 and Comparative Example 2 in which the heating temperature was higher than the transformation point of the reservoir, the reservoir dropped off at 70 or less times of heating and cooling, but in Examples 1 and 2 where the heating temperature was lower than the transformation point of the reservoir. No dropout was observed even after the temperature was raised and lowered 200 times.

[0018]

[Table 1]

[Examples 3 and 4 and Comparative Examples 3 and 4] Apart from those used in the above evaluations, an electron emission cathode was prepared according to the above-described procedure, mounted on a scanning electron microscope, and actually used. The number of heating and cooling cycles and the life under the circumstances were investigated. Note that Z
For rO / W, the operating temperature was 1800K (Comparative Example 3) and 1150K (Example 3), and for HfO / W, the operating temperature was 2150K (Comparative Example 4) and 1800K.
(Example 4)

The results are shown in Table 2. Comparative Examples 3 and 4 in which the operating temperature was higher than the transformation point of the reservoir were 600
Although the service life was less than 0 hour, in Examples 3 and 4 where the operating temperature was lower than the transformation point of the reservoir, a long service life of 8000 hours or more was obtained.

[0021]

[Table 2]

[0022]

The method of using the electron-emitting cathode of the present invention operates the electron-emitting cathode such that the source supplying the material constituting the coating layer always maintains a temperature below the transformation point of the material. Compared to the conventional method of use, the reservoir does not fall off even after repeated heating and cooling, and a long life is achieved stably. As in the case of analytical applications such as SEM and TEM, ON-
This method is suitable for an electron beam-using device with a severe OFF operation.

Claims (3)

[Claims] 1. A metal substrate and a surface of said metal substrate,
A method of using an electron emission cathode having a coating layer for lowering the work function of a metal substrate and a supply source for supplying a material constituting the coating layer, wherein the transformation of the material constituting the supply source is performed. A method of using an electron emission cathode, which is operated at a temperature lower than the temperature. 2. The method according to claim 1, wherein the metal substrate is one selected from the group consisting of tungsten, molybdenum, tantalum and rhenium. 3. The method according to claim 1, wherein the supply source is zirconium oxide or hafnium oxide.